Liquid Receivers

[nevinjohn]

Most of the super market refrigeration units are equipped with one of these storage equipment.
When the unit is turned off, will the refrigerent (in receiver) still be in the liquid state? Its only when the compressor is running, the pressure is maintained so the liquid stays liquid at that pressure.

If is stays liquid, wont it flood back to the compressor, considering the fact that the receiver is in the discharge/liquid line?

Can anyone explan what happens when the unit is shut down (the state of refrigerent)

[24057]

The liquid receiver is placed before the compressor to prevent liquid passing though and causing potential damage. Not sure what you mean when you say it's placed on the compressor discharge? Only hot gas is discharged. Also the way that receivers are designed means that only vapour leaves out the top, whilst the liquid sits at the bottom. This means that the only way for the refrigerant to leave is as a vapour.
Correct me if i'm misunderstanding the question.

[Rob White]

.

When the system is running the liquid in the receiver is flowing
and subcooled.

When the system is off the liquid meets the saturation temp
and is now a liquid vapour mix at what ever pressure it is.

When the system is off just think of the receiver as a big refrigerant
cylinder and in cylinders you have saturated liquid.

The receiver follows the condenser and it is used to store the liquid
refrigerant before it goes to the expansion valve/s, as the valve open
and close the liquid flows out of the receiver accordingly.

It is possible for the liquid to flow back through the condenser and
into the compressor, but this is unlikely if the system is designed
correctly and if it is a risk you can fit one way valves into the
compressor discharge and that will stop the problem.

When liquid enters a comp through the suction it is referred to as
"liquid slugging" or "liquid floodback".

When liquid enters the comp through the discharge (in the off cycle)
it is sometimes referred to as "liquid migration".

Any liquid is a problem so good design always minimises the risk.

Regards

Rob

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[chemi-cool]

When liquid enters the comp through the discharge (in the off cycle)
it is sometimes referred to as "liquid migration".

If that is what happening, than you need an NRV on the discharge line. It is a common problem in cases when condensers are placed above the compressors.

[chillerman2006]

pressures equalise, so in general any liquid in the high side prior to reaching saturation conditions, will continue to migrate towards the expansion device & low side

only as mentioned by Chemi, with higher level condensors do you get liquid migration back to compressors but this is either a design fault or component failure

ie: someone forgot to specify NRV in discharge line or compressor internal PRV failure can cause liquid to migrate back to compressor during shut down

R's chillerman

[Brian_UK]

The liquid receiver is placed before the compressor to prevent liquid passing though and causing potential damage. Not sure what you mean when you say it's placed on the compressor discharge? Only hot gas is discharged. Also the way that receivers are designed means that only vapour leaves out the top, whilst the liquid sits at the bottom. This means that the only way for the refrigerant to leave is as a vapour.
Correct me if i'm misunderstanding the question.

OK, I'll correct you as the others are too polite. You are confusing a receiver with an accumulator which is, indeed, on the suction side of a compressor to protect it from liquid intake.

A receiver collects the liquid refrigerant from the condenser prior to feeding the expansion device.

[nevinjohn]

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When the system is off just think of the receiver as a big refrigerant
cylinder and in cylinders you have saturated liquid.

Any liquid is a problem so good design always minimises the risk.

Regards

Rob

.

How can the refrigerent stay as liquid when the compressor is not running?

[Iceman717]

Pressures are going to equalize and you will have saturation conditions. Some liquid will evaporate as your high side pressure drops, this will cause the pressure to increase and vapor will condense.

[Rob White]

.

The same as if it was in a gas bottle.

Its called the saturation temperature or the saturation pressure.

Refrigerants contained in a sealed system expand to fill the space,
The liquid will vapourise until the vapour and liquid inside the sealed
system equal a pressure that is equivalent to the temperature the
system is sat in.

If a system with liquid inside is alowed to warm up to ambient temperature
the refrigerant will expand to a pressure that equals that temperature.

Say a system (or a cylinder) is allowed to stand in a room at 25 degC, the
refrigerant will expand to fill the space at 25 degC.

R22 at 25 degC = about 9.4 barg. That is the saturation temperature and as
long as there is at least one drop of liquid present, the refrigerant will always
follow that rule.

When the system is running you have liquid in the liquid side and vapour in the
vapour side, all at different pressures and quality, but the second you turn the
system off all the refrigerant will stabalise to the saturation temperature of the
ambient temperature the system (or cylinder) is sat in.

Regards

Rob

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[otrotabi]

Hi Rob,

Slightly off the topic I must say, but we have a supermarket installation where we suspect air to be present because we have high condensing pressures and high subcooling at the same time which for my opinion is an indication of non-condensables. My question is regarding your last post. I was trying to prove your point that in a gas bottle, if I read gas pressure and read the gas saturation table, I should get ambient temperature as a result, but I have a "small" - about 1 degrees Centigrades today, almost 3 degrees Centigrades 1 week ago - difference between the table output and actual ambient temperature as read from a thermometer.

The actual setup is as follows: I have a digital gauge (Testo 550) reading pressure from a R134 gas bottle. Reading as of the time I am writing this post is 77.6 psi which corresponds to 23.5 degrees centigrades. The instument has also a thermometer and reads 24.6 degrees centigrades, the other day this difference was bigger. I checked temperature with another instrument, it reads 24.9 centigrades.

First question: are these differences only a gauge error or are they due to something else ? Second question: in the supermarket installation, there are three condensers in parallel, all of them with an input and output ball valve so that we can isolate them. My idea of how to determine whether there is air in the system (R22 in this case) is to isolate one of the condensers, turn the fans on for let´s say 15 minutes and measure pressure and temperature afterwards, if there´s a big difference I assume there´s something else besides R-22 in the system, am I right ? Last question, should I try to do this experience in a clouded day or during night time ?

Thanks for your help.

Regards

José

[cadwaladr]

if you vent it to atmosphere you are breaking the law,if you think there is air in the system bleed air/refrigerant into a reclaim cylinder vac the bottle first then find the highest point in the condensor area do it when the system has been turned off for a while.

[Rob White]

Hi Rob,

Slightly off the topic I must say, but we have a supermarket installation where we suspect air to be present because we have high condensing pressures and high subcooling at the same time which for my opinion is an indication of non-condensables. My question is regarding your last post. I was trying to prove your point that in a gas bottle, if I read gas pressure and read the gas saturation table, I should get ambient temperature as a result, but I have a "small" - about 1 degrees Centigrades today, almost 3 degrees Centigrades 1 week ago - difference between the table output and actual ambient temperature as read from a thermometer.

The actual setup is as follows: I have a digital gauge (Testo 550) reading pressure from a R134 gas bottle. Reading as of the time I am writing this post is 77.6 psi which corresponds to 23.5 degrees centigrades. The instument has also a thermometer and reads 24.6 degrees centigrades, the other day this difference was bigger. I checked temperature with another instrument, it reads 24.9 centigrades.

First question: are these differences only a gauge error or are they due to something else ? Second question: in the supermarket installation, there are three condensers in parallel, all of them with an input and output ball valve so that we can isolate them. My idea of how to determine whether there is air in the system (R22 in this case) is to isolate one of the condensers, turn the fans on for let´s say 15 minutes and measure pressure and temperature afterwards, if there´s a big difference I assume there´s something else besides R-22 in the system, am I right ? Last question, should I try to do this experience in a clouded day or during night time ?

Thanks for your help.

Regards

José

Hi Jose, sorry I have not responded earlier.

If you have a discrepancy of less than 1 degree I would not worry
too much it could just be the variance of the system at the time.

With the Testo the temperature probe can be set for surface measurement
or internal measurement. I can't remember which one but one has a built in
deviation and it measures with an offset which changes as the temp changes.

If it wasn't a discrepancy with the equipment and if the probe set-point is the
same the only other discrepancy is air in the gauge line compressing at a different rate?

As for air in the working system you need to isolate the condenser like you suggest but
allow them to equalize to ambient temps naturally then do a pressure - temp comparison.
If the is Air, nitrogen or other non-condensable in the system it will show up on the gauges.

Regards

Rob

.

[The Viking]

Jose,

Look at the condensor as a long pipe, the duty it performs depends on the lenght of the pipe.
If you pump in superheated refrigerant at one end of the pipe (assuming the outside of the pipe is colder than the refrigerant) it will at some point have cooled down enough for all of the refrigerant to have transformed in to it's liquid phase. But as our expansion devices requires a solid liquid feed we do want a bit of a safety margin so we will make sure the pipe is slightly longer, cooling the refrigerant a bit below the temperature where it theoretically transformed in to liquid. This we call sub-cooling.

If you introduce a non condensable gas in to this pipe what you in effect are doing is to shorten the lenght of pipe available to cool the refrigerant. Yes, this would raise the head pressure but what would happen with the subcooling? Shorter distance = less heat transfer = ?

So in your post above where you state that the head pressure as well as the subcooling is "high", it is unlikely that this will be because of air in the system as this would lead to a high pressure but low subcooling.
For both the pressure and the subcooling to be high you are looking at a scenario where there is to much refrigerant in the condensor... Over charge?



Also, please have a look at THIS THREAD

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[icereaper]

hi all
i have an installation where my piping goes up from compressor and receiver set 1.5m over and down 1.5m to condenser, would it be a major problem with liquid not being gravity fed too receiver tank. thanx

[otrotabi]

I would say you probably could have a problem, it would be better with the receiver on the bottom. However most condensing units work this way, the receiver´s inlet being higher than the condenser´s output. If you have no other way to do it, at least make sure you have a inverted trap at the receiver´s inlet so that you make sure that once the liquid enters the receiver, it is not possible for it to get back into the condenser by gravity.

[Peter_1]

He's not breaking the law in Argentia I suppose when venting

[otrotabi]

He's not breaking the law in Argentia I suppose when venting

Hi Peter, I guess this question regards to something which was asked previously and I am afraid is still not solved yet (it´s winter here, so there was no need to do anything yet). Short answer is no, you are not breaking the law here. Long answer is : in our company we have a gas reclaimer and reclaim as much gas as we can, I can not say it´s a 100 % but I am actually not into field work myself, so it is not always on my hands to decide. It is both an economic decission (gas is not getting any cheaper) but also an environmental decission. Believe or not, some of us do care about nature down here, though I guess you should agree it is not us who caused the problem in the first place.

Regarding this supermarket, it is really not up to me to decide but I can certainly give an advise on how to get the job done. How would you solve the problem in Belgium ? Or USA ? How do you get the air out of a supermarket installation ?

Regards

José

[otrotabi]

So in your post above where you state that the head pressure as well as the subcooling is "high", it is unlikely that this will be because of air in the system as this would lead to a high pressure but low subcooling.
For both the pressure and the subcooling to be high you are looking at a scenario where there is to much refrigerant in the condensor... Over charge?

.

I have had overcharge problems in the past with air conditioners, no liquid receivers included. I assume unused liquid refrigerant is stocked at the liquid receiver (250 lts with 3 different liquid level sights). However I can suggest removing (not venting) some refrigerant and see what happens.

Also, please have a look at THIS THREAD

.

I went my whole university career listening to my professors speak indistinctly about degrees centigrades or degrees Celsius (actually in Spanish Celsius is hardly ever heard when speaking about temperature -Fahrenheit is never used here), however most of them were engineers and I will not argue about etymology, you are probably right. I just wanted to make clear it is not degrees Fahrenheit I am speaking about. Thanks for the tip anyway.