Manufacturers of hardwood pallets are faced with the prospect that they will be required to sterilize their pallets or pallet parts prior to export to Europe. The proposed regulations stipulate that the interior of the wood used in the pallets has to be heated to 56°C or 133°F for 30 minutes. Pallet manufacturers will be faced with decisions on how they should proceed. They can use conventional kilns to sterilize their pallets or they will be able to purchase specialized pallet sterilizing chambers. The purpose of this article is to give some insight into heat sterilization so you can make better decisions in the future.

There are three factors that will influence how well the heat sterilization process will work. They are energy, humidity and air circulation. Energy is the amount of heat supplied. Insects, insect eggs and fungi become less active with at higher temperatures and are killed when the temperature is increased above 130°F. Certain changes take place when the temperature of wood is increased. Wood becomes weaker increasing the risk of checking, cracking and honeycombing. Also, wood darkens as it is exposed to higher temperatures.

Humidity is the second factor that concerns us. Normally we discuss relative humidity. Relative humidity is the ratio of the amount of water vapor present in the air to the maximum capacity that the air could hold at the same temperature. When air is at 100 percent relative humidity it cannot hold any more moisture, thus lumber exposed to 100 percent relative humidity for practical purposes is not drying. Wood exposed to lower humidity dries faster than wood exposed to high humidity. Exposure to a too low relative humidity can result in cracks and splits due to a too rapid drying of the wood. During normal kiln drying the relative humidity is kept high at the beginning of the drying cycle in order to avoid checking. If your sterilization chamber does not have a method of raising the humidity, checking of the lumber used in the pallets may occur.

Directly measuring and controlling relative humidity at elevated temperatures is often complicated and expensive. A simple solution used in lumber drying is to use a combination of both a normal temperature sensor, called a dry bulb, and a temperature sensor covered by a wet wick called a wet bulb. The difference between the dry bulb temperature reading and the wet bulb reading is known as the wet bulb depression. The wet bulb depression is a measure of the evaporation potential of the air. The larger the wet bulb depression, the greater the evaporation potential of the air, and the lower the humidity is. There are standard charts that show the relationship between the dry bulb temperatures, wet bulb depressions and relative humidity.

The higher the humidity, the higher the wet bulb, the faster the heat transfer. The reason behind this is that if a low humidity is used, the water on the wood surface evaporates, cooling the surface of the wood. In practical terms this means that the faster you can achieve a high wet bulb temperature, the faster the wood you are trying to sterilize will reach 56°C or 133°F. When green lumber is exposed to an atmosphere, temperature wise it will act as if it was a wet bulb. That is, the lumber will increase in temperature to the wet bulb temperature of the surrounding air. Therefore the higher the wet bulb temperature is above 133°F, the faster the green lumber in a pallet will increase to 133°F. If the wet bulb temperature of the air in the chamber is below 133°F, the lumber in the pallet will not go above the wet bulb temperature until the wood begins drying out, even though the dry bulb temperature is above 133°F.

Air circulation, the third factor, is important in heat sterilization to insure there is uniform heat distribution in the chamber. This differs in drying in which air circulation is used both to bring energy to the stack of lumber and to remove evaporated moisture from the stack. Normally in lumber drying it is important to insure good airflow at the beginning of drying.

In the lumber drying literature, heat sterilization schedules have been known and used for years for controlling mold, fungal stain and decay, and insects.

Many pallet manufacturers who want to design a pallet heat sterilizing chamber or sawmill operators who want to use their lumber kilns to sterilize pallets need to have some idea of the required temperatures and times to obtain the internal temperatures required for sterilization.

Trial runs were conducted using different pallet designs, temperatures and humidities at North Carolina State University’s Hodges Wood Products Laboratory. Data from trials was used to develop estimates of the time required to elevate the internal temperatures of the wooden pallet components. In the study, kiln conditions and the internal temperatures of the test pallets and pallet parts were recorded versus time. The internal temperature was measured using a thermocouple, which was placed center mass of the largest component used. When interpreting the results it is important to consider that these trials were conducted in a small laboratory kiln (approximately 1,500 board feet capacity).

Two basic schedules were used in the study, a steaming schedules (trials 1 and 3) and a dry heat schedule (trial 2). For the steaming schedule (trial 1 and 3), the heating valve of the kiln was shut off. Setting the wet bulb temperature to 160°F controlled the high-pressure steam spray in the kiln. The wet bulb reached setpoint in 25 to 30 minutes. The actual wet bulb depression was one or two degrees. As Table 1 indicates, it took approximately 70 minutes for the conventional stringer pallets to be treated using these conditions (trial 1), 100 minutes for the block pallets to be treated (trial 1) and up to 16 hours (941 minutes) for pallet loads of cants to be treated (trial 3).

For the dry heat schedule (trial 2), the dry bulb was set at 180°F and the wet bulb was set at 160°F. The maximum recorded wet bulb was 150°F. The steam spray was valved off during this trial. It took approximately 85 minutes for the conventional stringer pallets to be treated using these conditions, and up to 165 minutes for the block pallets to be treated.

As the data indicates, the higher humidity air (the steaming schedule) is more efficient at transferring energy. Operators should remember, however, that the water-saturated air could be extremely detrimental to kiln structures (especially steel and block structures).

For both the steaming and dry heat schedules, the quality results were acceptable. The pallets treated with the steaming schedule exhibited some iron staining from the nails, while the pallets treated with the dry heat exhibited some checks and cracks around the nails (prolonged exposure to a dry environment will worsen the checks and cracks). The pallet components from deck boards to pallet cants showed surprisingly little damage due to checking using the steaming schedule.

Rapidly heating wood, such as pallets in a sterilization schedule under two hours, is relatively energy intensive compared to normal hardwood kiln drying. The reason behind this intensive energy use is that the wet wood has a high mass compared to air dried lumber and a very short heat up time used. It is critical in heat sterilization that your heating and humidification system be designed to meet your production schedule.

Typically the heating capacity of a hardwood kiln ranges from 0.5 to 1.5 boiler horsepower per thousand board feet of lumber. To get the rapid heating as in the tests, the boiler horsepower needs to be sized from 6.0 to 12.5 boiler horsepower per thousand board feet, depending on the lumber used and starting temperature.

Normally, in a conventional steam kiln we use heating coils to raise the dry bulb temperature and a steam spray system to raise the wet bulb temperature. If you want to use the rapid high humidity heat schedule that requires a high wet bulb temperature, one should consider an alternate humidification system such as steam baths. Steam baths produce steam by radiator pipes that are submerged under water in a container. The reason this system is preferred is that this is a closed loop system, where the condensate (and the boiler chemical) used in the steam making process, is returned to the boiler. In the conventional steam spray system boiler chemical is lost as the steam is sprayed into the chamber.

As mentioned above, the environment used for heat sterilization can be extremely corrosive and damaging to some structures. In addition to using the proper materials, a floor drain system should be used, especially when using the high humidity schedules.

Heat sterilization will not permanently protect your pallets from mold, fungus or insects. Heat sterilization only kills mold, fungus and insects that are present when the material is sterilized. In certain cases mold and fungus have rapidly infested heat sterilized lumber that was not dry. It is critical for the pallet operator and user to keep their production facility free of waste wood, minimize pallet inventory of heat-treated pallets, and insure there is some air movement around green pallets that have been heat-treated.