I. Overview
Currently, the majority of equipment used in the domestic feed industry is provided by Jiangsu Muyang Group as complete sets and integrated solutions. The Huanghai Grain and Oil Feed Factory also utilizes these products. While the overall performance has been satisfactory, the shrimp stabilizer frequently experiences issues with pasting. On average, about 1% of each batch ends up being pasty, which negatively affects both yield and product quality. This issue can be effectively addressed by integrating an automatic temperature control system into the process.
II. Process Introduction
The semi-finished shrimp material enters the stabilizer through a slip tube along with humid air. Steam flows into the steam coil inside the stabilizer via a pressure-reducing valve and comes into full contact with the moist material to achieve drying. The production process demands high-quality output with minimal moisture fluctuation and no pasting. Therefore, the drying temperature must be precisely controlled. Experience has shown that when the stabilizer’s air temperature remains within a 2°C fluctuation around 98°C, the control effect is excellent. Through analysis, it was found that the main cause of pasting is unstable steam flow and significant temperature fluctuations. This is largely due to the low precision of domestic pressure-reducing valves, leading to inconsistent steam pressure after decompression. The original design assumed constant pressure, and manual adjustment of the main steam line was used, resulting in inadequate temperature control.
III. Control Object Analysis
To address this, an automatic temperature control system was designed. Figure 1 illustrates the control system schematic. It operates as a single-loop control system where the controlled parameter is the stabilizer’s temperature, and the control parameter is the amount of heating steam.
Figure 1: The system works as follows: Under normal conditions, steam enters the stabilizer through a manual valve and a regulating valve to heat the raw material. When the stabilizer temperature exceeds the set value, the regulating valve automatically reduces its opening under the meter's control, decreasing the steam input and bringing the temperature back down. Conversely, if the temperature drops below the set point, the valve opens more, increasing steam flow and restoring the desired temperature. This ensures the stabilizer temperature stays within the required range. The instrument's control function principle is illustrated in the diagram.
IV. Control System
The automatic control system is divided into two parts: data acquisition and control. Controlled parameters are detected and transmitted to the control instrument, which analyzes the data and determines the control action based on a pre-defined control law. It then sends a signal to the actuator to perform the necessary adjustments. The basic block diagram of the system is shown in Figure 2.
Figure 2: The system consists of several key components. First, an automatic regulating valve is installed on the manual steam valve, with the original line serving as a bypass. The temperature must be controlled between 96–100°C, requiring high precision. We use a Fischer valve from Germany, which features an electric-pneumatic positioner that enhances positioning accuracy and sensitivity through negative feedback. This allows precise control, achieving a precision of 0.2%. The valve is widely used in industries such as grain, oil, food, and petrochemicals.
For temperature measurement, we selected German E+H temperature components and transmitters, which offer high precision and stability. These devices use a platinum resistance thermometer, ensuring accurate readings with minimal drift. A three-wire connection method is used between the sensor and transmitter to reduce the impact of wire resistance on measurements. The transmitter outputs a standard 4–20 mA DC signal and supports the HART protocol for easy configuration and calibration.
The controller used is the WT-80 model from Hong Kong Shangrun Instrument Co., Ltd. It employs microprocessor-based intelligent control, automatically calculating optimal adjustment parameters. It features optical isolation, strong anti-interference capability, and a 1% accuracy level. For this application, the controller uses a PID control law with parameters set using the 4:1 attenuation curve method. After tuning, the proportional band (P) was set to 28%, integral time (I) to 3 minutes, and derivative time (D) to 1 minute.
V. Conclusion
After over six months of operation, the system performs exceptionally well, meeting all production requirements. The working temperature is consistently maintained between 98–100°C, achieving a 100% yield for shrimp material. With an annual output of 10,000 tons, and an estimated loss of 4,500 yuan per ton due to pasting, the company saves approximately 450,000 yuan annually. This advancement places the facility among the industry leaders in terms of efficiency and quality.
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