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Drying Expert Answers Questions On Heat Treatment Technologies For Hardwood Pallets
Pallet Manufacturers Should Be Interested in Heat Sterilizing Pallets for the Lowest Cost Possible

By Staff
Date Posted: 6/3/2002

As the implementation of a global treatment standard for wood packaging comes closer to a reality, companies faced with the likelihood of having to treat hardwood pallets and wood packaging are starting to ask technical questions about heat treatment. Pallet Enterprise has discussed the issue with Joe Denig, a widely recognized drying expert and extension specialist at North Carolina State University’s College of Natural Resources.

Pallet Enterprise: What factors determine how well a heat treatment process works?

Denig: Pallet manufacturers should be interested in heat sterilizing pallets for the lowest cost possible. If you have enough demand, this equates to turning the sterilization chamber as many times as possible per day. So the focus needs to be on the lowest production cost per pallet (unit of production) and not the cost of the chamber.

There are three key factors in heat sterilization you should be aware of – energy, humidity and air circulation. Energy is the amount of heat supplied. Insects, insect eggs and fungi become less active at higher temperatures and are killed when the temperature exceeds 130oF. The properties of wood changes when the temperature is increased. Wood becomes weaker, increasing the risk of checking, cracking and honeycombing. Also, wood darkens as it is exposed to higher temperatures.

Humidity can impact lumber quality and treatment time. Relative humidity is the ratio of the amount of water vapor present in the air to the maximum capacity of water vapor the air can hold at a given temperature. When air is at 100 percent relative humidity, it cannot hold any more moisture. Lumber exposed to 100 percent relative humidity, for practical purposes, is not drying. Wood exposed to low humidity dries faster than wood exposed to high humidity. Exposure to a too low relative humidity can result in cracks and splits because the wood dries too rapidly on the surface. During normal kiln drying, 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 may occur in the lumber used for the pallets.

Measuring and controlling relative humidity at elevated temperatures can be 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. Standard charts show the relationship between the dry bulb temperatures, wet bulb depressions and relative humidity.

Higher humidity results in a higher wet bulb, which speeds up the heat transfer. The reason is that if a low humidity is used, 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 sterilizing will reach 56o Centigrade or 133o Fahrenheit. When green lumber is exposed to an atmosphere, it will increase in temperature to the wet bulb temperature of the surrounding air. Therefore, the higher the wet bulb temperature is above 133o Fahrenheit, the faster the green lumber in a pallet will increase to 133o Fahrenheit. If the wet bulb temperature of the air in the chamber is below 133o Fahrenheit, 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 133o Fahrenheit.

Air circulation, the third factor, is important to ensure uniform heat distribution in the chamber. This differs in lumber drying, where air circulation is used both to bring energy to the lumber and to remove evaporated moisture. Normally, in lumber drying it is important for good airflow at the beginning of drying.

Pallet Enterprise: How long will it take to dry a typical load of 48x40 pallets in a dry kiln?

Denig: The length of time it takes to heat sterilize green pallets depends on a number of factors. These factors include the temperature, wood thickness, heating capacity of the treatment chamber and humidity of the air. It takes longer to heat frozen pallets up to 133oF versus pallets that have an initial temperature of 70oF. The thickness of the wood used in the pallet construction is a factor because stringer pallets will come up to temperature faster than block pallets. Heat capacity of the sterilization chamber (how many BTUs the heat source can generate) and the humidity of the air being used to conduct the heat also greatly impact the treatment time.

Given the above variables, it took slightly over one hour to treat stringer pallets that were heat treated in a kiln with excess heat capacity and very high humidity. Under normal conditions, where the process has not been fine tuned, one should expect treatment to take three to four hours in a conventional hardwood dry kiln.

Pallet Enterprise: What are advantages/disadvantages of using direct vs. indirect steam?

Denig: The type of heat source plays a role in how fast you can sterilize pallets and the amount of checking and splitting present after treatment. There are four typical heat sources for a heat chamber – direct fired heating sources, indirect fired heating sources, indirect steam and direct steam.

Direct fired heating sources are probably the least expensive in terms of capital cost. The best analogy of this is heating your house by using gas-fired logs. You have to bring in outside air to burn with the fuel. Bringing in outside air uses more heat because you have to continuously heat the air. Because you are taking relatively cool air and heating it, you are increasing the water holding capacity of the air – or in other terms, decreasing the relative humidity. The heat in the air that is transferred to the pallets will initially be used to dry the surface of the pallets versus heating the pallets. Because the pallets are drying, this system will pose a greater problem of checking and splitting than any other.

Indirect fired heating is comparable to a hot air gas furnace in a house. A heat exchanger transfers heat from the burner to the air that is re-circulated through the house. Indirect fired systems are the second cheapest method. Because the air is re-circulated, water evaporated off the surface of the pallets eventually raises the relative humidity of the air, speeding the heating process and reducing the checking and splitting.

For indirect steam, water is heated in a boiler, and air flowing past steam heating coils transfers the heat in the steam to the air and then to the pallets. This is the heating system used in a conventional steam heated lumber dry kiln. Due to the need of a boiler, this system becomes more capital intensive. Steam is an efficient heat transfer method. By itself, there is no way to humidify the air except by the moisture contained in the pallets. The speed of heating and the amount of checking degrade is comparable to the indirect-fired heating system.

Steam also can be used directly from the boiler or a steam bath can be used in the chamber to generate the steam. Again, a boiler is required, so the capital cost is more. The advantage of this system is that the relative humidity is near 100 percent. Little drying of the surface of the pallets occurs, which translates into faster heat up times and minimal checking and splitting. Normally, conventional steam heated lumber dry kilns have a steam spray system to raise the humidity in the dry kiln in addition to the steam heating coils.

Pallet Enterprise: How can weather problems, such as frozen wood, or mold affect pallet treatment, and should a treatment system be designed to deal with these problems?

Denig: The time required to raise the temperature of the pallets is proportional to the difference between the initial temperature of the pallets and the desired final temperature. The cooler the temperature of the pallets initially, the more heating time required. In cooler climates it is important to size your heating source appropriately if you want short heating times.

The amount of heat lost through a heat chamber’s walls is directly proportional to the inside temperature versus the outside temperature. The colder the climate, more you are constrained by your heating source. If you have poorly insulated walls in a cold climate, the more water from the air will tend to condense on the walls of the chamber. This will keep the relative humidity of the chamber down, increasing the checking and splitting problems. Also, water condensing on the walls can lead to structural problems with the chamber.

Pallet Enterprise: What are various testing methods to measure the core temperature of the wood?

Denig: The time to heat up the pallets will vary with factors, including the initial temperature of the pallet and pallet design. Most certifying agencies will test your heat chamber or kiln initially using heat probes in the air and in the pallets. They will develop a correlation between the temperatures in your kiln and how fast the pallets meet the sterilization standards. The temperature chart of the chamber conditions for each run will serve as proof that you met the requirements.

Thermocouples are an inexpensive way of testing the internal temperature of the pallets so that you can more closely monitor or improve your process. A thermocouple is simply a two-strand wire made from two different metals. When heated, they generate a small electrical current. This current can then be translated into temperature. Depending on the types of metal used, different calibration constants are used. A thermocouple monitor with four inputs can be purchased for about two hundred dollars. The ends of the insulated thermocouple wire are stripped and twisted together. A hole is drilled into the center of the thickest part of the pallet and the twisted end is inserted. A toothpick pressed in the hole will seal the hole and hold the wire in place. The other ends of the wire are attached to the monitor.

If you do use a thermocouple system to monitor the internal temperature of pallets, your probes should be distributed over the charge so that you have a good chance of monitoring the slowest heating pallet.

Pallet Enterprise: Explain the basic parts of a dry kiln and how it works?

Denig: All of the components needed to heat sterilize pallets exist in a conventional steam-heated lumber dry kiln. The steam coils serve as the heat source. Fans are used to circulate the air throughout the kiln. Air circulation is critical because it used to transfer heat from the heating coils to the lumber and remove the moisture from the lumber. There is usually a steam humidification system to increase the humidity inside the kiln. Vents are used to vent off excess moisture from the kiln. All of this is contained in an insulated building.

Normally, in a conventional steam kiln, heating coils are used 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, you should consider an alternate humidification system such as a steam bath. If you are only going to heat sterilize pallets, only a steam humidification system is needed. If you also want to dry lumber, you will need heating coils in addition to the steam humidification system. 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.

A kiln controller is used to control the conditions inside the kiln. If the controller senses that the kiln is not up to the desired temperature, the steam valve to the heating coils are activated. If the temperature is too high, the controller turns the steam valve off. If the controller senses the humidity is too low, the steam spray valve is activated to humidify the kiln. If the humidity is too high, the controller activates the vents allowing the humid air to escape into the atmosphere. Fans are continuously on, and reverse directions, in order to insure uniform directions.

The conclusion you might draw from this information is that if you design an excellent heat-treating chamber, you are close to designing a conventional steam-heated lumber dry kiln. The additional cost to the pallet producer to add the flexibility to both heat sterilize pallets and dry lumber becomes small. One of the questions a pallet producer needs to ask when considering installing a heat sterilization chamber is do they want the additional flexibility. If you are considering using a kiln for heat sterilization, you should be aware the atmosphere used in sterilization could be extremely corrosive. The heat, high humidity and acids given off from the oaks can greatly shorten the life of a kiln if it is made out of bricks or steel.

Pallet Enterprise: Do you think checking, cracking or honeycombing could be a problem for any heat treatment system without humidity controls? What about a standard dry kiln?

Denig: In our tests at North Carolina State University, we did see more checks and end splits when we heat treated pallets using dry heat. Some humidity is built up from moisture evaporated off of the surface of the pallets. The greener the pallets are and the tighter the kiln structure, the faster humidity will build up. The defects we observed were concentrated around the nails and exposed ends of the lumber. With the conditions that we used, pallet degradation was not significant enough to concern the end user. There are conditions where heat-treating can degrade the pallet to such a point that the end user will not be satisfied. These include maintaining too dry of an environment through introducing cool air that is heated into the chamber either through the type of heater used or leaky construction, poor insulation, or just leaving the pallets too long in too dry of an environment. Having a system to raise the humidity, such as a steam spray line, is a definite plus. An indirect heating system, which allows the humidity in the air to build up as it is re-circulated through the system, is a minimum.

Pallet Enterprise: How important is even distribution of heated air or steam to successfully treating pallets?

Denig: Uniform airflow throughout the heat chamber is critical to insure that you have uniform conditions throughout your sterilization chamber or kiln. This holds true if you are using dry heat or air that is saturated with steam. The certifying agency that is giving the pallet producer the authority to use the "bug stamp" should test the pallet producer’s sterilization chamber or kiln for uniform heat distribution when first certifying the unit, and then on a periodic basis.

Uniform airflow achieves uniform heat distribution. A sterilization chamber should have a fan system that forces all of the air past the heating source and then uniformly distributes the air past the pallets back to the heat source. Airflow takes the path of least resistance. So proper loading and baffling of the chamber is important to achieve good airflow uniformity. Due to the fact that we are only heating the pallets and not drying them, extremely high airflow is not necessary. But we do want a uniform airflow (approximately 150 feet per minute) through the pallets.

Pallet Enterprise: What are the top factors that a company should consider when purchasing a treatment system for pallets?

Denig: Based on anticipated production, I would look at the alternative that gives me the lowest cost per unit of production. This includes long-term maintenance cost and operating costs. For a big producer I believe the best alternative will be a heat chamber that is very similar to a conventional steam heated lumber dry kiln. For a producer that will only intermittently have a need for heat sterilizing pallets, or low production, one of the less expensive semi-portable heat chambers may better serve the purpose.

From a financial standpoint, the amount of heat-sterilized pallet business will determine the best type of sterilization unit to buy. From a technical standpoint, a unit that is similar to a conventional steam-heated lumber dry kiln is the best choice.

Another crucial point is the ease of loading and unloading. Due to the fast cycle time, loading speed becomes critical. Copying a track kiln is not a bad idea. A charge is prepared and waiting on carts while the charge in the chamber is being sterilized. As soon as the sterilization process is done, the charge is rolled out of the doors on one end, and the new charge is rolled into the opposite end.

Pallet Enterprise: What can pallet manufacturers do to reduce quality degradation of lumber once treated?

Denig: Heat sterilization only kills the insects and fungus that are present during the time of heat-treating. Lumber that has been heat-treated is often more susceptible to mold attack. Small surface checks can grow into large splits.

If after being heat treated, pallets are moist with surface water, such as heat treating with steam, a short cool down period in which excess surface moisture is vented off will help to eliminate moisture problems. During the cool down period, the heat source and steam must be turned off, vents should be opened to allow fresh air to come into the chamber and the fans are turned on to circulate the air until the surface moisture is wicked off.

The yard where the pallets are stored should be free of debris to keep the pallets from getting re-infested by any wood-inhibiting insect. Storing pallets under roof will help keep the pallets from further checking. Air circulation is required in hot humid environments to prevent mold from developing. Proper stock rotation and minimum inventory will also greatly reduce further degradation.

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