Refrigerant compressor as vacuum pump

I am building a biodiesel reactor to convert french fry oil into diesel fuel to power my truck and hopefully a generator. Part of the biodiesel
making process involves distilling excess methanol (which is expensive) out of the fuel. I will apparantly need about 23 Hg of vacuum. I am too cheap to buy a proper vacuum pump and am looking for a solution from the scrap yard where I normally do all my shopping. Would a refrigeration compressor, the kind that is not enclosed in a steel case that can't be disassembled without a cutting torch, be able to supply this vacuum? Is there a better solution, maybe fron the automotive scrap yard?
stan
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A scrap ref compresson would likely pull 23" og Hg for a short time, but what are you going to do with the methanol? It will condense in the vacuum pump unless you flush it out with a lot of air (called ballasting). Maybe you can lower your vacuum requirements by heating?
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I was going to draw the vacuum through a condensor and let the methanol drip into a closed container.
stan
Nick Hull wrote:

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That would work if the condenser is at dry ice temperature. IIRC 32 degF is much too warm to condense all the methanol under vacuum. Look up the methanol vapor pressure curve to understand what is going on.

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Yes it will draw a good vacuum, not a lot of volume however.
Lane
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SBaer wrote:

It'll take hours to do the recovery with a small hermitic-type refrigerant pump. I've done this (you may have run across pictures of my recovery setup) using a condenser and liquid trap. I've found that using a diaphragm or floating-piston pump (Thomas or Gast) that do not rely on oil for lubrication works much better. They're quite affordable on E-bay, and eliminate the need for a condenser. The liquid is collected from the exhaust side of the pump in a cooled vessel. I really should update my web page.
Dale Scroggins (VeggiePup)
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wrote:

http://www.rdysales.com/home.html
check out their ebay sales for vacuum pumps. Or give them a call, ask for Yavone, tell her Gunner refered you and ask what they have in your price range.
Gunner
"To be civilized is to restrain the ability to commit mayhem. To be incapable of committing mayhem is not the mark of the civilized, merely the domesticated." - Trefor Thomas
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Hi Dale
I have referenced your website many times, what kind of vacuum pump is in the website picture? How powerful is the motor?
Stan

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I was under the impression that you had to ADD Methanol to make cooking oil suitable for diesel engines. I seem to remember seeing a TV article here in UK where they added turpentine to used chip oil to run a diesel car.
John
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Hi John
You do have to add methanol, but in excess to push the reaction to completion, about 20% of the methanol used can be recovered to reuse next time.
stan

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SBaer wrote:

about 30 yrs. ago i was trying to get into a/c work..... i needed a vacuum pump to pull a vacuum on the air cond. system to get all the moisture out.. the book i had shows how you take an ordinary refgerator motor and make a vacuum pump out of it.. well the next day i find an old ref. on the railroad tracks.. stopped and with a pair of pliers and a large screwdriver i pulled it out of the frame... went home and cut the tubing at the compresser about 2 ines away and pluggd in the compressor with a jumper wire set up.. it worked... put my finger on the various tubing until i could feel a vacuum.. that is the one i put a fitting on by brazing it to the tubing...... stood the pump upright and then brazed a piece of metal to it so it would stand up straight like it did in the old refrigerator..... so here was a $250 vacuum pump that cost me nothing other than the fitting and a little gas to braze it up.... i did not have to cut open the enclosed tank on it to get it to operate a vacuum for me....
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Hi Jim
The reason I was planning to use a pump that was not enclosed is that they tend to be larger. I don't know how the two types differ on the insides, but the unenclosed ones look like piston compressors.
stan

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pump that was not enclosed is that they tend to be larger. (clip) ^^^^^^^^^^^^ I have a friend who is a partner/serviceman in an ice machine rental company. He has converted a number of hermetically sealed refrigeration compressors to vacuum service. Some of them are quite large--will make hundreds of pounds of ice per day.
There is an issue with those that they depend on oil in the refrigerant for internal lubrication, but my friend tells me that in vacuum service they should last a very long time.
Out of curiosity, Stan, what is the step where you add methanol all about?
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A very good website for the whole biodiesel thing is http://journeytoforever.org/biodiesel.html
stan
Leo Lichtman wrote:

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Why not distill the methanol out at atmospheric pressure using heat? It will be more efficient than the vacuum pump and can be made more so with regenerative feed heating.
Mark Rand RTFM
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| |>I am building a biodiesel reactor to convert french fry oil into diesel |>fuel to power my truck and hopefully a generator. Part of the biodiesel |>making process involves distilling excess methanol (which is expensive) |>out of the fuel. I will apparantly need about 23 Hg of vacuum. I am |>too cheap to buy a proper vacuum pump and am looking for a solution from |>the scrap yard where I normally do all my shopping. Would a |>refrigeration compressor, the kind that is not enclosed in a steel case |>that can't be disassembled without a cutting torch, be able to supply |>this vacuum? Is there a better solution, maybe fron the automotive |>scrap yard? |> |>stan | |Why not distill the methanol out at atmospheric pressure using heat? It will |be more efficient than the vacuum pump and can be made more so with |regenerative feed heating.
And the recreational aspect is a plus :)
Rex in Fort Worth
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Automotive AC compressors of the 2-cylinder type work reasonably well as vacuum pumps as is, although there is a modification that can be done to the valves to improve efficiency. The way I used to do it was to remove the cylinder head and valve plate from the block and then drill a 5/16" hole from outside the block into each cylinder, down just far enough from the top of each cylinder for the hole to be uncovered by the piston at the bottom of its stroke. This drilling was done from the suction side of the block so that the drill passes through a built-in sediment chamber which will allow the use of the original intake fitting. The intake passages in the valve plate were sealed with epoxy and the outer ends of the cylinder holes were threaded and closed with 1/8" pipe plugs, and after the compressor was re-assembled the crankcase was filled with refrigeration oil. The last one I converted would do about 26" of vacuum and if you wanted more volume you probably could manifold several of them together on a common suction line. These compressors were made by York and some by Tecumseh, and were commonly used on Ford cars. Mike
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