What is the compression ratio of a refrigeration compressor?


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Old 01-06-07, 09:50 PM
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What is the compression ratio of a refrigeration compressor?

What is the compression ratio of a refrigeration compressor?
I have a DIY project going on that needs a compressor capable of a compression ration of 45 or so. It's a vacuum chamber to boil water in, basically..... actually it's for "no-heat bread-like substance".

The idea is I put the dough in the chamber and turn the pump on, then the water in the dough boils, and the bread rises like a "popover". I know it won't rise if the volume the pump can move is not high enough, or the dough is too thin, but if I can get a suitable pump... should work, no?
The vapor pressure of water at 20 degrees is about 17.5 torr or so (atmospheric pressure is abotu 760 torr aka mm Hg) so that's less than a ratio of 1:45.

Sorry if this is not a good place to put this, it looked like the most appropriate of what there was on the DIY forums, though.
Can anybody point me to a better place to find the answer?
Or a place to ask about such a project in general?

Or if anybody could suggest where I could get a suitable compressor for such a project, that would be very welcome.
 
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Old 01-07-07, 05:34 AM
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Usually in the a/c or refrigeration field, the compression ratio of a compressor is not talked about very much. This is figured out by the manufacturer. Compressors are rated by btu's, hp, tons, temperature range, and refrigerant type. To answer your question more directly, I believe the compression ratio is around 4 or 5 to 1. But don't hold me to it.

For your project, I would think a "real" vacuum pump would be the way to go. A refrigeration compressor running in a vacuum could be damaged by overheating and lack of oil return.
 
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Old 01-07-07, 12:48 PM
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Jim pretty much summed it up.
A vacuum pump is designed conserably different than a compressor.
The nature of a compressor is that the volume it can draw in a vacuum drops the deeper the vacuum goes.
Vacuum pumps normally use a vane or what is more commonly referred to as a rotary compressor.

Our trade speaks in inches of mercury and to boil water at 69 degF you need to achieve 29.5 "Hg.
The best a compressor would do would be in the low 20's which would not boil water.
Years ago Frigidaire had a line of fridge compressors that were a rotary design where enterprising refer techs would be able to assemble a vacuum pump for next to nothing.

Another thing to consider is that the range you would be working in to boil water at atmosphere is quite narrow.
If you wanted to monitor the progress of the vacuum you would need to use an electronic micron guage to read the level.

I use these tools regularly and can say you would be looking at a fairly high cost if this was a hobby project.

Here is a good site by Robinaire, a leading HVAC/R tool supplier.

http://www.robinair.com/acsolutions/acvacuum/acvacuum.php

Maybe you could describe your project in greater detail and we might be able to offer more.
(I promise not to tell a soul! )
 
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Old 01-07-07, 07:40 PM
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5 ?!
Thanks for the replies, though
I'll tell you a bit about the project, and maybe you can give me some better ideas and/or point me to where to get info on a suitable pump or something.....

As I said, the idea is to boil water out of dough, there's a bunch of reasons being able to make bread without heat would be cool, but I won't go into them here. Suffice it to say that's the goal of the project, and using a pump is just one way of doing that that I've though of.

So, I though of using a vacuum pump, but the thing is they pump such a low volume.... I think I might need something more like a turbine with a high compression ratio, if a refrigerator compressor won't do it .
Suppose I wanted to boil 250 grams (1 cup of water) from the dough

v=(n)(R)(T)/(P)
14521 l=(13.9 mol)(8314l(Pa)/mol(K))(293.15K)/(2333Pa)

2333 Pa is 17.5 torr, the water vapor pressure at 20 degrees Chttp://www.chemistrycoach.com/vapor_pressure_of_water.htm
so the volume I would need to pump from the intake, not including that required to get the pressure down from atmospheric pressure, would be about 14.5 cubic meters.

So I mean, if you take these pumps as an example http://www.usvacuumpumps.com/welcome/cp_series.html

that's like 26 minutes of pumping at 15cfm.

hm... not that bad. except these pumps can't get down to 17.5 torr, only 29.9. Besides, 800$ is way out of my price range.

So what can I do?
I can skimp on noise output, portability.... hm.... I can supply my own motor, maybe, but that's just about it.
 
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Old 01-07-07, 07:57 PM
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oh yeah, the rest of the project.... I would probably be using a plastic bucket from a brew-your-own store as the vacuum vessel. I tested one by closing it (they are already watertight), and putting it under 11 meters of water in a lake, and it survived. So really all I need is a suitable vacuum pump...... maybe I could buy used.

Actually, come to think of it, if I could just get the vacuum down with a pump, maybe I could get a decent vacuum that way, then to KEEP the vacuum that way, I could use an absorbant to absorb water vapor --> silica gel! You can buy it by the pound at flower shops, and it absorbs many times its own weight in water. Plus, it's food grade.

So, taking that approach, I just need a pump that will get me below 17.5 torr or so, even if it takes a while.

The point of it being below 17.5 torr is that the rate of water-from dough removal is limited by how fast the silica gel can absorb water vapor rather than how fast it evaporates from the dough (since it would be at boiling, the conversion to vapor is limited by the heat input).

Hm. I wonder how fast that is. Anybody?

Also, I wonder if the gel would absorb water vapor at such a low pressure, or would it come back out of the gel again?

I wish there was a forum/site more suited to discussion of such a project... does anyone know of one?
 
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Old 01-10-07, 05:33 AM
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C'mon, wher'd you guys go? Does anybody know a better place to discuss/get info on such a project?
 
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Old 01-10-07, 07:29 AM
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Sorry, this is well outside of my experience. Wish I could help.
 
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Old 01-10-07, 08:03 AM
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Wink

Have you checked out any AC shop's around you . I know that we had a vacuum pump around for a long time . That was used before we had to go to the refrigerant recovery units. That we have to use now.
I also worked for a company That used just a air compressor on the intake side of it as a vacuum pump. They used it too lift a 200lb drum with it It did get down to 27" with a very large Qu ft Pm.
 
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Old 01-18-07, 01:30 AM
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Where to buy an AC compressor

Where would I buy a new AC compressor? I have no idea where to start looking.... What kind of store/shop sells them? I am in Ottawa, Canada.

Actually I want it for use as a vacuum pump. I read at http://www.belljar.net/refrig.htm that they make fairly good ones.

I could get a used one, but I want to use it for a food-involving app, so I'd rather avoid all the gunk that comes with a used one.

Also, any idea how much they cost? I can get a whole AC unit for 200$ so.......
 
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Old 01-18-07, 05:01 AM
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You'll pay that much if not more for a compressor motor.
 
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Old 01-18-07, 01:16 PM
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How far do you have to pull it down. One place I worked at they used just a air compressor on the intake side for a vacuum pump. It would lift 200 lb cans . If high vacuum try old AC shopes for the old pumps we used years back to pull down the AC units. A AC compressor will have a very high but slow pull down.
 
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Old 01-18-07, 03:11 PM
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I want it to get to a vacuum below 17 torr. So, where can I get such a thing? What should I look for in the yellow pages, for example?
 
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Old 01-18-07, 04:39 PM
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check out the AC wholesale companys
 
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Old 01-18-07, 05:55 PM
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Moderator note:
I merged your new question on compressors with your original post on vacuum pumps as it is a continuation of the original question.

In your original discussion you calculated that you would need greater than 25 cfm at vacuum to do what you want.
The rotary compressor mentioned in your link I doubt would be 1/10th of 1 cfm at vacuum.

I am not trying to discourage you but pulling a deep vacuum requires specialized equipment that is nearly impossible to jury-rig.

One thing you have to be aware of is that in order for a vacuum pump to achieve deep vacuum the specialized oil the pump needs to function is very sensitive to moisture.
If for example you were trying to freeze-dry food you would need to deal with the moisture that will accumulate in the oil.
If you do not have a means of extracting the moisture before it gets to the vacuum pump you will need multiple oil changes to maintain deep vacuum.
A silica gel filter in the inlet may help pick up some of the water vapor.
 
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Old 02-15-07, 07:47 PM
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What is the refrigerant in a window mounted air conditioner unit?

What is the refrigerant in a window mounted air conditioner unit?

Is it freon-12?

What I really want to know is what volume an air conditioner compresser will pump. I am thinking of using one as a vacuum pump. Also, what is their compression ratio?
 
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Old 02-16-07, 04:37 AM
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R-22!!!!! Toooooo Shooooorrrrrrrttttttt
 
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Old 02-16-07, 04:49 AM
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Depends on the age of the unit. Most of them are R-22. and older ones are R-12..

As for Ratio... don't know.
 
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Old 02-16-07, 04:51 AM
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What are you trying to vacuum down ?? You can probably pick up a decent vacuum pump at a pawn shop for a $100 bucks or less.
 
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Old 02-16-07, 04:26 PM
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yfgcrldhtn,

I for the second time have merged your threads on vacuum pumps.
Please keep all posts on this subject here because we have already flogged this topic to death.

I will summarize what we know so far:

You want to bake bread without heat by lowering the pressure on raw dough so that the water will boil at room temperature and the bread will be cooked.
You also want to do this by only spending a minimum amount of money on equipment.

Honestly, you have come up with some clever math to work out the vacuum level required and the power it would take to do so.
We have also offered plenty of information on your project but you aren't getting it.
Using your own calculations which look like they could be in the ball park, you need a considerable amount of vacuum power in order to do this.

If you have read the suggestions already offered I suggest reading them again.
Maybe with all these facts in one large thread someone can pull a science rabbit outta a hat and defy nature.

Besides, what makes you thing that the bread would even cook.
My guess is that the Co2 in the dough will boil out like a fizzing champagne bottle and plug up your $1000.00 vacuum pump.
 
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Old 01-24-08, 07:02 AM
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acwriter

Yes, it is true that lowering the pressure does lower the boiling point of water. My question is (about the bread), would not the bread be more appetizing if it were browned by a browning element? What is the texture of the bread like and, does it taste very dry when the moisture is extracted using an evacuation process? Oven bread is steamy and very moist when it first comes out of the oven and it requires standing time for the moisture to vacate. I'd like some feedback. Thank you.
 
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Old 01-24-08, 02:37 PM
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Exclamation Hard to believe

that it has been a year since I thought this topic had been beaten to death.

To reiterate:
My trade and life experience tells me that when you lower the pressure on a ball of yeast charged dough it will blow up like a doughy balloon and no cooking of any kind will occur.
 
 

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