The art of air blast freezing: Design and efficiency considerations
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LEO PHAM, Document, Email sample
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. The initial guess of the fraction of the tube length required for desuperheating, F SH , is iterated until UA SH,model and UA SH,NTU converge. Upon convergence, the fraction of tube length required for desuperheating is known and the process is repeated for the two-phase section. The two-phase zone analysis starts by guessing the fraction of the tube length required for full refrigerant condensation (latent heat), F 2Ph . The resistances are solved for the tube length L 2Ph cor- responding to F 2Ph . The refrigerant saturation temperature and the HSC discharge pressure are used to determine the refrigerant properties. The average heat transfer coefficient during condensa- tion in horizontal tubes, required to determine the refrigerant-side resistance in the two-phase zone, is calculated using the correlation suggested by Dobson and Chato [53] . The refrigerant side resistance is determined with the area corresponding to the two-phase length, L 2Ph . The resistances are calculated in the same manner as described for the superheated region using L 2Ph . The conductance of the two-phase zone, as calculated by the model, is given by: UA 2Ph;model ¼ N tubes $ 1 R total;2Ph (11) The actual rate of heat transfer over the two-phase zone, _q 2Ph , is calculated by: _q 2Ph ¼ _ m 3 ðh x¼1 h x¼0 Þ (12) where h x¼1 and h x¼0 refer to the vapour and liquid saturation enthalpies respectively. The air-side capacitance rate is the minimum capacitance rate in the condensing section as the capacitance rate of the condensing refrigerant is effectively infinite. Therefore, the maximum possible heat transfer rate in the condensing zone is: _q max;2Ph ¼ _C air;sat T R;sat T air;in (13) The NTU conductance, UA 2Ph,NTU , is calculated by: UA 2Ph;NTU ¼ N tubes _C minNTU2Ph (14) P. Dempsey, P. Bansal / Applied Thermal Engineering 41 (2012) 71e83 77 The initial guess of the tube length required for the two-phase region F 2Ph is iterated until UA 2Ph,model and UA 2Ph,NTU converge. Upon convergence, the fraction of tube length required for phase- change is known and the process is repeated for the sub-cooled zone. With F SH and F 2Ph known, the fraction of tube length associated to sub-cooling is calculated by: F SC ¼ 1 F SH F 2Ph (15) The six resistances are calculated using the sub-cooling tube length given by: L tube;SC ¼ F SC $L tube;total (16) The conductance of the sub-cooled zone is calculated by: UA SC;model ¼ N tubes $ 1 R tải về 1.05 Mb. Chia sẻ với bạn bè của bạn:
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