Screw comp High Discharge temperature

[kfjoe]

Hi all

I have an issue with a screw comp going out on high discharge temp safety.

It has liquid injection cooling and an economiser circuit providing sub cooling to the liquid line.
The discharge pressure is not high at about 230psi and the suction superheat is 5 degrees or lower.
But discharge temp is going over 100 degrees c

The economiser / sub cool circuit doesn't seem to be doing much and the return to the economiser port is relatively warm at about 25 deg c

There is some flashing in the liquid line sight glass, which is before the T off for the economiser and liquid injection circuit, but the system has been charged to the correct weight of refrigerant and there are no restrictions / temp drop. But doesn't seem to clear unless the system is run to a low head pressure of like 170 psi or lower where the sight glass will clear and the sub cool circuit seems to work.
But I can't seem to clear the sight glass totally at a normal head pressure

Any suggestion / help much appreciated

Thxs

[Brian_UK]

Pressures don't help unless we know the refrigerant used.

[kfjoe]

Sorry it's R404a refrigerant

[moideen]

Restricted oil flow to the compressor will cause high dischargetemperature. Check for main injection valve, oil orifice or filter blockage. Has your compressor oil cooling system?

[kfjoe]

Yes it has liquid injection

[mbc]

as you know the oil and ECO both are important in outlet temperature.
So flow rate of oil and oil temp. and how much we charge liquid to ECO. is effecting to our temp.
check both of them.

[kfjoe]

Oil temp seems to pretty much be the same as discharge temp
Should it be lower ? I can understand it being lower with an external oil cooler , but with liquid injection into the comp ?

[RANGER1]

Oil temp seems to pretty much be the same as discharge temp
Should it be lower ? I can understand it being lower with an external oil cooler , but with liquid injection into the comp ?

Kfjoe,
Yes you are right, oil & discharge temps are the same on liquid injection.
You mention in first post charge is correct, but liquid line shows it's short of refrigerant.
So is there a charge size on this unit that you are following?
Seems like its short of refrigerant.
What type of condenser do you have, has it got a liquid receiver?
Can you give us liquid line temp & discharge pressure leaving liquid receiver to work out it's subcooling.
What else is in system, evaporators, their superheat, compressor suction temp & pressure etc, it may be very relevantrelevant

[kfjoe]

Yes charged to the correct weight charge listed
Air cooled condenser and liquid receiver , air cooler / evaporator
Suction superheat is actually very low , suction pressure about 1 bar at -25 air temp
Hp relatively low at 230 psi in 26 deg ambient

[kfjoe]

I am returning to take some temp / pressure logs
But trying to get an idea of what could be causing the high discharge temp

[Magoo]

Vapour temps from economiser adds to oil cooling, high vapour temps equals reduced oil cooling. Check why economiser is not performing.

[kfjoe]

Thxs for the advice
I am checking the economiser tomorrow
I am hoping it is not because it doesnt have a solid column of liquid feed ,
as it has the correct charge , so if my subcooling is low i am a bit confused .

[RANGER1]

Kfjoe,
You mention you also added a complete new charge, what happened for you to do that?
Was it, or has it been leak tested & 100% sure you fixed all leaks.

[kfjoe]

The system had a leak , repaired , pressure tested , evacuated and charged to weight , but still seemed to be showing some flashing in sight glass

[kfjoe]

Apparently the economiser heat exchanger has been replaced with a bigger one , but not sure if that would make much of a difference

[moideen]

what is your unit model.?what is the eco.saturates temp and eco. equipped with txv or exv?

[kfjoe]

Well my a sub cooling is 2 and the level gauge on receiver is reading very low , sight glass flashing , but correct weight charge went in .
Condensing pressure is 240 psi
Suction pressure 12 psi
Air return temp -26
Suction superheat 8
Economiser suction pressure 50 psi superheat 7

[RANGER1]

Well my a sub cooling is 2 and the level gauge on receiver is reading very low , sight glass flashing , but correct weight charge went in .
Condensing pressure is 240 psi
Suction pressure 12 psi
Air return temp -26
Suction superheat 8
Economiser suction pressure 50 psi superheat 7

So kfjoe, what would you do if this charge weight information did not exist?

[kfjoe]

Do what I want to do and charge to sub cool / sight glass
Getting that through to the customer is another matter

[RANGER1]

Do what I want to do and charge to sub cool / sight glass
Getting that through to the customer is another matter

kfjoe, you mention in your first post that if you lower head pressure to 170psi sight glass becomes clear.
Can you try it for a period & check system over again.
Your air cooler superheat to may be all over the place if it is short of refrigerant (hunting or not controlling).

The only symptom of low head pressure might be TX valve not big enough for smaller pressure differential across it.
Also is air cooler clear of ice,frost build up & all fans working etc?

[Magoo]

obviously the bigger sub cooler HX will consume more liquid and lower the receiver level. Now the liquid line is flashing short and starving the economizer secondary circuit.,
Bingo!! system low on charge, so rewrite the system charge label when you have finally stabilised system operation.
Tell the client what has to happen, do not pussy foot around. It is the clients capital plant investment that will fail ultimately.

[kfjoe]

yes somewhere between 150 - 170 psi the sight cleared,
but the economiser was bringing the liquid temp down to below 0 deg c , The valve seemed to close , loose superheat ( now oversized and DP lower ? )
this is first thing in morning when ambient is low , and previous engineer had wound down the condensor fan controls allowing it to run too low in my opinion.

the system runs relatively ok until ambient increases and then will start to trip on discharge temp , although the head pressure doesn't increase massively.

My thoughts are liquid injection akv not getting a solid liquid column and not very effective
does this sound feasible ?

[RANGER1]

Yes, it's short of refrigerant, how much who knows.

[kfjoe]

I can't think of anything else - low receiver level gauge (if working correctly), flashing sight glass, low subcooling, no sign of restrictions ,but low superheat showing evap filled, it can't all be in the evap ? Closing the receiver outlet and letting it pump down doesn't seem to move the receiver level gauge a great deal, so surely low on refrigerant.
Been told a previous engineer in past put in 90 kg possibly to try and cure a similar issue though , which is a little worrying

[RusBuka]

1 . Tell compressor model.
2 . You have only an injection or still there is an external cooler for oil?

[NoNickName]

Low on refrigerant, but also seems low on subcooling. Flashing may be caused by pressure drop on the liquid line equating or higher to the actual subcooling.

[kfjoe]

But how can I increase subcooling ?

[shaaf]

Dear kfjoe

There could be installation fault. Liquid injection line should be from the bottom of the liquid receiver. and one thing more there should be level switch on the liquid receiver. when liquid level below the desired level then stop the liquid supply to the evaporator. This safety for continuous supply to compressor for liquid injection otherwise compressor will trip on high discharge temperature.

may it help you out

[sterl]

This s production freezer or? Are there ordinary expansion valves on the evaporators or electronics or ?
Is the liquid injection valve a conventional TXV or something else?

With the superheat that low and the machine unloaded, the refrigerant could be fractionating and the evaporator could be retaining some of the less volatile component. That would make the evaporator lazy, but also "overcharged" locally.

[nike123]

If you have working evaporation temperature below -20°C than allowable pressure drop at permanently mounted liquid line filter drier is below 0,035 bar.

At liquid line temperature of 30°C that is about 0.1°C temperature difference between inlet and outlet.
How did you checked your filter drier for restriction?

Since you probably have replaceable core filter drier, I would change its cores anyhow!