Handbook of air conditioning and refrigeration / Shan K

22.2 CHAPTER TWENTY-TWO
22.1 RETURN AND EXHAUST SYSTEMS
Types of Return and Exhaust Systems
Return Ceiling Plenum
As described in Sec. 20.3, for any enclosed conditioned space in a building, if the difference of air
densities is ignored, the total volume flow rate of air entering the space must be equal to the total
volume flow rate of air leaving the space.
Among the three main categories of air systems described in Sec. 20.9 (Fig. 20.16):
● For most of the constant-volume systems, return air is often returned to the packaged unit
through door undercuts and a collective return grille in the closet, under the attic, or at the high
level in the basement. An exhaust fan is optional and is usually installed in the bathroom or
toilet.
● In many dedicated outdoor air and space recirculating systems, an exhaust system with a volume
flow rate less than the supply volume flow rate of the outdoor ventilation air is often employed in
restrooms and service rooms. This is discussed in Chap. 28.
Among the variable-air-volume and dedicated minimum outdoor fan systems, use of the ceiling
plenum as a return air plenum, as shown in Fig. 20.16b and c, is the most common return air
system. In a return ceiling air plenum system, return air is often returned to the ceiling plenum
through the return slots or grilles first. Usually a connected return duct connects the ceiling plenum
and the fan room. Part of the return air becomes recirculating air and enters the AHU or PU, and the
remaining air is exhausted outside.
For a return ceiling plenum, the following must be considered during design:
● Fire protection. Because the return air flows through the ceiling plenum, refer to federal and local
fire codes for the requirements, materials, and equipment installed inside the ceiling plenum, as
well as the fire wall and partitions that separate the ceiling plenum and adjacent fire compartments.
● Noise control. Fan noise that has been transmitted into the ceiling plenum is very difficult to
attenuate. Do not install silencers inside the ceiling plenum. Install silencers in the fan room.
Check both duct-borne noise and radiated noise; also check noise transmitted from both supply
and return air sides.
● Air leakage. Because most of the ceiling plenums use acoustical ceiling tile and T-bar construction,
the airtightness between the conditioned space and the ceiling plenum is hard to
maintain. Field measurement showed that the pressure differential between the conditioned
space and the ceiling plenum in a VAV system serving a small public library �p r � pl � 0.004
in. WC (1 Pa). The critical air leakage from a ceiling plenum is the air leakage to outdoors, or
vice versa.
Low-Level Return Systems and Enclosed Parking Garage Ventilation
Low-level return systems have been widely used in the following applications:
● In indoor stadiums and assembly halls, where there are many spectator seats. It is better to extract
the return air from the return grilles located at low-level seats to induce part of the diffused