Handbook of air conditioning and refrigeration / Shan K

Design Considerations
AIR SYSTEMS: VARIABLE-AIR-VOLUME SYSTEMS 21.55
If series fan-powered boxes and parallel fan-powered boxes are both operated continuously, the
energy use of the supply fan plus the fans in the parallel fan-powered boxes consume less than onehalf
of the kWh used by the supply fan plus the fans in the series fan-powered boxes in 45°F
(7.2°C) cold air distribution.
● Because there are two mixings (the mixing of outdoor air and recirculating air in the AHU or PU
and the mixing of cold primary air and plenum air in the fan-powered box), adequate supply of
outdoor air is critical in a fan-powered VAV system. This is discussed in Chap. 23.
● In a VAV system using cold air distribution, fan-powered boxes can be installed in both the
perimeter and interior zones, or in the perimeter zone only, for lower initial cost. Cold primary air
is supplied directly to the interior zone by high-induction ceiling diffusers, as shown in
Fig. 18.10a and b.
● The mixing ratio of cold primary air to zone supply air and the reset of the cold primary air supply
temperature for the perimeter zone are different from those of the interior zone. Different
AHUs or PUs are preferable for the perimeter zone and interior zone separately. An analysis of
energy savings and investment costs is often necessary.
● In a series fan-powered box, the supply fan in the AHU or PU offsets the system pressure drop up
to the inlet of the VAV box. The fan in the fan-powered box offsets the pressure drop of the VAV
box, elbows (if any), downstream heating coil, flexible duct, and diffuser.
In a parallel fan-powered box, the supply fan in the AHU or PU offsets the pressure drop of
the VAV box, flexible duct, diffuser, and straight-through mixing loss of the cold primary
airstream in the fan-powered box.
● At summer and winter design conditions, the ratio of the volume flow rate of cold primary air
supplied to the fan-powered box to the zone supply volume flow rate V˙ sn,c/V˙
sn,d and the ratio of
the volume flow rate of warm recirculating plenum air to the zone supply volume flow rate
V˙ s,rec/V˙
sn,d depend on the following factors: (1) zone sensible cooling load and heating load,
(2) zone supply temperature differential at summer and winter design conditions Trn � Tsn,d, or
Tsn,d � Trn, and (3) outdoor air requirements.
As mentioned previously, for cold air distribution V˙ sn,c /V˙ sn,d is about 0.6 for a fan-powered
box. The winter heating in the perimeter zone often needs greater supply volume flow to match
the requirement of the warm air supply temperature differential not exceeding 15°F (8.3°C);
V˙
sn,h /V˙ sn,d for cold air distribution is about 0.7.
● The supply main ducts, branch ducts, cold air passage in the VAV box, flexible ducts, and diffusers
must be well insulated especially for cold air distribution. Cold primary air and recirculated
plenum air should be mixed in a perpendicular flow direction to prevent downstream temperature
stratification.
● A low zone relative humidity and zone dew point are desirable to prevent possible surface condensation
during cold air distribution. When cold primary air at 44°F (6.7°C) is supplied directly
to the conditioned space through high induction diffusers, the surface of the induction diffusers
can be assumed to be 3°F (1.7°C) higher than the supply air; or the dew point of zone air should
be lower than 44 � 3 � 47°F (8.3°C), that is, a zone temperature of 75°F (23.9°C) and a space
relative humidity �r � 37 percent.
● At the start of cool-down period in a hot and humid summer, direct supply of 40 to 50°F (4.4 to
10.0°C) cold primary air to the fan-powered boxes may cause condensation because of the higher
dew point of stagnant air in the ceiling plenum before the cool-down period. The AHU or PU should
supply 55°F (12.8°C) air to the fan-powered boxes first, to reduce moisture and lower the dew point.
● If humidification is required to maintain an acceptable zone relative humidity in winter, a humidifier
can be installed in the AHU or PU.