Steam Conditioning Versus Mist Conditioning in Wood-Drying Kilns

Both are effective, and each has different quirks to consider. June 13, 2014

We are looking at a kiln system that would use hot water instead of steam for the heat source, but more importantly uses a water misting system instead of the more traditional steam for conditioning the wood at the end of the drying cycle. Does anyone have any information on this to share about the pros and cons?

Forum Responses
(Commercial Kiln Drying Forum)
From contributor W:
It doesn't make much difference whether the water is hot or cold in an atomizing system. There are types with high pressure systems where the water is pumped to 500-1000 psig. Then it atomizes very finely, almost like steam. The nozzles are about the size of a human hair so the water has to be filtered to 1 micron. Then the nozzles can be a maintenance problem. There is a cooling effect in the chamber from the water being sprayed. If steam is used, the evaporation takes place in the boiler so there is no heat load, often the steam overheats the kiln and needs some tricks to control. They are usually figured out pretty soon though. Another system is low pressure (80-120 psig) where a simple shallow well pump is used to raise water pressure and then the nozzles can be larger and less maintenance is needed.

From contributor F:
How big of kiln are you considering? The overheat problem with steam is not really a big concern. As Contributor W mentioned it is figured out very quickly. Also, if the kiln is small there is more surface area per volume for the heat to be transferred to the outside and the temperature does not increase as much.

From the original questioner:
The kiln size we are considering is 50,000 board foot. I am trying to find out if there are any issues with such a system and what they might be as I only have experience with a steam heated kiln system.

From Gene Wengert, forum technical advisor:
I have looked at the water mist systems and find that nozzle maintenance is a big issue for continued performance. Also, water drops on the lumber can be an issue. The temperature drop is seldom an issue. The relief of longitudinal stresses with water seems poor in my experience. Steam certain, if used with little super heat is very effective. You need to avoid super heat with either a low pressure boiler or a desuperheater.

From contributor G:
Cold water systems work well when properly sized, installed and maintained. The first step is a good filtering system. You'll basically need drinking quality water going to the nozzles. With the systems we supply, we include the filter package to be sure the water is of good quality. The size of the system is very important as well. Too many nozzles and you'll have water on the walls and floor, not enough and it will take too long to condition. If the nozzles are the right size, the atomized droplets will be small enough to be absorbed into the wood without forming a drop on the surface of the wood. Pressure and nozzle size are extremely important to getting a small enough droplet.

The mist nozzles need to be installed in a high velocity air stream in front of the fans, similar to a spray line to ensure good mixture of the water droplets with the airstream. Too close to the kiln wall and you'll get more water condensing on the wall. Kiln temperature is also important when conditioning with water. I have found that at temps under about 130 deg F the system is not as effective as it should be. In mills that dry eastern white pine we have found that they can successfully condition at this temperature with cold water, where in the past many have limited themselves to about 120 deg F. I think this is because when they condition with steam, although they set the dry bulb at 120 deg F, the actual dry bulb temp often climbs up over 130 deg F anyway because of the heat from the steam. With a cold water system the dry bulb cannot increase so they are able to better maintain their low db for conditioning. In most instances when the cold water spray is on the heat will come on as well in order to maintain the db setpoint.

As for stress relief, we have also found that the surface of the boards (at least in white pine) do not dry out as much, and customers have noticed they get less roller check in the planer mill when they use the cold water system. As for energy savings, using a cold water system may save you money over steam spray depending on how you produce your steam. It may also reduce the load on an overtaxed boiler, thus freeing up some energy for other kilns or other uses. I would encourage you to talk to people who have used the particular system you are looking at, and especially those who have switched to it after using a steam spray, and find out what improvements and/or issues they have noticed.

From contributor N:
I am running three kilns that are steam heated and two kilns that are hot water heated which hold 90 cu mtrs each. In my opinion the hot water kilns perform much better both in running cost and maintenance. The hot water kilns use about 35% less gas. For conditioning and equalization I use both steam and water to get the best results. The hot water kilns run at 80 deg c without any problems. As Gene said spray nozzle maintenance is a major issue. I use 0.5 micron nozzles at 150psi which does the job well. I use adjustable cat pumps that will do 2000 psi but running at higher pressure than needed seems to cause more blocked nozzles. We use a charcoal filter for the water but still had problems with the nozzles blocking up. It ended up after tests that the lime in the water was causing the problem. I think hot water for heating at the end of the day is far more cost effective and I could never imagine equalizing or conditioning without steam and water.

From contributor M:
I have both a water mist system and a steam spray system in a small research kiln. The spray system is one of the high pressure systems with fine nozzles, etc. For the final conditioning, both systems do a good job. Contrary to what some of the posters say above I find that with our small kiln the steam spray also overheats the kiln and it is hard to maintain the desired differential temperature between the wet bulb and the dry bulb. Maybe that is in our setup, but I know of commercial systems where that is a consideration also. I have had three research kilns and all of them tended to overheat on conditioning, making it hard or impossible to get the close wb-db differential required, so I think it can be a problem. The water spray system produces a slight drop in temperature when it comes on and the heat comes on a little, so the differential wb-db is maintained with ease.

I find that both systems do about the same job in conditioning. In the research kiln conditioning seems to go much faster than in some commercial operations. Maybe it is because we actually get to the set conditions and maintain them easily. What the water spray system does not seem to be good for is adding humidity to the kiln during the run at low temperatures. If I have enough nozzles set up for conditioning, then I spray too much water to evaporate at 130 or less. We dry oak at 110 and have to add humidity sometimes. I gave up on getting the water system to help here.

Maintenance for the mist system is higher than for the steam system. When the system in installed a good filtration system is a necessity. Do not skimp on the filtration. I guess my take is that if you have steam and are getting good results with it, go with steam. If you are having trouble conditioning, or don't have steam readily available for that purpose the high pressure-fine nozzle water will work at least as well and possibly better for conditioning. If you need to add a little humidity at lower temperatures you can do it with the spray system, but you will need a system that lets you cut out most of the nozzles for that. Solenoid valves/lowering the pressure does not help. You will need to budget a little more time for maintenance. You’ll probably have to flush the nozzles every load or so.

From Gene Wengert, forum technical advisor:
Research kilns tend to be better insulated than actual full size kilns, so overheating can be expected. Also, in your kilns do you use a desuperheater? That is critical, for even if the steam pressure is reduced before going into a kiln, the heat is still there, that is the steam is superheated.

Anyone that uses steam and has overheating problems should read page 100 in Drying Hardwood Lumber, which talks about cooling before steaming or using a DB 10 F higher than used for equalizing. Of course, a desuperheater is also a good piece of equipment and it uses a water spray in the steam line. I am surprised that a kiln company has not put together a hybrid system using both steam spray and water, which should be the best of both worlds.

From contributor S:
Steam conditioning in a commercial and production setting is the best way to go as far as maintenance is concerned. The more the kiln needs to be maintained the more it can set you back in production. Our steam system produced our record amount dried for a year 11.75mbf with two 40k bf a 65k bf, and an 80k bf kiln.

From contributor G:
Gene we have a customer getting ready to put both systems into a brand new kiln. Couldn't talk him out of either one!

From contributor W:
Nyle has installed several systems using both steam and water mist. We run the steam spray on a call from the wet bulb control and the water spray from a call on the dry bulb control. When the wet bulb is above set point the steam spray opens and the dry bulb and wet bulb both rise. As the dry bulb rises, the water spray turns on and cools and adds humidification causing the wet bulb to come up a little faster and reducing steam demand. The problem we have seen with this is getting kiln operators to understand what is going on, and why.