Thermostatic Expansion Valve (TEV) Installation, Inspection, Diagnosis & Testing

 Thermostatic Expansion Valve (TEV) Installation, Inspection, Diagnosis & Testing

Thermostatic expansion valves (TEVs) are designed to meter refrigerant into the cooling coil at the proper rate. This design can keep the proper dose of refrigerant entering the cooling coil for maximum air conditioning or heat pump system operating efficiency. TEVs are similar to automatic expansion valves (AEVs) discussed below, but incorporate the signal from a temperature sensor mounted at the end of the evaporator coil

Details: If you are diagnosing a problem with an air conditioner or heat pump and the TEV appears to be involved, check the TEV installation details against the information we list in detail in a separate article at TEV INSTALL & REPAIR

Example Refrigeration Equipment Field Diagnosis & Repair: Thermostatic Expansion Valve Inspection, Testing, Experiments with TEV and Pressure Control Switch

 How to Adjust the Thermostatic Expansion Valve

For most TEVs, adjusting the thermostatic expansion valve woks as follows:

• Turning the adjustment stem "clockwise" increases the superheat.
• Turning the TEV adjustment stem "counter-clockwise" decreases the superheat

Field Notes from TEV Adjustment

The following are from my [DF] notes from a refrigeration service call [1982] VERY early in my [DF] refrigeration training:.

Case outline & initial observations: Commercial cooler running too warm - (WACOOP), hermetically sealed compressor, Kramer W14, unknown refrigerant (thought from a label maybe it should be R22 but someone may have charged with R12), cooler running too warm, need to diagnose cooling coil and fan operation and control settings on an old, used cooler just brought in. Very common on old equipment like this: no labels, no data tags, not much information at all.

Fanco refrigerant pressure switch: found set at 35# and 10# differential, connected improperly to the low side service port, cannot fully shut off the service port as a result - maybe leaking?

Compressor pump: running continuously.

Ambient temperature about 90 DegF; R12 in my service canister is at 100 psi static.

Low side refrigerant pressure: measured 40 psi. If there is R12 in the system I'd expect about 45 deg. temp at proper charge, and if R22 in the system I'd expect about 20 degF temp at proper charge and operation. But there were NO frost lines on the equipment, so I know that there is little or no liquid refrigerant and the system is operating at about 45 degF so must be filled with R12.

Actions and Tests:

Set the TEV 8 quarter turns more open - out and down, to see what happens.
With the fan off the Low Side goes to 25 psi and 25 degF.

The frost line moved at least to the se3nsor bulb and the pump (compressor) shut off. The low side pressure went up to 36 psi and then the pump restarted. This is telling me what the pressure control switch is doing.

Further actions and observations:

Opened the TEV 1/4 turn more to see the effect.
System shut off at 23# and came back on at 37#

Turned the blower cooling fan back on since the Dx was iced coil. Low side went up to 50, then dropped to 39# and stabilized.

Kept a series of observations from 10:55 PM to 12:42 AM (service call made during hours the business was closed to avoid disruption)

5 then 9 more turns opening the TEV, low side up to 75# & can see gas in the SIGHT GLASS in the refrigerant line - this is "wide open" TEV setting

9 turns closing down the TEV to almost shut - so there are about 10 turns from wide open to fully shut on this TEV. At 9 turns towards shut from wide open, the low side pressure falls FAST!

9 turns back open at the TEV confirms gas bubbles again in the sight glass and 70# pressure.

Closed the TEV completely (about 9+ turns to the right or "up" or "in"). Suction lines closed, no gas in the sight glass, rapid low side pressure drop to 26#, compressor turns off at 20#.

I am convinced the pressure control switch is working properly, that is it does what it's pressure settings say it should be doing.

Set the TEV to 12 1/4-turns (in other words 3 full turns) open from fully shut. Suction line very cold, low side goes to 30#. 2 more turns open, low side goes up to 34#.

Finally decide to run the system with the TEV open 3 quarter-turns (about 3/4 of one turn) from fully shut. The system stabilizes with the cooler (a refrigerator) in the mid to upper 40's, no more oscillating, no coil frosting.

If I set the cut-in pressure way down the cooling coil ices over and the compressor will run continuously without cooling anything. See frost moving down the low side line. So that's not the right "fix".

I could set the pressure switch to 12.5 psi, left the TEV alone, and got the cooler down to our target of 32 degF.

The HI event sets the defrost cycle by setting the cut-in. The low event sets the cutout and therefore the lowest temp we will reach. A bigger low event means a lower target temperature, but the risk is that if you set it too low the compressor will run continuously and ice up the coil without ever running a defrost cycle.

The TEV seemed to be sometimes sticking. The low side pressure would hang at 28# or rise only very slowly as if the TEV was not opening when I expected it to. Have to be sure the blower fan is also running when checking this performance.

Final resolution of the cooler operation troubles:

Ultimately I replaced the TEV with a Singer TXV223FA 1/2 with a TE value of 9 (heat delta), installed a filter dryer (#082 PN 2003), set the pressure control to 35# on and 20# off. adjusted the system to get NO frost on the suction line near the compressor. Final pressure switch settings were 33# and 18# hi and low. We were able to get the cooler, charged with R12, to hold a stable 34 degF at cutoff, rising to 39 degF at which point the compressor would cut back on.