The art of air blast freezing: Design and efficiency considerations
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int ¼ _ m int ðh 3 h 6 Þ ¼ _ m evap ðh 2 h 3 Þ þ _ m evap ðh 5 h 7 Þ (25) 10.6. Complete system model The system model consists of the individual component models linked together. Fig. 8 shows the flow chart of the complete system model. The condenser model uses two loops to determine the fraction of the tube length required for the superheat and two- phase sections according to the condenser inlet air temperature, air mass flow rate and the refrigerant condensing pressure. The evaporator model has one loop used to determine the fraction of the tube required for the two-phase section according to the freezer air inlet temperature, air mass flow rate and the refrigerant evap- orating pressure. The complete model has an additional loop on the superheat to ensure the energy balance between the evaporator, condenser and compressors is met. The model is used as a tool to analyse the thermodynamic performance of the system (i.e. system COP). 11. Model validation and results The two-stage compressor model consisted of two individual compressor models, the LSC ( _ m 1 ) and HSC ( _ m 3 ). The compressor model predicted the LSC mass flow rate to within 7% of the nine measured mass flow rates. Similarly, the compressor model pre- dicted the intercooler mass flow rate to within 13% of the measured mass flow rates. The modelled condenser capacity agreed to measured capacity to within 5% of the 13 measured data points, as shown in Fig. 7 .3 ( Fig. 9 ). The evaporator model was validated using the measured refrigerant mass flow rate. The outputs of the evaporator model are the refrigerant superheat and the outlet temperature of the air. The modelled evaporator capacity agreed with the measured capacity to within 4%. The system model was validated with the recorded data from the unloaded freezer. The experimental data was collected over a 4 h period where the evaporator load dropped from 75 kW to 45 kW. The ambient air over this period varied from 17.4 C to 19.7 C. Fig. 7 .6 shows the model predicted COP against the measured COP, where the model COP agreed to within 8% of the 11 measured data points ( Fig. 10 ). P. Dempsey, P. Bansal / Applied Thermal Engineering 41 (2012) 71e83 78 It was very encouraging to note that the model predicted the evaporator air outlet temperature very accurately to within an error of 2%. This established the accuracy and the validity of the model, which now could be used to predict performance of a blast freezer as well as a design tool for future blast freezers in order to achieve higher energy efficiency. 12. Design improvements Odey [47] investigated performance enhancing measures of a batch air blast freezer. He found that generalised rules of thumb have been used for the design of air flow through blast freezers. Critical aspects of the design and implementation of the airflow circuit are often excluded from the refrigeration contract, resulting in poorly implemented and underperforming facilities. Typically, the refrigeration contractor’s response to poor freezer perfor- mance is to increase the fan capacity and power. It was found that simply increasing the air flow by increasing fan speed did not necessarily increase the air speed through the cartons in the freezer. The higher fan speed resulted in negative velocities at the fan inlet due to the formation of a large unstable vortex. As a result more heat was added to the freezer from the fans thus reducing the efficiency. The following modifications were installed on the air blast freezer: Baf fling on the top and sides of the freezing chamber Fan inlet cone and diffuser Air inlet and discharge vanes on corners Variable speed drive on fan Prior to the modifications the air flow entering the fan was highly unstable with significant flow reversal. This turbulence reduced significantly with the above modifications. Most of the pressure drop in the unmodified freezer occurred at the 90 turning points, whereas the modified freezer had the largest pressure drop through the product pallets. As a result of the experiment, the existing 11 kW fan motors drawing 8.7 kW were replaced with Fig. 8. System model flow diagram. tải về 1.05 Mb. Chia sẻ với bạn bè của bạn:
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