Design And Performance Research Of Mechanical Heat Pump System For Wood Drying

Wood drying is of great significance to use and save wood.Conventional wood drying with the wet air discharged into the atmosphere directly is a high energy consumption and environmental pollution process.In such a case,a new drying system based on mechanical heat pump theory,which can dry the wood through recycling the latent heat of the evaporated steam in the absence of external heat source,is proposed.Meanwhile,drying characteristics,energy contrast and economical analysis are done on the new designed system.Firstly,a new wood drying system based on mechanical heat pump technology is proposed against the existing problems in conventional wood drying system.The parameters of each state point of the system are determined by energy balance and mass balance.The proposed wood drying process can meet with the demand of the producing and the quality guaranteed by the conventional one.Secondly,the influences of drying temperature,air velocity,compression ratio and the outlet relative humidity of moist air on the performance of mechanical heat pump wood drying system are studied.Analysis shows that drying rate improves with the increasing of drying temperature,energy consumption of dehumidification decreases at first and then increases with the air velocity unchanged.When drying temperature is constant,the drying rate first increases and then decreases with the increasing of air velocity,and the highest is at 2.0 m/s.The energy consumption of dehumidification has the same trend,and the higher the drying temperature,the smaller the energy consumption of the dehumidification.The energy consumption of dehumidification increases with compression ratio,so the lower compression ratio is beneficial to improve the operating efficiency of the system when certain heat transfer temperature difference is satisfied.With the increasing the outlet relative humidity of moist air,the compression ratio increases first and then remains at a certain level,the energy consumption of dehumidification decreases first and then slightly increases and then decreases again,the main dehydration parts change from the condenser and expansion to the first heat exchanger and expander.In the meanwhile,the higher the drying temperature,the lower the energy consumption of dehumidification and air velocity has no influence on it.On the basis of the above foundation,a comparison is made between the mechanical heat pump wood drying system and conventional wood drying system.Results indicate that the dehumidification energy consumption of conventional wood drying system is 5341.39 kJ/kg H₂O,the minimum dehumidification energy consumption of mechanical heat pump wood drying system is 1017.67 kJ/kg H₂O with the same drying conditions.The minimum standard coal consumption in mechanical heat pump wood dying system is 54.6%less than that in conventional process on given boiler efficiency（70%）and generating efficiency（30%）.In addition,contrary to the conventional wood drying system,the mechanical heat pump wood drying system can better use of energy.Finally,in order to evaluate the wood drying cost of two different systems,the economic models are established.The results show that the minimum wood drying cost of the mechanical heat pump wood drying system is 122.5 yuan/t dried wood,which is 37.6%less than the conventional one.In addition,a new single-factor impact method is created to analyze how the factors influence on the wood drying cost.Results show that the investment cost,equipment life and annual output of dry wood are the main factors affecting the wood drying cost,operating cost,environmental cost are the secondary factors.