Handbook of air conditioning and refrigeration / Shan K

22.14 CHAPTER TWENTY-TWO
to balance the airflow, the space pressure is changed from � 0.03 to � 0.03 in. WG (� 7.5 to � 7.5
Pag), and the exfiltration changes to infiltration. The pressure loss at the outdoor air intake passage
(excluding the damper), like the intake louver and duct friction loss �p lou, should be taken into
account only in the outdoor air economizer cycle. During minimum outdoor air recirculating mode,
�p lou should be negligible.
Pressure fluctuation �p om at design flow and at 50 percent design flow causes insufficient outdoor
air intake. The effect of this pressure fluctuation can be reduced by using minimum outdoor
ventilation air control, as discussed in Chap. 23.
22.5 SUPPLY FAN AND RELIEF FAN COMBINATION
Recirculation Mode
Consider an air system with a supply fan, relief fan, and exhaust fan combination, as shown in
Fig. 22.3a, with the same operating characteristics as mentioned in Sec. 22.4. A relief fan is a fan
that is installed in the relief flow passage (exhaust flow passage), adjacent to the recirculating flow
passage leading to the mixing box, to relieve the undesirably high positive space pressure. A relief
fan is different from an exhaust fan, which is often used to exhaust fumes and contaminated or toxic
gases and to maintain a negative pressure in the space or in a localized enclosure. An axial fan is often
used as a relief fan because of its large volume flow and smaller fan total pressure. The control
systems used in such a combination are similar to those of a supply fan and exhaust fan combination,
except that a relief fan control is added. The relief fan has the following pressure-volume characteristics:
During minimum outdoor air recirculating mode operation, the relief fan is not operating and the
relief damper is closed. Outdoor air drawn into the air system is balanced by the exhaust air and the
exfiltration through the openings in the building shell. In this case, the system becomes a supply fan
and exhaust fan combination, and its operating characteristics are the same as those described and
analyzed in Sec. 22.4.
Air Economizer Mode and Design Volume Flow Rate
V · , cfm 10,000 15,000 16,640 17,250
�p t, in. WC 4.0 2.10 1.08 0.43
In a VAV system, there are often two outdoor air dampers: a smaller minimum outdoor air damper
and a larger economizer damper. Both dampers have a total pressure loss of 0.4 in. WC (100 Pa) at
100 percent outdoor air intake. During air economizer mode with 100 percent outdoor air at the design
volume flow rate, the outdoor dampers are fully opened and the recirculating damper is closed.
The relief fan relieves the space air pressure and maintains it within predetermined limits. Outdoor
air is extracted through the intake louver and both outdoor dampers, and flows through the filter and
coils. The conditioned air is then supplied to the space. A small part of the space air is exhausted,
and another part is exfiltrated. The majority of space air is relieved by the relief fan through the
ceiling plenum and relief passage.
At design volume flow, the duct friction loss of the outdoor intake passage is negligible, and the
pressure loss of the intake louver is 0.2 in. WC (50 Pa). The space pressure depends mainly on the airflow
balance in the space. The variable part of the pressure loss of the supply air system is then given as
�p var � 0.2 � 0.4 � 2.5 � 0.85 � 0.03 � 3.98 in. WC (990 Pa)