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3.4 Hot Gas Bypass
3.4.1 Operation
When applying hot gas bypass with split system condensing units, bypassing discharge gas to the
compressor suction line offers more flexibility than conventional hot gas bypass to the evaporator
unit.
The hot gas bypass valve is installed between the compressor discharge piping and suction piping,
bypassing the condenser and evaporator coils. The discharge gas mixes with the suction gas, raising
the suction temperature and pressure and decreasing the mass flow through the evaporator. The
higher suction temperatures could cause compressor overheating, therefore a separate liquid quench-
ing valve is provided to mix refrigerant from the system liquid line with the discharge gas before mix-
ing with the suction gas entering the compressor. (Refer to Figure 13).
During normal operation, when the evaporator is under full load the hot gas bypass equalizer pres-
sure will remain high enough to keep the valve port closed. If the evaporator load decreases the evap-
orator temperature and pressure will drop. When the suction pressure reduces below the hot gas
bypass valve setting, the hot gas bypass valve opens, diverting some of the refrigerant flow back to
the compressor suction. The liquid quenching valve bulb senses this increased superheat and opens,
allowing liquid refrigerant to mix with the discharge gas, desuperheating it.
Proper mixing of the three refrigerant paths ensures stable operation and system performance. The
liquid quenching valve bulb must be located downstream of all these connections to control superheat
at the compressor inlet. Superheat settings for the liquid quenching valve are chosen to maintain con-
sistency with the system expansion valve. During hot gas bypass operation higher superheats,
50-60°F (10-15°C), may be observed at the compressor. The liquid-quenching valve is internally
equalized and superheat is not adjustable.
To aid in lubricating the compressor, the hot gas bypass solenoid is delayed for 30 seconds on the ini-
tial call for cooling and de-energized for 30 seconds during every 60 minutes of continuous operation.
3.4.2 Adjustment
Upon deciding what evaporator temperature is desired, the following procedure should be used to
adjust the hot gas bypass valve:
1. Install the suction and discharge pressure gauge.
2. Adjust temperature setpoint to call for cooling so that the refrigeration compressor will run
continuously.
3. Remove the TOP adjusting nut from the valve.
4. Insert an Allen wrench in the brass hole at top of valve in adjusting port and turn clockwise if a
higher evaporator temperature is required. Adjust no more than 1/4 turn at a time. Let the
system stabilize for 15 minutes before determining if additional adjustments are necessary.
5. After obtaining the suction pressure required, reinstall cap tightly making sure there are no
leaks.
6. Let the system operate for approximately 10 to 15 minutes to make sure the suction pressure is
within the range desired.
7. There may be a variation of approximately 3 to 6 PSIG (21 to 41 kPa) on the evaporator due to the
differential on the hot gas bypass.
8. Return temperature setpoint to the desired setting.