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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5669
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have walk in cooler box Im working on. I cant get below

Customer Question

I have walk in cooler box I'm working on. I can't get below 40 deg. R-134a system. Suction 25psi (-9) 42 deg superheat. 120 head pressure with 11 deg sub cooling. Clean evap and cond. coils. All evap fans work.
Submitted: 2 years ago.
Category: HVAC
Expert:  Phil replied 2 years ago.

Phil :

All work to be done by licensed professional. WE KEEP GOING UNTIL YOU HAVE THE INFORMATION YOU NEED, I come and go. This is step ONE.

Phil :

Hello, Welcome to Just Answer

Phil :

So far it sounds like the compressor suction valves are going bad,

Phil :

You can do this test, close the liquid line service valve at the reciever, and hold the systems pressure switch closed or force the compressor contactor in manually so the compressor will keep running, Then tell me how low the suction pressure goes,

Phil :

If the compressor is good, it will pull into a vacuum

Phil :

If not, then the valves are bad,

Phil :

We can go from there with coaching on how to fix it after you get that test done,

Customer : It's a new compressor
Expert:  gotoman for ac replied 2 years ago.
Assist. Did you charge system with liquid or gas. The 134a refrigerant is charged in liquid state. Russ
Expert:  servtech50 replied 2 years ago.

Hello and welcome to just answer, I am servtech50 and will try to assist you today.

My first question to you is, was the evaporator and compressor sized properly for the load or area of the box? If this walk-in has been working properly for a long time then that is not an issue. (I am not sure if you are building a walk-in or repairing it). If this was a working system then I would check the TXV. 25psi suction pressure is equal to 21 to 22 deg. in relationship to a PT chart. (pressure = temp.) and if your suction line temp. is 42 deg. at the sensing bulb of the TXV then you have 21 to 20 deg. superheat. which means the evap. is starving. You should have a 10 to 12 deg superheat at sensing bulb. Sub-cool should be anywhere from 18 to 30 deg. depending on your ambient, if you have condenser fan control or a head pressure control (head master) for cold weather operation. So if you are building a cooler first make sure your evap and compressor are sized properly for the load. (for medium temp and your load your compressor and evap. should be sized for a 25deg evap temp.) Make sure you are using a TXV for 134a and feeds at the proper rate to achieve a 25deg. evap temp. and a 10 to 12 deg. superheat. other words if you are using a 1/2 horse compressor (6000 btu) then your TXV must be a 1/2 ton expansion valve not a 1/4 ton or a 3/4 ton expansion valve (one would starve the evap and the other would flood the evap. in either case you would not be able to adjust properly). Do not worry about the sub-cool at this point. your 11 deg. sub cool temp. is due to your condenser coil being flooded due to the starving of the evap. And after reading all of this you could also be just low on refrigerant. Does your system have a receiver and a sight glass? There should not be any bubbles in sight glass with 134a. But according to your head pressure of 120 psi the system should be full, unless you do have a headmaster control then it could be holding back the refrigerant.

Expert:  Phil replied 2 years ago.
Hello again, if its a new compressor, then it should pump down. I will be good to do that test regardless, it only takes a few minutes.

We can trouble shoot this step by step. Rus, the expert who posted with an assist, raised a good question... had you charged the system with liquid refrigerant, or vapor? R 134a is a refrigerant made of several different refrigerants that *separate in the vapor phase.. so that if you charge in vapor phase you are not putting R-134a into the system, but only a bad mix of one of its components. That can be a problem for the remaining R-134a in the cylinder of refrigerant as well... if someone had charged from it incorrectly earlier. What you put in the system in that case would not be 134a, but an unbalanced mix.

Regarding what our 'servtech50' posted. Those are all valid issues that need to be addressed. Can you answer to each of those... I will need that data to evaluate the situation.

Phil
Customer: replied 2 years ago.
This was working system. All I did was change the compressor. It is an exact replacement. I charged with liquid. I'm going to check superheat again
Expert:  Phil replied 2 years ago.
Thanks!

I will post the pressure /temp chart link here, just so we have it for easy reference in the thread.
http://www.csgnetwork.com/r134apresstempconv.html

You are on the right track with the super heat, however your earlier reading of 42F super heat unless I am misreading you... must have been a typographical error..

.... if the box was running at 40F when you took it, then the suction pressure/ temperature would necessarily be 42F cooler, or 2 degrees below zero...but it was not.... the suction pressure you reported at 25 lbs puts the evaporator at 28F or so.

As you know, sometimes the gauge needle sticks...be careful about that. And let me know if the system has a liquid line sight glass on it or not.

***
It is also possible that the wholesale house gave you a high temperature rated compressor instead of a medium or low temperature rated compressor.. checking those numbers closely will be a good idea.

We can go from there,

Customer: replied 2 years ago.
Rechecked today. Had 20-25 suction pressure. 39-40 deg suction line temp @ txv. 16 deg superheat. Box is 36 deg. Seems to be working ok.
Expert:  Phil replied 2 years ago.
Hello again, it appears your temperature measurement device is having problems, or you are not making good contact with what you are measuring.... we know that because your suction line temperature is reading warmer than the box temperature.

The suction line temperature for super heat purposes must be taken on further than a few inches from the evaporator coil... so that its not possible for the tube at that point to be warmer than the room its in.

If you are taking the suction line temperature at the condensing unit, you will get abnormally high super heat readings.

It sounds like you are on the right track, the fact that the box cooled down over night is a good sign, but you may still be loosing capacity, and could fail to cool sufficiently on a hot day... you should be sure that you are checking the super heat correctly, I run them between 8 and 10F super heat..if the condensing unit is within 10 feet of the box... if the condensing unit is further than 20 feet away, I run the super heat between 6 and 8F measured at the cooling coil suction connection, next to the TXV bulb.

Customer: replied 2 years ago.
Ok. I checked suction line temp in box with a clamp thermometer. Cond. unit is about 20ft away. Why is it that some suggest superheat to be between 20-30 and some 6-8?
Expert:  Phil replied 2 years ago.
Hello again, no competent service person will ever tell you that super heat on that sort of system should be between 20 and 30F..... super heat however *will run that high as a box is still warm and pulling down to temperature...but super heat is not properly taken in that case.

Don't confuse *sub cooling out of the *condenser with *super heat from the evaporator... many people do...

sub cooling can be quite high depending on the total load, and as the previous expert said in cases where head pressure control devices of certain types are fit.... (your system is not likely to have such a device however)... and if it did, would have no effect on super heat that we are involved with.

Regarding thermometer error... plus or minus 2F is common... depending on how the clamp on fit, it could be off a lot more than that... for super heat measurement I make sure I calibrate the thermometer used against a known source..and that can be tricky.

for instance the other day I put a thermometer into a glass of ice water mostly full of ice and not so much water. it read 36F... one would expect it to read 32 F... it only read 32 F when pushed into a bin full of crushed ice.

Thats a 4 degree split there, and if its not accurate you could add or subtract another 2 degrees or so.

Thats how it goes when checking these sorts of temperatures.



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