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Vacuum Pump Specifications for Discontious Vac

Started by Ga_Boy, May 04, 2004, 08:40:16 AM

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Ga_Boy

I have a line on a vacuum pump with the following specifications.

115V, 4.2 A, 60 Hz
Seller claims it holds a 25.5 HG.



Is this enough information to determin if this pump is sutiable for a discontinous vac kiln about the size of a 500 gallon LPG tank?

10 Acers in the Blue Ridge Mountains

Den Socling

These are guesses:
1) 4.2A probably means 1/2HP
2) 1/2HP means very low capacity and low capacity means slow drying
3) if it pulls 25.5" HG, that's about 114 Torr absolute which means your wood would never be cooled by evaporation to less than 130'F.
Not good.  ;)

Ga_Boy

Den,

Thanks for the info

I am guessing there is/are some old threads that outline the specs for a vacuum pump.  I remember reading threads that discussed what types of seals and matrerial of construction are best.

If I understand this process, the volume of the chamber will be one of the primary factors for pump horse power and capacity in addition to the capacity of the heating and cooling.

I will assume the volume of my chamber is 150 ft^3.  
10 Acers in the Blue Ridge Mountains

Den Socling

I think I have shown the calculations in a past thread. The volume of the chamber does matter as much as the volume of wood and the water in the wood.

This idea will work if you guys can 1) come up with an air circulation system that can heat the wood to raise the vapor pressure of the water in the wood and 2) connect a vacuum system that can pull chamber pressure low enough to use the elevated vapor pressure.

A pump with low capacity but that is capable of pulling the chamber to low pressure will work but it will work more slowly than a pump that can pull low quickly. Remember that liquid water increases in volume about 1700x when it turns into vapor. That vapor has to be moved by the pump.

Ga_Boy

Den,

Then this means the bigger issue is "how to heat".  True?

If so then this becomes a thermodynamics problem.  True?

If so I need to duast off the physics books as I do not remember the thermal properties of wood.

I need to think on this.  The direction my thoughts want to go is the properties of thermal transfer of the atamospher within the kiln.  Though, I am not sure how the atamospher of a vacuum effects the heat trransfer in terms of a gas (the atamospher) to wood given, if I remember correctly wood is a desent insulator for heat transfer.

Let me know if I am going in a direction that as already been traveled and goes no where for this application.




Mark
10 Acers in the Blue Ridge Mountains

Den Socling

You got it. Pulling the vacuum is easy compared to getting every piece of wood heated to the same temperature. Heat transfer in a vacuum is particularly difficult. That's why, with this discontinuous vac, we bleed air into the chamber during the heating side of the cycle.

Wood has a low thermal conductivity. While it's wet, free water in the wood helps to conduct heat. Conductivity can vary with species, grain, sapwood and defects. That's why my original idea was to have multiple RTD's in multiple samples. The wood will have to be stickered and warm air will need to be circulated throughout the kiln charge.

There is a table at our website.
http://www.pcspecialties.com/torroconvHC.html
There's a lot of numbers and it may look confusing so lets look at the top line, only. See where 30 Torr and 84'F fall on that line? That means that, if you heat (wet) wood to 84'F, the vapor pressure of the water in the wood is 30 Torr. Now, if you pull the chamber pressure to 30 Torr, the water boils out. There are complications, of course, but that is the basic idea.

Ga_Boy

Den,

I am not familiar with the electric plates used to heat that I have read about.  Does the wood need to be exposed to the atmosphere on both two major surfaces?  

Where I am going with this is using a heated fluid with thermal transfer plates attached to one of the major surfaces.  My thought is to use the system designed for radiant floor heat.  I have seen this used on TOH retrofits.  It is basically a plastic tube with an aluminum heat reflector.

Now here is the issue as I see it.  The heat is applied to one side only. This means that evaporation only occurs on the other major side of the material.  

Hypothesis:  There is uneven drying on thicker material such as 12/4 and up causing material defects.

Unknowns: Parameters for material defects to occur when using a vacuum to dry 12/4 and larger material with heat applied to only one side.

Is the above correct?  

As soon as the correct formula(s) are identified, I will build a spread sheet to graph and model the drying process.  I need my Physics books so I can research this.  I do not remember which law of thermodynamics applies.

Observation:  Heat transfer issues are probably why RF drying was developed.




Mark
10 Acers in the Blue Ridge Mountains

Den Socling

Mark,

Heating plates are used in our type of vacuum kiln that run with continuous vacuum. They are heated by circulating water through them. Electrically heated devices, like WoodMizer's electric blankets, are very difficult to control. We sandwich the layers of wood between two heating plates.

You might be able to make the radiant tube work with aluminum plate but you are heading toward a much more expensive and complicated system.

Den


Fla._Deadheader

  The Teech and myself have teamed up and purchased several Vac Pumps. Got a pretty good deal, AND, if our set-ups work like we hope, we will have a couple of pumps for sale.  We will also have the knowledge? to help someone build their system, the same way that Den is helping us.

  Stay tuned. We should start next week, cutting the tank and making the door hinge and figuring the best way to circulate the air in a round chamber ::) ;D :D :D :D :D :D

  
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

Ga_Boy

Harlod/Shopteacher,

I'll be interested in those vacuum pumps.



Den,

I am pondering the heat transfer to the wood.

Is it better to heat the air via an exchanger, then circulate the heated air to heat the charge?  

This seems inefficent although I have not done any calculations.  

It would seem that having the transfer occur by direct contact between the wood and heat exchanger is the most efficent in terms of heat exchange.  Here again this is just an assumption; not having done any calculations.

Now for the evaporation to occur it would be best if both of the major surface area of the wood is exposed to facilitate the drying.  But, if one side is in contact with the heat exchanger then drying can only occur on the exposed side.

Based on your limited description it appears this is how you do it in your continous vacuum systems.  If not; then:

Does this increase the probility of ununifirom drying and/or damage to the material?



Mark

10 Acers in the Blue Ridge Mountains

Den Socling

Mark,

I work with conventional kilns and vacuum kilns everyday. All conventional kilns heat the wood by blowing warm air through the stickered kiln charge. The air is reheated by fin pipe and recirculated through the charge. It works.

It certainly is better to have two heated sides in continuous vac kilns but I have seen lots of wood dried with heat on one side. If you cut squares or dimension on a circle mill, there will be enough variation in thickness that lots of pieces won't be touched by the upper plate. Or, if I'm double stacking lumber, I may end up with an odd number of pieces and one lays alone. Warp is more probable but it will dry.

Den

Ga_Boy

Den,

I think I got it.  I am putting way to much thought into the heating system design.  I'll use the KISS methodolgy. ::)



Mark

10 Acers in the Blue Ridge Mountains

Den Socling

The heating system is going to require some thought. Pumping warm water through a heat exchanger is easy enough but circulating air through a kiln charge that is packed into a propane tank? hmmm  ???

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