Condensing Question

[n00b 0f l337]

I was wondering a bit about condensing laws, and I honestly cannot find my text book anywhere for Chem.

If you were to have two gases, lets say r507 and r22 in a condenser. Would both condense assuming the condenser can handle it? Or will only the r22 condense leaving the r507 gaseous since it has no need. If this was applied to a mini autocascade you could use r22 an r507 to have the r22 chill the r507 later for the evap.
I think it falls under partial pressure or something.
Wdrzal?

[LeSnip3R]

If both can condense under the pressure and the temp your condenser put them, then yes thes will both condensate... no affirmative action in physics yah :stick:

I don't get it for the autocascade, you talk about r22 chilling the r507 ? They would be both liquid but they'll need a low pressure to be cold... and they will freeze together... like in a ss...

In an autocascade you've one gaz which doesn't condensate at amb temp, and which is chilled by the "1st stage gaz"...

[n00b 0f l337]

Right, but it doesnt always have to be that way I beleive.

[wdrzal]

You would have to run a simulation on "NIST cycle D" its a simulation program for 35 of the common refrigerants. you can simulate up to 5 at a time. The down side is the program cost 200.00 and updates are 100.00

Here is the users guide if you want to read thru it.

http://www.nist.gov/srd/webguide/CYCLE_D%20ver%203.0_files/CYCLE_D%20ver%203.0.htm

[n00b 0f l337]

Thanks wdrzal. :)

[Unknown_road]

the gasses will mix and condens like a mixture or the discharge pressure will rise because the r507 in the condensor has to go somewhere. So the r507 will also condens just because it can't go anywhere else.

if only the r22 would condens, leaving the r507 gaseous no r4** mixture would work ;) so you could already reason yourself that this isn't happening, don't need gasdynamics for it.

[n00b 0f l337]

Well r4** are a mix are they not? Mixing r22 and r507 they remain seperate things?
But good point, I geuss you'd have to.

[SoddemFX]

R22 is a true azeotrope, r507 is a blend very near to azeotropic, r404a is a zeotrope.

In an azeotropic blend all the composites have the same vapour phase and liquid phase points at the same pressure. In a zeotrope under a constant pressure you will have some constituents boiling at one temperature and some at another temperature. The difference bettween the boiling points gives the glide in a zeotropic blend.

If you have zeotropic blend in a auto you may have problems with phase separation if the glide is large.

Mixing r507 and r22 would create a zeotropic blend with about 5 degrees glide.

This is "as far as i know", could Walt, Unknown_road or one of the other experienced guys confirm?

On the condensing side i think Unknown_road is spot on.

Tom

[n00b 0f l337]

K thats all I needed to know. :)
I've seen 2 stagers which run r507/r507. The purpose to apply much more capacity to the 2nd stage.

[SoddemFX]

You'll decrease the discharge pressure of the 2nd stage and as the compressor has a set compression ratio this will give a lower pressure on the suction of the 2nd stage...

Tom

[n00b 0f l337]

Right, I beleive BB Mods's was doing one a while back.

[pythagoras]

Compressors dont have a fixed compression ratio:eek: The compression ration is discharge pressure/suction side pressure measured in psi absolute.

Regards

John.

[SoddemFX]

Hi John,

You are right, D'oh! :D I've always thought of it that way, but i was wrong. I was thinking of it in just one stroke with no valve or anything else attatched...

Would you mind explaining how a two stage with identical refrigerants opperates?

//hope you dont mind NOL :)

Tom

[n00b 0f l337]

The 2nd stage is just condensed much more easily, and with much lower high pressure side I figure you can run more into vacuum on low side without losing so much capacity.

[Jort]

do i read 5 threads on the 1st page started by noob of 1337?

;)

[n00b 0f l337]

And 2 of them un-answered.
Its the weekend beside my girlfriend i got nothing else to do.

[pythagoras]

Its more efficient for two reasons.

First the second stage refigerent is cooler so it can absorb more heat before it enters the phasechange region.

And secondly, because the compression ratio is lower, each sweep of the piston collects more gas fropm the suction side. There is a gap at the top of the cylinder the higher the high side pressure the more gas is trapped there. When the piston enters the down stroke the gas trapped fills the cylinder until its pressure is the same as the suction side. The higher the high side pressure the further down its stroke it gets before it starts to suck gas from the low side.

I'm sure thats not a very good explanation:confused: I'll do a better one in a seperate thread if you like.

Regards

John.

P.S. Adam.. Whilst we are already off topic in your thread, I picked up your evap today, is a filter drier and sight glass of any use to you?

[SoddemFX]

No, thats a very good explanation, thanks very much John :)

Tom

[wdrzal]

Think of 2 pots of water put on a stove burner. one at room temp,the other at 99c. the one at 99c will boil almost immediatly while the one at room temp may take 10 min. to boil. The fluid at room temp asorbs more capacity,this is analogious to sub-cooling, BUT the greatest engery transfer happens during phase change (boiling).sub-cooling, Mass flow along with velocity also play a important part in design. more sub-cooling is not always better. Remember the coldest temps are not at the exact exit of the cap tube. This is why chillys latest design work better since the refrigerant is fed in at the top and not the base which was the previous norm.

John explanation is correct, but if the velocity pushes the refrigerant past the optium phase change point,performace can be lost.

[Unknown_road]

R22 is a true azeotrope, r507 is a blend very near to azeotropic, r404a is a zeotrope.

In an azeotropic blend all the composites have the same vapour phase and liquid phase points at the same pressure. In a zeotrope under a constant pressure you will have some constituents boiling at one temperature and some at another temperature. The difference bettween the boiling points gives the glide in a zeotropic blend.

If you have zeotropic blend in a auto you may have problems with phase separation if the glide is large.

Mixing r507 and r22 would create a zeotropic blend with about 5 degrees glide.

This is "as far as i know", could Walt, Unknown_road or one of the other experienced guys confirm?

On the condensing side i think Unknown_road is spot on.

Tom


An azeotrope always has to be a mixture, since r22 is a pure gas it is not azeotrope. about the r507/r22 mix, your right.

[SoddemFX]

Ah ok, is there a class name given to a single gas refrigerant?

Tom

[Unknown_road]

pure gas :p:

[wdrzal]

Ah ok, is there a class name given to a single gas refrigerant?

Tom

Or a mix acting like a single gas is a :
Zerotrope= show no maxium or minimium vapor pressure at a constant temperature.

[pythagoras]

Have you missed this thread Adam?

Want a filter/drier and sight glass included?

Regards

John.

[n00b 0f l337]

Could you PM me that pythag, lets keep this off the boards.

[wdrzal]

The 2nd stage is just condensed much more easily, and with much lower high pressure side I figure you can run more into vacuum on low side without losing so much capacity.


No,as to vacuum part the rest is correct, but what will help is the sub-cooling, the rule of thumb is you gain 1/2% capacity for every degree.

The problem with to much sub-cooling in small condensors is the refrigerant already passed thru when its at it maxium asorbation rate...........

[mindnumb1]

Two stage systems:

It is more energy efficcient. Kind of like intercooling. Think of it like this: Imagine the Compressor piston going up 1/2 the way. wait until the compressed gas have reached surrounding temperature. What has happened? the gas pressure has gone down a little. Now, to complete the stroke it takes less energy than it would take to do it in one stroke. Now imagine that you put two compressor in series, with a cooling stage between them. this gives the same effect.

Now, for your implementation, its not exactly the same, but if some conditions are present, you might get some of the same effect.

Measure the outlet temp on high pressure side on the second stage. Is it higher than Room temp? If so, make sure you have a radiator that lets your 2. stage high pressure side cool down to room temp before cooling it further down with your 1. stage system. No point pushing heat into the 1. stage system that you can get rid of elswhere.

Side note: Imagine the piston going immensly slow so that the compressed gas temp stays at room temp during compression. What will get you the same effect? Right... A series of compressors with intercooling between them.


Ok, best of luck with your project, Best regards, Bernt.

[Unknown_road]

offcourse the discharge temp is higher as room temp. the compression and the heat from the compressor heat the refrigerant up (the compressor itself is normally about 60-80 *C) .

putting compressors in series is called a booster cascade or a compound cascade. the whole problem with these systems is you need 2 oil seperators. 1 for each compressor or else 1 compressor will run out of oil. thing is when you already have 2 compressors and an oil sep, the only thing you need is a HX and you can build a classic cascade which will give you far better temps as a compound cascade.

[mindnumb1]

Opps, I am guilty of pointing out the obvious, please bare with me :)
( punishes self by listening to gnarles barkley on norwegian public radio, ouch! :) )