[ryand]

I know how and where to check for super heat and subcooling , but i just dont know what numbers i am looking for. Is it different for a cooler or a freezer? Does it matter what refrigerant it is? thank you to whom ever replies every little bit of info helps so much!!

 

[cg2]

Hopefully someone will correct me if Im wrong, but i dont think subcooling is necesarry in refrigeration. Since most coolers/freezers have liquid recievers, you can't back up refrigerant in the condenser to achieve it. Just make sure there is a clear sightglass and you'll be fine. Superheat is important to keep liquid from flooding back to the compressor. I believe that most compressors are rated to run at 20 degrees. These recommendations are for txv systems, cap tubes will only operate at pressures set by the charge:expect high superheat when box is warm. superheat will decrease as the box pulls down, so make sure that you dont add so much refrigerant that the box will have 0 superheat when it gets down to temp.

 

[r404a]

I must respectfully disagree that subcooling is not necessary when working on low or medium temp equipment. The notion of subcooling is as important on TXV systems for low temp as well as it is fo Ac and medium temp. I think that I understand what CG is saying concerning the ability to "back" refrigerant up, or the lack thereof, and I believe that what he maybe saying (correct me if I am wrong CG) is that the ability to get a subcooling reading may be influenced by system components like a head pressure regulator, "warming up" your liquid. That maybe the case, but it is always a good idea to block off your condenser and put the HPR out of play to check liquid seal on the Liquid line. Keep in mind, subcooling can help you spot, in fact SHOULD ALWAYS be used to spot liquid stacking in the case of a bad txv powerhead or clogged txv strainer. Ryand, I guess that I am getting away from the original question, I do that a lot, so I mainly read, not write....but a ballpark answer is IF you have a TXV low or med temp system WITHOUT a condenser flooding device and you are condensing at design temp (100 degrees sat discharge temp) and you are close to box temp, 10 degrees of LL subcooling would show that you have ample liquid to the TXV.
Concerning superheat values I suggest that you try to always check superheat at two places on your systems, this is at the evap itself and at the compressor. This is very important in low and medium temp work, and I think it beehooves AC techs to do so as well. You only can do so much with compressor superheat readings, not that it is not important, but you are, as CG said looking for floodback and trying to make sure that the compressor is no going to get damaged. RULE OF THUMB across the evap on MT 8 to 10 degrees and 4 to 8 low temp. Perfect world you get 20 degrees at the compressor, but as long as you aren't scorching the hot gas line (230 plus hot gas discharge temp)you can probably be safe way upwards of that superheat number. KEEP IN MIND if these readings and methods are new to you, experience plays a lot into it.
I promise to quit writting after this last little paragraph....keep in mind on special lowtemp TXVs, known as pressure limiting (ZP charged heads) that you ABSOLUTELY should NOT goes monkeying with the superheat adjustment on pulldown. You have to wait until you are very close to box temp to get a proper superheat reading. Period.

Good luck

r404a

 

[cg2]

Maybe I can clear up a question I've had since my RSES classes twenty years ago here. How can liquid be subcooled coming out of a reciever, given that both liquid and gas exist together in the reciever?

 

[air2spare]

thanks r404a

that is some great info! keep up the good work

 

[r404a]

cg,
Concerning the receiver issue, what happens is there is a pick up tube that should tatke pure liquid out of the bottom of the receiver. Yes, there is a vapor and liquid mixture in the receiver, but the liquid will be on the bottom. You know, I just understood what you are saying- the refrigerant vapor and liquid are in cotact with one another so saturated discharge temp should be just that- not superheated OR subcooled. However, when you hav a reading of pressure on the liquid line and you are FULL of liquid in this line, you can read subcooling. I know what you are thinking, so look at it this way: If you had a refrigerant tank sitting in the shop, R22, and it was 90 degress, you would read a pressure on your gauge of 168 psig or so. This is considering that you had vapor AND liquid 22 in the bottle. Now, if you only had a pinch of gas in the bottle, well, you would read something less than that. Now, think of this, if you filled that bottle up totally with liquid, which we all know to be EXTREMELY dangerous, you would read a pressure that exceeds the ambient of that refrigerant, actually, you would be at some hydrostatic pressure. Now, I know it is a stretch, but I can only explain it in this way, well, because I am not too good at explaining things....lets try to imagine the liquid line along the same line of that tank full to the brim with liquid. You don't have the exceeding hydrostatic pressure because your receiver DOES have vapor on top, and you still have a gaseuos state in the condenser. Also, the longer the liquid line is, the more space you have to "cool" the liquid below that condensing temp that is occuring in the condenser/receiver. I have a feeling this post is going to get knocked around a bit, but I hope that I conveyed something. Good Luck.

r404a

 

[cg2]

I'll think about this today some more when I should be thinking about work. Anybody else? It just sems to me that any sybcooling which takes place could only be due to heat transfer from the full liquid line to ambient- or if it's strapped to the suction line or has a heat exchanger in the liquid line. I have seen some systems in the old days that had a separate subcooling pass in the condenser for the liquid line after leaving the reciever, but its been a long time since I've seen one of those. I have seen that in Trane AC condensers more recently, I suppose because they used multiple passes thru the condenser and had to accumulate the refrigerant at the bottom as liquid.

 

[icemeister]

cg2.........

Here's a thread from last year where subcooling and receivers were discussed:

http://www.hvac-talk.com/vbb/showthread.php?threadid=56495

There have been other threads as well.

 

[jacob perkins]

you can have bubbles in the sight glass,and be taking a sub-cool reading at the same time.

i try not to ask too many questions...

especially when i know that i wont remember the answer even if i do understand it

 

[cg2]

OK i read it

So it looks like subcooling can exist in receiver systems. The funny(?) thing is I was taught by RSES 20 years ago that it couldn't so I never used it as a diagnostic tool. I'll have to start checking it now and see if it helps.