Handbook of air conditioning and refrigeration / Shan K

23.32 CHAPTER TWENTY-THREE
ble cooling load in the interior zone during part-load operation in an office building needs cold
air supply.
4. During minimum outdoor air recirculating mode, when the sensed zone temperature Trn exceeds
75°F (23.9°C), DDC terminal controllers modulate the opening of single-blade dampers of
the reheating VAV boxes in the perimeter zones and VAV boxes in the interior zones, to increase the
zone, volume flow of supply air to maintain a nearly constant zone temperature Trn � 75°F
(23.9°C). During part-load operation, when Trn drops below 75°F (23.9°C), because of a reduction
in zone sensible cooling loads, DDC terminal controllers modulate the opening of single-blade
dampers of the reheating VAV boxes in the perimeter zones and VAV boxes in the interior zones, to
reduce the volume flow of the zone supply air to maintain a nearly constant zone temperature of
75°F (23.9°C), as discussed in Sec. 21.3.
5. During minimum outdoor air recirculating mode part-load operation, when reheating VAV
boxes and VAV boxes close their single-blade dampers to smaller openings, the pressure drop
across the dampers increases. This results in a higher static pressure in the supply main duct. As
such, an increase in duct static pressure is sensed by a pressure sensor located at a distance of
0.3Lmain from the farthest branch takeoff in the supply main duct, as discussed in Sec. 23.8; a DDC
controller modulates and reduces the speed of the supply fan through an adjustable-frequency
variable-speed drive to maintain a nearly constant duct static pressure set point, such as 1.2 in.
WG (300 Pag), at the location of the duct static pressure sensor. Therefore, the volume flow rate of
the supply fan is reduced to balance the reduction of zone supply volume flow rate during part
load.
6. During the minimum outdoor air recirculating mode, if CO2-based demand-controlled ventilation
is used, the relationship between the supply flow density of minimum outdoor air (as well as
the corresponding minimum outdoor damper opening) and the CO2 concentration in the return duct
(refer to Fig. 23.2) has already been determined during commissioning. Since as the supply fan is
modulated at a speed to maintain a preset duct static pressure of 1.2 in. WG (300 Pag), the DDC
controller opens the minimum outdoor damper to a setting to provide base ventilation V˙ bv first. Ac-
cording to the measured CO 2 concentration in the return duct, the DDC controller then resets the
minimum outdoor damper opening setting according to the determined relationship at a predetermined
time interval (such as 2 and 5 min).
Measured CO2 concentration, ppm Required minimum outdoor air, cfm/ft 2 (L/s�m2 )
� 800 V · � 400
o,sys � 0.14 (0.7)
V · bv � 0.07 (0.35)
The recirculating damper opens 100 percent, and the exhaust dampers open 30 percent.
For a VAV system with a supply and return fan combination, a pressure sensor is used to measure
the representative space pressure pr, in in. WG (Pag), and controls the space pressure in floating
control mode. Only when pr � 0.03 in. WG (� 7.5 Pag) does the DDC controller increase the
speed of the return fan, which raises the amount of exhaust air through the exhaust damper eu and
maintains pr ��0.03 in. WG (7.5 Pag). Also only when pr ��0.005 in. WG will the DDC controller
reduce the speed of the return fan to increase space pressure to � 0.005 � pr � � 0.03 in.
WG (� 1.25 � pr � 7.5 Pag).
When the measured return duct CO2 concentration is � 800 ppm, the minimum outdoor air intake
for a typical office building should be 0.14 cfm/ft 2 (0.7 L/s�m 2 ). Minus the exhaust through
the restroom of 0.03 cfm/ft 2 (0.15 L/s�m 2 ), the remaining part is 0.14 � 0.03 � 0.11 cfm/ft 2
(0.55 L/s�m 2 ). Such an amount of outdoor air is sufficient to maintain a space pressure of � 0.005
� pr ��0.03 in. WG (� 0.005 � pr � � 7.5 Pag) and an associated exfiltration from an average
leaky building as well as an amount of 0.015 cfm/ft 2 (L/s�m2 V˙
) of exhaust air through the exhaust
damper.