Most condensing units, both residential and for refrigeration have an outdoor ambient design temperature between 60° and 100°. When the outdoor temperature falls below 60° the condensing pressure begins to fall to a point where the metering device fails to operate properly.
When in the cooling mode, TXVs require a minimum pressure drop between inlet pressure and the outlet pressure to operate properly. The low pressure, found at the inlet of the TXV during low ambient temperatures, cause the valve to hunt for an equilibrium point. This erratic operation causes the valve to hunt, first starving the evaporator and then flooding the evaporator with refrigerant and diminishing the systems ability to cool.
In most cases this is not a problem for residential AC systems but, it does cause problems for refrigeration systems that operate year round..
By keeping the head pressure above the minimum temperature we solve the problem of the hunting TXV. The minimum temperature for proper metering is at about 90° saturation temperature in the condenser. For example: If the unit uses R134a, the minimum head pressure the minimum head pressure should be about 104 psig. Use your PT chart to verify.
To manage head pressure in low ambient conditions, fan cycling controls are often used. By monitoring the head pressure, most fan cycling control will stop the condenser fan when the head pressure drops to the minimum cut-out setting. With the fan disabled and the condenser still running, the head pressure rises. The fan cycling control will then start the fan when the pressure reaches the cut-in setting.
Example: An R134a condenser operating in a 45° outdoor ambient temperature. When the head pressure drops to 90° condensing temperature or 104 psig, the fan shuts off. This is the cut-out pressure. We would want to set the cut-in pressure to be about 40 psig above the cut-out pressure, or 144 psig for this example.
These settings keep the head pressure high enough for proper operation of the system. Depending on the application, the differential should be between 30 psig and 50 psig.
Note – The mechanical settings on the device are for reference, cut-in and cut-out pressures should be checked and adjusted based on actual pressures.
This information is part of our online HVAC course – Commercial Refrigeration for HVAC Technicians. For more information on HVAC training, contact me at 904-742-9511