Main fans:

Fan capacity is measured in cubic feet per minute(cfm hereafter). The cfm will decrease as the fan encounters higher resistance. The resistance is measured in static pressure between both sides of the fan blade.

Fan blades are typically manufactured for either low static pressure or high static pressure applications. The more fan blades and higher rpm, the more a fan blade operates efficiently at high static pressure. For example, a fan blade designed for higher static pressures will normally have cast blades. These blades often appear wing shaped. It is very important to remember that fan blades are seldom efficient at both low and high static pressure. Consequently, considerable attention should be granted to insuring the correct choice of fans. Main fans in a typical kiln environment will normally operate with a static pressure between 0.25" to 0.5". Above 1" static pressure, a blower is normally used.

The fan motor itself should be made for harsh environments, such as those rated hostile duty or high temp (explosion proof motors are probably overkill). Heavy cast iron frame motors are preferred over the less expensive aluminum body motors because of long term durability. In warmer environments-such as kilns--you should not load the motor 100% of rated Amperes. By only loading the fan motor at 80%, the temperature rise from surface to the core of the motor will be lower. The practice of decreasing the amperes will extend motor life in kilns. In addition, the use of lower rpm motors, such as 1200 rpm versus 1800 rpm, extends motor life in a kiln environment. Due to the lower rpm, the bearings last longer and consume less grease.

Axial fans mounted directly on the motor shaft will also require less maintenance than the belt driven type. In most cases, a directly mounted fan is also more efficient in the kiln environment with the 0.25" to 0.5" static pressure.

The fan blade can be made from several differing materials. For kilns, all aluminum fan blade are preferable. Higher quality blades can normally be recognized by the shape of the blades. Uni-directional blades are shaped like an air plane wing propeller and bi-directional resemble a S-curve. The best blades are normally cast in either aluminum or a glass fiber reinforced poly-propylene. For most kiln applications, a bi-directional fan is preferred. This is because fans commonly alternate between forward and reverse cycles. The reason kilns commonly reverse the cycle of air flow is the concern over uneven drying within the kiln chamber.

The "venturi" is the frame that holds the motor and fan blade. The shroud around the fan blade itself should resemble a "bell mouth" opening on one side for uni-directional fans or a "double bell mouth" opening for bi-directional fans. When a shroud is formed, the efficiency of the fan blade can be increased by reducing the clearance between the fan blade tip and the shroud.

The venturi are normally manufactured in either steel, aluminum, or stainless steel. Though mild steel is the least expensive option, it is prone to corrosion and requires a surface re-coating every 3-5 years. Aluminum and stainless steel are similar in cost, but stainless steel will last longer, easier to install, and not prone to corrosion as is aluminum.

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Heat unit:

The heating capacity is measured in British Thermal Unit (btu hereafter) or Kilowatt (kw hereafter). The more KW/btu heat available, the faster a load of lumber can be dried (providing sufficient venting and air flow is present). The output of a heat coil or heat unit is dependant upon the surrounding air temperature. This is also known as ambient temperature. For example, most coils can only produce half the KW/btu output at 60C/140F that they can produce at the lower ambient temperature of 25C/77F. Consequently, when comparing heat systems between different vendors, make sure you know at which temperature the units were factory rated. If the rating is not specified and/or vendor does not know, you should consider finding another brand or vendor.

Coils versus heat units.

Coils refer to a finned pipes placed in front of or on both side 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 could 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, strait forward, and traditional design. Though popular, this coils design does have some disadvantages which have become apparent. First, the coils holds 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. Second, a row of finned pipes in front of the main fans create significant turbulence. This, in effect, reduces the general air flow in the kiln (by increasing the resistance/static pressure) and fan efficiency (higher electrical cost). Third, as some prefer only the kiln to be designed and fabricated in aluminum and stainless steel, the cost of heat coils has risen faster than the cost of heat units using the same high quality materials.

Heat units are smaller and make better sense in today's high efficiency kilns. The only possible drawback is the extra fan/motor. Their biggest advantage are more accurate temperature control. Furthermore, the unit takes up less space. By positioning the heating units properly, the main fans meet less resistence and 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 expensive. Steel pipe with steel fins is more common and the least expensive in initial cost.

Heat units can be heated using steam, hot water, heated oil (120-150C or 250-300 F), LP gas, natural gas, #2 heating oil. The heat units using gas or #2 oil should be designed with a separate combustion chamber to prevent constant venting. To avoid condensate and extend the life of the unit itself, a small heat recovery unit should be installed on the venting system. A heat unit consists of a casing, a heat transfer area, and a fan. The casing is normally manufactured steel with an epoxy or powder coating, though several companies do make the unit casing in aluminum or stainless steel. The heat transfer areas are normally copper tubing with aluminum fins for hot water and stainless steel tubing with aluminum fins for steam. We at Wooddryer System prefer to manufacture using stainless steel casing, tubes, and aluminum fins for durability reasons. The gas and #2 oil units will normally have a steel air to air heat exchanger for the heat transfer area, but stainless steel is periodically available.

The Direct fired heating system is typically in use in pine kilns or other kiln applications where venting is nearly constant. Wooddryer System does not manufacture these units, as you loose some climate control. These units are generally cheaper in terms of initial purchase cost.

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.

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Venting:

Venting itself is normally measured in cubic feet per minute (cfm hereafter). However, in the case of passive venting in kilns, the cfm is determined by the main fan capacity. Therefore, when comparing passive venting, the surface area of the vents are normally used. For example, if you have four 12"x24" vents on both sides, you will have 8 square feet of vents for inlet and 8 square feet of exhaust.

• Less venting: As each cubic foot of air vented will carry more moisture.
• Lower heat requirements: As the air on the vacuum side is lower in temperature before it passes through the coils or heat units. In addition, less venting means that fewer cubic feet of fresh air will need to be heated.
• No reduction in air flow while venting: The last advantage is often the most overlooked. General speaking, as more venting is required, the more moisture is carried away from the lumber. Therefore, if a high rate of venting were needed, we should try to keep the air flow similarly high. On passive vent systems, the energy to vent air comes from the main fans. This means energy or fan capacity is diverted away from proper air flow and into the creating necessary venting. Conversely, a power vent system with exhaust on the vacuum side does not reduce air flow or divert main fan capacity away from providing air flow. Theoretically speaking, power venting actually adds to the air flow.

Some kiln equipment companies even go so far to advertise their power venting as a new idea in their ongoing effort to portray their kilns as operating more efficiently. Remember, changing species and schedules may also change the heat, vent, and fan capacity requirement. Furthermore, this shortcoming may not have been an original design flaw, but a victim the changing times in respect to drying temperatures and/or species to be dried. Some companies has been recently advertising this so-called "new method of Wet Air Venting" or "Power venting", but WOODDRYER SYSTEM has been practicing this method for nearly three decades. In fact, we probably should call our design "Wettest Air Venting", as we have taken this innovation a step further.

When choosing or comparing exhaust fans for power venting, the same characteristics are equally important with regard to choosing main fans. As such, please refer back to the main fan section to learn the important aspects of comparing fans. With exhaust fans, you would normally chose a uni-directional fan and expect a static pressure of about 0.5" to 1". Just as with a main fan system, you want an even exhaust and inlet across the complete width of the kiln as opposed to one large exhaust fan/inlet in the middle. The latter will create a dry zone in the middle and wetter areas near the walls, equaling an uneven drying spread across the load.

One of the most important aspects of any venting system component is the materials used in manufacture. Wooddryer System uses only aluminum, stainless steel, and glass-fiber components. We believe it is best to avoid any equipment made from ferrous metals, due to the high level of condensate these components will encounter when placed in a area where hot, humid air meets cold dry air.

Some vent systems are based on the concept of continuous/constant venting. The constant venting technic is normally done when using:

• Modulated vents: (adjusting the vent opening in percentages): This way the vents never close or open completely.
• Blower with air flow restriction: This system uses a blower which operates outside the kiln at full speed. Between the exhaust blower and the kiln chamber, a damper is installed to adjust or modulate the air flow. The best argument for this system is that, because a blower creates a constant vacuum, the kiln lasts longer. The argument seems to make sense.

Wooddryer System does not recommend the practice of constant venting. Although our controllers are fully capable of operating a kiln with modulated venting, we discourage the practice all together. For further information, please see the kiln theory section to understand the significant back draws of constant venting.

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Kiln controllers:

This area is probably one of the more interesting aspect of lumber drying. This is also the one area where kiln and control system companies try to differentiate themselves from one other. By trying to distinguish ourselves from our competitors, we are likely to point out features which nice to have but not essential to the operation.

Kiln controller networks:

The network computer system for "supervising" your kiln controllers has some very basic tasks to perform. The systems main functions are record keeping and possible automatic set point adjustments based upon moisture content or time based schedules. It should be mention that the best systems are decentralized ones. A decentralized network consists of individual controllers which can operate the kiln without or without the network itself. This arrangement allows both the individual controllers to capitalize upon the strength of network (such as kiln prioritizing) or continue operating should problems occur in the network itself.

List of important features on a kiln controller network.

• Ease of use (Windows: point and click)
• Large database for drying schedules
• Record keeping (more is not always better, the right information is the right quantity presented in a simple, but telling way, is more valuable).
• Automatic set point changes on kiln controllers from the supervising net work computer.
• Decentralized network design.

List of non-essential features, which will help you operate the kilns more efficient.

• On-line access from remote locations, allowing access from home or on-line help from the company.
• Integrated MC% reading and recording. (With temperature and specie correction)
• Heat management (hot water or steam management based on available supply).
• Energy consumption and cost calculations (both for cost and electrical cost).
• Support different types of kiln controllers. Furthermore, it is also supports steam chambers, fan sheds, and predryers.
• Change set points based on MC% readings from kiln controllers.
• Optional dial up system. Alarms operator when certain criteria are not met in the kilns.
• The software program can run in the background, enabling you to use/operate other programs while the kilns are being continuously supervised.

There are several reasons to upgrade existing kilns to newer computerized controls. The most important reason is that you will start to generate a database of kiln schedules and slowly build a broad body of kiln knowledge specialized to your specific organization. Though this may not be significant today or tomorrow, it will progressively offer you a kind of insurance plan against losing nearly all your kiln drying knowledge. Were your kiln operator to suffer an accident, leave the company, or somehow find himself unable to perform his job, the tremendous body of kiln knowledge would remain intact. In essence, computerized controls offer your operation both an affordable tool in accumulating valuable information and a invaluable insurance policy against the total loss of your operation specif drying experience.

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Specification sheets on Wooddryer System kiln equipment: