Vacuum Kiln

[shopteacher]

Den:
   I've been trying to decipher all the different posts on the constrution of a vac kiln and put together a pictorial of what I thought the components would look like grouped together.  I'd like you to look it over and see what additions(or deletions) and the rights and wrongs of it.  I just trying to get it composed so I can see what all parts will be needed.

Could you explain the operation of the controller.

[Den Socling]

DanG. I was half way through a lengthy post and my computer lost it. I will do this in short 'burst'.

I suggested discontinuous vac drying because it is the only technology within reach of even the best back yard mechanic. You have the basics in your diagram. The details that will make the system work better will be discussed in future post.

[Den Socling]

The controller that could make it automated could look like this. A controller will look at wood temperature with an RTD. The operator will compare different samples and decide which will be used for control. The controller will look for low temperature. When it is low enough, the vac will be switched off. A solenoid valve will bleed air into the chamber. A solenoid valve will open to let water into the heat exchanger. A contacter will start a fan. A heat control system will control the temperature of the water. A ball valve will open to let water out of the chamber.

[Den Socling]

When the wood is warm enough to build the necessary vapor pressure, the heating circuit shuts off. The vacuum pump starts with a contacter. The seal water to the pump starts with a solenoid valve. The pump pulls open a ball type check valve. Evaporation from the wood begins. As the chamber pressure decreases, evaporation occurs from within the wood, not just off the surface. This is what makes vac kilns capable of drying thick stock rapidly.

[Den Socling]

After a while, the pump has pulled as low as it will go. The wood has cooled because of the decrease in the boiling point of water. The controller shuts off the vacuum pump and switches back to the heating cycle.

[PatrickG]

The vacuum kiln looks very interesting and I will be anxious to see the rest of the information.  In the mean time, how much vacuum is required?  Why intermittent?

 :P Pat   :P

[Fla._Deadheader]

  Patrick, without Den having to repeat all the info, check back to the thread "A kiln for bowl drying", or something close, in this Drying Forum.

  This latest info is a diagram that will co-incide with that thread.   This is GOOOOOOD info, Den

  Biggest problem I have is finding a place to set this sucker up. : :

[Den Socling]

You need cooling water for the vacuum pump. This could be a partial recovery system. The water from the pump goes into a tank. When the water in the tank gets too warm, fresh water is added as warm water is dumped. This could be done with a temperature controller and a float valve. I use little cooling towers. When the water gets too warm, it's pumped to the top of an evaporative cooler. When too much water has evaporated, a float valve lets fresh water in.

If you are hot rodding the process with condensers, the same water is used. A cooling water pump runs constantly to circulate water through the system.

[Minnesota_boy]

Fla._Deadheader,
I have 230 acres up here with at least half of it open if you want to set it up here on shares.  I can even give you a chance to try freeze drying for half the year.  :D :D ;D

[Fla._Deadheader]

  That's an offer that's hard to refuse, but, naaahhh.  With Gasoline going to $3.00 a gallon, I can't afford the hauling : :

[shopteacher]

Den:  What makes up the brain of the controller?  In other words what is telling the contactors and solenoids to operate?
Can there or should there be more than one RTD?  By RTD are we talking a thermocouple or a gas filled bulb and capillary tube?  
   In one of your earlier post you described the condenser.  If I understand your description the vapor being pulled from the wood and chamber will condense into a liquid and run back into the chamber to later be exhausted through the drain. Is that correct?  
   Could the drain also be a solenoid valve connected to the controller?

[Den Socling]

I would say that the 'brain' of a controller is a module called a 'loop controller'. The devices have a keypad, display, input buffers, built in software and everything needed to survive the real world. Jason (where is he?) would say to use a PLC and build your own circuit and software. Loop controllers cost between $250 and $350 so I don't mess around with PLC's.

I would have more than one RTD. RTD's are neither thermocouples nor capillary devices. They are precise resisters which change with temperature. Most all controllers can read an RTD. RTD's don't need special wire like thermocouples.

The drain could be a solenoid valve but actuated ball valves work better in this application. With our commercial vac kilns, a PLC watches water level in a tank that collects water from the chamber and condenser. I use a PLC here because it can do math calculations. When the tank is full, the PLC bleeds air into it. A ball-type check valve closes off the chamber. A little pump removes the condensate. The PLC calculates and displays the approximate MC and drying rate.

[PatrickG]

Patrick, without Den having to repeat all the info, check back to the thread "A kiln for bowl drying", or something close, in this Drying Forum.

I read the bowl drying thread.  I didn't see the part where it told how much vacuum.


 :P  Pat   :P

[Den Socling]

Instead of thinking vacuum, think reduced pressure. The atmosphere over your head is heavy enough to push a column of mercury 760 mm high. Vacuum reduces pressure.

When there is 760 mm of pressure on water, it has to be heated to about 212'F to make it boil.

If you reduce pressure to 50 mm, water will boil around 115'F.

'Millimeters of mercury' is a mouthful so the term 'Torr' is used. One Torr is one millimeter of mercury.

A two-stage, liquid ring vac pump with cold seal water can reach to around 28 Torr. If you pulled this low, you would be boiling water around 82'F. If you can pull 50, you're still safe with most wood.

[Den Socling]

If I've said it once, I've said it a thousand times.  :D

[shopteacher]

Den:  How hard is it to setup or program the loop controller?
         I found a site on PLC's that is very helpful with the understanding of how they work.  Plan to spend a little more time trying to figure out what it would take to get one of them to operate correctly.
http://www.plcs.net/

[Den Socling]

Loop controllers are very easy. You set the hardware so it knows what the inputs and outputs will be. Then tell it if control is direct (response to an increasing process value) or reverse (response to a decreasing process value). There's a whole bunch of possible parameters but most can be ignored.

With our hypothetical controller, we could tell it to look for an RTD input and respond to a decreasing temperature. A setting that is often called hysteresis sets the deadband. We might set the hysteresis to energize a double throw relay at, say, 105'F. The relay could energize the actuators needed to run the heating cycle. When the controller sees the other end of the deadband, maybe 115'F if our setpoint is 110'F, the relay output de-energizes and starts the vac cycle.

I think that Garrett has links to Partlow's 1160 and Future Designs' 9300 from our website. They are two very good, low cost loop controllers.

[Jason_WI]

Den,

Would a vane type vacuum pump work from a old milk machine system. Our 7 HP vac pump for our milk system can pull 28-29 inches of mercury and has a 100 cfm capacity. It uses an oil drip lubrication system and a reclaimer to recycle the oil. They seem to run forever as long as the oil is flowing.....

These can be had at farm auctions for about 200 bux because they are too small for the corporate farms.

Jason

[Den Socling]

Hi Jason,

I've been wondering where you were.

That vac pump you mention sure sounds like it would work but 7 HP is a lot to burn for a little kiln charge. You should be able to dry 1000 bf with about 2 HP.

Den

[shopteacher]

Den, I looked up the partlow controller and printed out the operators manual for it. I'm part way through the manual and was wondering if it would require more than one of these to operate the different devices on the kiln? The price on the web site was $162.35.  

[Den Socling]

You might use one Partlow to switch back and forth from heating to vacuum. A second Partlow would be useful in holding the heating water temperature where it belonged.

A third Partlow wouldn't be needed for the vac cycle but a third Partlow with a pressure transducer could tell you where the pressure was.

I'm not sure where that price came from but it sounds a little low.

[Den Socling]

The Partlow's output would set off a chain of events. They could all be done with relays. Some relays would be time delay relays (TDR's). This is where a PLC could look better but you would need to write the ladder logic that would set the relay output. A PLC is a programmable bank of relays.

[Jason_WI]

Jason (where is he?) would say to use a PLC and build your own circuit and software.

Not a PLC but a PIC microcontroller.


Jason

[Den Socling]

Not defined as a 'PLC'?
Just an itbity one?

[shopteacher]

Den, I got that price from TTI, Inc.

http://partlow.ttistore.com/

What is the price range of the PLC that would be reqired to operate this kiln?

Jason how much trouble would it be to come up with a working PLC for this application?