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Thread: Subcool and Superheat
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04-08-2001, 06:11 PM #1
Subcool and Superheat
What is the correct method to check subcool. I take it for a correctly charged system subcool would be in the region 15K. Also how can one tell when an expansion valve is "hunting". I know these questions seem pretty elementary but subcool, superheat are two of the major aspects which control the stability and balance of a system, and also show up if the system is not designed correctly, ie: mismatch of components.
If only these aspects were emphasised enough, Im not sure if there is a simple foolproof guide which us "techs" can understand and carry out each and every time we install. As Marc says, the designers are putting together components whilst seated in their offices, its the techs in the field who have to do the "fixes" Often they dont are not able to, they have too much confidence in the design engineer. Before the installers and the design engineers "polarise" the techs need to be able to assert their knowledge of system operation, backed up with readings and data and if necessary "challenge" the wisdom of the designer.
Am I right here?
Any opinions, statements and facts expressed in this message do not constitute legal advice in any shape or form and is given for a general outlook in nature. You are advised to seek appropriate and specific professional assistance from a regulated and authorised advisor for definitive advice.
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07-08-2001, 04:02 AM #2
subcooling
Dear Aiyub,
Let me run this by you as every condenser has slightly (at least) different characteristics. You must always have 100% liquid at the expansion valve for maximum evaporator net refrigeration effect.
That includes the hottest day of the year. If you know the ambient and its corresponding leaving refrigerant pressure with a clean condenser and all fans in operation, you can simulate the hottest day of the year by running up the head pressure artificially by blocking the condenser. It is not an exact science but will get you close enough so you know on the hottest day of the year, you will have maximum net refrigeration effect due to 100% liquid in the liquid line (or you cannot achieve it). You will have to sustain the artificial head for at least 5 minutes to get a more accurate reading. By maximizing your charge, the liquid will "stack" on cooler days and be as cool in temperature as possible leaving the condenser. This gets refrigerant temperature as close as possible to entering air loads.
Just a thought of course. Some will have a much better way.Tom D.
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07-08-2001, 06:12 AM #3Let me run this by you as every condenser has slightly (at least) different characteristics. You must always have 100% liquid at the expansion valve for maximum evaporator net refrigeration effect.
The bubbles in the liquid line disappear between 10-15F (5.5-8.5K) subcooling. More than 15F (8.5K) subcooling indicates that liquid is starting to back up into the condenser, effectively reducing its surface area and driving up the head pressure. A sight glass won't tell us if the system is overcharged. The best way is subcooling.
Whatsmore, subcooling changes very little with increased head pressure. In cool weather, I will block the condenser to simulate warm weather, but not drive it up to the max. That just isn't necessary.
And there are lots of variables that should be checked before we even put our gauges on the machine, much less attempt to charge it.Last edited by Gary; 07-08-2001 at 03:18 PM.
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07-08-2001, 05:33 PM #4
Oh foolish me, I took it for granite again that expansion valve buls are cleaned and supreheat checked thus not allowing a flooded coil as you stated Gary. I will be more careful.
Tom D.
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07-08-2001, 06:01 PM #5Oh foolish me, I took it for granite again that expansion valve buls are cleaned and supreheat checked thus not allowing a flooded coil as you stated Gary. I will be more careful.
However, there are less experienced people watching, and I try to avoid teaching them just enough to be dangerous.
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07-08-2001, 10:06 PM #6
[QUOTEHowever, there are less experienced people watching, and I try to avoid teaching them just enough to be dangerous. [/B][/QUOTE]
Yes, but unfortunately, every frige firm I've met adopts exactly that philosophy - ie: Make 'em dangerous!!
I say: "Make it illegal for accountants to control budgets!"
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12-08-2001, 11:30 PM #7
----------------------
I'd recomend you get a copy of the "TECH Method Lesson Series" for an interesting and worthwhile study. You wont regret the purchase and the book is something you can carry with you for all your required on site reference covering all the aspects of fridge and a/c. Someone like yourself needs to have a book like this.
visit http://techmethod.co.uk
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Ok, Ill buy the book........ahem........bible......
But.......Can I buy it in the UK ?
I am loathe to post my visa numbers all over the web
Also saves me converting pounds to dollars...Any opinions, statements and facts expressed in this message do not constitute legal advice in any shape or form and is given for a general outlook in nature. You are advised to seek appropriate and specific professional assistance from a regulated and authorised advisor for definitive advice.
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12-08-2001, 11:58 PM #8
A bible it is not. It is basic fundamentals of trouble shooting as it ought to be taught in the schools, covering everything from high temp systems to cascade freezers.
Marc is converting the books to metric for the European market, and will soon be the UK distributor.
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14-08-2001, 02:02 PM #9
Other good reads
I would also recommend the following
Depending on where you did your course you should also consider Principles of Refrigeration by R J Dossart from Wiley and if its still in print Refrigeration Technicians Pocket Book by F H Meredith from Butterworths. There is also an electrical handbook for refrigeration but I don't have that to hand it's the BTEC course book so check your local college bookshop.
For thermo' in general the old HNC course book Applied Thermodynamics for Engineering Technologists by Eastop and McConkey from Longman.
The ASHRAE fundamentals (I just got the new one) is the minimum 'coffee table' reference and ideally you could have the complete handbook series on your laptop. Great for that what's the best storage temp for unripe kiwi fruit question.Derek
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19-09-2001, 04:48 PM #10
super heat on cap tub.
how does one charge cap tube system with super heat method? For many years I charge cap tube system by pressure - and it always work. Thats how I show all the new hires. Recently, a new tech question my method. Although I've doing it for years, but I find it very difficult to explain in words. can someone here give a clear, concise explaination on " charge cap tub system by super heat method" ?
Be Happy
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19-09-2001, 08:27 PM #11
When considering any charging method, whether it is TEV or cap tube, we need to define what we are trying to accomplish.
At design temperature/load:
Our first goal is to flood the coil.
Our second goal is to cool the compressor.
Our third goal is to NOT flood the compressor.
Our fourth goal is to NOT back liquid up into the condenser.
We need to keep in mind that the charge is most accurate AT DESIGN TEMP.
Limits:
The compressor can flood at less than 10F inlet superheat. (Tecumseh recommends at least 15F inlet superheat. Copeland recommends at least 20F inlet superheat.) I recommend at least 20F inlet superheat.
More than 15F subcooling indicates that liquid is starting to back up into the condenser, driving up the head pressure.
These must be the limits within which we charge any system, and so we must monitor both subcooling and superheat.
This said, the TEV system should be close to the subcooling limit, while the cap tube system should be close to the superheat limit AT DESIGN TEMP.
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29-10-2001, 02:23 AM #12
bubbles in the sight glass
A coil can be completely flooded while there are still a few bubbles in the sightglass.
With the sight glass near the expansion valve you should see no bubbles if charged correctly. Just my opinion.
:D
Dean
Subzero*psia
Extinction is simply proof of failure to adapt.
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02-12-2001, 04:26 AM #13
O.K. lets use an R-12 3 door(self-contained) reach in cooler that maintains a box temerature of 36F.
Lets say box temperature is at 50F and we start the compressor. What can we expect our pressures to be?
As I refer to my P.T. chart I would expect my suction pressure to be in the range of 28-31 psi. This would
give me an average coil temp of say 32F. We know that
with a cap tube system our superheat will be fairly high
on start-up knowing that the evaporator is designed to be
starved. Its my opinion that the subcooling value (liquid temperature/condensing pressure) would be consistant reguardless of box temp. So if surrounding ambient around
box is say 75F, what could we expect our condensing pressure
to be? Again, going back to P.T. chart, I would expect the condensing pressure to be around 125-130psi, giving us an average cond coil temp of 106F. So here's the final question
? I like to shoot for 15F subcool which would give me a liquid temp feeding the evaporator of 91F. This I feel
is the magic bullet theory of being able to walk away
with out knowing superheat on cap tube systems. Please,
everyone chime in on their thoughts as to my theory.
Is my 15F subbcool a good number? Is there a better way?
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02-12-2001, 05:22 AM #14
that's the way i've always done it bernie.nowhere have i ever heard of a rule of thumb for superheat in a cap tube system. has anyone else? everytime i take a superheat reading on a cap tube system it's between 40f-65f on good working unit.
Last edited by aaron crimmins; 02-12-2001 at 05:27 AM.
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02-12-2001, 08:39 AM #15
another subcooling question for you wiz kids
On a TXV system with a receiver, does the receiver have to be full before ***** back up into the condenser (increase in subcooling)?. rephrasing above question, At what point do we see subcooling change; before the receiver is full or after it is full?
Last edited by dan wong; 02-12-2001 at 08:43 AM.
Be Happy
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02-12-2001, 06:26 PM #16
superheat subcool
there would be a change in subcooling if there was a blockage before the receiver, the receiver than would not have to be full for a change.
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03-12-2001, 04:17 AM #17
Re: superheat subcool
Originally posted by aaron crimmins
there would be a change in subcooling if there was a blockage before the receiver, the receiver than would not have to be full for a change.Be Happy
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05-12-2001, 06:16 AM #18
Useful superheat is required to ensure that liquid doesn't reach the compressor and cause damage as we know that liquid cannot be compressed- but any further increase in suction gas after this is known as non-useful superheat and serves little purpose other that to increase the amount of energy input required at the compressor. On a capillary system it is not normal to use a sight glass and therefore some other method of checking the correct charge has to be used.
Careful measurement of the temperature across the last pass of the evaparotor cannot always be done either especially if it is a flooded plate as in a small domestic so attention should be given to the suction line from the evaporator and a good indicator of the correct charge is to watch for the suction line to start sweating.
On the other hand - if the manufacturer has put a charge weight on the name tag just weigh in the correct charge!
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05-12-2001, 08:08 AM #19
Frank, I agreed with your post entirely.
....any further increase in suction gas after this is known as non-useful superheat and serves little purpose other that to increase the amount of energy input required at the compressor.
On the other hand - if the manufacturer has put a charge weight on the name tag just weigh in the correct charge!Be Happy
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06-12-2001, 05:42 AM #20
Dan
If a compressor is running hot then I would certainly be looking at the charge as this would indicate a shortage.
On systems where the metering device is a TEV then you would have the benefit of a sight glass and, provided it is fitted close to the valve you can charge to a full sight glass.
Here in the UK most of the a/c equipment comes with a factory charge tag and additional charging weights depending on the length of the pipework.
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06-12-2001, 12:58 PM #21
Now let us suppose that sometime has gone by and a technician has changed the liquid drier. The tech uses what he has in his truck, not what the original equipment manufacturer used or specified.
And lets also say this is a captube fed system.... how are you going to charge the system? Your only real choice is using the superheat or subcooled method. Myself I prefer the superheat method.
Aaron was right, captube systems do often run very high superheats.... but if you wait until the cabinet temperature is at or near the control setpoint... you will see superheats that rival a TXV. (providing the OEM actually sized the capillary correctly or that the capillary hasn't been changed to a non OEM size). I always stick around to check evap superheat and monitor the system for any irregularities while the system is pulling down anyway.:D
Dean
Subzero*psia
Extinction is simply proof of failure to adapt.
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14-12-2001, 04:43 AM #22
Re: bubbles in the sight glass
Originally posted by subzero*psia
While I agree with this, I dissagree also. It depends on where the sight glass is physically located. Most sight glasses I see on my journey are located outside just after the liquid receiver which will give you a FALSE reading everytime... ie you see bubbles even though the charge is correct.
With the sight glass near the expansion valve you should see no bubbles if charged correctly. Just my opinion.
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15-12-2001, 02:09 AM #23
excuse me
Sorry forum members for interrupting your thread speak
I know that this forum is in the Internet world so a lot of real life rules do not apply.
Would send me a reply to let me know if there is anything odd to you about our descriptions under our avatars.
Mine is improver, I was an apprentice at 17 postings and apparently your description according to webram depends on the number of postings that you make. It’s all a bit of webram fun
Thinking at first the whole thing very bizarre I thought what a good idea to do it in real life conventions and swap over every bodies job descriptions labels and see what happens in the ensuing misunderstandings and failures of communication.
Kinda makes me feel like a guinea pig though.
OK so webram having a joke and it aint April fools day
So come on what’s going on with avatars and descriptions
I have a brilliant avatar one of those animated giffs you can send as easy as pictures and attachment, like funny pictures its all promised in the future by webram.
I v seen the world iv been around but I aint never seen anything like it in my life.
Welcome to jacks wheelyworld refrigeration forum
www.geocities.com/jackkybird/watercycling.html
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15-12-2001, 02:41 AM #24
Here is another scenario regarding a hot compressor... with a capillary tube fed system, has POE type oil. The system is small, less than one ton. It has been exposed to high ambients. The drier has broken down and waxing has occurred in the capillary(s).... but of course you don't know this for sure without a visual or a more complete test and diagnosis. (And yes, I know that in restaraunts etc. it can be very difficult just to get them to unload one or two shelves!)
The system has been worked on a few times over the last several months and everytime the technician has observed the symptoms you discribed ... they even checked the high and low side pressures... which only coroborated what they believed was a low charge (an obvious leak right?)... so they added "just enough" refrigerant each time to get those pressures where they belonged.
Have you ever seen a 5 pound sack hold 10 pounds... you will if you witness one of the above service calls. I have seen over 40 ounces of refrigerant come out of a system that was OEM designated to hold only 19 ounces of R-404a!:D
Dean
Subzero*psia
Extinction is simply proof of failure to adapt.
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16-12-2001, 09:50 PM #25Originally posted by subzero*psia
so they added "just enough" refrigerant each time to get those pressures where they belonged.
Have you ever seen a 5 pound sack hold 10 pounds... SNIP
Should we therefore try and pre-charge our gauge sets before connecting or promise not to connect up such big bottles when we top-up a systemsBrian - Newton Abbot, Devon, UK
Retired March 2015
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17-12-2001, 03:42 AM #26
Now let me see, I think the origonal "?" was how do I check super heat and/or sub cooling. I have a real disadvantage here has I do not spell has well has I can speak.(Which isnt all too good at times either). Has Im in USA and I use PSIA and degrees F .
First the definition of super heat (I think) is the amount of heat added to a substance above its boiling point. Take a temp reading at the suction - return line of your compressor. Next Take a pressure reading at the suction line. Next (I usually skip this and go to the 4th step) if possable take temp reading at evaporator inlet.
In theroy all of the refrigerant is at the same pressure. Convert the pressure to temperature.
Subtract the true suction line temp from the temp you converted (the pressure conversion). The differance will be your super heat.
The advise of our other collegues here is also good. A refrence is required. refrigerant Manufacturers may have a PT chart with a referance on it.
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17-12-2001, 03:57 AM #27
Oh yes your 2nd and 3rd "?" Subcooling is the temp below saturation temp to which a substance is cooled.
Take temp of liquid line out of condencer. Take pressure reading of liquid (not hot gas but liquid line pressure). The differance is your sub cooling.
Differant manufacturers and applications require differant subcooling and superheat settings so if in doubt call the manufacturer. 8 to 12' SH is usually what our mfgs need to make any refrigeration system operate. Remember your load will also have an effect on SH and SC if it is high.
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17-12-2001, 04:02 AM #28
Has for hunting; This is mostly up to individual interpetiation.
Hunting is just that. A valve is "looking" for the optimum opperating setting. It usually is most evident on correctly charged (or close to correct) with at least a slight load. The closer a Valve gets to satisfying the load the less hunting it has to do.
Hunting valves to me usually do not have high super heat cause the valve is wide open. If the system is overcharged the valve will tend to remain closed due to the refrigerant cooling the bulb and it tending to stay closed.
One way to detail a "hunting valve is watch the suction pressure. It will rise on an increase in SH and within a few minutres will go lower and SH will decrease. Also U may be able to hear one open and throttle closed.
Several factor can cause valves "not to hunt" though. Its mostly worthless has a trouble shooting tool to me.
Hope this helps
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24-12-2001, 06:55 AM #29
Assuming proper airflow through both coils, the idea is to flow enough refrigerant in the high side to feed the metering device, but not enough to back up liquid in the condenser.
We can test the upper limit by subcooling. More than 15F SC means liquid is starting to back up into the condenser. The lower limit (starved coil) is evident in the superheat.
In the low side we want enough refrigerant flow to flood the coil, but not enough to flood the compressor.
The compressor is in the danger (floodback) zone below 10F SH (near compressor inlet). I like to see about 20F SH (at design cutout temp). If the inlet SH is more than 30F, we need to check the coil outlet SH to see if the coil is properly flooded.
Bottom line: At design cutout temp, if the SC is 15F or less and the compressor inlet SH is 20-30F (preferably closer to 20F), the system will function properly, be it TXV or cap tube, and regardless of what the sightglass indicates.
But there are exceptions to these general rules.Last edited by Gary; 24-12-2001 at 05:37 PM.
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14-01-2002, 04:51 AM #30
With respect to superheat method of charging cap tube systems, Gary hit the nail on the head when he says, the bottom line is "at design cutout"!
The problem is, when most tech's wrap up after a service call, the system/house is not down to "design cutout temperature".
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03-03-2002, 08:27 PM #31
subcooling superheat
check the superheat by temp of suction pressure subtract suction temperature
subcooling is saturated discharge temperature minus liquid line temperature
sorry if i have typed this and its hard to understand but its something i do all the time like a robot on site but when i come to explain it by writing it down etc i seem to make it sound so complicated
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04-03-2002, 05:50 AM #32
No Icepick, it is not a complicated computation. What is ironic is that subcooling is so very important but so few seem to know much about it. The younger fellows I run across here in the States tell me the instructors in various schools for basic air conditioning and refrigeration just seem to "gloss" over it. For years now, TXV manufacturers, especially Sporlan (since I first saw theirs in 1964) have widely published capacity charts and in the very beginning of the publications in the catalogs they give design liquid temperatures (and a full liquid column) entering the valves and PDs for capacity ratings of different refrigerants.
Perhaps the various instructors should present these publications as learning tools with solid explanations. Although, some school boards almost forbid any other teaching aids except what they say can be presented. Oh well. have a super day today and an even better tomorrow.
TomTom D.
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