Coils refer to finned pipes placed in front of or on both sides of the fans cross the width of the kilns. Main fans circulate the air in the kiln through the heat coils, thereby transferring heat from the coils into the air. Heat units consist of a small coil section with much denser fin area than the standard kiln coils. In addition, the heat unit utilizes a fan to maximize the coil surface area.
Coils are generally chosen because they offer a simple, straight forward, and traditional design. Though popular, the coils design does have some disadvantages which are rather apparent:
-- The coils hold a large volume of steam or hot water. When the heat valve is closed, the coils still hold significant heat and thereby promote temperature overrun. This drawback is most noticeable on high pressure steam systems especially when drying slower species.
-- Each row of finned pipes in front of the main fans create significant turbulence and resistance to a smooth air flow from the back of the kiln through the fans to the front of the kiln, or opposite. This, in effect, reduces the general air flow in the kiln (by increasing the resistance/static pressure) and fan efficiency (higher electrical cost).
-- Naturally, the kiln operators prefer all the equipment, including the coils, to be manufactured in all stainless steel or aluminum for maximum protection against corrosion. Kiln-direct can supply you with both heat coils and heat units, but we are not sure why other kiln manufacturers only offer coils, except that off-the-shelf heat units in aluminum and stainless steel are very expensive. Kiln-direct has designed heat units specifically for lumber kiln environments and introduced these at very competetive prices compared to the coil setup.
Heat units are smaller and make better sense in today's high efficiency kilns. The only possible drawback is the extra fan motor; however this small disadvantage is easily offset by their biggest advantage of more accurate temperature controls. Furthermore, the unit takes up less space. By positioning the heating units properly, the main fans meet less resistence and will operate more efficiently.
Coils can be fired using steam, hot water, or heated oil (120-150C or 250-300 F). They can be manufactured using steal or stainless steel pipes with steel or aluminum fins. Stainless steel pipes with aluminum fins are the best solution, though more expensive. Steel pipes with steel fins are more common and the least expensive in initial cost. Kiln-direct offers only the SS pipes with aluminum fins.
Another kind of heat unit is the direct fired heating system which is typically used in pine kilns or other kiln applications where venting is nearly constant. However, Kiln-direct is has completed the development of a direct fired heating system, which can even dry oak efficiently when used with our kiln control system and
interval fan operation. A direct fired kiln uses nearly 100% of the heating value from the gas instead of the usual 85-95% of a boiler. Please return soon for more information on this subject.
The last type of heating available through a heat unit with electric heat is costly to install and expensive to operate unless your KW price is extremely low. In situations where the cost of energy is unusually low, electric does work well. When electric heat is used we strongly advise that you install
heat recovery units to reduce the heat consumption to an absolute minimum.
