Everyone wants to know "how fast is the wood drying?" There are various methods for computing the answer. Temperature drop across the load () is a particularly useful for estimating the progress of the whole kiln charge. Although it doesn't tell much at all about moisture distribution or the remaining moisture content in individual boards there are other analogs available for developing those estimations.

The basic "Rule of Thumb" here is:

• One degree (Fahrenheit) of will correspond to the absorption of one pound of moisture into 63,000 cubic feet (ft³) of air circulating through the stacks of wood in the kiln.

This is useful in several ways:

• If you know the across the load and the volume of air moving through the lumber stacks, you can readily calculate the drying rate.
  
  
• If you already know the approximate drying rate and the across the load, you can "back calculate" the effective volume of air moving through the lumber stacks which will in turn allow you to calculate the air velocity through the sticker openings.
  
  
• If you know the across the load and the approximate volume of air moving through the lumber stacks, you can calculate what the should be to check for possible saturation problems.

Example

Consider the typical situation in a kiln drying 50,000 board feet of oak lumber. We know that we want to achieve an average drying rate of 2% per day and that we want to make sure the across the load does not exceed 2º F. What air velocity through the load will achieve this?

Presuming oak has a specific gravity of 0.56, the "bone dry", or "basis" weight of the lumber is calculated to be 145,600 pounds. A drying rate of 2% per day would be 2,912 pounds per day or just over 2 pounds of moisture per minute. Since the "rule of thumb" states that 63,000 ft³ absorbing 1 lb. of moisture creates a 1º F. , absorbing 2 lbs. in this case would result in a of 2º F.

To calculate air velocity one must remember the simple formula:

velocity (ft/min) = volume (ft³/min) / area (ft²).

Calculating the area of Sticker Openings

The area we are looking for is the actual area of the sticker openings. There are two ways to do this: (a.) the rigorous method and (b.) the shortcut method.

Rigorous method: first the sticker and bolster opening areas have to be calculated and then the area represented by the stickers and bolsters have to be subtracted to arrive at a net area:

For this example we will presume the total lumber length to be 24' comprised of three 8' long packages, each package having 26 layers of 4/4 lumber separated by ¾" thick by 1 ½ " wide stickers on 2' centers. The packages are stacked 4 high separated by 4" x 4" bolsters.

The total sticker area, therefore, is 25 openings x ¾" (0.0625') x 4 packs high x 24' long = 150 ft². The total bolster area is 4" x 4" (0.33') x 24' long = 32 ft². Aggregate total opening area is 182 ft².

There will be 5 stickers across in each package x 25 openings high x 12 packages = 1,500 stickers, each with an area of 0.75" x 1.5" / 144 in²/ft² = 11.72 ft². There are also 5 bolsters across x 12 packages, each with an area of 4" x 4" ) 144 in²/ft² = 6.67 ft². Aggregate total blocked space is 18.39 ft².

By the rigorous method the net sticker opening area is therefore 163.61 ft².

Shortcut method: uses the number of layers per package instead of the number of openings per package to approximate the effect of the larger bolster openings and the blocking effect of the stickers and bolsters.

Using the shortcut method, 26 layers x ¾" (0.0625') x 4 packs high x 24' long = 156 ft² net sticker opening.

Since the difference between the two calculations is only about 4%, the shortcut method is the one usually used.

Now that we have volume and area, the required air velocity can be calculated as:

63,000 ft³/min / 156 ft² = 404 ft/min.

A word of caution

We generally consider that even in a well loaded, well baffled kiln, only about half the air volume produced by the fans actually ends up traveling through the sticker openings. Therefore, to the stated air velocity, the volume produced by the fans should be about twice as much: 126,000 ft³/min.

Once the basic calculations are done for a particular scenario, scaling up and down is completely linear. At 200 ft/min, for example, the )T would be approximately 4º F. which would be perfectly acceptable in the later stages of drying when the temperature and wet bulb depression are both quite high. This is one of the reasons why it is possible to save lots of energy by reducing fan speed and output using variable frequency drives - the next subject.

Over the last ten years the use of variable frequency (VF) drives has become nearly universal in new kilns for one simple reason: when used effectively, they can pay for themselves in as little as one year and seldom more than two!

How and why they work:

The speed of an alternating current (AC) induction motor is fixed by the number of poles with which it is built and varies according to the frequency of the alternating current. Normal frequency in the United States is 60 cycles per second (60 Hertz). A two pole motor, which will make one complete revolution during each cycle, will therefore run at 60 revolutions per second or 3,600 revolutions per minute (rpm) when running on 60 Hertz AC power. This is called the synchronous speed of the motor. The actual speed is slightly less because of slip by which torque is produced.

Similarly, a four pole motor will take two cycles to make a complete revolution, so its synchronous speed is 30 revolutions per second or 1,800 rpm; a six pole motor takes three cycles, therefore 20 revolutions per second or 1,200 rpm and so on.

When the frequency is changed using a VF drive, the same rule applies. At 30 Hertz the speed is half, for example.

There is a set of "laws" in physics called the "fan laws" which state the following:

• Output of a fan varies in direct proportion to the speed of the fan. Doubling the speed of a fan will double the output, usually measured as cubic feet per minute (cfm).
  
  
• Power required varies as the 3rd power (cube) of the change in speed. If the speed is doubled, the power needed is 2 x 2 x 2 = 8 times as much as before.

The fan laws work in both directions. If the speed is cut in half, so is the output, but the power is reduced to ½ x ½ x ½ = 1/8 of its original value!

In most cases, during the last 1/3 of drying, below fiber saturation and when the temperature is highest and relative humidity lowest, it is possible to reduce airflow by some amount without affecting the drying time or quality. To the extent this can be done, power savings can be enormous!

Other benefits of VF drives are:

• they are current limited to maximum running current of the motors, so "soft starting" is an inherent feature, avoiding the normal 300% starting current of "across the line" starting.
  
  
• they automatically correct power factor to near unity, improving the energy efficiency of your plant.