SEQUENCES OF OPERATION CONTINUED
Reheat Operation
The hot gas reheat option includes differential pressure
regulator, check valve, hot gas solenoid operated valve,
VARISPEED variable speed condenser head pressure
control, single stage thermostat (factory mounted) and a
one row heat reclaim coil.
The adjustable differential pressure regulator is factory
set based on the reheat coil capacity and may require
further field adjustment after testing.
The VARISPEED head pressure control permits
stable refrigeration pressures over a wide range of
evaporator and ambient temperatures.
Refrigerant Flow
The VARISPEED Fan Control System basically
controls the compressor discharge pressure. The speed
control module responds to discharge pressure; it
speeds the condenser fan up as pressure rises and
slows the fan down as pressure falls due to load
conditions or as outdoor ambient temperature falls.
The Differential Pressure Regulator (DPR) is used to
maintain a constant differential of approximately 10 psig
across the reheat coil to assure the desired hot gas flow
through the coil. The DPR is field adjustable to allow
more or less gas to flow through the coil, giving a higher
or lower reheat temperature rise.
The Reheat Solenoid Valve (RSV) is installed ahead of
the reheat coil, and opens when reheat is required (see
Controls Section below). The Reheat Check Valve
(RCV) acts to prevent any back flow of hot gas into the
reheat coil when the RSV is closed.
The Hot Gas Bypass Valve (HGBPV) is not directly
associated with the reheat circuit, but serves to limit the
evaporator pressure. By introducing hot discharge gas
into the cold expanding refrigerant entering the
evaporator, a false load is imposed, thus keeping the
compressor suction pressure from falling to dangerously
low levels, and preventing coil freeze-up. The HGBPV is
field adjustable to satisfy a broad range of application
conditions.
The Hot Gas Bypass Solenoid (HGBPS) opens
whenever the compressor starts, and closes when it
stops. The Liquid Line Solenoid (LLS) opens when the
unit starts. If the system has a pumpdown circuit (larger
units, reciprocating compressors), when there is no
longer a call for cooling, the LLS closes, the unit pumps
down, and the compressor is shut down by the low
pressure control. On a call for cooling the LLS opens,
and rising evaporator pressure allows the low pressure
control to start the compressor.
If the unit does not include a pump down circuit, the LLS
& HGBPS serve as antimigration valves to keep
refrigerant from collecting in the evaporator and the
compressor crankcase during the off cycle.
Controls
There are many ways to control hot gas reheat. You
should choose the means that best fits your specific
application:
Thermostat – Terminals may be furnished in the control
panel of outside air units to allow control of the reheat
from the conditioned space. The factory mounted leaving
air thermostat will serve as a high limit control to prevent
adding heat into the space
Humidistat – Terminals may be furnished in the control
panel to switch the compressor and the reheat coil on or
off providing optimal humidity control. On outside air
units the humidistat will control the reheat coil only.
DDC – Terminals may be furnished, or the factory may
wire Direct Digital Controls to control the unit depending
upon the algorithm of the control.
If additional space temperature control is required,
additional circuits (up to four) may be controlled by an
electronic reset or DDC controller with both room and
duct sensor input. On a drop in space temperature, the
controller would energize or de-energize the hot gas
solenoid(s) in sequence to raise or lower the leaving air
temperature.
Operating Pressures
The operating pressures of the system may vary
somewhat from conventional operating characteristics.
Please refer to the Cooling Performance table on page
17 for approximate operating pressures.
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