Lumber dry kiln control systems control the environmental conditions of heat, relative humidity, and air circulation necessary for the proper drying of hardwood and softwood.  These control systems are usually personal computer (PC) or programmable logic controller (PLC) based.  On some older systems, cam follower control is used.  Dry kilns typically dry lumber to between 3 and 19 percent moisture content.

These controllers monitor and operate the kiln system on a pre-determined schedule basis or by calculating moisture content or temperature drop across the loads.  The controls for a dry kiln is divided into zones side to side and along the length for more accurate drying. Each zone's heat and humidity are controlled by regulating the heat and roof vents. Venting occurs when the relative humidity is higher than the set point.

The temperature within the dry kiln is measured with a dry bulb.  The dry bulb is a resistance temperature detector (RTD).  The RTD's are mounted on both sides of the loads in each dry kiln zone.  The dry bulbs allow measuring the dry kiln temperature as well as the temperature drop across the load.  This temperature measurement allows the kiln controller to operate the heating system to maintain the temperature in the kiln.

Humidity in a dry kiln is measured with the use of wet bulbs.  A wet bulb is an RTD covered with a cloth sock.  The sensor mounts over a water box with a controlled level of water.  The sock hangs in the water and stays wet by wicking moisture up from it.  There is typically one wet bulb per zone in the dry kiln.  By measuring the humidity in the dry kiln, excess moisture can be vented to the atmosphere with automatically controlled vents.  Moisture can also be added with a water mist or low pressure steam (15 psi).

Air circulation is also controlled by the dry kiln controls.  Air is circulated through the loads between 600 and 1000 cfm.  The circulating air picks up moisture off the surface of the lumber as the lumber is drying.  The kiln controller will cycle the fans forward or reverse in a predetermined pattern.

There are also capacitance systems that will measure moisture content of the lumber both above and below the fibre saturation point.  This is used to automatically shut down the dry kiln based on actual moisture content rather than a timed schedule.

English Terminology: Dry kiln controls, kiln control, kilns, doors, baffles, vents, wet bulb, dry bulb, lumber drying, timber, boards, sawmill, planer mill, wet mill, dry mill.