Purpose of a receiver in a refrigereation system ?

Eugene Silberstein 3 · March 2006

Keepo those awesome questions coming.... It'll be spring before you know it!

Fat Eddy_2 · February 2006

Liquid line receivers

Is it possible to have sub-cooling if there is refrigerant vapor present with the liquid inside a liquid line
receiver ?

Would it be at saturation if both liquid and vapor are present ? And would I be able to use a TP chart to calculate the pressure in the vessel if I knew the temperature ?

Eugene Silberstein 3 · February 2006

Awesome Question

Yes, you can definitely have subcooled refrigerant in your receiver. Consider this:

You have an R-22 system that is in the off position. It's 80-degrees outside and the (static) pressure in the system is about 144 psig. The sytem starts up and the high side pressure begins to increase. We now have vapor refrigerant in the receiver at a pressure of 226 psig (roughly the saturation pressure that corresponds to a saturation temperature of 110 degrees, 80 degrees + 30 degrees).

Let's assume that your condenser is operating with 20 degrees of subcooling, so the refrigerant is leaving the coil at a temperature of 90 degrees and flowing into the receiver. The pressure in the receiver is 226 psig, which corresponds to a saturation temperature of 110 degrees. The subcooled liquid is at a temperature of 90 degrees which is well below the temperature (110 degrees) that corresponds to the pressure in the vessel. In order for the subcooled liquid to vaporize, the pressure in the vessel must be reduced to about 168 psig.

Think of this as kind of a pressure cooker. We keep the top of the pressure cooker secured to build up vapor pressure over the water to make it boil at a hotter temperature. In our case, the 226 psig vapor refrigerant keep the subcooled liquid, well..... subcooled.

The answer to your second question is, therefore, no. You cannot use a PT chart to determine the pressure in the receiver if you know the temperature. If we could, however, that would be very cool as we would no longer need our gauge manifold.

Awesome question... thanks for playing...

Fat Eddy_2 · February 2006

Thank you , that was a very clear and concise explanation.

Fat Eddy_2 · February 2006

LL Receiver Continued

Since sub-cooled liquid and refrigerant vapor can simultaneously be found in a LL receiver.

Then can we assume that this holds true for all vessels or storage devices in a refrigeration system, e.g. condensers, evaporators ?

.. And if this is true, Then would it be true that there are very few points in an operating refrigeration system where there is actually a condition that corresponds to a PT chart ?

don_156 · February 2006

Thats

true fats...the difference is subcooling and superheat.

Where one is above the pt chart and the other is below.

However where both liquid and vapor normally exist then the temp is equal to pt relationship.

Does that make sense?

Fat Eddy_3 · February 2006

What I'm trying to figure out is exactly how many points in the system and over what size of an area does saturation exist, I know that there are a couple of points in the system where it does exist, but I think the exact area could be as small as a pinhead, follow me ?

Going by the Prof's first reply, which I thought was very good and kind of explained what I've always pictured to be the case,

I think if he is right then the rest of this must be true, we now know that sub-cooled refer and superheated vapor can and will exist in of a vessel simultaneously,, this defies what most of us were taught in voc school..

"That where there is vapor and liquid there is saturation and the PT chart applies",,,,

I say it's not true unless it is in a container and at rest.

What do you think ?

Eugene Silberstein 3 · February 2006

At saturation

When a refrigerant is saturated, the temperature and pressure correspond to each other as the pressure-temperature chart dictates.

Let's take a look at the condenser and the three processes that take place therein. When hot gas is discharged from the compressor, the vapor is superheated well above the saturation temperature on the high side of the system. For example, let's say that the high side pressure is 226 for an R-22 air conditioning system. The temperature of the discharge gas may be as high as 220 degrees. Now, in order for the refrigerant to condense, the superheated gas must give up sensible heat until it is cooled down to 110 degrees, which is the temperature that corresponds to our 226 psig saturation pressure. Up until that point the refrigerant is not saturated and does not follow the pressure temperatrure relationship.

Now, once the refrigerant has cooled down to 110 degrees, the change of state from a vapor to a liquid begins. It is here that the pressure temperature relationship holds true. As the refrigerant continues to flow through the condenser coil, more and more of the vapor refrigerant condenses to a liquid. SINCE THE PRESSURE AND TEMPERATURE OF THE REFRIGERANT CORRESPOND AS PER THE P/T CHART AT THIS POINT, ANY CHANGES IN TEMPERATURE WILL RESULT IN A CHANGE OF STATE WITH NO CHANGE IN TEMPERATURE.

The difference between this "vessel" and the receiver is that the vapor pressure above the liquid is 226 psig and the temperature of the subcooled liquid is 90 degrees, which is about 20 degrees cooler than the corresponding saturation temperature. Since the pressure above the subcooled liquid is higher than the saturation pressure that would correspond to saturated refrigerant at 90 degrees (168 psig), the liquid refrigerant remains subcooled as it cannot vaporize because of the higher vapor pressure above the liquid.

In the evaporator, we have the same story. Saturated refrigerant flows through the evaporator and, since the vapor pressure above the liquid refrigerant is the same as the corresponding pressure to the boiling temperature of the refrigerant (PT realtionship), any heat added to the refrigerant will result in a change of state.

So, to sum up, the PT relationship holds true whenever the refrigerant is saturated. This is the case in the condenser after all of the vapor has condensed and in the evaporator before all of the liquid has vaporized. Once all of the liquid has vaporized in the evaporator, the PT relationship no longer holds true. The same is true for the refrigerant at the outlet of the condenser and the refirgerant in the liquid line, receiver, etc.

Hope this helps, Eddy.

Eugene Silberstein 3 · March 2006

Too Good to Let Fall Off

This question is just too good to let fall off the deep end... so I'll bump it up!

BUMP!

Fat Eddy_3 · March 2006

Thanks for the replies prof, very informative and easy to understand, Thank you.

Purpose of a receiver in a refrigereation system ?

Comments

Categories