Dryer Components - Controls and Accessories
DRYER CONTROLS
The heart of the timber dryer is the control system. The controller needs to measure 3 variables in timber drying- the air temperature, the relative humidity and the wood core moisture content (MC). Relative Humidity (RH) can be measured directly using RH sensors. RH sensors used for normal ambient temperature (max 55oC) and humidity is cheap and readily available. For higher temperatures and for use in very dusty and acidic conditions, the sensors are very expensive and requires frequent cleaning.
A thin piece of Cellulose paper can be used as a RH sensor. The cellulose paper behaves exactly like the wood surface. It will absorb or gives out moisture depending on the Temperature and Humidity of the air. This cellulose sensor is call Equilibrium Moisture Content (EMC) Sensor. When the water vapor pressure in EMC sensor is in equilibrium with the water vapour pressure in the air, no evaporation or absorption will take place. Just like wood moisture content, the moisture content in the EMC paper can be measured electrically in Ohms.
A piece of damp cotton cloth placed around a temperature sensor can be used to measure RH. Like the EMC paper, it will absorb or gives up moisture depending on the Temperature and RH of the air. When moisture in cloth evaporates, it needs energy to evaporate. The energy is derived from the tsensor’s body. This hhas a cooling effect on the temperature sensor. When the vapour pressure in the cloth is in equilibrium with the air vapour pressure, the evaporation stops and so does the cooling.
The temperature of the sensor at which the cooling stops is known as the Wet Bulb Temperature. The temperature of the surrounding air is at a higher temperature and this temperature is known as the Dry Bulb Temperature. The Dry Bulb temperature and its difference with the Wet Bulb temperature can be used for measuring RH too.
These relationship between Dry Bulb Temperature, Wet Bulb Temperature , EMC and Relative Humidity has been scientifically established. Using this chart you can see how the dry and wet bulb temperature difference can affects the EMC with respect to the dry buld temperature. A typical chart is shown below.
Therefore, Timber dryer controllers have 2 different platforms , one is based on EMC method of measuring RH and the other is based on Dry Bulb and Wet Bulb Temperature.
EMC basically tells us how the wood surface would behave at different temperature and humidity conditions. The ratio between MC and EMC is called Drying Gradient (DG) or Drying Speed (DS). This ratio is an indicator of how fast the moisture in the core will diffuse to the surface.
Pine timber may have a recommended value DG = 3.0 , while Red Oak may have a DG value = 2.2 for example. This DG value also indicates how fast or difficult the wood can be dried. If EMC or RH is too low, the measured DG (MC/EMC) becomes larger than the recommended value, the timber will dry too fast and drying defects will occur.
EMC values with respect the resistance (ohms) values of a cellulose paper at differnet humidity and tempearture conditions have been fully established by controller manufacturers and rwesearch institutions. Therefore, Timber dryer controllers can be based on direct EMC measurements or be based on Dry Bulb and Wet Bulb Temperature difference to determine the drying conditions in the dryer.
EMC basically tells us how the wood surface would behave at different temperature and humidity conditions. The ratio between MC and EMC is called Drying Gradient (DG) or Drying Speed (DS). This ratio is an indicator of how fast the moisture in the core will diffuse to the surface.
Pine timber may have a recommended value DG = 3.0 , while Red Oak may have a DG value = 2.2 for example. This DG value also indicates how fast or difficult the wood can be dried. If EMC or RH is too low, the measured DG (MC/EMC) becomes larger than the recommended value, the timber will dry too fast and drying defects will occur
CONTROL ACCESSORIES
Dryer Control accessories includes the following items :
- Temperature Sensors
- EMC Sensor Holder
- EMC Cellulose sensors
- MC Cables ( for connection from probe box to wood measurement pins)
- Probe Box with MC measurements connection sockets
- Probe Box Cable ( for connection to dryer controller carrying EMC or wet bulb temperature measurement vslues , dry bulb temperature measurement values and the various MC measurement values)
The quality of the MC measurement cables is the most important. Normal PVC cables CANNOT be used, as the material can absorb moisture that will give false MC readings. Only Teflon and Silocone cables can be used as they are high temperature and moisture resistant.