Bet you never saw a user-adjustable CP TEV!!

[havertyp]

An unusual application, this, I have stripped away all the electronics, PCB's, wiring, insulation & panels to get to this, it is an Emerson 047680 constant-pressure TEV and is used on a high-accuracy (to 0.001 deg C!!) oil-bath made by Hart (now part of Fluke) and used a lot in the semiconductor industry to calibrate temperature sensors etc. High stage uses R507 and an Embraco J2212GK3-RV hermetic comp. and low stage uses a Copeland KAJB-010E-CAV-800 semi-hermetic comp, running on R508b. The CP valve is adjustable by the user (note the non-standard threaded rod & knurled knob which replaces the usual adjuster screw & sealing cap on the valve: this knob is accessible by the user and a look-up table printed beside it allows the user to set the evaporator pressure (& thus temperature) to suit the setpoint they need: the idea is to balance the heating & cooling power as close as possible to prevent the heater's PID controller constantly over/undershooting and thus unacceptable temperature instability of the test chamber oil (a special non-conductive perfluorinated fluid). This valve was shot, it was not regulating the evaporator pressure, constantly cycling and thus the temperature stability was poor. Just replaced it with a Sporlan ADRI series valve and new charge and all is well. I dont like the idea of these valves being adjustable by the user though, they really are designed to be adjusted by a refrigeration tech and capped off. I think that is what killed the last valve, since it is being tweaked every day and the mechanics must have suffered, it was impossible to turn the knob in the end and the regulation was shot. Would be interested in your opinions on having this type of valve open to user-adjustment on a daily basis and if there are better solutions.

[Gary]

That's an AEV (automatic expansion valve), not a TEV.

[havertyp]

Thanks for the tip! - had a long day doing stuff on the PC today and never noticed the mistype - I stand corrected!
(must go to the principals' office for a caning and 100 lines!!)

[Gary]

Thanks for the tip! - had a long day doing stuff on the PC today and never noticed the mistype - I stand corrected!
(must go to the principals' office for a caning and 100 lines!!)

Better yet, with the proper name you can go to google and find lots of information.

[havertyp]

Just wondering if it is a good application by the manufacturer of this piece of kit (they made up the knurled adjuster and threaded rod and subbed it for the AEV's original capped - off adjuster. I know for definite that this AEV is tweaked at least once or twice a day by the technician using the bath. Are these valves up to this type of constant tampering? Getting worried as I see another similar unit showing the same symptoms. Happily the repair on this unit was successful, but for how long...even my kitchen taps wear out their spindles at least every 18 months!!

[Gary]

An AEV works best where there is a constant heat load. If the heat load varies from the norm, the AEV will be very slow to respond. When extra heat load is added, this tends to raise the pressure. The AEV responds by reducing the refrigerant flow to drop the pressure. With less refrigerant flow the pulldown takes longer.

[havertyp]

The heat load on this unit is constant, it is basically a tub with approx 15 litres of a special low-conductivity fluid into which a PCB with temperature sensors or RTD's for calibration is placed. Problem is, the user may want this fluid to be anywhere between -40 C and 100 C and at a particular temperature, the accuracy must be within approx 0.01C! To get this accuracy, the cooling power can be changed (by varying the evaporator pressure via the jury-rigged AEV adjuster knob) and thus the evaporator temp. Tuning the cooling power to the heater power avoids excessive instability (the bath has a temperature sensor driving a 2KW heater via a PID controller). if a different temperature setpoint is needed, the AEV has to be adjusted, using a lookup table printed beside it and a pressure gauge on the suction
side (also visible to the user). Main concern is the wear introduced to the AEV internals by constantly adjusting this, is thee an electronic version of an AEV available which is more tolerant of regular adjustments..

[Gary]

If there isn't, there should be.

Seems like an EEV could be set up to do the job. It's all in the controls.