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jakb21599
11-01-2011, 03:05 PM
Hello friends, Need some help with a freezer that i cant get to pull down to temp ( -20F)
The freezer has been working fine until last week and starting loosing temperature . Air flow is good across evaporator and is clean, all fans working. It has 2 txv valves shared by one common liquid and suction line.1 txv valve is in the middle of freezer right at the inlet of suction and liquid line entering freezer from out side, the other txv valve is located at the very end of freezer. The refrigerant is 404A. Here are some temp and pressures that i have.
The freezer box temp right now is 20(F) It was keeping -20(F)
Outside air temperature is 36(F)
suction line at compressor 22psi
liquid line at compressor 90psi
suction line temp at compressor 29(F)
liquid line temp discharging out of compressor 101(F)
liquid line temp after condensor 45(F)
suction line temp at evaporator 22(F)
suction line pressure at evaporator 20 psi
I think the charge is a little high but, when i get the suction line pressure around 15psi, you can here the lines in the freezer hissing. At current suction line pressure you can here one txv still hissing. I have taken apart and cleaned the screens but still same result. The site glass has a steady flow and no flashing or bubbles can be seen.
Any suggestions on what is wrong and how to fix? thanks for your help

chemi-cool
11-01-2011, 04:35 PM
Replace the refrigerant with R-507 and see how it goes.

R-404 behaves funny sometimes.

Gary
11-01-2011, 04:53 PM
Why would you think the charge is high? It looks low to me.

Gary
11-01-2011, 04:55 PM
The line between the compressor and the condenser is the "discharge line". The line between the condenser and the TXV is the "liquid line".

jakb21599
11-01-2011, 05:54 PM
Hi Gary, what should the charge be? I was shooting for around -15 to -20 evaporator temperature. on the pressure chart for 404A it is around 16 to 20 psi. that is why i was thinking that maybe the charge was a little high. Should it be something different to get the -20 temperature back in the freezer? If so can you tell me what it should be so i can adjust. thanks

Brian_UK
11-01-2011, 07:46 PM
The pressure is 'high' because the load is 'high'.

Perhaps start concentrating on your sub-cooling to keep a solid liquid line.

Whereabouts is the sight glass by the way?

jakb21599
11-01-2011, 08:00 PM
After the receiver and drier but their is a liquid line cylinoid valve after the site glass for hot gas defrost( no temperature drop across valve) What target sub-cooling should i be looking for?

Gary
11-01-2011, 08:05 PM
Hi Gary, what should the charge be? I was shooting for around -15 to -20 evaporator temperature. on the pressure chart for 404A it is around 16 to 20 psi. that is why i was thinking that maybe the charge was a little high. Should it be something different to get the -20 temperature back in the freezer? If so can you tell me what it should be so i can adjust. thanks

There is no temperature or pressure anywhere in the system that tells you anything useful. All useful information (dT, TD, SC, SH, approach, etc.) comes from comparing two temperatures to each other.

For example, on a very hot day we might expect the condensing temperature to be higher. On a cold day we would expect it to be lower. So... it's not just about condensing temp or outdoor temp, it is about a comparison of the two.

In this case, it is about subcooling and superheat, not evaporating temp.

Gary
11-01-2011, 08:10 PM
After the receiver and drier but their is a liquid line cylinoid valve after the site glass for hot gas defrost( no temperature drop across valve) What target sub-cooling should i be looking for?


That is a liquid line solenoid, not a defrost solenoid.

This may help bring you up to speed:

http://www.refrigeration-engineer.com/forums/showthread.php?t=19701

rasper1
11-01-2011, 08:14 PM
if your not sure of the charge take your ambient temp. then take the air temp after the condensor should be 15 degrees higher if it is 15 degrees higher your charge is close to being right

padraic
11-01-2011, 08:26 PM
90psi in the liquid line is very low maybe you are over condensing in the cold weather and not getting enough pressure drop across the valve... we had a few problems like this in cold weather and fitted alco fan speed controller on codenser fan and problem sorted...a tempory fix to see if it the problem block off half of the condenser with a bit of cardboard and see how it behaves try and get around 200psi in liquid line

Gary
11-01-2011, 08:32 PM
if your not sure of the charge take your ambient temp. then take the air temp after the condensor should be 15 degrees higher if it is 15 degrees higher your charge is close to being right

15K TD = 27F TD

A fair starting point, but less so in cold ambients where the TD may be lower. Low loads and/or high efficiency systems also tend to have lower TD's.

I would start with a few degrees of subcooling, wait for the box temp to drop down near design temp and then trim the charge according to subcooling and superheat.
.

transcoolchris
11-01-2011, 08:39 PM
Wise words Gary, you need to be looking at superheat and subcooling, also I would spend some time looking at and understanding the system, Gut feeling is it is probably light on refrigerant but it needs to be diagnosed.

jakb21599
11-01-2011, 08:46 PM
the box temp now is 8 degrees (F) after adding 2 pounds of refrigerant. Sub-cooling is 2 degrees. What am i aiming for on sub-cooling and super heat at design box temperature?

yorkshire
12-01-2011, 12:01 AM
Agree with padraic 90psi in liquid line is too low

HVACRAU
12-01-2011, 06:42 AM
90psi in the liquid line is very low maybe you are over condensing in the cold weather and not getting enough pressure drop across the valve... we had a few problems like this in cold weather and fitted alco fan speed controller on codenser fan and problem sorted...a tempory fix to see if it the problem block off half of the condenser with a bit of cardboard and see how it behaves try and get around 200psi in liquid line

I agree with this

Gary
12-01-2011, 07:19 AM
I would agree... if not for the lack of subcooling.

In any case, it would be a good idea to have a condenser fan control for cold weather operation.

DCoker
14-01-2011, 04:56 AM
The system is low on gas. Your head pressure should be much higher. The hissing sound coming from the valve is the sound of a starving valve. The unit being 404a should have either a head master with a set point of 210 psi and or fan cycle switch. The cut in on the fan cycle switches should be set up as follows : cut in should be 275psig and the cut out should be 210psig. The cold temp probably condensed the refrigerant making it run like it is low on gas. When you charge the unit a rule of thumb is to clear the sight glass and then add 20% more of the net weight it took to clear the sight glass.:o

DCoker
14-01-2011, 05:04 AM
the box temp now is 8 degrees (F) after adding 2 pounds of refrigerant. Sub-cooling is 2 degrees. What am i aiming for on sub-cooling and super heat at design box temperature?
Super heat on a low temp application should be 6 - 8 deg. The superheat should have swing and this should be the average. If it is dead on... Then your valve is probable flooding.

Gary
17-01-2011, 05:12 PM
Install a condenser fan control to get the condensing temp up to about 90F, then let us know what the subcooling and superheat temps are.