Refrigeration compressors will not pump liquid!!! I'm sure that's no major revelation!!!
But when I begin finding busted valves, pistons or crankshafts, I start thinking liquid refrigerant.
All of the refrigeration compressors I service are in pump-down systems.
What's a pump-down system? It simply means the condensing unit starts and stops on the low pressure control.
When the control thermostat calls for cooling it opens a liquid line solenoid valve.
The refrigerant begins to flow into the evaporator. The pressure rises in the compressor until the low pressure control closes, starting the compressor.
When the thermostat reaches set-point it closes the valve.
The compressor continues to run, pumping the refrigerant out of the evaporator. As the pressure falls in the compressor the low pressure control opens. The compressor stops.
The liquid refrigerant is stored in the receiver until the thermostat calls again, opening the solenoid valve. The process is repeated.
Ever heard of a flooded evaporator???
Refrigerant migrates to the coldest place in a system. If it is not isolated in the receiver, it tends to accumulate in the evaporator.
In properly operating pump-down systems, this never happens. Excess refrigerant leaking into the evaporator is quickly pumped out by the refrigeration compressors as the pressure rises.
But what happens if the thermostat calls. The solenoid valve opens. But the compressor doesn't start???
We now have an evaporator slowly filling with liquid refrigerant. It becomes flooded!!!
Along comes an unsuspecting technician. Finding the reason for the failure, he restarts the compressor.
At first all appears to be normal. The compressor starts running. But suddenly all heck breaks lose. Literally!!!
A huge slug of liquid suddenly enters the compressor. If you've never hear liquid being pumped by refrigeration compressors, it's a horrible sound.
Loud clanging and clacking noises along with violent shaking, will scare the heck out of you!!!
If you're lucky the slug is small enough to clear the compressor with minimal damage. But large ones can destroy valves, pistons, and crankshafts.
When you are familiar with pump-down systems, flooded evaporators are not as big deal.
Remember, on pump-down systems, you should never have low side pressure higher than the cut-in setting of the low pressure control.
That's the first hint you may have a flooded evaporator.
Here's the procedure I use.
First I attach the low side refrigeration gauge. Next I front seat the compressor suction line service valve. Close it completely.
I lightly bump the starter of test the compressor. If it clangs or clacks, I stop!!! It's also flooded. The refrigerant must first be removed.
If the compressor bump starts normally, then I turn it on. It should run with the service valve closed and shut off on the low pressure control.
I'm now ready!!!
I turn off the solenoid valve and gently crack open the closed service valve. As the pressure increases on the attached low side gauge the compressor will eventually start.
While running, I meter the refrigerant using the service valve. I open the valve stem when pressure nears the low side cut-out.
I then close the valve and severely limit the amount of refrigerant entering the compressor.
How do I know when I have it right???
A frost line forms on the compressor suction bell. I am now metering liquid refrigerant into the suction line!!!
How do I know when all the liquid is gone???
The frost line disappears. I can now begin to slowly open the service valve all the way until the compressor stops.
The system is now in normal operating mode. Time to open the solenoid valve and pack my tools!!!
It's amazing how many refrigeration compressors I seen damaged by liquid refrigerant on pump-down systems!!!
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