Refrigeration 101

[Gary]

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.

[Grizzly]

Very nicely put Gary.
Should be helpful to anyone who has a grey area!
Very easy to understand terminology as well.
Grizzly

[chillyblue]

Great one mate.

Just one question.

Originally Posted by Gary

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.

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

[Gary]

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.

[Gary]

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.

[chillyblue]

Originally Posted by Gary

I first check the delta-T's to confirm adequate airflow through both coils.

What would you expect these to be during high loads, do they differ from when the load is low???

[Gary]

Assuming a fixed speed compressor, only insufficient airflow can raise the delta-T beyond normal limits.

[chillyblue]

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

[Gary]

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.

[nathan...]

hell yeah. this is the good stuff
keep it coming

[Magoo]

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

[desA]

Thanks Gary - excellent stuff.

I'd appreciate if you could touch on refrigerant charge determination, at some point in the thread.

[Gary]

Originally Posted by desA

Thanks Gary - excellent stuff.

I'd appreciate if you could touch on refrigerant charge determination, at some point in the thread.

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.

[Gary]

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.

[desA]

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.

What figures for evap outlet superheat & condenser sub-cooling should be used for an air-to-water heat-pump?

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.

[Gary]

Originally Posted by desA

Thanks so much, Gary.



What figures for evap outlet superheat & condenser sub-cooling should be used for an air-to-water heat-pump?

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.

You should start a new thread for this, as it is specific to your particular system.

[philfridge]

Seems like Gary knows his stuff on paper but anygood in the field ?

[Karl Hofmann]

Originally Posted by philfridge

Seems like Gary knows his stuff on paper but anygood in the field ?

Of course he's not.... He aint REFCON certified..

[Gary]

Worse yet... I'm retired.

... after 40 years in the field.

[nike123]

Originally Posted by philfridge

Seems like Gary knows his stuff on paper but anygood in the field ?

Originally Posted by philfridge

Always hold your head up, but be careful to keep your nose at a friendly level.

Always keep your words soft and sweet, just in case you have to eat them.

.........................

[waisk]

Originally Posted by Gary

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.

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?

[Gary]

Originally Posted by waisk

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?

The compressor raises the pressure and this raises the boiling point/condensing temperature.

[Gary]

Originally Posted by waisk

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?

This is beyond the scope of "Refrigeration 101" so you should start a new thread for your project.

Design is not my forte, but you will find there are others on this forum more qualified than I to guide you.

[gwynn jones]

Originally Posted by Gary

SNIP............Adjusting a TXV aka TEV is almost always a mistake, especially when the refrigerated space is above design temp.

Oh I very much agree -I work with a few lads who will fiddle with the TEV every call - makes even more trouble

[gwm121]

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....

[Josip]

Hi, gwm121

Originally Posted by gwm121

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....

.... they want to show how much you can save changing evaporator temp for 1 degree only ...



Yes, you are right, Gary is making those things very simple in the best possible way ... regarding a good book refer to Gary

Best regards, Josip

[chillyblue]

Originally Posted by Gary

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.

What would be high and low???????

[chillyblue]

TEV adjustment is chicken & egg.

Do you open the tev to get the duty from the evap to get the room temp or do you wait for the room temp to come down before opening it (which may never happen).

If the superheat is high and the room temp is high! where do you start??

CB

[superswill]

Originally Posted by gwm121

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....

try

http://techmethod.com/

a link to gary's web site

[Gary]

Originally Posted by chillyblue

TEV adjustment is chicken & egg.

Do you open the tev to get the duty from the evap to get the room temp or do you wait for the room temp to come down before opening it (which may never happen).

If the superheat is high and the room temp is high! where do you start??

CB

You start by leaving the TXV adjustment alone. Don't even think about it. Adjusting a TXV is almost always a mistake. They do not get out of adjustment... unless somebody screws with them.

If the room temp is high, the superheat is supposed to be high. That's how a TXV works. If the room temp doesn't come down, then something else is wrong.

TXV's should be non-adjustable.

[chillyblue]

Originally Posted by Gary

If the room temp is high, the superheat is supposed to be high. That's how a TXV works. If the room temp doesn't come down, then something else is wrong.

How high is high, 8, 10 ,20, 30, 50???????????
and if it is high what indication can you get from the evap that it is performing correctly,
Some rooms i've worked on can take hrs to pull down, there must be some indication via evap td or air dt that indicates correct operation.
Mistakenly so i've always gone by evap superheat, and now i'm trying to correct myself and an understanding of why is always helpful.

Thanks CB

[lowcool]

mate if you fit tev with changeable orifice and open or close it as to what oneself might think it should be.near to required value why not,apart from experience,where is the bible that says every valve matches every coil?

[luckee]

Very nicely written Gary. Thanks for taking the time to write this thread.

[chillyblue]

Originally Posted by lowcool

mate if you fit tev with changeable orifice and open or close it as to what oneself might think it should be.near to required value why not,apart from experience,where is the bible that says every valve matches every coil?

Thats a great explanation mate!!!!

[Gary]

Originally Posted by chillyblue

How high is high, 8, 10 ,20, 30, 50???????????
and if it is high what indication can you get from the evap that it is performing correctly,
Some rooms i've worked on can take hrs to pull down, there must be some indication via evap td or air dt that indicates correct operation.
Mistakenly so i've always gone by evap superheat, and now i'm trying to correct myself and an understanding of why is always helpful.

Thanks CB

Let's say you have a room that is very warm. You increase the orifice size and you turn the adjustment screw out until you have what you believe to be the right superheat at the bulb. You have matched the refrigerant flow to the heat load.

You are currently overloading the compressor and condenser by absorbing a very heavy heat load in the evaporator. But if the compressor can handle the overload, the room temp will drop quickly, leading you to believe that you have done the right thing.

As the room comes down to its design temperature the TXV starts hunting (oversized orifice) and flooding (incorrect TXV adjustment). Why? Because the heat load is now insufficient to boil off the excessive flow of liquid refrigerant through the TXV.

If you have already left the job site, you may not even be aware that the system is hunting, flooding, flushing the oil out of the compressor and potentially going into self-destruct.

[Gary]

Originally Posted by chillyblue

How high is high, 8, 10 ,20, 30, 50???????????
and if it is high what indication can you get from the evap that it is performing correctly,

Your indication comes from the condenser. Given a normal load, and assuming good condenser airflow, the SCT should be 20-35F/11-19K above the ambient (condenser air in) temperature (20-35F/11-19K cond TD), depending upon the relative coil sizing. If the heat load being rejected by the condenser is normal, then the heat load being absorbed by the evaporator must also be normal.

[chillyblue]

Originally Posted by Gary

Your indication comes from the condenser. Given a normal load, and assuming good condenser airflow, the SCT should be 20-35F/11-19K above the ambient (condenser air in) temperature, depending upon the relative coil sizing. If the heat load being rejected by the condenser is normal, then the heat load being absorbed by the evaporator must also be normal.

Thanks Gary, thats great help thanks.
could you confirm what happens if you have a high load and a low load??
I'm assuming with a high load the SCT will reduce below 11-19K above air on to condenser and with a low load the SCT will increase to more than 11-19k above air on to cond.
I'm also assuming that under high conditons your subcooling will be less and with low load conditions your subcooling will more

[Gary]

Originally Posted by chillyblue

Thanks Gary, thats great help thanks.
could you confirm what happens if you have a high load and a low load??
I'm assuming with a high load the SCT will reduce below 11-19K above air on to condenser and with a low load the SCT will increase to more than 11-19k above air on to cond.

Just the opposite. High load increases SCT and low load decreases SCT... and SCT above ambient (cond TD) should remain within the 11-19K range.

[Gary]

Originally Posted by chillyblue

I'm also assuming that under high conditons your subcooling will be less and with low load conditions your subcooling will more

There will be relatively little change in subcooling as the liquid line temp will tend to rise and fall with the SCT and a TXV system stores sufficient surplus liquid refrigerant to compensate for variations in load without substantial changes in subcooling.

[chillyblue]

Originally Posted by Gary

Just the opposite. High load increases SCT and low load decreases SCT... and SCT above ambient (cond TD) should remain within the 11-19K range.

If the ambient air onto the condenser remains the same and the sct decreases with low load then the cond td must get bigger.
If the ambient air onto the condenser remains the same and the sct increases with high load then the cond td must get smaller.

do you mean that with a high load the cond td would be around 11k (smaller) and low load would be around 19k (larger)

CB

[Gary]

Originally Posted by chillyblue

If the ambient air onto the condenser remains the same and the sct decreases with low load then the cond td must get bigger.
If the ambient air onto the condenser remains the same and the sct increases with high load then the cond td must get smaller.

do you mean that with a high load the cond td would be around 11k (smaller) and low load would be around 19k (larger)

CB

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 .

[chillyblue]

Thats right, thanks

I assume that is the same with your evap the
high load big td
low load small td

CB

[Gary]

Originally Posted by gwynn jones

Oh I very much agree -I work with a few lads who will fiddle with the TEV every call - makes even more trouble

In another thread Magoo has offered a rule for judging TXV superheat. He says the superheat should be 60-70% of the TD. This makes very good sense. I like it... a lot. :

[Frikkie]

Very helpful and very nicely written. Thank you Gary.

[gazsweet]

Excellent, great to read. just need an apprenticeship now. its so hard to find!!!

[Rich1281]

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

[Gary]

Oops... hit the wrong button.

[Gary]

Originally Posted by Rich1281

Hi Gary, it’s been a long time since we had lunch in Michigan at Bob Evans :-) Hope you have been well.

Hi Rich. A very long time. Nice to hear from you again.

Originally Posted by Rich1281

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?

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.

Originally Posted by Rich1281

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. ?????

Are you measuring subcooling on the liquid line near the receiver?... or on the drip leg between the condenser and the receiver?

[keepmecool]

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

[chillyblue]

Originally Posted by Gary

Are you measuring subcooling on the liquid line near the receiver?... or on the drip leg between the condenser and the receiver?

Does this make a difference?? if so why??

Originally Posted by Rich1281;

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. ??????

I've seen this, but i am very doubtfull how the system will cope when the ambient air into the condenser reaches 32 deg C (design temp) I can see hp switches tripping.

CB