by Ron Walker
Head pressure control is required because refrigeration equipment unlike air conditioning equipment must be able to run in very low ambient outdoor temperatures.
There are various ways of falsely loading the condenser to raise high-side pressure to of the most common are:
- Fan cycling controls
- Head pressure control valves.
- Fan Cycling
Fan cycling controls are pressure switches that sense high-side pressure, or temperature controls that sense liquid line temperature that turn the fans off when the condensing pressure-temperature falls, and on when the pressure-temperature reaches the upper setting on the fan cycling switch.
In a 4 fan system, fans 1-2 are connected to 1 fan cycling control and fans 3-4 are connected to another. If you are wanting to maintain 180 psig. head pressure the controls would be set as follows:
- Cut-in Cut-out
- Fans 1-2 180 psig. 160 psig.
- Fans 2-4 200 psig 190 psig.
As you can see, this causes fluctuation of the head pressure and can result in expansion valve hunting and poor superheat control.
Another problem with fan cycling controls is that when the fans are turned on and off, the liquid line pressure after the receiver fluctuates, which may cause the refrigerant’s saturation point to change and result in flash gas, or bubbles in the liquid line. This decreases the capacity of the TX valve due to vapor bubbles passing through the expansion valve orifice and decreased pressure drop.
Condenser Flooding (see Headmaster in the component section)
The ability to maintain constant head pressure (liquid pressure) during varying periods of low ambient operation is ideal. One method of achieving this is to use condenser flooding valves. In larger systems, two valves are required.
The portion of the condenser that is full of liquid refrigerant (flooded) no longer serves as a condenser. Flooding a portion of the condenser reduces its effective condensing surface, and therefore its capacity. When the appropriate amount of condenser flooding has occurred, the reduced condenser capacity will cause the pressure to increase to 180 psig.
The benefit of condenser flooding is the ability to provide very consistent liquid pressure in the receiver during periods of low ambient operation. Consistent liquid pressure will result in very stable TEV operation during the winter months.
There are drawbacks to this method of head pressure control. Extra refrigerant is required to accomplish condenser flooding. During extremely low ambient conditions, it may be necessary to flood nearly 85% of the condenser. Depending on the size of the condenser, this may require several hundred