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Timber Drying Kiln Control System

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Information about Timber Drying Kiln Control System
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Published on February 11, 2008

Author: Manfred

Source: authorstream.com

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Timber Drying Kiln Control System:  Timber Drying Kiln Control System Brett Percy – CPE Systems Melbourne LabVIEW User Group 18th May 2005 Requirements:  Requirements Control four kilns at two sites Two identical smaller kilns in lab process bay Two larger, dissimilar kilns in the factory on Commercial Road Kilns are used for piloting timber seasoning processes Control and log temperature and humidity for periods of days to weeks Automated control via a drying schedule External access to user interface via Web for monitoring outside hours Terminology:  Terminology Dry bulb temperature Standard temperature in degrees Celcius Wet bulb temperature Temperature in degrees Celcius measured using a moist sensor Usually expressed as a depression below the dry bulb temperature Average Moisture Content Expressed as a percentage Derived from the current mass of the timber and the expected dry mass Relative Humidity:  Relative Humidity Relationship looks like this RH (DBT, WBT) PS = 6E25 * (Exp (-6800 / (273.15 + DBT))) / ((273.15 + DBT) ^ 5) PSW = 6E25 * (Exp (-6800 / (273.15 + WBT))) / ((273.15 + WBT) ^ 5) WS = 0.622 * PSW / (101325 – PSW) Y = WS – (1005 + 1810 * WS) * (DBT – WBT) / (1810 * TDB – 4187 * WBT + 2502400) P = Y * 101325 / (0.622 + Y) RH = P / PS % Relative Humidity = 100 * RH Solution:  Solution Fieldpoint unit running LabVIEW RT to control each pair of kilns UPS to keep Fieldpoint running in the event of power failure PC to handle datalogging Fieldpoint unit handles logging if host PC is not available PC also handles web and email interfaces Lab Kilns:  Lab Kilns Two identical smaller kilns in Lab process bay Electric heater elements On/Off steam input On/Off venting Fan with speed and direction control 50kg Load cell Factory Kilns:  Factory Kilns Conventional Kiln Dual and Single Coil steam heaters Steam and water spray humidification Dampers for dehumidification Variable fan speed and direction No load cell Two pairs of RTDs Pair in use is determined by fan direction Conventional Kiln:  Conventional Kiln Factory Fieldpoint unit RTD Senders Factory Kilns:  Factory Kilns Container Kiln Steam heater Steam humidification Dampers for dehumidification Load cell Variable fan speed only, direction is fixed Single pairs of RTDs Container Kiln:  Container Kiln Control Challenges:  Control Challenges Nonlinear control effects Steam humidification adds to heat input Water spray humidification reduces heat input Rate of heating is much higher than rate of cooling Significant lag in steam response in lab kilns Uses PID control for both control loops Heat loop runs faster to reduce effect of humidity control on temperature Drying Schedule:  Drying Schedule Schedule controls dry bulb and wet bulb setpoints, fan speed and fan direction There is a default ramp time for changes in setpoints Each stage can be controlled manually, by a time delay or by the calculated moisture content Also a Ramp type for specific ramp times Drying Schedule:  Drying Schedule Datalogging:  Datalogging Data is logged to the PC’s local drive and to a network drive Comments can be added to the logfile Header includes PID and calibration values used Temperatures, setpoints, average moisture content and mass are logged Logfiles are manually rolled over by operator User Interface:  User Interface User interface is common to both kilns Allows manual and automatic control of kilns Live charting of data Calibration of mass and temperature readings User Interface:  User Interface Web Interface:  Web Interface Initially used the LabVIEW web server Problems with the server disconnecting because the PC was too busy Some problems with redrawing text indicators Used VNC instead Like PC Anywhere but free! Error messaging:  Error messaging An email message is sent when Temperature or humidy goes ‘out of control’ Fieldpoint detects a problem with an RTD Also provision for a dialer to be connected Questions ?:  Questions ? Thankyou for listening

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