The current state-of-the-art of commercial enhanced surfaces for thin film evaporation and condensation on vertical tubes is exemplified by fluted tubes having overall heat-transfer coefficients of 1300 to 2000 Btu/hr ft 2 `F at atmospheric pressure . Experimental surfaces have been tested which gave overall coefficients up to 5000 Btu/hr ft 2 'F.
Oxidative fouling factors as small as 10 -4 (Btu/hr ft 2 ยฐF) -' have been reported for fluted enhanced surfaces . It is hypothesized that such small fouling factors occur because high shear forces developed in the thin-film region prevent formation of all except very thin oxide films . Limited data also suggest that the fouling resistance may decrease with increasing heat-transfer coefficient (i.e. . with increased shear rate) ; if true this would provide an important incentive for development of improved enhanced heat transfer surfaces .
Although the mechanism responsible for high performance on the condensing side of fluted or finned tubes seems relatively well understood and upper limits can be estimated, the mechanism on the evaporating side seems to be much more complicated . Consequently an upper limit for heat transfer rates on the evaporative side was estimated assuming that the upper limit occurred when the water film thickness was just sufficient to prevent dry spot formation . Assuming a metal wall resistance of 0.00004 and upper limits of 18,000 and 14 .200 Btu/hr ft' 'F on the condensing and evaporative sides, respectively, gives an estimated upper limit on performance of enhanced surfaces of 6000 Btu/hr R' `F .
,nf -Constant in Eq . (3) = 1 .72 x 10 -`, dimensionless โข Research sponsored by the Office of Saline Water under Union Carbide Corporation's contract with the U .S . Atomic Energy Commission .