Berger et al. (1983) found that helically coiled tubes are superior to straight tubes for heat transfer applications. Because of the curvature of helical coils, centrifugal force is introduced resulting in the development of secondary flows, as reported by Dravid et al. (1971). Thus, the movement of the outermost fluid tends to be faster than that at the inside of the coil, which increases the turbulence and thereby increases heat transfer. As reported by Cowley et al. (1962), a reduction in the time taken for cooldown of cryogenic equipment was seen when metallic components were coated with materials of poor thermal conductivity. Allen (1966) suggested that heat transfer by virtue of forced convection between the entering liquid and the transfer line wall results in a sudden temperature drop during the initial phases of the chilldown process. After a brief time period, the rate of temperature drop reduces to a minimum and is then maintained until chilldown is attained. During this period, the flow encountered is film boiling with relatively low-velocity gas flow. When low-conducting coating materials such as Teflon are introduced between the transfer line wall and the cryogen, a thermal gradient is developed resulting in the early attainment of a temperature corresponding to a nucleate boiling regime. This eventually results in higher rates of heat transfer, leading to faster chilldown of the line. Maddox and Frederking (1966) reported that heat removal rates
comparative study of the chill down times of transfer lines with and without coatings under low-velocity fluid flow, concluded that the coating can enhance heat transfer. In their study on Critical Heat Flux (CHF) in helically coiled tubes, Jensen and Bergles (1981) concluded that CHF initially increases with mass velocity, but then decreases after achieving a maximum value; they found that CHF occurs at the inside surface first. Because of higher centrifugal force and secondary flow, a coil with smaller radius was found to have a higher critical value for the same heat flux, mass velocity and tube length.