Dave's Tech Tip for July 2010

Why are you adjusting that TXV? To answer this question we first need to look at the main
functions of the TXV. The TXV is designed to keep the evaporator as full of saturated
refrigerant as possible while at the same time protecting the compressor from slugging or
overheating. This is accomplished by maintaining a set superheat. The typical TXV comes
factory set for 8-12 degrees of evaporator superheat, which in most cases should be fine. What
is the correct setting for evaporator superheat? While it is true lower superheat will make the
evaporator more efficient, we need to be more concerned with compressor or total superheat.
If we check the superheat on the suction line 6” from the compressor we should normally
have at least 20 degrees of total superheat. This will ensure the compressor is not slugging
with liquid even when frost is present. The suction line temperature at the compressor should
not exceed 65 degrees to ensure proper compressor cooling. The last but most important
reading is the discharge line temperature taken 6” from the compressor. This temperature
should never exceed 220 degrees, as higher discharge temperatures will lead to compressor
failures. Reducing superheat will lower both the compressor suction line and discharge line
temperatures.
The TXV can not be adjusted open or closed, it is a modulating valve. Turning the adjustment
stem clockwise will only increase spring pressure causing a higher superheat. Turning
the adjustment stem counterclockwise will decrease spring pressure reducing superheat.
When an operating refrigeration system stops working, the TXV may have lost its power head
charge or it may have become restricted, but it’s not likely that it came out of adjustment.
Clean or replace the malfunctioning valve, don’t try to readjust it. When replacing a TXV,
protect it from overheating while brazing as this will damage the new valve. The sensing bulb
should be mounted snuggly to the suction line on the evaporator outlet at the 9:00 or 3:00
position for smaller suction lines 5/8 or less. For suction lines 7/8 and larger the bulb should
be mounted at the 4:00 or 8:00 position. Adjustments may be needed on a new system at start
up, especially when you have long line runs.

 

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