Refrigeration Pump-down Cycle

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This article is written by Jeremy Smith CM, experience refrigeration tech and all around great dude. Thanks, Jeremy


A very common means of control seen on refrigeration equipment is the pump down control. Why do we use this rather than just cycling the compressor off and on like a residential HVAC unit?

Since most refrigeration equipment tends to be located outdoors, it comes down to ambient temperatures and the basic properties of refrigerant we all understand about temperature and pressure and how they can conspire to kill a compressor.

During periods of low ambient temperatures, if we were to just cycle the compressor off, it can easily get colder at the compressor than it is inside the space.   If the compressor cycles off for long enough as it would during a defrost cycle, refrigerant vapor will start to condense within the crankcase.  If we are lucky, the extent of this problem will be a unit that doesn’t start because the pressure of the refrigerant is lower than the cut in setting of the pressure control.  What typically happens, though, is that enough refrigerant will condense to start to settle under the lubricating oil causing a lack of lubrication on restart leading to bearing wear and premature failure.  If enough refrigerant condenses within the compressor housing, the resulting damage could cause valves, pistons and other internal parts to break if liquid gets into the cylinders.

How can we prevent this?

One thing that is applied across almost all sectors of our industry is crankcase heaters.   These small heaters, either immersion style heaters or wrap around style heaters add a small amount of heat to help keep the compressor oil warm and help to prevent vapor from condensing there. The effectiveness of these are limited by the wattage of the heater, the ambient temperature and the size of the compressor.   Too low an ambient or too large a compressor and they start to lose some effectiveness.

So, how else can we prevent condensation within the compressor?  Let’s look to the pressure/temperature relationship of refrigerant for the answer.   If we lower the pressure in the crankcase to a point where the saturation temperature of the refrigerant is below the ambient temperature the compressor is in, refrigerant cannot condense.   This is why we use a “pump down” type system.

In operation, a pump down control consists of little more than a liquid line solenoid valve, a thermostat control and a low pressure control.   When the thermostat or defrost control opens, the solenoid de-energizes, stopping the refrigerant flow and allowing the system to pump the suction pressure down before the low pressure control turns the compressor off.

How low should we set that cut out?   The Heatcraft installation manual has us setting the cut out as low as 1” Hg vacuum, depending on the minimum expected ambient.  I like to set the cut in just below the lowest expected ambient temperature so that you don’t wind up in a situation like I mentioned earlier.   If the ambient gets too low and the cut in is too high, your unit won’t cycle on until it warms up enough resulting in a preventable service call.

Combining a pump down control with a crankcase heater and ensuring that all controls work properly at all times can save your compressor from damage in cold weather.

 

Jeremy Smith, CM

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