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View Full Version : A/C Design spec. Help I'm a bit rusty



lonemascot
04-03-2010, 03:23 PM
I'm a little embarassed to be asking these questions as I really should know this stuff but as a past installation engineer (hardly ever doing any service work) and having quite a few years break from the industry, I have forgotten it all :confused:

I recently started a new job as an air con inspector and need to know a few bits of info.

1) I'm asked to record the temperatures of the suction and discharge temp and comment on what it means. Is there a quick way to get these temps without taking the casing off the units i.e. taking temp at the top of the condenser and on the suction pipe entering the unit. Will the reversing valve on heat pumps cause dodgy results?

What exactly am I looking for here?

2) When I was at college we were given design spec sheets which listed probably temp increases/decreases across the system:
air on - air off temps,
design superheat,
temp gained across the compressor,
sub cooling temps etc.
Does anyone have a link to similar info? With this info I could establish from my results where the system is not performing.

3) Also, I've lost my trusty slidey comparitor. Does any one have a link to a chart of refrigerant pressures at different temps?

I understand I should be shot at dawn for asking these questions but I'm so rusty its not even funny.

Thanks for all your help.

Jon

Brian_UK
04-03-2010, 08:40 PM
No offence Jon but if you have just completed your expensive inspectors course and are asking these basic questions I would query the effectiveness of the course.

3) There are hundreds on the web.

lonemascot
05-03-2010, 01:12 AM
No offence Jon but if you have just completed your expensive inspectors course and are asking these basic questions I would query the effectiveness of the course.

3) There are hundreds on the web.

You are right, the course was a pile of crap. I was hoping for a quick refresher and what I got was a bloke playing a slideshow and reading off the board in the most boring voice ever. Nothing to do with the A/C process at all, just a load of waffle about the types of units we might come up against etc.

Like I said in my original post. I was an installation engineer and very, very rarely did any service work. Therefore, I never had to take temps or pressures. Plus I haven't worked in the field for ages and like they say, if you don't use it you lose it. Which I have.

You obviously know the answers, so instead of confirming that I should know this stuff, how about helping me out.

Thanks

Gary
05-03-2010, 02:51 AM
Maybe this will help get you started:

http://www.refrigeration-engineer.com/forums/showthread.php?t=19701

lonemascot
05-03-2010, 04:06 AM
Hi Gary thanks for the link but I have already read the entire thing. The opening post is very good. You should definitely add to it. I understand the principles of how an air con system works very well, pressure/temperature relationships, states of gases etc.

What I'm looking for is typical values for the following:
temp difference across coils for evap/cond air on/off
temp difference between air off and evap/cond temp
amount of superheat added to evap temp
amount of sub cooling taken from cond temp
temp difference of suction/discharge and ambient
temp difference across expansion device

Regarding where I can take the temp for suction and discharge; on packaged air con units the visible pipes are both the low side (to and from the evaporator with the txv inside the condenser unit) so i cant use these. Will i get accurate suction/discharge temp readings by taking the temp from the top of the condenser coil and the suction line pipe that is easily accessible. I dont want to be removing casings all day long?

Once I have these 2 temps. I'm supposed to make a comment upon the units functionality being normal or abnormal.

My memory is vague, is it right to assume that if the discharge is unsusually hot there may be a blockage in the TXV, an air flow restriction across the condenser coil, the system is overcharged or a pump down valve has been shut. If the suction is unsusually low then the system may be undercharged, there is an air flow problem across the evaporator, the TXV is seized in the almost shut position or a pump down valve is closed. If the temperatures of the suction and discharge are similar then the compressor valves are faulty or the reversing valve is seized halfway between modes. Please tell me if I'm completely wrong or have missed out anything.

You obviously know your stuff from reading 101 so I'd be grateful for your help. Actually, I'm sure your answer to these questions could find a relevant spot in there for the newbies.

Thanks

mad fridgie
05-03-2010, 04:19 AM
I do not know if this will help, but you can judge a system of a couple of temperatures.!
Why do you not purchase one of those Testo, AC analisers (spelt wrong but could true) I think they have some sort of pretermined rule. You connect to the system, hey presto, gives all the answers need.
Never used one, but have seen them mentioned here on RE

Gary
05-03-2010, 04:48 AM
Regarding where I can take the temp for suction and discharge; on packaged air con units the visible pipes are both the low side (to and from the evaporator with the txv inside the condenser unit) so i cant use these. Will i get accurate suction/discharge temp readings by taking the temp from the top of the condenser coil and the suction line pipe that is easily accessible. I dont want to be removing casings all day long?

Let's start with this:

The discharge line temperature drops rapidly between the compressor and the condenser. The line temp 6" from the compressor is generally considered to be the "discharge temp".

Discharge temp is not a symptom I use for diagnostics. I prefer to use compressor inlet superheat, which tracks well with discharge temp.

I'm not sure what you mean by 'suction temp'. There is the suction line temp at the evap outlet, compressor inlet and everywhere in between.

lonemascot
05-03-2010, 04:02 PM
I meant suction line temperature.

I don't know why they want the suction and discharge line temps but they do. The report is made up by CIBSE pencil pushers that have little or no field experience. They even call the compressor a processor in the report!

All I would have done was take temps of air on and off of both coils, take a measurement of the super heat in the suction line and maybe check that the condenser was operating adequately by checking the sub cooling. But if its superheating correctly then it should be sub cooling corectly...I think.

Gary
05-03-2010, 05:48 PM
I meant suction line temperature.

I don't know why they want the suction and discharge line temps but they do. The report is made up by CIBSE pencil pushers that have little or no field experience. They even call the compressor a processor in the report!


Or possibly they are referring to saturated suction temp (SST) and saturated condensing temp (SCT). You may want to contact CIBSE for clarification.

Gary
05-03-2010, 06:41 PM
temp difference across coils for evap/cond air on/off


This is called the coil delta-T (dT). If I were looking for a quick way to give a thumbs up/thumbs down for an A/C system it would be the dT's.

Generally, an A/C system should be able to achieve design room temp at about 50%RH. In order to do this, the temp of the air leaving the coil (air off temp) should be about 11K below the room temp. The air entering the evap may or may not coincide with the room temp, so for accuracy I would use the temp at the thermostat instead of the evap air on temp.

It is best to measure the air off temp close to the coil, as the temp may rise downstream. An alternative would be to use supply air wetbulb temp, as this does not change from the coil to the supply vent.

If the evap dT is much more than 11K, there is an airflow problem. If the evap dT is much less than 11K, there is a system problem or high humidity problem.

Which brings us to the cond dT. I would expect the cond dT to be somewhere between 11-17K. More than 17K would indicate an airflow problem. Less than 11K would indicate a system problem (not picking up a normal heat load).

If the evap dT is low, but the cond dT is normal, then it is picking up a heavy latent load (high indoor humidity)... or leaking return air duct... or possibly the evap fan speed is way too high.

Of course, all of this assumes the compressor is fixed speed (or running at 100%). Altering the compressor speed and/or fan speeds changes everything.

lonemascot
05-03-2010, 07:46 PM
It is strange why they would want the discharge line temp. I shall contact the accreditation scheme people to find out exactly what they want to establish because as you say, your method would be a quick and easy way to establish system performance.

What about inverter units where the compressor runs at different speeds to compensate for different loads. can you not do the same tests then? Surely you would still expect similar dT. The velocity of refrigerant would slow through the system with a lower compressor speed and would allow for greater heat pick up throught the evaporator?

lonemascot
11-03-2010, 02:27 AM
I've done a bit of research and I think I've sussed it.

I reckon I'm supposed to take temperatures to find out the evap and cond temps so that i can work out the Co-Efficient of Performance (COP). After all its an inspection to improve energy efficiency so this would give me a quick indication of current efficiency.

Please tell me if I'm talking rubbish but so far I have it as follows:

Actual COP= (Evap temp (K) / (Cond temp (K) - Evap temp (K))) x 0.5

Eg. For an R22 system with outdoor ambient of 30C:
COP = (278K / (318K - 278K)) x 0.5
COP = 3.475

Which gives me 3.475kW of heat transfer power for every electrical kW put in to the compressor.

To get the Evap temp I would take the suction line temp at the compressor and take away the 3.5-5.5K of super heat. In practice this would mean taking the temp of the suction line entering the outdoor unit before the compressor.

To get the Cond temp I would take the liquid line/sub cooled temp and add 5.5-8.5K of sub cooling. In practice this would mean taking a temp right at the bottom of the condersor coil or liquid line before the expansion device. Bear in mind that the exp device is usually in the outdoor unit in packaged systems.

In the report they actually ask for suction and discharge line temps so if I was to take the discharge temp right next to the compressor, what temp would i need to subtract from it to give me the condensing temp?

Am I on the right track here?

I could attach gauges to get these pressures/temps easily but I'm not Fgas registered and secondly I don't think its a good idea attaching gauges for no reason due to a slight refrigerant loss every time it gets done.

Ok, so say i have the COP. What is the best attainable COP for an air con system providing comfort cooling only?

I really appreciate any help you guys can give me.