An automatic pump down circuit effectively stores refrigerant in the high side of an HVAC/R system when cooling is no longer needed, preventing liquid refrigerant from returning to the compressor during the off cycle.
In a pump down system, a critical component is a solenoid valve located in the liquid line. This valve controls the flow of liquid refrigerant to the evaporator coil. The automatic nature comes from how this valve and the compressor are controlled, typically by a thermostat or temperature control and a low-pressure safety switch.
Understanding the Pump Down Process
The process begins when the thermostat or temperature control reaches its prearranged set point, indicating that the cooling demand has been satisfied.
The Sequence of Operation
Here's a step-by-step breakdown of how an automatic pump down circuit operates:
- Cooling Call Ends: The thermostat or temperature control satisfies the cooling requirement.
- Liquid Line Solenoid Closes: The control signal closes the solenoid valve in the liquid line. This immediately stops the flow of liquid refrigerant into the evaporator coil.
- Compressor Continues Running: Crucially, the compressor continues to operate for a short period after the liquid line solenoid closes.
- Refrigerant is Pumped Out: While the compressor runs and no new liquid enters the evaporator, the compressor actively works to pump the remaining refrigerant out of the low-pressure side of the system (the evaporator and suction line).
- Refrigerant Storage: This pumped refrigerant is pushed through the compressor and into the high-pressure side, primarily the condenser coil and the receiver (if present). The system effectively "pumps down" the low side.
- Suction Pressure Drops: As refrigerant is removed from the low side, the pressure in the suction line begins to fall.
- Low-Pressure Switch Trips: A low-pressure control switch, connected to the suction line, monitors this pressure drop. When the pressure falls below a predetermined safety threshold, the low-pressure switch opens its contacts, interrupting the power supply to the compressor and shutting it off.
- System is Pumped Down: At this point, most of the system's refrigerant is safely stored on the high side, and the compressor is off. This prevents liquid refrigerant from migrating back to the compressor during the off cycle, which could cause severe damage upon restart.
Restarting the System
When cooling is needed again:
- Cooling Call Begins: The thermostat or temperature control signals a demand for cooling.
- Liquid Line Solenoid Opens: This signal powers the liquid line solenoid valve, causing it to open and allow liquid refrigerant to flow into the evaporator.
- Suction Pressure Rises: As refrigerant enters the low side, the pressure begins to rise.
- Low-Pressure Switch Resets/Timer Starts Compressor: The rising pressure in the suction line causes the low-pressure switch to close its contacts (resetting). Depending on the control scheme, this either directly starts the compressor or enables a timer that starts the compressor after a short delay (allowing the low side to partially re-pressurize).
- Compressor Restarts: The compressor turns back on, and the normal refrigeration cycle resumes.
Why Use Pump Down?
Automatic pump down circuits are commonly used in commercial refrigeration systems, such as those found in supermarkets or cold storage facilities. They offer several key benefits:
- Compressor Protection: The primary benefit is preventing liquid floodback to the compressor, a major cause of compressor failure.
- Reduced Start Loads: The compressor starts against a lower head pressure (the suction side is already low), potentially reducing electrical load on startup.
- Controlled Start-Up: Ensures a smoother system start by controlling refrigerant flow.
By utilizing a liquid line solenoid valve that closes upon reaching the temperature set point, and allowing the compressor to run until the low-pressure switch trips, the automatic pump down system effectively clears the low side of refrigerant and protects the compressor during idle periods.
