| With the rapid development of urbanization and industrialization,the output of sewage sludge has soared,and the rational disposal of sewage sludge has become a key task to be completed urgently.In order to realize the harmless and resource treatment of sludge,it is necessary to dry the sludge to reduce its moisture.Compared with the traditional drying technology,heat pump sludge drying has become a research hotspot due to its characteristics which are low energy consumption,high safety and not being affected by the external environment.The main factors,which affect the heat pump sludge drying,are the temperature and humidity of the drying medium,the air volume and the thickness of sludge,etc.However,current researches on the overall performance of the low-temperature heat pump sludge drying system are mainly through experiments,few theoretical studies,which are based on the structural parameters and operational parameters,are carried out.Therefore,a method which combining experimental research and theoretical model is adopted to optimize and analyze the operational parameters and structural parameters of the heat pump sludge drying system,so as to determine the system’s parameters under the best operating status.First of all,an experimental platform for the heat pump sludge drying system was built to study on the relationship between the temperature and humidity of air at the inlet and outlet of key components,air volume,systematic power consumption,and dehumidification amount with the drying time,so as to provide a basis for subsequent theoretical model verification.Secondly,based on the law of conservation of energy and conservation of mass,and according to the operational mechanism of heat pump sludge drying system,a steady-state model for the heat pump sludge drying system,which interconnecting the refrigerant circulation and the air circulation,was established.The distributed parameter method,which was invited to establish the models of condenser,evaporator and drying closet,attempted to reflect the complex phase change or heat and mass transfer process.The flow states of refrigerant and air were reasonably simplified to improve the model’s accuracy and calculative precision.Three variables which are coefficient of performance(COP),dehumidification rate(DR),and specific power consumption(SPC)are simultaneously introduced to evaluate the system’s operating and drying performances.Comparing the results of DR and SPC measured by the experiment with the corresponding simulated results,it is found that the fitting degree of the model is very high,and the average error for above two variables are less than 4%.Finally,based on the verified theoretical model,the single variable method was employed to obtain the performance variables of the system under different working status by changing the operational parameters and structural parameters.Effects of the operational parameters and structural parameters to above three variables are analyzed.The results show that the DR increases with increasing the air volume,fin spacing and the number of horizontal tube rows,while decreases with increasing the number of longitudinal tube rows.The COP decreases with increasing the air volume and the number of horizontal tube rows,whereas increases with increasing the fin spacing and the number of longitudinal tube rows.The SPC increases with increasing the air volume and the number of longitudinal rows,yet decreases with increasing the fin spacing and the number of horizontal rows.Based on the sensitivity analysis,the heat pump sludge drying system achieves the best operating and drying performances when the fin spacing is 3.85 mm,the numbers of horizontal tube rows of evaporator and condenser are 18 and 14,and the numbers of longitudinal tube rows are respectively 12 and9.At the same time,the corresponding air volume is 3300 m~3/h. |
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