Experimental Study On Defrosting Characteristics Of Heat Pump Alternative Defrosting System For Two Channel Continuous Air Supply

In the food purification workshop with low temperature air supply,glycol water solution of different concentration is generally used as the indirect cooling and refrigeration system of the refrigerant,while the environment with low temperature and high humidity is very easy to cause the frost of the evaporator and affect the normal operation of the refrigeration system.In order to avoid the influence of frosting and defrosting process on the cleanliness and constant temperature and humidity environment of the air conditioning area,the evaporator is often made into two groups of parallel connected in the system with the same processing capacity,and the double air ducts are used to control the connection or isolation with the system with the help of the front and rear air valves to realize the continuous air supply of the refrigeration system.The defrosting methods for indirect cooling system,such as electric heating defrosting,hot water defrosting and steam defrosting,have the disadvantages of complex defrosting system,poor environmental adaptability and high energy consumption.In this paper,based on the research of defrosting method of two channel continuous air supply refrigeration system,a heat pump alternative defrosting system with condensation heat as defrosting heat source is developed.Compared with the traditional defrosting method,it has better environmental adaptability,reliability and convenience,and its defrosting characteristics are explored through experiments.This paper describes the frosting mechanism and its harm,the defrosting control method,etc.are described in theory.The operation principle and design selection of the two channel continuous air supply refrigeration system and the heat pump alternative defrosting system are introduced.The frosting defrosting experiment is carried out in the refrigeration enthalpy difference laboratory.The pressure drop at both ends of the inlet and outlet of the heat exchanger is taken as the judgment condition of defrosting control.In the experiment,the initial pressure drop of the heat exchanger under the condition of normal air supply without frosting is recorded as P initial,and the initial pressure drop of 1.5p,2.0P,2.5P and 3.0p is taken as the control condition variable at the start of defrosting to explore the best start of defrosting.Under the condition of the best start time of defrosting,taking the ventilation area of defrosting air valve as 10%,20%,30% of the original area,and the defrosting time of 20 min and 30 min as the experimental control variables,the optimal ventilation area and end time of defrosting air valve are explored.The experimental results show that the frost degree is directly proportional to the pressure difference,and the growth characteristics of frost layer accord with the theoretical law.Based on the analysis and comparison of the time-varying trend of the pressure drop parameters at the inlet and outlet of the heat exchanger,the temperature and humidity parameters at the outlet side,the air volume,the cooling capacity and the frosting defrosting characteristics under various pressure difference experimental conditions,and combined with the previous literature experience,it is finally determined that the 2.5P initial pressure difference condition is the optimal defrosting control condition at the beginning of defrosting.The experiment was carried out under the condition of the best defrosting start time,and the residual condensation water on the fins and coils after defrosting was used as the evaluation index of defrosting degree in each defrosting experiment.Through the experimental study and analysis,it can be seen that the decisive factor affecting the defrosting degree is the temperature.The higher the condensation pressure maintained by the defrosting system during the defrosting process,the less the residual condensation water on the fins and coils,and the more thorough the defrosting degree.However,the analysis of the change trend of the temperature and humidity parameters on the air side and defrosting side shows that the defrosting process has little impact on the normal operation of the refrigeration system.When the defrosting ventilation area is 10% of the original area and the defrosting time is 30 min,the maximum value of the condensation pressure is 19.2bar when it tends to be stable,and the remaining condensation water on the fins and coils is 0.154 kg.Compared with other experimental conditions,the defrosting degree is the most thorough,and finally determined by combining the defrosting characteristics,the change trend of condensation pressure and the determination index of defrosting degree The optimal ventilation area of the defrosting air valve is 10% of the original area,and the size of the area is 10 mm × 600 mm.The defrosting time of 30 minutes is determined as the optimal end time of defrosting,and the relevant matching relationship between the opening and closing degree of the air valve in a frosting defrosting cycle and the optimal defrosting characteristics is obtained.