Questions about Vacuum Pumps

[Fla._Deadheader]

  There is a Robinair Vacuum pump advertised in our local newspaper. What kind of questions should a person ask about a used pump ???

  Most of us interested in building a Vacuum Kiln, have no idea what we are looking for. Any advice would be greatly appreciated, in common language, please ;D

[Den Socling]

We won't worry about 'type' of pump. lets just ask about capacity.

The smallest pump that would work on a couple/few hundred bf would need a capacity around 10 to 20 CFM (cubic feet per minute). You need this capacity when the pump is down in the 60 Torr (28" HG) area. If capacity is unknown, HP is an indicator. It takes roughly a horse to move 10 CFM. So, if anybody talks to you about fractional HP, the pump probably doesn't have the capacity. Unless you're using the propane tank from the backyard grill.  :D

[Fla._Deadheader]

That's clear as mud : :D :D :D :D

  Thanks Den. I can understand that. I gotta call the guy and see what he has.

[old3dogg]

Make sure the inside is stainless steel.Acids in a lot of species of hard wood will eat up a carbon steel pump in a hurry!

[woodhaven]

FD,
I am glad you ask that question. The numbers that Den gave were some I had been wondering about. As far as Robinair Vacuum pumps go I have been using them for 34 years and the only reason I have ever bought a new one was for higher capacity. I still have the first one I ever bought and it still works fine and will pull down to 50 microns even if it is 34 years old.

[Fla._Deadheader]

Then, yer sayin, if this one is big enough, it would work on a 1000 bdft kiln ??? Moisture no problem???  Price is $300.00

[woodhaven]

Don't know that one is better for Den. I wonder about moisture to. In my work the oil in the pump does all the real work. I will change my oil after each use. No more than twice between oil changes.

[shopteacher]

FDH: I got my vacuum problem almost solved. As soon as I figure out where to connect my hose I'm going to connet on to that big sucking sound created by NAFTA. Mexico needs a little moisture down there anyway. Mighty dry place. :D :D ;D

   I think your going to find that Robinair to be about 5 CFM and probably a 2 stage pump.  Very good for refrigeration work, but won't be able to draw down the size of the chamber your talking about. I have one I used for air conditioning and it takes a good while to evac. a central air system.

[woodhaven]

Hey Teach,
Robinair does make a 10 cfm. From what Den said that wouldn't be big enough either.

[Fla._Deadheader]

Anybody out there that can calculate the cu/ft in a chamber that measures about 5' X 12' inside??? How long to draw the vacuum using a 10 CFM pump???

[Don_Lewis]

That type of pump won't last in a dryer. I have used them in experiments and they will only last a few weeks. It is for refrigerant and not good for handling a lot of water vapor.

[woodhaven]

2 Dim. will only give you square feet. You need 3 Dim. to get cubic feet. Length X Width X Depth = CuFt
3x3=9 sqft
3x3x3=27 cuft

[Fla._Deadheader]

Uuhhhhhhh, it's one of them long round jobs. It ain't square ??? ???

  Thanks Don. What IS the one we need, that liquid ring thing ???

[sawwood]

 I was talking to a guy about his vacum pump on his lathe
 any he was gitting a new one from htt://www.surpluscenter.com. you might try them.

 Sawwood

[Den Socling]

A liquid ring vac pump doesn't care about a little water vapor because it slings water outward into a ring to create a vacuum.

A gallon of water creates about 1700 gallons of vapor. That's roughly 227 cubic feet of vapor. How many gallons per hour do you want to move? To dry 1000 bf of Red Oak at a rate of 0.5% per hour, you have to move about 2.25 gallons. That's about 511 cubic feet per hour or 8.5 CFM.

Moving the air out of the chamber is duddly squat.  :D That's why I said in an earlier post, get any pump to test a chamber.

[Ga_Boy]

FDH,

To calculate the volume of a cylinder the formula is:
Pi X r^2 X h

Where pi = 3.14  Pi is much longer than 3.14 but this will work for what we are doing.
  
           r = the radius which is squared (^2)
             Lets assume r = 5'

           h = height (or length)
             Lets assume h = 12'

Now doing the calculation it will look like this:

   3.14 X 5'^2 X 12' this produces 942 cubic feet or 942 ft^3

To calculate how long a 10 Ft^3/min pump will take I must make an assumption. I hope Den, O3D or Jason will jump in to either validate or correct my assumption.  The assumption is the drawn time for the pump is linear, meaning a straight time line.

Now to the calculations.

We have a pump capable of 10 ft^3/ min and a cylinder containing 942 ft^3.  With the pump removing 10 ft^3 every minute, on paper it will take 94.2 minutes to evacuate all the air.  I do not know if you want to or this pump is capable of pulling a complete vacuum; here is where Den, O3D or Jason needs to jump in.

Any clearer? :P


Mark

[Ga_Boy]

FDH,

After I posted the how to calculate the volume, something occurred to me while I was in the gym.

In the example used, I cited radius as 5'.  For your chamber the diameter is 5', therefore the radius is 2.5'.

Let run the calculation again using your numbers.

volume (V) of a cylinder is stated as V = Pi X r^2 X h

Where
Pi = 3.14159265359
r = 2.5'
h = 12'

Now

V = Pi X 2.5'^2 X 12'  Please note that the value for Pi used is what is in my TI-85 calculator.

So....

V = 235.619449019 ft^3

Rounding, the  volume of your chamber is:
 236 ft^3 (read as 236 cubic feet)

Please note that if you use 3.14 for Pi the result is 235.5 ft^3

Using the previous assumption for pulling a complete vacuum the time is 23.6 minutes.  However you do not need to pull a complete vacuum.  Here is where Den, Jason or O3D need to chime in.


Mark

[old3dogg]

I dont think that you will be able to find a chamber good enough to pull a complete vacuum.Ive seen chambers down to 20 Torr.Scary thought.We used to make the new guy open the bleed valve!
Getting the air out of a chamber isnt really the problem here.The problem is when you apply heat to the wood and the vacuum pump has to pull the vapor out of the chamber.If the pump is to small you are not going to be doing much drying.At least you wont dry to fast.
Ga-Boy.
Im flattered that you think Im that smart!
I will let Den tell you if your calculations are correct.
Mike.

[Jason_WI]

This is the company that makes our rotary vane vacuum pump for dairy applications:

http://www.masportpump.com/index.php?option=com_frontpage&Itemid=1

They also have liquid ring sealing pumps. There website is currently under construction though ???

Jason

[shopteacher]

I gots one those liquid ring thingys with the indoor pluming.  Ya just pulls the little handle and liquid goes around the ring and the vapors is all sucked away.  Don't have much lag time though before it fills back.  Haven't figured how to hook the kiln up to it yet, but it sure would be convenient. 8) :D 8)

[Den Socling]

I know nothing of this company but a Google search for used liquid ring vacuum pumps delivered this company among others.

http://www.aaronequipment.com/inventory.asp?categoryID=63&subcategoryID=15

They sure have a lot of used pumps.

[old3dogg]

Shopteacher.
All of us vac kiln guys spend a lot of time on those liguid ring thingys only beause we have a lot to offer up to one and we are full of it!!

[Ga_Boy]

Shopteacher,

I believe those two guy on Myth Busters busted your idea about using that liquid ring thingy to pull a vacuum. ;)

Back to study hall. :P


Mark

[shopteacher]

Ah, get out. Really!  Just when I thought I had that problem beat.  Well back to rummaging the junkyard. ;D  Let's see now, I need something that sucks big time.  Anyboby know how to run a pipe to the IRS?