Direct gas heating systems for lumber kilns has been available for many years. Wooddryer System and Kiln-direct was not interested in developing direct gas fired heating systems for several reasons:
- Perceived fire hazard.
- Unable to control humidity(especially on slow drying hardwoods).
- Personnel safety in regards to cabon monoxide.
Fortunately, several years ago Wooddryer System was asked to upgrade the control system and install power exhaust. The customer was located in France and had four kilns using hot water heating with our kiln controls and design plus 24 Predryer sections. Because we did not specialize in turn-key installations the customer went with a local kiln company and decided on a direct gas fired heating system. After a few years of the kilns poor performance on fresh-sawn oak the customer asked us for a solution. We voiced our general concerns, as stated above, about our perceived shortcomings of direct gas fired heating systems. The customer insisted on trying to improve the kilns by installing our
power exhaust system and kiln controls with
interval operated fans plus upgrading the controller software to the latest version. After several months of operation, with the upgraded kilns, we were very surprised by the results. All our concerns proved to be unjustified and we were also able to solve the problems of drying fresh sawn oak. We were very puzzled by the excellent results.
Lets try to answer our own concerns from above:
Perceived fire hazard.
As we studied the design and how it worked we could see how this could be a safe heating system. We also knew that this heating system had been used safely for many years by many kiln competitors.
Unable to control humidity.
This was our primary concern for most of our kiln systems that had been installed in the hardwood lumber industry. We always knew that this system would work well on fast drying species, such as pine and of course, softwoods. The reason for our concern when drying slower species, such as oak, relates to the fact that a direct gas fired heating system has to have fresh air for the combustion. When you are drying oak or similar species there is very little venting/air exchange when compared to faster drying lumber. Therefore we believed that the incoming air for the combustion would create too dry of a climate in the kiln and potentially increase the drying defects, thus reducing the quality of the drying: when this did not happen we investigated and came to two conclusions:
- Efficient heating vs. moisture evaporation is the primary physical reason that it is possible to stay ahead of the potential problem. First the theory: In order to produce a 1000 BTU (293watt) you will need ten cubic feet (0.3 m3) of air. It also takes approximately 1000 BTU to evaporate 1 lb. (454g.) of water. Since a pound of water can raise the relative humidity from 25% to 85% at a temperature of 120F/49C in 50 cubic feet of air we know that it would be theoretically possible.
- Internal main fan operation is also a considerable contributor making this direct gas fired heating system possible. Theory behind the interval operated fans. Since the main fans are stopped for shorter periods when the climate in the kiln gets too dry, it prevents three causes that creates a drier climate. First, when the fans run they naturally heat the air which lowers the relative humidity. Secondly, when the fans run they increase the heat loss through the walls and floor which will require more heating and more fresh air to enter the kiln thus reducing relative humidity further. Thirdly, when the fans are running there will be some natural ventilation to the outside, even if the vents are closed, from small openings around the doors and other places, which also helps reduce the relative humidity. The interval fan system is important when drying slow species and especially so, if you are using direct gas fired heating.
Personnel safety in regards to cabon monoxide
Whenever you have a gas combustion chamber you will create some gases such as carbon monoxide (which is the most lethal). On several kilns with direct gas fired heating we did not see any safety systems or warning signs. Kiln-direct believes it is a must to have warning signs on all entries into a kiln with direct gas fired heating. In addition, we have built a forced venting feature into our control systems. By activating this feature you will disengage the heating and start the power venting thereby bringing fresh air into the kiln before a person should go inside. It is also essential to the operation that the gas heating system is adjusted periodically to ensure that the combustion is as clean as possible both for maximum efficiency and to minimize the amount of lethal combustion gases.
There are several advantages to a direct gas fired heating system. First, it operates at 100% efficiency since all the combustion energy is added to the air in the kiln opposed to a boiler which sends part of the combustion energy out through the chimney/stack. Furthermore, on a boiler system for either hot water or steam you will have some additional energy loss through the piping to and from the kiln. Secondly the installation is much simpler and a large portion of the investment is saved when compared to a traditional boiler system. Lastly it is easier to increase the heat input into the kiln. It does not cost much more for the additional heat output plus a direct gas fired heating system does not lose as much of its heat capacity as the temperature rises in the kiln. Actually, this system is nearly immune to the temperature changes in the kiln as it relates to the heat output.
In combination with a direct gas fired heating system, Kiln-direct encourages the customer to consider upgrading the venting. This can help you reduce the heat consumption with 25 to 35%. A heat recovery system can help you reduce your energy consumption with upto 25% to 35% savings.
The bottom line is that a direct fired kiln will often make economical sense for smaller kiln installations. Once you get about three to four 50 mbf or 100 m3 kilns, you should start considering a wood waste heating system provided you have wood waste available.
When compared to traditional kilns using an oil or gas boiler the total energy savings can be as much as 40 to 50% when compared with a kiln using direct heat and a heat recovery system.
The first 30% comes from the heat recovery and another approximately 15 to 25% comes from the high efficiency of the direct gas heating system.
