Trane TRG-TRC014-EN manual System-Level Ventilation

period five

Application Considerations

notes

System-Level Ventilation

RA

OA				PA
	VAV #1		VAV #2		VAV #3
primary air (PA)	1000 cfm		1000 cfm		1000 cfm		3000 cfm
	[0.47 m3/s]	+ [0.47 m3/s]		+ [0.47 m3/s]		=	[1.41 m3/s]
OA required	200 cfm	+	200 cfm	+	300 cfm	=	700 cfm
	[0.09 m3/s]		[0.09 m3/s]		[0.15 m3/s]		[0.33 m3/s]
ventilation fraction	0.20		0.20		0.30		Figure 96

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.

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