Research On The Frequency Conversion Control For Marine Refrigerated Container And Its Performance Characteristics

In this paper, the research object was refrigeration unit of marine refrigerated container. Through comprehensive analysis on the system composition and capacity control method of current mainstream products on the market, a new type of refrigeration unit using in the marine refrigerated container was developed, R134 a was used as refrigerant, and variable-frequency compressor, electronic expansion valve and hot-gas regulating valve were chosen to regulate the cooling capacity. Meanwhile, an appropriate control strategy was designed for lager operating range and avoiding large temperature fluctuation caused by frequent on/off control due to low load in the container. Then, an experimental rig for the new-developed refrigeration unit was built, and a series of experiments were carried out to test its performance characteristics.Firstly, according to the rated cooling capacity and the designed schematic of refrigeration unit, key components of refrigeration unit were selected to establish an experimental rig. And according to the control requirements of the refrigeration unit, key electric components were selected to build control box by its designed layout and the electrical schematic diagram of the system. In addition, a data acquisition system was built to record all necessary operating parameters with various sensors installed appropriately.Secondly, six operation modes including pull-down with economized cooling control in chilled mode, modulated cooling in chilled mode, pull-down with economized cooling control in frozen mode, modulated cooling in frozen mode, heating control and defrost control were designed for the new-developed refrigeration unit. The corresponding control strategy was established to adjust the cooling capacity of refrigeration unit by variable-frequency compressor, electronic expansion valve and hot-gas regulating valve control in chilled mode and frozen mode, then the control strategy was programmed in the PLC. The control parameter settings of PID in the PLC program were obtained by the trial engineering method.Finally, a series of experiments including pull-down experiments, load step-change experiments and setpoint step-change experiments in chilled and frozen mode, and heating experiments were carried out. Experimental studies showed that the new-developed refrigeration unit could obtain satisfactory pull-down characteristics. In chilled mode, the temperature in the container reduced from 35℃ to 0℃ within 17.3 minutes, about 2℃reduction per minute; in frozen mode, the temperature in the container reduced from 27℃to-18℃ within 35.1 minutes, about 1.3℃reduction per minute. Control precision of supply/return air temperature in container was ±0.2℃, especially in chilled mode, the precision of supply air temperature in low load(even empty container) could be kept within ±0.2℃ due to using new-developed capacity control strategy. Moreover, the controller designed for the new-developed refrigeration unit could achieve desirable stability, fast response and good anti-interference ability.