Superheat adjustment or Incorrect charge

[Tradewinds]

Hi Everyone,
I'm pretty sure this is a common question within the industry, but what should we be looking for to differentiate between an incorrect refrigerant charge or whether we need to adjust the superheat on a txv. I'm currently working on a R22 system (coolroom +3'c and freezer -25'c with common condenser) and the TXV is approximately 5 m away from evap coils (ie outside of freezer room). The coolroom gets down to temp but the freezer only gets to -14'c. I have recently installed new TXV's so I don't really want to change the factory setting, however given the location of the valves I may have to. I have tried adding refrigerant to the system to get a clear SG, superheat of 6'c and subcooling of 6'c but keep getting frost back at the compressor. I have adjusted the charge to get differant readings but none of which I am completely happy with.
I guess I just need some clarity on the two proceedures.
Also, should the EPR valve for the coolroom be set to -5'c?
Any thought's?

[Pooh]

You make the statement that you are getting "frost back" at the compressor, what is the measured superheat at the compressor suction? If you do not have a full sightglass you will not have any subcooling, if the pipe run is very short and the superheat is set to 6deg C you will still get frosting at the compressor as the suction vapour will be at -29deg C anyway. The EPR on the chillroom is set about right for the required temperature so that is not the problem however which room controls the running of the compressor or is it on individual solenoids and pumpdown control on the compressor.

Not an ideal set up really but should work.

Ian

[Tradewinds]

Hi Ian,
Thanks for the feedback.
The compressors just cycles off on pump down when the solenoids close. Here's all the figures:
Suct press. 25psi= -18'c
Suction Temp(with surface probe)=-13'c
Disch Press.=200psi =38'c
Disch. Temp.= 37'c
Liquid line temp=36'c
Ambient 24'c
Superheat=6'c
Subcooling=2'c
Sight glass is bubbling when both coolroom and freezer are operating but goes clear when coolroom cycles off.

Based on the above, I am suspecting it is just short of gas but I am also wondering if the reciever is too small. A new condensing unit has just been installed. The original system had a larger remote receiver with the compressor and condenser being seperate. They have been removed and a complete condensing unit has been installed. This new unit only has a small receiver on it so I was wondering if that could be a problem.

[The Viking]

Based on the above, I am suspecting it is just short of gas but I am also wondering if the reciever is too small. I was wondering if that could be a problem.

I agree.
:cool:

[Tradewinds]

Ok.
Sorted. Turned out to be incorrect size of TXV orifice.
Thanks for your help and what a great forum.
Keep up the good work fella's.
Stuart.

[Pooh]

Stuart
looking at the figures I still reckon it is short of gas, I presume you have fitted a smaller orifice in one of the valves, this has hidden the problem rather than sort it. How would the wrong sized orifice explain that the sight glass clears when the chiller cycles off. Also if the receiver was too small the system should trip on HP on pump down. I would expect at least 10 degC of subcooling if the system was correctly charged.
Where did you take the superheat readings at the comp or after the evaporator?

Ian

[Boatfrig]

Be careful when using a sight glass to determine proper charge. If there are any components between the outlet of the receiver and the sight glass, or line length, or bends that may cause a pressure drop in the line, the sight glass may show bubbles when an increased volume of refrigerant is required.

To check for bubbles caused by a line pressure drop, increase the head pressure about 5 psi and watch for the sight glass to momentarily clear. This would indicate that there is liquid ***** at the outlet of the receiver.
The increase of pressure results in the liquid in the line to be momentarily sub-cooled relative to the line pressure, hence the bubbles (flash gas) caused by the restriction are momentarily cleared. To increase head pressure partially block the condenser cooling air or water flow. As the pressure and temperature of the liquid in the receiver equalize, the bubbles will return.