After the required amount of supply air is transmitted to the conditioned space, it is essential to distribute the air properly within the conditioned space. Thus it is important to design suitable air distribution system, which satisfies the following requirements:

a) Create a proper combination of temperature, humidity and air motion in the occupied zone. The occupied zone is defined as all the space in the conditioned zone that is from the floor to a height of 1.8 m and about 30 cms from the walls. In the occupied zone, the maximum variation in temperature should be less than 1°C and the air velocity should be in the range of 0.15 m/s to 0.36 m/s.

b) To avoid draft in the occupied zone. Draft is defined as the localized feeling of cooling or warmth. Draft is measured above or below the controlled room condition of 24.4°C and an air velocity of 0.15 m/s at the center of the room. The effective draft temperature (EDT) for comfort is given by:

EDT = (DBT − 24.4) − 0.1276(V − 0.15)

where DBT is the local dry bulb temperature (in °C) and V is the local velocity (m/s). For comfort, the EDT should be within –1.7°C to +1.1°C and the air velocity should be less than 0.36 m/s.

Air Distribution Performance Index (ADPI)

The ADPI is defined as the percentage of measurements taken at many locations in the occupied zone of space that meets EDT criteria of –1.7°C to +1.1°C, that is:

where N is the total number of locations at which observations have been made, and Nθ is the number of locations at which the effective draft temperature is within –1.7°C to +1.1°C.

The objective of air distribution system design is to select and place the supply air diffusers in such a way that the ADPI approaches 100 percent. The ADPI provides a rational way of selecting air diffusers. Studies show that the value of ADPI depends very much on space cooling load per unit area. A large value of space cooling load per unit area tends to reduce the value of ADPI.

Space Diffusion Effectiveness Factor (SDEF)

The effectiveness of air distribution system is sometimes assessed using Space Diffusion Effectiveness Factor (SDEF). It is defined as:

where Tex is the temperature of the exhaust air, Ts is the supply air temperature and Tr is the temperature of the room air (at the measuring point). A SDEF value of ≤ 1 implies that some amount of cold supply air has not mixed with the room air and is leaving the conditioned space as exhaust. The space air distribution is considered to be effective if SDEF ≥ 1.0.

Table below shows the recommended supply air velocities for diffusers. Since the air velocity at the supply air outlet is normally much higher than 0.36 m/s and its temperature is much lower than 24.4°C, it has to mix properly with the room air before it reaches the occupancy level. This depends on the effective design of the air distribution system.

Design of air distribution systems

The objective of air distribution system design is to choose the location and type of supply air diffuser and the location and type of the return air grilles. The parameters that affect air velocity and temperature at a given point in the conditioned space are:

a) Velocity of air at the inlet to the supply diffuser: Noise criteria to be observed

b) Supply to room temperature difference (Ts−Tr)

c) Geometry and Position of air supply outlet

d) Position of return air inlet

e) Room Geometry

f) Room surface temperature: Lower the surface temperature (e.g. with glass) stronger are the natural convection currents.

g) Internal heat sources (e.g. people, appliances)

h) Room turbulence

The exact prediction of velocity and temperature profiles inside the conditioned space requires simultaneous solution of mass, momentum and energy equations for the conditioned space. However in general this task is extremely complicated due to the several factors that affect airflow and heat transfer inside the conditioned space. However, a basic understanding of room air distribution requires the understanding of, buoyancy effects, deflection of air streams and behaviour of free-stream jets. Normally the location and type of return air grilles do not affect the air distribution significantly.