Constant Temperature Control Method And Its Improvement In Liquid UHT Sterilizer

Liquid UHT(Ultra High Temperature)sterilizer is used to keep the liquid in a constant flow status to pass through heat exchanger which will heat the liquid to sterilization temperature(130~150?),preserve for a certain time,and then achieve a commercial sterile level of liquid.As the nutrition and bacteria are very sensitive to temperature in beverage,there is strict control in sterilizing temperature,normally the temperature accuracy is ±1?.The imported sterilizers used for dairy or other high end beverages can ensure its temperature in an accuracy of ±1? and the running is very stable.While,China made sterilizer,the temperature accuracy is ±1.5? or even higher.As temperature fluctuation will affect the stability of sterilizer,so the efficiency cannot be guaranteed.If temperature is too high,machine would stop,meanwhile,cause the waste of raw materials,labor etc.Therefore,after studying constant temperature sterilizing control system,we optimize the hardware and soft ware of control system,temperature collection and filter program,replace single loop PID control system into dual loops PID control system,so as to reduce the interfere of steam pressure,hot water temperature change to sterilizing temperature.The study methods and innovation are in below aspects:1)Compare the measuring temperature value of PT100 platinum resistance thermometer sensor and standard mercury thermometer,apply linear regression method to analyze,and obtain the transfer equation between PLC digital figure and actual temperature in the temperature range of 42.9~66.8? and 67.9~91.4?.This method increases the inspection accuracy of constant temperature sterilizing,the tolerance is less than 0.1? during temperature exchange in sensor's full stroke.It solves the certain range temperature exchange mistakes through one linear relation when PT100 platinum resistance thermometer sensor is used to measure component.2)Use flow rate overshoot as test index,conduct three factors and three level constant PID control orthogonal test: ratio index KP,integration time TI and rate time TD.The test result testifies,the effect degree from large to less is TD?TI?KP.According to test result,choose the optimized PID parameters combination's controller,so the setting flow rate tracking is more accurate and the overshoot of maximum flow rate is less than 2.75%.Stable product and hot water flow rate benefits the constant temperature sterilizing.3)Combine temperature exchange transfer equation and PID control parameters and apply Siemens Step 7 software to write constant temperature sterilizing program with S7-300 PLC as its control core.Use this system in real production,after checking,the transitional time,subsidence ratio,maximum deviation,residual,vibration circle,frequency etc.all meet the production requirement.This sterilizer achieves the target of quick restore after interfere,overshoot small,maximum deviation is ±1?,which is 30%~50% higher in temperature control accuracy comparing with other domestic sterilizer.System runs stable,production efficiency increases,the production cost decreases.