Question
We are considering purchasing a vacuum kiln (hot water platen type). We only deal in high grade lumber. Our goal is to provide premium kiln dried lumber and components to cabinet suppliers. Being a small manufacturer, is this type of kiln the best option for our business? The drying speeds of these kilns are very fast as opposed to conventional kilns. They are also far superior for drying thick stock. I would like to further understand the downside (if any) of using a vacuum kiln. I'm not concerned about the capital investment. I'm only interested in providing the best quality material with the most reliable control. Will vacuum do this for me or should I further consider DH?
Forum Responses
(From WOODWEB's Sawing and Drying Forum)
Drying thick stock such as maple is a good application for vacuum. It is unlikely that quality of 4/4 material will be as good as with a conventional or DH kiln. If quality is your concern, for 4/4 to 8/4, DH or conventional is the best way to go. If speed is your main concern or you are drying very thick material, vacuum is a good choice. Evenness of moisture content is harder to achieve with vacuum. Operating costs and capital costs are higher with vacuum. There is a good reason that 99% of the kilns sold in the US each year are conventional and DH kilns.
I would visit a kiln drying the same lumber that you are interested in before investing in "new" technology. If you cannot visit, could you send a load of your lumber to one of their kilns and have them dry it for you? Stay with it so you can record drying times, etc. Check energy consumption.
The rejection of wood for high MC depends on the customer. Many customers do not even check the MC.
A well-run conventional drying operation can have very little degrade if run properly and if the wood is good to begin with. A vacuum kiln cannot do better for any products that I have seen, except for thick red oak.
The person who is selling you the kiln… if you ask them for the exact procedure for drying your species and thicknesses, can they tell you and write it down? Will they give you a performance guarantee? Is this just a guess, or do they have experience with this being dried in a vacuum? I have seen a lot of lumber ruined while an operator is trying to figure out how to dry in a vacuum.
Vacuum drying patents exist that are over 120 years old, I am told. Vacuum has been around for years, but has not been economically viable for standard lumber products. I have yet to see a vacuum kiln dry to uniform final MCs, except for VacuTherm.
Gene Wengert, forum technical advisor
As far as heat is concerned, you don't need to heat a structure, you don't lose heat through walls and you don't dump heat with vents. It takes less than 20,000 btu's per hour per 1000 BFt to dry red oak at 1/2% per hour.
Gene, tell us about your experience with VacuTherm and their uniform MC.
Cost, of course, is always a concern, but one would have to check that out for a specific situation. With thick lumber, conventional costs would be extremely high, so vacuum will win in most cases. For 4/4, I doubt that vacuum will win, especially with a wood waste boiler that is also disposing of wood waste. (Stop burning and you would have to pay to have the wood hauled away.)
Is the electric energy cost you gave above just for the vacuum pump? With 1000 BF of red oak, you need to evaporate about 3000 pounds of water, which will require 3 million BTUs or about 870 kWh per MBF.
Gene Wengert, forum technical advisor
We have no problem with heat from wood waste boilers. Steam works great and helps get RH up in cold chambers. And, we have a customer in New York who uses an outdoor furnace. Since his vac kilns don't need a lot of heat, they rarely drop in temperature when the fire goes low.
You're slightly high on btu's but close enough. Divide 3 million by 64 (% change in MC) and divide that by 2 (1/2% per hour). What do you get?
Vac kilns are great for drying thick wood and yes, there are a few down sides. If you don't care about the cost of buying a vac kiln, the cost of operating one should be no problem, either. If you need to dry thick hardwood fast, go for it.
The ability to dry thick stock will allow me to target specialized, better-paying markets, which will maximize the advantage of being a small manufacturer. The ability to take an order for any profiled dimension, dry the material, machine, and ship it in less time than it takes a large millwork business to dry the material alone would give me an obvious competitive edge. I believe the operating cost is comparable to a conventional kiln but the speed is what impressed me the most.
Stress is caused by bad schedules and/or the inability of the control system to hold the schedule.
"Drying too slow" causes honeycomb? Drying slowly without keeping the RH up causes shell-drying and that leads to honeycomb. If you're drying something like a 4-1/2" x 9" dimension, you have to go slow because of the uneven shrinkage. But you avoid shell drying by keeping RH up.
You don't need weight to avoid warp, but if you need to resist cup, and you have a rectangular chamber (instead of cylinders, like you have), you can put an inflatable bladder on top of the load and let a little air into it.
Gene, the Vacu-Therms I have seen have had very poor pressure control and little or no RH control. There is no reason to consider "moisture movement variations" or "initial MC variations" or "heartwood/sapwood." I don't believe that Vacu-Therm has any equalization process.
Contributor D, I know that the Vacu-Therm kilns that Jim Parker used to dry my wood had equalization included. As a result, he achieved excellent uniform final MC.
Anyone, there was a report from Clemson University about the moisture gradients and stresses in rf vacuum drying (Power Dry Kilns). Indeed, they were fairly high. Honeycomb did occur.
In the Wood-Mizer kilns, we were able to create drying stresses and also had a wide range of final MCs.
Gene Wengert, forum technical advisor
As I said, you certainly can cause stress in vac kilns but Power Dry's junk is not in the same class as today's kilns.
Common Lumber Name | A | B | C |
Hardwoods | |||
Alder, Red | 9.9 | 19.2 | 2506 |
Apple | 10.9 | 31.7 | 4132 |
Ash, Black | 9.3 | 23.4 | 4132 |
Ash, Green | 14.3 | 27.6 | 3590 |
Aspen, Bigtooth | 10.3 | 18.7 | 2439 |
Aspen, Quaking | 10.3 | 18.2 | 2373 |
Basswood | 6.2 | 16.6 | 2174 |
Beech, American | 8.9 | 29.1 | 3793 |
Birch, Paper | 8.8 | 25.0 | 3260 |
Birch, Sweet | 11.9 | 31.2 | 4065 |
Birch, Yellow | 9.2 | 28.6 | 3723 |
Buckeye | 8.9 | 17.2 | 2235 |
Butternut | 11.3 | 18.7 | 2440 |
Cherry | 13.8 | 24.4 | 3184 |
Chesnut, American | 11.6 | 20.8 | 2708 |
Cottonwood | 8.5 | 16.1 | 2102 |
Dogwood | 6.8 | 33.3 | 4331 |
Elm, American | 10.2 | 23.9 | 3116 |
Elm, Rock | 12.2 | 29.6 | 3860 |
Elm, slippery | 11.5 | 25.0 | 3251 |
Hackberry | 11.8 | 25.5 | 3319 |
Hickory, Bitternut (Pecan) | 14.7 | 31.2 | 4062 |
Hickory (True) | |||
Hickory, Mockernut | 9.1 | 33.3 | 4332 |
Hickory, Pignut | 9.3 | 34.3 | 4332 |
Hickory, Shagbark | 10.9 | 33.3 | 4333 |
Hickory, Shellbark | 6.6 | 32.2 | 4195 |
Holly, American | 8.3 | 26.0 | 3387 |
Hophornbeam, Eastern | 7.9 | 32.8 | 4266 |
Laurel, California | 15.1 | 26.5 | 3456 |
Locust, Black | 21.2 | 34.3 | 4470 |
Madrone, Pacific | 7.8 | 30.2 | 3925 |
Maple (Soft) | |||
Maple, Bigleaf | 12.8 | 22.9 | 2980 |
Maple, Red | 13.1 | 25.5 | 3318 |
Maple, Silver | 12.4 | 22.9 | 2981 |
Maple (Hard) | |||
Maple, Black | 12.3 | 27.0 | 3523 |
Maple, Sugar | 12.3 | 29.1 | 3793 |
Oak (Red) | |||
Oak, Black | 11.7 | 29.1 | 3792 |
Oak, California black | 16.4 | 26.5 | 3455 |
Oak, Laurel | 6.3 | 29.1 | 3791 |
Oak, Northern red | 13.6 | 29.1 | 3793 |
Oak, Pin | 13.0 | 30.2 | 3928 |
Oak, Scarlet | 13.2 | 31.2 | 4065 |
Oak, Southern red | 9.6 | 27.0 | 3520 |
Oak, Water | 10.4 | 29.1 | 3793 |
Oak, Willow | 6.4 | 29.1 | 3790 |
Oak (White) | |||
Oak, Bur | 15.4 | 30.2 | 3928 |
Oak, Chestnut | 10.1 | 29.6 | 3858 |
Oak, Live | 17.5 | 41.6 | 5417 |
Oak, Overcup | 10.7 | 29.6 | 3860 |
Oak, Post | 11.0 | 31.2 | 4063 |
Oak, Swamp chestnut | 10.7 | 31.2 | 4063 |
Oak, White | 10.8 | 31.2 | 4062 |
Persimmon | 7.0 | 33.3 | 4332 |
Sweetgum | 8.9 | 23.9 | 3115 |
Sycamore | 10.7 | 23.9 | 3115 |
Tanoak | 9.0 | 30.2 | 3926 |
Tupelo, Black | 10.4 | 23.9 | 3116 |
Tupelo, Water | 12.4 | 23.9 | 3115 |
Walnut | 13.4 | 26.5 | 3454 |
Willow, Black | 8.6 | 18.7 | 2438 |
Yellow-poplar | 10.6 | 20.8 | 2708 |
Common Lumber Name | A | B | C |
Softwoods | |||
Baldcypress | 13.2 | 21.9 | 2844 |
Cedar, Alaska | 14.4 | 21.9 | 2844 |
Cedar, Atlantic white | 10.9 | 16.1 | 2100 |
Cedar, eastern red | 16.4 | 22.9 | 2981 |
Cedar, Incense | 13.1 | 18.2 | 2371 |
Cedar, Northern white | 11.1 | 15.1 | 1964 |
Cedar, Port-Orford | 12.6 | 20.2 | 2641 |
Cedar, Western red | 12.2 | 16.1 | 2100 |
Douglas-fir, Coast type | 12.3 | 23.4 | 3049 |
Douglas-fir, Interior west | 13.2 | 23.9 | 3116 |
Douglas-fir, Interior north | 14.0 | 23.4 | 3048 |
Fir, Balsam | 9.9 | 17.2 | 2236 |
Fir, California red | 10.6 | 18.7 | 2437 |
Fir, Grand | 10.7 | 18.2 | 2371 |
Fir, Noble | 10.1 | 19.2 | 2507 |
Fir, Pacific silver | 10.4 | 20.8 | 2711 |
Fir, Subalpine | 10.5 | 16.1 | 2101 |
Fir, White | 12.2 | 19.2 | 2506 |
Hemlock, Eastern | 12.6 | 19.8 | 2573 |
Hemlock, Western | 11.5 | 21.8 | 2847 |
Larch, Western | 11.3 | 25.0 | 3251 |
Pine, Eastern white | 12.3 | 17.7 | 2303 |
Pine, Lodgepole | 11.5 | 19.8 | 2576 |
Pine, Ponderosa | 12.6 | 19.8 | 2573 |
Pine, Red | 12.2 | 21.3 | 2777 |
Southern yellow group | |||
Pine, Loblolly | 12.9 | 24.4 | 3183 |
Pine, Longleaf | 15.0 | 28.1 | 3658 |
Pine, Shortleaf | 12.9 | 24.4 | 3183 |
Pine, Sugar | 12.6 | 17.7 | 2302 |
Pine, Western white | 10.0 | 18.2 | 2370 |
Redwood, Old growth | 14.9 | 19.8 | 2573 |
Redwood, Second growth | 13.2 | 17.7 | 2302 |
Spruce, Black | 11.3 | 19.8 | 2575 |
Spruce, Engelmann | 10.0 | 17.2 | 2234 |
Spruce, Red | 10.6 | 19.2 | 2506 |
Spruce, Sitka | 10.8 | 19.2 | 2506 |
Tamarack | 12.0 | 25.5 | 3318 |