Handbook of air conditioning and refrigeration / Shan K

Controls
AIR SYSTEMS: FAN COMBINATION AND SMOKE CONTROL 22.21
Point o Point m Paint r Point ru
p, in. WG
V
0 � 0.6 � 0.03 1.0
· , cfm 20,000 20,000 20,000 16,650
During air economizer mode, if the total pressure loss of the return system is increased to 0.70
in. WC (174 Pa) and the return air is exhausted through the return fan and exhaust damper, at 50
percent of design volume flow, then the fan total pressure of the return fan is
�p ret.f50 �� 9650
20,000� 2
(0.70 � 1.0) � 0.03 � 0.37 in. WC (91 Pa)
and the pressure-volume characteristics at the key points are as follows:
Point o Point m Point r Point ru
p, in. WG
V
0 � 0.15 � 0.03 0.23
· , cfm 10,000 10,000 10,000 9650
As in the supply and relief fan combinations, the controls for supply and return fan combination
systems operated at minimum outdoor air recirculating mode and air economizer mode operations
are discussed in Chap. 23.
22.7 COMPARISON OF THREE FAN COMBINATION SYSTEMS
An air system with supply and exhaust fan combination is simpler and less expensive. Such a system
is not suitable for operation in air economizer mode for a high space pressure. In a supply and
exhaust fan combination, a comparatively lower pressure drop in the return system is necessary to
prevent an unacceptable low pressure in the mixed plenum, such as p m ��1 in. WG (� 250 Pag).
Such low mixed plenum pressure induces air leakage and may impede minimum outdoor air
control.
The operating characteristics of the supply and relief fan combination and the supply and return
fan combination can be compared as follows:
● In a supply and relief fan combination in recirculating mode, total pressure at point ru may be
negative; in a supply and return fan combination, total pressure at ru must be positive, in order to
overcome the pressure loss of the exhaust passage.
● With a relief fan, the mixed plenum pressure p m is always negative. With a return fan, p m may be
positive if the return fan is too large. A positive p m means that there will be no outdoor air intake
into the mixing box. Such a condition should never occur.
● When a negative space pressure must be maintained, a relief fan causes the pressure in the mixed
plenum to be smaller (more negative) than a return fan during minimum outdoor air recirculating
mode.
● When there is a considerable pressure drop in the exhaust or relief flow passage, a return fan requires
high positive pressure at point ru, so energy is wasted across the recirculating damper.
● A supply and relief fan combination with a lower total pressure loss return system is often more
energy-efficient than a supply and return fan combination during recirculating mode operation,
for the following reasons: (1) The pressure drop across the recirculating damper is smaller.