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 Kiln-direct 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 consists of two basic processes:
Evaporation of moisture from the surface of the lumber:
The kiln operator manipulates the temperature, relative humidity, and the air flow in the kiln chamber to insure the correct rate of moisture removal.
Internal water/moisture movement inside the lumber:
The rate of water/moisture movement inside the lumber can be attributed to two general factors. Difference in moisture content between different zones in the lumber and the temperature of the lumber itself.
The kiln operator creates a dryer surface than the core of the lumber and, in simple terms, pulls out the moisture. The second factor is the temperature of the lumber.
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" In case hardening, 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 operator's 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 is within the safe rate of drying for the species being dried. It is important to always remember each species has its own individual maximum drying rate; hence, each species dries best under differing kiln conditions. To discover the optimum rate per species, we adjust the three variables (temperature, relative humidity, and air velocity) to achieve the maximum drying rate.
A vacuum in a vacuum kiln is also an adjustable variable; however, vacuum kilns are not discussed in this section.
How will changing the variables effect the drying process:
- Temperature.
- Relative Humidity.
- Air velocity.
How do we measure and determine the variables in the kiln:
- Temperature.
- Relative Humidity.
- Air velocity.
How do we control and manipulate the variables in the kiln:
- Temperature.
- Relative Humidity.
- Air velocity.
We hope you found the above information useful and helpful in understanding the different variables of lumber drying. 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 Kiln-direct exist to help serve the kiln drying operators and their owners in making their current or future kilns as economical and profitable as possible.