Trane TRG-TRC014-EN manual Constant Volume-PartLoad, What Is Variable Air Volume?, period one

period one

What Is Variable Air Volume?

notes

Constant Volume–Part Load

SupplyDB	= SpaceDB	–		Sensible Heat Gain
				Constant × Supply Airflow
Supply	= 75°F –		20,000 Btu/hr		=	65°F
DB	1.085 × 1,840 cfm
Supply	= 23.9°C –			5,862 W	=	18.3°C
DB
		1,210 × 0.87 m3/s

Figure 5

Rearranging the equation and using the constant supply airflow of 1,840 cfm [0.87 m3/s], the supply air temperature required to balance a lesser 20,000 Btu/hr [5,862 W] space sensible load is:

Supply DB = Space DB –			Sensible Heat Gain
			Constant-----------------------------⋅-----Supply-------- Airflow--------------------------------
Supply DB = 75°F –		------20,000--------------------Btu/hr----------------------		= 65°F
		1.085 ⋅ 1,840 cfm
	Supply DB = 23.9°C –		5,862W
		1,210 ⋅ 0.87m3 /S

Therefore, as the space sensible load drops from 40,000 Btu/hr to 20,000 Btu/hr [11,724 W to 5,862 W], this system modulates the temperature of the constant 1,840 cfm [0.87 m3/s] supply air from 55°F to 65°F [12.8°C to 18.3°C].

Removing less energy from the air takes less energy from the central system. Therefore, with this particular system, refrigeration energy savings are realized at part load conditions, although control of space humidity levels suffers due to the warmer supply air condition.

TRG-TRC014-EN 3