High head pressure no duty

I've been racking my brains on this one all night trying to think about why this is happening.

Remote condensing unit within 25 mtrs of cold store evaporator. Engineer has reported that the head pressure is 325psi on R404a and 40 psi on suction at the moment ambient is 13c and the liquid line is untouchable (to hot). The coldstore is 10 degrees off design temp,(chiller +1c) condenser clean and fans all ok.

Evaporator clean, fans going right way etc, engineer changed valve and orifice no difference, system has never been broke into so no incondensables i hope,we installed it 5 years ago.

Heres the strange thing if we had a partial blockage on the liquid side the system would pump down and go out on LP switch.

Told the engineer too switch off the evap fans to see if the coil would ice over, it appears some of the distributers are blocked and the coil is only frosting at the top and bottom and within 3 mins its pumped down and gone out on LP. But i can't understand the High discharge temps and pressures.

Heres my thoughts tell me what you think?

Yes some distributors are blocked but still getting a little duty out of system?

A restriction in the high pressure side causing the high head pressures.

I have had this problem on a system years ago but at my age the memory recall is not as good as it should be from sniffing R12:D:D

I think its just the partial restriction on the evaporator what do you think.

Regards
Lrac

Restriction on high side, does it have discharge mufflers or oil separators? Kinked line?

Any restriction after the condenser on the liquid line, including a blocked evap, would give low pressures and cool liquid lines. The refrigerant stays in the condenser and liquid line longer, so it loses more sensible heat. Also there's less refrigerent in the suction line feeding the compressor, therefore pressures don't rise in the high side as there's little vapour to add to the discharge side. The discharge temp might increase, but the liquid line wouldn't be hot.

Only other thing I can thing of, is if it has a pump down solenoid, a leak could have occured between the solenoid and compressor, and it's been sucking in air, explaining high discharge pressures and temps and maybe any blockages.

Unless of course someone on site stuffed it up and got incondensibles in there and aren't admiting to it.

So discharge blockage before the condenser or incondensibles are the only thing I can think of.

Quote:

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Originally Posted by jollycold

Personally I would reclaim new drier and a fresh charge first...

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I'd do this, but blow through with nitrogen after reclaim to see if there's a restriction before the TX.
edit: and leaks on the low side that might have been sucking in air if the LP cut out was set at 0kpa.
Plus also give it a good evacuation.

Quote:

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Originally Posted by LRAC

I've been racking my brains on this one all night trying to think about why this is happening.

Remote condensing unit within 25 mtrs of cold store evaporator. Engineer has reported that the head pressure is 325psi on R404a and 40 psi on suction at the moment ambient is 13c and the liquid line is untouchable (to hot). The coldstore is 10 degrees off design temp,(chiller +1c) condenser clean and fans all ok.

Evaporator clean, fans going right way etc, engineer changed valve and orifice no difference, system has never been broke into so no incondensables i hope,we installed it 5 years ago.

Heres the strange thing if we had a partial blockage on the liquid side the system would pump down and go out on LP switch.

Told the engineer too switch off the evap fans to see if the coil would ice over, it appears some of the distributers are blocked and the coil is only frosting at the top and bottom and within 3 mins its pumped down and gone out on LP. But i can't understand the High discharge temps and pressures.

Heres my thoughts tell me what you think?

Yes some distributors are blocked but still getting a little duty out of system?

A restriction in the high pressure side causing the high head pressures.

I have had this problem on a system years ago but at my age the memory recall is not as good as it should be from sniffing R12:D:D

I think its just the partial restriction on the evaporator what do you think.

Regards
Lrac

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Hi Lrac:)

the last time I seen this it was a fast food joint where the condensing unit was above a false ceiling, basically it is flash gas in the liquid line
One of our engineers rang me, he just could not get the room down in temp, all he had done was changed the compressor, and recharged.
I went down took a look at it, stripped the armafex off the suction cable tied the liquid to the suction, taped the armaflex back over it and ran it up.
Room started to come down. I left the engineer to monitor and it came down in a few hours:D

The flash gas causes the poor distributor performance, the gas chokes the liquid line to causing the high head, any suction pressure you see is mainly from gas pressure rather than liquid expanding.

Hope this helps.

Kind Regards Andy :)

One or all the condenser fans running backwards, due to duff capacitor, faulty fan speed control, or sparky has been doing some work on site and swaped the phases over on three phase.
325psi should give a condensing temp of about 50C.

Jon

The refrigerant may be having some non condensable gases. Was the system ever topped up.

Unlikely but...

Blocked circuit in condenser? Combined with incorrect TEV setting.

404a @ 40 PSI= -14 deg c sounds like valve isn't feeding properly.

I'm changing camps and voting for non condesibles. Had a leak on a tev, system on pumpdown, lp switch set too low, leaked out some r22, sucked in air and water. System was cycling on hp switch. Was still maintaining temp, customer complained of a burnt smell and though evap fan motor was burning.

Jon

What is the condenser delta-T (air out temp minus air in temp)? High delta-T narrows it down to (or eliminates) an airflow problem.

Quote:

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Originally Posted by LRAC

Engineer has reported that the head pressure is 325psi on R404a and 40 psi on suction at the moment ambient is 13c and the liquid line is untouchable (to hot.

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325 psig is roughly 123°F (50.5°C) for R-404a. If the ambient is 13°C and the liquid line temperature is HOT, and the condenser is clean, and the fans are turning the right direction, etc, etc....

This sounds like you have hot gas flowing through the liquid line (as in insufficient charge) for some reason.

If the refrigerant charge was weighed in for the correct amount, then the liquid receiver may have lost the seal on the diptube. There may be just enough liquid getting forced out the receiver to provide some cooling, but not enough.

Quote:

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Originally Posted by LRAC

Yes some distributors are blocked but still getting a little duty out of system?

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The distributor tubes may not be blocked, just not enough liquid is flowing through.

Flash gas stays at a relatively constant temperature.

Noncondensable gases will of course raise the pressure significantly, but they won't drastically increase the liquid lne temperature.

Whatever is causing the hot liquid line is contributing to the problem....

I would replace the expansion valve and make sure the bulb is insulated and fastened tightly. Try to regulate the suction pressure. In South Africa we use R404 for low temperature application only. If you running your compressor out of the designed suction parameters e.g. low temp compressor -20°C too -40 °C suction for a long period of time your condenser is designed to handle a set amount of heat to reject. Once the compressor operates out of its suction parameters for a long period of time e.g. 0 °C the compressor will start to over heat and you will have the same effect as a undersized condenser.

Regards

Lyle
Cape Town

Check the condenser. It may be blocked or is undersized.

Is the sight glass clear??

Thanks for all the replies on this post, i will attend site with the engineer on Monday. I have this niggle in the back of my mind that the valve selection might be wrong or we have incondesables?

Always open to further suggestions.
Kind regards
Lrac

If I had to guess, my money would be on non-condensables. But I would rather know what's wrong than guess what's wrong... and knowing requires more information than you have provided (dT's, TD's, subcooling, superheat, etc).

As ever, Gary's pragmatic approach will be the decider.

I vote "uncondensibles" but there,... I'm not the King!