Brubaker, William

Western Dry Kiln Clubs

Western Dry Kiln Clubs. Meeting (23rd : 1972)

Undetermined

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POSITIVE HUMIDITY CONTROL FOR DIRECT FIRED KILNSWilliam BrubakerMoore Dry Kiln Company of OregonAshland, OregonIn recent years, there have been two predominate methods ofheating kilns in the western United States -- namely, steam heating anddirect firing. Steam heating, the most widely used, can be categorizedinto low pressure systems (15 psi or less) and high pressure systems(100 to 150 psi). Direct firing, on the other hand, traditionally has beenwith natural gas as a fuel, primarily because of its convenience, lowcost, and ready availability -- up to now. "Up to now," we say becausethe picture of natural gas availability is quickly changing in most of thecountry.We'll quickly abandon steam heating from this talk, because agood steam heating system is pretty hard to build a case against.The purpose of this presentation is to discuss humidification sys-tems to be used in direct fired kilns. Please keep in mind the differ-ence between humidification and humidity control. Humidificationwould be the process of adding moisture to the kiln atmosphere or en-vironment. Humidity control, on the other hand, would be adding orsubtracting moisture to maintain a desired humidity level, or what wecall a wet bulb temperature set point. So, while humidification concernsitself with a steam spray or some other device, humidity control in-volves the control and operation of ventilators, as well as steam sprays.Let's sidetrack here a moment, to preface this talk a little further,let's put to rest a few superstitions that are commonly held in the indus-try regarding wet bulb control and high temperature drying (schedulesin which the dry bulb temperatures are over the boiling point of water).We have found that high temperature drying requires no humidificationsource, other than the lumber itself. There are some minor excep-tions, but a kiln used only for high temperature schedules would needno humidification system ; and we can further state that it needs nohumidity controls whatever only a dry bulb control. This is because,whether you like it or not, you have absolutely no control over the wetbulb temperature if the dry bulb temperature is over the boiling point.The kiln, the lumber inside, and the dry bulb temperature, all in com-bination, produce a situation which will dictate the wet bulb temperatureat any one moment and there's nothing you can do to alter this condition,except make it worse (or more severe, to use a milder term). In otherwords, you're best to leave it alone to take care of itself. But let'sargue about it some other day, because it's an entirely different subject.Now, having removed high temperature drying from this discus-sion, let's get back to what we were talking about. We can now statethat humidification in direct fired kilns is a matter of concern in sched-ules utilizing dry bulb temperatures of less than 210°F to pick a roundnumber, or what we refer to as conventional temperature schedules.Up until recently, the only practical means of humidification was throughthe use of a steam spray system. Scads of other methods were triedwhere the mill had no steam generation facilities. There was even thewater spray line; much the same as a steam spray line, but with atomi-zers spaced along the pipe, in hopes of facilitating the vaporization ofthe water in the 230° temperatures of the kiln (even Moore Oregon was26guilty of this one). To add to the confusion, some operators even triedto spray water into the flame of the direct firing burner. Needless tosay, they came up with resounding failures. The result, a flood where-ever the water was applied enough to make old Noah wish he werearound to build a bigger ark, was the wet bulb just keeps on dropping.Why did these attempts not work? Basically because we wereignoring the laws of physics that we should have learned in high school.A few minutes ago, we began heating exactly one pound of wateron a hotplate which is transferring an average of about 38 BTU's perminute to the water. If I've not read this paper too fast, it had betterbe boiling by now. We know from the laws of physics that it takes oneBTU to heat a pound of water 1°F. higher in temperature. So, in thiscase, starting with water at approximately 70°, it took 142 BTU's tobring this pound of water to 212°F.Now that the water is at 212° and boiling, we also know from thelaws of physics that it'll require another 970 BTU's to completely con-vert this pound of boiling water to steam, or nearly 7 times as much asit did just to come from room temperature to boiling. By the way, theburner on this hotplate is well over the 500° maximum exit temperaturewe allow on our direct fired burner controls. What we're trying to sayhere is that even with a very hot heat source, it's taking almost 7 timesas much effort to take the water from 212°, to steam at the same tem-perature, as it did to bring that water from room temperature to 212°.If it takes that much effort and BTU's to boil out a pound of steam on thishotplate, why have we been trying to blow a bunch of water into a directfired burner?This cannot be done successfully in one pass through the flamebecause of the lack of time required to transfer the necessary BTU's tothe droplets of water injected. So, why not spray water into the heatedexit air and recirculate it over and over again inside the burner untilenough BTU's have been absorbed to convert the water to steam -- andthen release it to the kiln? Well, this is exactly what we are now doingwith the Moore Oregon Vapoaire Heating/Vaporizing Unit. Granted,it's a lot more expensive than putting a spray pipe in to shoot water onthe burner, but it works! It humidifies as well or better than a steamspray line, or two steam spray lines.If you had a supplementary boiler and steam sprays for your directfired kilns, you'd have the usual operational and maintenance duties as-sociated with a boiler and steam lines. You would be saddled with watertreatment and would receive only 80 cents or less out of every dollarspent on fuel for the steam generated.With Moore Oregon's Vapoaire, steam generation is almost 100%efficient, and no water treatment is needed. Add to this its easy adap-tation to light oil firing, as well as natural gas (or even LP gases) andyou have the most versatile heating system available for kilns. Also,we found that the large recirculation blower in the unit could success-fully be utilized to take care of all venting needs, eliminating the neces-sity of roof vents. This is not to be confused with past attempts with"pressure venting." Lastly, we are anticipating using the Vapoaireconcept in direct firing kilns with wood wastes in the near future.Moore Oregon's Vapoaire is available in 8 sizes, ranging fromapproximately 350,000 BTU's per hour to 10,000,000 BTU's per hour.(A 10,000,000 BTU size would nicely handle the requirements of a 104'double track kiln for conventional temperature operation). So if you'vegot a kiln, or are going to buy one soon, we've got the King of DirectFired Burners to fit your drying requirements.27

Positive humidity control for direct fired kilns

https://ir.library.oregonstate.edu/downloads/8049g6377

Positive humidity control for direct fired kilns

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