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Thread: Refrigeration 101
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05-07-2009, 05:21 PM #1
Refrigeration 101
I was thinking it might be a good idea to set up a basic thread to send the newbies to:
Refrigeration 101:
Let's start from the beginning:
Wet your finger and wave it in the air. What you are feeling is a refrigeration effect. When a liquid turns to a vapor it absorbs heat. In this case it is sucking the heat out of your finger.
The opposite is also true. If that vapor then loses that heat, it will turn back into a liquid.
In a refrigeration system, we force a liquid to become a vapor in the evaporator, thus absorbing heat from the refrigerated space.
We then use a compressor to pump that vapor to the condenser.
In the condenser we force that vapor to reject the heat and thus turn back into a liquid so that we can re-use it.
We then meter the liquid back into the evaporator to complete the loop and do it all over again and again and again.
How do we force a liquid to become a vapor?... or a vapor to become a liquid? By manipulating its boiling point.
The boiling point is the temperature at which the liquid turns to vapor when heat is added. It is also the temperature at which a vapor turns to liquid when heat is removed.
Boiling point = saturation temp = evaporating temp = condensing temp
When we think of the boiling point of a liquid it is the boiling point at zero psi pressure. If we increase its pressure we raise its boiling point. If we decrease its pressure we lower its boiling point.
In the evaporator we force liquid to become a vapor by lowering its pressure until its boiling point/evaporating temperature is lower than the air it is trying to cool.
In the condenser we force the vapor to become a liquid by raising its pressure until its boiling point/condensing temperature is higher than the air it is trying to heat.
Different substances have different boiling points at different pressures.
We can tell what the boiling point/saturation temp/evaporating temp/condensing temp is at various pressures for common refrigerants by checking a pressure/temperature chart.
Okay... let's go a step further: Superheat and subcooling.
If we boil off a liquid into vapor and then add heat to that vapor its temperature will rise above the saturation temperature. This is called superheating the vapor. When its temp is 10 degrees above the saturation temperature it is superheated 10 degrees. When its temp is 20 above saturation it has 20 degrees of superheat, etc, etc.
Similarly if we condense a vapor into liquid and then further cool the liquid this is called subcooling. When the temp gets 10 degrees below saturation it has 10 degrees of subcooling. When its temp is 20 degrees below saturation it has 20 degrees of subcooling.
Refrigerant flows very rapidly through the evaporator coil into the suction line. Many people believe that you can't have superheat until the liquid has all turned to vapor, but this is not true. Because of the velocity of the refrigerant flow it is possible to have liquid droplets surrounded by superheated vapor at the outlet of the evaporator... and in fact this is what happens. All of the liquid droplets are gone by the time there is 5-10F/3-5.5K superheat.
We want the superheat at the evaporator outlet to be low enough to ensure that we are fully utilizing the coil, thus maximizing its ability to absorb heat, but we do not want liquid droplets to be sent back to the compressor.
Similarly, it is possible to have vapor bubbles surrounded by subcooled liquid at the outlet of the condenser. All of the vapor bubbles disappear at about 10-15F/5.5-8.5K subcooling.
We want the subcooling to be high enough to ensure that we are sending sufficient liquid to the metering device, but not so high that we are backing up liquid into the condenser, thus reducing its ability to reject heat.
On a cap tube system there is a fixed amount of liquid flowing into the evaporator. When the load is heavy there is warmer air flowing through the coil and thus the liquid is all boiled off long before it reaches the outlet of the coil, thus the superheat is high when the load is heavy. If properly designed and charged, the superheat will be just right when the design temperature (design load) is reached.
Many people believe that a TXV will maintain a fixed superheat, regardless of load. This is just simply not true. When the load is heavy the superheat rises and more liquid is fed to the evaporator. The superheat remains high as long as the load remains high. And again, the superheat is just right when the design temperature (design load) is reached. But the design temp will be reached sooner because of the extra refrigerant feed.
As we see, when the load decreases the superheat decreases... so what happens when the filter gets dirty, or the evap coil... or the blower wheel? Less airflow means less load therefore the superheat drops, even though the refrigerated space may be at design temp.
When the load is high the superheat is high, and when the load is low the superheat is low... even with a TXV.
Everywhere, throughout the system, there are opposing forces balancing against each other, and it can be very difficult to tell which of these forces is out of balance.
And yes, there is more... much much more... but that's enough for now.
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05-07-2009, 06:05 PM #2
Re: Refrigeration 101
Very nicely put Gary.
Should be helpful to anyone who has a grey area!
Very easy to understand terminology as well.
Grizzly
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05-07-2009, 06:10 PM #3
Re: Refrigeration 101
Great one mate.
Just one question.
As you say the superheat will be high when the load is high (thats why they say you should only set the valve at design temp).
scenario, Your client is complaining of high room temperatures, So you check everything and you've got good sub-cooling, everythings is running well, but you have high air on temps and high air off temps and your superheat at the evap oulet is also high as you said.
What actions do you take??? or where do you start to confirm that there is not a problem, it's just high superheat due to pull down. I think most engineers would assume the TEV requires opening to reduce the superheat,
Like with the cappillary system, you arrive on site cabinet temps high, suction superheat high what are you checking for to confirm there is not a fault.
I'm sure you'll have the answer for us and i'm sure i missing something totally obvious.
CB
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05-07-2009, 06:27 PM #4
Re: Refrigeration 101
I first check the delta-T's to confirm adequate airflow through both coils.
Then I check for high subcooling.
Then I check for low superheat.
Then I check for low subcooling.
Then I check the condenser TD to see if the high side is rejecting a heavy heat load.
If the high side is rejecting a heavy load, then the low side must be absorbing a heavy load.
Adjusting a TXV aka TEV is almost always a mistake, especially when the refrigerated space is above design temp.Last edited by Gary; 05-07-2009 at 06:34 PM.
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05-07-2009, 07:01 PM #5
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15-07-2009, 06:14 PM #6
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30-07-2009, 04:34 PM #7
Re: Refrigeration 101
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27-05-2010, 07:30 PM #8
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28-05-2010, 02:11 PM #9
Re: Refrigeration 101
This will very much depend on the magnitude of the TD itself.
For instance:
TD=10K ; SH = 0.5*10=5K; SH = 0.7*10=7K
TD=20K ; SH = 0.5*20=10K; SH = 0.7*20=14K
TD=30K ; SH = 0.5*30=15K; SH = 0.7*30=21K !!!
That's asking for a fair-sized SH. In all likelihood, you'd have a tremendously high Tc,discharge temperature.
The Magoo rule seems to be applicable in low TD applications & may need to be adjusted for high TD applications.
Like everything in the rhvac game, some level of sound engineering judgement will need to be made in each specific application.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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19-11-2010, 03:25 AM #10
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27-07-2009, 05:13 PM #11
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19-08-2009, 07:25 AM #12
Re: Refrigeration 101
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
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19-08-2009, 09:00 AM #13
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Re: Refrigeration 101
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.Last edited by nike123; 19-08-2009 at 09:03 AM.
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05-07-2009, 06:48 PM #14
Re: Refrigeration 101
Here is a list of the temps/pressures needed to troubleshoot a system:
Evap air in temp
Evap air out temp
Low side pressure or saturation temp
Suction line temp at evap outlet
Cond air in temp
Cond air out temp
High side pressure or saturation temp
Liquid line temp at receiver or condenser outlet
The more information provided the more accurate the diagnosis.
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08-01-2010, 02:33 AM #15
Re: Refrigeration 101
Respected Gary
Where I can get the books Tecch method troble shooting? Is there any agent in India?
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05-07-2009, 07:12 PM #16
Re: Refrigeration 101
Assuming a fixed speed compressor, only insufficient airflow can raise the delta-T beyond normal limits.
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05-07-2009, 07:29 PM #17
Re: Refrigeration 101
If the superheat is high would the delta-T be much higher than when you have a normal superheat??
Am i correct in saying you should have approx (commercial refrigeration) 3 to 6 Deg C temp difference (air on to air air off)
Would the temp difference (air on to air off) be the same regardless of the high or correct room temp???
I.e design room temp 0 deg C
will the air on-off temp still be 3- 6 deg C difference when the room temp is 20 deg C
And will it still be 3-6 deg C when the room is at 1 deg C
CB
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02-11-2010, 07:14 AM #18
Re: Refrigeration 101
Delta=T upper limit differs for various types of systems as well as for different design temperature and humidity requirements. And this goes beyond the scope of "Refrigeration 101".
Suffice it to say that if you are familiar with the delta-T limit for a particular type of system, anything above this limit is an unambiguous indicator of an airflow problem.
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05-07-2009, 07:37 PM #19
Re: Refrigeration 101
Delta=T upper limit differs for various types of systems as well as for different design temperature and humidity requirements. And this goes beyond the scope of "Refrigeration 101".
Suffice it to say that if you are familiar with the delta-T limit for a particular type of system, anything above this limit is an unambiguous indicator of an airflow problem.
Excessive delta-T can ONLY be caused by insufficient airflow.
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06-07-2009, 12:29 AM #20
Re: Refrigeration 101
hell yeah. this is the good stuff
keep it coming
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06-07-2009, 05:28 AM #21
Re: Refrigeration 101
Hi Gary
A good post, should benefit alot of people.
I have this " pet " thing, that a large percentage of service tech's do no understand TEV superheat, let alone how to test and adjust. But then again I am old.
I have a fail proof test procedure that I stated years ago on this site, only works if designer was worth his salt.
Do you have a similar procedure.
What I use is good for pull-down performance or at design, but takes time.
magoo
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07-07-2009, 08:12 PM #22
Re: Refrigeration 101
Thanks Gary - excellent stuff.
I'd appreciate if you could touch on refrigerant charge determination, at some point in the thread.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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11-07-2009, 05:47 PM #23
Re: Refrigeration 101
Think of the system charge as consisting of low side charge and high side charge.
Assuming there is proper airflow through both coils (and that everything else is functioning normally):
On the low side, at design refrigerated space temp, if the coil outlet superheat is right, then the low side charge is right.
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.
On the high side the subcooling should be no more than 15F/8.5K because this is the point at which liquid will start to back up into the condenser.
On a cap tube/fixed orifice system, if the superheat is right (at design space temp) and the subcooling is not more than 15F/8.5K then the system has the right charge.
Note that minimum subcooling is not a factor on cap tube/fixed orifice systems. They are critically charged.
On a TXV system there is a minimum and maximum charge. When the superheat is right (at design space temp) and the subcooling is not more than 15F/8.5K then there is enough refrigerant in the system to do the job. This is the minimum charge.
Additional refrigerant can then be added to bring the subcooling up to 15F/8.5K. This is the maximum charge.
It is good practice to bring the charge up to maximum because vapor bubbles flowing through the TXV will wear out the needle and seat. This also provides additional refrigerant for heavy loads.Last edited by Gary; 11-07-2009 at 05:55 PM.
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12-08-2009, 04:40 AM #24
Re: Refrigeration 101
Hi Gary, it’s been a long time since we had lunch in Michigan at Bob Evans :-) Hope you have been well.
Quote:
Originally Posted by Gary
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?
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.
......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
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12-08-2009, 06:34 AM #25
Re: Refrigeration 101
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?Last edited by Gary; 12-08-2009 at 06:43 AM.
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16-08-2009, 10:55 AM #26
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24-07-2011, 02:13 PM #27
Re: Refrigeration 101
On AC systems its the outlet side of the condenser, on refrigeration units its usually at the drip since I ususally can't get onto the outlet of the condenser. Wouldn't the liquid line just outside the reciever be the larger/greater subcooling difference?
BTW, someone stoll my books, where can I get replacements?
Rich
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24-05-2010, 06:57 AM #28
Re: Refrigeration 101
Gary,
When you say "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."
At what abient temp are we talking here?
75 degree's?
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25-05-2010, 06:05 AM #29
Re: Refrigeration 101
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28-05-2010, 06:56 AM #30
Re: Refrigeration 101
In my heat-pump trials, I have observed an influence on evaporator SH, due to ambient air temperature movements.
If Ta,in falls during the range of test, SH will roll down a little more than if Ta,in is constant.
If Ta,in rises during the range of test, SH can often end up being almost constant over the test duration.
The influence can be in the range of ~ 1-3K, depending on the operating SH of the machine.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-05-2010, 08:22 AM #31
Re: Refrigeration 101
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-05-2010, 08:31 AM #32
Re: Refrigeration 101
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19-10-2010, 03:17 AM #33
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19-10-2010, 03:16 PM #34
Re: Refrigeration 101
Gary this is great info I do appreciate it could you tell me what is meant by " if the superheat is right at( design space temp) and the subcooling is not more than 15F/8.5K then the system has the right charge.
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11-07-2009, 06:14 PM #35
Re: Refrigeration 101
OR
You can weigh in the charge as marked on the nameplate.
The manufacturer has done all of the work for you, setting up the system under tightly controlled conditions, determining the right charge, and then pulling all of the refrigerant out, weighing it and marking that weight on the nameplate.Last edited by Gary; 11-07-2009 at 06:18 PM.
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02-08-2010, 12:34 PM #36
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12-07-2009, 08:30 AM #37
Re: Refrigeration 101
Thanks so much, Gary.
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.
At what operating temperature would the refrigerant mass charge determination be most appropriate - startup Tc,sat 35'C; or hot end Tc,sat 70'C?
I have no given mass charge to work off in this case & have to determine it in-house.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-07-2009, 03:32 PM #38
Re: Refrigeration 101
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12-07-2009, 07:04 PM #39
Re: Refrigeration 101
Seems like Gary knows his stuff on paper but anygood in the field ?
Transvestites are men who like to eat, drink, and be Mary.
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12-07-2009, 07:20 PM #40
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12-07-2009, 09:01 PM #41
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22-08-2009, 03:10 AM #42
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12-07-2009, 07:27 PM #43
Re: Refrigeration 101
Worse yet... I'm retired.
... after 40 years in the field.Last edited by Gary; 12-07-2009 at 07:47 PM.
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15-07-2009, 11:18 AM #44
Re: Refrigeration 101
Hi, Gary, I am new here and as well new to refrigeration. I have a couple of question about the condenser. Because as I know, the refrigerant pump out from the compressor is in high pressure, then I am confuse after I read through your article. As you said, "need to raise its pressure in condenser until its boiling point temperature is higher than the air"... what exactly is this mean and how to do that actually?
Ps*I am a student which involves in a project which needed to build a refrigeration system, and the part that I am responsible to, is design an air cooled condenser which do not have a fan to cool it, because the system is an outdoor unit, so we plan to use natural wind to cool it, and I do not have any clue on how to start.. can you guide me?
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15-07-2009, 05:34 PM #45
Re: Refrigeration 101
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15-07-2009, 05:37 PM #46
Re: Refrigeration 101
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17-07-2009, 11:50 AM #47
Re: Refrigeration 101
very nocely written. Where can we find a book written like this for a guy with experience ONLY on small splits who neds to raise knowledge to pass the 2079 test...?? Oh im confused by the sample question raie evap temp 1 degree and achieve eff increase 3 degres... why not 2 degres, 3 degres 4 degrees....
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17-07-2009, 12:08 PM #48
Re: Refrigeration 101
It's impossible to make anything foolproof because fools are so ingenious...
Don't ever underestimate the power of stupid people when they are in large groups.
Please, don't teach me how to be stupid....
No job is as important as to jeopardize the safety of you or those that you work with.
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27-07-2009, 05:42 PM #49
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30-12-2009, 07:14 PM #50
Re: Refrigeration 101
yes! I have one of Gary's TECH books and they cut to the chase to real t/s, a great purchase if u can!!!
just a pup on the porch:)
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