Subcooling

[samisonfire]

I've been working in the refrigeration industry for 10yrs or so, and as a service and maintenance engineer for the last 3yrs.

I get by fine using the techniques i have picked up along the way but i feel i could better myself massively.

Unfortunately i don't have anyone to look upto as a mentor so i'm finding it hard to progress as an engineer.

I would like to learn how to use subcooling and superheat as a diagnostic tool.
I've been looking up on it through text books and the internet and have found some helpful stuff, but when attempting to put it into practice today i struggled.

I was working on a freezer cold room.
The evaporator had excessive ice build up in patches.
Ice was forming at the top left (inlet) and in a V shape in the top centre. Also the the TEV and surrounding pipes were covered in ice but i don't see that being a problem.
I forced electric defrost and manually defrosted with warm water. Checked defrost settings on controller and found 0 drip time. I adjusted to 10 mins.

After doing all that i thought i'd check the subcooling.
I connected my gauges to the liquid line access port and taped my thermometer probe to the liquid line near the condensor.

Liquid pressure was 12 bar, so 26c in temp, with an ambient of 14c.
Liquid line temp was 30c.
This confussed me as it seems to of GAINED 4k rather than lost any???
Suction pressure of 1 bar.

I know the liquid pressure is low and i have suggested fitting a fan speed controller, but would this effect the subcooling?

The system is coming down in temperature very well and cycling as it should do.

I didn't get round to checking the superheat as i would of had to take the evaporator casing off which was a bit of a task and i was running out of, what with it being a Friday!

I've got literature explaining how to measure subcooling/ superheat and what temperature differences to expect but nothing about abnormal readings.

I know i should know all these techniques but have forgotten them from my college days and have not had much guidance/help from my employers.

If anyone has any answers for me i would be very grateful.

[Gary]

Maybe this will help:

http://www.refrigeration-engineer.com/forums/showthread.php?t=24475

[cool runings]

I've been working in the refrigeration industry for 10yrs or so, and as a service and maintenance engineer for the last 3yrs.

I get by fine using the techniques i have picked up along the way but i feel i could better myself massively.

Unfortunately i don't have anyone to look upto as a mentor so i'm finding it hard to progress as an engineer.

I would like to learn how to use subcooling and superheat as a diagnostic tool.
I've been looking up on it through text books and the internet and have found some helpful stuff, but when attempting to put it into practice today i struggled.

I was working on a freezer cold room.
The evaporator had excessive ice build up in patches.
Ice was forming at the top left (inlet) and in a V shape in the top centre. Also the the TEV and surrounding pipes were covered in ice but i don't see that being a problem.
I forced electric defrost and manually defrosted with warm water. Checked defrost settings on controller and found 0 drip time. I adjusted to 10 mins.

After doing all that i thought i'd check the subcooling.
I connected my gauges to the liquid line access port and taped my thermometer probe to the liquid line near the condensor.

Liquid pressure was 12 bar, so 26c in temp, with an ambient of 14c.
Liquid line temp was 30c.
This confussed me as it seems to of GAINED 4k rather than lost any???
Suction pressure of 1 bar.

I know the liquid pressure is low and i have suggested fitting a fan speed controller, but would this effect the subcooling?

The system is coming down in temperature very well and cycling as it should do.

I didn't get round to checking the superheat as i would of had to take the evaporator casing off which was a bit of a task and i was running out of, what with it being a Friday!

I've got literature explaining how to measure subcooling/ superheat and what temperature differences to expect but nothing about abnormal readings.

I know i should know all these techniques but have forgotten them from my college days and have not had much guidance/help from my employers.

If anyone has any answers for me i would be very grateful.


As for the ice build up.... We will deal with that later.

Subcooling.

The 26 deg on the gauges if correct is the temp that the refrigerant is changing from vapour to liquid (the condensing temp) if your thermomiter reads higher then the refrigerant is not totaly condensed to pure liquid (there is still some vapour in it).
Refrigerant directly out of the comp would be at about 50 or 60 deg C so if the condensing temp is 26 and the liquid line is 30 deg then somthing is very wrong ;)

Now you need to find out what is wrong:p

Is the condenser blocked?
is the system short of gas?
Is the condenser fan motor running OK?
Is the fan blade the correct one?

You need to find out why the liquid line is higher in temp than the condensing temp.

On a normal air cooled condenser the subcooling will be about 3 to 6 deg C bellow, not much more.

Cheers cool runnings.

Ps you are right in your thinkings and you can never learn too much.

Cheers cool

.
.

[lana]

Hi there,

Measuring Sub cooling is little bit tricky??!!

First, if there is a liquid receiver then you have to measure the liquid line temp before that, i.e. exit of the condenser.
Second, when there is a head pressure control like fan cycling then this issue becomes more complicated.
Third, your thermometer must be accurate enough for this purpose. We are talking about around 5 or 6°C here and if the thermometer is not accurate then you will get confusing results.

NOW this happens when you have fan cycling :
At a set condensing pressure a fan is turned off and pressure starts to rise again. Normally after around 30Psi fan starts again. During this time liquid temperature changes and your thermometer needs time to register the temp.
In this case you have to maintain the head pressure at a constant value for a few minutes so you can get a correct readings of the liquid temp.
What you have to do is this : turn on and off the fan manually so you can get a constant head pressure, then let the thermometer register the correct temp. this requires Patience!??
In my experience you need a reliable thermometer in this job and a correct type. Here is a guide :
There are two types of thermometers 1- Thermocouple 2- Pt-100
1-thermocouple type has a very short response time (about 3sec) but low accuracy.
2- Pt-100 has a long response time (about 40Sec)but high accuracy.
Whatever type you use, you have to know their characteristics.
I suggest that you get a Pt-100 type.

Hope this helps a bit.
Cheers

[Lowrider]

good contact between ur thermometer and the pipe helps getting a good measurement too. and adding some insulation around it, to prevent measuring part ambiant, part pipe temperature.
there are good thermocouple available on the market. Good 3-point temp sensors are available for reasonable prices

[samisonfire]

Thanks to everyone for the comments.

Regarding your comments Cool runnings:

Condensor coil clean as a whistle.
Suction pressure of 1bar which is spot on for a freezer running on R404a, full sight glass and reaching -21c with no problems so not short on gas.
Condensor fan motor working fine and the fan blade has never been changed.

Surely if the condensor was blocked or had fan motor/blade issues then my head pressure/temperature would be higher than expected, not lower.

Am i correct in saying that i want to have a head pressure of around 15bar on this system?
If i did have 15bar then that would convert to 35c and assuming the liquid line temp stayed the same, (30c) that would give me an exceptable subcooling of 5k.

Not sure if the liquid line temp would rise as the condensing temp did?

I have 2 thermometers, tested one against the other and there was 1 degree difference between them so i don't think either of them are too far out of calibration.

I taped the probe, using armaflex tape to the liquid line about 300mm from the condensor.
The air flow from the condensor has no effect on my readings as there is a panel seperating the condensor coil/fan from pipe work/compressor etc.
There is a liquid receiver and my probe was positioned before that.
There isn't currently any fan speed controller so that is not effecting my readings.
I am thinking out putting a fan speed controller on this system so that i can maintain a decent head pressure.
Is this the right thing to do?
What are the effects of over condensing?

Thanks everyone, Sam

[rkmoorthy]

hi any one give me somre details of pressure for suction side and discharge side for refrigerants of given below. because of i dont have much knowledge of reefer containers,chiller, and freezers also.

134a chiller.......freezer.........reefer........

404a chiller.......freezer.........reefer........

r22 chiller.......freezer.........reefer........

r12 chiller.......freezer.........reefer........

r11 chiller.......freezer.........reefer........


r407c chiller.......freezer.........reefer........

i will be thankfull for if you give more details about what i ask.

[Gary]

I have 2 thermometers, tested one against the other and there was 1 degree difference between them so i don't think either of them are too far out of calibration.


Hook your gauge up to a refrigerant container. Strap your thermometers to the side of the container. They should all indicate the same (ambient) temperature.

In particular, I suspect your gauge in out of calibration.

[cool runings]

Thanks to everyone for the comments.

Regarding your comments Cool runnings:

Condensor coil clean as a whistle.
Suction pressure of 1bar which is spot on for a freezer running on R404a, full sight glass and reaching -21c with no problems so not short on gas.
Condensor fan motor working fine and the fan blade has never been changed.

Surely if the condensor was blocked or had fan motor/blade issues then my head pressure/temperature would be higher than expected, not lower.

Am i correct in saying that i want to have a head pressure of around 15bar on this system?
If i did have 15bar then that would convert to 35c and assuming the liquid line temp stayed the same, (30c) that would give me an exceptable subcooling of 5k.

Not sure if the liquid line temp would rise as the condensing temp did?

I have 2 thermometers, tested one against the other and there was 1 degree difference between them so i don't think either of them are too far out of calibration.

I taped the probe, using armaflex tape to the liquid line about 300mm from the condensor.
The air flow from the condensor has no effect on my readings as there is a panel seperating the condensor coil/fan from pipe work/compressor etc.
There is a liquid receiver and my probe was positioned before that.
There isn't currently any fan speed controller so that is not effecting my readings.
I am thinking out putting a fan speed controller on this system so that i can maintain a decent head pressure.
Is this the right thing to do?
What are the effects of over condensing?

Thanks everyone, Sam


Hi Sam.

404a would be ok at 15 to 20 bar ( I've seen it as low as 10 using liquid amplification) so no probs there.

Liquid temp on a normal air cooled cond can't be as low as the ambient air temp so about 5 k subcooling and your ambient would be somewhere around 25 deg C??

But your liquid pressure was 12 bar, so 26c in temp, with an ambient of 14c.
Liquid line temp was 30c.

So I would look to unknowns??

If you are confident about you thermomiters are you confident you gauge is callibrated??

But your logic and reasoning is sound :)
You just have to find the problem :p

cheers

cool

.

[RANGER1]

If your ambient is 14 deg C your head pressure is OK . If TX valve was struggling and high superheat then maybe worry about increasing head pressure .
TX valves are selected with a certain min pressure drop . If it gets below that then you may expect problems with s/heat etc .
I think Gary said it somewhere but measure the subcooling on liquid line out of reciever not inlet and see what you get .
Do you have a sight glass to see what liquid line is doing as well ?
Without checking superheat you also still don't know whats really going on .

[nike123]

:off topic:
@rkmoorthy

Do not steel other members threads! Start your own, it is easy. If you don't know how, read FAQ (http://www.refrigeration-engineer.com/forums/faq.php).
Regarding your question:

http://www.refrigeration-engineer.com/forums/showthread.php?t=11383&highlight=pressures+meaningless

[samisonfire]

I've hooked my gauges upto my 404a bottle and strapped my thermometer to the side to find equal readings so no problem with either of those i don't think.
As far as i can tell there isn't actually a problem with the cold room i'm refering to, apart from the ice build up of course. I just thought i would take the oppertunity to check the subcooling as i have never taken such a reading before. Unfortunately i didn't get the readings i was hoping for so thats why i asked you pro's. I'm back on site tommorrow with any luck so i shall gather all the information needed to make a proper judgement. I'll try taking the liquid line temp from after the receiver and see if that makes any difference also.
All the information i have been given has been very helpful, thanks to everyone.
It has made me realise that there is so much more information that can be gathered from a system to diagnose a fault. I'm going to try my best to learn these techniques then maybe one day i'll be up there with you pro's, ha!

cheers guys, Sam

[Gary]

With the following, we could do a full evaluation:


Evap air in temp
Evap air out temp
Saturated suction temp (convert from pressure)
Suction line temp at evap outlet
Suction line temp at compressor inlet

Cond air in
Cond air out
Saturated condensing temp (convert from pressure)
Liquid line temp at receiver outlet

The most difficult to measure is usually the suction line temp at the evap outlet. I generally go to the compressor inlet first.

If the compressor inlet superheat is good then the evap outlet superheat is probably good.

If the compressor inlet superheat is not right, then I need to check the evap outlet superheat.

[El Padre]

Hi Sam,
The subcooling value will tell you how much liquid refrigerant is in the condenser, you will obviously have to take other readings as Gary reccommends to diagnose the system as a whole.

Cheers

[samisonfire]

I was attempting to take all the measurements you suggested Gary but have come to the conclusion that maybe my thermometers are not as good or as accurate as i may of thought.
I've got a k-type digital thermometer which seems to fluctuate far too much, and a multi meter with a probe which doesn't seem to work very well in low temperatures. It's a good multi meter but not sure if it doubles up as a particulary good thermometer.

I've got some more probes coming for the k-type then i'll check the calibration. Until then i think i'm going to stop confusing myself with possible false readings.

I'm very grateful for everyones help

Thanks, Sam

[nike123]

You need this:
http://www.fluke.co.uk/comx/show_product.aspx?pid=9472&type=1&locale=uken
http://www.fluke.co.uk/comx/show_product.aspx?pid=21454&locale=uken&product=DTHE

Or this with probes:

http://www.testo.com/online/abaxx-?$part=PORTAL.INT.SectorDesk&$event=show-from-menu&categoryid=1223923
0602 4592
0628 0020

Or this:
http://www.trutechtools.com/Digi-Cool-1250-Dual-Temp-wback-light_p_742.html

Or this:
http://www.hotektech.com/Tes5602.htm

Dont vaste money on others.

[nike123]

The most difficult to measure is usually the suction line temp at the evap outlet. I generally go to the compressor inlet first.



Gary, what tool you use for temperature measurements?

[Gary]

Gary, what tool you use for temperature measurements?

I use a Fluke.

[nike123]

I use a Fluke.

I presume 80PK-8 probe!
Are you satisfied with its accuracy and repeatability?

[Gary]

Two probe... one of them a clamp-on. I've had it a long time... no problems.

[nike123]

Two probe... one of them a clamp-on. I've had it a long time... no problems.
Thanks Gary! I trying to decide between Testo and Fluke.

[Gary]

Thanks Gary! I trying to decide between Testo and Fluke.

I told the salesman if he could bounce if off the ceiling/floor and it still worked, I would buy it.

He did... it did... and I did. :)

I'm not nice to my tools.

[nike123]

I

He did... it did... and I did. :)

I'm not nice to my tools.

Me too!
That is why I like tools like this:
http://www.shookmfg.com/images/technician_whitepaper_img_5.jpg


http://www.youtube.com/watch?v=yqGAMurcfTQ
:D:D

[desA]

You blokes are tough customers to please. :D

[nike123]

You blokes are tough customers to please. :D

We have privilege to not work in laboratory, like you, poor developing engineers.;):D

[FreezerGeezer]

For what it may be worth, I was taught that your condensing pressure should be within 1/2 a Bar of 10*C above air on temp, and the suction pressure at the evaporator outlet within 1/2 Bar of 7*C below the air off temp.
This is for refrigeration only. Air con is a bit different.
Also, I am concerned about the frozen over TEV. A frozen outlet and distribution line into the evaporator shows a probable lack of refrigerant.
A completely frozen over TEV shows a blockage in the valve. A completely frosted over evaporator shows lack of air flow or maybe lack of heat load.
From your initial description it sounds like there's a lack of refrigerant. Have you checked for leaks and / or blockages in the system?

[samisonfire]

Thanks for the links Nike, if i had the money i would definitely invest in the digi-cool, that looks like a very good bit of kit.

[samisonfire]

For what it may be worth, I was taught that your condensing pressure should be within 1/2 a Bar of 10*C above air on temp, and the suction pressure at the evaporator outlet within 1/2 Bar of 7*C below the air off temp.
This is for refrigeration only. Air con is a bit different.
Also, I am concerned about the frozen over TEV. A frozen outlet and distribution line into the evaporator shows a probable lack of refrigerant.
A completely frozen over TEV shows a blockage in the valve. A completely frosted over evaporator shows lack of air flow or maybe lack of heat load.
From your initial description it sounds like there's a lack of refrigerant. Have you checked for leaks and / or blockages in the system?

Thanks for your message freezergeezer,

The rule of thumb about the head pressure is very helpful for me.
I use the 10c below satuarated suction temperature for suction pressure rule but did not know of the head pressure one.

Last week when i first started this thread the system in question had various areas of ice build up. The top centre and top left had quite servere ice, and the entire side, where the TEV is located was full of ice.
The ice around the TEV had been gathering over a long period of time, (i know this as i have been maintaining it since it was installed). I had noticed this for some time but didn't bother doing anything about it as it wasn't causing any issues. Seen as i was defrosting the coil i spent some time on that aswell.

The kitchen staff, when loading the cold room, open the door and hang the curtains over the top for easy access thus allowing a boat load of moisture in.
I have obviously recommended that they don't do this but i can only recommend.

I thought it was normally to get a certain amount of ice build up on/around the TEV in a freezer? and with this amount of moisture getting in it would gradually get worse and worse.

Last week the suction pressure was 1bar,(R404a) had a full sight glass and the temp was coming down to -21 in no time. Head pressure was 12bar and ambient was 14c so that's fine according to your rule of thumb.

This week suction pressure was down by about 5psi but sight glass was still full and cabinet temp fine. Ambient and head very similar.
I'm not sure whether to trust my thermometer readings so i'm going to hold out until i get a new probe. I actually got a new probe today so i'll check the subcooling/superheat next week, then hopefully i get correct readings.

I checked the defrost settings on the controller to find no drip time so i adjusted it to 10mins, this week the evap coil was clear of ice but i didn't check the TEV.

Is what i'm saying about the TEV getting some ice on it correct or is it always an indication of a lack of refrigerant?

Thanks, Sam

[samisonfire]

Sorry, i meant 10c below desired cabinet temperature for sst/suction pressure.