vapour compression cycle

[brian connolly]

hello all

am new to this but here goes. im doing an f gas course course at the moment and am not fully understanding the vapour compression cycle. i have asked the lecturer a couple of questions but his answers dont clear it up for me. i think im right in saying that a certain refrigerant is always a certain temperature at a certain pressure ie r404a is around 5.1 bar at zero degrees and always will be. my question is, is it possible for a refrigerant to be a vapour or a liquid at the same temp/pressure.

[monkey spanners]

Hi Brian,

Welcome to RE

A fluid has three states, subcooled (below boiling point at the given pressure), saturated (at boiling point, fluid is a mix of vapour and liquid), and superheated (fluid is a vapour)

Lets take water as an example as its one we are all familiar with, you can have a pan on the stove (as a heat source) its in your kitchen so we'll say pressure is at one bar or 14.7psi, all the time the temperature is below 100C the water is subcooled, if the water is at 90C we would say it has 10K subcooling.(K for Kelvin as its a temperature difference and not an actual temp)

Once the water reaches 100C and starts to boil we say its saturated, there is neither subcooling or superheating.

Now that all the liquid is gone and only vapour remains its temperature begins to rise, say its at 120C we would now say iy has 20K superheat. ( in your pan all the vapour would likely dissipate but in a vapour compression cycle its piped back to the compressor to begin its trip round the system again)

So to answer your question i would say it would depend on the temperature, pressure, type of refrigerant, quantity, and volume of the container.

Jon :)

[al]

Brian

check out this thread, been very good for beginners and experienced guys too.

Who are you doing fgas with?

http://www.refrigeration-engineer.com/forums/content.php?115-Refrigeration-101

al

[brian connolly]

thanks monkey spanner,
it doesnt quite clarify it for me though. you said it depends on the temp/pressure but i was wondering is it possible that a refigerant can be a vapour or a liquid at a certain temp/pressure. for example lets take r404a at 0 degrees is 5.1 bar (according to a slide rule i have). is it a vapour or a liquid at this temp/ pressure or can it be either?

the reason i ask this is the lecturer drew a cycle on a flip chart with different temperatures at the different stages. at -10 the refr was a liquid, at -4 it was a vapour, at 55 degrees it was still a vapour and at 33 degrees it was a liquid. looking at these figures if you take a refrigerant at -10 and add some heat to -4 it becomes a vapour, add some more heat to 33 degrees and its a liquid and add some more heat to 55 degrees and its a vapour. how can it be a vapour at -4 degrees and at 55 degrees but yet be a liquid at 33 degrees.

this is what i cant get my head around

[brian connolly]

thanks al.

doing it with skillnet in blanch

[Brian_UK]

Think of the refrigerant in a storage cylinder, it is at one pressure and one temperature yet, it contains liquid with a gas on top.

The example your tutor has given is also correct but not with at one temperature as asked in your first question.

Your second description is of a working system.

As one Brian to another, what is your overall knowledge of refrigeration systems? We tend to assume that you are doing the F-Gas as a fore-runner to something else.

edit:
Have a look at this Sporlan pipework diagram, ignore most of the equipment but concentrate on the main components, namely, compressor, condenser, expansion device and evaporator. The different states of the refrigerant are shown in different colours and now try a relate those states to the temperatures and pressures given by your tutor.
http://www.sporlanonline.com/5-158_092008.pdf

[brian connolly]

[QUOTE=Brian_UK;244854]

The example your tutor has given is also correct but not with at one temperature as asked in your first question.

thanks brian, not sure how to do these quotes and things so bare with me. not sure what you meant by above quote. and you said my second description was of a working system which is correct but im still not sue how a refrigerant can be a vapour at a low temp and high temp and a liquid in between. obviously im picking something up wrong but can you see what im trying to say? i havent looked at your link yet as am tired and probably wouldnt be able to take it in.

to answer your question im a plumber and have installed a lot of heat pumps which got me interested in refrigeration, i have done a basic course on the vapou compression cycle but the tutor only explained at what stages the refrigerant was a vapour and when it was a liquid. now im doing the f gas course and there is temperatures involved and they are confusing me.

[brian connolly]

looking at your post again brian, your point about the cylinder may be the thing that clears it up for me. so it is possible to have a refrigerant at a certain temp/pressure and be either a vapour or a liquid??????

[Brian_UK]

so it is possible to have a refrigerant at a certain temp/pressure and be either a vapour or a liquid??????Yes, simply.

Refrigeration cycle, in simple terms...Pressures and temperatures for indication only.

Compressor sucks in low pressure vapour, 1bar 5°C, compresses vapour and discharges it (act of compression generates heats, think bicycle pump) now 10bar at 60°C.
High pressure/temperature vapour enters condenser at 10bar 60°C, air blown over condenser to cool hot vapour. Vapour now cools and condenses back to a liquid, temperatures has dropped to 30°C but pressure still at 10bar.

10bar liquid now passes through expansion device which lowers it pressure from 10bar to 1bar to enter the evaporator. Act of dropping pressure reduces temperature and you have a mix of liquid/vapour at the start of the evaporator. Pressure of 1 bar remains and temperature now down to 5°C as the vapour leaves the evaporator and gets sucked in again by the compressor.

A very rough description, yes it is late, but hope it helps a bit.

[brian connolly]

thanks brian

im not sure if you have cleared it up for me or confused me even more. you mention the refrigerant being at 10 bar and 30 degrees and 10 bar and sixty degrees. if this is the case i have got this completely wrong. our lecturer gave us a slide rule and told us the pressure /temp relationship was constant so looking at the slide rule r404a at 10 bar is 20 degrees c. because of this slide rule i assumed that when r404a was at 10 bar it always had to be 20 degrees c. i dont expect a reply as its but will be up for another while anyway trying to get my head around this

thanks again brian

[stufus]

I think you maybe getting mixed up with "latent heat"
LATENT HEAT = The heat required to change a liquid to a gas (or the heat that must be removed from a gas to condense to a liquid )without any change in temperature.
I.E. You can bring about a change in state without having a change in temperature
A better definition of latent heat would be...
Latent heat is the heat given up or absorbed by a substance as it changes state.It's called latent heat because it's not associated with a change in temperature. Each substance has a characteristic latent heat of fusion,latent heat of vaporization,latent heat of condensation and latent heat of sublimation.

I'm also guessing the diagram drawn by the tutor is a pressure enthalpy chart and it's the condenser section that's thrown up this question.

Cheers

Stu

[chillerman2006]

Hi Brian

take the discharge pressure in a system

look at this pressure on your slide rule (comparetor)

the temperature you read is a saturated vapour ( a mix of liquid and vapour)

then take the temperature of the tubework on the system

if the temperature is the same its a mixed vapour/liquid

if its below its a liquid (subcooled)

if its above its a vapour (superheated)

compressor pumps high pressure vapour (above saturation temp - superheated)

the condensor rejects the heat/temperature to below the saturation temp @ that pressure

so it becomes a liquid

the liquid is metered through the expansion device into the low pressure side

still a liquid.... it absorbs heat which takes it above its saturation temp and becomes superheated vapour

this superheated vapour returns to the compressor and the cycle begins again

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

the bit to get your head around is the slide rule (comparetor)

(regardless of high or low pressure)

is showing you the saturated (mixed vapour/liquid) temperature for that pressure

when you add heat to increase this temp... its superheated (vapour)

when you reject/remove heat to lower this temp.... its subcooled (liquid)

there are a number of pics in my album, this is a standard system

the solid colour represents liquid and the dotted is vapour

http://www.refrigeration-engineer.com/forums/album.php?albumid=528&attachmentid=7955

R's chillerman

[knighty]

your lecturer has made a mistake or miss spoken...

there's no reason to link the pressure/temperature like that.... (without taking state into consideration liquid or gas)

a liquid won;t change temperature just because you pressurise it...

if you filled a can with water and then blew it up with a foot pump you could keep increasing the pressure but the temperature wouldn't change


I'm not a frigie, but I'm assuming your pressure/temperature graphs/ruler it just to tell you what state the refrigerant will be at a given temperature/pressure

so
at 20'c and 50psi it'll be a gas
at 20'c and 90psi it'll be a liquid

at 60'c and 90psi it'll be a gas again


when something is liquid it's molecules are close together (they're zooming round like ping pong balls but close together because they're attracted to each other)

when you heat it up, they have more energy so zoom around faster and start to spread out.... once they spread out enough it turns into a gas

if you pressurise it... you squash the molecules closer together... if they get close enough it turns back into a liquid


hope this helps :-)

[MikeHolm]

Yes, simply.

Refrigeration cycle, in simple terms...Pressures and temperatures for indication only.

Compressor sucks in low pressure vapour, 1bar 5°C, compresses vapour and discharges it (act of compression generates heats, think bicycle pump) now 10bar at 60°C.
High pressure/temperature vapour enters condenser at 10bar 60°C, air blown over condenser to cool hot vapour. Vapour now cools and condenses back to a liquid, temperatures has dropped to 30°C but pressure still at 10bar.

10bar liquid now passes through expansion device which lowers it pressure from 10bar to 1bar to enter the evaporator. Act of dropping pressure reduces temperature and you have a mix of liquid/vapour at the start of the evaporator. Pressure of 1 bar remains and temperature now down to 5°C as the vapour leaves the evaporator and gets sucked in again by the compressor.

A very rough description, yes it is late, but hope it helps a bit.


Brian, the one point in your description that stands out for me is the fact that upon entering the condenser, you can go from 60C/10bar to 30C/10bar (the pressure does not reduce). It would seem to me that in order to NOT have a pressure reduction, the volume in the system, condenser and subsequent pipework would have to be much smaller than that which moves the vapour and this is why the pressure does not change much.

The act of condensing the vapour to liquid should, at first glance, reduce the pressure as well as the temp (regardless of area volume). The non changing pressure side of this can be confusing to people.

[brian connolly]

thnks stufus
i think i understand the whole latent heat thing but to go back to brians post where the refrigerant is at ten bar and 60degrees and also ten bar and 30degrees.i tought it was always the same temp at the same pressure.

[brian connolly]

exactly whats confusing me, if you look at the gauges a certain pressure has a certain temp,is this not always the same regardless of state

[brian connolly]

hey knighty, im a plumber so i understand that if you pressurise water you dont change its temp but according to my lecturer there is a pressure/temp relationship when dealing with refrigerants

[corkman]

Hi Brian

take the discharge pressure in a system

look at this pressure on your slide rule (comparetor)

the temperature you read is a saturated vapour ( a mix of liquid and vapour)

then take the temperature of the tubework on the system

if the temperature is the same its a mixed vapour/liquid

if its below its a liquid (subcooled)

if its above its a vapour (superheated)

compressor pumps high pressure vapour (above saturation temp - superheated)

the condensor rejects the heat/temperature to below the saturation temp @ that pressure

so it becomes a liquid

the liquid is metered through the expansion device into the low pressure side

still a liquid.... it absorbs heat which takes it above its saturation temp and becomes superheated vapour

this superheated vapour returns to the compressor and the cycle begins again

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

the bit to get your head around is the slide rule (comparetor)

(regardless of high or low pressure)

is showing you the saturated (mixed vapour/liquid) temperature for that pressure

when you add heat to increase this temp... its superheated (vapour)

when you reject/remove heat to lower this temp.... its subcooled (liquid)

there are a number of pics in my album, this is a standard system

the solid colour represents liquid and the dotted is vapour

http://www.refrigeration-engineer.com/forums/album.php?albumid=528&attachmentid=7955

R's chillerman well thats cleared it up for me thanks

[monkey spanners]

Brian Connolly, have a look here,

http://www.youtube.com/watch?v=x9yirfC8niI

In fact you would do well to watch the whole series.

Jon :)

[Brian_UK]

Brian, the one point in your description that stands out for me is the fact that upon entering the condenser, you can go from 60C/10bar to 30C/10bar (the pressure does not reduce). It would seem to me that in order to NOT have a pressure reduction, the volume in the system, condenser and subsequent pipework would have to be much smaller than that which moves the vapour and this is why the pressure does not change much.

The act of condensing the vapour to liquid should, at first glance, reduce the pressure as well as the temp (regardless of area volume). The non changing pressure side of this can be confusing to people.Good point Mike, I was trying to keep it as simple as possible just before bedtime. ;) I did say however that the act of cooling the vapour has reduced it's temperature, pressure losses are more than needed here I think.

Mind you, I'm always open to criticism.

[chillerman2006]

The act of condensing the vapour to liquid should, at first glance, reduce the pressure as well as the temp (regardless of area volume). .

Hi Mike

download these moving pics mate

boyles law https://rapidshare.com/files/3621119676/Boyles_s_law.gif

charles law https://rapidshare.com/files/1749579138/Charle_s_Law.gif

picture paints a thousand words as they say !

R's chillerman

[brian connolly]

thanks monkey spanners,
started watching it but to be honest got fed up.

my main question is, is it possible for a refrigerant to reduce in pressure but not in temp, or alternatively reduce in temp but not in pressure?

just watched a basic video on you tube and they kept saying " low pressure/low temperature and high pressure/high temperature." but from what i can gather on this forum it is possible for a refrigerant to be a fixed pressure at different temperatures.

[chillerman2006]

thanks monkey spanners,
started watching it but to be honest got fed up.

my main question is, is it possible for a refrigerant to reduce in pressure but not in temp, or alternatively reduce in temp but not in pressure?

just watched a basic video on you tube and they kept saying " low pressure/low temperature and high pressure/high temperature." but from what i can gather on this forum it is possible for a refrigerant to be a fixed pressure at different temperatures.

boyles law

charles law

[monkey spanners]

my main question is, is it possible for a refrigerant to reduce in pressure but not in temp, or alternatively reduce in temp but not in pressure?



Have a look here, at the top of the coil there is superheated vapour, the temp drops till the saturated/condensing temperature and stays constant through most of the rest of the coil and in the last few bends it is cooled below its saturation temperature and is subcooled a few degrees.
All through this process, the pressure is near constant apart from losses in friction etc in the pipework, as the temperature drops it changes from a vapour to a liquid.

http://www.youtube.com/watch?v=UUpwH-NHkuY

Jon

[mad fridgie]

thanks monkey spanners,
started watching it but to be honest got fed up.

my main question is, is it possible for a refrigerant to reduce in pressure but not in temp, or alternatively reduce in temp but not in pressure?

just watched a basic video on you tube and they kept saying " low pressure/low temperature and high pressure/high temperature." but from what i can gather on this forum it is possible for a refrigerant to be a fixed pressure at different temperatures.

Lets bring down to your level.
Ever had a mug of hot fresh tea, then you go to the loo, to read your porno mag, after a tug you go back to the mug of tea, it is now cold. The pressure has not changed. Refrigerant is the same. The conditions determine what the refrigerant will be. I would advice that you do not but a tea bag in a refrigeration system.

[frank]

if you filled a can with water and then blew it up with a foot pump you could keep increasing the pressure but the temperature wouldn't change

You can't compress a liquid further than the point where the vapour becomes a liquid because, at this point it becomes what is known as a 'Compressed Liquid' or a 'Subcooled Liquid'

Lets look at the process of phase change with water in an open system where the pressure exerted on it's surface is 1 Atmosphere at sea level or thereabouts (i.e. the pressure acting on the liquid surface does not change)

Consider a pan of water without a lid on a stove at atmospheric pressure.

We add heat from the gas burner, gradually increasing the temperature of the liquid (water) up to a temperature of 100C. During the heating (energy input) process, the only thing that has happened is an increase in temperature (sensible heat).

When the water reaches it's phase change temperature for the pressure (14.9 psi - 1 bar), the water is still a liquid, but with the continued (additional) heat input, (latent heat) the phase change takes place. Liquid about to vaporise is called Saturated Liquid.

Once the water starts to vaporise, the temperature does not rise but remains constant at 100C.

If the heat input continues, the entire pan of water will phase change into vapour at a constant temperature and constant pressure and the pan will run dry.

If the same were to happen in a closed system, (as in a pressure cooker) once the liquid has all changed to a vapour, the vapour would then absorb the constant heat input (sensible) leading to an increase in both temperature and it's specific volume. The increase in specific volume also leads to an increase in pressure as the expanding volume of vapour is contained within the same fixed volume container. The vapour would be classed as 'superheated' as it's temperature would be above saturation.

If the vapour was allowed to be heated up to say, 300C (200K superheated) then allowed to cool to say 200C (100K superheated) it still wouldn't condense back into a liquid but it's specific volume and hence pressure would reduce.

As the temperature decreased down to 100C, the phase change process reverses and the water becomes sub-cooled at anything less that 100C.

The same process applies to any fluid capable of phase change.

[corkman]

well what happens in hot water system when you heat the water the pressure goes up

[brian connolly]

hey corkman.

know what you mean. i would imagine if you were heating liquid in a closed system (ie pressure cooker) once you start to heat the water the pressure would increase due to expansion. i would also imagine that frank presumed this was obvious and was only referring to pressure changes when the liquid was becoming a vapour and afterwards???????? maybe

[brian connolly]

ok i think i may have if figured out thanks to all you guys.

i think i was looking at the whole pressure/temp relationship wrong from the beginning. the lecturer showed us slide rules telling us that at a certain temp the refrigerant was at a certain pressure. he confirmed this buy showing us the same info on the gauges.
i assumed that if you knew the temp of a particular part of the cycle all you had to do was look at a gauge or the slide rule and it would give you the pressure at that point. but now i hope im right in saying the gauge is only telling us what the pressure would be in a particular point in the system (saturated vapour)

[MikeHolm]

well what happens in hot water system when you heat the water the pressure goes up

Corkman,

You can't really compare the boiler system with a fridge system. There is a built in expansion tank that will allow the liquid expansion without much change in pressure. If we didn't have the tank...well.. I have seen faulty boiler systems that have blown apart.

I worked on a very high temp solar thermal system where the liquid was directly in the glass tube. There was was a blockage that prevented the expansion tank from taking up the liquid expansion and the 1000L of glycol in the panels started to go to steam at about 140C. The glass was rated at 25bar and there was no place for the pressure to go. Sounded like a bomb and some glass was found 60-80m away.

Point is, conditions change when you have a finite volume system (refrigeration) and a variable system (boiler). Not easily comparable.

[brian connolly]

understood mike. can you tell me if i was correct in my last post?

[chillerman2006]

. but now i hope im right in saying the gauge is only telling us what the pressure would be in a particular point in the system (saturated vapour)

No

read my other posts, the answer is there !

[brian connolly]

hey chillerman
i was actually referring to your posts but think i may have worded it wrong. what i meant was when we read the slide rule (or press/temp info on gauge) this info is only correct when the refrigerant is a saturated vapour(on discharge gauge) and doesnt mean the refrigerent will always have the same pressure/temp relationship as on gauge/slide rule. i think thats what you were trying to explain to me earlier

[brian connolly]

hey again chillerman.

i had a look at the pics you linked. i have seen a few of these and understand the cycle as far as vapour/liquid goes. it was the diff temps and pressures that had me confused. i think if the lecturer had of gave us one of these drawings with the different pressures/temps during the diff stages of the cycle, i wouldnt have got it into my head that a refrigerant was always at a certain temp at a certain pressure and vice versa.

to quote my slide rule. r404a at 20 degrees is ten bar. but this doesnt mean at ten bar it always has to be 20 degrees or at 20 degrees it always has to be ten bar which is what i had assumed.

thanks for the patience chillerman

[chillerman2006]

Hi Brian

the system is just 2 pressures - high & low sides

you determine the condition of the refrigerant by looking at the pressure on your slide rule

and seeing if it matches the temp (saturated - mix vapour/liquid) higher (superheated vapour) lower (subcooled - liquid)

for instance 17 bar discharge pressure, will remain from compressor to expansion valve

made up figures for example

discharge pipe @ 80*c (superheated vapour)

liquid line @ 40*c (subccoled liquid)

same with suction pressure

5 bar

2*c evap inlet (liquid)

12*c compressor inlet (superheated vapour)

but still both at 5 bar

I know it takes a lot at first to get your head around but if you keep going over the posts here, it will eventually click

R's chillerman

[Emmett]

Where is the Jedi :cool:when you need him!!!:D

[brian connolly]

thanks again chillerman i have it now.
our lecturer never elaborated on the scale rule he gave us and without your explanation it was easy to assume wrongly. i asked all these questions of the lecturer but he still never explained it like that.
thanks again

[mad fridgie]

You have to understand what pressure/temperature relationship is.

You have sealed container, half filled with liquid and half filled with vapour. You roll the container around.
The liquid and vapour mix. They are at the same temperature and pressure. This is saturated pressure/ temperature or (when both liquid and vapour exist at the same conditions)
If the container was just filled with vapour and you heated it up. "the vapour is superheated" Your are surrounded by superheated vapour "The air that you breath" the saturated temp at atmosperic pressure would be -194C give or take.
If the container is filled with liquid and you cool it down, the liquid becomes sub-cooled, again this somethink you come in contact every day "water" as it saturation temp at atmospheric pressure is 100C.
So you use your gauge (pressure) to give a theoretical saturated temperature. You then measure the actual temperature, if there in no difference you are likely to have a lquid/vapour mix, if there is a difference, if a positive diff, you have superheated vapour, if a negative difference you have sub cooled liquid.
But you can have other things come into play supercooling , glide, velocity carry over and the biggy "poor measurement of temperature and pressure"

[MikeHolm]

Boy, ya go to work and others answer all the questions....:D but thats OK cause they are more qualified than me anyway.

[brian connolly]

thanks mad fridgie and dont worr mike ill think of lots more questions