Troubleshooting vacuum kiln problems

      An extensive discussion on vacuum pumps and vaporization of organic compounds in wood. February 28, 2001

Q.
When I suggested that the wenge in our vac kiln might "gum up" our condensers, I was joking. One load couldn't do that. However, when I ran the 8 PowerDry RF/V kilns at VELDA (nearly 20 years ago), I had a problem with the vac pumps. It turned out to be a sticky residue on the condensers. I believe that a lot of organic compounds vaporize (evaporate) when ambient pressure is reduced. I also believe that the wenge dropped its final 15% in about a day because I set the chamber pressure to swing from a high that built vapor pressure in those gums to a low that "evaporated" enough to get the water out. What do you think?

The "swing" was 60 torr from top to bottom. It was taking about 2 hours to get from top to bottom on Christmas Day. It was taking 20 minutes to get from top to bottom when they shut it down the next day.

Forum Responses
Your decompression rates (Torr/min) are too low to expect any direct effects, like bubble formation or strong cooling.

Gums will plug the vessels. Extreme conditions (low EMC's) are needed to vacuum dry resinous woods. At pressures below 40 Torr the supply of heat of evaporation may become a limiting factor in the drying rate.

My vacuum kiln is continuously heated in the 60-150 Torr range, enough for powering continuous de-watering rates of 4 liter/cbm wood/hr. I have never seen a beneficial effect on the drying rate using a pressure "swing" in my vacuum kiln. Maybe you need the high pressure phase of the "swing" to heat the wood, to power the evaporation process during the low pressure phase with insufficient heat supply.

The fast decrease in MC readings below FSP is a typical behavior in vacuum drying and is possibly not related to the pressure "swing".



From the original questioner:
We have built vacuum kilns with view ports. We do indeed see bubble formation on the decompression side of a pressure "swing". Perhaps you could use a larger pump or you may be missing components in your piping.

I'm not convinced that we can dry resinous wood at an extreme speed. I didn't see the wood going in or coming out. But I've got a customer who says it worked and a computer interface that logs every detail every minute of every drying run.

Our kilns are also continuously heated. We size all the components to dry a minimum of 2.25 gallons/hour/MBF. I've seen them do it a lot faster.

I never mentioned MC readings. I mentioned an indication from the computer generated graphs.



Wood does not vaporize at room temperatures--otherwise, you would see it happen, albeit slowly, to a piece of wood just sitting around. (An exception would be the aromatic species like cedars and resinous softwoods. Do I recall that VELDA only did oak?)

When drying wood, other things are happening, such as evaporation of acids, movement of fine dust, and so on. I suspect this is what was causing the deposition on the coils. The organic compounds in wood (except as noted) do not vaporize at 100 F or so.

It is possible to "open" the wood structure with large pressure swings, actually resulting in micro cracks. I do not believe that you had enough of a pressure change to create such an effect. Perhaps the surface dried very quickly, giving you an average loss that seemed high, but the core was still quite wet?

Gene Wengert, forum technical advisor



From the original questioner:
I decided to look for the vapor pressures of the compounds in wood. Gum, sap, pitch, resin--just what are we talking about? I couldn't find any specifics. There's all kinds of polymers and alcohols which seem to be common in wood. So you definitely are not going to vaporize cellulose in a vac kiln (unless it's one of those flamin' RF/vac kilns), but what about this other stuff? Maybe vac drying would be harmful to something with desirable natural "oils".


If wood vaporized at temperatures under 100 C (212 F), the oven-drying test that is the basis for measuring MC would be inaccurate. (I am not talking about vaporization of one millionth of a gram, but substantial vaporization.) In fact, the oven test is even used for resinous woods such as pine, with the knowledge that there will be slight inaccuracy.

Gene Wengert, forum technical advisor



Pressure swings in a vac kiln do nothing but slow the drying process and cause "case hardening".


Moisture moves in wood as a result of a vapor pressure gradient (this is called diffusion-- For example, you smell perfume after a few minutes from a bottle that has been opened on the other side of the room) and by a total pressure gradient (called mass flow or total pressure flow, which would be similar to smelling perfume if it was squirt at you. There is pressure in the squirt mechanism that is higher than in the room). Total pressure flow is usually, unless the wood is totally occluded and has no internal flow, much faster than diffusion. In a vacuum drier, the moisture moves quickly when the vacuum is first started, because inside the wood the pressure is atmospheric, but outside (in the kiln) there is a vacuum with very low pressure.

Gene Wengert, forum technical advisor



From the original questioner:
Pressure "swing" can not cause the drying process to slow and it can not cause case hardening.

In a vacuum kiln, you can either let the pump pull constantly or cycle the vacuum. Pulling constantly is a bad idea because the chamber pressure will vary with the amount of vaporization caused by heat input. If chamber pressure rises, boiling point rises and wood temperature rises. Furthermore, (and this is an opinion) constant pulling of pressure causes an increase in MC gradient between shell and core and results in case hardening. A "swing" occurs when a timer or an on/off pressure controller is used. Some people operating a vac kiln don't realize that they are operating with a "swing". It's hysteresis, deadband, cycle time, vac duty or something with another name but it's there. All vac kiln manufacturers have known the minimum: pull on a dry load and you will cavitate your pump. Cavitate enough and the pump is junk.

The question is this: Does the amount of the "swing" affect drying time and drying quality? As far as drying time, I'm not sure. If everything else is under control, drying rate is directly proportional to the rate that we add heat. Quality is the big question. Does a wide swing reach deeper into the wood? Does a deeper reach decrease MC gradient? Does a wide swing affect the organic compounds that started this discussion?

I believe, after 20 years of study, that there is no question that a controlled "swing" is the key to consistently short drying time and consistently high drying quality. I don't believe that there is an optimum pressure and hysteresis that will cover all species and thicknesses.



It is possible to achieve high moisture removal rates with constant pull vacuum drying. You need active control of pressure and temperature in the vac kiln to keep deviations within sufficiently narrow margins. The MC gradients are controlled using a gradual decrease of pressure as drying proceeds, in much the same way you'd do with the RH in a conventional kiln.

I agree that the pressure swing will only slow the drying process and can give severe case-hardening (when using a large pull).

There is a peak moisture removal at the high-to-low transition of the pressure; during the high pressure stage of the swing, drying will cease. The net effect is negative compared to continuous drying. This is because the pressure swing disturbs the delicate (psychometric) equilibrium between the evaporating moisture and its ambient.

Expanding air bubbles on decompression give an enhanced moisture flow. People investigating this effect use multiple "swings" of 10 to 100 ato. That is some 1,000 times the swing magnitude you use. It is not possible with this method to dry wood to less than 40% MC.



From the original questioner:
You can not control MC gradient by decreasing chamber pressure.

From what pressure would you begin? If you started by holding pressure steady above 80 torr, for example, the actual temperature of the wood would exceed 120 F (47 C). Vapor pressure is going to follow the laws of physics and MC gradient will be unaffected by your pressure setting. Furthermore, you can not decrease chamber pressure if you have a vac kiln full of warm, wet wood. Again to the vapor pressure. It will develop in the water in the wood to the degree that you add heat. That vapor pressure sets the minimum chamber pressure as long as water is available. If you try to pull under that pressure, water vapor fills the vacuum. Chamber pressure will not go down until heat is used and vapor pressure goes down. Also, you're running the wrong direction on the performance curve of vac pumps. Capacity greatly decreases as you go to lower pressures. In a properly controlled vac kiln, you can increase drying rate as MC decreases. This requires the pump to move an increasing amount of vapor.

If you have the right controller, pressure never goes so high that drying will cease, although drying will slow. When drying slows, wood temperature increases and vapor pressure increases. I believe that this decreases the MC gradient. When decompression begins, rate of vaporization increases as gradient increases. When chamber pressure approaches vapor pressure, drying rate is at the maximum. If you keep pulling, drying rate is going to decrease until it matches the rate that you're adding heat. (I assume that we are talking about a kiln with hot water platens.) This would contribute to case hardening because you never give the wood a chance to equalize temperature or MC (shell to core). Another assumption I've made is that we are discussing thick stock. Our customers usually dry material from 2" to 3 1/2" (55mm to 90mm) thick. 4/4 is so easy that any approach will work.



Maybe "swings" will work, but... Vapor pressure is the pressure exerted by a vapor when the rates of condensation and evaporation are equal between the liquid and vapor state.

If the chamber pressure goes above "schedule" when the MC of the wood is above 30%, the process temperature (wood temp) is going to go up also. The warmer the wood fibers, the more susceptible they are to cracking and warping caused by internal stresses. So, if the vacuum slips from 30 torr to 60 torr, and you pull it back down too fast, you will evaporate water off the surface too fast and cause "shell drying," which again results in stress and cracks. Also, if you lower the heat, you're not bringing water to the surface fast enough to keep an even moisture gradient between the core and shell and again, this will cause damage.

Of course, if the MC is under 25%, things change. Vac kilns don't dry even so the shell and core of all of the boards won't be the same. You'll find some boards that were dried too slowly and they will have wet cores. Some will dry too fast and they will have wet shells--the damage is already done. You will find a few that are just right. The fibers are already set.

So, I believe that pressure swings in a vac kiln slow the drying and cause case hardening.



From the original questioner:
Vapor pressure is exerted by solids and liquids. Vapor pressure is a function of temperature. Vaporization increases when ambient pressure is decreased. A liquid boils when vapor pressure equals ambient pressure.

If you were pulling from 60 to 30, why would you feel that you were "shell drying"? If you're dealing with something like red oak, you're decreasing pressure down in those vessels. If you reach way down in these vessels, wouldn't you be decreasing MC gradient between shell and core?



The point that I am trying to make is this. If you are "boiling" water, sap, resin or whatever from wood at a set pressure and heat, then you come along and raise the chamber pressure to whatever, everything that you were evaporating from the wood comes to a complete stop. It will do this until the temp of the "plates" catches up with the new atmosphere that you have created.

Most resins in wood will become hard when heated and then cooled very quickly. When these resins dry, they block the "pores" in the wood. Wouldn't this make it impossible to move any water through?

Controlling the phlem temp around the load to evaporate the moisture you are bringing to the surface of the wood seems more logical to me.



From the original questioner:
Vapor pressure increases as a result of movement of the water molecules. As temperature rises, movement increases and vapor pressure rises. Do you suppose that this movement could assist in diffusion? Then, if this kinetic energy is used while pulling back down, nothing is lost. Increased diffusion is gained.

Temperature at the high side is much lower than what is needed to set any "resin".



The vapor pressure and temperature in a vac kiln define an equilibrium moisture content (EMC), just as temperature and RH do in a conventional kiln. In fact, you can calculate the RH in a vac kiln directly from the (air-free) pressure: RH = pressure / sat.pressure x 100%.

So, at any fixed temperature, RH is proportional with the pressure. Pressures, slightly under saturation, give mild conditions (high RH); very low pressures (low RH) correspond to low EMC's, which can cause severe case hardening. At any stage (MC) during the drying process a suitable pressure (EMC) can be chosen, to prevent large MC-gradients. This allows me to control the MC-gradient.

A pressure swing leads to an RH-swing. I will pose a new question to the forum on the effect of an RH-swing in conventional kilns.



From the original questioner
It has been a common mistake for vac kiln operators to apply conventional kiln operating procedures to their vac kilns. The dynamics in a vac kiln are completely different.

EMC is a major factor in conventional kilns because you're skimming water off the surface.



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