What does your pressure gauage actually measure?

[acnerd]

Hello everyone, I am new and finding all these topics very interesting. Here's 2 questions for you: I was speaking to a seasoned fridge engineer a few months ago, and told him I needed to measure the liquid pressure to work out subcooling, but there wasn't an access point for me. He told me that it would be ok to measure compressor discharge pressure, because the pressure gauges actually measure liquid pressure in the system, and not vapour. Is this true?

Second question : how do you work out suction pressure drop if you are working out superheat on the compressor suction return due to an inaccessible evaporator? (ie. 6 meters in the air)

[littleyapper]

do you know you are breaking the law?
get your seasoned fridgie to tell you about certification and f gas regs
you cannot break into a system even if you are an ac nerd

[acnerd]

do you know you are breaking the law?
get your seasoned fridgie to tell you about certification and f gas regs
you cannot break into a system even if you are an ac nerd

When I said new, I meant on this forum! I hold a current refrigerant handling certificate and ten years experience in a/c and fridge, I was just starting a discussion.
Do you have an answer???????

[taz24]

Hello everyone, I am new and finding all these topics very interesting. Here's 2 questions for you: I was speaking to a seasoned fridge engineer a few months ago, and told him I needed to measure the liquid pressure to work out subcooling, but there wasn't an access point for me. He told me that it would be ok to measure compressor discharge pressure, because the pressure gauges actually measure liquid pressure in the system, and not vapour. Is this true?

Second question : how do you work out suction pressure drop if you are working out superheat on the compressor suction return due to an inaccessible evaporator? (ie. 6 meters in the air)

To know the answers to your questions you need to understand fully how a refrigeration system works.
AC systems are different to fridge systems because most splits start the evaporator effect in the outside unit. The supply line to the indoor evap is at evaporator pressure.
The suction is measured at the comp end on most AC systems and therfore any measured superheat will have an amount of discrepency built into it, dependant of the pressure drop over the suction line.
On most splits you have no control over the superheat because the system is set up for a minimum and maximum distance of pipe run and then for protection they install suction slop pots to protect the comp from any liquid floodback.


With the high pressure systems (R410) they only fit one gauge port because the pressure on the high side is extremly high.

You need to go back to basics and work from there.

Cheers taz.

[acnerd]

Thanks for the reply Taz, I agree with what you are saying. That's why they insulate the "liquid line" on a/c units, because it starts evaporating at the outdoor unit.

But my original questions remain unanswered. Say for example you have a downflow cabinet with a remote condenser and you want to measure subcooling. The ideal would be to measure actual liquid temperature and pressure at the same point, but not always possible, so is it ok to measure compressor discharge pressure and assume it is liquid pressure? That is what I have been told, but not sure if it is correct. What about pressure drops, and how do you calculate them?

PS: Never heard an accumulator been called a slop pot before, but it describes it perfectly! ;-)

[chillyblue]

Thanks for the reply Taz, I agree with what you are saying. That's why they insulate the "liquid line" on a/c units, because it starts evaporating at the outdoor unit.

But my original questions remain unanswered. Say for example you have a downflow cabinet with a remote condenser and you want to measure subcooling. The ideal would be to measure actual liquid temperature and pressure at the same point, but not always possible, so is it ok to measure compressor discharge pressure and assume it is liquid pressure? That is what I have been told, but not sure if it is correct. What about pressure drops, and how do you calculate them?

PS: Never heard an accumulator been called a slop pot before, but it describes it perfectly! ;-)

Hello mate
Sounds like youv'e had a right roasting for your first post (unlucky)
Firstly your gauges measure pressure (wether it's liquid under pressure or gas)
Subcooling can be measured using your discharge pressure rather than the liquid pressure, but your measurements will not be quite as accurate (depends on pipes lengths and sizes)
Also the same with superheat, yes the best place to read superheat from your evaporator is at the outlet of the evap right next to the tev bulb, but i'm the same as you it's not always that simple if you have a evap 6 mtrs in the air, normally i would connect my gauges as close as poss to the evap but if thats not possible i would connect my gauges to the compressor suction valve if no other place nearer to the evap is possible and work from there, but again your measurements will not be 100% accurate, but near enough.
Please bare in mind if the problem is so bad then you will have to get to the evap, also you need to make sure you have no other controlling valves (cpr's, epr's or sv's etc) on the returning suction line as these could make a massive difference.
I assume that if you can't get up to the evap you can't adjust the tev anyway (assuming it's close to the evap as it should be.)

hope thats a start for you.

CB

[Peter_1]

do you know you are breaking the law?
get your seasoned fridgie to tell you about certification and f gas regs
you cannot break into a system even if you are an ac nerd

Why he is breaking the law? I haven't read anything in his post which tells me he's breaking the law.
This besides, hold quotes as those better for yourself and try to contribute in a positive way by responding the initial question.
Prove you're qualified and give this man a proper answer.

[lowcool]

measure superheat with a six metre ladder it wont do it by itself,setting etc unless its eev

[chillyblue]

Well said peter_1

[acnerd]

Thanks Chillyblue for a good answer!! I did puzzle over how the gauges could differentiate between liquid or vapour pressure, but as you have said, it's PRESSURE after all.

So if I understand you correctly, unless I have very long pipe runs, or a big height difference, pressure drop shouldn't be a concern?

[acnerd]

Thanks Peter_1. It was a "rough" induction to the forum, but we'll leave it at that! :-)

[chillyblue]

So if I understand you correctly, unless I have very long pipe runs, or a big height difference, pressure drop shouldn't be a concern?

you will always get a pressure drop, because even the flow through the pipes causes a pressure drop, nearly everything produces a resistance to flow, but this resistance is made worse by incorrect pipe sizing, short radius turns, heigth differences and components.
The only way to do it properly is to take both readings (pressure & temperature)as close to the tev's bulb as possible.
I've never experienced it to be a major problem and if it was me i would set the tev using readings at the compressor.
A word of caution though set the superheat higher than you would if you was doing it directly at the valve and "air on the side of caution" to avoid the risk of flood back. i.e. set your superheat 4-5deg high than you would normally.

Big question How are you going to set the valve if you can't get to it, and if you can get to it why can't you take the readings there???

CB

[lowcool]

if taking superheat settings at the compressor you can only be asking for trouble especially if compressor is oversized even fractionally.you can have liquid coming back without even knowing it let alone temperature losses in suction line.you may as well open the valve until suction line feels cold to touch and not worry about calculations and go by guess work

[taz24]

Thanks Peter_1. It was a "rough" induction to the forum, but we'll leave it at that! :-)

I hope I did not come over as sarcastic in my post, sorry if I did.

It is a fine line we tread with new posters and we try to evaluate who they are, what they know and what they want in a few sentences.

You came over as quite knowledgeable and understanding of the basic systems and you asked a specific question which I hope was answered eventualy.

Ac systems are special to themselves and proberbaly no 2 installs will work identical purly because of the differance in pipe diamiter and pressure drops.

As for you initial question I think Chilly answered it best and I think you understand the problem more.

Basicaly you estimate the length of the string then you guess a bit and then you come to an educated descision about the length of the string .

The other alternative is to drill lots of holes all over the system and braze in some gauge fittings so you can stop guessing.

All the best taz.

.

[Gary]

If the compressor inlet superheat is in the ballpark, then the coil outlet superheat will be in the ballpark as well... unless you have a very long or very restricted suction line.

When just checking the general health of the system, I use compressor inlet superheat.

[Peter_1]

...you can have liquid coming back without even knowing it let alone temperature losses in suction line.you may as well open the valve until suction line feels cold to touch and not worry about calculations and go by guess work

When you measure SH, you can always detect liquid coming back, even with an oversized compressor.
Touching until it feels cold is a dangerous method. You can only do this as a rough estimate after years of experience and knowing the working range of the compressor. A suction line for a blast freezer will not feel cold when touching

[Peter_1]

you will always get a pressure drop, because even the flow through the pipes causes a pressure drop, nearly everything produces a resistance to flow, but this resistance is made worse by incorrect pipe sizing, short radius turns, heigth differences and components.
The only way to do it properly is to take both readings (pressure & temperature)as close to the tev's bulb as possible.
I've never experienced it to be a major problem and if it was me i would set the tev using readings at the compressor.
.....

To avoid misunderstanding, you're speaking of SH and acnerd talked about SC.

But you are completely right abut the pressure drop in the discharge and condenser - every flow provokes a pressure drop - but this fault is minor, especially if you compare it to the measuring fault of your thermometer.

Therefore I show students the measuring fault of their meters. They know the evaporating temperature of the system. They have to measure on several bends (the first ones) of an evaporator. They never measure evaporating temperature but some degrees higher (+/-3 to 5°C higher) This is partially due to the thermal resistance of the copper. This is then their measuring fault and this value can also be used at the outlet of the evaporator when measuring SH.

[lowcool]

peter how can i always tell if liquid is coming back,suction line does not have to be cold for this effect.only thing i am trying to highlight is that correct superheat across evap cannot not be taken at compressor,set up a room on a single system that requires high humidity and see for yourself.domestic fridges dont have long suction lines

oh yeah tell the kiddies to insulate probes as it does help or is this minor as well as temp transfer on copper.i think your being paid to much to alter these students lives.
you could always have liquid injection to preserve the life of blast compressor,especially in eskimo land where gloves are worn

[Peter_1]

...i think your being paid to much to alter these students lives....

Paid to much?? Not at all

Big advantage is that all the students are following this in evening classes to improve their skills or to learn new ones.

So everybody who's there is at least 25 years old and is willing to do some efforts to learn something.
The motivation is completely different as you see with students in day classes where they sit there because they're obligated by law.

[acnerd]

Thanks for all the replies guys. Another year on this forum and I'll be half an expert!!

[Magoo]

Hell if you cannot understand a set of gauges ..Who am I to judge. At least you put the gauges on the system, which I suppose is a positive.

[acnerd]

Cheeky blokes from Australia aren't they.

[acnerd]

Cheeky blokes from Australia aren't they.

I meant New Zealand, sorry buddy!!!!!

[lowcool]

what are you apologising to magoo for,what about us skips lol lol lol

[acnerd]

what are you apologising to magoo for,what about us skips lol lol lol

I know exactly how annoyed you Aussies and Kiwis get when people mistake you for each other. I humbly back out of this conversation, apologising profusely, head held low!!

[hd88]

guys, you sound like you are trying to fix the space shuttle. lol. I dont want to pee in your cheerios but what is he fixing? a/c or refrigeration? Any good journeyman hvac tech (and there isn't many) should know you need to worry about compressor superheat on any system with a long lineset so that the compressor will last for years. Nobody has even mentioned DISCHARGE LINE TEMPERATURE at the compressor. You need good compressor superheat (15 to 20 degrees) so you are cooling that refrigerant cooled compressor, and in turn, not flooding it. Oil burns at 250 degrees. You need to be around 190 - 200 degrees to make sure you are not going to burn up that compressor. Read a Copeland book or Tecumseh book and learn the right stuff, and remember that without a lot of field experience you dont know jack. Dont give any bad advise and guessing, or assuming on these forums and then the forums will be helpful to all.

[lowcool]

if evap superheat is correct and compressor is not then you would have selected the wrong components,i believe that is why we have sub coollers for compressors also,and head fans and oil coollers to compensate for this flaw considering refrigerant types etc,long pipe runs etc,high ambient temps etc.

[hd88]

Poor installation and trouble shooting is the largest problem here in Florida, and every other state. Unqualified, unlicensed, uneducated overpayed techs that wont call in for help when they need it. They wont admit it when they are lost in space and the customer and the employer ends up with the s...t end of the stick in their hand.

[Gary]

Poor installation and trouble shooting is the largest problem here in Florida, and every other state. Unqualified, unlicensed, uneducated overpayed techs that wont call in for help when they need it.

Unfortunately most qualified, licensed, educated, underpaid techs don't know what they are doing either, and the person they call in to usually knows less than they do.

Qualifications are superficial, licenses are superficial, and worst of all service education is superficial.

Installation apprentices at least have the benefit of adult supervision, but service apprentices are on their own. They survive or not based on their level of mechanical aptitude and little else.