Intermittent flash gas in liquid line help.

[Contactor]

Hello

Small cold room set point 5c holding at 6.5c

R22

Condenser on:26c Off:34c subcooling 6c (measured once only)

Evaporator on:7c Off: 5c superheat 12c (measured once only in wrong place)

LP 3.5 bar / -3c
HP 15 bar / 42c


Can anyone explain why the sight glass on this job is full for about five minutes, then flashes off by about 50% then slowly fills up again after 5 mins or so?

I don't want to replace TEV until i'm sure.

Any ideas anyone?

Thanks All

[monkey spanners]

Sounds like the valve is hunting a bit, does the suction pressure go up and down a bit too?

I have a guide book by danfoss on setting up valves somewhere, but can't find it to check but i suspect it could be the orifice is too big.
Wait for more knowledgeable members opinions before a trip the wholesalers though

What was the reason for you attending the cold room?

Jon

[Contactor]

Called out as room too warm, they want 4 degrees (dairy fridge).

Is it normal for the sight glass to empty when a valve is hunting or could this also be slightly short of gas?

Thanks

[Brian_UK]

Could be that someone has adjusted the TEV out of range.

Sort fault finding from Sporlan here...
http://www.sporlanonline.com/Form10-143.pdf (260kb)

[Gary]

I would add refrigerant until the subcooling is about 8-8.5K, then take new readings.

[Peter_1]

it's a phenomena where your liquid is leaving your condenser at near 0 SC. On a log p/h graph, your left point on the condensing line is just on the saturated line.

When it starts flashing cover once fast your condenser with a piece of paper, you will see that the bubbles will disappear. Leave it there for 1 minute and then remove it, the bubbles will com back within 5 seconds.
Is the fan perhaps HP controlled?

[Peter_1]

I would add refrigerant until the subcooling is about 8-8.5K, then take new readings.

How will he do this when there's a receiver? The additional added refrigerant will store in his receiver because the refrigerant it's pushed out at the bottom of the condenser by the HP stream upwards the condenser.

[Peter_1]

Is it an Unithé Hermétique group?

[Contactor]

It's a Unitie Hermetique with no fan speed control

[Peter_1]

Typically, A UH has no SC battery in its condenser coil.
Is your TEV sized not too big

[desA]

It would be interesting to measure evap superheat & sight-glass response simultaneously. If they move together, then perhaps TXV hunting could be investigated.

Could the evap fans be hunting slightly, or even the condenser fan? In other words, something else is causing the system disturbance.

[Magoo]

Possibly an undersized liquid line creating the hunting effect.

[desA]

Possibly an undersized liquid line creating the hunting effect.

An excellent suggestion.

[icecube51]

did they move the external unit for say a half meter and make a bend in the liquid pipe that narrows the pipe diam. someone pushed the pipes aside because the needed the space, ore it looks nicer that way, and reformed the pipes. i can go one for ever like this you now. in 35jr you see some strange things happening that they don't tell you straight away.

Ice

[Gary]

I would add refrigerant until the subcooling is about 8-8.5K, then take new readings.

How will he do this when there's a receiver? The additional added refrigerant will store in his receiver because the refrigerant it's pushed out at the bottom of the condenser by the HP stream upwards the condenser.

Receiver outlet subcooling.

[Contactor]

I'm new to this unit but the customer says it's been working fine until now, there have been no recent changes.

[Magoo]

Put a sightglass into liquid line at the TX valve, that will tell you what is entering the TX and creating the hunting effect. Plus re-check the evap superheat conditions. TX valves do go awol