| Summary, etc |
Helical coils find applications in multiple industries such as nuclear power plants, food processing industries, chemical manufacturing plants, HVAC industries etc. as heat exchangers, compact heat exchangers and boilers for heating or cooling fluids. They are also used in mixing of fluids, fluid-solid, liquid-liquid, liquid-gas etc., and mass transfer applications. Helical coils offer several advantages over straight tubes owing to which they are becoming increasingly popular. Helical coils due to coiled tube have a compact design and occupy much less physical volume than straight tubes. The thermal stress caused due to temperature variations are swiftly mitigated in helical coils, whereas, straight tubes experience serious problems. Heat transfer efficiency of helical coils is better than straight tubes due to secondary flow induced by coiled tubes which promotes better fluid mixing. Although currently major applications of Helical coils are in high pressure systems, they have several at low and sub-atmospheric pressure as well. They are used as heat exchangers in cryogenic cooling and analyzer pre-cooling in liquid processing plants, as condensers within a process loop, in vacuum systems as inter- and after- condensers, and also as a thermosyphon.It has now been established in literature by many researchers that the HTCs during single-phase heat transfer in helical coils are larger when compared with the straight tubes. However, it is not possible to conclude anything with certainty in case of two-phase heat transfer. Many researchers observed their experimental data to be in agreement with correlations for straight tubes, whereas many others concluded that their data were not in line with straight tube correlations and expressed a need for new correlations. Some of them suggested their own correlations for boiling in helically coiled tubes. Moreover, the literature available on boiling in sub-atmospheric pressure conditions is further limited and inconclusive. This leads us to a conclusion that more experimental studies need to carried out on helical coils in boiling process, especially boiling under low system pressure.In the present work, experimental study of the heat transfer in flow boiling was conducted in a helical coil once through steam generator. The distribution of convective HTC along the length of the helical coil was studied and the difference in its distribution between the inner side and the outer side of coil was analysed. A closed loop experiment facility was used for the purpose of this study. In the experiments, the absolute pressures used were 0.3 bar, 0.5 bar, 0.75 bar and 1 bar. For each system pressure, heat transfer phenomenon was studied for volume flow rates (measured at the reciprocating pump used to drive flow) of 90 ltr/hr, 126 ltr/hr and 162 ltr/hr, with the vapor quality ranging from ≈ - 0.1 to 0.1 for each of the flow rates. A thermal imaging camera was used to measure the outside wall temperature of the helical coil which was painted with black paint of known emissivity. Thermodynamic variables such as temperature, pressure and pressure drop were measured using RTDs, PTs, DPTs respectively.From the resulting experiment data, it was evident that the inner side wall temperature is significantly larger than outer side at any cross-section of the helical coil. It was observed that value of inner side HTC is small as compared to outer side at any cross-section. HTC variation across the circumference was observed to be independent of the mass flux. However, circumferentially averaged HTC along the length of the coil was a function of mass flux. In case of system pressure variation, the HTC across the circumference as well as averaged HTC along the length of coil were observed to decrease with a decrease in the system pressure.<br/> |
