does the liquid line has subcooling if the system has a receiveer?

[comfortable air]

a rooftop has a two seperate system. one don't have receiver , another one installed a receiver. i measured both system subcooling. found the one which has receiver has no subcooling. another one has 20 degree subcooling. my friend told me if the system has a receiver, it has no subcooling unless the receiver is full. in that case, we can not judge the charge by the subcooling. becuse receiver is not supposed to be full. Is it ture?

[ej45]

Yes, that is true. Its physics, a reciever that is not full has to be at saturation and will not have subcooling. (both gas and liquid are present)

[Ph-D]

Yes you are correct, if liquid and vapor are present the published thermodynamic tables that are produced will be correct. This is only true where you have an interphase of liquid and vapor. The liquid below the interphase could have some sub-coling this depends on many factore. What if the ambient temperature aroundvthe reciever? What is the dwell time of the refrigerant in the receiver? Is the receiver insulated? Is the reciever greatly oversized and what is the refrigerant flow rate through the receiver? Ther are many variables that must be truly considered. It is a generally accepted fact that you should not count on the any sub-cooling that you may obatined from the receiver. Any such gain is a bennifit.

[comfortable air]

Thank you everybody. I did the trouble shooting rely on the subcooling for couples of year untill now I know that the unit which has the receiver can not charge by the subcooling. if the receiver is under the condensor, is it possible the receiver full? how to deceide how much refrigeration charging in this situation?

[skpkey9]

With a system that utilizes a reciever you charge to a full sightglass to ensure that you have a full column of liquid to the TEV.

You will only use subcooling measurements with these systems to diagnose a TEV or to make sure you have no flash gas at TEV...

Hope this was helpful

[jayguy]

a typical reciever should be piped "inside the condensor". as the refrigerant travels through the condensor, it leaves the condensor and enters the receiver. it leaves the receiver and re-enters the condensor to get sub-cooled. the receiver liquid level changes with charge, load, outdoor air temp, etc.

another version is where the receiver is in the liquid line but the unit has a condensor head pressure control valve (valves), but that version is much more complicated. if you have that version...let us know and we can give you more info on how it works.

if you have a receiver in the liquid line, without any head pressure control valves, then all you have is an 18" diameter liquid line.

[heatingman]

The top of a reciever is at saturation presure and temp. The bottom will be subcooled when the ciruit is active. b/c there is flow, the refrigerant was sub-cooled, and there is not enough time to gain back the heat lost to the condensor, there will and should be subcooling from the reciever during operation. At rest after time of heat gain in the reciever, then there might be no subcooling. If you had a unit running w/o subcooling, I suspect you'll be back out there soon to add charge.

[freonguy]

[QUOTE=comfortable air;1504716]Thank you everybody. I did the trouble shooting rely on the subcooling for couples of year untill now I know that the unit which has the receiver can not charge by the subcooling. if the receiver is under the condensor, is it possible the receiver full?

To determine what the level is in the receiver either running or static, try this one;

Move the outlet hot air from a hot air gun ( not a hair drier ) up and down on the end of the receiver for about a minute, stop the heat and starting at the bottom, feel as to where the steel warms up considerably - that is your liquid level. If you are unsure, practice on a refrigerant bottle that you know has a level of refrigerant in it - works like a dream.

The SAFETY MESSAGE here though is do not ever concentrate the heat in one spot - play the heat up and down in an even fashion - In chilly Saskatchewan, it was essential to know the liquid level in a receiver on condenser flooding style systems for charging purposes; I was taught using a small open flame from a torch, however it is risky at best and found the hot air gun will work, just a bit slower and you need 115 volts.

[skpkey9]

Heatingman & Comfortable Air ~ Here's some reading for both of you and anybody else to get you on the right page. Not tring to get anybody upset , but to get you guys up to speed on recievers and subcooling. That way you are armed with the correct information in a bad situation....

Hope this Helps
Follow this link:
http://hvac-talk.com/vbb/showthread.php?t=133135

[controldude]

I'm With Jayguy on this one we just installed a receiver w/ori &ord valves from condensor to receiver back to subcooler on condensor coil to liquid line then yes you can do sub cooling and get better performance from your system.

[bertoh]

i am with heatingman on this one. even after reading the other link skpkey9 refered to. if you have 10 degrees of subcooling leaving a condenser draining down into a receiver and the receiver is only 10% full, you still have 10 degrees SC. the only thing that will change that is pressure or temp.their are many ways of piping receivers, but what i am desribing is a basic condenser with or without head press control. but if it is a set up where hot gas is piped to the top of the receiver, it may well have little or no subcooling leaving the receiver because of the amount of hot gas that is condensed in the receiver has nullified the amount of SC produced by the condenser.
the receivers purpose is not a performance issue ie. gain SCooling, it is for storing the entire charge of the system for service, for storing pumpdown volumes on long line size systems , storage of excess liquid during low ambient operation etc.

[heatingman]

I'm not arguing here that outlet of reciever subcooling is a charging benchmark, all I'm saying is that it exists. I understand there may be exceptions when you pumping in hot gas before the inlet of the chamber. I'm speaking of a storage style only. piped directly between the condenser outlet and the txv. No bells. No whisles. No head pressure controls, or anything.
To change the subcooled liquid going to the reciever to saturation point either heat must be added, or pressure must be removed, and if either happens, what is it. Think about a suction accumulator, will that cause 0 superheat because both liquid and gas are pressent within the chamber?

[heatingman]

I'm With Jayguy on this one we just installed a receiver w/ori &ord valves from condensor to receiver back to subcooler on condensor coil to liquid line then yes you can do sub cooling and get better performance from your system.

The hot gas is causing the lack of subcooling Because its the heat energy absorbing the subcooling. Check when its hot enough to close the hot gas line completely.

[ej45]

I'm not arguing here that outlet of reciever subcooling is a charging benchmark, all I'm saying is that it exists. I understand there may be exceptions when you pumping in hot gas before the inlet of the chamber. I'm speaking of a storage style only. piped directly between the condenser outlet and the txv. No bells. No whisles. No head pressure controls, or anything.
To change the subcooled liquid going to the reciever to saturation point either heat must be added, or pressure must be removed, and if either happens, what is it. Think about a suction accumulator, will that cause 0 superheat because both liquid and gas are pressent within the chamber?How can you say that a receiver cant be at saturation? the outlet liquid line could give off heat and subcool. So when I have a suction line flooding back to a compressor at said saturation there is nothing to worry about because its mostly gas and there cant be liquid there? Your explaining something that goes against everthing we do every day. If your pressure and temps are at saturation for said gas your at saturation (liquid and gas). Its physics. You cant have liquid and vapor present and have subcooling. In a dynamic operating system where heat is moving, its possible to have subcooled liquid post receiver full sure because its ALL liquid.

[ej45]

Subcooling... check your PT chart

To increase crease sub cooling you can do two things period...
1. increase the pressure with no temperature change with all liquid present will increase sub cooling
2. remove heat from all liquid with no pressure change will increase subcooling.

Its physics look at your PT chart, thats why we cycle fans in low ambient operation to maintain pressure to maintain subcooling. cycle on fans and lose subcooling. Think about a condenser with three condenser fans operating a low ambient temps. Just an example...

1. 250lb head, 15 subcool, one fan on
2. 200lb head, 5 subcool, two fans on
3. 150lb head, 0 subcool, saturation, three fans on

We could also enter in refrigerant density, As the temp changes the liquid volume changes. The volume increases with temp increase.

[refertecbd]

Subcooling is not an acurate way to trouble shoot charge in a TXV system with a reciever, rely more on SH and DT, OD ambient, things like this. Recievers store excess liquid not use under certain conditions.

[heatingman]

How can you say that a receiver cant be at saturation? the outlet liquid line could give off heat and subcool. So when I have a suction line flooding back to a compressor at said saturation there is nothing to worry about because its mostly gas and there cant be liquid there? Your explaining something that goes against everthing we do every day. If your pressure and temps are at saturation for said gas your at saturation (liquid and gas). Its physics. You cant have liquid and vapor present and have subcooling. In a dynamic operating system where heat is moving, its possible to have subcooled liquid post receiver full sure because its ALL liquid.

You are arguing my point. Subcooling exists post reciever, read some of the previous posts. people think that once you hit the reciever magically all the subcooling is gone because of "physics" (no further expanation) by the time it leaves, and I argue just the opposite.

[heatingman]

Yes, that is true. Its physics, a reciever that is not full has to be at saturation and will not have subcooling. (both gas and liquid are present)

Youre first post

[heatingman]

[QUOTE=ej45;1508468. So when I have a suction line flooding back to a compressor at said saturation there is nothing to worry about because its mostly gas and there cant be liquid there? Your explaining something that goes against everthing we do every day.[/QUOTE]

No there is alot to worry about. But w/ your logic on recievers having to be at saturation (liquid and gas) then you must apply your own logic to a accumulator magically desuperheating the same way a reciever will magically cause a loss of subcooling( because theres liquid in the accumulator).
Reality of dynamic flow does not opperate under the same rules as a static system. Superheated gas and Subcooled liquid can and do reside in the same chamber during dynamic flow. Look at the evaporator and the condenser, they are the same chamber inlet to outlet, yet both gas and liquid exist within the given space for two reasons one dynamic flow and two heat absorbtion, or rejection. So I'll ask again if what you say is true about a reciever, then what causes the heat gain to remove subcooling.

[bertoh]

maybe thats where the subcooling went.
have you ever seen bubbles in a sight glass even though you have good subcooling????????? i suppose at some elevated SC you would clear that sight glass, but you have reduced your condenser capacity, and drove your HP so high their is no way it is more effeicent. just because you have 10 deg SC leaving a condenser into a receiver doesnt mean that RC is 100% full.
their is probably a much more scientific way of getting the point across, but dont discount common sense. thats what we do every day!!

[ej45]

No there is alot to worry about. But w/ your logic on recievers having to be at saturation (liquid and gas) then you must apply your own logic to a accumulator magically desuperheating the same way a reciever will magically cause a loss of subcooling( because theres liquid in the accumulator).
Reality of dynamic flow does not opperate under the same rules as a static system. Superheated gas and Subcooled liquid can and do reside in the same chamber during dynamic flow. Look at the evaporator and the condenser, they are the same chamber inlet to outlet, yet both gas and liquid exist within the given space for two reasons one dynamic flow and two heat absorbtion, or rejection. So I'll ask again if what you say is true about a reciever, then what causes the heat gain to remove subcooling.Maybe I am missing something hear but I thought an accumulator was to prevent a flood back or flooded start (to protect the compressor)? I would think that under normal operating conditions, no liquid would be in the accumulator. If there is liquid in the bottom boiling off during a startup then we would have saturation. If I have liquid in my low side manifold and hook up to a running circuit, the circuit pressure boils of the liquid to vapor, the liquid does not stay there, so how could liquid be pressent in a superheated gas pipe like an accumulator? Only if your flooding back and then again your at saturation.

Sorry about the run-on. This is getting interesting

[heatingman]

Maybe I am missing something hear but I thought an accumulator was to prevent a flood back or flooded start (to protect the compressor)? I would think that under normal operating conditions, no liquid would be in the accumulator. If there is liquid in the bottom boiling off during a startup then we would have saturation. If I have liquid in my low side manifold and hook up to a running circuit, the circuit pressure boils of the liquid to vapor, the liquid does not stay there, so how could liquid be pressent in a superheated gas pipe like an accumulator? Only if your flooding back and then again your at saturation.

Sorry about the run-on. This is getting interesting

You are absolutely right about the accumulator, I was not thinking clearly when I used that example. during operation unless there was a system malfunction bring back liquid, there would be no liquid other than oil, and if there was a problem there would not be subcooling anyway.
But I still want an answer from someone about what would cause the sucooling to go away? If its pressure drop, would that not stablize after a couple of minutes runing.