[Gasification] Vacuum drying (was: RE: Drying)

[Mark Ludlow]

List,

The primary motivation for "vacuum" (actually, reduced-pressure drying) is
that the boiling temperature of water is reduced as the pressure is reduced.
This makes for increased heat transfer when the source of heat is "low
grade"--waste heat, for example, as described by Fourier's Law of Heat
Transfer. But the heat still needs to be transferred to the wood itself and
in a "vacuum" convection transfer, as used in a hot air dryer, is not
available; only conduction and radiation can transfer energy.

Vacuum driers for wood have the ability to extract some free moisture but
when this phase of water removal has run its course, water must be extracted
as vapor. At any significant rate of water removal a condenser becomes
essential and the heat sink cooling the condenser must be cold enough to
condense water at the saturation temperature of the referenced pressure. For
instance, at 0.25 psia the boiling temperature of water is 59F so any
cooling water running through a condenser must be at least this cold and
preferably colder. Similarly, the seal water fed to a liquid ring vacuum
pump (the most common industrial design) must be even colder than this to
maintain a vacuum seal. Trying to remove water vapor with a vacuum pump
alone is very inefficient because the high specific volume of water vapor at
low pressures requires a large displacement vacuum pump and lots of
mechanical energy to drive it.

Using electricity to remove water is probably one of the most expensive
methods because electricity is one of the most expensive forms of energy.
Kevin correctly observes that the wood would get colder as water is removed
and 970 BTUs (0.284 kWh) worth of energy must be added to the dryer (at
minimum) for every pound of water removed, not to mention the energy used to
drive the vacuum pump and the pump for cooling water recirculated through
the condenser.

Tracing the feed inside the dryer with tubing through which steam or hot
fluid is circulating helps things a bit, as would tumbling the chips in a
rotary vacuum dryer that has an external shell heated by steam or hot fluid.
In this case heat transfer is by conduction through contact with the heated
shell.

Mark

-----Original Message-----
From: gasification-bounces at listserv.repp.org
[mailto:gasification-bounces at listserv.repp.org] On Behalf Of Kevin Chisholm
Sent: Thursday, December 14, 2006 6:51 PM
To: doug.williams; Rodenhuis, E.J. (Erik Jan,Student TBK); Adam Carr,
Renergy; gasification at listserv.repp.org
Subject: Re: [Gasification] Vacuum drying (was: RE: Drying)

Dear All

Wood Mizer makes a vacuum lumber drying kiln. Basically, boards are stacked 
in a vacuum chamber, and a vacuum is drawn.

As the wood dries, it cools. The heat of evaporation must be replaced, for 
the process to continue. 1 BTU/pound of water removed. 1 calorie per gram of

water removed. Wood Miser replaces the heat lost through evaporation by 
inserting layers of electrical resistance tape between the boards.

This system works very well indeed. One problem is the high cost of 
electricity. Another cost is labor to stack the wood neatly. And then 
unstack it carefully after it is dry, so as to not tear the resistance 
strips.

We designed and built a 6,000 square foot solar collector to preheat air 
going to a new kiln holding 32,000 FBM hardwood boards. Worked great.

Best wishes,

Kevin



----- Original Message ----- 
From: "doug.williams" <Doug.Williams at orcon.net.nz>
To: "Rodenhuis, E.J. (Erik Jan,Student TBK)" 
<e.j.rodenhuis at student.utwente.nl>; "Adam Carr, Renergy" 
<adam.carr at renergy.org.uk>; <gasification at listserv.repp.org>
Sent: Thursday, December 14, 2006 10:36 PM
Subject: Re: [Gasification] Vacuum drying (was: RE: Drying)


> Erik,
>
> You ask;
>
>> The article mainly discusses drying of sludge, how effective and
>> efficient would it be for woody biomass?
>
> Vacuum drying of timber, is a commercially developed technology, and I had

> a
> series of meetings with a developer here in New Zealand, about back in
> 1996-7. I have no further info to offer, but sure that  I  have seen
> referances to vacuum  timber drying in various publications.
>
> Regards,
> Doug Williams.
>
>
> _______________________________________________
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> 



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