English for Thermal engineering by Huy N.Q, Manh N.P - Danang University of Science and Technology

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English for Thermal engineering

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Chapter 1: Introduction to Thermodynamics ......................................................................... 4

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Contents

Closed and open system: ................................................................................................ 4

Closed system ......................................................................................................... 4

1.2

Open system: .......................................................................................................... 4

2.2

Atmospheric pressure: ........................................................................................... 5

2.3

Gage pressure (Pg) .................................................................................................. 6

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Temperature and the Zeroth Law.................................................................................... 7

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Heat transfer, Q [J] ......................................................................................................... 8

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Latent heat .............................................................................................................. 8

4.2

Sensible heat ........................................................................................................... 8

4.3

Modes of heat transfers: .......................................................................................... 9

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4.1

Work, W [J] ................................................................................................................. 10

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Energy E [J] ................................................................................................................. 12

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Chapter 2 Energy and Environment ..................................................................................... 14

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2.1 Energy and environment ................................................................................................ 14

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2.2 Acid Rain Formation ..................................................................................................... 15 2.3 The Greenhouse Effect .................................................................................................. 15

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Absolute pressure: ................................................................................................. 5

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2.1

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Pressure ......................................................................................................................... 5

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2.4 Global Warming and Climate Change ........................................................................... 16 2.5 Negative impacts of global warming.............................................................................. 17 2.5.1 Increase in average temperatures and temperature extremes .................................... 17 2.5.2 Extreme weather events .......................................................................................... 17 2.5.3 Ice melt .................................................................................................................. 17 1 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

2.5.4 Social effects .......................................................................................................... 17 Chapter 3: The laws of thermodynamics.............................................................................. 19 3.1 The first law of thermodynamics ................................................................................... 19

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3.2.1 Introduction to the second law of thermodynamics ................................................. 21

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3.2 The second law of Thermodynamics ............................................................................. 21

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3.2.2 The second law of thermodynamics states: ............................................................. 22

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Chapter 4: Reversed cycles and refrigeration/air conditioning system.................................. 24

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4.1.2 THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE .................... 26

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4.1.3 ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE ........................ 29

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4.2 Air conditioning and ventilating systems ....................................................................... 32 4.2.1 How does an air conditioner work .......................................................................... 32

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4.2.2 Moisture Removal - Dehumidification .................................................................... 33

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4.2.3 Refrigerants ............................................................................................................ 33

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4.2.4 Types of commercial air conditioners ..................................................................... 35

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1) Window or Wall Units .............................................................................................. 35

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2) Split System Air Conditioners .................................................................................. 36

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3) Ductless Split Systems.............................................................................................. 37

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4) Evaporative Coolers ................................................................................................. 37

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5) Package Air Conditioning Units ............................................................................... 39

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4.1.1 THE REVERSED CARNOT CYCLE .................................................................... 25

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4.1 The fundamentals of refrigeration.................................................................................. 24

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3.3 Thermodynamic cycles .................................................................................................. 22

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4.3 Ventilating system ......................................................................................................... 40

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CHAPTER 5: BOILER AND THERMAL POWER PLANT .............................................. 41 5.1 Rankine Cycles ............................................................................................................. 41 5.1.1 The simplest Rankine cycle .................................................................................... 41 5.1.2 Rankine cycle with vapor superheating ................................................................... 41 2 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

5.1.3 Rankine cycle with vapor reheating ........................................................................ 42 5.1.4 Regenerative Rankine Cycle with Feed Water Heating ........................................... 43 5.1.4 Losses in Rankine Cycles ....................................................................................... 43

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5.2 Boiler ........................................................................................................................... 45

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5.1.5 Combined Brayton-Rankine Cycle.......................................................................... 43

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5.2.1 Introduction to the steam generator ......................................................................... 45

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5.2.3 Main parts of boiler ................................................................................................ 46

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3) Superheater .............................................................................................................. 47

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4) Reheater ................................................................................................................... 48

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5) Drum or Separator .................................................................................................... 48 6) Air heaters ................................................................................................................ 48

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7) Types of Pressures ....................................................................................................... 49 Superheater (SH) Outlet Pressure ............................................................................. 49

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Drum Pressure .......................................................................................................... 49

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Design Pressure ........................................................................................................ 50

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Reheater (RH) Inlet and Outlet Pressures .................................................................. 50

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Calculation Pressure ................................................................................................. 50

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SH Outlet Temperature ............................................................................................. 50

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8) Outlet Temperatures..................................................................................................... 50

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RH Outlet Temperature ............................................................................................ 50 Feed Water Temperature .......................................................................................... 50

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2) Economizer .............................................................................................................. 47

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1) Furnace .................................................................................................................... 46

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5.2.2 Reasons for superheating steam .............................................................................. 46

3 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

Chapter 1: Introduction to Thermodynamics The aim of this chapter is to understand the definition of basic thermodynamic parameters.

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It (also known as a control mass) consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass can enter or leave a closed system, as shown in Fig. 2. But energy, in the form of heat or work, can cross the boundary; and the volume of a closed system does not have to be fixed. If, as a special case, even energy is not allowed to cross the boundary, that system is called an isolated system.

Figure 2: a closed system

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1.2 Open system:

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1.1 Closed system

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Figure 1: System, boundary and surroundings

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A system is defined as a quantity of matter or a region in space chosen for study.

The mass or region outside the system is called the surroundings.The real or imaginary surface that separates the system from its surroundings is called the boundary.

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1 Closed and open system:

An open system,or a control volume,as it is often called, is a properly selected region in space. It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. Flow through these devices is best studied by selecting the region within the device as the control volume. Both mass and energy can cross the boundary of a control volume. This is illustrated in Fig. 3.

4 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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2 Pressure

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2.2 Atmospheric pressure: The air surrounding the earth can be treated as a homogeneous gas, called atmosphere. Atmospheric pressure (Pa) is the pressure due to the force by the atmosphere’s mass. Standard atmospheric pressure is 101.325 kPa (at sea level and the temperature is 150C).

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2.1 Absolute pressure: The absolute pressure (P) is the force of a fluid acting on unit area. In the SI system, the unit for pressure is Pascal (1Pa = 1 N/m2),. In the English system, it is psi.

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Figure 3: An open system

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English for Thermal engineering

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Figure 4: Atmosphric pressure

5 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

2.3 Gage pressure (Pg) is the difference between the absolute pressure and the atmospheric pressure if the difference is positive. If the difference is negative, it is called vacuum pressure (Pv).

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Pa = ρ g h

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Barometer is a device used to measure the atmospheric pressure.

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where:

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ρ = 13,595 kg/m3 g = 9.807 m/s2 h = 0.76 m = 760 mm

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In case of mercury:

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Particularly,

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Other units

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Figure 5: a basic Barometer

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1 bar = 105 Pa; 1 at = 1 kG/cm2 = 10 mH2O = 9.807×104 Pa; 1 atm = 101.325 kPa

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ρ = The density of the working liquid, kg/m3 g = The acceleration of gravity, 9.8 m/s2 h = The height of the working liquid in the tube, m

Figure 6: Absolute, gage and vacuum pressures

6 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

3

Temperature and the Zeroth Law

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If two bodies at different temperatures are brought together, the hot body will warm up the cold one. At the same time, the cold body will cool down the hot one. This process will end when the two bodies have the same temperatures. At that point, the two bodies are said to have reached thermal equilibrium.

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a. Definition: The measurement of the degree of hotness or coolness is temperature.

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c. Thermometers measure temperature, by using materials that change in some way when they are heated or cooled. In a mercury or alcohol thermometer the liquid expands as it is heated and contracts when it is cooled, so the length of the liquid column becomes longer or shorter depending on the temperature. Modern thermometers are calibrated in standard temperature units such as Fahrenheit or Celsius. Three practical points for using thermometer are:  The thermometer should be isolated to everything except the body which temperature is measured. The general method is to immerse the thermometer in a hole in a solid body, or directly in a fluid body.  When thermal equilibrium is reached, the thermometer can indicate its own temperature as well as the body measured. The thermometer should be small relative to the body so that it only has a small effect upon the body.  The thermometer must not be subject to effects such as pressure changes, which might change the volume independently of temperature.

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Notice: The Zeroth Law of thermodynamics is a basis for the validity of temperature measurement.

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The Zeroth Law of thermodynamics states: Two bodies each in thermal equilibrium with a third body will be in thermal equilibrium with each other.

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Conversion of temperature scales: 0C = 5/9(0F-32); K = 0C + 273.15; 0F = 9/50C + 32

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b. Temperature scales: Celsius (0C), Fahrenheit (0F), Kelvin (K)…

7 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

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4.1 Latent heat

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Unit conversion: 1 Btu = 1055.056 J; 1 cal = 4.184 J; 1N.m = 1 J

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Latent heat is related to changes in phase between liquids, gases, and solids. Latent heat is defined as the energy absorbed by or released from a substance during a phase change from a gas to a liquid or a solid.

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4.2 Sensible heat Sensible heat is related to changes in temperature of a gas or object with no change in phase.

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a. Definition: Heat is energy transferred from one system to another solely by reason of a temperature difference between the systems. Heat exists only as it crosses the boundary of a system and the direction of heat transfer is from higher temperature to lower temperature. b. Heat Unit: Heat is measured in terms of calorie, defined as the amount of heat necessary to raise the temperature of 1 g of water at a pressure of 1 atm by 1 degree Celsius (ºC). In mechanical engineering practice in the United State and the United Kingdom, heat is measured in British thermal unit or Btu. One Btu is the quantity of heat required to raise the temperature of 1 lb of water to 1 degree Fahrenheit (ºF) and is equal to 252 cal.

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Figure 7: Thermometers. (a) Liquid-in-glass. (b) Infrared - sensing ear thermometer.

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4 Heat transfer, Q [J]

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Digital thermometers almost replace the mercury ones in nowadays because they are more accurate and easier to use.

For thermodynamics sign convention, heat transferred to a system is positive (Q > 0); Heat transferred from a system is negative (Q < 0). The heat needed to raise an object's temperature from T1 to T2 is: Q = Cp m (T2 - T1)

(2)

8 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

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Convection: Heat transfer between a solid surface and an adjacent gas or liquid.

It is the combination of conduction and flow motion. Heat transferred from a solid surface to a liquid adjacent is conduction. And then heat is brought away by the flow motion.

Figure 8: Three types of heat transfer

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Radiation: The energy emitted by matter in the form of electromagnetic waves as a result of the changes in the electronic configurations of the atoms or molecules.

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Conduction: Heat transferred between two bodies in direct contact.

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4.3 Modes of heat transfers:

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+ Specific heat C: the heat capacity, or the measure of the amount of heat required to raise the temperature of a unit mass of a substance one degree is known as specific heat. If the heating process occurs while the substance is maintained at a constant volume or is subjected to a constant pressure the measure is referred to as a specific heat at constant volume or at constant pressure. The latter is always larger than, or at least equal to, the former for each substance. In case of water and other approximately incompressible substances, it is not necessary to distinguish between the constant-volume and constant pressure specific heat, as they are approximately equal. Generally, the two specific heats of a substance depend on the temperature.

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where, Cp is specific heat of the object at constant pressure; and m is mass of the object.

9 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

5 Work, W [J]

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In thermodynamics sign convection, work transferred out of a system is positive with respect to that system. Work transferred in is negative.

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Work, like heat, is an energy interaction between a system and its surroundings. In other word, work is the energy transfer associated with a force acting through a distance.

A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work interactions.

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A gas in a piston–cylinder assembly undergoes an expansion process for which the relationship between pressure and volume is given by =

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The initial pressure is 3 bar, the initial volume is 0.1 m3, and the final volume is 0.2 m3. Determine the work for the process, in kJ, if (a) n = 1.5, (b) n = 1.0, and (c) n = 0.

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Known: A gas in a piston–cylinder assembly undergoes an expansion for which p =

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Find: Evaluate the work if (a) n = 1.5, (b) n = 1.0, and (c) n = 0

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Example:

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W=

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For a change in volume from V1 to V2, the work is obtained by integrating (3):

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The work expression can be written as: δW = Vdp

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English for Thermal engineering

Schematic and Given Data: The given p–Vrelationship and the given data for pressure and volume can be used to construct the accompanying pressure– volume diagram of the process:

Analysis: The required values for the work are obtained by integration of Eq. (4) using the given pressure–volume relation.

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(a) Introducing the relationship p = into Eq. (4) and performing the integration: The constant in this expression can be evaluated at either end state. p =

=p

: The work expression then becomes

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p

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3. The expansion is a polytropic process.

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2. The moving boundary is the only work mode.

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1. The gas is a closed system.

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Assumptions:

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English for Thermal engineering

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a. Definition: Energy is the capacity of doing work. It may exist in a variety of forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear. All forms of energy are interconvertible by appropriate processes. In the process of transformation either kinetic or potential energy may be lost or gained, but the sum total of the two remains always the same. This is known as the principle of conservation of energy: energy cannot be created or destroyed; it is transferred from one type of energy to another. For example,

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Heating water by gas: Chemical energy ---> thermal energy Running nuclear power plant: Nuclear energy ---> electric energy Flying rocket: Chemical energy ---> thermal Energy ---> Kinetic Energy

Total energy of a system consists of the kinetic, potential, and internal energies and is expressed as: E = U + KE + PE

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Heating water by electricity: electric energy ---> thermal energy

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6 Energy E [J]

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(c) For n = 0, the pressure–volume relation reduces to p = constant, and the integral becomes W = p(V2 - V1), which is a special case of the expression found in part (a). Substituting values and converting units as above,W = 30 kJ.

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To evaluate W, the pressure at state 2 is required. This can be found by using which on rearrangement yields or p = (b) For n = 1.0, the pressure–volume relationship is p = / . The work is Substituting values

(6)

or, on a unit mass basis, 12 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

e = u + ke + pe

(7)

Closed systems whose velocity and elevation of the center of gravity remain constant during a process are frequently referred to asstationary systems.The change in the total

Note: In the thermodynamics, total energy of a stationary system is its internal energy.

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Enthalpy, H:

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H = U + pV In the thermodynaics, we usually find this group. For simplicity, they were gathered into a new property called Enthalpy. It has the same properties of internal energy.

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1. The temperature of 150 kg of water is raised from 150C to 850C by addition of heat. How much heat energy in kilojoules is supplied?

2. 20 kg of water at 650C are supplied with 5000 kJ of heat energy. What the mass of water will be vaporized? Note: r = 2256.54 kJ/kg; Cp = 4.184 kJ/kgK. 3. Determine the quantity of heat in kilojoule required to vaporize 50 kg of water at a saturation temperature of 1000C

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Internal energy (U): The energy associated with the random, disordered motion of molecules. It is the sum of the kinetic and potential energies of all molecules.

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PE = mgh

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Potential Energy (PE): The energy that a system possesses as a result of its elevation in a gravitational field or change of configurations.

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where, m = mass of the system; v = velocity of the system

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KE = mv2/2

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b. Classification: - Kinetic Energy (KE): The energy that a system possesses as a result of its motion.

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energy E of a stationary system is identical to the change in its internal energy U.

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English for Thermal engineering

Chapter 2 Energy and Environment

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The aim of this chapter is to give a background about energy, environment, the greenhouse effect and climate change.

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(Source: Fundamentals of thermal-fluid science, Y. A. Çengel)

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2.1 Energy and environment

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Hundreds of elements and compounds such as benzene and formaldehyde are known to be emitted during the combustion of coal, oil, natural gas, and wood in electric power plants, engines of vehicles, furnaces, and even fireplaces. Some compounds are added to liquid fuels for various reasons (such as MTBE to raise the octane number of the fuel and also to oxygenate the fuel in winter months to reduce urban smog). The largest source of air pollution is the motor vehicles, and the pollutants released by the vehicles are usually grouped as hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO). The HC emissions are a large component of volatile organic compounds (VOC) emissions, and the two terms are generally used interchangeably for motor vehicle emissions. A significant portion of the VOC or HC emissions are caused by the evaporation of fuels during refueling or spillage during spit back or by evaporation from gas tanks with faulty caps that do not close tightly. The solvents, propellants, and household cleaning products that contain benzene, butane, or other HC products are also significant sources of HC emissions.

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The conversion of energy from one form to another often affects the environment and the air we breathe in many ways, and thus the study of energy is not complete without considering its impact on the environment. Fossil fuels such as coal, oil, and natural gas have been powering the industrial development and the amenities of modern life that we enjoy since the 1700s, but this has not been without any undesirable side effects. From the soil we farm and the water we drink to the air we breathe, the environment has been paying a heavy toll for it. Pollutants emitted during the combustion of fossil fuels are responsible for smog, acid rain, and global warming and climate change. The environmental pollution has reached such high levels that it became a serious threat to vegetation, wild life, and human health. Air pollution has been the cause of numerous health problems including asthma and cancer. It is estimated that over 60,000 people in the United States alone die each year due to heart and lung diseases related to air pollution.

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English for Thermal engineering

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You have probably noticed that when you leave your car under direct sunlight on a sunny day, the interior of the car gets much warmer than the air outside, and you may have wondered why the car acts like a heat trap. This is because glass at thicknesses encountered in practice transmits over 90 percent of radiation in the visible range and is practically opaque (nontransparent) to radiation in the longer wavelength infrared regions. Therefore, glass allows the solar radiation to enter freely but blocks the infrared radiation emitted by the interior surfaces. This causes a rise in the interior temperature as a result of the energy buildup in the car. This heating effect is known as the greenhouse effect, since it is utilized primarily in greenhouses.

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2.3 The Greenhouse Effect

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The sulfur oxides and nitric oxides react with water vapor and other chemicals high in the atmosphere in the Figure 2.1: Formation of acid rain presence of sunlight to form sulfuric and nitric acids (Fig. 2.1). The acids formed usually dissolve in the suspended water droplets in clouds or fog.

These acid-laden droplets, which can be as acidic as lemon juice, are washed from the air on to the soil by rain or snow. This is known as acid rain. The soil is capable of neutralizing a certain amount of acid, but the amounts produced by the power plants using inexpensive highsulfur coal has exceeded this capability, and as a result many lakes and rivers in industrial areas such as New York, Pennsylvania, and Michigan have become too acidic for fish to grow.

Forests in those areas also experience a slow death due to absorbing the acids through their leaves, needles, and roots. Even marble structures deteriorate due to acid rain.

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Fossil fuels are mixtures of various chemicals, including small amounts of sulfur. The sulfur in the fuel reacts with oxygen to form sulfur dioxide (SO2), which is an air pollutant. The main source of SO2 is the electric power plants that burn high-sulfur coal. The Clean Air Act of 1970 has limited the SO2 emissions severely, which forced the plants to install SO2 scrubbers, to switch to low-sulfur coal, or to gasify the coal and recover the sulfur. Motor vehicles also contribute to SO2 emissions since gasoline and diesel fuel also contain small amounts of sulfur. Volcanic eruptions and hot springs also release sulfur oxides (the cause of the rotten egg smell).

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2.2 Acid Rain Formation

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In a 1995 report, the world’s leading climate scientists concluded that the earth has already warmed about 0.5 0C during the last century, and they estimate that the earth’s temperature will rise another 2 0C by the year 2100. A rise of this magnitude is feared to cause severe changes in weather patterns with storms and heavy rains and flooding at some parts and drought in others, major floods due to the melting of ice at the poles, loss of wetlands and coastal areas due to rising sea levels, variations in water supply, changes in the ecosystem due to the inability of some animal and plant species to adjust to the changes, increases in epidemic diseases due to the warmer temperatures, and adverse side effects on human health and socioeconomic conditions in some areas.

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The global climate change is due to the excessive use of fossil fuels such as coal, petroleum products, and natural gas in electric power generation, transportation, buildings, and manufacturing, and it has been a concern in recent decades. In 1995, a total of 6.5 billion tons of carbon was released to the atmosphere as CO2. The current concentration of CO2 in the atmosphere is about 360 ppm (or 0.36 percent). This is 20 percent higher than the level a century ago, and it is projected to increase to over 700 ppm by the year 2100. Under normal conditions, vegetation consumes CO2 and releases O2 during the photosynthesis process, and thus keeps the CO2 concentration in the atmosphere in check. A mature, growing tree consumes about 12 kg of CO2 a year and exhales enough oxygen to support a family of four. However, deforestation and the huge increase in the CO2 production in recent decades disturbed this balance.

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The greenhouse effect makes life on earth possible by keeping the earth warm (about 30 0C warmer). However, excessive amounts of these gases disturb the delicate balance by trapping too much energy, which causes the average temperature of the earth to rise and the climate at some localities to change. These undesirable consequences of the greenhouse effect are referred to as global warming or global climate change.

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2.4 Global Warming and Climate Change

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Figure 2.2: Greenhouse effect

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The greenhouse effect is also experienced on a larger scale on earth. The surface of the earth, which warms up during the day as a result of the absorption of solar energy, cools down at night by radiating part of its energy into deep space as infrared radiation. Carbon dioxide (CO2), water vapor, and trace amounts of some other gases such as methane and nitrogen oxides act like a blanket and keep the earth warm at night by blocking the heat radiated from the earth. Therefore, they are called “greenhouse gases,” with CO2 being the primary component. Water vapor is usually taken out of this list since it comes down as rain or snow as part of the water cycle and human activities in producing water (such as the burning of fossil fuels) do not make much difference on its concentration in the atmosphere (which is mostly due to evaporation from rivers, lakes, oceans, etc.). CO2 is different, however, in that people’s activities do make a difference in CO2 concentration in the atmosphere.

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English for Thermal engineering

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English for Thermal engineering

N

In the 1997 meeting in Kyoto (Japan), the world’s industrialized countries adopted the Kyoto protocol and committed to reduce their CO2 and other greenhouse gas emissions by 5 percent below the 1990 levels by 2008 to 2012. This can be done by increasing conservation efforts and improving conversion efficiencies, while meeting new energy demands by the use of renewable energy (such as hydroelectric, solar, wind, and geothermal energy) rather than by fossil fuels.

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2.5 Negative impacts of global warming

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2.5.1 Increase in average temperatures and temperature extremes

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Scientists project that extreme weather events, such as heat waves, droughts and rainstorms will continue to occur more often and with greater intensity due to global warming, according to Climate Central. Climate models forecast that global warming will cause climate patterns worldwide to experience significant changes. These changes will likely include major shifts in wind patterns, annual precipitation and seasonal temperatures variations.

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2.5.3 Ice melt

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One of the most dramatic effects of global warming is the reduction in Arctic sea ice: In 2012, scientists saw the smallest amount of Arctic ice cover ever recorded. Most analyses project that, within a matter of years, the Arctic Sea will be completely ice-free during the summer months.

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Extreme weather is an effect of global warming. While experiencing some of the hottest summers on record, much of the United States also has been experiencing colder than normal winters. Global warming may also lead to extreme weather other than cold or heat extremes.

For example, hurricane formations will change. Though this is still a subject of active scientific research, current computer models of the atmosphere indicate that hurricanes are more likely to become less frequent on a global basis, though the hurricanes that do form may be more intense. This is because hurricanes get their energy from the temperature difference between the warm tropical ocean and the cold upper atmosphere. Global warming increases that temperature difference.

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2.5.2 Extreme weather events

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One of the most immediate and obvious effects of global warming is the increase in temperatures around the world. The average global temperature has increased by about 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past 100 years, according to the National Oceanic and Atmospheric Administration (NOAA). In 2014, some cities in the United States had the warmest summers on record, according to Scientific American. A report by the World Meteorological Organization released July 3, 2014, said that deaths from heat increased by more than 2,000 percent over the previous decade.

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2.5.4 Social effects

As dramatic as the effects of climate change are expected to be on the natural world, the projected changes to human society may be even more devastating.

Agricultural systems will likely be dealt a crippling blow. Though growing seasons in some areas will expand, the combined impacts of drought, severe weather, lack of snowmelt, greater number and diversity of pests, lower groundwater tables and a loss of arable land could cause severe crop failures and livestock shortages worldwide.

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English for Thermal engineering

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In addition to less nutritious food, the effect of global warming on human health is also expected to be serious. The American Medical Association has reported an increase in mosquito-borne diseases like malaria and dengue fever, as well as a rise in cases of chronic conditions like asthma, are already occurring, most likely as a direct result of global warming.

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English for Thermal engineering

Chapter 3: The laws of thermodynamics

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Thermodynamic, a field of physics that describes and correlates the physical properties of macroscopic systems of matter and energy. The principle of thermodynamics are of fundamental importance to all branches of science and engineering.

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We have considered various forms of energy such as heat Q, work W, and total energy E individually, and no attempt has been made to relate them to each other during a process. The first law of thermodynamics, also known as the conservation of energy principle, provides a sound basis for studying the relationships among the various forms of energy and energy interactions. Based on experimental observations, the first law of thermodynamics states that energy can be neither created nor destroyed; it can only change forms. Therefore, every bit of energy should be accounted for during a process.

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We all know that a rock at some elevation possesses some potential energy, and part of this potential energy is converted to kinetic energy as the rock falls. Experimental data show that the decrease in potential energy exactly equals the increase in kinetic energy when the air resistance is negligible, thus confirming the conservation of energy principle.

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Consider a system undergoing a series of adiabatic processes from a specified state 1 to another specified state 2. Being adiabatic, these processes obviously cannot involve any heat transfer, but they may involve several kinds of work interactions. Careful measurements during these experiments indicate the following: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process.

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3.1 The first law of thermodynamics

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When a macroscopic system moves from one state of equilibrium to another, a thermodynamic process is said to take place. Some processes are reversible and other are irreversible. The laws of thermodynamics govern the nature of all thermodynamic processes and place limits on them.

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A central concept of thermodynamic is the macroscopic system. The state of a macroscopic system in equilibrium can be described in terms of such measurable properties as temperature, pressure and volume, which are known as thermodynamic variables. Many other variables (such as density, specific heat, compressibility, and the coefficient of thermal expansion) can be identified and correlated, to produce a more complete description of an object and its relationship to its environment.

19 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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Considering that there are an infinite number of ways to perform work interactions under adiabatic conditions, this statement appears to be very powerful, with a potential for far reaching implications. This statement, which is largely based on the experiments of Joule in the first half of the nineteenth century, cannot be drawn from any other known physical principle and is recognized as a fundamental principle. This principle is called the first law of thermodynamics or just the first law.

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Taking heat transfer tothe system and work done bythe system to be positive quantities, the energy balance for a closed system can also be expressed as:

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They can also be expressed in the rate form as:

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The first law of thermodynamics is essentially an expression of the conservation of energy principle, also called the energy balance. The general mass and energy balances for any system undergoing any process can be expressed as:

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A major consequence of the first law is the existence and the definition of the property total energy E. Considering that the network is the same for all Figure 9: The energy change of a system during a process is equal to the network and heat transfer adiabatic processes of a closed system between two between the system and its surroundings. specified states, the value of the network must depend on the end states of the system only, and thus it must correspond to a change in a property of the system. This property is the total energy.

Note that the first law makes no reference to the value of the total energy of a closed system at a state. It simply states that the change in the total energy during an adiabatic process must be equal to the net work done. Therefore, any convenient arbitrary value can be assigned to total energy at a specified state to serve as a reference point.

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Therefore, it is reasonable to conclude that a process must satisfy the first law to occur. However, as explained here, satisfying the first law alone does not ensure that the process will actually take place. It is common experience that a cup of hot coffee left in a cooler room eventually cools off (Fig. 2). This process satisfies the first law of thermodynamics since the amount of energy lost by the coffee is equal to the amount gained by the surrounding air.

Now let us consider the reverse process—the hot coffee getting even hotter in a cooler room as a result of heat transfer from the room air. We all know that this process Figure 10: A cup of hot coffee does not get hotter never takes place. Yet, doing so would not violate the first law as long as the amount of energy lost by the air is equal to the amount gained by the coffee.

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It is clear from this argument that processes proceed in a certain direction and not in the reverse direction. The first law places no restriction on the direction of a process, but satisfying the first law does not ensure that the process will actually occur. This inadequacy of the first law to identify whether a process can take place is remedied by introducing another general principle, the second law of thermodynamics.

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We applied the first law of thermodynamics, or the conservation of energy principle, to processes involving closed and open systems. As pointed out above, energy is a conserved property, and no process is known to have taken place in violation of the first law of thermodynamics.

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3.2.1 Introduction to the second law of thermodynamics

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3.2 The second law of Thermodynamics

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The use of the second law of thermodynamics is not limited to identifying the direction of processes. The second law also asserts that energy has quality as well as quantity. The first law is concerned with the quantity of energy and the transformations of energy from one form to another with no regard to its quality. Preserving the quality of energy is a major concern to engineers, and the second law provides the necessary means to determine the quality as well as the degree of degradation of energy during a process.

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For a constant-pressure process, Wb +U = H.Thus,

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3.2.2 The second law of thermodynamics states:

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All important thermodynamic relations used in engineering are derived from the first and second laws of thermodynamics. One useful way of discussing thermodynamic processes is in terms of cycles – processes that return a system to its original state after a number of stages, thus restoring the original values for all the relevant thermodynamic variables. In a complete cycle the internal energy of a system depends solely on these variables and cannot change.

Therefore, the total net heat transferred to the system must equal the total net work delivered from the system.

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An ideal cycle would be performed by a perfectly efficient heat engine – that is, all the heat would be converted to mechanical work. The 19th century, Nicolas Carnot, who conceived a thermodynamic cycle that is the basic cycle of all heat engines, showed that such an ideal engine cannot exist. Any heat engine must expend some fraction of its heat input as exhaust.

The second law of thermodynamic places an upper limit on the efficiency of engines; that upper limit is less than 100 percent. The limiting case is known as a Carnot cycle.

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3.3 Thermodynamic cycles

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Figure 11: A refrigerator that violates the Clausius statement of the second law

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Kelvin–Planck statement: “It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.”

The Clausius statement is expressed as follows: “It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher- temperature body.” (See Figure 3)

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Carnot cycle is composed of four reversible processes—two isothermal and two adiabatic— and it can be executed either in a closed or a steady-flow system. Carnot cycle principles: 1. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs.

22 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.

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Figure 12: Execution of the Carnot cycle in a closed system

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Figure 13: P-Vdiagram of the Carnot cycle

23 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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Chapter 4: Reversed cycles and refrigeration/air conditioning system (Source: Fundamentals of thermal-fluid science, Y. A. Çengel)

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Another device that transfers heat from a low-temperature medium to a high-temperature one is the heat pump (Fig. 1b). Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only. The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it. Meanwhile, the objective of a heat pump, however, is to maintain a heated space at a high temperature.

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Refrigerators are cyclic devices, and the working fluids used in the refrigeration cycles are called refrigerants. A refrigerator is shown schematically in Fig. 1a. Here, QL is the magnitude of the heat removed from the refrigerated space at temperature TL; QH is the magnitude of the heat rejected to the warm space at temperature TH, and Wnet,in is the net work input to the refrigerator.

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We all know from experience that heat flows in the direction of decreasing temperature, that is, from high-temperature regions to low-temperature ones. This heat transfer process occurs in nature without requiring any devices. The reverse process, however, cannot occur by itself.

The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.

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4.1 The fundamentals of refrigeration

Figure 14. The schematic of Refrigerator and heat pump The performance of refrigerators and heat pumps is expressed in terms of the coefficient of performance (COP), which was defined as: 24 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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Since it is a reversible cycle, all four processes that comprise the Carnot cycle can be reversed.

Reversing the cycle does also reverse the directions of any heat and work interactions. The result is a cycle that operates in the counterclockwise direction on a T-s diagram, which is called the reversed Carnot cycle. A refrigerator or heat pump that operates on the reversed Carnot cycle is called a Carnot refrigerator or a Carnot heat pump.

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Figure 15. The schematic of Carnot Refrigerator and its T-s diagram

Consider a reversed Carnot cycle executed within the saturation dome of a refrigerant, as shown in Fig. 2. The refrigerant absorbs heat isothermally from a low-temperature source at TL in the amount of Q (process 1-2), is compressed isentropically to state 3 (temperature rises to TL), rejects heat isothermally to a high-temperature sink at TH in the amount of QH (process 3-4), and expands isentropically to state 1 (temperature drops to TH). The refrigerant changes from a saturated vapor state to a saturated liquid state in the condenser during process 3-4.

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The Carnot cycle is a totally reversible cycle that consists of two reversible isothermal and two isentropic processes. It has the maximum thermal efficiency for given temperature limits, and it serves as a standard against which actual power cycles can be compared.

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4.1.1 THE REVERSED CARNOT CYCLE

The coefficients of performance of Carnot refrigerators and heat pumps are expressed in terms of temperatures as: 25 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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Notice that both COPs increase as the difference between the two temperatures decreases, that is, as TL rises or TH falls.

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4.1.2 THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE

1-2: Isentropic compression in a compressor 2-3: Constant-pressure heat rejection in a condenser 3-4: Throttling in an expansion device 4-1: Constant-pressure heat absorption in an evaporator

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Many of the impracticalities associated with the reversed Carnot cycle can be eliminated by vaporizing the refrigerant completely before it is compressed and by replacing the turbine with a throttling device, such as an expansion valve or capillary tube. The cycle that results is called the ideal vapor-compression refrigeration cycle, and it is shown schematically and on a T-s diagram in Fig. 3. The vapor-compression refrigeration cycle is the most widely used cycle for refrigerators, air-conditioning systems, and heat pumps. It consists of four processes:

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It seems as if these problems could be eliminated by executing the reversed Carnot cycle outside the saturation region. But in this case we have difficulty in maintaining isothermal conditions during the heat-absorption and heat rejection processes. Therefore, we conclude that the reversed Carnot cycle cannot be approximated in actual devices and is not a realistic model for refrigeration cycles. However, the reversed Carnot cycle can serve as a standard against which actual refrigeration cycle are compared.

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The two isothermal heat transfer processes are not difficult to achieve in practice since maintaining a constant pressure automatically fixes the temperature of a two-phase mixture at the saturation value. Therefore, processes 1-2 and 3-4 can be approached closely in actual evaporators and condensers. However, processes 2-3 and 4-1 cannot be approximated closely in practice. This is because process 2-3 involves the compression of a liquid–vapor mixture, which requires a compressor that will handle two phases, and process 4-1 involves the expansion of high-moisture-content refrigerant in a turbine.

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The reversed Carnot cycle is the most efficient refrigeration cycle operating between two specified temperature levels. Therefore, it is natural to look at it first as a prospective ideal cycle for refrigerators and heat pumps. If we could, we certainly would adapt it as the ideal cycle. As explained below, however, the reversed Carnot cycle is not a suitable model for refrigeration cycles.

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In an ideal vapor-compression refrigeration cycle, the refrigerant enters the compressor at state 1 as saturated vapor and is compressed isentropically to the condenser pressure. The temperature of the refrigerant increases during this isentropic compression process to well above the temperature of the surrounding medium. The refrigerant then enters the condenser as superheated vapor at state 2 and leaves as saturated liquid at state 3 as a result of heat rejection to the surroundings. The temperature of the refrigerant at this state is still above the temperature of the surroundings.

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The saturated liquid refrigerant at state 3 is throttled to the evaporator pressure by passing it through an expansion valve or capillary tube. The temperature of the refrigerant drops below the temperature of the refrigerated space during this process. The refrigerant enters the evaporator at state 4 as a low-quality saturated mixture, and it completely evaporates by absorbing heat from the refrigerated space. The refrigerant leaves the evaporator as saturated vapor and reenters the compressor, completing the cycle.

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Notice that unlike the ideal cycles discussed before, the ideal vapor compression refrigeration cycle is not an internally reversible cycle since it involves an irreversible (throttling) process.

This process is maintained in the cycle to make it a more realistic model for the actual vaporcompression refrigeration cycle. If the throttling device were replaced by an isentropic turbine, the refrigerant would enter the evaporator at state 4’ instead of state 4. As a result, the refrigeration capacity would increase (by the area under process curve 4’-4 in Fig. 3) and the net work input would decrease (by the amount of work output of the turbine). Replacing the expansion valve by a turbine is not practical, however, since the added benefits cannot justify the added cost and complexity.

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cycle.

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Figure 16. Schematic and T-s diagram for the ideal vapor-compression refrigeration

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27 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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Heat pump mode

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4.1.3 ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE An actual vapor-compression refrigeration cycle differs from the ideal one in

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several ways, owing mostly to the irreversibilities that occur in various components. Two common sources of irreversibilities are fluid friction (causes

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pressure drops) and heat transfer to or from the surroundings. The T-s diagram

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Figure 18: Schematic and T-sdiagram for the actual vapor-compression refrigeration cycle.

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In the ideal cycle, the refrigerant leaves the evaporator and enters the compressor as saturated vapor.In practice, however, it may not be possible to control the state of the refrigerant so precisely. Instead, it is easier to design the system so that the refrigerant is slightly superheated at the compressor inlet. This slight overdesign ensures that the refrigerant is completely vaporized when it enters the compressor. Also, the line connecting the evaporator to the compressor is usually very long; thus the pressure drop caused by fluid friction and heat transfer from the surroundings to the refrigerant can be very significant. The result of superheating, heat gain in the connecting line, and pressure drops in the evaporator and the connecting line is an increase in the specific volume, thus an increase in the power input requirements to the compressor since steady-flow work is proportional to the specific volume.

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of an actual vapor-compression refrigeration cycle is shown in Fig. 5

29 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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throttling valve. We do not mind this at all, however, since the refrigerant in this case enters the evaporator with a lower enthalpy and thus can absorb more

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heat from the refrigerated space. The throttling valve and the evaporator are usually located very close to each other, so the pressure drop in the connecting line is small.

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Example:

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the refrigerant to the throttling valve before the refrigerant is completely condensed. Therefore, the refrigerant is subcooled somewhat before it enters the

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that the refrigerant is a saturated liquid at the end, and it is undesirable to route

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Also, it is not easy to execute the condensation process with such precision

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The compression processin the ideal cycle is internally reversible and adiabatic, and thus isentropic. The actual compression process, however, will involve frictional effects, which increase the entropy, and heat transfer, which may increase or decrease the entropy, depending on the direction. Therefore, the entropy of the refrigerant may increase (process 1-2) or decrease (process 1-2) during an actual compression process, depending on which effects dominate. The compression process 1-2 may be even more desirable than the isentropic compression process since the specific volume of the refrigerant and thus the work input requirement are smaller in this case. Therefore, the refrigerant should be cooled during the compression process whenever it is practical and economical to do so. In the ideal case, the refrigerant is assumed to leave the condenser as saturated liquidat the compressor exit pressure.

In actual situations, however, it is unavoidable to have some pressure drop in the condenser as well as in the lines connecting the condenser to the compressor and to the throttling valve.

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Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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4.2 Air conditioning and ventilating systems 4.2.1 How does an air conditioner work An air-conditioner consists of several primary components as shown in Fig. 6:

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• A compressor that is driven by an electric motor (typically located outdoors)

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• An outdoor fan to circulate air over the hot condenser fins and tubing

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The second law of Thermodynamics

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• A refrigerant fluid to operate at the needed pressures and temperatures

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• An indoor fan to circulate air over the cold evaporator tubing and fins

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Figure 19 – Schematic of an air conditioner

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The compressor “sucks” the refrigerant from point 4 through tubing in the evaporator coil. This action causes the liquid refrigerant to “evaporate” and become cold. The evaporating refrigerant inside the tubes cools the air being circulated over the outside of the tubes and fins by the indoor fan.

In order to move the refrigerant from point 1 to point 2, it must be raised to a higher pressure by the “compressor”. The compressing causes the refrigerant to become hot (a similar effect occurs with an air compressor and this can be verified by quickly and carefully touching the discharge line).

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• An evaporator (or indoor) coil with tubing and many fins to cool the air

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•An expansion device (usually located indoors) to lower system pressure

The hot refrigerant is sent through the tubing inside the condenser. Outdoor air circulated by the fan cools the refrigerant and causes it to return to a liquid (condense). Even though the air may be warm it is cooler than the hot refrigerant. In some cases, water can be used to cool the hot refrigerant if higher efficiency is needed. The warm liquid leaving the condenser (point 3) 32 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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passes through an expansion device which lowers the refrigerant pressure before it returns to point 4 to repeat the cycle.

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Commonly used refrigerants are carbon dioxide, ammonia and chemicals called nonhalogenated hydrocarbons, with the type of refrigerant selected based on the specific cooling application. In the past, chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC) were used, but CFCs were phased out of production in the 1990s and HCFCs will be phased out of production by 2030 because of their ozone depletion potential.

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Some of refrigerants are being used widely Pictures of storage tank

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R-134A is the only approved alternative refrigerant to retrofit R-12 air conditioning systems in vehicles, which is also its most common use.

Ozone Depletion Potential of zero.

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A refrigerant is a heat carrier that is used to move heat from a room to be cooled to the outside.

With regard to the vapor-compression cycle, the refrigerant is the working fluid of the cycle which alternately vaporizes and condenses as it absorbs and gives off heat respectively.

Generally speaking, a good refrigerant have to meet following properties: inexpensive, nonpoisonous, nonexplosive, noncorrosive, nonflammable, stable, high latent heat of vaporization, easy to vaporize and condense, easy to detect leaks, no environment pollution.

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4.2.3 Refrigerants

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In addition to cooling internal spaces, air-conditioners also provide dehumidification. The original intent behind the invention of the air conditioner was to remove humidity from industrial spaces, with the cooling of the air considered a secondary effect. The removal of moisture during air-conditioning unit operation occurs when the relatively warm air of the building interior is pulled across the cold evaporator coils. Since physics dictates that warm air can hold more water than cool air, the cooling of the building air as it contacts the evaporator coils causes it to release moisture which forms as condensation on the coils. This condensation eventually drips off and is collected and drained off to the building exterior or to a sewer connection. A reduction in building humidity tends to improve the comfort level of the occupants by enhancing the effectiveness of the body’s natural cooling system. The combination of humidity removal and temperature decrease defines the “conditioning” of the air.

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4.2.2 Moisture Removal - Dehumidification

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Ozone Depletion Potential of zero.

New equipments.

An alternative solution for R-22

R-404A

R404A is a HFC blend, used as a replacement for R502 and R22 in low and medium temperature refrigeration applications. Although it is a popular refrigerant, R404A is under great scrutiny due to its high Global Warming Potential (GWP). This has led to an increased focus on lower GWP alternatives such as R407F and R407A, along with newer alternatives such as R448A and R449A.

R-410A

R410A refrigerant is a leading high pressure alternative to R-22, comprising R-32 and R-125, for air conditioning and refrigerant applications for new equipment.

R-22

R-22 refrigerant is one of the most commonly used refrigerants in the air conditioning systems. R-22 is the short name for the halocarbon compound CHClF2 (monochlorodifluoromethane), which is used as the refrigerant. R stands for the refrigerant.

In the number “22" second “2" denotes the number of the fluorine atoms in the compound.

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Advantages of R22 Refrigerant: - Has less effect on ozone layer.

- Low compressor displacement.

- Greater water absorbing capacity.

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R-407C

34 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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4.2.4 Types of commercial air conditioners

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Window units house all the parts of an air conditioner – compressor, condenser, expansion valve, coil, and more – in one box that fits into the window of your home. Their installation is simple and they’re very portable. Wall units function similarly to window units, though their installation is not as simple since they require wall modification.

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1) Window or Wall Units

Figure 8

a. Benefits include:   

Simple wiring allows the user to locate the unit near a home electrical outlet. They are compact and don't require a lot of space.

The primary maintenance is to keep the filter cleaned or replaced periodically.

Compared to other air conditioning equipment, they are low in cost.

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Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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 

They are self-contained and generally require few tools for installation.

Although they are commonplace items, window or wall units can have drawbacks as well as advantages, depending on what the buyer needs.

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In order for split system AC units to function, they have to have a delivery system for cool air.

That delivery system is ducting. As the cooling process in the indoor evaporator coils takes place a blower or air handler blows across the coils to distribute cooling. The cool air enters ducts, channeling that air to every room in the home. So, unlike other air conditioners, split system AC units are able to cool an entire home with only one combined system. The cost of a split system is higher in comparison than other air conditioners but the cost difference is offset by the need for only one system to cool an entire home or building.

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a. How they work – Ducting

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Noisy when window mounted. These units can cause loose sashes to vibrate and the equipment itself can often vibrate and generate noise.

Require tight sealing whether wall or window installed. Air leakage around the unit reduces their effectiveness and drives up the cost to operate.

Because they are a single unit, window air conditioners invite easy ingress into unprotected homes by thieves if not properly installed.

Older units require more energy to run.

Multiple units are required for whole-house cooling.

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Because window or wall units are limited in size, they are generally limited in effectiveness as well. They are usually ineffective in cooling more than one room. Other limitations are:

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b. Disadvantages:

Figure 20

b. Advantages: While split AC systems are expensive, they can allow an entire structure to be cooled. Other benefits to split AC units revolve around the ducts:

36 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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Properly installed ducts deliver cool air anywhere in the home, even to the room farthest away from the evaporator coil.

Well-circulated air pushes out stale air into the return ducts and removes contaminants to be trapped in the filter, keeping indoor air clean.

When ducting is located inside walls or other conditioned areas or is well-insulated, energy loss is minimized and a split system AC is an energy-efficient method of cooling.

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Ductless split air conditioning systems are comprised of an indoor unit mounted to the wall, and an outdoor component that sits next to your home. While ductless systems don’t use ducts, as their name implies, they do use hoses to run from the room(s) you want to cool to the outdoor condenser unit.

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3) Ductless Split Systems

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Poor installation of ducts can lower the efficiency of your equipment. The shortest distance between your inside unit and the various rooms in the home is best. If ducting winds around objects, has sharp turns or is improperly sized, this slows down airflow speed, wastes energy and reduces comfort.

Metal ducting transfers heat and cold quickly. If the ducts are not inside conditioned spaces or properly insulated, there is a transfer of cooling into areas of the home where energy is wasted.

Filters must be maintained in split system AC units to ensure good airflow and proper comfort. Filters should be changed every three months or when dirty to prevent damage to the system and clogged air circulation.

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Since split system AC units are so dependent on a balanced installation of ductwork, there can be disadvantages to these units as well:

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4) Evaporative Coolers a. The Concept 37 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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Dry air is much more receptive to evaporation than humid air. These cooling units perform best in areas with low humidity which allows for quick evaporation.

A modern evaporative cooler is usually mounted on a roof or in a window. It operates with a fan blowing air across pads that are soaked with water and the pads must have a constant water supply to maintain their moisture.

The pads must be cleaned regularly or changed to remove thick, mineralized deposits since the water evaporates but leaves any minerals behind. The units must also be drained in winter to prevent damage to the equipment.

Since both humidity and temperature fluctuate, evaporative coolers work best on some days, poorly on others. c. The Benefits Air conditioning requires having a sealed home to maintain efficiency. Evaporative coolers are healthier because they use a constant flow of fresh air. The fan draws outside air in, and open windows or door, keep a breeze of healthy air flowing through the home.

They have a low environmental impact. The cost of using evaporative cooling is about one-third the cost of standard air conditioning.

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Evaporation can take place only in climates with a low humidity because of the following issues:

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b. The Drawbacks

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Evaporative cooling works best in dry climates by using the evaporation of water to draw heat out of the air. As a breeze or fan blows over a moist medium, such as fabric, the water molecules disperse into the air, absorbing the surrounding heat. If you come out of a pool or lake into the surrounding air on a hot day, as the water evaporates from your body, you feel cooled. It's the same concept.

Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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

Those concerned with healthy air for both the home and the planet have a solution for their needs with this type of cooling unit.

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Since they are a single unit designed for a confined space, their efficiency is unmatched for the job they are designed to perform. Package AC units are an inexpensive option for dorm rooms and other facilities giving the occupant full control over their environment. With no need for ducting and no requirement to cool a large multi-room area, package AC units are a smart way to bring cool comfort to such areas as:

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a. Self-Contained Efficiency

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These air conditioners are sold and installed as a single unit. The compressor is located on an outside wall or rooftop much like any other central air conditioning unit. Just inside the wall or ceiling space, the blower section and controls sit. Since the equipment is a package unit, there is no need for ducting. The refrigerant circulates the short distance from the outside compressor to the inside air handler and delivers cool air to the interior room instantly. There is no distance for cool air to travel, thus the units are very efficient in cooling quickly with little to no waste in efficiency.

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5) Package Air Conditioning Units

Motel and hotel rooms Campus dormitories Single and double occupied nursing home rooms Garage apartments Guest houses Small office spaces

a. Disadvantages 39 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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While they are excellent for certain spaces, cooling a whole house room by room with package AC units can be costly. Large rooms or multiple room areas don't lend themselves to good air distribution with package cooling equipment. Cooling energy can become wasted in large, spacious areas. One end of the room can be quite comfortable while the farthest areas do not receive sufficient cooling. Package units can also be noisier than other methods of air conditioning.

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Simple ventilation devices include fans or blowers that are arranged either to exhaust the stale air from the building or to force fresh air into the building, or both. Ventilating systems may be combined with heaters, filters, humidity controls, or cooling devices. Many systems include heat exchangers. These use outgoing air to heat or cool incoming air, thereby increasing the efficiency of the systems by reducing the amount of energy needed to operate it.

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Engineers estimate that for adequate ventilation the air in a room should be changed completely from one and a haft to three times each hour, or that about 280 to 850 liters (about 10 to 30 ft3) of outside air per minute should be supplied for each occupant. Providing this amount of ventilation usually requires mechanical devices to augment the natural flow of air

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Factory ventilation systems must remove hazardous airborne contaminants from the workplace. Nearly all chemical processes generate hazardous waste gases and vapour, and these must be removed from the workplace environment in a cost-effective manner. Chemical engineers, in particular, are involved in ventilation design for factories and refineries.

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Buildings in which people live and work must be ventilated to replenish oxygen, dilute the concentration of carbon dioxide and water vapor, and minimize unpleasant odors. A certain amount of air movement or ventilation ordinarily is provided by air leakage through small crevices in building’s walls, especially around windows and doors. Such haphazard ventilation may suffice for homes, but not for public buildings such as offices and theaters, or factories

40 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

CHAPTER 5: BOILER AND THERMAL POWER PLANT 5.1 Rankine Cycles

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By closely examining the T-s diagram of the simple Rankine cycle (Figure 1), it is possible to notice that at the exit of the expansion machine (turbine) a mixture of liquid and vapor is present (state 5). Usually, a vapor quality at and below around 90% can cause damage to the turbine blades by erosion due to the impact of droplets at high velocity on them. The way to get around the blade impact problem is done by introducing a first modification on the simple Rankine Cycle. Usually, a superheater is installed at the exit of the steam generator in order to superheat the saturated vapor to higher temperatures T as seen in Figure 2. Usually, the superheater is an additional piece of equipment integrated to the steam generator.

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5.1.2 Rankine cycle with vapor superheating

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Figure 24: Schematic diagram of the simplest Rankine cycle and its T_s diagram

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The simplest Rankine cycle is the one based on four reversible processes as shown in Figure 1. Saturated liquid 1 undergoes an isentropic compression process to reach compressed liquid at state 2. Next, the compressed liquid is driven to the steam generator, where heat Q is added to obtain saturated vapor at state 4. Useful work is produced in an expansion machine, such as a steam turbine, in an isentropic process yielding fluid at state 5. Finally, there occurs condensation by removing heat Q in the condenser to close the cycle and the fluid returns to the initial state 1. All processes are ideal. The diagram T-s also shows the corresponding Carnot Cycle 1’-3-4-5-1’. Clearly, one can see that the Carnot cycle has a higher thermal efficiency than the simple Rankine cycle by simply reasoning that heat is delivered to the Rankine cycle at an average temperature (between T2 and TH) lower than the one for the Carnot cycle (TH).

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5.1.1 The simplest Rankine cycle

41 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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Clearly, by heating up the working fluid to higher temperatures, a higher thermal efficiency will also be obtained without any additional increase in the working pressure. However, there is an additional cost of the superheating stage installation.

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The previous Rankine cycle configuration can solve the problem of wet steam at the turbine exit. However, it brings about a new problem that is to superheat the turbine inlet temperature to a considerable high value. To solve this, the solution is to expand the vapor to an intermediate pressure and direct the vapor back to the steam generator to reheat it. Next the superheat vapor is expanded in a second stage of the steam turbine. The schematics of this configuration can be seen in Figure 3. What really is done is to expand the vapor in stages so that the expansion process progresses around the vapor saturation curve in a way such vapor quality is not too high in the end of each stage. Figure 3 shows a two-stage steam turbine, but additional stages are also possible.

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Figure 25: Schematic diagram of the vapor superheating Rankine cycle and its T_s diagram

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5.1.3 Rankine cycle with vapor reheating

Figure 26: Schematic diagram of the vapor reheating Rankine cycle and its T_s diagram

42 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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friction with turbine blades, aerodynamics losses as the vapor flows around the blades as well as entrance and exit losses deviate the expansion process from the ideal one. Pumps are also not ideal pieces of equipment and it is required to take that into account. Other losses are associated with working fluid flow in pipes and accessories which cause distributed and local pressure losses. Finally, heat addition and rejection are not isothermal.

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5.1.5 Combined Brayton-Rankine Cycle

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a. Gas turbine Cycle

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Figure 27: Schematic diagram of the Regenerative Rankine cycle and its T_s diagram

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The most common machine to produce net shaft work in Rankine Cycles are steam turbines.

As an actual device, a steam turbine is not an isentropic one and internal losses due to fluid

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5.1.4 Losses in Rankine Cycles

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Neither one of the previous Rankine cycle variations solves the problem of lower thermal efficiency of the Rankine cycle when compared with the equivalent Carnot cycle. However, one of the other problems is related to the heat addition at a low temperature liquid that enters the steam generator coming from the pump. In order to solve this, a regenerative cycle was conceived. Usual practice consists of making partial expansion in the steam turbine, extract part of the vapor and mixes it with condensing water in a feedwater heater as indicated in . The corresponding T-s diagram is shown in Fig. 19b. The thermodynamic analysis is a little more difficult because the equations now must take into account that only part of the vapor continues the expansion (1-m1), while the other part (m1) will undergo mixing with the condensing water.

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5.1.4 Regenerative Rankine Cycle with Feed Water Heating

43 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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As analyzsed in section (a), a relatively high exhaust gas temperature is obtained in gas turbines. Considering economic and environmental issues it makes sense to recover such amount of thermal energy to produce other useful effect. The heat from exhaust gas in gas turbine cycle can be recovered for following purposes:

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(1) Using the exhaust gases to power an absorption refrigeration cycle. As a result air conditioning can be obtained for controlling the environment in working areas or, even, to cool off the gas turbine inlet air Temperature.

(2) Using a compact heat exchanger to heat a fluid for any process purpose, including hot water and steam.

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b. Combined Brayton-Rankine Cycle

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Gas turbines are complex turbo machines made up of thousands of parts. Nevertheless, gas turbines have three main parts that perform the fundamental thermodynamic processes involved in the mechanical shaft power production from the fuel chemical energy as illustrated in Figure 5. First, the income atmospheric air must undergo a compression process in the compressor section where both pressure and temperature are increased. Next, the compressed air is driven to a combustion chamber where fuel is injected into the compressed air stream and burnt increasing the temperature at a constant pressure process. Finally, the combustion products at a high temperature and pressure are expanded in the power turbine section generating shaft power to drive the compressor as well as an electrical generator or any other rotary device attached to the rotary shaft. The combustion products are exhausted through a nozzle into the atmosphere.

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Figure 28: Schematic diagram of Gas turbine Cycle

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English for Thermal engineering

(3) Using a heat recovery steam generator (HRSG) to produce steam at a temperature and pressure enough to power a steam turbine (or other expansion machine) in a Rankine cycle.

In high capacity gas turbine plant, there is only the latter case is utilized. Figure 21 shows aschematic of a combined Brayton-Rankine cycle. Notice that steam to power the Rankine cycle is produced in the HRSG, which is driven by the exhaust gases from the Brayton cycle.

The combined cycle efficiency is given by equation: 44 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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where, WB is the net shaft power produced by the gas turbine (Brayton cycle), WR is the net shaft power produced by the Rankine cycle, and QH is the thermal power due to fuel combustion. The pumping power in the Rankine cycle has been neglected. Overall, the combined cycle efficiency is higher than that of Brayton cycle or Rankine cycle.

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Figure 29: Schematic diagram of Combined Brayton-Rankine Cycle

5.2 Boiler 5.2.1 Introduction to the steam generator Boiler or Steam generator helps to burn the coal and transfer the heat to water, thereby generating steam at the required pressure and temperature. Coal is fed to the Raw Coal Bunkers 45 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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that is sufficient to store one day requirement. Then, the coal is pulverized into a fine powder in the pulverisers or mills and carried away into the boiler by hot air at 60-70 supplied by Primary Air Fan. The fuel/air mixture is fed in the four corners of the boiler. For complete combustion of coal in the furnace, secondary air is supplied by Forced Draft fans. Feed water is supplied to the boiler drum situated at the top of the boiler from the economizer which make use of the heat in flue gas to preheat the feed water to increase the boiler efficiency. Water converts into steam in the furnace area and the dry and saturated steam is collected in the top portion of the drum. The saturated steam is then flows through superheat tubes that hang in the hottest part of the combustion flue gases path as it exits the furnace. Here the steam is superheated to about 5400C before being routed into the high pressure steam turbine.

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English for Thermal engineering

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Place for combustion and enclosure of fire flame Heat exchange between fire and water to heat water to high temperature.

Protection of leakage of fire flame

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The furnace is a place where supplied coal and air are burning out to change its energy from chemical to heat. Its purpose is to achieve the most efficient transfer of heat energy from the fuel to the working fluid. Water wall generally consist of tubes and fins which are welded each other and allow for water to go through it. Followings are the purpose of it:

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The combustion chamber are constructed entirely of water-cooled tubes (water riser tubes).

These are formed into four walls which surround and form the furnace. The water contained within these tubes forms the boiler circulatory system and heat energy is transferred to the water by radiation, conduction and convection. Coal in form of pulverized fuel os admitted to the furnace via burner nozzles.

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5.2.3 Main parts of boiler

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By superheating the steam, most of the work is done by the steam before it begins to become wet steam as only sensible heat an approximately 12% of the latent heat is able to produce work within the turbine.

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If the steam was dry saturate steam, it would immediately begin to become wet steam as it followed through the turbine. In the early stage of the turbine the steam would drive the blades but as the moisture in the steam increased the blades would drive the steam. Also the impact of the water droplets on the blades would cause erosion along with the reduction in turbine efficiency.

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When steam is supplied to a turbine it gives up heat as it travels through the blade system. As it gives up heat, work occurs on the turbine blades.

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5.2.2 Reasons for superheating steam

46 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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This is for preheating feed water by reusing exhaust gas at the end of boiler. Accordingly the more heat absorption the more efficiency the boiler can get. And by reducing the temperature difference between heated water from water wall and feed water to drum, it is possible to reduce a thermal stress of drum shell.

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The economizer utilizes heat from exhaust gas to raise the temperature of the feedwater entering the boiler to near boiling point. If the economizer raises the feedwater temperature above boiling point then the economizers is said to be a steaming economizer. During starting up or light load conditions the economizer can be damaged by corrosive effect of flue gasses below the dew point temperature. To minimize this corrosive effect a bypass damper may be fitted to bypass flue gasses around the economizer under these conditions.

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English for Thermal engineering

3) Superheater

The steam from water wall is a saturated steam which means water and steam are coexisted.

The steam separated by drum or separator is superheated by superheater to increase the plant efficiency and to protect the turbine from water inflow. Superheaters may be the horizontal self-draining or the pendant non-draining type.

47 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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In the horizontal type the pipes are arranged in a continuous loop which progress in a vertical manner thus making them self-draining. However, with the pendant type the hole elements are vertical and may be located in a top section of the furnace where they receive heat by radiation as well as convection

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The steam from high pressure turbine has low pressure but high temperature. The reheater is a item to reuse the steam with high calorie for intermediate pressure turbine and low pressure turbine.

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Figure 31: Boiler Drum

Figure 31: Tubes Air heater

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The boiler drum will contain steam baffles and steam scrubbers to ensure that under normal steaming conditions the steam is almost completely dry saturated condition when it leaves the drum for superheaters. Steam washing equipment may also be provided so that any dissolved impurities in steam/water mixture exiting the boiler drum can be removed.

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 The separation of steam from the mixture of steam and water being fed into it  The boiler drum houses equipment for the purification of the steam after separation.

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The boiler drum of a subcritical boiler serves two following functions:

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5) Drum or Separator

6) Air heaters

To assist combustion the air entering the furnace combustion chamber is preheated. This is achieved by passing the inlet air through a device called an air heater (air preheater). There are a number of different types of air heater in use today, but the objective is the same for all and that is to elevate the furnace inlet air temperature to aid combustion.

48 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

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Superheater outlet pressure is the pressure at which steam is available for further consumption.

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Usually it is ~5% higher than process or turbine inlet pressure to account for the piping loss, that is, pressure losses in piping, bends, flow nozzle, and valves.

In fixed-pressure boilers, this pressure is held constant as the turbine inlet valves vary the steam pressure to the inlet nozzles. In variable-pressure boilers, where steam is supplied directly to the turbine inlet nozzles, the steam outlet temperature varies with load.

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a. Superheater (SH) Outlet Pressure

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7) Types of Pressures

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Two main types of air heaters have been used in large power stations for many year with comparatively little change in basic design. They are type of tube and rotating regenerative design.

b. Drum Pressure Drum pressure is the pressure at which the circulation of steam and water takes place in evaporator circuits. Drum pressure is ~10% higher than steam outlet temperature (SOP) to account for steam pressure loss in SH and attemperator.

49 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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English for Thermal engineering

c. Design Pressure

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Reheater design pressure is the highest safety valve lift pressure (SVLP) on the inlet side of the RH. Boiler codes may vary slightly in defining the design pressures for boiler and RH.

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In principle, the boiler design pressure is usually the lifting pressure of the second safety valve (or the highest set safety valve lifting pressure if there are more than two safety valves) on the drum and it forms the basis for the strength calculations of the boiler pressure parts (PPs)in drum-type boilers. The design pressure limits the maximum operating pressure of a boiler.

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a. SH Outlet Temperature

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Superheater outlet temperature is the temperature at which the steam leaves the SH. It is usually the highest temperature attained by the steam in a boiler. RH steam is sometimes slightly higher than the SH steam in large utility boilers. Superheater outlet temperature up to ~350°C is usually not controlled, being used mainly for process heating. Then SOT reduces as the load reduces. For power application, the SOTs are generally ~400°C and are required to be controlled in two ways:

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1. By limiting the temperature within a band of ~5°C for the safety of the turbine casing by means of an attemperator

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2. By keeping the temperature constant up to usually 70% load by sizing the SH larger (a range of 70–100% is then known as the steam temperature control [STC] range of the boiler)

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Reheater outlet temperature (RHOT) is the temperature at which steam leaves the RH. Reheater inlet temperature is turbine-dependent and it is lower at lower loads. The RHOT is required to be held constant for optimum cycle efficiency. Since RH pressure is one-third of the main steam pressure, the RHOT is usually equal to or preferably higher than the SOT.

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8) Outlet Temperatures

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Calculation pressure is the design pressure, adjusted for factors such as the static head and used for calculating the strength of individual components. There may be a single boiler design pressure, whereas the calculating pressures can vary for different boiler parts.

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e. Calculation Pressure

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These pressures are turbine-dependent based on the power cycle design. Unlike SOP, which is held constant in drum-type boilers, RH pressure slides downward at lower loads.

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d. Reheater (RH) Inlet and Outlet Pressures

c. Feed Water Temperature Feed water temperature (FWT) varies from as low as 85°C for very low-capacity process plants with no feed heating (when there is no FW contamination) to as high as 270°C in very high50 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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1. At 85°C (usually return condensate from process and not deaerated), the FW needs to be dosed with adequate quantities of sodium sulfite to protect the internals from pitting. It can enter the economizer (ECON) thereafter only if the fuel contains negligible sulfur to cause no corrosion. Otherwise it needs to be heated i n the lower drum or in the external heat exchanger (HX) to safe temperature.

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pressure (HP) power plants with several stages of feed heating. Feed water temperatures to boilers are usually at five levels.

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4. When HP heaters are in line, the FWTs are usually between 160 and 270°C, depending on the size of the turbine, as it should be large enough to accommodate the steam extractor nozzles.

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3. The next level is between 130 and 140°C, common with small power plants of ~15 MWe where HP heaters are avoided, and increased heating is done in the deaerator itself.

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2. At 105°C, the FW is deaerated and heated in a deaerator. This is the most common temperature with process industries. Depending on S in fuel, the FW may be heated in the deaerator itself or in lower drum or in external heater by 10–15°C to protect the ECON or the ECON can be avoided by use of an AH.

51 Designed by: Huy N.Q, Manh N.P – Danang University of Science and Technology (DUT)

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