A pump down refrigeration control system is often used in industrial applications.
What is a pump down system?
A pump down system simply uses a solenoid valve in the liquid line to isolate the refrigerant in the receiver.
How is this accomplished?
The control thermostat opens or closes the valve.
When cooling is required, the valve opens and refrigerant enters the evaporator. The pressure increases until the low pressure control closes.
When the low pressure control closes, the condensing unit starts.
It continues to run, cooling the product.
When the desired temperature is reached, the thermostat opens and the solenoid valve closes.
The condensing unit continues to run, pulling the refrigerant out of the evaporator.
When the low pressure control opens, the compressor stops.
The refrigerant is now stored in the liquid receiver awaiting the next cycle.
Why not simply stop the condensing unit with a thermostat or pressure control?
Why use a liquid line solenoid valve to isolate the refrigerant in a receiver?
Why use a pump down refrigeration control system?
Industrial refrigeration units use large amounts of refrigerant.
Remember, refrigerant naturally migrates to the coldest place in the system.
Given the opportunity, the evaporator would begin to fill with liquid during the off cycle.
So what happens when the condensing unit starts?
All of this liquid refrigerant is sucked out of the evaporator into the compressor!
Refrigeration compressors do not pump liquid.
Small amounts can be tolerated with little or no damage.
But, the large amounts found in industrial refrigeration equipment destroy compressors!
Busted valves, pistons, or a crankshaft, are common if liquid refrigerant floods the evaporator.
What are some common problems you'll find when using pump down systems?
If the solenoid valve sticks open due to trash or damage, the temperature continues to drop. The thermostat is useless.
It is always a good idea to have a separate thermostat.
This low limit thermostat kills the condensing unit directly.
Low limit alarms can also be used to alert when temperatures drop below a set point.
A defective low pressure control can also create a problem.
Either the unit doesn't start, allowing refrigerant to flood the evaporator, or the control fails to open causing the compressor to run in a vacuum.
A properly working low pressure control is very important!
A pump down refrigeration control system is a very simple and effective way of controlling large industrial applications.
But they must be properly understood!
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