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Thread: Walk in cooler issues
03-18-2012, 01:18 PM #1Regular Guest
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- Jan 2011
Walk in cooler issues
I have a client with a walk-in cooler problem or bad pressures, the unit cools but I don't like it. There are 2 evaporators and the cooler size is about 3000 sq ft, anyhow the one evap i replaced in the winter, the other is original, the compressor was replaced 2 years ago as well as the oil sensor and pump. The system was going off on low oil pressure, due to the differential being around 10 psi, the sight glass on the crankcase was clear, you can see the droplets of oil when the system is running, I figured that i needed to add oil because maybe the old coil had a lot of oil left in it that never went back to crankcase so i added a quart, the oil pressure went up to about 60-65 psi with a suction of 40-45, the suction press on r-22 is 40, the hi side is 200, ambient is 70 deg, receiver has 65 pd capacity, i have 53 pds in it with a full sight glass. I can't get the suction press up, the coils are clean, and the txvs are cranked open, the superheat is still too high at 18 deg, subcooling is 6 deg , could need more refrigerant but the sight glass was full 5 pds ago, so i have not added anymore, but the signs point to the system still being low, the box temp is down to 39 deg with these pressures, but i don't like the low suction pressure. Any help is appreciated.
03-18-2012, 01:30 PM #2
Did you independantly check each evap coils' suction superheat? If not, one could be flooding, and the other starving, resulting in an avg 18 deg SH.
Another thing could be pressure drop in suction line if it is not sized properly. To properly check evap SH, you have to have a port on the suction leaving the coil, so you can determine if there is a pressure drop from there to the condenser.
03-18-2012, 06:47 PM #3Regular Guest
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- Jan 2011
there is a port on the suction line at the end of the evaporator, thought of the underfeeding, i opened up that txv all the way, the old evap i did nothing with the txv yet and there is no drop, both evaps have 18 deg superheat, around 36 deg suction lines
03-18-2012, 06:51 PM #4
03-18-2012, 07:53 PM #5
40# sounds about right to me for r-22 at 39 degrees
03-18-2012, 08:18 PM #6
So is the oil pressure 20 or 60-65, both of which are on the high and low end respectively? Is there an oil return issue? Reciever has 65lbs capacity at 100% full or 80% full? If that's at 80% full sounds like your pretty close to a full charge. Subcooling #'s will vary if your bypassing thru the headmaster or unit is not working at design temp. Are the txv's sized properly? I also believe there is nothing wrong with 40psi on the low side.Local 597 Service Fitter
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03-18-2012, 08:26 PM #7
Id check the screens in the TXV's and confirm they are the right size like he said.
03-19-2012, 08:36 AM #8Regular Guest
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- Jan 2011
03-19-2012, 09:01 AM #9
The TD (not 'split') is determined when it is engineered.
It isn't always 10 degrees, so using that rule of thumb isn't always good.
40 pounds is a fairly normal R-22 medium temp suction pressure, give or take a pound or two.
The superheat is an issue. I'd like to see 8-10 degrees SH at the coil.
03-19-2012, 09:02 AM #10
As far as charging the system goes, use this as a guideline.
This ass-u-mes that the unit has a headmaster.
03-19-2012, 09:07 AM #11
Yea 40 is about right.
I start at 6 deg SH at the txv bulb. That will usually be real close and what a new evaporator spec sheet will tell you to set it at. From there ill adjust to get the least swing or most stable point.
If your backed all the way out and still have 18 deg SH then your evaporator isnt flooded. You need to figure out why
03-19-2012, 09:38 AM #12
Well; let's think about this a little -
Water cooled design-condensing would be 105º. Air cooled design-condemsing would be 125º. You are at 101º. Lack of oil return may be low refrigerant flow / low head pressure / poor head pressure control. I would start with making it control at about 117º.
The SSH is too high at the coils. This commonly results from either insufficient subcooling or insufficient refrigerant flow despite having adequate subcooling. See paragraph 1 above. The TXV's are only so big. After you get the head pressure to 117º - get the subcooling to 10º if it's not already there - and see how the TXV's flow then. The TXV's may be too small but let's make them do as much as they can before condemning them.
BTW: Did you really say that you thought oil was logged out in the system somewhere - so you added More oil? <g>