Firewood kiln design Home Kiln theory Heat treat and dry firewood

To have an efficient firewood kiln you need to engineer and design it to be efficient. So take your time when you design your kiln and seek advice, since we have seen many companies that have constructed their own firewood kilns only to realize that it does not work. We will look at some poor as well as good practices in designing a firewood kiln.

Poor design specifications:

• Container or van-body kiln.
  This seem like a logical choice, but they really do not do the job well. It is hard to get the right air circulation, fairly difficult to load and the ones we have seen on the market are under power on air flow and undersized for the heat system.
• Main Fans not located overhead.
  Regular kiln design nearly always has the main fans overhead and this simply creates the best air flow.
• Main fans under sized.
  For our 10 cord unit we installed 10 HP  main fans with a total of 40000 CFM (cubic ft per minute) at 1/2in static pressure. (Most companies will only tell you the air volume in free air = 0 static pressure). Most of the firewood kilns on the market seem to have less than this capacity.
  
• Heat system undersized.
  A firewood kiln needs to have about 2-3 times as much heating capacity as a regular hardwood kiln. We have doubled our heat system for firewood kilns plus we have heat recovery on the venting to recapture the heat, which indirectly boosts our heat systems.
• Only hot water heating.
  We have not seen an efficient firewood kiln that relied on only hot water heating to meet the 160F inside wood temperature required by USDA.
• Venting, undersized (and no heat recovery system).
  This is ony required if you are drying firewood, because for heat treatment purposes only venting does not matter.
  If you are drying, then you need to be sure that you have a sufficiently large vent and hopefully an exhaust fan.
  Not having a heat recovery system on the vents will make the process longer as more heat is needed to keep the chamber hot.
• Poorly insulated chamber.
  The kiln need to be a minimum of R-20 and preferable R-30+ in the walls. This is one of the reasons a van-body is a bad idea. They normally have R10 to R16.
• Insulated using the wrong insulation.
  There are many insulation types on the market. We would not recommend using open cell foam, or mineral/glass-fiber insulation as this can get wet. We use a close-cell foam that cannot absorb moisture plus it has a class 1 fire rating.
• Control system only relying on chamber temperature.
  Actually for drying this is not so important. However, if you are only heat treating firewood, not using internal wood temperature sensors may extend your treatment time and gas consumption (or wood usage).

• True kiln design.
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• Main fans location and performance.
  For our 10 cord unit we install 10 HP main fans with a total of 40000 CFM (cubic ft per minute) at 1/2in static pressure.
  For our 24 cord unit we install 18 HP main fans with a total of 72000 CFM at 1/2in static pressure.
  
• Heat system (General).
  Hot water from wood waste can be used to dry firewood, but you probably need something else to get the kiln temperatures up to 180-200F to reach 160F inside the firewood. We have found that a direct gas system does this efficiently. You can choose to just use the gas at the end of the process.
• Heating with gas.
  A small heater will not do the job. You will need between 50000 to 60000 btu/hr per cord heat output capacity.
  Also the use of indirect heating will use more energy as you are not utilizing all the combustion energy in the chamber (hot air out of chimney is an energy waste).
  10 cord kiln has 600000 btu/hr heating output and 24 cord kiln has 1200000 btu/hr heating output
• Heating with wood waste / hot water.
  Normally the hot water stove is the limiting factor and we suggest getting the biggest commercial hot water stove available from your local dealer. Then you have to insure that you install sufficient amount of hot water fin tubing inside the kiln.
• Heating with wood waste / direct flue gasses.
  We have not made this as we are not convinced it is safe. We are working on this.
• Good venting, preferable with heat recovery system.
  We would strongly suggest that you install a heat recovery system on the vent to reduce the overall heat requirements and speed up the drying process. Naturally if you are using gas it will reduce your heating costs.
• Well insulated chamber.
  The kiln needs to be a minimum of R-20 and preferable R-30+ in the walls.
  We would not recommend using open cell foam, or mineral/glass-fiber insulation as this can get wet. We use a close-cell foam that cannot absorb moisture plus it has a class 1 fire rating.
• Control system designed and optimized for heat treating with internal wood temp sensors.
  You should have a computerized control system preferable with a link to an office computer for recording the temperatures and to print HT reports for certification. Some units use chart recorders on the kilns for this.
  Internal wood temperature sensors that are integrated into the control parameters insure certification and keep the HT process to a minimum.