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Alternative lumber drying techniques

Are freeze drying and RFV practical methods for drying lumber? April 25, 2001

What are your opinions on freeze drying and RFV for high value lumber? Do the benefits outweigh the expense?

Forum Responses
Freeze drying involves freezing the water and then removing it under vacuum by sublimation (so it never turns to water). It would be a dumb idea for lumber drying. As for RF, it does work but isn't really practical. It is Gee-Whiz, though, and some people will buy it anyway.

There is another method that would go with these two--drying lumber in limestone caves in Virginia--a patented process in which, supposedly, the vapors from the cave (limestone vapors?) cause the wood to dry perfectly.

More seriously, both drying methods have been around for quite a while but have limited practical application to lumber drying. One reason is that conventional drying systems work so well and with little quality loss (if operated well). When systems are not properly operated, they will not always dry well. But do not blame the drying system--it is like blaming General Motors when your Corvette keeps getting speeding tickets!

Gene Wengert, forum technical advisor

I thought about offering a water heated vac kiln with a limestone liner (vacuum enhanced limestone vapors for maximum gee whiz), but maybe you've had enough.

I ran between one and eight RFV kilns at one time. The number running depended on my supply of spare parts, supply of wood and ability to pay the electric bill. I used to be in contact with several companies running RFV. As far as I know, they are all gone except three patched-up junkers from my original 8. I think that two companies are still trying to sell RFV. I haven't come across an installation by either.

The idea of RFV is great. I once thought that it was the only way to dry thick wood. But the RF oscillators are expensive to build and very inefficient. And the RF field that heats the charge of wood acts a lot like your microwave. A guy at HeatWave claimed to have these problems solved, but I only heard the claims.

American dry kiln operators (unlike the Europeans) have heard so many bad stories about vac kilns (because of unscrupulous vac kiln manufacturers) that the entire technology is tarnished in the US.

WoodMizer was one major exception to the crowd of unscrupulous. Their only mistake was trying to build a vac kiln that complimented their portable mills. A low cost little bugger that you could tow with your pickup and set up in your garage. They actually came pretty close.

I know of two small vac kiln owners. Both seem to be happy with them. One says he wouldn't go back to DH.

The key to the small vacuum dryers (after the price of the equipment and the high operating costs) is that the markets must be able to use wood that will have some stress. Small vacuum kilns typically do not have a stress relief technique. Of course, with any kiln, if you air dry first, stresses will be very small.

A second concern is that the final MC is quite variable due to the variability of the wood's permeability (or else the entire load is over-dried to achieve a uniform, but very low final MC). Small vacuum kilns typically cannot do any equalizing. This can be solved by pre-drying before final drying--the incoming MC is then very uniform.

One person solved this variability problem by using the vacuum kiln to reduce the green MC to a fairly low level and then went into the standard kiln to finish the drying, equalize, and condition. This is a good idea if time is critical and if costs are not too important.

The appeal of a small vacuum dryer is that it typically requires little monitoring to operate.

In tests I did years ago, the quality of vacuum, RF vacuum (RFV), and pre-dryer/kiln were compared and we saw no basic difference in final quality (except as noted above). The color in a vacuum kiln was brighter for many pieces, but this brightness caused finishing problems for the kitchen door manufacturer who wanted a more uniform appearance from piece to piece. (We solve this by color matching now, but it is an extra processing step.)

Of course, any drying system cannot fix problems with the resource--gray stain, shake, etc. Also, a poorly operated conventional or vacuum system will always be poor.

Gene Wengert, forum technical advisor

Professor Wengert,

I have to suggest again that your opinion of mean deviation of MC in vacuum drying is outdated. And operating cost can be under conventional drying.

A customer recently told me that one of his customers preferred vacuum dried wood because it is so stress free.

There is a growing market for vacuum dried wood in North America. In my opinion, many manufacturers of furniture and stair parts are going to refuse discolored conventionally dried wood and use vacuum dried only.

An improperly operated dryer will cause stain. We had stain develop in the Wood Mizer dryer. The key is the equipment and its operation.

I would be delighted if vacuum drying were cheaper than conventional. It is hard to beat the low capital cost (even $ per BF per year basis). I await getting good information on any new system's drying costs. If you look in DRYING HARDWOOD LUMBER, you will see some of the items that go into a cost determination. If we are to compare two systems for drying, we need to use the same cost comparison system.

Gene Wengert, forum technical advisor

Costs are important, so are results (MC), stress, etc. But no one has really talked about the comparisons of energy sources.

In Europe things are changing. People are talking about energy--carbon emissions have to drop, cfc's, we 'woodies' should care more than most.

We don't build vac kilns for 4/4 (25 mm) lumber except in special cases. The foothold of conventional drying is too strong. However, for my own interest, I have calculated the cost of big conventional kilns versus much smaller vac kilns. The costs are surprisingly close.

The fog rolls in when you start working with the big stuff. Ask these guys how long they spend drying 3" oak. If you get an answer, it will probably be in the neighborhood of a year. We can do it easily in 14 days. How do you compare the drying cost? If you get an answer from a conventional kiln operator drying 3" material, ask about degrade. Ours is less than 1%. Often, there is zero degrade.

No question that vacuum is ideal for thick red oak.

One of the problems with vacuum is that past units have generally been very poor with salespeople saying that they will do everything, when in fact they do not. So, the data I have is based on these units. Perhaps we will have some more modern equipment and better costs.

Gene Wengert, forum technical advisor

Do vac kilns use less energy than conventional?

What is the most efficient source: diesel, wood waste, peat, gas, electric?

How do vac kiln do with softwoods - say 6 x 2 rafters, 7 x 3 beams?

Energy cost and energy usage are two different things. Dehumidification kilns use much less energy than any other kiln but how much that translates to savings depends on electric rates. A couple of years ago, when you could buy oil for 50-60 cents a gallon or gas for $2-3 a million BTU, the costs in some cases were about even. Now that those prices have increased much faster than electric rates, even in California, the differences are greater and it is very rare when dehumidification does not cost a lot less for energy than any other system.

Even wood burning in many cases costs more. I saw figures last week from a Canadian mill that showed that a wood fired system drying 50 million BF a year would spend more than $1 per MBF more running a conventional wood fired system than DH. One customer in the US who switched from DH to conventional with a wood fired boiler told me that he would never save enough to repay the investment. But there are cases where it comes out the other way and I pride myself on presenting that honestly to customers.

As with all economic calculations, there are many ways to add the numbers. If you are burning wood waste, you have the capital cost for a boiler--do you depreciate it over 10 years, 20 years, or ? Then there is the cost of fuel preparation and storage. Wood burning is expensive. On the other hand, if you can only get interruptable natural gas, or bunker C oil that requires extra heating to get it to flow, wood burning can look cheaper. But if you do not burn your waste, what do you do with it and how much does that cost? Often getting rid of volumes of wood waste is expensive--hogging, transport, and so on. With a boiler, of course you may need 24 hour attendant, special permits, and so on.

With respect to electricity, the rates here have gone up faster than the gas rates. So, even though DH uses less energy than a gas fired steam kiln, the energy cost is more than the alternate fuel ON A LARGE SCALE. When dealing with smaller scale units, gas or oil systems will be more expensive. So, when comparing energy costs, you also need to throw in economy of scale. Then in Canada, there was a period when the utilities gave tremendous incentives to use electricity--costs were several cents per kWh! There have been similar incentives in Georgia and Alabama, but not quite as attractive.

Also, remember when looking at electric rates, most commercial rates are about 50% electrical energy usage and 50% demand charges. So although the quoted rate may be 5 cents per kWh, the actual cost will be 10 cents when demand charge is added. And they can add a few other charges, too (reactive, meter, etc.).

The truth of the matter is that it takes about 2,500,000 Btu of heat to evaporate the water in wood no matter what drying system you use. A DH unit or heat exchangers on the vents recycle some of the energy, making such systems very low energy users. Then there are heat losses through the structure (27%) and vent losses (if you have vents) (23%). Certainly a well insulated DH kiln will have lower losses than a conventional, old steam kiln. The best way to save energy (and not lose quality) is to reduce the MC by shed drying or quality air drying. Out west, vent heat exchangers can reduce energy use in conventional systems by 30% or more. (In the south, energy (wood waste) is so plentiful and cheap that there is no financial benefit in heat exchangers for most mills.)

Although saving energy is seldom economically beneficial in the U.S. or Canada without tax incentives (expect to see incentives with George W. in charge now), if you have a boiler that is "max-ed out", then saving energy is a way to delay having to make a capital investment.

Hopefully, now you appreciate why it is usually possible to find someone that says they are saving costs with this or that energy system. The picture is very complex with many different facets to the energy picture.

Gene Wengert, forum technical advisor

Speaking of drying cost, a vac kiln customer told me today that when he shuts down all his conventional kilns, he will be saving $25,000 a year by not buying end sealer.

This sounds fishy, as the cost of end sealing is $2.50 per MBF. So, to save $25,000, you would be end sealing 10 million BF per year. I do not think that we have a vacuum dryer that is drying 10 million BF per year??

Gene Wengert, forum technical advisor

I'll check on their cost per 1000. His vac kilns will dry around 2MMBF. He wants to increase vac kiln capacity to around 6MMBF.

I had another customer tell me that he was glad to increase vac drying capacity and reduce air drying. The quantity of end sealer that he was using for air drying had put his company in a special class with government regulators.

He must not have been using Anchor Seal or similar type coatings, as these are not a concern (water and wax) to regulators.

Gene Wengert, forum technical advisor

A lot of high production people don't like the emulsions because they make everything slippery.

Seems to me that the money one would save on end coatings, one would spend on water treatment for the boiler and cooling tower. The EPA gets real concerned on the "water" going down the drain.

RF/V drying does work, but unless you can move a lot of wood in a short time, the cost just isn't worth it.

The customer that I mentioned spending $25,000 on end sealer saw this post. He went back and checked. Between 7/1/98 and 6/30/99, he actually spent $39,000 on sealer, thinner and freight.

His end sealer is not the commonly used Anchor Seal from UC Coatings. It costs only $2.50 to $3 per MBF to use their coating. So at $39,000 per year, that is over 13 million BF of lumber. The thinner used for Anchor Seal and similar coatings is water, so I am confused about his costs for thinner and shipping it!

Perhaps he is referring to a coating put on dried lumber and dried parts to prevent the ends from drying out.

Perhaps he himself can clarify directly what is actually going on.

Gene Wengert, forum technical advisor

My company manufactures squares and very little lumber. We cut ash, beech, cherry, hard maple, soft maple, red oak, and white oak. We cut our squares to length green to maximize kiln capacity (kiln capacity had always been a limiting factor until our recent installation of vacuum kilns). Since they are cut to lengths ranging from 8" to 84"(not many of the latter), there is much more surface to coat than an equivalent amount of lumber. We use a product made (sold) by Pierce and Stevens in Buffalo, NY. For many years it has given us excellent results in the prevention of end checks. I should add that we do not put any of our squares outside, they stay inside out of the sun. The squares we cut range from 4/4 to 14/4. Most of the stuff we dry is "off the charts" of standard kiln drying schedules. Most of our stock is 10/4 or larger.

We have tried Anchor Seal and unfortunately have found problems enough to discontinue its use. First we found that with our large squares we got excessive checking in the dry kilns. We could not live with that. Second, since our operation is inside, the floors from the spray booth area to the area where we piled the squares on pallets for drying got so slippery that it was dangerous and unproductive. We probably could have solved the slipperiness problem if the product had prevented checking of the wood. Anchor Seal is cheaper (much cheaper), more environmentally friendly and safer (our end seal is very flammable), but it did not do a good enough job of preventing end checks for us.

The use of our vac kilns has freed up enough conventional kiln space that we are now drying more squares "in the blank" and we will use less end seal there. Of course we don't end coat the squares we vacuum dry, so we will save on end seal there also.

Thanks for the clarification. I have seen many short square manufacturers dipping the ends directly in hot parafin wax, but have not heard of the coating that you are using. When we did some vacuum drying tests we coated the ends of the lumber and also squares but then removed the coating before vacuum drying at the request of the vacuum kiln manufacturer. Without coating the ends, we noticed very little checking in the vacuum kiln.

Gene Wengert, forum technical advisor

The mention of paraffin wax evokes another thought. We have purchased squares form other manufacturers that had wax on them. One of our dowel machines (Hawker D350) just could not feed them due to slippage. We get very little end checking when vacuum drying if all the details are taken care of before and during drying. We do not end coat that material. The story I was told many years ago is that the Pierce and Stevens product was developed to prevent end checking of hard maple bowling pin blocks, hence we call it pin block seal. They currently call it endseal, product number E1009.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Ref = "One of our dowel machines (Hawker D350) just could not feed them due to slippage"

The next time this presents a problem, please check the infeed rolls to make sure the knurl is not worn out. Also that the outfeed rolls are set on the proper diameter. The knives must be kept sharp and at the proper setting.