What are your responsibilities as a kiln operator and how can you implement them?

Operating one or more kilns is a big responsibility and should never be taken lightly. A kiln operator controls a process which can seriously effect the economy of the company. The cost of inattention to the daily operation can easily amount to more loss in value than an operator's own yearly pay. This job and its responsibilities require a person with a high level of self discipline and a flexible work schedule (extra overtime in order to attend to the kilns).

It is wise for any kiln operation to have a trained assistant available to relieve the head kiln operator from time to time; in addition, the lumber company will benefit from having trained and knowledgeable personnel as backup should it loose the main kiln operator.

Normally the kiln operator's responsibility is expanded to the extended drying process consequently, the kiln operator is responsible for quality control throughout a lumber yard. He is the one that the company will depend on to solve problems not related to the sawmill and secondary manufacturing, such as moulding and planning. The kiln operator’s job is very important and his responsibility is not limited to the four walls of a kiln chamber and it’s controls. The kiln operator must/should follow the lumber from the day it enters the lumber yard/sawmill to the day it ships, in order to spot and correct problems that are normally considered drying related problems. Many times these problems occur before and sometimes after the kiln drying process itself.

Partial list of the things the kiln operator should look for

We hope this information meets with your expectation and approval. We always welcome your opinion and suggestions. Feel free to email us anytime. Thank you for your continued interest.

In this section we will introduce you to some of the concepts of kiln drying in a very concentrated form. The subject is far too great to try to be too specific with detailed explanations for this web-page. However, there is much literature available (both in books and on-line) on the subject and we encourage everyone to educate themselves in the art of drying lumber. It is an exciting subject. We are always interested in improving our ability to advise you and value your input, so we welcome opinions and suggestions. We will always be available with suggestions, guidelines, and explanations to our customers.

First, it is important to understand that there is a general relationship between how fast the lumber is dried and the quality you get as a result of doing so; however, on some species slow drying can be fatal to the final outcome through stain and mold.

Drying lumber is in it’s nature a controlled removal of water from the lumber at a speed which does not damage the lumber by checking, collapse, splitting, etc. while trying to maximize production through the equipment. This is a place where the lumber company will loose profit if it cannot dry lumber with minimum degrade and still ensure adequate production through the kilns.

When we are drying lumber we have some control parameters that we can utilize:

Drying defects and their probable causes.

Normally you can adjust your drying program/schedule by the defect/s you see on the kiln dried lumber. See a list of corresponding drying defects and their probable causes.

Safe rate of MC% drop per day versus drying gradient.

Safe rate of MC% per day.
Each specie and thickness has an approximate rate at which it can lose water (moisture) per day at a safe rate. The “safe rate” is different from specie to specie and from thickness to thickness. Most kiln drying manuals will focus on the “safe rate” for maximizing the drying quality and production.

Safe drying gradient.
Safe rates are a good measure, however; we do look at the safe rate of drying per day, but we prefer to focus on the maximum safe drying gradient. In kilns with adequate air velocity and venting it is possible to use drying gradient, whereas with older kilns the kiln operator must sometimes focus on the safe rate even if he has to exceed the safe drying gradient.
The climate in a kiln has a corresponding equilibrium moisture content (see graph). This means that by controlling the climate you can control the moisture content on the surface of the lumber. Since lumber shrinks during the drying process, the surface which dries first will try to shrink more than the core with the higher MC%. The difference in the inside and outside MC% (the EMC% of the air) is called the drying gradient.
When operating a kiln with more than adequate air flow and venting, you can control the climate in the kiln to create the exact EMC in the air. This also enables you to control the surface MC% of the lumber and thereby the drying gradient. So it is important to have a kiln with sufficient heat, vent and air flow capacity.

Example: With a drying gradient of 2, and core MC% of 24%, the EMC should be 12% (24/2).

The maximum drying gradient allowed is different depending on the specie and thickness. Slow drying and harder species; such as oaks, have a low drying gradient, whereas faster drying and softer species; such as pines, have a high drying gradient. On softer species the wood can flex and stretch more, which allows for greater shrinkage difference between the core and the shell, therefore the drying gradient can be higher than on softer species.

The difference between pulling moisture out and forcing moisture out of the lumber.

Normally when you have well air dried lumber in a kiln you are trying to force the moisture out through the drier outside shell, in contrast to drying fresh sawn lumber. Here we try to pull the moisture out without over drying (case-hardening) the surface.

This aspect is much more important than it sounds.
Let us explain: Just as you know from a wet wick or sponge , once they dry completely they are actually hard to get wet again. You must dip them in water and squeeze, thereby forcing the water/moisture back into the fabric. This is nearly the same thing that happens to lumber that is over dried. Once the shell of a board has been over dried, either through air drying or drying too fast in the beginning of the drying process it becomes much harder to get the moisture out through the dry shell. This will normally mean a higher temperature is required to force the moisture out.

The best possible drying quality is achieved
by starting the drying process from fresh sawn lumber and keeping the temperature as low as possible. The lower temperature also makes the wood slightly stronger and thereby the chance of degrading is further reduced. This low temperature approach to drying can have many additional advantages.

We hope this information meets with your expectation and approval. We always welcome your opinion and suggestions. Please feel free to email us anytime.  Thank you for your continued interest.

Since 1920, Universities in Europe and the United States have been making suggested drying schedules on different species and thicknesses. There are three general types of drying schedules available:

North American:
Is based on constant fan operation and Wet/Dry bulb temperatures. These can easily be converted into a Temp/RH% schedule for those kiln controllers which use an RH sensor instead of a wet bulb.
These schedules have numbers like T4-E6, T2-C1, etc.

British:
These schedules are also based on a Wet/Dry bulb temperatures and can be used the same way as North American schedules.

Drying gradient:
The species are normally divided into 4 groups depending on the drying characteristic. Each group and thickness is then given a maximum drying gradient that one can use and still expect a high quality drying.
The temperature will depend on the specie you are drying. Generally the slower and harder the specie dries, the lower the initial temperature is.

There is much good literature available on drying lumber; but you can also find some which is less clear and sometimes has questionable accuracy. When you are studying the subject always use your common sense and try to compare it to your own experience.

We hope this information meets with your expectation and approval. We always welcome your opinions and suggestions. Feel free to email us at techsupport@kiln-direct.com.

Developing drying programs/schedules is a continuing process of trying to perfect the drying quality and the efficiency of the kilns. On some occasions the same specie will require a little different drying schedule if it comes from a different region. One example is the difference between oak grown in warmer climates and oak grown in colder climates.

Finding the right temperature schedule.

In most cases finding the correct temperature for drying is a choice between:

As a general rule the following temperature for most hardwood species can be used:

Over 25%MC (predrying phase):

Developing drying programs/schedules is an ongoing process of trying to perfect the drying quality and the efficiency of the kilns. On some occasions the same specie will require a little different drying schedule if it comes from a different region. One example is the difference between oak grown in warmer climates and oak grown in colder climates.

Finding the right relative humidity schedule (wet bulb) based on the drying gradient.

The relative humidity (and temperature) is closely related to the equilibrium moisture content on the air. The equilibrium moisture content of the air is a measure of the moisture content the lumber will react to if placed in a certain climate. This is due to the fact that lumber is a hygroscopical material, which permits it to absorb and retain moisture depending on the outside climate (temp and relative humidity). The relative humidity setting is very important since the wrong setting can cause either stain (to high RH%) or surface checking (to low RH%). We suggest you calculate the RH% setting by using the drying gradient.

Table with different species divided into 4 groups used to determine drying gradient in table.
Guidelines for approximate drying gradient for different groups of species and different thicknesses

Example on determining the RH% setting using drying gradient:

If the MC% of the wood is above 28% you must use 28% in your calculation. When drying lumber, the shrinkage starts around 28%, this is the reason why MC% above 28% is considered 28% for purposes of calculating the RH settings. We are trying to prevent the excess shrinkage of the shell (in comparison to the core) to be stretched beyond the breaking point when surface checking starts.

Kiln-direct believes this is the best method to determine the relative humidity or depression schedule for species and thicknesses that you cannot find good and accurate existing data. Using this method does require a thorough understanding of kiln theory and very close attention during the first drying cycles to insure that drying defect and other degrade becomes excessive.

We hope this information meets with your expectation and approval. We always welcome your opinions and suggestions. You can email us at techsupport@kiln-direct.com. Thank you for your continued interest.

Developing drying programs/schedules is an ongoing process of trying to perfect the drying quality and the efficiency of the kilns. In this section we focus on some of the parameters which do not relate to temperature and relative humidity. Kiln-direct's kiln controllers have been designed to allow the kiln operator more methods of fine tuning the drying process. These parameters can be used to optimize the kilns to the best drying quality and lower operating cost. Many of the parameters in this section are probably not relative to the standard kiln controllers.

Learn more about the advantages of kiln-direct controllers.

This system has been designed for drying all types of hardwoods and softwoods. The setup parameters described in this section can be used to fine-tune the kiln controller for each type of load you will be drying. When using the Kiln Link Direct network you can build a database for each type of wood and thickness. Each drying schedule/program can have their individual setup parameters, which is automatically sent to the kiln controller when a load is started initially. This system makes the operation of kilns very simple, without compromising your ability to take advantage of all the fine-tuning you need to maximize the kilns.