Filter
Top Products
TRG-TRC014-EN 69
period five
Application Considerations
notes
System-Level Ventilation
During operation, a typical VAV system delivers a mixture of outdoor air and
recirculated air to the multiple spaces it serves. The volume of supply air that
enters a space is controlled by a temperature sensor in that space.
In contrast, the ventilation requirement for a space typically remains constant
as the supply airflow varies with thermal load. The challenge is to maintain the
proper amount of ventilation air to each individual space while varying the
supply air to that space in response to its load.
ASHRAE Standard 62–1999, Ventilation for Acceptable Indoor Air Quality,
includes a procedure that can be used to calculate the quantity of outside air
required at the central air-handling unit in order to satisfy the ventilation
requirements of the individual spaces. At part-load conditions, VAV systems
typically require a constant quantity of ventilation air, resulting in an increased
percentage of outdoor air at the air handler. Fixed-position dampers will result
in underventilated VAV systems.
System-Level Ventilation
OA
RA
PA
primary air (PA)
VAV #1
VAV #2
VAV #3
+
+
=
=
1000
1000
cfm
cfm
[0.47 m
[0.47 m
3
3
/s]
/s]
1000
1000
cfm
cfm
[0.47 m
[0.47 m
3
3
/s]
/s]
1000
1000
cfm
cfm
[0.47 m
[0.47 m
3
3
/s]
/s]
+
+
3000
3000
cfm
cfm
[1.41 m
[1.41 m
3
3
/s]
/s]
+
+
=
=
200
200
cfm
cfm
[0.09 m
[0.09 m
3
3
/s]
/s]
200
200
cfm
cfm
[0.09 m
[0.09 m
3
3
/s]
/s]
300
300
cfm
cfm
[0.15 m
[0.15 m
3
3
/s]
/s]
+
+
700
700
cfm
cfm
[0.33 m
[0.33 m
3
3
/s]
/s]
OA required
ventilation fraction
0.20
0.20
0.20
0.20
0.30
0.30
Figure 96