Answer on how do we get it.
To measure evaporator superheat, get your low side pressure reading and convert it to dew point temperature, subtract from that the actual evaporator outlet temperature reading (taken with a probe), and the difference is your evaporator superheat.
To measure suction line superheat take two physical temperature readings... evaporator outlet and again approximately 6 inches from the compressor inlet. The difference between these two is your suction line superheat.
To measure compressor superheat (all superheat prior to the compressor) get your low side pressure reading and convert it to dew point temperature, subtract from that the compressor inlet line temperature. The difference is compressor superheat, [not to be confused with heat of compression which is work added (kinetic energy adding more superheat) to the already superheated refrigerant.]
Note, you will get negative numbers depending on how you perform your calculations. I cheat and reverse the formula... same thing but less confusing.
Thanks for posting Dan, jump right in here!
how to check sh on multiple evaporator
questions for you superheat experts, what is the best way to check a system with a single compressor with two or more evaporator? more to the point- how does one go about adjusting superheat on individual evaporator?? medium temp cabinet (35F-40F) dan
[Edited by dan wong on 21-04-2001 at 11:11 AM]
little mor e detail on problem
continue from my previous post.
The unit is a cold food storage with a counter top, commonly used in restaurant or pizza prep-table. both evaporator perform same function, medium temp. TXV, originally, a DANFROSS 1/4T internal eq. valve. I changed it to Sportland 1/4t balance port, internal equalize, does not have LL solenoids, evaporators simply hook up in parallel, suction line in parallel with a single line return to compressor. This is the way it came out of factory. unit is old, no tag avaible to identify manufacturer or amount or charge.
Original problem: from time to time, one or both evaporator will ice up completely, changing the txv did not solve the problem, I've adjusted superheat same proceedure as decussed on post here, situation is better but not solved(I set it to 18F sh) I replace the temp control with a low pressure control, make the TURN-ON and TURN-OFF wide, that seen to work ok now.
my question is, since I am taking pressure reading at the compressor-which is common to both evaporator-not knowing if one is over feed or under feed, by taking temp at outlet of each evaporator, It is not accurate adjustment, How can I make an accurate ajustment with this type of set up? dan
Re: little mor e detail on problem
Quote:
Originally posted by dan wong
Original problem: from time to time, one or both evaporator will ice up completely, changing the txv did not solve the problem, I've adjusted superheat same proceedure as decussed on post here, situation is better but not solved(I set it to 18F sh) I replace the temp control with a low pressure control, make the TURN-ON and TURN-OFF wide, that seen to work ok now.
my question is, since I am taking pressure reading at the compressor-which is common to both evaporator-not knowing if one is over feed or under feed, by taking temp at outlet of each evaporator, It is not accurate adjustment, How can I make an accurate ajustment with this type of set up? dan
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Most undercounter refrigerators I am familiar with have defrost timers. I recall older designs that did not, and I added timers to them. All commercial refrigerators should have scheduled defrosts. I highly recommend a defrost timer if you don't already have one.
But for sake of discussion, I like the input you have received, after asking some good questions. It makes for fine theoretical discussion and wonder. The manufacturer in me says that you are poking your head into things you shouldn't. The tech in me is with you all the way.
But let's step back. Was the only problem you had originally, that one coil or another or both were icing up? If so, why did you choose to change TEV valves? Why not open up the low pressure control then, as opposed to after you performed major surgery?
Dan
should net require a defrost on med temp.
Dan, This cold table is approx. 15year old,(working ok for 15 year w/o a timer). medium temp 35F-40F. (I also had two other customers with same type of table, with three evaporators, no defrost timer, Therefore I don't beleive I needed one.) I replace the Txv becuse I thought they maybe bad, I was hoping a pair of BALANCED PORT VALVE would solve the problem.
The two other customer is now with another company, also having similiar problem.
Back to my question, If I disconnect the fans on one evaporator, let it ice up, allow that TXV to shut off, then adjust the other, would that work??? dan wong