Patent GB-1326575-A (GB)
Title: Kiln system
IPC: F27D19/00
Filed: 1970-11-04
Abstract: 1326575 Automatic control of temperature and humidity MOORE DRY KILN CO OF OREGON 4 Nov 1970 [14 Nov 1969] 52377/70 Heading G3R [Also in Division F4] A direct-fired kiln system having a kiln chamber for drying and conditioning materials e.g. timber, is provided with a control system to automatically maintain temperature and humidity at desired values. The kiln comprises a kiln furnace 20, Fig. 2 (detailed in Fig. 1 (not shown)) having a gas or oil fired burner 30 and an outlet duct coupling heated furnace gases to the kiln chamber 10. Wet and dry bulb sensors 60, 65, respectively monitor the instantaneous temperature and humidity in the chamber. A nozzle 90 in the furnace gas outlet duct injects water for vaporization in the heated furnace gas under the control of an automatically adjustable valve 94. A controller 110 regulates the valve 94 motors 125a 125b which control the position of fresh air intake vents 42, 44, motors 135 which control the positions of a main damper 46 and a by-pass damper 48, the speed of a fan motor 82 and the air intake to the burner whereby to regulate the flow rate and recirculation of gases through the kiln furnace and provide predetermined temperature and humidity conditions in the kiln chamber. The instantaneous temperature of the chamber is sensed by the dry bulb sensor 65 the resulting electrical signal from which is compared with a pre-set reference value in a regulator 100 which produces a pneumatic signal in a line 102 if the temperature is below the reference value. The regulator 100 also receives the wet bulb sensor 60 electrical signal and compares the actual humidity sensed with a predetermined reference value and produces a pneumatic signal significant of deviation therefrom. The resulting heat demand and humidity demand pneumatic signals are fed to the controller 110. A sensor 70 e.g. a thermocouple, senses the furnace gas outlet duct temperature, which typically will be several hundred degrees hotter than the dry bulb temperature.  Over Temperature Protection As shown, Fig.3, the output of sensor 70 is fed to a limit controller element 106 together with a heat demand signal. When the temperature sensed by thermocouple 70 exceeds a safe limit e. g. 450‹F, the limit element 106 acts as a shut off valve decreasing the amplitude of the pneumatic signal to the burner air supply control 38 irrespective of the presence of the heat demand signal, and a pressure difference will exist across the limit element. This is sensed by a control element 108 which produces an output accordingly to a restrictor tee 114 thereby reducing the amplitude (e. g. to zero) of a pneumatic signal in input line 116 of a relay 118. The relay 118 also receives as input the humidity demand signal and functions as an OR gate to pass via line 130 the higher of its two inputs whereby to move the dampers 46, 48 toward their closed positions and increase the speed of fan motor 82. This causes a decrease in the proportion of tempering air and an increase in the circulating flow rate and a consequent reduction in temperature of the furnace gases to within the safe range.