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flyboy3b
29-09-2007, 03:08 PM
ill try to make this as short as possible, and thanks for reading.
i bought a used norlake that is claimed to have a bad compressor. my first check found low enough pressure that i had to lower the low pressure limit to get it to fire up. there were no useable reading to be taken at that time.
i charged it with 70 lbs nitrogen and found a leak in the evap coil which i repaired. i refilled it and left it for a few days and it held pressure.
when i returned, i evacuated the system to 500 microns, and started adding a charge of 404a.
the nameplate reads 42 0z as a reccomended charge, but i dont know if that is the original condensing unit, so i start adding refrigerant slowly and watch my gauges and thermometer readings. at one point, i see the evap coil frost up (a breif moment of elation) and i continue to slowly add the liquid. as i continue adding, i see the coil warm up and sweat and when i reach 42 oz,, i stop.
at this point, i have the following readings;
1. ambient 76 (it was a nice day in wisconsin)
2. full load on the coil (this unit is self contained and its just sitting on the floor)
3. temp after the reciever 97
4. pressure after the reciever 275 lbs
5. low side 75 (at the compressor)
6. temp at the low side of compressor 86.6
7. temp at the entrance of the evaporator coil 48
8. temp at the evap outlet 56
ok,, so the subcooling looks ok, but the superheat is really high, and the coil is not getting cold enough. so i pull 12 oz out of the system and the high side pressure drops to 200 and the amount of subcooling drops to 7 Deg. the superheat drops a little, and the first couple runs on the evap coil ice up, but overall,, still not cold enough.
i try adding 28 oz of refrigerant (16 oz more than the nameplate) still no noticeable difference. the high side pressure stays at about 280, the subcooling is around 10 deg, the superheat is high, and not much cold air
with a 16oz overcharge (supposedly) the system has the following readings;
1. evap outlet temp 56
2. compressor in temp 75
3. compressor in pressure 75
4. condensor in temp 125
5. condensor out temp 98
6. pressure after the reciever 280
7. no noticeable temp change at the drier
8. appears to have a new txv (which i tried adjust with no appreciable difference).
the compressor is an extended medium temp application and everything else appears ok.
any thoughts on this would be greatly appreciated. fortunately, this is a spare walkin and it doesnt need to go into service right away, but ive been spending alot of hours trying to figure this thing out (its my first system).
thanks again in advance,
mike

750 Valve
02-10-2007, 12:47 PM
Is there a suction service valve on the compressor to shut down and check the condition of the valves - efficiency test?

flyboy3b
02-10-2007, 02:23 PM
no,, unfortunately there is not. at this point i have the unit pumped down and am looking for a new compressor. the only other thing i can imagine is that perhaps the oil is so low that the valves arent sealing very well. is there an easy way to check, and or, change the oil on a hermetic compressor?
thanks,
mike

Bones74
12-11-2007, 02:46 AM
You sound confused. Go to my profile and call me.:)

Gary
12-11-2007, 06:29 PM
Add refrigerant until the subcooling is 15F, then go away for awhile. The superheat is supposed to be high until the temperature inside the walk-in drops to normal range.

Gary
12-11-2007, 08:09 PM
2. full load on the coil (this unit is self contained and its just sitting on the floor)


I'm confused. Does this mean the evaporator is not mounted inside a walk-in box? Is it just a condensing unit and evaporator coil sitting on the floor?

BTW, at this point the condensing temperature is running about 36F over ambient, which means the compressor is pumping a very heavy load so it is probably OK.

jack_lizhaoyang
12-11-2007, 08:42 PM
is there a valve to shut off liquid after condenser before tev? shut off liquid to check if the compressor can pump down. if can't pump down means compressor gone. otherwise check tev
what type of TEV?
if possible change to a smaller orifice.
you can't expect freeze on coil when suction pressure above 60 for 404a.
it's useless to check superheat when totally there is no freeze.

M.Amer
12-11-2007, 11:40 PM
If their is port @ crank case conect gauge n pump dwn system if da pressur starts increasing after p/d change valve plate n piston sings...for oil charge unit 4om liq recivr proper amaont...chk txv stainer.

Gary
13-11-2007, 05:28 PM
There is nothing wrong with this compressor. What we are looking at here is a very heavy heat load on the evaporator.

Gary
13-11-2007, 06:27 PM
8. appears to have a new txv (which i tried adjust with no appreciable difference).

No doubt you were taught in refrigeration school that "on a medium temp system the evaporator outlet superheat should be 8-10F". That's half right. Here's the missing half:

"On a medium temp system the evaporator outlet superheat should be 8-10F if/when the air entering the evaporator is at design temp"

If the entering air temp is higher, the superheat should be higher. If the entering air temp is lower, the superheat should be lower.

You cannot adjust superheat until the refrigerated space is at or very near its design temperature.

flyboy3b
21-11-2007, 08:47 PM
sorry i wandered off for a bit. i got busy with work. i think gary had it correct. i was trying to adjust the superheat right off the bat, and the load was too high. i ended up just pulling a new vacuum and charging the reccomended 26oz and leaving it. its been installed for a few weeks and holding a near perfect 40f.
thanks again for all of your input, im learning, little by little.
mike

Gary
21-11-2007, 09:45 PM
If you would care to take a few temperature measurements for us, we could help you to fine tune the system. Specifically, we would need to know:

Low side:

Evaporator air in temp
Evaporator air out temp
Saturated evaporating temp (low side pressure converted to temp from a P/T chart)
Suction line temp at the evap outlet
Suction line temp near the compressor inlet

High side:

Condenser air in temp
Condenser air out temp
Saturated condensing temp (high side pressure converted to temp from a P/T chart)
Liquid line temp near the receiver outlet

Various combinations of these temperature measurements can reveal a multitude of system problems.