Cryogenic Tests on a Teflon-Tube Heat Exchanger
The use of heat exchangers with tubes made of Teflon for application at moderate temperatures has been discussed by Githens, Minor, and Tomsic . This type of heat exchanger was developed for use with fluids which normally would foul metal surfaces, but not the flexible walls of Teflon tubes, Since Teflon retains some resiliency at cryogenic temperatures, the heat exchanger was tested with liquid hydrogen and liquid nitrogen in order to Investigate some advantages that would be offered by Teflon tubes for cryogenic heat transfer. Because of its low thermal conductivity, a reasonable wall thickness of Teflon can accommodate a sufficiently large temperature difference for a cryogen to absorb heat through nucleate boiling instead of film boiling, As shown by the cryogenic data cited by Brentari and Smith , the rate of nucleate boiling heat transfer can be an order of magnitude greater than the rate of film boiling heat transfer. The possibility of attaining nucleate boiling with a reasonable thickness of Teflon is illustrated in Fig. 1, which is based on data cited by Brentari and Smith for pool boiling at 1 atm. The wall to saturated liquid temperature differences used for Fig. 1 were 8°K for nitrogen and 2°K for hydrogen. The nucleate boiling heat fluxes were 10 and 8 W/cm2 for nitrogen and hydrogen, respectively. The thermal conductivity of Teflon was obtained from the literature . The figure indicates that nucleate boiling can occur in a water to liquid hydrogen heat exchanger with a Teflon tube wall thickness of 0.035 in., in a water to liquid nitrogen heat exchanger with a thickness of 0.02 in., and in a liquid nitrogen to liquid hydrogen heat exchanger with a thickness of 0.006 in.
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