WASTE HEAT UTILIZATION
2
Heat Content of Substances Certain calculations are required to determine the heat content of waste heat sources and hence a variety of equations are presented below for the use of the reader. As will be seen, the applicability of any particular equation may depend on the material itself as well as the state of the material. Sensible and Latent Heat When heat energy is added to, or removed from a substance, either the temperature of the substance changes or the substance changes state. Heat energy associated with a change in temperature is called sensible heat. The amount of energy is dependant upon the temperature change, the amount of material, and the specific heat of the material expressed in kJ/(kg.°C). Heat energy associated with a change of state (or phase) is called latent heat. Figure 1 illustrates the change of state process for ice-water-steam. When the change of state is from steam to water, the latent heat of condensation is removed. When the change of state is from water to steam, the latent heat of evaporation is added. For a given material, the latent heat of evaporation is equal to that of condensation at a given temperature and pressure. Latent heat is generally dependent on material properties, temperature and pressure. The units for latent heat are expressed in kJ/kg.
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Example of Change of State Figure J
Enthalpy
The quantity of heat energy in a substance depends upon its heat capacity, the temperature change it experiences and whether or not it goes through a change of phase. This energy content can be expressed quantitatively by the use of a term known as heat content or "enthalpy", commonly denoted as "h" (kJ/kg). For ease of calculations, it is assumed that the enthalpy of all substances in a system is zero at a base temperature. For example, this base temperature is O°C for steam tables. For most refrigerant tables, -40°C is the base temperature. Enthalpy is a property of state of a substance; that is, for a given material its value is always the same at a given temperature and pressure. Change in enthalpy can be calculated as follows: Dh
=
hfinal
-
hinitiaJ
• The enthalpy of water (h[) is a measure of the amount of heat energy contained in the water (sensible heat) at a specific temperature. • The enthalpy of evaporation (hfg) (correctly called the latent heat of vaporization) is the quantity of heat energy required to convert one kg of water to one kg of steam at the given pressure. • The enthalpy of steam (hg) is the total energy contained in dry saturated steam at the given pressur
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