Process Sensors Corporation

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Moisture Measurement in Wood Chips

Knowing the accurate amount of moisture in wood is extremely important in production of various types of wood products. The amount of moisture at various stages of the production process is also important; as is knowing when and where to measure depending on how the wood is being processed. For example, moisture measurement in wood before it is dried and when it is being processed in a batch digester or a continuous digestor all have distinct implications on final wood products.  

Green Wood

Monitoring the moisture content of the green wood prior to entry into the dryer can greatly improve the efficiency of oriented strand board (OSB), medium density fiberboard (MDF) and particleboard production processes in terms of reduced energy costs, increased production capacity, lower scrap and more consistent product.

Green wood is the term used to describe wood that comes from recently felled trees. As it has not aged, moisture content is high—70­­-75% in chips that come from fresh logs and in the general range of 30-60% if the wood has been sitting in storage. In MDF production, green wood chips from the chipper are routed to storage bins where they reside until entering the refiner. Knowledge of the moisture of the incoming chips enables informed decisions regarding the blender additive mix and control of the dryer. In particleboard production, the raw material is frequently a blend of recycled material and virgin wood. Once again, it is important to know the moisture content of the mix prior to entry to the dryer in order to optimize dryer efficiency.

Measurement Location and Installation

Measurement can be made either through a viewing window in the side of a storage bin or on a conveyor/weigh belt pre-dryer. When installed over a viewing window, the gauge must be positioned at a 15-degree angle to the window to eliminate specular reflectance off the surface of the glass window. In either location, the gauge is mounted approximately 200cm from the product.

Measurement Performance

Typical analog range for green wood is 30-75% moisture. Dry weight is 43-30%. Accuracy in measurement of green chips is typically 1-3%.

Wood Chips in a Batch or Continuous Digester

Moisture content of wood chips going into the digester is an important measurement. In a batch digester, loading the correct amount of wood will maintain a consistent chemical addition and ensure a proper cook time and pulp quality. For a continuous digester, knowing the moisture content of the chip feed will determine the correct continuous flow of chemicals to the digester. Proper cooking time and temperature can then be calculated to assure good pulp quality.

Measurement Location and Installation

For both types of digesters, the transmitter should be located over a conveyor transporting the wood chips. The sensor bottom should be mounted about 8 inches from the surface of the wood chips, near the weigh scale in a batch digester. The most important factor in the transmitter’s installation is that the transmitter be allowed to view the moving product continuously and without gaps or interruptions.

Depending on the sensor location, dust can be a problem. If the area is dusty, care should be taken to use an air purge to prevent dust from depositing on the transmitter’s lens. Process Sensors supplies a very effective air purge assembly that requires low air pressure and is extremely effective.

Measurement Performance

Moisture in wood chips is a very well understood and documented application; therefore, calibrations for measurement wavelengths, algorithms and sensor optical requirements are all preset at the PSC factory. Typical accuracy is about ± 0.5% moisture. Typical analog range is 20-65% moisture. The transmitter analog output can easily be adjusted to optimize PLC or recording requirements. Options are available to provide a bone-dry feed rate signal.

Process Sensors is very experienced with the requirements of these application. For additional information or to discuss your application further, please contact the specialists at Process Sensors Corp. or fill out the form below and someone will reach out to you soon!

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