Component Operation and Maintenance
6.4.4Thermostatic Expansion Valve Operation
The thermostatic expansion valve performs one function. It keeps the evaporator supplied with enough refrigerant to satisfy load conditions. It does not effect compressor operation.
Proper valve operation can be determined by measuring superheat. If too little refrigerant is being fed to the evaporator, the superheat will be high; if too much refrigerant is being supplied, the superheat will be low. The correct superheat setting is between 10 and 15°F (5.6 and 8.3°C).
Adjustment
To adjust the superheat setting, proceed as follows:
1.Remove the valve cap at the bottom of the valve.
2.Turn the adjusting stem counterclockwise to lower the superheat.
3.Turn the adjusting stem clockwise to increase the superheat.
NOTE
Make no more than one turn of the stem at a time. As long as 30 minutes may be required for the new balance to take place.
6.4.5Hot Gas Bypass Valve Operation
The hot gas bypass is inserted between the compressor discharge line and the leaving side of the expansion valve through the side outlet distributor. The system, with normal operation when the evaporator is under full load, will maintain enough pressure on the leaving side of the hot gas valve to keep the valve port closed.
If the load on the evaporator decreases, the evaporator will get colder. When the coil is too cold, the internal pressure in the evaporator drops and allows the hot gas bypass valve to open. Hot gas then mixes with the liquid coolant on the discharge side of the expansion valve raising the temperature and pressure in the evaporator. The net result is a reduction in the cooling capacity of the unit to match the load.
To aid in lubricating the compressor, the hot gas bypass solenoid is delayed for 30 seconds on the ini- tial call for cooling and
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 the temperature setpoint to call for cooling so that the refrigeration compressor will run.
3.Remove the TOP adjusting nut from the valve.
4.Insert an Allen wrench in the brass hole at top of the valve in adjusting port, and turn CLOCKWISE if a higher evaporator temperature is required.
5.After obtaining the suction pressure required, reinstall the cap tightly making sure there are no leaks.
6.Let the evaporator operate for approximately 10 to 15 minutes to make sure the suction pressure is within the desired range.
7.There will be a fluctuation of approximately 3 to 6 PSIG (21 to 41 kPa) on the evaporator due to the differential on the hot gas bypass.
8.Return the temperature setpoint to desired number.
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