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Abstract: Cross-counter flow coiled finned tube heat exchangers used in medium capacity helium liquefiers/refrigerators were developed in our lab. These heat exchangers were developed using integrated low finned tubes. Experimental studies have been performed to know the pressure drop characteristics of tube side and shell side flow of these heat exchangers. All experiments were performed at room temperature in the Reynolds number range of 3000–30,000 for tube side and 25–155 for shell side. The results of present experiments indicate that available correlations for tube side can not be used for prediction of tube side pressure drop data due to complex surface formation at inner side of tube during formation of fins over the outer surface. Results also indicate that surface roughness effect becomes more pronounced as the value of di/Dm increases. New correlations based on present experimental data are proposed for predicting the friction factors for tube side and shell side. [Copyright &y& Elsevier]

Abstract: Performance of highly effective heat exchangers is governed by the various internal and external irreversibilities. In low temperature applications, the performance of these heat exchangers strongly depends on the irreversibilities such as ambient heat-in-leaks, longitudinal heat conduction through separating wall of heat exchanger and conduction through high temperature connecting tubes when they are integrated to the system. The special focus of present analysis is the study of effect of these irreversibilities on the performance of heat exchangers through second law analysis. It is observed that the effect of ambient heat-in-leak is different for the balanced and imbalanced counter flow high NTU heat exchangers. Study also makes it possible to compare the different irreversibilities for varying range of NTU and analyze the influence of external irreversibilities on the performance of heat exchangers when either hot fluid or cold fluid is minimum capacity fluid. [Copyright &y& Elsevier]

Abstract: Coiled finned-tube heat exchangers have been used in small and medium helium refrigerators/liquefiers, miniature J–T refrigeration systems for many years. The efficiency of these cryogenic systems strongly depends on the thermal and pressure drop performance of these heat exchangers. A considerable improvement in the performance of heat exchanger is possible by choosing an appropriate geometrical configuration for a given process requirement. In the present study, geometry of heat exchanger has been derived taking into consideration the clearance provided for manufacturing of the heat exchangers and an optimized geometrical configurations have been find out. The results show the possibility of adjusting the thermal and pressure drop performance by varying the clearance. The predictions of four end temperatures from present design method have been compared with the actual experimental results of one of the prototypes fabricated in our lab. [Copyright &y& Elsevier]