Adjusting expansion valves for correct superheat can be frustrating.
By the book it seems simple enough. Just read the pressure and temperature of the suction line as it leaves the evaporator.
Cross reference the pressure-temperature chart to see what the temperature should be for that pressure.
Then simply find the difference between the two temperatures. The temperature of the suction line and the temperature according to the chart.
Need more superheat? Simple, just open the valve's superheat adjustment stem to allow more refrigerant into the evaporator.
Need less superheat? Simple, just close the valve adjustment stem reducing the amount of refrigerant entering the evaporator. Hey, this ain't rocket science!!! Right???
So I thought when I first arrived in the field. Fresh out of refrigeration school. Turn the adjustment stem. Go get a cup of coffee. All is well in the world!!! Another refrigeration repair accomplished.
If only it was that easy. Ever heard of a searching expansion valve? Want to occupy yourself for many hours? Just sit there and watch a valve in the real world do it's thing.
What a mess!!! The superheat goes down to 3 or 4 degrees then slowly rises to around 12 to 15 degrees. Then begins to repeat the cycle.
Over and over and over it goes. It never settles on one value and stays!!!
So, what's you superheat? Who the heck knows!!! Take your pick! Take an average! Flip a coin! Throw darts at a chart!
Want to know the dirty little secret? Who cares!!! Want to know what really important?
Pay attention to the compressor!!! It will tell you what you need to know about the superheat.
Assuming the rest of the system is operating normally, the compressor on a medium temp system should have a moderately cool suction bell around the suction line.
If the compressor body is sweating or cool up to the heads, superheat too low. If the suction line is warm at the compressor, superheat too high.
The whole idea of superheat is to get just the right amount of refrigerant into the evaporator.
Too much, it floods back and damages the compressor. Too little, you starve the evaporator, losing cooling capacity.
Simple, simple, simple, that's my motto. Keep that refrigeration repair simple.
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