In air conditioning systems that use air as the fluid in the thermal distribution system, it is essential to design the Air Handling Unit (AHU) properly. The primary function of an AHU is to transmit processed air from the air conditioning plant to the conditioned space and distribute it properly within the conditioned space. A typical AHU consists of:

1. A duct system that includes a supply air duct, return air duct, cooling and/or heating coils, humidifiers/dehumidifiers, air filters and dampers
2. An air distribution system comprising various types of outlets for supply air and inlets for return air
3. Supply and return air fans which provide the necessary energy to move the air throughout the system

Transmission of air:

In an AHU, air is transmitted through various ducts and other components with the help of fans. Since the fan motor consumes a large amount of power, and the duct system occupies considerable building space, the design of air transmission system is an important step in the complete design of air conditioning systems. In the end the success of any air conditioning system depends on the design of individual components as well as a good matching between them under all conditions. In order to design the system for transmission of air, it is important to understand the fundamentals of fluid (air) flow through ducts.

The fundamental equation to be used in the analysis of air conditioning ducts is the Bernoulli’s equation. Bernoulli’s equation is valid between any two points in the flow field when the flow is steady, irrotational, inviscid and incompressible. The equation is valid along a streamline for rotational, steady and incompressible flows. Between any two points 1 and 2 in the flow field for irrotational flows, the Bernoulli’s equation is written as:

where p/ρg is the pressure head, V^2/2g is the velocity head and Z is the static head respectively.

Each of the heads has units of length. The above equation can also be written in terms of static, velocity and total pressures as:

The above equation implies that for frictionless flow through a duct, the total pressure remains constant along the duct. Since all real fluids have finite viscosity, i.e. in all actual fluid flows, some energy will be lost in overcoming friction. This is referred to as head loss, i.e. if the fluid were to rise in a vertical pipe it will rise to a lower height than predicted by Bernoulli’s equation. The head loss will cause the total pressure to decrease in the flow direction. If the head loss is denoted by Hl, then Bernoulli’s equation can be modified to:

To overcome the fluid friction and the resulting head, a fan is required in air conditioning systems. When a fan is introduced into the duct through which air is flowing, then the static and total pressures at the section where the fan is located rise. This rise is called as Fan Total Pressure (FTP). Then the required power input to the fan is given by:

The FTP should be such that it overcomes the pressure drop of air as it flows through the duct and the air finally enters the conditioned space with sufficient momentum so that a good air distribution can be obtained in the conditioned space. Evaluation of FTP is important in the selection of a suitable fan for a given application. It can be easily shown that when applied between any two sections 1 and 2 of the duct, in which the fan is located, the FTP is given by:

Thus to evaluate FTP, one needs to know the static pressures at sections 1 and 2 (p1, p2), air velocities at 1 and 2 (V1, V2), datum at 1 and 2 (Z1, Z2) and the head loss Hl. Normally, compared to the other terms, the pressure change due to datum is negligible. If the static pressures at the inlet and exit are equal, say, to atmospheric pressure (p ρg(z2 − z1) 1= p2 = patm) and the duct has a uniform cross section (v1=v2), then FTP is equal to the pressure loss due to friction. Thus to find FTP, one has to estimate the total pressure loss as air flows through the duct from one section to other.