Refrigeration 101

[Gary]

Taking this a step further, "dewpoint temperature" and "bubble point temperature" are terms we use to describe a mixture of refrigerants which have different saturation temperatures (zeotropic mixture).

Let's say we have two refrigerants.

The saturation temperature for refrigerant "A" at 0 psi is 10C/50F.

The saturation temperature for refrigerant "B" at 0 psi is 15C/59F.

If we mix these two together and are in the process of adding heat (at 0 psi), the mixture will start to bubble at 10C/50F and will finish evaporating at 15C/59F.

If we are in the process of removing heat from the mixture (at 0 psi), it will start to form dew at 15C/59F and finish condensing at 10C/50F.

The bubble point for this mixture at 0 psi is 10C/50F.

The dewpoint for this mixture at 0 psi is 15C/59F.

The difference between bubble point temperature and dewpoint temperature is called the "glide" which in this case is 5K/9F.

Note that evaporating finishes at the dewpoint temperature, so when looking at the low side pressure, we want to know the dewpoint.

Note also that condensing finishes at the bubble point temperature, so when looking at the high side pressure, we want to know the bubble point.

[Gary]

And then there is pressure.

If we raise the pressure, we raise the saturation temp. If we lower the pressure, we lower the saturation temp.

The pressure/temperature (P/T) chart (aka comparator) for our refrigerant tells us the saturation temp for a given pressure. Or for zeotropic refrigerant mixtures, it tells us the bubble point and/or dewpoint.

For those who confuse zeotropes with azeotropes, here's how I remember which is which:

Just as theist means with religion and atheist means without religion, zeotrope means with glide and azeotrope means without glide.

[desA]

Thanks Gary - excellent.

[faiaz]

Re. Refrigeration 101
great information...,would you like to emphasize some thing on setting the superheat on a refrigeration system.
Fai

[nike123]

If I recollect correctly, it is already explained as "Magoo rule" of 0,65xTD
But more detailed is here:
http://www.kueba.de/en-us/Tools/Küba...s/default.aspx

[Indy2010]

Excellent explanation, thanks Gary

[joe magee]

I will concur with Gary. Keep away from the expansion valve. I've been at this a long time and have started up many supermarkets with long line ups of cases. very rarely do I have to touch a valve. Whats nice now is that some of the cases have sensors on the inlet and the outlet of the coil and supply and return air sensors. Back at the E-2 you can read superheats and splits. Makes it very convenient.

[mickel]

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.

[Windseeker]

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.

AH-HA Moment. Thank You, Thank You, Thank You.

Now, I know why I over-charged the system during pull-down and found 350 PSI hi pressure and frosted lines when I came back.

[patrickj]

Dear Gary,
I have been encountered with aproblem of Oil nonreturning back to the compressor in an R22 system connected to an flake ice plant evaporating at -20C.The flaker is fed through 7nos TXVs and a hand regulating valve.I recon itis an TxV problem and what sort of superheats should yu allow. At the moment ir is set at 7 C.
patrickj

[Gary]

Subcooling should be measured on the liquid line after the receiver. Keep in mind that the liquid line cannot be cooler than the air surrounding it. If the liquid line is in the hot air stream leaving the condenser, it will measure abnormally high.

Move down the liquid line until it enters an area that is surrounded by ambient air. That's where you measure the subcooling.

[exxu]

First of all thank you Gary and all of you for sharing your knowledge.

Second of all, picture this:
- a sistem used in air conditioning: 5 C evap, 40 C cond, txv with no MOP, runing, in normal conditions with SH = 5k and SC = 4K, about 30 kw compressor capacity (it's a purely ipotetic system), R407c
- according with txv producer (let's say Danfoss - TEZ 5 with orifice no.2), the valve has a capacity of 30 kw at a TXV DP of about 12bar; the capacity rises with the TXV DP rise
- with a high load, the superheat rises, the txv opens and the TXV DP decreases thus it's capacity decreases, so the sistem capacity decreses too, leading to a increasing suply air temperature.
What is wrong ?

[chillerman2006]

Subcooling should be measured on the liquid line after the receiver. Keep in mind that the liquid line cannot be cooler than the air surrounding it. If the liquid line is in the hot air stream leaving the condenser, it will measure abnormally high.

Move down the liquid line until it enters an area that is surrounded by ambient air. That's where you measure the subcooling.

Question Sir <Gary>

Liquid line (on water chiller) should be subcooled liquid coming from condensor

Sun beating down on tube-work

I am expecting now saturated liquid (lots of flash gas)

Could you explain for me please (technically or simply put)

What is in this liquid line now ?

R's chillerman

[Gary]

Question Sir <Gary>

Liquid line (on water chiller) should be subcooled liquid coming from condensor

Sun beating down on tube-work

I am expecting now saturated liquid (lots of flash gas)

Could you explain for me please (technically or simply put)

What is in this liquid line now ?

R's chillerman

I think you are grossly overestimating the solar input.

[chillerman2006]

Question Sir <Gary>

Liquid line (on water chiller) should be subcooled liquid coming from condensor

Sun beating down on tube-work

I am expecting now saturated liquid (lots of flash gas)

Could you explain for me please (technically or simply put)

What is in this liquid line now ?

R's chillerman

I think you are grossly overestimating the solar input.

Hi Gary

could you explain further for me

& obviously others would benefit from your expertise here

R's chillerman

[Gary]

I think you are confusing TXV capacity ratings with system output.

Let's say an elevator has a capacity rating of 10 people. Does that mean there are 10 people on the elevator? Nope.

[exxu]

I'm well aware that the things are not going that way. The txv will open to increase the system capacity, to reduce the superheat and the thv dp will decrease. Then what is the table i've attached from Danfoss doc. saying ?

[Gary]

The table is telling you the maximum flow (valve wide open) for a given dP... and you are still confusing capacity with output. Opening the TXV does not increase the capacity of the system, it increases the flow and thus the output.

[Gary]

Given the curvature of the liquid line, along with the reflective properties of the metal and the relatively small size, I suspect the solar heat gain would be negligible. I would certainly not expect lots of flashing.

[chillerman2006]

Given the curvature of the liquid line, along with the reflective properties of the metal and the relatively small size, I suspect the solar heat gain would be negligible. I would certainly not expect lots of flashing.

Thankyou very much Sir,

you have put to bed, once & for all a miss-conception which has been argued back & forth for far too long !

R's chillerman

[Uni]

A basic question, Gary.

Given a system running in Cooling mode, how to raise evaporating temp (or pressure)? Different ways to do so?

My condensing ~50C but evaporating ~ -3, quite low for an A/C.

Try to open the TXV apperture until getting low limit SH but failed to raise Tevap muchly.
Anything to do with indoor air flow rate? I was trying to maintain a 10% lower air flow rate than initial calculation for noise benefit.

[Gary]

A basic question, Gary.

Given a system running in Cooling mode, how to raise evaporating temp (or pressure)? Different ways to do so?

My condensing ~50C but evaporating ~ -3, quite low for an A/C.

Try to open the TXV apperture until getting low limit SH but failed to raise Tevap muchly.
Anything to do with indoor air flow rate? I was trying to maintain a 10% lower air flow rate than initial calculation for noise benefit.

Given a fully active evap coil, the pressure/temperature will depend upon the amount of heat being transferred from the air to the refrigerant. The more heat transfer, the higher the pressure/temperature.... and vice versa. The rate of heat transfer depends upon the surface area, the volume of air flowing through the coil and the temperature of the air flowing through the coil.

It is a mistake to believe that a pressure/temperature is, in and of itself, too high or too low. In truth, the temperature/pressure may or may not be too high/too low for a particular design under a given set of conditions.

In this case, the evap being the outdoor coil, on a cool day -3C is probably not low at all. It depends.

[Gary]

Anything to do with indoor air flow rate? I was trying to maintain a 10% lower air flow rate than initial calculation for noise benefit.

Thereby throwing the entire system out of balance. Air flow is IMPORTANT. Earplugs are cheap.

[Uni]

Thanks for your insights, at least I know that my thought was not far off to explain the situation.
Agree with you, airflow is so important and I hate forcing myself reducing it. But have to factor in noise level, can't make it quiet, cant sell.

I considered Tevap low compared to initial calculated system balance. Such a low evap temp will affect system efficiency as well. Small compressor is now designed for high efficiency and is most efficient at approx Tevap ~ 5C or more. Stucked with airflow and coil size so could not improve it. And if you read my other post, it affects heating mode as well (high head pressure). Too much compromises just for noise.

[texas64]

Gary, what do you think about using compressor superheat to analyse system performance? Checking discharge line temps and subtracting saturated liquid pressure. Something like 50 to 70 degrees as normal range.

[Gary]

Gary, what do you think about using compressor superheat to analyse system performance? Checking discharge line temps and subtracting saturated liquid pressure. Something like 50 to 70 degrees as normal range.

Discharge superheat is directly related to compressor inlet superheat. The higher the inlet superheat, the higher the discharge superheat... and vice versa. I use inlet superheat and it tells me what I need to know. I don't know much about discharge superheat, because it is not something I use.

[spoon man]

Sorry to but in. I work on chillers for industrial. Supplying cooled water for various machines. A sub cooling of 15oc should be acheavable with any system there or about and 5oc superheat give or take. If you are not acheiving this its the same route. Gas charge/condensor fault/evap fault/ tev fault/ and abnormal ambient range. Your gauges are your friend and a simple logical reason. Look for signs like liquid return to compressor(bottom iced up) or tev iced up, mass dt across evaporater, low diff across condenser etc

[spoon man]

In that case check pump curves - completely different game but very easy to grasp.

[chillerman2006]

Sorry to but in. I work on chillers for industrial. Supplying cooled water for various machines. A sub cooling of 15oc should be acheavable with any system there or about and 5oc superheat give or take. If you are not acheiving this its the same route. Gas charge/condensor fault/evap fault/ tev fault/ and abnormal ambient range. Your gauges are your friend and a simple logical reason. Look for signs like liquid return to compressor(bottom iced up) or tev iced up, mass dt across evaporater, low diff across condenser etc

Spoon Man

a condensor has 3 jobs to do when rejecting heat

de-superheating (specific heat rejection)
condensing (latent heat rejection)
subcooling (specific heat rejection)

the last subcooling depends if the coil has been sized accordingly to subcool the refrigerant and to what degree, industrial refrigeration will subcool the already de-superheated/condensed refrigerant but most package chillers dont subcool to 15*c, in fact many struggle to reach 6*c

Exactly what make/model are you referring to ??? @ 15*c many will be overcharged

R's chillerman

[spoon man]

Oh and possibly water blockage on water side or pump failure all water / process side. Anything slowing down heat exchange

[Gary]

Subcooling at 15K is way too high, causing liquid to back up into the condenser, reducing it's effective area and therefore it's efficiency. I would recommend max 8.5K/15F subcooling or clear sightglass, whichever comes first.

[corkman]

i like this post

[L-PLATES]

Hello everyone i am new here and have just passed my 2079 and good to be one of the lads now
this is a great site and really look forward to making new friends

[Fett]

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

[Gary]

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.

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.

[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.

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.

[exhussmann]

This thread is a wealth of good information. It is a shame that the people who need this info the most do not look at / subscribe to free sources of education such as this web site.
Thanks
From the west side of the pond.