No... No... and No.
Let's say the ambient is 25C and the SCT is 36C. The TD is 36-25=11K.
The SCT then rises to 40C. The TD is 40-25=15K .
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Thats right, thanks
I assume that is the same with your evap the
high load big td
low load small td
CB
Very helpful and very nicely written. Thank you Gary.
Excellent, great to read. just need an apprenticeship now. its so hard to find!!!:rolleyes:
Hi Gary, it’s been a long time since we had lunch in Michigan at Bob Evans :-) Hope you have been well.
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Originally Posted by Gary
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Coil outlet superheat varies with type of system. Generally speaking, a freezer should have 6-8F/3.5-4.5K superheat, a cooler should have 8-10F/4.5-5.5K superheat, and an A/C should have 12-16F/6.5-9K superheat..
We installed a Russell freezer system last week and I thought of you. They are saying Copeland now wants 30F superheat at the compressor MINIMUM. They don't seem to care about the superheat outlet at the coil. What happened to Copeland’s 12F standard?
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On the high side the sub cooling should be no more than 15F/8.5K because this is the point at which liquid will start to back up into the condenser.
I have yet to see a system that can obtain a 15F sub cooling, the most I have seen ever was 10 and the newer systems seem to be around 4f. Is this due to a design issue? I always test a system before leaving to make certain it can pump down under load and not be a problem. Over 10f sub cooling I have yet to see possible. ?????
Also, I'd like to mention to newbie’s that these numbers may not be obtainable on converted systems. On a converted system you may have an overly charged system trying to obtain even small sub cooling temperatures. Also, one must be concerned with pump down capacity where LONG liquid lines are used and the solenoid valve is located inside the condenser. I have one customer with a 7/8 liquid line where the original installer put the solenoid valve at the unit and the line was over 100 actual feet. Needless to say, we moved the solenoid valve to the evaporator and the systems pump down capacity was ample since we didn't have the entire refrigerant in the liquid line pumping into the condenser.
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......Additional refrigerant can then be added to bring the sub cooling up to 15F/8.5K. This is the maximum charge.
Is this a new standard? Like I stated, I've seen systems overcharged at 10 Sub Cooling.
Eagerly waiting for your guidance.
Richard
Hi Rich. A very long time. Nice to hear from you again.
As far back as I can remember, Copeland has called for a minimum of 20F superheat and a maximum of 30F superheat at the compressor inlet.
Being a compressor manufacturer, their primary concern is the superheat at the compressor inlet rather than the evaporator outlet. No surprise there.
Are you measuring subcooling on the liquid line near the receiver?... or on the drip leg between the condenser and the receiver?
Gary, thanks for directing me to your thread on the basics. It really brings it all together for me in plain simple language. Much appreciated!
Michael
I apologize for the confusion. It seems further clarification is needed here:
Subcooling is the cooling of liquid below saturation. In order for this to occur the liquid must be subjected to a temperature that is below saturation.
In order for the subcooling to be 15F/8.5K the liquid must travel through an area where the temperature is at least 15F/8.5K below the SCT or the desired subcooling cannot be achieved.
If the receiver outlet is directly in the path of the air leaving the condenser, then the surrounding air is not cool enough to sufficiently subcool the liquid.
In this case, the liquid line temperature must be measured further downstream where the surrounding air is cooler.
Similarly, in order to achieve sufficient superheat at the TXV bulb the vapor must travel through an area where the temperature is sufficiently high to cause that superheating. If the suction line is directly in the path of the air leaving the coil, then the surrounding air temperature will not be warm enough to achieve the desired superheat.
Hope this helps end the confusion.
you are an author of tech methods books?
Hi Gary
Brilliant artical !!! Iam new to this A/C but I find it very interesting. One problem I do have is some of the Buss words you experienced engineers use.
eg delta T, TD. pulldown etc could you explain some of the more basic ones for me please. Also we had a leak on a compressed air cooling system last week all the 407 gas had been lost. repaired the leak and recharged every thing ok now,but noticed a bypass valve had been fitted to the system,how do I know if this valve is working?
One last point could I have lost some of the oil from the system,how do I check it.
Regards Eamonn
Delta t ( Δt) is difference between two temperatures of same media.
For example, air entering and leaving condenser, or air entering and leaving evaporator, or water entering and leaving condenser, or water entering and leaving evaporator.
TD is difference in temperature of two different media.
For example, difference between saturation evaporation temperature of refrigerant in evaporator and evaporator air entering temperature or difference between condenser air entering temperature and condensation saturation temperature.
http://www.refrigeration-engineer.co...75&postcount=5
Pull down is time period passed from starting equipment to reaching design condition.
For your problem please post question in new thread since that is not refrigeration 101.
Thanks for the info! I read it three times!
No, it would not be incorrect, but it may not tell you what you need to know.
The ideal is to have solid liquid at the TXV inlet, but to not back liquid up into the condenser.
Since there is solid liquid at 10-15F/5.5-8.5K SC, we can tell where along the liquid line there is solid liquid and where there is excess SC.
Excess SC is fine at the TXV inlet, but if there is excess SC at the receiver outlet, then we start backing liquid up into the condenser, which drives up the high side pressure.
Measuring SC at the TXV inlet can tell you if there is sufficient refrigerant to feed the coil, but it doesn't tell you if the system is overcharged. That's why I want to check it closer to the receiver.
Some terminology clarification:
http://sporlan.jandrewschoen.com/Col...erminology.pdf
http://sporlan.jandrewschoen.com/Col...00%20Terms.pdf
Take note that all useful information about a system comes from subtracting one temperature from another temperature.
I can't think of any single temperature or pressure anywhere on any system that tells me what I need to know about that system.
But given temperatures at various points in the system, we can subtract different combinations of temperatures to figure out exactly what the system is doing or not doing.
There has been a big move towards self-diagnostic systems in recent years, but as far as I know they haven't gotten it right yet... apparently their computers haven't learned how to subtract yet.
On the other hand, I am retired now, so maybe things have changed since then.
Hmm it just seems like there is enough technology now that they should be able to at least design a simple system to read subcooling and superheat at a pre-determined point.
But then again they probably don't do this because we still need jobs. hehe
Gary,you are a legend in your own mind. heheh!
Well... that's true enough... lol
The manufacturers don't know how to troubleshoot because that isn't their area of expertise. Their area of expertise is design.
Here is a lesson for you. Commercial refrigeration is a science of vague assumtions, based upon debatable figures, taken from inconclusive experiments, performed with instruments of problematical accuracy by persons of doubtful reliability and questionable mentality !!!!!!!!!!!!!!!!!!!!
I got that at a week long Manitowoc factory school in Manitowoc, Wisconson, in 1996 and it is so TRUE.
I see you all over this forum, and just wanted to mess with you. no harm intended bro. just from one old tech to another. hd88
Well, I think I do... Stick a thermistor in the suction line at a pre-determined spot and one in the liquid line at a pre-determined spot. Then stick a calculator inbetween the two, and WHALA self diagnosing.
Well... not self diagnosing but at least it would tell you the temps and the difference between them.
Hey Gary,
What are the effects of excessive superheat/subcooling and lack of superheat/subcooling.
Perhaps we can run a few example system diagnoses here.
Example:
Time of Day: Noon
Type of System: Rooftop Packaged
Outside Air Temp: 103F
Indoor Air Temp @ Return:86F
Indoor Air Temp @ Supply:83F
You inspect each section of the system visually.
Indoor Coil is clean and clear of obstruction and the air filter is new and clean.
The Outdoor coil is slightly soiled, but nothing to be seriously concerned about.
Outdoor Fan: Operating
Indoor Fan: Operating
Compressor: Operating
Complaint: The unit is blowing warm air.
This should be a pretty simple one... you don't need superheat or subcooling to figure it out.
Here in Arizona one out of five calls is this problem in my experience.
-Fett
Yes... you do.
Could be undercharge, could be restriction, could be blown valves in the compressor.
You might think you know what the most likely problem is, but you don't really know what the problem is until you check the subcooling and the superheat.
While you're at it, check to see what type of metering device you have.
Hi All;
enjoy the site, I'm a sparky working on hot water heat pumps and have a couple of questions regarding fundamentals.
We were called to a fault, the outlet from the TX valve was frosting up. The fridgie explained that the TX valve was the problem.
The unit was working fine, the reported fault (rattly). Both pressures LP and HP were a little low, there was some oil but no leaks could be found. I suggested topping up refrigerant, it was explained that doing this would only put more refrigerant into the TX outlet (and frost it up more).
I have read some posts about checking superheat temp in the fault finding of a TX, not sure how this is done. If someone could give me advice on checks to carry out if TX valve outlet frosts up (including adjustment) I'd appreciate it.
Frosting of valve outlet at air-con unit could indicate at problem, but at refrigeration unit that is normal. What is that unit used for?
If there is oil, there is leak!Quote:
The unit was working fine, the reported fault (rattly). Both pressures LP and HP were a little low, there was some oil but no leaks could be found.
How much low, we need numbers?
We also need temperatures of air in and out of evaporator and condenser to tell if pressure is low or not.
Also we need pipe temperature at evaporator outlet, compressor inlet, condenser outlet.
Topping up refrigerant could be done only after is established that refrigerant missing.That is done by measuring what I said above and judging from acquired measurements.Quote:
I suggested topping up refrigerant, it was explained that doing this would only put more refrigerant into the TX outlet (and frost it up more).
If measurement say that subcooling is low, than refrigerant is missing in condenser. It could be leak or restriction on low side. If you have oil spots and nobody before is not added refrigerant you are probably short of refrigerant.
If you have sight glass, than you will be having lot of bubbles in glass.
Please do not touch TXV before we establish that it is cause of trouble, especially, do not adjust it.Quote:
I have read some posts about checking superheat temp in the fault finding of a TX, not sure how this is done. If someone could give me advice on checks to carry out if TX valve outlet frosts up (including adjustment) I'd appreciate it.
Aye, you may be right, but for the problem I have suggested it is not necessary.
But say we plug in 18-20F superheat and lets say 3-4F subcooling.
Would you say it is undercharged or just a high heat load on the system?
I think that alot of people overcharge systems before considering other factors. A system at proper charge should have no problem bringing down the temp from 86F to 78 or 80 in an hour to an hour and a half. Think of a company that just installed a brand new unit with new ductwork on a relatively old house (35-40 year old house). So they get done on the install and all the ductwork was run. Maybe two or three weeks goes by and a 2x4 which is part of the roof support breaks away from its mounting and falls on some ductwork tearing a hole in it. So now the unit is sucking in that hot air inside the attic and fails to cool the home. Or maybe the installer stretched the duct too much and didn't fasten it together very well and the joint seperated.
Having that hot air will give you a high superheat, well assuming that its a cap tube system but even a TXV would open up pretty wide to compensate for the added heat load.
Hey Gary
How would you determine the correct refrigerant charge in a water cooled system when water flow is controlled by a water regulating valve and it depends on head pressure to regulate. Say we are working with R404 and condenser water temp. of 50F?????
Dear Gary sir,
You are really a genius, The way you have explained Refrigeration to newbies, this was very useful as a beginner for me. Thanks a lot..... :)