I. Overview
Currently, most of the equipment used in the domestic feed industry is provided by Jiangsu Muyang Group, offering complete sets and comprehensive solutions. The Huanghai Grain and Oil Feed Factory also utilizes the group’s products. While the overall performance is satisfactory, the shrimp stabilizer frequently experiences pasting issues. On average, about 1% of batches are affected, which negatively impacts yield and product quality. This issue can be effectively addressed by implementing an automatic temperature control system.
II. Process Introduction
The semi-finished shrimp material enters the stabilizer through a slip tube along with humid air, while steam passes through a pressure-reducing valve into the steam coil inside the stabilizer, ensuring full contact with the wet material for drying. The process demands consistent product quality, with moisture content kept stable and no paste formation. Therefore, precise temperature control is essential. It has been observed that the stabilizer's air temperature fluctuates by less than 2°C around 98°C, which is ideal. Our analysis shows that the main cause of pasting is unstable steam flow and significant temperature fluctuations. This occurs because the domestic pressure-reducing valves lack precision, leading to inconsistent steam pressure after decompression. Originally designed for constant pressure conditions, manual valves were added to the main steam line, resulting in improper temperature control.
III. Control Object Analysis
To address this, an automatic temperature control system was developed. Figure 1 illustrates the control system schematic. It is a single-loop system where the controlled parameter is the stabilizer temperature, and the control parameter is the amount of heating steam. Under normal operation, steam enters the stabilizer through a manual valve and a regulating valve. When the temperature exceeds the set point, the regulating valve automatically reduces its opening under the controller’s command, decreasing steam flow and bringing the temperature back to the desired level. Conversely, if the temperature drops below the set value, the valve opens more, increasing steam flow and restoring the temperature. This ensures the stabilizer remains within the target range.
IV. Control System
The automatic control system consists of two parts: data acquisition and control. Controlled parameters are measured and sent to the control instrument, which analyzes the data and sends control signals to the actuator based on the control law. A basic block diagram of the system is shown in Figure 2.
Figure 2: Specific implementation details include the installation of an automatic regulating valve on the manual steam valve, with the original line serving as a bypass. Given the requirement to maintain a temperature between 96–100°C, high-precision control is necessary. The selected temperature control valve is a Fischer valve from Germany, featuring an electric-pneumatic positioner that improves positioning accuracy and sensitivity. The valve has a control accuracy of 0.2%, making it suitable for various industries such as grain, oil, food, and petrochemicals. The valve is designed to close automatically when power is lost, ensuring safety.
Temperature measurement components and transmitters are produced by E+H, using a platinum resistance thermometer with high precision and stability. A three-wire connection method is employed to reduce errors caused by wire resistance. The transmitter outputs a 4–20 mA signal and supports the HART protocol, allowing for easy configuration and calibration.
The controller used is the WT-80 model from Hong Kong Shangrun Instrument Co., Ltd. It features advanced microprocessor-based control, with optical isolation for interference resistance. The PID control law is applied, and parameters are set using the 4:1 attenuation curve method. After tuning, the system achieves a control accuracy of 1%, with P=28%, I=3 minutes, and D=1 minute.
V. Conclusion
After over six months of operation, the system performs reliably, maintaining the working temperature between 98–100°C. The shrimp material yield reaches 100%. With an annual output of 10,000 tons, the savings from reducing paste-related losses amount to 450,000 yuan annually, meeting industry-leading standards.
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