Low-pressure pump down shutdown is a process in refrigeration systems that involves collecting the refrigerant from the liquid line, passing it through the evaporator and suction line, and storing it in the compressor discharge or condenser. This procedure places particular emphasis on the liquid receiver.
Purpose of the Pump Down Shutdown
The primary goal of this strategy is to prevent flooded compressor start-up and avoid liquid slugging, which can occur due to excessive refrigerant migration trapped in the evaporator.
Additionally, low-pressure shutdown is recommended for systems that use hydrocarbon or flammable refrigerants. Its main purpose is to reduce the amount of flammable refrigerant mass in the refrigerated space, especially in the suction line and evaporator. In such cases, pump down is considered an essential safety measure.
Essential Components to Implement the Pump Down System
To implement a low-pressure pump down system, the following components are essential:
- Low-pressure switch (preferably dual): Detects pressure changes and controls the compressor’s start and stop functions.
- Liquid solenoid valve: Regulates the refrigerant flow in the system.
- Electronic refrigeration controller: Monitors and coordinates the system’s operation.
Low-Pressure Pump Down Procedure in 10 Steps
Below is a step-by-step explanation of the low-pressure pump down shutdown procedure:
1. Shutdown Activation
When the system needs to shut down, for example, during a defrost cycle, the controller closes the solenoid valve.
2. Interrupting the Refrigerant Flow
When the solenoid valve closes, the normal flow of refrigerant gas is interrupted, but the compressor remains running.
3. Refrigerant Suction
The running compressor continues to suction the remaining refrigerant from the evaporator and suction line.
4. Pressure Reduction
As the compressor continues to suction, the system pressure drops below normal levels, as less refrigerant remains in the suction line.
5. Low-Pressure Detection
When the pressure reaches a set value, the low-pressure switch detects this drop and disconnects the compressor.
6. Compressor Shutdown
When the compressor shuts down, the pressure in the suction line is very low, significantly reducing the amount of refrigerant in that area.
7. System Reactivation
At the end of the defrost cycle, the system must be restarted. The controller opens the solenoid valve, but the compressor remains off.
8. Refrigerant Re-entry
When the solenoid valve opens, part of the refrigerant returns to the evaporator and suction line due to the pressure difference with the stopped compressor.
9. Pressure Increase
When the pressure in the suction line increases to a predetermined level, the low-pressure switch detects the higher value and reconnects the compressor.
10. Compressor Start-Up
The compressor starts and begins to operate with a minimal amount of refrigerant, preventing issues such as starting with excess refrigerant and reducing compressor stress. At this point, the system’s operating pressures begin to normalize.
Benefits of Low-Pressure Pump Down Shutdown
- Prevents flooded compressor start-up
- Avoids liquid slugging
- Reduces risks associated with flammable refrigerants
- Minimizes compressor stress
