Emerson PFH020A-L, User Manual

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A temperature-compensated heater maintains the liquid refrigerant pressure during Off cycles. A liquid
pressure switch is also installed to turn the heater Off during operation, when the receiver pressure is
high. The heater pressure switch has a cutout of 150psig (1034kPa) and a cut-in of 100psig (690kPa). The
receiver includes a pressure relief valve set for 475psig (3275kPa).
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 quenching valve is
provided to mix refrigerant from the system liquid line with the discharge gas before mixing with the
suction gas entering the compressor. (Refer to
General piping arrangement on page 9).
During normal operation, when the evaporator is under full load the hot gas bypass equalizer pressure will
remain high enough to keep the valve port closed. If the evaporator load decreases the evaporator
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 consistency
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 initial
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.
Vertiv | Liebert PFH Installer/User Guide | 30