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TRG-TRC014-EN 73
period five
Application Considerations
notes
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.
Part-Load Supply Air Tempering
dry
dry
-
-
bulb temperature
bulb temperature
humidity ratio
humidity ratio
mixed air
mixed air
68°F
68°F
[20°C]
[20°C]
55°F
55°F
[12.8°C]
[12.8°C]
B
BB
B
B
BB
B
overcooled space conditions
overcooled space conditions
68°F, 74% RH
68°F, 74% RH
[20°C]
[20°C]
supply air
supply air
A
AA
A
A
AA
A
Figure 101