This document covers air-handling systems in HVAC design, focusing on fans and duct systems. It discusses fan types, performance characteristics, testing procedures, and duct design principles. The content provides essential knowledge for HVAC engineers and designers to select appropriate fans and design efficient air distribution systems.
by GxP Cellators Consultants Ltd.
In most HVAC systems, the final energy transport medium is moist air—a mixture of dry air and water vapor. This is conveyed through filters, heat exchange equipment, ducts, and various terminal devices to the space to be airconditioned. The power to move the air is supplied by fans. This chapter discusses fans and duct systems, together with related subjects such as grilles, registers, diffusers, dampers, filters, and noise control.
According to Air Moving and Conditioning Association (AMCA) Standard 210, "A fan is a device for moving air which utilizes a power-driven, rotating impeller." The three fan types of primary interest in HVAC systems are centrifugal, axial, and propeller. The fan motor may be directly connected to the impeller, directly connected through a gearbox, or indirectly connected by means of a belt-drive system.
The fan law equations are used to predict the performance of a fan at some other condition than that at which it is tested and rated. The HVAC designer is particularly interested in the effects on horsepower, pressure, and volume consequent to varying the speed of the fan in a system.
The fan laws expressed in the following equations relate only to the effect of varying speed, assuming that fan size and air density remain constant:
CFM2 = CFM1 × (RPM2 / RPM1)
SP2 = SP1 × (RPM2 / RPM1)^2
TP2 = TP1 × (RPM2 / RPM1)^2
BHP2 = BHP1 × (RPM2 / RPM1)^3
Expressed in simple language, the fan laws say that when fan size and air density are unchanged:
The airflow rate varies directly as the change in speed
The pressure developed by the fan varies as the square of the change in speed
The power required to drive the fan varies as the cube of the change in speed
The complete fan laws also include terms for changes in fan size and air density. The laws are valid only when fans of different sizes (diameters) are geometrically similar. These equations include:
(CFM2 / CFM1) = (RPM2 / RPM1) × (D2 / D1)^3
(SP2 / SP1) = (VP2 / VP1) = (RPM2 / RPM1)^2 × (D2 / D1)^2 × (d2 / d1)
(TP2 / TP1) = (Same as SP and VP equation)
(BHP2 / BHP1) = (RPM2 / RPM1)^3 × (D2 / D1)^5 × (d2 / d1)
Where D = fan diameter and d = air density.
A centrifugal fan creates pressure and air movement by a combination of centrifugal (radial) velocity and rotating (tangential) velocity. These two effects combine to create a net velocity vector. When the fan is enclosed in a scroll (housing), some of the velocity pressure is converted to static pressure.
The fan characteristics can be changed by changing the shape of the blade. Typical shapes are:
Forward-curved
Straight radial
Backward-inclined (straight or curved)
Airfoil
The geometry of the fan wheel, inlet cone, and scroll also has an effect on fan performance and efficiency. For a backward-inclined (BI) or airfoil (AF) fan wheel:
For a given wheel or diameter, as the blade gets narrower and longer, higher pressures can be generated but flow rates are reduced.
The inlet cone is shown curved (bell-mouth) to minimize air turbulence. Straight cones are also used, at the cost of some reduction in performance.
The clearance between the inlet cone and the wheel shroud must be minimized for efficiency, because some air is bypassed through this opening.
The forward-curved (FC) wheel usually has a short, wide blade and a flat shroud. The inlet cone is curved or tapered and is mounted to minimize the clearance between the inlet cone and shroud. This type of fan handles large air volumes at low pressures.
The illustrations shown in previous sections depict single-width, single-inlet (SWSI) fans. However, double-width, double-inlet (DWDI) fans are also manufactured and used in HVAC systems.
Fans for HVAC applications should be tested and certified for performance rating in accordance with AMCA Standard 210, promulgated by the Air Moving and Conditioning Association. Also, ASHRAE Standard 51 prescribes the test setup and data-gathering procedures for fan testing.
For a line of several sizes of geometrically similar fans, only the smallest fan in the line is actually tested. Performance of all other sizes is calculated, by using formulas based on the fan laws. The testing setup and procedures are designed for ideal inlet and outlet conditions, with a minimum of turbulence.
The test procedure includes measuring the airflow and horsepower against varying pressures, for a constant fan speed. Pressure is measured in inches of water, by using an oil- or water-filled manometer. Airflow is measured in cubic feet per minute. The data can then be plotted as a series of curves.
The normalized typical curves for a BI fan show the relationships between static pressure, horsepower, and airflow.
Airfoil fan curves are similar with slightly higher efficiencies.
The curves for an FC fan have a different shape compared to BI fans. These curves illustrate the unique performance characteristics of forwardcurved fans, which typically handle large volumes of air at lower pressures.
When the fan speed is varied, the result is a family of parallel curves. This relationship is important for understanding how fan performance changes with speed adjustments, which is crucial for variable air volume (VAV) systems and other applications requiring adjustable airflow.
The HVAC system in which a fan is to be installed has a system-curve characteristic relating to the HVAC system geometry. Understanding the interaction between fan performance curves and system curves is essential for proper fan selection and system design.
Various factors can affect fan performance in real-world applications, including:
System effect factors due to non-ideal inlet and outlet conditions
Variations in air density due to temperature and altitude
Changes in system resistance due to filter loading or damper adjustments
The fan selection process involves several steps:
Determining the required airflow and static pressure1.
Selecting the appropriate fan type based on the application2.
Choosing a fan size that meets the performance requirements3.
Considering factors such as efficiency, noise, and space constraints4.
Verifying the selection using manufacturer's performance data5.
Proper duct design is crucial for efficient air distribution. Key principles include:
Minimizing pressure losses through proper sizing and layout
Balancing the system to ensure proper airflow to all terminals
Considering noise control in duct design and layout
Providing access for cleaning and maintenance
Common duct sizing methods include:
Equal Friction Method
Static Regain Method
Velocity Reduction Method
T-Method
Each method has its advantages and is suitable for different types of systems and applications.
Air distribution devices are crucial for proper air delivery in conditioned spaces. Common types include:
Grilles: For return and exhaust air
Registers: Similar to grilles but with adjustable vanes
Diffusers: For supply air, designed to mix room air with supply air
Dampers play a vital role in air-handling systems for control and balancing. Types of dampers include:
Volume dampers: For system balancing
Fire dampers: For fire safety
Smoke dampers: For smoke control
Combination fire/smoke dampers
Control dampers: For modulating airflow
Air filtration is essential for maintaining indoor air quality and protecting HVAC equipment. Key considerations include:
Filter efficiency ratings (MERV)
Pressure drop across filters
Filter maintenance and replacement schedules
Special filtration requirements for specific applications
Noise control is an important aspect of HVAC design. Strategies for noise control include:
Proper fan selection and operation
Use of sound attenuators and duct lining
Vibration isolation for equipment
Proper duct design to minimize turbulence
Improving energy efficiency in air-handling systems can lead to significant cost savings. Strategies include:
Use of high-efficiency fans and motors
Variable frequency drives for fan speed control
Proper system balancing and commissioning
Regular maintenance and cleaning of components
System effect factors can significantly impact fan performance in real-world installations. These factors arise from non-ideal inlet and outlet conditions, such as:
Abrupt transitions or elbows near fan inlets or outlets
Obstructions or duct fittings that cause turbulence
Improper fan housing or plenum design
Understanding and accounting for system effect factors is crucial for accurate fan selection and system design.
The configuration of air-handling units can vary based on application requirements. Common configurations include:
Single-zone vs. multi-zone units
Constant air volume (CAV) vs. variable air volume (VAV) systems
Draw-through vs. blow-through fan arrangements
Horizontal vs. vertical unit orientations
Each configuration has its advantages and is suited to different types of applications.
The field of air-handling systems continues to evolve. Some future trends include:
Integration of advanced sensors and controls for improved system performance
Use of computational fluid dynamics (CFD) for optimizing air distribution
Development of more energy-efficient fan and motor technologies
Increased focus on indoor air quality and ventilation effectiveness
Integration with building management systems for holistic building performance optimization