Expansion valves on medium temp systems normally do not have any frost or ice accumulation. If you see ice, think problem.
1. Low Refrigerant Charge: A system that is low on refrigerant will have ice or frost on the valve. Check the sight glass.
Also listen for a hissing noise coming from the valve body. Both are indications of low refrigerant.
2. Debris: After determining that the system is not low on refrigerant, I look for debris in the valve. The only way to know for sure, is to disassemble the valve.
Look for a fine white powder. This powder is normally from a deteriorating filter-drier core. The filter-drier must be replaced.
I have also found small metal shavings left over from installation. They accumulate and block refrigerant flow.
Valves on low temp systems will normally have frost or ice. But these valves can also have an additional problem.
Moisture in low temp commercial freezers will eventually surface at the valve. The valve will actually form a small amount of ice internally. This ice will restrict or eventually stop the refrigerant flow.
Want a quick and easy way to check? Simply warm up the valve!!!
Take a torch and lightly apply heat! You will probably hear a slight gurgling or hissing as the ice melts. The refrigerant flow then returns to normal.
However you must get the moisture out of the system by changing the filter-drier and changing the compressor oil if possible
A good moisture indicator in the sight glass is very important. It will tell you when the system is moisture free.
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