This is a broad and well studied question. We will attempt to cover the basics in this chapter while placing a special emphasis on the drying variables the kiln operator can control and how they influence the drying process. As you may conclude, we view the kiln drying process from a slightly different angle. We at Wooddryer Systems believe our approach to kiln drying will bring more operational clarity to the subject. This is especially the case for an individual who is just getting involved in lumber drying.
Drying lumber consist of two basic processes:
Important: Lumber can be dried too quickly. If a board is dried too fast, the surface may become a type of insulator. This is called "case hardening" If case hardened, the core moisture can not pass through to the surface easily; thus, preventing moisture from properly "wicking" its way to the surface of the board and evaporating. Lumber dried too quickly in the initial stages of drying will suffer increased degrade and slow the overall drying process.
Kiln operators job: To establish the correct settings of temperature, relative humidity, and air flow (air flow is only an adjustable parameter in some kilns). This is a careful balance between evaporating of surface moisture at a safe rate and encouraging maximum moisture movement inside the lumber. If the proper balance is achieved, drying occurs at the fastest possible rate without inflicting excessive degrade. This operational balance makes the kiln both efficient and profitable for the lumber company.
This section will deal with the three variables kiln operators can manipulate to adjust the drying rate.
Important: When lumber is dried with little degrade, kiln operation the within the safe rate of drying for the specie being dried. It is important to always remember each specie has its own individual maximum drying rate; hence, each specie dries best under differing kiln conditions. To discover the optimum rate per specie, we adjust the three variables (temperature, relative humidity, and air velocity) to achieve the maximum drying rate.
Vacuum in a vacuum kiln is also an adjustable available; however, vacuum kilns are not discussed in this section.
We need to examine the effect of each variable on the following:
Temperature: (How will changing the variables effect the drying process)
The temperature is closely related to the drying rate of the lumber and is one of the
three controllable factors we can utilize to either slow or increase the drying rate.
An increase in kiln temperature will dry the lumber faster. This is accomplished because
of two factors.
The above rule would suggest that higher drying temperature will always be an advantage;
however, there are noticeable drawbacks to using higher temperatures . . .especially in
the beginning of the drying process (when removal of free water). Some species, such as
maple and beach, will turn darker when higher temperatures are used. On oak, degrade
(checking, honey comb, warp) is significantly increase when temperatures above 37-46C
(100-110F) are used on green oak. In addition, it is generally true that lumber dried
lower temperature is stronger, consequently, lumber dried at lower temperature is able
to withstand more stress before checking than lumber dried at higher temperature.
Over last several decades, the average drying temperature has decreased in order to
improve both the quality and appearance of the lumber. Wooddryer System is a strong
advocate of low temperature drying and believes that one of the major obstacles in using
lower temperatures efficiently is poor and/or outdated kiln design and specifications.
Relative humidity. (How will changing the variables effect the drying process)
In order to understand the role of relative humidity, we must first appreciate the close
relationship between wood and relative humidity. Wood is a hygroscopic material; meaning
it loses and gains moisture until the amount of water in the wood is in balance with the
surrounding climate. The moisture content at this balance point is call the equilibrium
moisture content (EMC). Therefore, when we are drying lumber we are creating a climate in
the kiln with a lower EMC than the wood being dried. The lower EMC first begins to remove
moisture from the surface of the lumber, after which the moisture in the wetter core will
begin to migrate to the drying surface area. The difference between the EMC and the core
moisture content is also called the DRYING GRADIENT. Each specie has a different safe
drying gradient. Slow drying species (such as oak) have a lower drying gradient than
faster drying species (such as Poplar). Simply put, the safe drying gradient of a specie
is measured by the amount of drying stress we can safely create between the core and the
surface without creating excess degrade.
A decrease in RH% will also dry the lumber faster. This is accomplished because of
two factors.
Rule of thumb: Drying rate = constant * (100 - RH).
*Constant is a specie with a certain moisture content and size.
This rule is a little more complicated than the aforementioned temperature "rule of
thumb". For example, if drying lumber at 70 R.H.%, you could increase the drying rate
by two if you lower the R.H. to 40%.
One of the problems with dry lumber is that you are trying to create a wick effect from
the core to the surface where moisture is removed at the optimum pace.
Trying to dry the lumber too fast using lower relative humidity (an increased drying
gradient) may well "case harden" the lumber (over dried surface). In addition, the drying
process will eventually slow and potentially require not only higher temperatures,
but excessive heat, to force the moisture out.
One could conclude that operating the kiln at higher than needed relative humidity is a
safe way to insure the best possible quality. However, with white species the higher
relative humidity can cause excessive surface moisture content and actually promote mold
and fungi. The consequence of this condition is often stained lumber. As mold and fungi
cannot begin growing on lumber below 20% moisture content, the proper use both EMC/RH% and
adequate air flow prevent stain by achieving the required moisture removal.
Air velocity. (How will changing the variables effect the drying process)
The effect air velocity has on the possible drying rate is simple. The air flow in the
kiln chamber can be seen as a transportation system which lifts moisture
from the surface of the lumber and carries it out of the chamber through vents (or to a
DH unit). Providing adequate vent capacity; if you double the air velocity, you double
the potential removal of moisture. We use the word "potential" to describe
moisture loss because removal is dependant upon the surface moisture available.
There are generally two contradicting conditions we like to achieve with air flow.
Though lower air flow saves electricity, it decreases uniform drying across the load.
Hence the question "why does high air flow produce more even air flow throughout the
kiln load?" In short, the air chooses the path of least resistance. This normally
occurs when air escapes between the lumber packs, between the 4x4 kiln blocks
(runners), over the load, etc. As you increasing the air volume, you increase the
static pressure on both sides of the kiln load. As static pressure increases, the
air will start utilizing more path ways until we achieve nearly uniform air flow
through all pathways. This equalizing effect is due to the exponential resistance
when velocity is increased through a path way.
It is normally said that higher air flow is required when the lumber is above fiber
saturation point (approx 25% MC) and lower air flow below fiber saturation point.
This is not because the drying slows down in the second half of the drying
process, but rather because higher temperature and lower relative humidity is
used to make each cubic foot of air capable of removing more moisture. The
resulting condition is one in which less air volume is required to achieve the
same drying rate.
Temperature (How do we measure and determine the variables in the kiln)
In most cases, the temperature sensor and indicator is called the "dry bulb".
Kiln temperature is measured using a standard RTD sensor(resistance temperature
detection, also call PT100 in europe). This method of determining temperature
is so popular that it has almost become a industry standard. For short distances
between dry bulb and control panel, a two wire version can be used. For longer
distances between dry bulb and control panel, the three or four wire is preferred
(as the controller compensates for the resistance in the longer wire). In some cases,
the signal is converted to either voltage or mA signal. For example, the 4-20mA
current-loop is a signal type which can both power the transmitter and provide a
nearly interference free signal.
In most standard kilns, two dry bulbs are used(one in the front and one in the back).
By having two dry bulbs, the controller can read the entering air temperature and
thereby provide a more accurate temperature control. In some cases, a relay is tied
into the forward and reverse fan cycle. Dependent upon fan direction, the relay sends
one of the dry bulb signals to the controller. Though uncommon, a technique
sometime used is that of reading both dry bulb signals into the controller.
Practically speaking, there is no real benefit of such a network unless one sensor
were to fail. Were a sensor to fail, the second sensor would simply keep operating.
If a two sensor relay system lost one sensor, the controller would simply
disregard the reading and not try to adjust the temperature.
If a chart recorder of similar method of constant recording is used, you will often
be able to determine the problem, just be analyzing the chart.
Relative humidity. (How do we measure and determine the variables in the kiln)
There are four methods of determining the relative humidity in the kiln:
The first method of using the dry and wet bulb is the most common in the industry today,
mainly for historical reasons. However, there are very good arguments for changing to
one of the other three methods. (Wooddryer System prefers the relative humidity sensor)
The last three methods are similar in function and they are alike in the way they
operate the kiln vents and exhaust system. The biggest problem with the dry and wet bulb
method is that when the heat source is disabled, the wet bulb is falling even though
the relative humidity is rising. This could result in conditions which promote mold
and fungi stain if the heat source is out for a longer period. On the contrary, the
other three systems measure the relative humidity related readings and the venting/exhaust
continue independently of the accuracy and reliability of the heat source and temperature
control. Objectively speaking, it is the ability to independently control the venting
without interference from the heat controls that makes the best operational sense.
Air velocity. (How do we measure and determine the variables in the kiln)
This is the only controllable option in some of the newer kilns through frequency
controllers. The velocity is normally measure using a small instrument readily available
from several vendors. On a rare occasion, you can see actual air velocity meters tied
into the controller, although such a arrangement seems excessive in cost and equipment.
Normally speaking, when referring to the fan speed in kiln with frequency controllers,
we cite the fan speed as a percentage of the full rpm. For example, a 1200 rpm motor
reduced to 900 rpm would be operating a 75%.
Temperature. (How do we control and manipulate the variables in the kiln)
The temperature can be manipulated through several different heat sources and mechanical
controls. Some operate the heat as an ON/OFF system and others as a modulated system.
Some heat sources available are:
Note 1: The steam or hot water can be generated through several different means. The
most common fuels are LP or Natural gas, #2 oil, wood waste. Wood waste system are
normally only economical on larger systems. We at Wooddryer System believe the fully
modulated valve system is an unnecessary extra cost, as we have invented the ON/OFF sequence
modulation using standard valves. All our controllers support this feature.
Relative humidity. (How do we control and manipulate the variables in the kiln)
The relative humidity is generally adjusted through 2 methods: Vent/heat or dehumidifier.
Dehumidification systems can be installed on both large and small kiln. These systems do
seem to have their strongest market in the smaller kiln market for hardwood, as they do
not require a boiler system. The major drawback of dehumidification systems is the high
cost of operation (even though they are suppose to be more energy efficient). The
higher operation cost is because the complete system is electrically operated. As heating
cost with electricity is much higher then gas, oil, or wood waste, the dehumidification
kiln proves uneconomical in the final analysis.
The dehumidifier system is somewhat similar to an air conditioning unit. The units have
a cold coil and a hot coil. In a dehumidification system, the humid air in the kiln is
pasted over the cold coil whereby humidity is condensed and run off. The excess heat
created is channeled to the hot coil where the same air is past through and some energy
recovered. This system works best in high humidity climate.
The vent/heat kiln are the work horse of the lumber industry. These units are not as
affordable in smaller kiln size below 45 m3 (20,000 bf) because a external heat system
is required to generate the hot water or steam. (Wooddryer System new
gas heat unit with separate combustion chamber allows for smaller kilns economically).
The vent/heat system relies on the fact that when a cubic foot of air is heated,
the relative humidity drops. By using this principle in a vented kiln, the humid
warm air in the kiln is vented out and replaced with cold air. This new cold air is then
heated and the relative humidity is then lowered. One of the biggest advantages of the
vent/heat system is that it is relatively inexpensive to make the kiln more powerful
than a dehumidifier when it comes to potential water removal. For venting and exhaust
systems there are the standard roof (or top of wall) vents. This has been the common
method of venting kilns for decades. Recently, more and more kilns are
manufactured with power venting to increase capacity. Furthermore, they also save energy
by removing moisture from the wettest area in the kiln (instead of the driest as with
standard roof vents). In addition, some manufacturers (including Wooddryer System)
offer heat recovery system which reuses the warm humid exhaust to heat the incoming air.
These heat recovery units can make a heat/vent system nearly as energy efficient as a
dehumidifier, but will operate at much lower cost by using an less expensive heat source
than electricity.
Air velocity. (How do we control and manipulate the variables in the kiln)
The air velocity can be adjusted using two methods.
The first method of using frequency drives for changing the electrical frequency going
to the motors is slowly becoming more common. On some occasions the frequency drive is
tied into the control system, which then can help adjust temperature, relative humidity
and the air velocity.
A often overlooked method of controlling air velocity on average is the technique of
operating the fans in intervals. This method has several lesser known advantages; such
as more even drying and less costly than frequency drives. We at Wooddryer System offer
controllers that incorporate these interval operated fan systems. Without getting
complicated, we know that the air flow is more throughly distributed at higher air
velocity; therefore, the load dries more evenly. We also know the reason for slowing
the fan speed is the energy savings ( the claim is sometimes made that a fan operating
at 50% only used 25% of the energy is not always true. This is especially not the case
in a kiln where you have to deal with static pressure as well. The only credible study
we have seen demonstrates a proportional relationship between rpm and ampere use).
In most cases, we at Wooddryer System believe you will be better off using the interval
system as opposed frequency controllers for the fans.
We hope you found above information useful and helpful in understanding the somewhat
complicated, yet simple way, lumber drying can be understood. We hope you understand
this is only a brief treatment of the basic forces involved. Should you intend to operate,
or are currently operating, a lumber kiln, we will look forward to helping you in the
future with our knowledge, equipment, controllers, and on-line support.
We at Wooddryer System exist to help serve the kiln drying operators and their owners
in making their current or future kilns as economical and profitable as possible.
Interesting connections to other part of the WOODDRYER SYSTEM web-site:
Wooddryer Link System
Rule of thumb: A 20F or 10C increase in temperature can dry lumber up to twice as fast
(with the same RH% and air velocity).
How do we measure and determine the variables in the kiln.
How do we control and manipulate the variables in the kiln
Wooddryer System Kiln Equipment.
Common sense to our original kiln design.
