Transactions

550 TRANSACTIONS OF TH E A.S.M.E. AUGUST, 1941
from jacket No. 6 was consistently lower than th a t in either of the
adjacent jackets. Even when steam was not supplied to the first
three jackets, jacket No. 6 (which was then the third heated
jacket) gave less condensate than jackets Nos. 5 or 7. I t was concluded
that the conditions for heat transfer in jacket No. 6 were
not as favorable as in the adjacent jackets, possibly due (a) to a
partial blocking of the air vent, or (b) to a partly clogged condensate
drain line which caused the lower part of the tube to be
immersed in condensate. Accordingly, all data from jacket No. 6
have been deleted from the subsequent correlations, except that
the actual rate of heat transfer in jacket No. 6 was used to calculate
the cumulative vaporization in subsequent jackets.
The local over-all coefficient of heat transfer in the boiling
section is plotted in Fig. 8 versus the cumulative weight per cent
of the feed vaporized, for all of the runs on water. The feed rate
ranges from 58,000 to 185,000 lb per hr per sq ft of cross section,
corresponding to inlet velocities of 0.27 to 0.85 fps. Arrows are
used to designate data taken on a jacket immediately following a
sure, he observed that high coefficients were obtained at the lowest
temperature differences, that the heat-transfer coefficient went
through a minimum a t 8 to 10 F, and that further increases in
temperature difference resulted in an increase in the coefficient.
The high coefficients at low temperature differences may be
specific to low temperature differences, or may possibly result
from (a) small errors in measuring the temperatures, which would
cause a large percentage error in the small temperature difference,
or (6) the use of a length-mean temperature difference, despite a
variation in local heat-transfer coefficient along the tube. For
purposes of comparison with the data of this paper, in which
temperature differences below 14 F were never employed, the
coefficients obtained by Stroebe when boiling water at 200 F have
been plotted in Fig. 9, versus the total per cent of the feed vaporized,
arbitrarily deleting all coefficients based upon temperature
differences of less than 3 F. On the same plot are shown the data
of this paper for the boiling section of all runs on water, calculated
as average coefficients (cf. Table 2) by dividing the total heat
flux in the boiling section by the length-mean temperature
difference.
Both sets of data plotted in Fig. 9, indicate a decrease in the
heat-transfer coefficient as the discharge end of the tube becomes
vapor-bound, due to excessive cumulative vaporization. In
fact, Stroebe observed that as the fluid left the tube, under some
conditions (particularly high over-all temperature difference),
all of the liquid seemed to be carried as a spray up the center of
F i g . 9 P l o t o f A v e r a g e H e a t - T r a n s f e r C o e f f i c i e n t i n E n t i r e
B o i l i n g S e c t i o n V e r s u s T o t a l P e r C e n t o f F e e d V a p o r i z e d ,
a n d C o m p a r i s o n W i t h S im i l a r D a t a T a k e n a t U n i v e r s i t y o f
M ic h i g a n (5 ) i n a V e r t i c a l T u b e
return bend when the cumulative vaporization was in excess of
60 per cent.
In Fig. 8, over a range of cumulative per cent vaporization
from 2 to 70 per cent, the coefficients fall in a band of points having
a maximum deviation of about 25 per cent from the average
coefficient at any given value of p. At vaporizations of less than
10 per cent, the higher steam pressures (about 24 psi gage) give
coefficients higher than the lower steam pressures (about 10 psi).
This beneficial effect of increasing temperature difference fades
out as higher cumulative vaporization is encountered, and the
heat-transfer coefficients increase with increase in p and go
through a flat maximum at about 40 per cent vaporized. At high
values of p the heat-transfer coefficients decrease to well below
1000. Based on meager data in this range, the coefficients obtained
when using higher temperature differences (resulting from
the use of higher steam pressures) are lower than when using
lower temperature differences. This increase in vapor-binding at
high cumulative vaporization is similar to that obtained with
benzene, Fig. 7.
In confirmation of the results obtained on water are the data of
Stroebe, Baker, and Badger (5) who boiled water under various
pressures inside a 1.76-in. vertical copper tube, 20 ft long.
Mass velocities ranged from 14,400 to 126,000 lb per hr per sq ft
of cross section, corresponding to inlet velocities of 0.065 to 0.58
fps. However, the heat-transfer coefficients (based on an average
temperature difference as measured with a traveling thermocouple)
were correlated in terms of physical properties of the
liquid and the temperature difference, independent of feed rate,
except as varying the feed rate varied the mean temperature
difference and, hence, the average liquid temperature. Film
heat-transfer coefficients for boiling water varied from 1160 to
2640.
When Stroebe plotted his heat-transfer coefficients versus the
temperature difference, for a given feed rate and discharge pres­
F i g . 1 0
H i g h - S p e e d P h o t o g r a p h o f R e t u r n B e n d s a t E n d o f
F i r s t a n d T h i r d P a s s e s
(Taken between surges at a feed rate of 530 lb of water per hr and a steam
pressure of 20 psi gage. Calculated cumulative vaporization equals 8 per
cent of feed [by weight], in first return bend [at the left] and 47 per cent in
third. Note quiet liquid layer in bottom of first return bend. A cardboard
background was placed behind return bends. Photograph taken by Professor
H. E. Edgerton with exposure of 1/100,000 sec.)
F i g . 11 H i g h - S p e e d P h o t o g r a p h T a k e n a F e w S e c o n d s A f t e r .
P h o t o g r a p h S h o w n i n F i g . 10
(Photograph taken during a surge in first return bend by Professor H . E .
Edgerton.)