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nyc_badboy
08-07-2011, 04:08 PM
The issue that I'm having recently at my job is that I know the proper way of charging a txv system is by subcooling and fixed irifice like capillary or piston by superheat. The issue that I'm having with this and pls correct me if I'm wrong when I'm doing refer systems I almost always have a liquid receiver. When we have a liquid receiver then we also dont have any subcooling because gas and liquid coexist inside the receiver. How do we charge the proper way in those situation? Any help would be appreciated .

skippedover
08-07-2011, 04:15 PM
I think this fall outside the public site rules to get any meaningful assistance. I'd recommend you spend some time getting some more post credits and then apply for pro status. There you can get detailed explanations and lots of education. C'mon over!

beenthere
08-07-2011, 04:56 PM
Moved to refrigeration and Ice making forum.

nyc_badboy
08-07-2011, 08:36 PM
well im trying to get posts but i recently became a member even though ive been reading this forum for a while ill do my best what is the minimum post 15 or sthing? i really would appreciate an answer to that question regarding subcooling my teachers nor my coworkers know the answer to that they just charge by sightglass...:gah:

icemeister
08-07-2011, 09:07 PM
For systems with a TXV but without a receiver (like a residential A/C), as you add more refrigerant to the system, the condenser starts to stack up with liquid resulting in an increase in subcooling.

If the system has a receiver, the condenser constantly drains to the receiver so the resulting subcooling is constant as you add refrigerant to the system. All that is necessary is to charge by the sightglass to assure a liquid seal at the receiver outlet.

BTW...in a dynamic system, subcooling is not lost in the receiver. ;)

trippintl0
08-07-2011, 10:26 PM
BTW...in a dynamic system, subcooling is not lost in the receiver. ;)

Stop saying things like that! They make me have to slow down and visualize a refrigeration system and think about whats really going on inside of it, and it's too late and I'm too tired to do that!

lynn comstock
08-08-2011, 04:55 AM
BTW...in a dynamic system, subcooling is not lost in the receiver. ;)Unless it is black in color, located in the sun, and being washed with condenser discharge air.:D (But otherwise, I agree with you.)

dave1234
08-08-2011, 11:27 AM
All that is necessary is to charge by the sightglass to assure a liquid seal at the receiver outlet.

BTW...in a dynamic system, subcooling is not lost in the receiver. ;)

Couple questions. What is meant by "dynamic system"? also I can understand having saturated liquid in the receiver but isn't the liquid refrigerant subcooled before is drains to it? Also why isn't it subcooled on the liquid line after the receiver? If you have a solid column of liquid in the sightglass would you not have 100% (subcooled) liquid feeding the TXV? :couchhide:

icemeister
08-08-2011, 04:54 PM
Couple questions. What is meant by "dynamic system"? also I can understand having saturated liquid in the receiver but isn't the liquid refrigerant subcooled before is drains to it? Also why isn't it subcooled on the liquid line after the receiver? If you have a solid column of liquid in the sightglass would you not have 100% (subcooled) liquid feeding the TXV? :couchhide:

This is a subject which has been discussed at very great length here in the past. I suggest you set aside some time to study this thread (http://hvac-talk.com/vbb/showthread.php?t=56495) and you will then better understand where I'm coming from on this.

A "dynamic" system could be described as an operating refrigeration system. The opposite would then be a "static" system which could be that same system at rest, or better, one might consider an isolated vessel like a refrigerant cylinder.

The main point here is if the system is active...ie, dynamic, then if there is subcooled liquid entering the receiver then there will be subcooled liquid leaving...minus any losses in between.

Such losses might be attributed to what Lynn mentioned earlier like heat gain through the receiver shell or perhaps from a pressure loss. Either would tend to reduce the amount of subcooling of the leaving refrigerant liquid.

That the refrigerant liquid will lose all of its subcooling merely because it is in a receiver is a fallacy which the above-linked thread is attempting to contradict.

As Andy Schoen stated in post #7 of that thread:


For subcooled refrigerant to return to saturation in the receiver either: (1) pressure must be reduced in the receiver, and/or (2) the receiver absorbs sufficient heat to eliminate the subcooling.

I totally agree with that statement. :)

jpsmith1cm
08-08-2011, 08:26 PM
The issue that I'm having recently at my job is that I know the proper way of charging a txv system is by subcooling and fixed irifice like capillary or piston by superheat. The issue that I'm having with this and pls correct me if I'm wrong when I'm doing refer systems I almost always have a liquid receiver. When we have a liquid receiver then we also dont have any subcooling because gas and liquid coexist inside the receiver. How do we charge the proper way in those situation? Any help would be appreciated .

I don't want to give you too much information, but let me ask you a few questions for you to chew on....

What kind of system are we talking about?

Does it have a headmaster?

What is the purpose of the liquid receiver?

-frozen-ocean-
08-08-2011, 09:07 PM
Remember a receiver has a dip tube in it so LIQ!UID will exit. Who cares whats happening up on top with vapour/liquid.

Andy Schoen
08-08-2011, 09:10 PM
I don't want to give you too much information, but let me ask you a few questions for you to chew on....

What kind of system are we talking about?

Does it have a headmaster?

What is the purpose of the liquid receiver?

If the purpose of the receiver is to support a headmaster (aka flooded head pressure control), and if the headmaster is bypassing, can we still have subcooled liquid in the receiver?

The problem is one has to estimate the rate of condensation of bypassed hot gas into the receiver. I don't know of a way to do this other than to place a flow meter into the bypass circuit, in addition to a pressure transducer and thermocouple.

I would expect even in this scenario you would find subcooled liquid in the receiver under steady state operating conditions.

BTW, flooded head pressure controls are rated assuming saturated conditions in the receiver. I know, as I did them for Sporlan. ;)

Andy Schoen
08-08-2011, 09:29 PM
The problem is one has to estimate the rate of condensation of bypassed hot gas into the receiver. I don't know of a way to do this other than to place a flow meter into the bypass circuit, in addition to a pressure transducer and thermocouple.

I would expect even in this scenario you would find subcooled liquid in the receiver under steady state operating conditions.



By definition, a vertical receiver would have less hot gas bypass condensation than a horizontal receiver. And a properly charge system must have a least saturated liquid in the receiver while the flooded head pressure control is in bypass mode. Ergo, it must be possible to have subcooled liquid in the receiver while the flooded head pressure control is in bypass mode at steady state conditions simply on the basis of receiver design alone. I see another article on this subject. ;)

nyc_badboy
08-09-2011, 12:15 AM
I don't want to give you too much information, but let me ask you a few questions for you to chew on....

What kind of system are we talking about?

Does it have a headmaster?

What is the purpose of the liquid receiver?
__________________


The refer system i usually encounter are 1-5 hp walk jn boxes and freezers purpose of the receiver in those systems is usually hold refrigerant for service purposes or when system pumps down when llsv de energizes... I just hate that whole sight glass thing and I would like to find out if any other way exist .... What I was roughy in school and correct me if I'm wrong pal there can be no subcooling at the surface of the liquid in the receiver. The reason for this is that when liquid refrigerant and vapor exist together, they must obey the P-T relationship and the refrigerant must be saturated. In order the get the proper subcooling a heat exchanger must be employed or you go straight to the expansion valve....

jpsmith1cm
08-09-2011, 06:07 AM
The guys that you work with know more than you may think.

When dealing with a system with a receiver, the FIRST thing that I do is to toe sure that the sightglass is completely full of liquid.

There are other steps that may be required, but that is the first

itsiceman
08-09-2011, 08:03 AM
Have you ever found an inefficient compressor that you thought was low on gas because a sight glass that wasn't full?

The few times I've measured SC at the receiver outlet with hot gas bypass it hovers around 1-0 but the sight glass is always full.
My meter is low res on temp so I recon its always reading positive but just barely from what you are used to seeing when a system isn't bypassing.