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TRG-TRC001-EN 45
period six
Psychrometric Analyses
notes
This is the manner in which a constant-volume, variable-temperature system
with a modulating coil performs. It provides a constant quantity of air to the
room and responds to part-load conditions by varying the supply air
temperature. This is performed by modulating the flow of the cooling fluid
through the coil, typically using a two-way or three-way control valve controlled
by a thermostat that senses the room dry-bulb temperature. Such a system can
provide good dry-bulb temperature control. As the sensible heat ratio changes
from full load, however, it may lose control of the room relative humidity.
One method of improving the constant-volume system’s ability to control room
humidity is to reheat the supply air. In this example, reheat is provided by a
heating coil located downstream of the air handler. This reheat coil is controlled
by a thermostat sensing the room dry-bulb temperature, while the cooling coil
is controlled to provide a constant leaving-air temperature.
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