15.14 DIY Vacuum chamber

Not only does a vacuum chamber or vacuum oven speed things up, but there are some things that you need one to accomplish.  For those brothers and sisters whom are not bucks up enough to afford to buy one, consider building your own.

Lots of ways to skin a Duck Billed Platypus, and the simplest vacuum chamber that we've found, is a simple pint or quart Bell jar with a bulkhead fitting through the lid.  Don't try this with the larger jars, as they may implode.

In this case we are using vacuum to cold boil away the alcohol and the jar would be sitting in warm water.

 

Surplus US military MASH vacuum pump

We've also made vacuum chambers out of sanitary spools, as well as Terpenator tanks.  Here is the prototype for a simple lid that we developed for a Terpenator tank.  It is held in place by the vacuum.

 

Plexiglas lids


The prototype shown is a 6", but with 1 1/2" Acrylic or Polycarbonate, you can go up to 12".

We also purchased a Pyrex vacuum bell from American Scientific and added a hot plate inside, which allowed us to process up to 6"  Petri dishes.  As we were using a rotary vane AC pump for vacuum, we had to frequently change the oil, as the alcohol condenses in the gear case.


 

Prototype vacuum chamber with hotplate

We also did cold boiling with it, but used a military surplus MASH pump, which doesn't have a crankcase to flood.  It also only pulls to around 25" Hg, so it was a slow process.

 

Cold boiling with MASH pump

We tried heated paint pots, which worked, but has a rounded bottom and you can't see what is going on inside with the solid lid:

 

Pressure paint pot with cable heat and insulation


Next we decided to build one based on a used 22 quart Mirror pressure cooker donated to the cause.  We tossed the lid, but saved the gasket.  I scored some reminant 1 1/2" polycarbonate from XXXX Plastics, left over from a bank teller cage job for the lid.

 

Prototype chamber with 2” polycarbonate lid

That brings us to the subject of lid deflection and cracking from high cyclic fatigue.  14.7 psi atmospheric pressure pressing down on a lid is not much, but when you multiply it by the area of the lid, you can see that it mounts up quickly as size increases.

 

The real issue is not the deflection, but its effect after repeated cycles from high cyclic fatigue, which manifests itself in multiple fine cracks in the lid, followed as it progresses by a lid failure by implosion.

 

Makes a mess, and except for any prestige involved, I encourage you not to do it…............

 

Use a straight edge to check for deflection, and if you are starting from scratch, you can determine the deflection ahead of time.

 

Search for formulas from the Thirteenth addition of the Machinery Handbook for calculating stress at the extreme fiber and total deflection for a round plate simply supported, we find the following:

 

  • The formula for stress at the extreme fiber is:

       

 

 

         S= 0.39W

                  t2

 

 2.0  The formula for deflection is:

 

          d= 0.221 W R2

                      Et3

Assume:

 

            S = Stress at the extreme fiber

            T=       Ultimate Tensile of Polycarbonate

            p=        Pressure in psi = 14.7 psia atmospheric pressure

            W=      Total load on plate= p X area

            t=         Plate thickness

            d=        Deflection

            R=       Radius of plate= ½Diameter

            E=       Modulus of elasticity = 375,000 psi

            D=       Diameter of pot

 

Polycarbonate mechanical properties courtesy of:  http://www.boedeker.com/polyc_p.htm

 

You can of course just buy a ready built chamber from someone like Best Value Vacs at:  https://www.bestvaluevacs.com/categories/vacuum-chambers/stainless-steel-chambers.html

 

BVV vacuum chamber

That leads us to the subject of pot heat, which works better if it is controllable in the 85F to 125F range.  We experimented with a reptile mats controlled by a light dimmer switch and it worked, though not smashingly well, so switched to silicone heat mats, controlled by a Variac variable transformer.

Heat mats available at: 

http://briskheat.com/ 

http://www.omega.com/ 

Our CAT Scientific hot plate will easily hold steady at those temperatures, but common hot plates or grills typically do not have precise control at those low temperatures.  One solution to this is to buffer the temperature fluxuations and average them out using sand.  Here is a picture posted by a brother whose name I didn't record, of his solution using an electric skillet:

 

Vacuum chamber with hot sand bed

 Sooo, how about vacuum pumps?  That depends a lot on whether you are pulling off residual LPG or cold boiling alcohol.  The cheapest vacuum pump capable of about -28.5" Hg is a simple faucet vacuum aspirator.

 

Faucet vacuum aspirator

 

Aspirator connected

Such a setup works slick for vacuum filtration, but doesn't pull low enough for vacuum chambers. 

Faucet aspirators also contaminate the discharge water steam, and most faucets like the one above lack the pressure and flow required, so they use one in conjunction with a sump pump sitting in a tub or barrel of ice water.  The cold water is also required to achieve maximum efficiency.

Our surplus MASH pump doesn't pull low enough either at -25" Hg, so now mostly used for vacuum filtration.

A rotary vane AC vacuum pump will work for purging LPG extracts, but the gear box will flood when using one for cold boiling.  Either way, frequent oil changes are required to preserve both pumping efficiency and the integrity of the pump itself.

An issue often ignored when selecting an AC vacuum pump, and that is its rated duty cycle.  Unless it is rated at 100%, it will overheat and be short lived.  I noted on like that a number of brothers and sisters were using Harbor Freight vacuum pumps and just returning them for another when they broke.  A tribute to Harbor Freights returns policy and sense of humor, which I predict will change if the pump returns continue in volume.

Here is the original CPS Pro-Set that we got extensive use out of before it was stolen.

 

CPS Pro Set rotary vane vacuum pump

What is required for a professional installation of course is a chemical duty pump capable of the vacuum levels needed, and as you might expect, none of those are inexpensive.

 Our Cascade TEK TV-02 came equipped with a Welch chemical duty diaphragm pumps, which did an excellent job at around $3,900.  Here is a link to their cut sheets and nomenclature:

 

Welch 2052 Diaphragm pump

 https://welchvacuum.com/collections/laboratory-vacuum-pumps-chemical-duty

 Other options:

There is also the Scroll pump and the Turbo-Molecular vacuum pumps.  Here is Pharmer Joe's analysis as to suitability by design:

 

Pharmer Joe's graph

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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