Temperature stalling at -20

[gozoboy]

We have problem with a cold room that has 2 systems. Both use a Dorin K750CS compressor and are watercooled. The first system works fine, reaches the set temperature of -22 in a short time.
The second will take about an hour to go from -19 to -20 and will just stop there. This has been going on for about four months now. First we changed the tev because the suction line began frosting up so we thought that it might be faulty. When the problems persisted we called an engineer to check the compressor and he told us that the piston rings had to be changed. Installed the compressor with new rings, the system works fine for about 10 days but then started to stall at -20 again. After another visit by the engineer we changed the valve plates but are still having the same problem. Pressures are 16.5 bar/255psi and 0.6bar/9psi. Refrigerant is R404a.
Any idea what might be wrong?

[Gary]

What are the air temperatures entering and leaving the coil?

[cadwaladr]

how do the pressures /temperatures compare with othersystem that works ok sounds like you have an internal problem with compressor maybe the bores are worn if new rings cured it for 10 days.

[Gary]

I'm thinking thoroughly defrosting it cured it for 10 days.

[US Iceman]

Any idea what might be wrong?

You are allowing the people to change too many parts without fixing anything. At that rate they will have rebuilt the system that does not work.

Seriously, this statement gives you the information you need (just as Gary mentioned).

...the system works fine for about 10 days but then started to stall at -20 again.

That is about how long it would take for the evaporator to develop sufficient ice to prevent adequate cooling. The defrost cycle may be operating, but not properly or for long enough.

If you can defrost the coil completely without changing anything else, then try the system again. If it pulls down to proper temperature once again... you have found the problem.

The next problem is find out why the evaporator is not defrosting. The ice may not be visible from the outside of the coil when you look at it. Insufficient defrost have a nasty habit of cleaning the coil surface you can see, but not the inside of the coil where you can't.

[Magoo]

At 0.6 bar suction at compressor is way too low for room at -20'C, thats 20 + degrees out of wack.
I agree with US Iceman and Gary, start at the evaporator, after a defrost get up and look at the evaporator, with a torch you should be able to look straight through. If not, then a defrost problem, get an electrician to check defrost system, heaters etc., do a comparison between one system against the other, as in amperage draw during defrost. Another thing to check is that evap defrost system may have defrost termination thermostats that stop defrost before total defrost.
magoo

[gozoboy]

The evaporator is not blocked with ice, you can see through it. the little ice that it has is at both ends, that is around the loops. When the valve plates were changed the bores seemed fine.

[Gary]

And the evaporator air in and air out temperatures?

[abet_meneses]

gozoboy,

At 255psi and 9psi operating pressure for a water cooled condenser seems that it has an unbalance pressure.Since the regulator was already replace,an adjustment of superheat setting should be done.Lower the superheat so that discharge pressure goes down while the back pressure goes up.It takes considerable field experience in adjusting the regulator.

[Yuri B.]

One has a luck who have two analogous systems/devices/units/ whatever - working alongside : just look more attentively, compare everithing and you should see, inevitably, what makes the difference.

[Gary]

One has a luck who have two analogous systems/devices/units/ whatever - working alongside : just look more attentively, compare everithing and you should see, inevitably, what makes the difference.

Good point.

Check the air in and air out temps on both evap coils.

[Magoo]

Could be moisture floating around the system, restricting TEV operation.
As Gary says "air on versus air off " what is the systems comparisons.
Out of left field perhaps the TEV are different from one system to the other, have you checked them.
magoo

[Andy_CTC_Ref]

TEV bulb insulated/mounted correctly with no ice build up on it??
Are your getting a fully frosted evap??

No oil or moisture in orifice or TEV??