Page 550 - Mechanical Engineers' Handbook (Volume 4)

2 Air-Handling Processes 539

1 2,3,4 SHF 0 0
W 1 m a W 2 ∆i/∆W 3
T T i=const. i =i s 4
w
1 2 2
i
1 i 2
m w 1
i water or steam
Figure 6 Adiabatic humidification processes.

Adiabatic mixing of two moist air streams is a process often used in AHUs because it
enables major energy savings for HVAC systems. For example, instead of taking the entire
amount of circulated air from outside in a cold winter day, a certain portion can be recir-
culated at the room air temperature. A certain percentage of recirculated air intake versus
fresh air intake is recommended based on indoor air quality requirements to be discussed in
more details in a subsequent chapter. A typical recirculation rate for U.S. commercial build-
ings is around 80%, which means about 20% fresh air is supplied to the building for dilution
of contaminants and occupants’ breathing. Figure 7 shows an example of adiabatic mixing
for an air stream at state 1 (fresh air) and an air steam at state 2 (recirculated air). The
resulting mixture is at state 3 on the mixing process line that connects states 1 and 2. The
exact position of the state 3 is proportional to the ratio of the flow rates for the incoming
air streams.

2.2 A Simple Air-Handling Unit

A simple AHU incorporates all of the above-mentioned processes for climates that require
heating and cooling in different seasons. Nevertheless, individual AHU components may be
active only in certain seasons, such as a humidifier in winter and a cooling coil in summer.
The simplest AHU would serve a single zone, such as a single space (room), in a building
and would be controlled by a thermostat placed in that zone. This kind of HVAC system is
typically used for computer rooms, small department stores, or other small individual spaces
with heating/cooling loads that are uniform and relatively stable throughout the zone. For
nonuniform loads or larger spaces, multiple units could be installed with or without a duct
system. Figure 8 presents an example of the single-zone system. Except for the cooling coil,
filter, or supply fan, all other devices are optional, depending on the type of space and
purpose of the system. The separation of the heating coil into preheat and reheat coils has

1 3
W 1 m 1 a m a 3 W 3 2
T 1 T 3 3
i m a 2 W i m 1 a
1 2 3 1
2 T m a 3
2 m a 2
i 2 m a 3
Figure 7 Adiabatic mixing of two moist air streams.