Handbook of air conditioning and refrigeration / Shan K

AIR SYSTEMS: MINIMUM VENTILATION AND VAV SYSTEM CONTROLS 23.33
Similarly, when the measured CO2 concentration is � 400 ppm, the minimum outdoor intake
0.07 cfm/ft 2 (0.35 L/s�m 2 ), and the remaining part is 0.07 � 0.03 � 0.04 cfm/ft 2
(0.2 L/s�m 2 ) which can provide a space pressure around � 0.005 in. WG (1.25 Pag).
7. During the minimum outdoor air recirculating mode, if a mixed plenum pressure control is
used, either a pressure sensor located in the mixed plenum measures the static pressure difference
between the mixed plenum and outdoors �pm,p, as shown in Fig. 23.4a, or a pressure sensor measures
the pressure drop of the outdoor air damper and the louver, as shown in Fig. 23.4b. If the measured
static pressure difference or pressure drop decreases and is less than a set point, such as � 0.4
to � 0.6 in. WC (� 100 to � 150 Pa), the DDC controller closes the recirculating damper until
�pm,p equals the set point and an amount of minimum outdoor air of o,sys � 0.14 cfm/ft 2 (0.7
L/s� m2 V˙ o,min � V˙
bv �
V˙
) is extracted in the AHU. The minimum outdoor air damper is opened a fixed amount, such
as 100 percent.
Mixed plenum pressure control provides an approximately constant minimum outdoor air with an
amount of V˙ o,sys. During minimum outdoor air recirculating mode, for VAV systems with a supply
and return fan combination, the minimum outdoor damper opens 100 percent, the exhaust damper
opens 30 percent, and the economizer damper is closed. A space pressure sensor measures the representative
space pressure pr, in in. WG (Pag). If it exceeds � 0.03 in. WG (7.5 Pag), a DDC controller
increases the speed of the return fan through a variable-speed drive, and thus reduces the amount of
minimum outdoor air intake as well as the space pressure and maintains pr at less than � 0.03 in.
WG (7.5 Pag).
When the zone load reduces, the speed and volume flow of the supply fan reduce accordingly.
As the measured pr drops below � 0.005 in. WG (1.25 Pag) because the return fan has not followed
the supply fan to reduce its volume flow appropriately, the DDC controller then reduces the speed
of the return fan, and therefore space pressure pr increases and is maintained at a value not less than
� 0.005 in. WG (1.25 Pag), as discussed in Sec. 23.6.
8. When the outdoor temperature 55°F � To � 75°F (12.8°C � To � 23.9°C), the AHU is operated
in 100 percent outdoor air economizer cycle, as shown in Fig. 23.7b . The DDC controller
opens 100 percent the minimum outdoor air damper, economizer damper, and exhaust damper. The
recirculating damper is closed. When 100 percent outdoor air is used for free cooling, it overrides
the minimum ventilation control.
The cooling and dehumidifying capacity of the chilled water cooling coil is also required at the
same time. The DDC controller modulates the opening of the two-way control valve of the cooling
coil to maintain a discharge air temperature Tdis � 55°F (12.8°C).
9. When the outdoor temperature To,mix � To � 55°F (12.8°C), the AHU is operated in the air
economizer cycle with mixed air control. Here To,mix is the outdoor air temperature at which the
AHU or PU starts the mixed air control (such as To,mix � 30°F, or � 1.1°C) to provide a mixture
of outdoor air and recirculating air at discharge air temperature Tdis directly, as discussed in Sec.
23.7. (Refer to Fig. 23.7.) So To,mix depends on the percentage of outdoor air among the supply
air and on the load ratio. In an air economizer cycle mixed air mode, a DDC controller opens 100
percent the minimum outdoor air damper and exhaust damper and modulates the economizer
damper and recirculating damper to maintain a mixed air temperature equal to Tdis � 55°F
(12.8°C) directly.
Since the amount of outdoor air intake in the AHU during mixed air mode is always greater than
the amount of minimum outdoor air V˙ o,sys in recirculating mode, the mixed air control overrides the
minimum ventilation control.
To provide cold air supply for the interior zone even in winter, the DDC controller resets Tdis �
55°F (12.8°C).
10. In the perimeter zone
● If the perimeter zone temperature T rxn and interior zone temperature T rin drop below 75°F
(23.9°C), the terminal controller reduces the opening of the single-blade damper and thus the sup-