Why Aren't All Lumber Dry Kilns Painted Black?

[Lumber Grader]

All my life I have noticed that most lumber dry kilns are silver on the outside or painted a lighter color. I noticed at your larger sawmills and lumber concentrations, most lumber dry kilns are silver? Millions and millions of dollars are spent to heat the lumber dry kilns to dry the lumber.  If they are silver then you are reflecting light away from the dry kiln and not absorbing the sun's solar heat and that does not make a much sense to me. I realize a lot of things we do are because Dad did it that way and then Grandpa did also and Great-Grandpa before him. If we want our lumber dry kilns to work more efficiently, it would seem to me they all should be painted black TODAY, so they would absorb heat and therefore not require as many $$$'s to maintain heat during the drying process.  Can anyone give me a good reason that all lumber dry kilns should not be jet black on the outside, other than they would look ugly??? 

[Dave Shepard]

Because they are insulated, and I doubt it would make a lot of difference.

[Lumber Grader]

I KNOW that all lumber dry kilns are insulated, but why would you not want to absorb heat, instead of reflect it away???? I disagree that it would make little difference, as there is a tremendous amount of heat in solar energy. If you are absorbing that heat back into the kilns you a lowering the amount of energy required to heat the kiln, no doubt. We want to warm up kilns, NOT LOWER HEAT, in order to dry kiln lumber. Energy costs over the long haul are going up, up, up.  I have touched the bright silver coating on lumber dry kilns in the summer, and you can burn yourself badly if you touch it for long, as it is very hot, BUT they are reflecting this heat AWAY from the dry kiln and not absorbing it, which would lower drying costs.   I have thought about this for years and I can think of no logical reason that all lumber dry kilns are not painted black on the outside, except for the fact they look ugly! Anyone have a better answer????

[submarinesailor]

Too answer your question about solar gain with black on the outside - you would lose FAR more heat then you would gain from the sun.

It has to do with a term called "Emissivity" and color can have a great deal to do with it.   A quote from Wikipedia "The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation."  Basically it is the ease that material releases heat into the atmosphere around it.  Even with good insulation, you still have some heat reach the outer surface and if you can reflect this heat back into the kiln/boiler, you can save energy/money.

Good example would be that if you took 2 pieces of aluminum, painted one flat black (an emissivity of 1) and left the other bare (an emissivity of about .04 to .6 depending on the surface conditions), heating both of them in an oven until they are both at 400 degrees – thermal equilibrium, and then look at them with an infrared camera.  You would see the one painted flat black reads  400 degrees or very close to it, while the other one reads somewhere between 60 to 240 degrees.  NOTE: The flat black one would cool much fast than the bare one because it is releasing its heat at a much faster rate.

In the case of the silver painted boiler/kiln, as the heat travels through the skin of the boiler/kiln, it hits the silver paint on the outside and is reflected back into it, saving energy/heat.  If they were painted black, the heat would be released into the atmosphere instead being of reflected back inside.

On a side note:  If you ever use one of those small inexpensive IR cameras.  Be very careful because 2 pieces of material can be at the same temperature, HOT, but look totally different in the camera due to their Emissivity.   Here is a link to an Emissivity chart:  http://www.thermoworks.com/emissivity_table.html.  BTW - Human skin has an emissivity of about .98 – releases heat very easily.  While looking at the chart, notice the difference between the polished metals and the flat metal, with galvanized pipe being in the middle.

Having worked (spend many hours/days) in many large boiler/power plants as a Certified Thermographer – looking for heat related problems, I have only seen one plant that painted their boilers any color other than silver, the new Pentagon Heating Plant.  The designers (the Corps of Engineers) would not listen about the heat release due to colors.  The color, Blue, was selected to make the new plant look pretty.  I could also tell you some very interesting stories about heat problems in many of the steel plants between Gary, IN. and Pittsburgh, PA.

Bruce

[Lumber Grader]

Wow Bruce. That is very interesting!!!! Thanks so very much for sharing that and much appreciated.  I know a man that ran a huge hardwood lumber concentration for almost 40 years.  This guy has a "Mensa IQ" and 3 PHD's!!! Extreme amount of education, but he also has common sense!!!! All of his lumber dry kilns WERE Silver!!!!!!!!!!!!!!!!!!  I am going to get him to read your posting. He is also a expert in wood-fired boiler systems.  I am going to get him to read your post and give me his opinion on it.  What you said seems to make common sense to me and I really do appreciate you taking the time to share your knowledge and experience and MUCH Appreciated, my friend!!!  I will make another post, after I talk to my buddy to see if he is in agreement with your post.  Thanks again Bruce!!!!!!!!!!!!!!!!!!

[submarinesailor]

One more thing to think about.  Why are the coolest roofs white – low heat gain?   White reflects heat (in both directions) better than galvanized metal.  Zinc/galvanized metal has an emissivity of about .4 while white is about .01 to .04.

Bruce

[Lumber Grader]

Wow Bruce, I wish we lived closer, as I would love to share coffee with you or a meal and talk.  What you have posted is very interesting to me and thanks again for sharing!!!

[submarinesailor]

Wow Bruce, I wish we lived closer, as I would love to share coffee with you or a meal and talk.

OK, a question for you ??? ??? ???  Where are you ???  If your profile was complete, we all would know where you are ??? :D :D :D :D :D :D :D ;D ;D ;D ;D ;D ;D ;D

[Lumber Grader]

Sorry, I am in Bartlett, TN, which is Memphis, TN metro area.

[hacknchop]

Dry kilns are all about control and  not just heating but to the right temps at the right time, dry sheds now where lumber is air dried only without the sides being closed in usually have dark roofs not so much for heat but rather to clear snow.

[Lumber Grader]

Lots of knowledgeable (very) folks on this Forum that are kind enough to share their valuable experience and knowledge.  I REALLY do appreciate it and I hope to have a lumber dry kiln in the not-to-distant future, so I may be back many times asking lots of questions, so thanks guys for some incredible insights and for taking the time to post and share!!!

[GeneWengert-WoodDoc]

Indeed, many kilns are not aluminum, but are masonry.  Masonry kilns are the color of the brick used.  Masonry kilns are not painted on the outside as that paint will act like a vapor barrier and then moisture will be trapped within the masonry wall...leading to poor insulation and deterioration of the wall.  So deterioration and increased heat loss will cost much more than the energy savings.  Many solar kilns are black on the outside.

Do you live in a sunny location?  If so, you will quickly learn that a dark colored car will get much hotter inside than a white car and aluminum will reflect the heat and make the car even cooler; that is why we use an aluminum reflector pad on the front window when parked.  The dark versus light color effect is smaller for a car that is well insulated (expensive).

Lets get technical:  The amount of energy in an insulated kiln that is lost through each wall is about 4% of the total energy input to the kiln.  Other energy losses are the roof, floor, vents and evaporation.  Now, at 9AM, the east wall if painted black might absorb some solar energy and become heated.  The other three walls would have no gain at this time of day.  This might reduce the loss for the east wall by 10% for a few hours during the day.  By noon, the east wall would no longer see the sun, so the difference between black or white color would be only for a few hours.  In fact, at noon, all four walls would not see the sun because the sun is overhead.  So, a 10% savings on a wall that loses 4% would be 0.4% total savings.   Of the 3 million BTUs per MBF used for drying, we might save at most  12,000 BTUs per MBF with a painted wall.  A million BTUs costs (using natural gas; less with wood waste heat and more with electric) around $5, so the potentials savings from one wall being painted black is 6 cents per MBF or $0.60 for a 10 MBF kiln.  With ten loads a year, that is $6 per year.  Can we paint a kiln wall for $6 or so?

Consider an analogy:  An insulated kiln wall is like having a tank of water with a really small pipe leading out of the tank.  Does it make much difference if, after the pipe, we have a large pipe or medium pipe on the outside?  The main resistance to flow is the small pipe.  With a kiln wall that is insulated, the main resistance to heat loss is the insulation, so what happens on the surface (solar heating) will not have much effect on the overall heat flow.  That is why spending money on insulation has a better payback than spending money on paint, in general, from an energy standpoint.    There are many studies that show a black vs. light colored house roof does not really affect energy use in a house that is well insulated.

The solar input for a flat surface (not a vertical surface) is around 1000 BTU per day per square foot.  A vertical wall is about 8% of that value (depends on wall direction, sun angles, etc.).  Even if the wall absorbed 8% of the solar, or 80 BTUs per square foot per day, and the wall was 900 square feet (big kiln), there would be IR heat losses and convection losses, plus with the insulation, not much of the captured solar heat would go into the kiln.  (This is why solar collectors use two clear covers in order to control and limit IR and convection losses.)  So, the solar input overall would be very small.

Many kilns are made of aluminum, so that is why they are silver.  Paint does not stick well to this aluminum used unless the wall is specially treated.  Painting is also expensive.  So, there must be a benefit to painting that will pay the cost.  Vertical walls are very poor solar collectors as the sun is usually hitting the wall at a steep angle.  The sun is usually only on one or two walls and seldom is on the north wall.

Emissivity to refers to the IR spectrum property of the material, not the visual reflectance.  So, it is hard to judge the emissivity by the visual color.  A material with a high emissivity will emit more IR radiation than a lower emissivity product AT THE SAME TEMPERATURE.  At the same time, in a kiln which is insulated and will have a wall temperature close or equal to the environment, the high emissivity surface will emit more heat.  However, it also will absorb more IR from the surrounding objects.  If the surrounding objects are close to the wall temperature, which is true for a kiln, it is a break even event...as much energy is emitted as is absorbed, whether the emissivity is high or low. So, at 100 F or so, the emissivity of the paint is not a big factor in the temperature or heat losses from a kiln wall.  But, for very hot surfaces, such as a boiler, where the energy emitted is increased by temperature to the fourth power, so as soon as the wall gets hot, such as with a boiler, the wall is no longer close to the temperature of the surroundings, but is much hotter.  In this case, the higher emissivity will lose more energy than what comes in, especially when compared to low emissivity material.

Do you know how a car will get frost on it at night, especially the roof that "looks" at the cold nighttime sky?  A white or black car gets the same amount of frost.  Again, this is so because the temperatures are low...not hot like a boiler.  Of course, once the sun hits the black car, the frost will melt more quickly than the white one.

So, at the cool temperatures of a kiln wall, you can do an energy balance equation using various emissivities and various colors and will find out that with insulated walls, the emissivity of the wall is not a big issue.  The color, however, is important due to solar heating.  I saw a paper written in 1966 about predicting exterior wall temperatures using an energy balance approach and then comparing predicted with actual.  Due to the huge reduction in energy transmission through a wall with good insulation, the slight benefit in using a dark color is very small.

Further,  and this is probably the main reason we do not paint, in most kilns, the energy usage or cost is not an issue (not true in DH kilns); that is, the mill has plenty of low cost energy available such as wood waste.  Stated another way, energy in the kiln is too cheap to save.  The cost of saving by using a dark color paint is greater than the energy saved even when amortized or spread out over many years.

Note that a DH kiln using electricity, which costs much more than natural gas or wood waste, there is a larger benefit from using a dark colored wall.  In fact, in a reefer kiln, painting the roof black will have a benefit for many hours a day.  Even so, a benefit on a small area is not, overall, that important.  It would be better money-wise to spend the money on more insulation and not on paint.

[Lumber Grader]

Thanks Gene! Much Appreciated!!!
Great information!!!!
Lumber Grader

[Saga Dan]

The most important factor involved is the use of insulation.  That has the biggest impact on any of the heat transfer that we are talking about.  Increasing the insulation will negate most of the gains or losses of having a highly reflective surface whether it be in the visual or the infrared spectrums. 

I do not want to hijack the thread but I do need to try and clear a few things up....  Reading through the other posts I am seeing some very good information and some not so good information in terms of visual and infrared radiation.  Not that I don't trust Wikipedia but the definition is a bit fuzzy.  The definition for Emissivity I have is; Emissivity is the ratio of the actual amount of electromagnetic radiation emitted by an object to the same amount emitted by and ideal blackbody at the same temperature.

In submarinesailor's example of the oven heated aluminum, the reflectivity and reflected temperature is not factored into the situation in regards to the emissivity and thus the temperature reading.  A question I have is, if a thermocouple were attached to bare aluminum and its reading was 220 degrees after 2 minutes of cooling what would the actual temperature of the painted piece be and would you pick it up with your bare hand?  Maybe the the Army Corp wants to be able to take accurate temperature readings with their IR camera on the pretty blue paint.

Color does not have any bearing on the infrared spectrum.  The paint used in automotive applications is all very similar in emissivity, between .88 and .95.  It does not matter if it is black or if it is white, it the makeup of the paint itself.  The difference comes in the visual spectrum as alluded to in Gene's reference to the frost melting off the black paint quicker. 

[GeneWengert-WoodDoc]

It might be helpful for those wanting more technical issues now to appreciate that the sunlight hitting the earth is 5% ultraviolet, 43% visible and 52% near infrared.

What this means that for an effective solar absorber, the surface must be black in the visible range in order to absorb the visible light (a technical expression for this is the surface should have an albedo of 1; an albedo of 0 means no absorption) and must also absorb near infrared, which is where the emissivity comes into play (emissivity is the absorption number, as mentioned in the previous postings).  A high emissivity means high absorption of near IR.  technically, the emissivity is sensitive to wavelength- a near IR emissivity can be different than far IR.   The hotter a surface is, the more near IR and even visible it emits...example is when heating metal, it gets dull red and then redder (more visible) and you can feel from a distance the IR being emitted.

Now, the covers (glass, plastic, etc.) of a solar collector should pass most of the visible light through and should not reflect or absorb visible. 

Likewise, it would seem to be a good idea if the covers passed the near IR from the sun through as well.  But, the incoming visible solar energy is converted to heat (and the hot surface emits the energy as infrared, as well as convection and conduction) so it would be best if the covers reflected the IR rather than passing it out.  If not, then there would be more IR going out than coming, so the best cover is one that transmits near IR from the sun but reflects the IR reflected from the cool absorber...cool compared to the sun.  Glass with low iron does this very well.

So, now we have the visible solar absorbed in the dryer and converted to heat.  We generally want to get the heat off this surface rather quickly.  That is, we want to use this heat as soon as we collect it.  The heat will come off eventually; if it didn't then the absorber would get really, really hot.  So, we remove heat from the absorber by convection...blowing air across the hot absorber surface.  Although some folks blow air across the top and bottom of the absorber, over a 24 hour period, the air will get all the heat off in either case.  So air on one side is just as good as air on two sides.  With air on one side, with slower removal of heat compared to blowing air on two sides with air, the single surface will get a few degrees hotter so that convection will transfer more energy from less area.

Finally, once we have hot air, we want to use as much of the heat ,or energy , in the air for evaporation and minimize conduction from the building (by insulating walls and floor), by radiation (small because the surfaces are not very hot), and by excessive venting (but the more we vent, the more the kiln will mimic air drying, which does indeed dry wood fairly well).  Note that if we get a really hot kiln, then it is clear we are not using the heat for evaporation...so do not judge a kiln's performance by how hot it gets when it is drying.

A solar kiln with the vents wide open is sometimes called a single pass dryer, as the air comes in, is heated, evaporates water, and then passes out to the environment.  In order to control drying rates and therefore quality, we like to limit venting and recirculate the air, in the dryer,  thereby keeping a higher humidity in the kiln, if the wood is wet.  But if the wood is dry, then we will get heating of the air in the dryer which can help drying.

[Deese]

 smiley_goofy_face smiley_goofy_face
Dr Wengert, I believe your last post hurt my simple brain a little.  :D
Thanks for providing such reliable, research backed data/info on this stuff.

I have printed off the VA Tech Solar Kiln pdf document and plan on starting my kiln build in the next week or so...I have a bunch of 2" pine lumber that has been stickered for over 18 months now that I will use as building material, the only problem is that it's all 8'5" length. I will make it work. Also getting the plywood next week. I will probably start my own thread once construction begins.