| With the refrigeration system running, frost appears on the surface of the device when the wet air temperature is lower than the saturation temperature of the corresponding pressure at the inlet cooling equipment.Frosting can decrease the heat exchange efficiency of the evaporator and worse the refrigeration performance, so the frost layer must be removed. Compared to several common methods of removing frost, the hot air continuous melting frost has clear advantages, since it use the obvious heat and latent heat to remove frost. Two evaporators are carries out heat when the refrigeration system is in the cooling conditions. When the system is in defrosting condition, one of the two evaporators is used as an evaporator,but the other is as the condenser. As a result, two evaprorators work alternately between the refrigeration cycle and defrosting mode. To a certain extent, the refrigeration evaporator can make up the heat load of the deforsting evaporator, which can ensure the temperature fluctuation in the storage pool as small as possible. The study of continuous defrosting shows great practical value.The frost formation mechanism, theoretical model of several common frost and the influence of the refrigeration system frosting are analyzed. As the frost layer thickening, static pressure difference of the evaporator before and later increases. Combined differential pressure with time factor,a defrosting method which is time pressure differential combined intelligent control is presented. On the basis of the above condition, there have been carried out works that are from load calculation to equipment selection. Two air coolers with the same evaporation areas of 7.5 m2 were designed and maked as the experimental cooling equipment. In order to complete the experiment convenient, the electric control is designed and finished personally. The MCGS is as operation platform which connected the module such as 4AI4 AO,PD3065-T,PD3065-PT100 orderly. The programmable logic controller is as hypogynous machine.In this auto system, the following communication protocol such ais MODBUS—RTU is used and the communication mode such as 485 and 422 are utilized. The compressor’s speed is controlled through frequency transformer in real time. Combined with the control system, a series of works including the leak checking, pressure maintaining, irrigating fluorine and debugging operation are completely. Finally, the experimental equipment could work smoothly.Finally, guidanced on the theory of the load change, defrosting time which is at a low-frequency is longer than high frequency. The low-frequency causes that the defrosting evaporator outlet temperature is low around zero degrees. While the high frequency defrosting is in high mode, the defrosting time is short obviously. The emperature fluctuation is larger than common. As a result, there is a optimal defrosting frequency qualitatively. The paper studies the effect of variable load to defrost by choosing the 50 Hz and 45 Hz as the defrosting research object when the cold storage’s temperature is 18 degrees below and 12 degrees below zero. The results show that the temperature fluctuations could reduce 1 ℃ compared to 50 Hz when the frequency is 45 Hz which the cold storage’s temperature is-18 ℃. When the cold storage’s temperature is-12℃,the temperature fluctuations is lower 0.7℃. On the basis of experimental data, frosting factors impacting unit system parameters is been analyzed. And the main criterion of defrosting beginning and the end of defrosting is been elicited. Defrosting control time and setting leeway is closely related to the rise of temperature of the muffler. |
