Trane TRG-TRC014-EN manual Part-LoadSupply Air Tempering, Application Considerations, period five

period five

Application Considerations

notes

Part-Load Supply Air Tempering

overcooled space conditions 68°F, 74% RH

[20°C]

		mixed air
supply air	B	A
55°F	68°F
[12.8°C]	[20°C]

dry-bulb temperature

humidity ratio

Figure 101

Minimum space ventilation requirements often dictate minimum primary airflow settings on VAV terminal units. If the space cooling load requires less airflow than this minimum, the space may overcool (to B = 68°F [20°C] in this example instead of the desired condition, A) unless the supply air is tempered. Overcooling the space causes comfort problems and causes the relative humidity to increase (to 74% here).

To avoid this, a method of tempering the supply air is needed. This can be done using a terminal heating coil or by mixing in warm air from the plenum or from a warm primary air stream of a dual-duct system.

Realize that if mechanical equipment is used to provide this supply air tempering, it must be available not only to handle the space heating requirements, but also to handle the tempering requirements during these part-load cooling conditions.

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