View Full Version : New student trying to understand subcooling in reciever

13-10-2007, 02:13 AM
I have completed an intro to ac / refrig and am now in a second class for a refrigeration operators license, i am having a problem with subcooling and how it works as far as once the refrigerant enters the reciever, please tell me if i am correct. subcooling takes effect in the last stages of the condensor, the refrigerant is then discharged into the reciever, because the reciever is at the same pressure as the condensor the refrigerant does not flash or have a increase or decrease in temp. so you can have a subcooled refrigerant in the reciever. that is the liquid refrigerant at the bottom of the reciever will be subcooled and the refrigerant at the top in contact with the vapor should corispond to PT chart. The liquid line draws it's suction from the bottom of the reciever where the refrigerant is at a subcooled temp and is subcooled even more as it travels through the liquid line toward the TXV. Please tell me if i am correct and where i can find something like that in writing. Thanks.

13-10-2007, 05:13 AM
Hi Donjahner. Your question is one of the stickiest wickets in our trade.

Saturated liquid and saturated vapor exist at the same conditions within a static environment. Let's start with something that isn't flowing and changing pressures and such as a dynamic refrigeration system.

A half full tank of R507, for example. If you put a temperature probe to the liquid it should reflect the pressure your pressure gauge indicates. But if you put your temperature gauge toward the top of the tank where it is vapor, will you see a difference? Test this out in the sunlight where the top of the tank should be hotter than the bottom is. Let me know what you observe. Let's do 3 measurements:

Very bottom
Assumed liquid level

After you post your findings (any refrigerant will do) we can puzzle this out further.

I look forward to the exercise.

13-10-2007, 05:32 AM
all i have is a 100lb of r22 bottom of tank temp is 65.3 f top of tank is 67.7f 123psi. does that mean their is 1.5 degrees subcooled refrigerant in bottom of cylinder. also could not locate to sunny location. in a powerhouse. Thanks

Abby Normal
13-10-2007, 04:48 PM
I view a receiver or a jug of ***** for that matter as being like a big heat pipe.

Where the liquid and vapour contact one another it is saturated.

Vapour will condense on the 'top of the tank' and drip back down to the liquid.

Vapour that evaporates, draws its heat from the liquid it leaves behind.

The liquid at the bottom is under slightly more pressure than the liquid at the surface ( that contacts the vapour)

The liquid cooled as vapour evaporates would also be more dense and tend to fall to the bottom.

Abby Normal
13-10-2007, 04:51 PM
Liquid entering a receiver is contained by gravity, its pressure can push down and side ways. It cannot push up so to speak and it cannot fill the volume of a receiver, so some vapour has to be there. That vapour needs energy to exist, it draws the energy from the liquid, and it therefore cools the liquid.

Just some thoughts on the matter, it is always a hotly debated subject

13-10-2007, 06:35 PM
Abby, your comments are inspired. Even a "static" tank of refrigerant is a working heat exchanger. And it is key to note that the only truly "saturated" vapor and liquid are at the point of contact... call it the "film" of molecules being exchanged at the surface of the liquid.

So we can expect to see slight differences in temperature even though we perceive a single pressure. Truth is, that gravity head is an additional pressure but is minor(think of the ocean), as is vapor head (Lower pressures as you rise from the surface (think of the earth's atmosphere). Let's look at donjahner1's measurements.

all i have is a 100lb of r22 bottom of tank temp is 65.3 f top of tank is 67.7f 123psi. does that mean their is 1.5 degrees subcooled refrigerant in bottom of cylinder. also could not locate to sunny location. in a powerhouse. Thanks

67.7-65.3=2.4 deg F.
123 psig = 70.8 deg F... as much as 5.5 deg F and as little as 3.1 deg F. On a P/T chart this indicates a potential disparity of 10 psig.

Where are the disparities?

First thought is that the pressure gauge and thermometer are not precisely tuned. For practical purposes not in bad shape, but for our discussion a margin of error that could take us either way.

Second thought is that the exterior of the tank is an insulator and will not precisely reflect the interior temperature. I forgot to ask for one important piece of information. Ambient temperature! My pardon. But, if the tank is an insulator, then I have to think that the ambient temperature is below the saturated temperature of 67.7. Any chance this is true?

Last thought for today is that the floor is cooler than the air temperature... So we add up possible reasons for the so-called disparities:

. Margin of Error between Temperature and pressure instruments

. Effect of liquid head (1 psig per 2 feet, round numbers) on subcooling the bottom of the tank

. Surface effect of floor temperature and of air temperature toward top of tank (Interesting if you took measurements of those)

. Insulating effect of the steel tank because it is actually acting as a heat exchanger as temperatures in the motor room change

Abby's observation of the tank being a heat pipe is a good one. It reminds me of the partial bottles of water we often see with droplets of water on the interior bottle surface of the vapor area. What this is showing us is that the water bottle is now in a condensing mode... that the ambient temperature has fallen below the water temperature (Dew point). A bottle of water is not much different than a drum of refrigerant.

The challenge we face in a truly dynamic system is that we have to feel confident we can understand a so-called stasis example before we get into the various subcooling schemes that abound in the refrigeration trade.

You could get crazy and try another set of readings including the floor and air temperature, insulate your thermocouples from the air and compare your gauge to others for accuracy...then, also, let's look at the vapor outlet pressure and the liquid outlet pressure of the tank. We should be able to discern a pound or so.

Or maybe that's is just too much trouble.

The term "Subcooling" is a relative word and probably the most misused word in our trade. People often use it when the only word that should be used is "cooling."

You cannot subcool a refrigerant tank, but you can cool it. You cannot have any sustained subcooling in a receiver that has vapor in it, but you can have cool liquid in it... and you can artificially apply pressure to it to cause subcooling... and ironically warm the liquid in the process.

Liquid does not necessarily become subcooled travelling to the expansion valve. It does only when you refrigerate it mechanically, or you are able to feed the evaporators from the roof or a mezzanine to take advantage of gravity head. Liquid to suction heat exchangers can also add some subcooling to the liquid at the evaporator. Pressure drop through the liquid line will tend to take away subcooling.

As far as refrigeration principles go, you should only strive for sufficient "sub"cooling to ensure a solid column of liquid at the expansion device. Any more "sub"cooling and you are wasting energy because you are operating your compressors at higher discharge pressures than you require. But any "cooling" you can provide the liquid will increase efficiency because there is less work required for the refrigerant to attain the evaporation temperature as it passes through the expansion device, and you can also take advantage lower discharge pressures.

Abby Normal
14-10-2007, 08:45 PM
Thanks Dan

I always teach when it is saturated, both liquid and vapour are present, and changes in state are occuring.

You can wrap a wet newspaper around a 20 pounder of propane, and it functions as a liquid level indicator.

The newspaper dries quickly along the vapour area of the tank and stays wet longer around the outside of the liquid area on the tank.

Colder surface dries slower.

15-10-2007, 02:37 AM
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 receiverís surroundings, then the liquid will reject heat to the surroundings and subcool even more.

However, if the subcooled liquid is cooler than the receiverís surroundings, heat will be gained by the liquid and subcooling will be lost. Often a receiver bypass will be used so the subcooled liquid will not sit in the receiver and lose its subcooling.
This bypass simply bypasses liquid around the receiver and directly to the liquid line and filter-drier. A thermostat with a sensing bulb on the condenser outlet senses liquid temperature coming to the receiver to control the bypass solenoid valve if the liquid is subcooled to a predetermined temperature, it will be bypassed around the receiver to the filter-drier.

15-10-2007, 03:40 AM
I must say, the main advantage to mantain subcooling is to minimize flashing in your expansion device. As Dan says, there is no point in subcooling more than you need to.

15-10-2007, 09:57 AM
Hi donjahneri
If you can get hold of a copy of "Principles of Refrigeration" second edition by Roy J Dossat it would be very helpful. This is a pretty good book on refrigeration, the relevant chapter is No 4 "Saturated and Superheated Vapours".