AAON HB Series, Инструкция по установке и эксплуатации

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8. Hot Gas Bypass
The purpose of external hot gas bypass (HGBP) is to
prevent coil freeze-up and compressor damage from
liquid slugging during periods of low airflow operation,
or with low entering air temperatures.
HGBP is useful when the air conditioning system is
subject to variations in load caused by varying air
volume or large proportions of outside air. The HGBP
valve meters discharge refrigerant gas to the
distributor downstream of the expansion valve, and at
the entrance to the evaporator distributor tubes. The
quantity of gas varies to control a constant suction
pressure, allowing more gas to flow as suction
pressure decreases.
HGBP is available as a factory installed option on HB
package units, in order to meet various design
conditions.
9. Hot Gas Reheat (Modulating)
Although the evaporator reduces moisture content
from warm, moist air being conditioned, the space
thermostat is a dry bulb device and will not call for
refrigeration if outdoor and space temperatures are
mild but very humid and the space temperature is
satisfied. However, the humidity level may cause the
space to be uncomfortable. A reheat system is used
to correct this condition. To prompt operation of the air
conditioning system, a humidistat is required and to
avoid cooling the space excessively while removing
moisture, a coil which accepts discharge gas from the
compressor is located downstream of the evaporator.
The function of this coil is to heat the air that has been
cooled by the evaporator to approximate room
temperature. A reheat valve is installed in the
compressor discharge line to divert discharge gas to
the reheat coil when the humidistat calls for
dehumidification but returns all discharge gas to the
condenser when cooling is required.
After the room temperature thermostat is satisfied and
the humidistat continues to call for moisture removal,
the modulating valve will allow a controlled amount of
hot gas to enter the reheat coil. A discharge air
temperature sensor mounted within the unit provides
input to an electronic control board. The valve position
is controlled to provide a specific supply air
temperature set point that is set on the control board,
or sent to the control board by a remote 0 to 10 VDC
signal.
The modulating hot gas valve is factory mounted and
wired. The control board is shipped with a default
setting for a neutral discharge air temperature of 75
°
F.
The factory setting can be overridden by connection to
a 0 to 10 VDC signal from another control system.
Figure 28a, AAON’s Modulating Reheat System
Condenser
Coil
Reheat Coil
Evaporator
Coil
Compressor
Reheat
Control
Valves
Control
Board
Supply Air
Sensor
Wiring to
Reheat Control
Valve
Suction Line
Liquid Line