High head pressure AND low discharge temperature. r404a

[nandoanalog]

Hello


Compressor was icing up, and discharge temperature was at 62C. Head pressure was a bit high at 2.2 mpa, low pressure 0.15 mpa, so I guessed TXV's (2 of them) must be a bit too open, and maybe the filter on the liquid line was clogged. Changed filter, and gave it half a turn (clock wise if looking straight at the spindle) to both txv's, and the compressor starts flooding. I kept on closing the txv's, but flooding seems to persist. Bringing them back to their previous setting also didn't seem to make any difference.

Any ideas? Txv's wen't bad?

[Rtic]

From Emerson, for when flooding occers;

TXV seat leak. If the pin and port do not seat properly, liquid refrigerantwill flow through the valve whenthere should be none. Inspecting thevalve may reveal dirt and debris, inwhich case the TXV can be cleanedand put back into service. Inspectionmay also reveal damage to the pin orport due to wire drawing or erosion ofthe pin. In such a case, the valveshould be replaced;

or

Incorrect bulb position. Ideally,the power element bulb should beattached to a horizontal run of suctionline—immediately after the evaporatoroutlet. It should be in close proximityto the equalizer connection, butupstream of it. The bulb needs to befirmly attached to the suction line tomaintain good thermal contact.Additionally, the bulb must not beinfluenced by external sources of heat.

[nandoanalog]

I'm very tempted to switch these txv's for axv's. They've been annoying since day one.

[monkey spanners]

How did you know they were flooding back?

[nandoanalog]

valve knocking?

[monkey spanners]

Could there be moisture in the system? Read where it can freeze and hold the valve open.
Is there a liquid/suction heat exchanger? Have heard where they can leak internally.
I think i'd fit some new txv's with new orifices. Check the sizing.
Also, is the evaporator clean? Fans working ok etc?

[nandoanalog]

It's an ice maker. I find the whole design pretty odd. But I guess the design engineer knows best!

It has a liquid/suction line heat exchanger. I don't think it's faulty tho.
It's a fairly large water cooled condenser (read over 2 meters long) that's subcooling the gas (get this) 34C ! If I close the cooling water, the pressure goes way up.

Both txv's are 067N6162 TR13/45KW, which have flooded the compressor on startup since day one. I was able to manage this by controlling the liquid line solenoid with a cycling timer on startups for 10 minutes until the txv's stabilized.
They have worked the whole summer reliably, but now, since the intake water is colder, hence lower load on the evaporator, superheat was a bit low. A tiny adjustment on one valve broke all hell lose. And to complicate it more, any adjustment on one txv, changes the behavior of the other one.
I'd think that TR11 txv's would be more suitable (or even AXV's like every other flake ice machine I have ever seen). But the engineer says noooo no no no.

[Magoo]

2.2mpa discharge = 50'C SDT. High discharge will over rate TXV selection and performance,

[nandoanalog]

2.2mpa discharge = 50'C SDT. High discharge will over rate TXV selection and performance,

Thought about putting some sort of pressure regulator on the liquid line and be be done with it. Or... AXV's.

[Magoo]

Establish why you have such a high discharge pressure.

[dkemper]

High head pressure and massive subcooling at the condenser are diametrically opposed symptoms.

I'd suggest non-condensables in the condenser, possibly to include excess liquid R404a (system over charged).

[nandoanalog]

High head pressure and massive subcooling at the condenser are diametrically opposed symptoms.

I'd suggest non-condensables in the condenser, possibly to include excess liquid R404a (system over charged).

I've managed to bring sub cooling down to 10C. Discharge is at 2mpa.
There's a valve on the liquid line, which I partially closed to bring pressure down to 0.5 mpa ( I know, I know, silly me. But this machine can NOT stop working until December), and it's now stable. The only superheat I can get on these txv's is 4C. Any small adjustment in either direction will cause them to act erratically.

Both evaporating coils are making ice homogeneously. Wouldn't condensables clog a coil/txv?

Now, this high head pressure might be caused by low flow cooling water/dirty condenser. I could increase the cooling water flow, but superheat would be a bit silly.

I'll recover some more gas from it and see how it goes.

[dkemper]

If high head was caused by inadequate water flow, I'd expect no subcooling at all.

Non-condensibles can include a lot of things, water, air, nitrogen, or what I suspect in your case, a back up of liquid R404a in the condenser. The excess refrigerant reduces the capacity of the condensor resulting in the high pressures, yet that same excess is then cooled by the water because it hangs around inside the condenser longer to a point of very high subcooling.

That same high pressure with high subcooling can then over feed/over capacity the TXV causing the flood back to the compressor.

[Magoo]

Hi dkemper.
I totally agree, definitely non condensibles,

[nandoanalog]

If high head was caused by inadequate water flow, I'd expect no subcooling at all.

Non-condensibles can include a lot of things, water, air, nitrogen, or what I suspect in your case, a back up of liquid R404a in the condenser. The excess refrigerant reduces the capacity of the condensor resulting in the high pressures, yet that same excess is then cooled by the water because it hangs around inside the condenser longer to a point of very high subcooling.

That same high pressure with high subcooling can then over feed/over capacity the TXV causing the flood back to the compressor.

Thought about that too. But I can't mess with the machine until next weekend.

With that said, one year ago, this machine ran with lower pressures. Something must have changed.

[Kenneth199]

as they said: none condensable gases. Reclaim the refrigerant and check it.
does it ever stop on low pressure switch? If so, whats the cut out?

[nandoanalog]

as they said: none condensable gases. Reclaim the refrigerant and check it.
does it ever stop on low pressure switch? If so, whats the cut out?

Good thing you mentioned that!
I checked it, and the contacts were oxidized! It was also set to almost 0mpa. It is now at 0.1 mpa.
But no, it doesn't have anything making the pressure go so low. If anything, the low side jumps to 0.3 mpa if I let the liquid flow at will.

[Allrounder]

How, how are you getting on? Can I suggest you get Emerson involved, they can come to site, take responsibility for the problem and you can take some knowledge from them at the same time. Everyone is a winner.

Allrounder