Why Measure Head Pressure using Temperature instead of Pressure?

Many people assume that the best way to control the head pressure from a compressor is to directly sense the line pressure using a sensor inserted into the liquid line.

But measuring pressure requires a sensor to be inserted into the liquid line which is an invasive procedure. Also, when the refrigerant type is changed, the head pressure values used for control must be changed to match the refrigerant.

A better process in our opinion is to measure the temperature of the liquid line to calculate the amount of subcooling taking place due to the head pressure. All commercially used refrigerants condense and sub-cool at the same temperature. Therefore the temperature that corresponds to the desired head pressure does not change when the refrigerant is changed, even though the desired head pressure is dependent on the type of refrigerant. Sensing the temperature of the sub-cooled refrigerant of various refrigerant types produces a consistent control signal.This temperature value can to used to control the head pressure and the value is independent of the type of refrigerant being used in the system.

These are the benefits of measuring head pressure using temperature of the liquid line:
  • The installation is simplified since the temperature sensor is placed on the outside of the line, not inserted into the line
  • The cost of the temperature sensor is low
  • The dependability of the temperature sensor is high
  • The control algorithm does not need to be modified when the type of refrigerant is changed

Speed regulation normally begins at 80°F liquid line temperature (ambient = 60°F) for commercial/industrial type condensing equipment. As the liquid line temperature falls with ambient temperature, the Controller reduces condenser fan motor speed to maintain head pressure. At 50°F liquid line temperature (ambient = 30°F) minimum motor speed is reached. The Controller removes power from the motor when operation of the condenser produces liquid line temperatures below 50°F.

When the liquid line temperature increases to 53°F, the Controller will “soft start” the motor. As temperature increases to 80°F, the motor speed proportionally increases. For all liquid line temperatures above 80°F (ambients above 60°F) the control allows the motor to operate at full synchronous speed.

The Controller always “soft starts” the motor when connected to the line and then reverts to either full speed, modulated speed, or off.

The control function maintains a minimum pressure differential for proper expansion of the refrigerant that maintains proper system operating conditions, while assuring proper suction pressure (evaporator temperature) over the anticipated ambient operating range.

The minimum allowable controlled ambient temperature is determined by the lowest controllable condenser flow rate (minimum RPM).
“Dallas