Critical cold room

[tomt]

Have a cold room must keep 4deg c, problem is that chart recorder reading 1.5 deg plus and minus from setpoint. Liquid injection valve was replaced by another contractor. I removed all 134-A, evacuated system to 500 microns. Weighed in virgin 134-A. Adjusted HGBV and liquid injection per manual. Started unit temp of box 7c, SP 25# all looked well. Came back 2 hours later doing same thing with swing in temp. SP 5# controller cycling liquid line valve. Has a EPR valve set for 26# I am leaning toward this that it is not holding back evap pressure at low load. Also has a redundunt unit that works just fine, controller controls both units for setpoint. Pharmacutical unit in a clean room so problem is getting in and out to check equipment.

[sedgy]

hi tomt, have you tested the temp recorder to see if its calibrated correctly?

[tomt]

There are 2 units A and B on the same recorder. A unit is fine keeps temp at 4c. They cycle them every 12 hours. Honeywell controller operates both units. Not looking in that direction. It is when B unit running cannot get stable temp. I am more a A/C service guy not to familiar with cold rooms.

[multisync]

If it does this all the time surely the best way is to stay with the unit and watch what actually happens??:confused:

[tomt]

did that it is in a clean room cannot see temp change at recorder.

[chemi-cool]

When unit stops, there is still refrigerant in the system and it takes time till its all boiled. In that short time the temperature will drop a bit, especially if the room is only part loaded.

The differential between start and stop cuts in about 1 > 1.5 °C.

That is what you see on the temp recorder.

The critical temp is of the products and not the air in the room.

To get exact temp of the substance that being kept in the cold room, you need to put the temp recorder's sensor in the product being stored so you will get the real temp.

[multisync]

System A runs fine and charts ok. System B is the problem child.

I wonder if it's cycling on the LLV rather than the stat?

Sit by the condenser and check the system how it runs when the stat goes off does it pump down and stop. Compare cycles with the good v bad and see.

[smoggy]

does the system have a liquid line solenoid fitted

[smoggy]

should of said on last post , how far away from exp device is the liquid line solenoid valve sited