The electrical cost related to operating a kiln is very significant and will normally represent about 20-60% of the total energy cost, depending on many factors:
-- Species and thickness you are drying.
-- Local energy cost for both heating and electrical.
-- Kiln design. Older kilns tend to have less fan power and will use less energy. (However we are not suggesting installing less fan power.)
-- Drying schedules.
-- It is amazing that many keep denying the potential benefits of interval operated main fans. Especially when one considers all the benefits. Kiln-direct has supported this feature which can be engaged or disengaged on our kiln control systems. The benefit of the interval operation can be placed in three major catagories:
-- Improve drying quality.
-- Reduce electrical consumption/cost.
-- Reduce heat consumption/cost.
Although the
interval fan operation was initially designed for hardwood kilns together with the power exhaust; it has also been used successfully in both fan sheds and predryers, where it greatly increased the flexibility of the systems. We will not attempt to go into details about the specific working of the interval systems on this page in order to keep it understandable for as many as possible.
General description of interval fan system.
Wooddryer System invented the concept of automated interval operated main fans based on the moisture evaporation from the lumber. What does this exactly mean?. Well, imagine the futility of blowing dry air over a dry surface. This is a waste of energy, since you cannot remove moisture from a dry surface. The ingeniuos idea behind interval operated main fans are this: "When the controller senses that there is no moisture evaporation from the load it stops the fans for a preset period in order to prevent more case hardening and reduce energy consumption". You, the kiln operator, determines the parameters for initiating the pause and the length of the pause. These parameters will change from species to species and on different species.
Improves drying quality.
The improved drying quality will be most noticeable on slow drying and color sensitive species. As an example we have chosen a color sensitive species such as European Beech or Hard Maple. These species tend to stay the brightest when we keep a low temperature schedule and preferable no live steam. On 27mm (4/4) this is fairly easily obtainable in most modern kilns, but a little harder on older kilns. Should we begin to consider 51mm (8/4) it would become very difficult to dry the lumber without adding steam in order to keep the relative humidity high on your standard. However, when using the automatic interval operated main fans based on the registred moisture evaporation from the lumber it suddenly becomes much easier and we can avoid adding steam to the process.
How is it possible to avoid adding steam to the process, when you would have to when using standard full-time main fan operation. First, we have to recognize that nearly all kilns leak energy and a little air (we call this unintentional venting) whenever the main fans are running. Secondly, It is important to remember the only two purposes of the air in a kiln are: (1) a medium to carry moisture from the surface of the lumber to the outside and (2) a medium to carry heat from the heating coils to the lumber. If no moisture is evaporating you are not consuming any heat; therefore, why not stop the fans.
It is the unintentional venting that carries the most importance when trying to understand the purpose of stopping the fans. Since any leak in a kiln works as a vent, one could consider the standard vents nothing more than large controlable leaks. On a normal kiln setup the steam spray (or cold water spray) would come on when the lumber is no longer able to replenish the humidity, which is lost through the unintentional venting. However, with interval fans we simply stop the fan for a preset period when there is little or no moisture evaporation. (The vents work by the positive and negative pressure generated by the fans and when the fans are not operating there is little to no unintentional venting). How does stopping the fan make sense? Well, if there is little to no moisture evaporation we can assume the surface of the lumber must be close to the EMC (equalibrium moisure content) of the air and that no heat is needed, since no moisture is evaporating from the lumber.
Some will claim that this would slow the drying process. If we are drying a species and thickness at a rate of 5% per day, we would move 0.05% from the inside to the outside in a 15 minute interval. In the same 15 minute interval the actual change in lumber temperature will hardly be noticeable. Since the moisture movement inside the lumber is determined by two factors, the temperature of the lumber and the gradient from inside to the outside. Now, in 15 minutes neither the gradient nor the lumber temperature will change sufficiently to reduce the moisture flow from the core to the surface; actually, the fact we are preventing the surface to case-harden may help the water flow later and possibly reduce the conditioning period.
Reduce electrical consumption/cost.
It should be obvious that there are circumstances when one can stop than main fans in shorter or longer intervals without reducing the drying time. If the fans' operating time were reduced from about 100% to 90% by using the interval system you would have reduced the electrical operating cost by 10%. Actually, on airdried oak it is not uncommon to see the main fan operating less than 50% of the time without compromising the overall drying time. Now that is a considerable energy cost reduction.
Our kiln controllers networked onto a computer with our software helps you by providing tools and methods to optimize this interval fan option to maximum profitability. Part of the "snap-shot" report is specifically designed to help you reduce your fan operation.
Want to learn more about our kiln controllers?.
Reduce heat consumption/cost.
The interval fan operation does not directly reduce your heating requirements; however, indirectly the savings can be significant. This savings comes mainly by avoiding steam or cold water spraying. In either case your are evaporating water to increase the humidity in the kiln. In order to evaporate one liter of water you need 0.6 kw of heat (1 lb of water is 1000 btu) therefore; it is easy to see how the energy waste can be prevented, if one can avoid steam or cold water spraying during the drying process.
