Vacuum Drying (Long)

Re: Vacuum Drying (Long)

JL work

>Typically the reading is "inches of mercury". A vacuum of 27" would be a reasonably high vacuum (with 31" being almost no air at all). That is of course, if the gauge has atmospheric pressure at "0". A possible reason for more water loss at night would be lower relative humidity. Warm air can hold more moisture than colder air.

Re: Vacuum Drying (Long)

Gary in Zirconia,NC

>The measurement is millimeters (mm) of mercury. The humidity at night could be lower.

Re: Vacuum Drying (Long)

Robert Cox

>The meassurement can be in inches or millimeter. Both of mercury.

760 in millimeters is 31 in inches.

(It's been a while since I did vacuum drying (nuclear fuel storage) so the 31 I am just taking from above.)

In an oddity though 27 is low for either IIRC. 31 inches of vacuum is a lot because we use 0 as atmospheric and progress to 31 as we puill harder.

In metric we start at 760 and head for 0.

Our spec for fuel storage is 3 torr (mm of Hg).

Even then, with the fuel giving off heat, we get icing. When we isolate the vacuum pump the pressure rises a good bit. So we pull it down several times to remove essentially all moisture before backfilling with helium.

So I guess my point is that I am not surprised that you see fluctuations. I'd expect them.

Re: Vacuum Drying (Long)

Robert Cox

>Oh yeah, moisture loss is greater at lower temps because of the amount of moisture the air can hold. Cold air holds less water.

Question, how do you know the moisture loss varies with temp? Are you opening this thing frequently?

Re: I missed somthing here . . .

Mark Mandell - Gone Round In Jersey

>Bob,

Have understood the "warmer/moister" relationship for many years; dehumidifiers run in the Summer, humidifiers run in Winter. But, I don't see how that effects the moisture loss in a sealed container like his "partial vacuum" chamber. If his pump will pull 27", but no more, it doesn't mean he's getting an air-change from dryer ot more humid air from outside, it only means that that's all his pump will pull.

If his wood is reacting to ambient room temperature then is the question perhaps whether evacuated wood releases moisture at different rates depending only on its temperature? In "normal" kiln-drying operations the temperature is raised and lowered according to the program with more water being removed at the higher temps. Southern/Northern Hemisphere issues (and how water drains) aside, the post seems to be saying that he looses more water at cooler temps, and that would seem opposite of what I would expect.

So, What am I missing?

M

Re: I missed somthing here . . .

Sam Simpson

>Hi Y'all,

Dan, the best way I know to understand what is happening is to think of the air as a carrier. You are cudgeling the moisture out of the wood and into the air. Whatever air is in the kiln will hold X amount of water. (more if it is warm and less if it is cold) For the water to fall out of the air and be collected by you there needs to be a temperature differential, between the air inside and the air outside the kiln. The greater the temperature differential the more water will fall out of the air and into your collecting vat. So, it would be odd if you did not see a greater rate of collection as temperature fell at night. But this does not mean that you are releasing that much more water from the wood, only that you are having more of it fall out of the air and into your collection vat. The water that is extracted during the day time is the same amount but more of it remains buoyant in the air around you therefore less falling into the collection vat.

Regards Sam Simpson.

If on the other hand?

Sam Simpson

>Hi Y'all,

Dan, if on the other hand, what you are reading is the actual moisture content of the wood, via a remote sensor inside the kiln - then I would expect you to also have a considerable fluctuation of the inside temperatures. (leaking) This would effect the efficiency of the kiln and the accuracy of the sensors. The wood might either be sweating or pursing.

In this scenario I would expect you to have a leaky kiln. Pump running almost continuously. If so you have a condition whereby the wood has been heated during the day and is now surrounded by cooler air inside the kiln. This could set up a situation whereby the wood surfaces are reading as dryer than several hours prior, even though the overall moisture content by weight is not that much different. Just different closer to the surface where the readings are taken.

Regards Sam Simpson.

Re: If on the other hand?

Dan Faux

>Hi Sam and others

In answer to Robert, no I am not opening kiln at all, and this also answers Sam's question, I have a probe inserted in the wood and I take readings from it at various times.As this is the first test of this kiln it is all new to me. I understand that there would probably be some leakage so the pump is set up to run continuously and it does keep the kiln at a constant level. I appreciate all the input and will certainly post results. This is a very simple way of drying out turning blanks and part turned bowls. If anyone is interested I'll post a pic of the device.

Dan F

Re: If on the other hand?

Robert Cox

>Pics would be cool.

My point with the nuclear fuel thing is that vacuum drying is weird.

In a container full of water the spent fuel gives off heat to boil the water off. But put it under vacuum and ice forms. Hold the vacuum and you see a pressure rise as the ice sublimates into vapor.

None of that is intuitive to me, I'm just a poor old civil eng. But it is reality.

Perhaps what you are seeing is just the way it works?

Or maybe not.

Re: Vacuum Drying (Long) *LINK*

walta100

>I suspect that the air that leaks in the day has more moisture in it than the night air. In vacuum drying you are trying to boil the water into a vapor and then remove the vapor from the chamber see the link for a better description. To boil the water at room temperatures you will need a very high vacuum. As the water boils it will absorb heat from the wood then the boiling will stop until the temperature increases or the pressure decreases. The pressure temperature chart shows for a boiling point of 101 F you need to get the pressure of -27.95 HG, 84F=-28.74HG. To make this work you need a perfect seal on the door and a very large vacuum pump. A perfect vacuum is -30 HG -31 is not possible on earth. This is a link to the pressure temperature chart page 2 is the info for water http://www.refrigerants.com/pics/ptchart.pdf


Basic Refrigeration

Re: Vacuum Drying (Long) *LINK*

Bill Turpin in WNC mountains

>At 27 inHg(inches of mercury)vacuum, water boils at 114�F (45.6�C). To remove H2O from your kiln you need to more or less boil the water at room temperature. This is possible with a low enough vacuum. To get this to happen at 33�C (91.4�F) you need a vacuum of about 28.5 inHg. Cheap vacuum guages are notoriously inaccurate. Plus or minus 2 inHg is quite normal. They are also sensitive to shock. A combination of a little heat with your vacuum could be much faster.

Your compressor is exhausting against atmospheric pressure. Does the barometric pressure fall at night? Refrigeration compressors are not rated for continuous duty. Normal run times are probably closer to 40%. Does your compressor overheat during the day, and shut off due to overload when you are not looking? And then starting up again after cooling. With cooler temps at night, does the compressor continue to run...therefore more run time? Do you have a fan blowing on the comp. while running? If not, install one. The comp. will live longer.

The attached link is a discussion on vacuum and vacuum pumps for the refrigeration trade. These are used for removing moisture from inside the tubing of freon systems. Remember that making your suction tubing (or suction's smallest orifice) half size makes pull down time FOUR times as long. Doubling the length of tubing also has the same negative effect.

Bill Turpin in WNC mountains


Vacuum

Re: Vacuum Drying (Long)

Steve Worcester

>You may want to look into some article on vacuum kilns. I know there is a guy in Texas, Robert Hensarling www.mesquiterocker.com runs a large vacuum kiln for drying his mesquite. He uses some sort of electric blanket to raise the temp in the kiln.

If you do a google search on vacuum kilns, you will stay up most of the night.