What is the refrigerant state in a receiver?

[MintJulep]

thread403-279014 got side tracked on an unrelated question.

What is the state of refrigerant in a receiver tank.

I am of the opinion that you have liquid and vapor together, and therefore you are saturated.

Others disagree.

When a system is operating, subcooled liquid enters and exits the receiver quicker than the pressure and temperature can achieve an equilibrium.

Given that every receiver I've ever seen has the entrance above the liquid level, and in many cases have an inverted dip tube I'm having trouble seeing how there is anything except a saturated condition.

 

[Compositepro]

There would generally be some degree of sub-cooling in the the receiver but for calculation purposes it would be considered saturated. The gas from the compressor is superheated at condenser pressure. The gas condenses in the condenser tubes and usually only liquid exits the condenser. The tubes are flooded at the condenser exit. This allows sub-cooling of the liquid in the condenser.

To have sub-cooling would mean that the capacity of of the system is limited by condenser capacity. This can certainly happen.

 

[imok2]

Once the subcooled liquid exits the condenser, the receiver receives and stores the liquid. The liquid level in the receiver will vary if the metering device is throttling open or closed. Usually receivers are on systems when thermostatic expansion valves are used as metering devices. The subcooled liquid in the receiver may lose or gain subcooling depending on the temperature of the area surrounding the receiver. If the subcooled liquid is warmer than the receiver’s surroundings, then the liquid will reject heat to the surroundings and subcool even more.

 

[Compositepro]

I meant to say that having NO sub-cooling means that the condenser is limiting capacity.

 

[SAK9]

As fas as my understanding goes a receiver is a simple storage device for storing refrigerant in the liquid state and located down sream of the condenser.They are required where there are large pipe runs and therefore the refrigerant charge is large.The condenser may not be large enough in such instances so reciever is required.

As for the condition of regrigerant in the condenser( I mean a water cooled Shell and tube here),it enters the vessel on top as a superheated gas and leaves the condenser at the bottom as a liquid(100%).The degree of sub cooling(it can be 8~10C)depends on the design of the condenser.About 15% of the water tubes are for sub cooling and are located at the bottom of the condenser receiving the coldest condenser water.2/3rd down the tube bundle refrigerant reaches liquid state and during remainder of the journey down it is subcooled.

I have just regurgitated what I learnt during my first job with an equipment manufacturer

 

[gscrivener]

Here's how I understand it...

There is a saturated condition in every receiver. The vapour and the vapour/liquid interface is saturated.

The liquid refrigerant enters the receiver subcooled so it is colder than the vapour it contacts at the receiver inlet. This temperature difference causes some vapour to condenser and the liquid temperature to increase. In a storage tank or a very large receiver with a comparatively low flow, this process would have enough time to affect all of the liquid. In a properly sized refrigerant receiver, the liquid is flowing enough that this process does not increase all of the liquid to the saturation temperature. Therefore, the amount of subcooling lost in a receiver is a function of liquid "dwell" time in the receiver

In my experience there is indeed a loss of subcooling as refrigerant liquid transits a conventional receiver, but there is still subcooled liquid leaving the receiver. This makes sense from another perspective. If we had saturated liquid leaving the receiver all of the time, refrigeration systems would require additional subcoolers in order to prevent evaporation of the liquid in the liquid line due to the decrease in pressure caused by the piping friction losses or vertical risers.

I personally have measured 35F liquid coming out of a receiver with a pressure corresponding to an 80F saturation temperature.

 

[HVAC68]

It's generally called a liquid receiver ! Does it answer the question ?

HVAC68

 

[MintJulep]

The concept of dwell time in the receiver is interesting. I'll admit that I never really thought much about time coming into things - I figured the transition from sub-cooled to saturated would be nearly instantaneous.

However, the other day I was blowing the crud out of my keyboard with one of those cans of "air".

As you blow the thing gets cold - the liquid is sub-cooling. When you stop blowing you can hear the liquid boiling for a reasonably long time, reestablishing a saturated condition.

So it seems that time is in fact a factor. But how much so?

I've seen receivers with inverted dip tubes on the liquid entrance. I've got to believe that the resultant "fountain" must be pretty effective at allowing the sub-cooling to disappear as the refrigerant splashes down.

Where the refrigerant stream entering the receiver is more laminar maybe less sub-cooling is lost.

 

[Compositepro]

You do want the liquid in the receiver to have some subcooling. Otherwise the liquid being drawn from the receiver will will flash as soon as there is any pressure drop due to line friction or elevation gain. This would be very undesirable. Subcooling does occur in the condenser.

 

[berkshire]

Composite pro
If the liquid were to flash off, as it is being drawn from the receiver, would that not indicate a lack of superheat on the system?
B.E.

 

[gscrivener]

I'm not familiar with receivers that have an inverted dip tubes for the refrigerant inlet. Most of the ones I've worked with have had the liquid enter at the top and leave through the bottom (the actual connection is on the bottom or there is a dip tube) but I must admit, my experience is mainly on the "system" not the actual construction of the individual components. I would think that a receiver with an inverted dip tube would have gas binding problems if it was installed in a warm ambient unless it was provided with a seperate vent line.

I'm not sure how much time is a factor; if you find out, let me know. I do know that I have seen some relatively large receivers with small loads (low flow) still maintain some subcooling. All arbitrary observations though...

 

[gxmplx]

MintJulep: People disagree on this matter only because they make different assumptions and the problem is they are not even aware they made them!

Thermodynamics teach that if there is thermodynamic equilibrium you MUST have a saturation conditions.

But a liquid receiver is seldom at a state of thermodynamic equilibrium unless you are either getting a vapor-liquid mixture from the condenser (i.e. it is not working properly) or there is no refrigerant flow and it reaches equilibrium (i.e. the system stopped).

Most of the time you get subcooled liquid from the condenser and it exits the receiver having lost SOME of its subcooling but (generally) not all of it, because of pressure drop and because a mixture vapor-liquid will tend to equilibrium.

Only think that the subcooled liquid can (and will) force condensation of the vapor in the receiver heating the liquid cooling the vapor to drag the system to a thermodynamic equilibrium and if the vapor is colder then some liquid will evaporate to drag it in the opposite direction but before it reaches equilibrium new refrigerant at the same initial temperature comes in and the liquid you had leaves before reaching saturation temperature.
You almost always have saturated conditions when the system starts if and only if it was off long enough to have reached thermodynamic equilibrium. This will depend on the heat transfer coefficients of the receiver.

So what you get at the receiver exit really depends on pressure drop, heat transfer coefficients but also on the time constants of the heating and cooling process of the liquid inside the receiver and the flow´s average velocity.

The only way to be sure is to measure subcooling but if you have a system working properly I would bet you do get subcooled liquid leaving the receiver, most of the time.

gxmplx