saturation?

[marc5180]

can someone clarify what "saturated" means Iv always thought that saturated was a mixture of both vapour and liquid like in the condensor but iv read things recently that contradict this.

[US Iceman]

The way I look at this is: the refrigerant is saturated if....

: Adding heat causes boiling
: Removing heat causes condensation
: Or, a mixture of vapor and liquid are present

You also have to remember this occurs at a constant pressure.

[Pooh]

Saturated state is the point when vapour is in contact with the liquid it has come from at a fixed pressure ie. water at 100 degC at sea level (boiling kettle on Skegness beach). If you keep adding heat and all of the liquid boils you end up with superheated vapour or if you remove heat and all of the vapour condenses you end up with subcooled liquid. However how can you have subcooled liquid in a receiver if there is vapour above it??

Ian

[gonefishing]

Out in West Texas, USA I lay it out like this........

~Saturation describes a condition at which a mixture of vapor and liquid can exist together at a given temperature and pressure. The temperature at which vaporization (boiling) starts to occur for a given pressure is called the saturation temperature or boiling point. The pressure at which vaporization (boiling) starts to occur for a given temperature is called the saturation pressure. For all refrigerants in use in our industry, there is a definite relationship between saturation pressure and saturation temperature.

As the pressure exerted on the refrigerant is increased, there is a proportional increase in the saturation temperature. The representation of this relationship between temperature and pressure at saturated conditions are found on a standard Pressure/Temperature Chart which has been plotted for the refrigerant in question.

Later........GF

[Abby Normal]

it means you have a vapour and a liquid of a substance in contact with one another, it is evaporating and condensing, at its saturation temperature.

[BigJon3475]

Think about it like the dewpoint. The air is so sat. with moisture at the a certain temps it condenses to a liquid. And more heat added it would go back to a vapor. That's why when you use gauges your looking for the saturation point as opposed to the pressure in the system.

[Peter_1]

However how can you have subcooled liquid in a receiver if there is vapour above it??
Ian

But, Pooh's question is still not answered.

[gonefishing]

However how can you have subcooled liquid in a receiver if there is vapour above it??


A receiver is a collecting spot, much like a pond at the end of a stream. Temperature and pressure have been satisfied to a point where liquid is allowed to remain in state. The vapor above the liquid point is holding a stable pressure to allow the liquid to remain in state. In a pond, we have atmospheric pressure above condensed and sub-cooled water. If we were to continue cooling the receiver, all vapor would condense, leaving no vapor in the receiver.

There is always liquid leaving, but the rate is slow enough to allow the stream to continue to replenish the pond. By holding a steady pressure at a somewhat stable temperature, we are able to maintain a calculated level of liquid in the receiver, or pond.

If the condenser were to loose it’s ability to continue to produce liquid, the level in the receiver will fall, much like a pond drying up if too much demand is placed on it. An expansion valve is our dam on the pond. If we open the gates too wide, the level in the receiver will drop, and the pond will dry up. If we create a condition which allows the receiver to completely fill, our pond becomes too full and it will flood our little town and crack our pistons.:D

Probably about as clear as the mud at the bottom of a pond, but I tried..................Not real scientific though.

GF

[Pooh]

gonefishing
that sounds fine in the case of the pond but the gas in contact with the liquid is not the vapour of that liquid unlike a receiver.????

Ian

[gonefishing]

gonefishing
that sounds fine in the case of the pond but the gas in contact with the liquid is not the vapour of that liquid unlike a receiver.????

Ian

Actually, it seems quite the same to me. Both are in stabilized environments.

The atmosphere above a pond, is stabilized, on a clear day.
On a dreary foggy day, there is a water layer with 100% saturated
atmosphere overhead. The water in the pond is sub-cooled and in holding.

In a receiver, at design charge, there is liquid, with vapor directly above.
If we over cool the receiver, it will stack full of liquid if flow
to the expansion valve is not increased.

GF

[Abby Normal]

Saturated state is the point when vapour is in contact with the liquid it has come from at a fixed pressure ie. water at 100 degC at sea level (boiling kettle on Skegness beach). If you keep adding heat and all of the liquid boils you end up with superheated vapour or if you remove heat and all of the vapour condenses you end up with subcooled liquid. However how can you have subcooled liquid in a receiver if there is vapour above it??

IanThe subcooling in a receiver is always a big debate.

Liquid can only expand so much

Liquid at the bottom of the receiver is under a little more pressure. Vapour at the top is not in contact with the liquid.

Vapour evaporates, contacts the shell, loses heat to ambient. Evaporating vapour gets heat from the liquid below.

A lot of points to look at. Receiver is like a heat pipe in some ways

[TRASH101]

why has no one given a definitive yes/no answer :confused:

the answer seems as clear as the mist above gonefishings' pond :eek:

surely if the vapor exits within the reciever it must also change state within the reciever thereby conforming to the general accepted definition of saturated vapor

[Pooh]

But the air above the pond cannot condense into the pond and raise the level.

Ian

[Abby Normal]

is it possible that the top of the receiver shell is warmer than bottom of the receiver shell?

How about the temperature of the bottom of a 30 pounder that is half full compared to the top?

[gonefishing]

But the air above the pond cannot condense into the pond and raise the level.

Ian

I guess we are looking at things way different. I thought
when it rained, we had achieved saturation, condensing,
thus liquid falling out into a sub-cooled pond.

The way I look at it, vapor compression refrigeration is just
the act of controlling when and where things are saturated,
condensed, sub-cooled, and superheated. Is it not all related
to the thermodynamic world we live in?

GF

[Pooh]

Sorry folks I will stop this before it gets too silly, from what I have read most of you are correct in part, but without getting into lots of maths and big words I cant explain it fully and really the question was only paosted for a bit of fun. Basically the reason is that the outlet from the condenser although subcooled liquid does contain vapour due to the pipe size and variations in flow, the main point is why do we fit a receiver other than to store refrigerant during pumpdown or low load, to serve as a liquid seal so we get pure liquid to the metering device.

Ian

[lana]

Hi there,


is it possible that the top of the receiver shell is warmer than bottom of the receiver shell?

Actually it is. Check it this time. The bottom of the receiver is colder than the top. This means that at the bottom we have sub-cooled liquid which goes via the dip tube to the TEV. At the top there is saturated vapour which condenses because of lower ambient temperature.

This is of course my opinion.(DON'T SHOOT PLEASE :eek::D)

Cheers

[expat]

Sorry folks I will stop this before it gets too silly, from what I have read most of you are correct in part, but without getting into lots of maths and big words I cant explain it fully and really the question was only paosted for a bit of fun. Basically the reason is that the outlet from the condenser although subcooled liquid does contain vapour due to the pipe size and variations in flow, the main point is why do we fit a receiver other than to store refrigerant during pumpdown or low load, to serve as a liquid seal so we get pure liquid to the metering device.


..............Yes.

[expat]

As for this business of why there is vapour above the liquid, well I quite liked gonefishing's thing with the pond but if you're not convince...Imagine you have an un breakable piston and chamber. Infront of it you've got your liquid vapour mix. If you keep pushing at a constant temp (or otherwise for that matter) you theoretically should get 100% liquid. Keep pushing and it should turn to a solid (remember I said unbreakable), I think after that it will become so dense it will turn into a Black Hole and swallow you.

ps don't try this at home kids unless you have freinds like Jesus, Mohammet, Buddah or anyone else with a direct line to God, and if you do choose to say hello to Douglas Adams for me.

[marc5180]

Thanks for all the help guys, didn't realise that it would have caused such a debate:rolleyes:

[Lord Kelvin]

Saturated means in that state it cant take heath to increase its temperature.

[Gary]

Imagine a half full (half empty?) container of refrigerant sitting on a block of ice with a heating pad on top.

We have subcooled liquid on the bottom, superheated vapor on the top, and saturation where the vapor is in direct contact with the liquid.

[Lord Kelvin]

e.i., water at 100 ºC and 1 atmosphere is saturated of heath,any additional heath will be used for a phase change.

Ice at 0 ºC and 1 Atmosphere will melt without changing its temperature, all energy added as heath will be used to melt it.
Both are "saturated".

[Pooh]

Not convinced

Ian

[Lord Kelvin]

Convinced about what ?

You can do the experiment, put some ice at say ... -- 25 º C in a beaker and start heathing it while you register its temperature , it will increase until it gets to 0 ºC , at that point it becomes "saturated", because it will not change its temperature until it is completely melted.If you can measure the amount of heath taken by mass unit of ice at that point, you will realize that what is called specific heat of water is higher at that point ( 0 ºC and 1 atmpsphere ) . Once all ice is melted down,water will start to increase its temperture as it gains heath.That will happen until it gets up to 100 ºC ( 1 Atm), when again it will become saturated of heath, any Calorie added will be used to drive its change of state.
If you want it clearer, just put some water in it !!

[BigJon3475]

Here ya go a chart for ya.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html

[wambat]

As long as there is liquid molecules present in the vapor mixture it is considered saturated, however the % of saturation would be minimal.

[Gary]

If we look into a liquid line sightglass, we might see vapor bubbles traveling with subcooled liquid. At the surface of the bubbles the temperature coincides with the pressure. IOW, there is saturation at the vapor/liquid interface, even though the overall mixture is measureably subcooled. That's why we need considerably more than 1 degree of subcooling to assure a solid column of liquid at the metering device.

And in fact, it is entirely possible for the vapor at the very core of these bubbles to contain some miniscule amount of superheat.

[BigJon3475]

Good explanation Gary. So basically saturated conditions contain possibly two different states with 3 possible temps.


I also like the explanation about the pond.