rF
l
INST.
HEA'ATTRANI..ER TO BOILING
LI\JUIDS
LI,/R'R~
VACUUifi
UNDEH
by
Richard
arding Braunlich
B.S., M.I.T.
1940
Submitted
in Partial Fulfillment
Reiuirements
of the
for the Degree of
Master of Science
from the
MIvassachusettsInstitute of Technology
1941
Signature of Author
I
-
-
-
Department of Chemical Engineering, May 10, 1940
Signature of Professornn
MAY23 1942
Charrre of
Research
Signature of Chairman of Departmant Committee
on Graduate Students
T.I.T.
Graduate House
Cambridge, Mass.
May 1, 1941
Professor George 14.Swett
Secretary
of the Faculty
Massachusetts Institute of Technology
Cambridge, assachusetts
Dear Sir:In partial fulfillment of requirements
for the degree of Master of Science, I submit
herewith a thesis entitled "Heat Transfer to
BoilingLiquids;E
%r an."."
Very truly yours,
Richard h. Braunlich
.-
ACKNOWLE)DG1ENT
The author
acknowledges
with gratitude the
kind assistance and advice of Professor ?..H.McAdams
and Mr. G.Williams under whose direction
was done.
this research
"-=
ram
TABLE OF CONTENTS
Page
I.
SU¢IvtRY
II .
INTHODUCTION
Ii1.
PROCEDURE
7
IV.
RESULTS
13
V.
D)ISCUSION
1
O
RESULTS
26
VI.
CONCLUSIONS
36
VII .
RC;COI¢ElNDATIONS
38
VIII.
APPENDIX
39
A.
DETAILS OF BOILER CONSTRUCTION
40
B.
THERLMOCOUPLEINSTALLATION
43
C.
NOMENCLATURE
49
D.
SAMPLE CALCULATIONS
51
DD.
SUJiLifRY
E.
PRECISION
F.
BIBLI OGRP-PiIY
OF DAT-A iAND CALCULATIOINS
OF MEASUREMENTS
53
56
57
TABLE OF FIGURES
Figure No.
Pag
I.
DIAGRAli OF APPAR.TUS
II.
DIAGRAM~OF BOILER
III.
HEAT FLUX VS.
8
41
TElIP.
D1IFF.
SHOWING
CONSISTENC'Y OF RESULTS
IV.
LOCATION OF ATM. CHECK POINTS U'ITH
TO STAilDARD ATM.
V.
HiEAT FLUX VS. TEMP . DIFF.
tirfT
ESPECT
CURVE
foOILING AT 212 0 F.
VI.
15
16
FOR WATER
17
FLUX VS. TEMP. DIFF.
FOR ¥WATER
BOILING AT 190°F.
VII.
iEAT FLUX VS.
3BOILINGAT 170
VIII.
iHEAT FLUX VS,
18
TEMP. DIFF.
110 0 F.
FOR WATER
19
TEMiP. DI'FF. FOR WATER
BOILING AT 150°F.
IX.
HEAT
FLUX VS.
TEMP.
20
DI FF . FOR
BOILING AT 130 0 F.
X.
XI.
XII.
VVATER
21
COMPOSITE OF FIGURES V TO IX
AlX. HEAT FLUX VS.
22
BOILING TEMP.
23
OVERALL COEFFICIENTS VS. OVERALL TEMP.
DIFF. - STEAM TO UWATER
XIII-XV.'JARIOUS
METHOJDSOF PLOTTIIIG TE
DATA
24
25
XVI.
CALIBRATIONJ
XVII.
DETAILS OF THERMllOCOUPLEINSTALLATION
47
XVIII .
THERiOiCOUPLECALIBRATION
48
OF STEAvii GAGE
42
SUIv MARY
The
urpose of this investigation
ue the work of C. K.
an experimental
was to contin-
alker9 who designed and built
boiler in 1940 for a study of heat
transfer to boiling liquids under vacuum.
Distilled
water was boiled in the all copper
cylindrical boiler by means of a horizontal, submerged,
chrome-plated copper tube internally heated by steam.
using a vertical reflux condenser, heat flux was measured for various values of Et (temperature drop from
the side surface of the tube to boiling water) and for
various boiling temperatures. The following results
were obtained:
1.)
, the heat flux obtained with
For a given At
a clean tube was reproducible within~ 15%.
.)
For values of
t ranging from 12 to 30 F., a
slight fouling of the surface increased the
heat flux 40% above that obtained for the clean
tube. 'This agrees quantitatively
with a sim-
ilar finding by Akin1 .
3.)
For values ofAt
ranging from 1J to 35°F.,
the heat flux varies approximately as the 2-2.5
power of ht, depending on the boiling temperature.
-2-
4.)
The maximumflux occurs at a t of 450 F. for
all boiling temperatures used (110 to tiO F.),
and is an exponential function of the boiling
temperature rather than of' the absolute pressure
5.)
or viscosity of the boiling liquid alone.
For a givenAt
over the range of 45 to 150 0 F.,
the coefficients and also heat flux decrease
with a decrease in the boiling temperature.
.)
Using octyl thiocvanate to promote the inside
copper surface, very high steam side coefficients were obtained. The side surface
temperatures of the tube wall were practically
the same at the three points as shown in
ure XVII.
ig-
II. Ii:THfODUCTION
The use of evaporation as an important part
of many industrial processes has naturally led engineers
liquids.
to a study
of heat transfer to boiling
The designing engineer is greatly aided
br experimental data in designing and
the performance
of new equipment.
redicting
'The Fwide use of
multiple effect evaporators, requiring boiling at
pressures lower-than atmospheric illustrates the
importance of data on heat transfer to liquids boiling under vacuum.
It has been only recently, however, that any
work has been done on the subject so that today the
phenomenon of boiling under vacuum still remains
one of the most important phases of heat transfer
about which our knowledge
is quite meagre.
Early investigators in the field of boiling
liquids studied the phenomenon at atmospheric pressure and at low temperature differences;
i.e., the
temperature of the heating surface was very near
that of the boiling liquid. These workers found
that as the temperature difference between the heating surface and the liquid was increased,
ficients
the coef-
of heat transfer increased markedly.
-4-
Later it was found that as the range of tem-
perature difference is increased beyrondthat studied
by the first investigators, a point is finally reached at which the heat transfer rate becomes a maximurn. The temperature difference
corresponding
to
this point is often termed the "critical"' value. iis
the temperature of the heating surface is still
further increased, the rate of heat transfer falls
off at a rapid rate.A."ttemperature differences below
the critical value, the boiling is vigorous and has
been
described in the literature as "nuclear" boil-
ing, while above the critical value boiling is much
slower and has been called "film" boiling because
the heating surface is insulated by a film or blanket of superheated vapor of the liquid being boiled.
The factorsaffectingeat transfer
are numerous and very little
in~- liquids
as to the
1bJ
to boilis knowvn
degree in variation of'heat transfer caused
variation
of tese
facto:-Cs. The most i.rortant
factors seem to be temperature difference,
absolute
nressure, viscosity, surface tension, surface wetting,
thermal conductivity, and surface roughness.
erous euations
um-
have been ro)osed to correlate
isting data on heat transfer to boiling liquids,
ex-
-5-
but no one equation fits all of the data.
The general effect of pressure on heat transfer to boiling liquids has been studied in the
Claassen,
in 1902,
ast.
using an experimental coil
evaporator, showed that the overall coefficients
from steam to boiling liquid decreased with a decrease in the pressure on the liquid. For the same
rate of heat transfer the overall coefficient obtained for water boilin?-y
at 1580 F. was 25%. lower than
that obtained at k'120F
Badger and Shepard , using a vertical tube,
basket type evaporator,
found a 30 to 45
decrease
under the same conditions.
CryTderand Finalborgo , using an electrically
heated copper tube boiled various liquids under
vacuum. Their results show a decrease in coefficients
as the pressure was lowered, but the data were ob-
tained over a low range of temperature drop ( tube
wall to boiling liquid).
Sherman and Kaulakis , using a steam heated,
nickel-plated copper tube, boiled several alcohols
and wter under vacuumn.Their results also show a
markec.decrease in coefficients with decrease in
pressure. The lowest boiling termperaturereached
in runs with water was 155 F.
Insigner and Bliss , using an electrically
heated, chromium-plated copper tube, boiled water
under vacuum. The hifhest temperature difference
used was 16.60 F., and the lowest boiling point was
1-7 0 F.
Their results show a marked decrease in
coefficients with vacuum in spite of the low temperature difference range.
In 1940, C.K.
VWalker9 designed and built an
experimental boiler housing a horizontal, steam
heated, chromium-plated copper heating tube. iHewas
able to obtain pressures low enough to boil water
0 F.
at 78°
s time was short he was unable to obtain
data of a comprehensive nature. An examination of
the original data indicated that measurements had
been made before equilibrium conditions had been
reached in the boiler.
It was the purpose of this investigation
to
continue the work of ??alker with the same apparatus
exploring wide ranges of temperature difference
p res sure.
and
-7-
III.
A. ) Description of
PROCEDURE
pparatus
The apparatus used in this investigation
was
designed and built by C. K. Walker9 in 1940. It
consists primarily of a cylindrical copper boiler
containing a horizontal, chrome-plated, copper
tube internally heated with condensing steam. Vapors
are totally condensed in a vertical copper reflux
condenser.
dagram
of the apparatus
is shown in
Figure I. The details of the boiler construction
are presented in section A of the appendix.
The steam line consists ( in order from the
steam main) of a condensate trap, a two inch high
pressure tee containing glass wool for filtering
purposes, a pressure regulator, a thermometer well,
a Bourdon pressure gage, the heating tube itself,
another thermometer well, two auxiliary condensers
to aid in condensing steam under vacuum, and a vented condensate trap fitted with a water driven asoirator to draw vapor out of the trap when the steam
line is run under vacuum.
ahe boiler is about a foot long and eight
inches in diameter. The heating tube is standard
half-inch
copperpipe, chrome-plated outside,
inches in outside diauleter,0.109
ness, .c12l
nchles in len,th,
0.840
inch wall thick-
,-vilug
an 3outside
-8-
,
.'0
0to
b
"IL,
2-
b
I.
q)
'I-,
0
1
b
b
r,
..C
0
o
01
Q
C
k
Q
cn
Oc4
_.
_b
I-
I-
-9-
LEGEND
B
- Boiler
C
- Condenser
F
- Steam Filter
G
- Pressure Guage
'FORF1IGURE I
h,J - Auxiliary Condensers
K, T - Traps
- Needle Valve
B
- Pressure Regulator
V
- Vacuum Regulator
-10-
eatr
area ofi' .2-
ar..Cinstalled
souare fet.
Thermocou'pes
in the side tube vialls to m.ea.sure
surflace terpejatures. ietails of the termocouple
installat-ion are presented in sectionU]~of the
TiLe vertical
co-p.-:;l'
tuube
inside
T'1ie c.Jn(eiQtisi"-l
a
.iC
telrfllu;
area
eedlc
water
is 2..2 s,-u$?e
feet.
il
r a.Spirator
valve
of a Lwo incl
steel
a
is
,act
,1.sed
e tO :maintin
iven
t-'lG vacuum being regulated
VaClUu'
U
Joiler,
i' '0e
1Y' :I.a
conisists
u.n...
t.a
va(cuu:; regulator
( a pressure
're..'s:_','.o)r A-/.i
th the connections
reversed).
B. ) Experimental Procedure
Before each run ( runs 10, 18,19,20 excluded)
the heating tube was removed from the boiler and
cleaned on both sides.
he chrome-plate
was
olish-
ed lightly with medium grade steel wool. The inside
was cleaned with a stiff wire brush and then coated
with octyl thiocyanate by means of a bristle brush
wliichhad been dipped in the promoter. Promoter was
also added to the filter from time to time. The
boiler was filled with distilled water until the
top of
about 1
the heating tube was submerged to a depth of
inches. If the run was to be a vacuum run
-l1-
the vacuum line was connected to the top of the
condenser and the water aspirator started. If low
temprerature steam (below 212 F.) was desired a
water aspirator was applied to the condensate trap
T. The steam valve was then cracked to give the
desired pressure in the heating tube. After boiling
had started the vacuum on the boiler was adjusted
roughly with the vacuum regulator and accurately
by adjusting the cooling water rate. The cooling
water rate was regulated so that there was never
less tan
a
0° . rise from inlet to outlet.
Tube -,alltempneratureswere measured by use
of thermnocouples and a Brown portable potentiometer,
Departi;ent Serial No. 150. jA thermocouple located
about two inches from one side of the tube gave
the biling
temperature.
Heat flux was measured br noting the rate of
flow and the temperature rise of the cooling water
flowing through the condenser. The rate of flow was
measured b
determining the time, using a stop watch,
to fill a 1000 cc. graduated cylinder. If the rate
was too high to give sufficient accuracy,
a cali-
brated bucket ( 8100 cc. ) was used in place of the
1000 cc. clinder.
exit
Thermometers
in the inlet and
ater lines of the condenser
ave the temper-
-12-
ature rise.
A run consisted of obtaining values of heat
flux at various
ht's (temperature drop from tube
wall to liquid) for a
articular boiling point. In
all instances, at least fifteen minutes were allowed
to elapse between
taking
data for two pints
in
order that the apparatus mightreach a steady state.
Data for water boiling at 212°F. were taken first
and served as a check on the reliability
of sub-
sequent vacuum runs. After each vacuum run one or
two atmospheric points were taken and compared with
the atmospheric
curve.
If these pints checked the
curve within 15%I the vacuum data were considered
reproducible. Data were taken for water boiling at
212°,1900,1700,1500,1300,
and 110°F.
A slight foul-
ing of the tube at one period resulted in increased
values of heat flux. Runs 18,19 and 20 were made
to learn more if possible about this phenomenon.
-13-
IV. RESU'LTS
The results of this investigation
are shown
in Figures III to XV. The following summarywill
expedite a study of the results.
1.)
For a given At from tube to distilled water
boiling at 212 0 F., Figures
III and IV show that
the heat flux varies less than
l5?1for a clean
tube and is increased 40X) by a slight deposit of
scale.
2.)
Figure V is a plot of heat flux versus
t
from heating surface to boiling liquid for distilled
water boiling at 2120 F. Data of other observers are
included and run as low as seven-tenths of the values
obtained by the author below the critical t (450F.)
and three-tenths
3.)
at the highest At
(900 F.).
Figures VI to IX present plots
similar to those
in Figure V for water boiling at 1900, 170,
1300, and 110°0F. In general,
1500,
the data of other ob-
servers lie below the present data.
4.)
Figure X is a composite of the authors data
given in Figures VI to IX. The effect of reducing
the boiling point by the use of vacuum, which in
general reduces the flux at a given
t, is shown.
-14-
5. )
Figure XI shows that the maximum flux may
be correlated by an exponential function of the
boiling temperature. Contrary to the statement in
the literature the maximum flux was not found to
be an exponential function of the absolute pressure;
neither was it found to be an exponential function
of the viscosity of the boiling liquid.
6.)
Figure XII shows that the overall coefficients
depend on the overall
t ( from steam to boiling
liquid) and the boiling temperature. Overall coefficients as high as 6000 were obtained at atmospheric pressure.
7. )
Figures XIII to XV show the three usual meth-
ods of plotting
from 1
the data. For values of' t ranging
to 35°F.,
the slopes of the three curves
are as follows:
d(//A)/d(bt)
=
2.5
= nil
d( h )/d(,t)
.
1.5
n
d(h)/d(q/A)
=
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d~~oQr
)
,O0
/,o = :O0
-=----
(),
to = 1:0°0.0
q/,
-
1' . O)
-26-
V. DI$CUSS IoL'4 O' RL-ESjLTS
ien
.
.
alker designed the boiler wJhich
was used in this investigation
he assumed that the
uleatng:itube would remain clean as long as the liq-
uids employed were free from contamination - the
boiler and condenser having been carefully cleaned
initially.
herefore, he made no allowances in the
final design for an occasional
removal of the heat-
ing tube for cleaning. After a short period of oper-
ation he found it necessary to dissemble the boiler
to replace a broken thermocouple
and he observed a
thin scale on the heating surface.
he character of
surface had changed in spite of all his
the heating
precautions.
hen the tube was removed again during
of the apparatus for this investi-
a reconditioning
gation, a thin greenish scale was observed on the
surface. It was suspected that scale formation had
been the cause of the trouble
attempting
to obtain consistent
reason considerable
alker experienced in
data and for this
time was devoted to making changes
in the apparatus which would facilitate frequent
cleanings of the boiler and heating tube.
The heating
tube was cleaned on both sides
prior to each of the first four runs and the data
were consistent
as is evidenced br Figure III. The
boiler was not shut donm after run 4 but allowed to
run overnight. Atmospheric
run 5 was made on the
following daT and the points obtained checked those
of the first four runs within 10'-
(Figure III). It
was then thought that good results might not require
a cleaning of the tube before each run.if the boil-
ing were maintained between runs. Therefore, boiling
was maintained
between runs 8,9, and 10. The four
points obtained in run 10 are shovm in Figure III
and are connected by a dotted line. These points,
strangely enough, represent an approximate increase
of 50/ in heat flux. An examination of the heating
tube revealed a thin, brownish scale which proved
that foreign
aterial may deposit on a heating surface
despite the violent agitation provided b
rising
vapor bubbles. Akin1 experienced the same phenomenon
with a similar apparatus.
e noticed
that a scale
formed on the heating tube if water were allowed
to stand in the boiler for any length of
run made to determine
time. A
the effect of the scale resul-
ted in extremely high coefficients. This phenomenon
might be the result of increased
wetting of the heat-
ing surface due to the chemical nature of the scale.
It is a well known fact that increased wetting of
a heating surface is accompanied by smaller bubbles
-28
and correspondingly higher heat fluxes.
tuns 18,1,
possible,
tile
and 20 were made to determine, if
effect of scale formation over a long-
er period of time. The boiling was continued at the
conclusion of run 17 and runs 18-30 were made at
intervals of two days. The points obtained in these
runs are plotted in Figure IV against the standard
atmospheric
curve. The maximum deviation
be less than
the conclusion
15%. The heating
is seen to
tube was examined at
of run 20 and found to be perfectly
clean. It is felt that the scale formation in earlier
runs was the result of dirt from the condenser which
gradually
worked down into the boiler. All of the
atmospheric points including vacuum check points are
shown in 1Figures III and IV.
All of these points,
with the exception of those obtained in run 10, are
consistent, the maximum deviation from the curve being
leas than 15o.
It is concluded
that for a givenAt
(tube wall to boiling liquid), the heat flux obtained
with a clean tube is reproducible
that for values of At
within-15%j,
and,
ranging from 12 to 30 F., a
slight fouling of the surface may increase the heat
flux 40'u, above that obtained for the clean tube.
Figures V to IX provide a comparison
curves obtained in this ivestigation
of the
with those ob-
-29-
tained by Sherman - Kaulakis
and
alker
The at-
mospheric curve agrees very well with that obtained by Sherman - Kaulakis with a Ni-P1. copper tube.
The curves deviate from each other to the right of
the hump and it is seen that this is also a char-
acteristic of the curves obtained with vacuum. The
curves obtained by Sherman - Kaulakis are lower than
those of the present investigator without exception.
The best agreement is found between the atmospheric
and 1700 F. curves. Walkers curves are always lowest.
This is undoubtedly because he failed to clean the
heating surface. The scale which continually accumulated acted as a resistance
to heat transfer and
reduced the flux. The Sherman - Kaulakis and Walker
data for water boiling at 1300 F. agree very well as
is seen in Figure IX.
Figure X is a composite of the authors data
presented in Figures V to IX. In agreement with previous work these results show that in general the
heat flux at any given temperature difference is
lowered as the pressure
on the boiling liquid is de-
creased, but that the critical temperature difference, corresponding
to the point of maximum flux, is
in all cases, approximately
the same at a value of
about 450F. This value agrees with that reported by
-30-
many other investigators. Heat flux is lowered
roughly ;20% as the boiling point is decreased from
212 to 190°F., 40% from 212 to 170 0 F., 50
212 to 130°0F.,
from
and 65% from 212 to 110 0 F. It must
be remembered that these percentages are only apnroximate and apolr only to heat flux as determined
this ivestigation.
in
The curves are all straight lines
to the left of the hump. The slopes of the :12, 190,
and 170)'F.curves
and 2.52
are
respectively.
about
equal
being 2.48, 2.55,
T-he slopes of the 150, 130,
0 F. curves are 1.77, 2.19 and 2.00 respectiveand 110°
ly. It is noticed that the 1500 curve crosses the
170 and 1900 curves. This unreasonable
and not easily
explained. The original data yield no source of error
and as consistent check points were obtained upon
three different occasions
(see Figure VIII) it is
felt that this condition may actually exist. As the
boiling point of water changes, physical properties
such as thermal conductivity,
viscosit7,
density, etc.
also change and it is possible that heat flux may
increase
with
decreasing pressure in the vicinit- of
130 F
curve also has a tendency to cross the
1 . The 130°
other curves. Refering
to Figure VIII we see that the
slope of the 1500 curve obtained bar Sherman and Kaulakis
is
even less than the slope of the curve obtained by
-31-
the present investigator while the slope of WSalker's
curve is about the same.
]maximumheat flux may be correlated with boiling temperature by plotting the former against the
latter on semi-log paper as shovm in Figure XI. The
equation of the straight line through the points is:
Log (q/Ao
)m ax
= .0038 T - 4.76
where T is expressed in degrees F. and heat flux as
B.t.u./(hr.)(sq.ft.)( . An attempt to correlate maximum heat flux with absolute pressure as
one by
Walker and Insigner - Bliss was unsuccessful.
corr-
elation with viscosity was also unsuccessful.
i s the slopes of the curves shown in Figure X
vary widely,it is useless to attempt a correlation
of heat transfer with boiling temperature by an equation of'the type proposed by Cryder and Finalborgo
,
which is in the form:
Log h/h
where h. and h n
temperatures
= b(t-tn)
are coefficients
t and t,
of heat transfer at
where tn is the boiling
oint
at atmospheric pressure, and b is a constant.
Overall coefficients are plotted against overall termerature drop in Figure XIII. T'he shape and
-32-
location
of thecurves are consistent with the data
as so wnvin 1ii re Jr leaving no doubts as to the
reliabili tr of'the thermocouole readings.
Benzyl merncaptan nwasused as a rolmoter in the
first fenw practice runs and did
not
produce
a uniform
steam side coefficient as was evidenced by thermocouple readings which varied widely. Octl
t-
ate was substituted and found to be much more satisfactory. In obtaining points to the left of the hump
thermocouple readings were always within one or two
degrees of each other when this promoter was used.
In the vicinity of the hump thermocouple readings
were subject to sudden and large fluctuations.
For
this reason points could not be determined accurately in the vicinity of the critical temperature.
No serious trouble was experienced while run-
ning steam through the heating tube under vacuum,
and no appreciable pressure drop was encountered until the condensing steam temperature reached 140-1500 F.
Even at this temperature, the largest temperature drop
observed was 80 F.
The steam side coefficients obtained were very
high, ranging from 10,000 to 100,000 B.t.u./(hr.)(sq.
ft.)(0 F.). The corresponding temperature drops from
-33-
from steam to tube wall were so low ( in most cases
1 to 5 F.) that these coefficients,
as calculated, 4re
approximate.
Heat losses from the boiler were determined
by Walker for water boiling at 212°F. This was accomlplished by using an electrical coil heater in place
of the regular heating tube, measuring heat input
by the wattage in the heater andoutput in the usual
manner by the heat removed by the condenser. The difference between these values was taken 9. the heat
loss.
it low fluxes the losses were less than 5% of
the total flux. As the losses would even be less for
higher fluxes and lower boiling temperatures ( well
within the limits of experimental error) heat losses
were neglected in all calculations by the author.
Three methods of plotting
the data are shown
in Figures XIII, XIV, and XV. The data have been
plotted as q/A versus
t, h versusl t, and h versus
q/A respectively. Since by definition h equals q/(A)(lt),
the second method involves a plot of q/A(t) versus
At,
and the third q/A(At) versus q/A. The first meth-
od involves a direct plot of the resulting flux, q/A,
and, since it cannot distort the basic results in com-
paring the data of different observers, it has been
used in making up the plots shown under results.
In comparing the three plots it would seem offhand as though h versus q/A offers the best method of
correlating the data and q/A versus
Actually,
t the next best.
any one of the three methods gives the same
result. For purposes of illustration
related to
t
assume that h is
by an equation of the type
h - a n
( since the curve is a straight )
line
but,
~ = hA
and - /
t-erefore,
q = an+
also
q = a
h
hn*l
Rearranging
the above equation
h *at
1
n
qn+
in other words, if the slope of h versus
the slope of q/A versus &t will be nl
t is n,
and the slope
of h versus q/A will be n/ntl . From Figure XIV the
slope of h versus at is 1.52. Therefore, if the other
curves have been drawn correctly their slopes should
be 2.52 and 1.52/2.52 or .605,respectively. By measurement they are 2.48
h versusAt
curve
absissa. Therefore,
and .610 . iAnr ordinate
ields q/A if multiplied
%deviations
of the
by the
should be the same
in either case. The fact that the q/A versus At
better is merely an optical illusion.
looks
-35-
iA few pictures of boiling water at atmospheric
pressure
were taken using a high speed technique.
well and time did not
'hese did not turn out vervY?
permit another attempt.
set of prints are included
in the library copy of the thesis.
-36-
VI. CONCLUSIONS
The following
of distilled
conclusions are based on the boiling
water outside a single, submerged,
horizontal tube. (chromium-plated copper)
For a givenA t (tube w.-allto boiling liquid),
1.)
the heat flux obtained with a clean tube is
reproducible withinl 15.
2.)
For values of
t ranging from 12 to 300F.,
a slight fouling of the surface increased the
heat flux 40% above that obtained for the clean
tube. This agrees quantitatively with a similar
finding by Akin1
3.)
For values of
t ranging from 13 to 350F.,
the heat flux varies approximately as the
-'2.5
power of At, depending on the boiling tempera-
ture.
4.)
The maximumflux occurs at a t of 450F. for
all boiling temperatures used (110 to 112° F.),
and is an exponential function of the boiling
temperatures rather than of the absolute pressure
or viscosity of the boiling liquid alone.
5.)
For a given
t over the range of 45 to 1500°.,
the coefficients and also heat flux decrease
with a decrease in the boiling temperature.
-37-
6. )
Using octyl thiocyanate to promote
the in-
side copper surface, very high steam side coef-
ficients were obtained.
peratures
of the tube wall were practically
the same at the three
ure XVII.
he side surface temp-
oints as shown in Fig-
VI.
REC MMIRI.;DATI
ONS
It is recommrended that:
1.)
The investigation
of heat transfer to
boiling liquids be continued using liquids
other than water. These liquids should be chosen
carefully in order that the separate effects
of factors controlling the heat transfer such
as viscosity, surface tension, thermal conductivity, etc. may be determined.
2.)
The position of the curve ( heat flux
versus temperature drop from tube to boiling
liquid) for water boiling at 1500 F. be checked.
3.)
Octyl thiocyanate be used as a steam
promoter when condensing steam on copper.
-39-
r
VIII. APPENDIX
-40-
A.
A diagram
of construction
DETAILS
OF BOILER
of the boiler
is presented
CONSTRUCTIOIN
showing the details
in Figure II. The heat-
ing tube is brazed into one end of the boiler and
passes through a packing gland at the other. The
pyrex glass windows are held in place by large brass
fittings, well gasketed, and therefore offer little
chance for air leakage. Thermocouples
wall are brought
from the tube
through small holes in one end of
the boiler and are soldered into place. The end of
the boiler to which the heating tube is brazed is
removable from the rest of the boiler, being held
in position by two large flanges securely fastened
with heavy bolts. Asbestos rope was used in the
.packing gland.
As it was necessary
to remove the heating tube
frequently for cleaning,it was impossible to cover
the removable end of the boiler with
insulation.
permanent
layer of magnesia-asbestos
nsulation
was molded over the removable end which had previous1? been covered with vaseline and allowed to dr?.
This layer was then pried from the boiler in one
piece, reinforced with adhesive taTpeand removed
thereafter as often a te tube as cleannecd.
-41-
4k
__
0
.7-.
0
0
0
Co
o
.-t
4
o
0
0
1-4
8
C)
F .1
0
r4
o
E-4
03
Ci:
1-1
I
I
oI
t
p
15:"-T
,
i·--·
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i
-- ·
: --1-1'1--1
1.1.11,,....i..,..t:
_._"_.
i...
·-- ,· ··- -o----r··
if·i
'·r
Cr' '3;! "1 1-r'1'l- i'
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ii-r i.
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· (
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4…-i
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t~~~~~~~~~~~~~~~~~~~~~~I
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~..
c
Ž
:~~~
)i
11i
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.
---- 4 ....-----
.. i'
:~....
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I..............
-
'~ - '
~
'.L.~~~~~~~~~~
:
::- .......
~."~--7--..~~'
--
r7
.
4--
;
.
-.......
. . ........................
_5:_ _it ;t;-;: ,; tr ::
-
. i4 ,. ,
-! t: .r_.L__
.
.--i-
-~j
-··."i~--t·:
.
.
-I
-
,': :' ..j. ......
.
-4
L~~~~~~~~~~~~~~~~~~
c,
1i
I-
.
.
'J:-:- ':
i?.-iI.... -
.....
i
-
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-
'
L
.
.. .
S. THEiiOCOUPLE I STALLATION
Investigators have used mranyr
different thermocouple
nstallations in obtaining surface coefficients
of heat transfer to boiling liquids. The surface
temperature
of a heating tube at any
point is measured
by placing the hot junction of a thermocouple directly under the surface. This is accomplished b
the junction in a slot ( milled lengthwise
tube or around the circumference)
soldering
along the
or in a chordal hole
(drilled tangentially to a circle between the inner
and outer walls of the tube). The leads from the junction are taken from the same or ppposite
sides of the
slot or chordal hole and are properly insulated from
each other.
A technique applicable to short tubes with thick
walls is described by Insigner and Bliss6 . A hole is
drilled in the pipe wall lengthwise
to the pipe. A
wire of 1/16 inch diameter is filed down to a smaller
size except for a short knob left on one end. A hole
is drilled into this knob, and the other lead of the
thermocouple
is soldered into the hole, the two leads
being insulated from each other along their length .
The couple is then used as a traveling thermocouple
being placed in different positions along the tube
-45-
A
inches, the couple then being soldered into a chordal
hole in the pipe.
When Walker's apparatus was tested the thermocouples were found to be shorted and it was decided
to replace them. A great deal of trouble was encountered in bending the assembled couples without breaking the glass capillary and consequently causing
short circuits. This difficulty was circumvented by
insulating the constantan wire from the copper capillary with a coating of Insalute cement over a water
glass base. Number 35 constantan wire was used as
#°6 was not available. The wire was first stripped of
silk and enamel and coated with water glass by means
of a wet folded cloth. This coating dried quickly and
was followed by a coating of Insalute cement applied
in a similar manner. The cement was allowed to dry
for twenty four hours at the end of which time another
coat of water glass was added to insure complete in-
sulation. The insulated wire was threaded inside the
copper capillary tubing and the hot junction made with
soft solder. The couples were then soft soldered in
the chordal holes. A few test runs on the boiler in-
dicated extremely low tube wall temperatures corresponding to relatively high steam temperatures. Rapid
-46-
(
dissipation of heat from the hot junctions via the
copper capillary seemed to be the logical explanation
for this conldition. Therefore, the couples were
removed from the chordal holes and soldered in two
inch slots milled lengthwise
in the surface of the
heating tube to effect greater immersion. The solder
was filed level with the tube surface and polished
with steel wool and fine emery cloth. Details of
this installation
and the arrangement of the thermo-
couples are shown in FigureU.
Figure
is
a plot of the thermocouple
cal-
ibration obtained by comparing e.m.f. readings on
a potentiometer
with a calibrated
both thermometer and thermocouple
thermometer when
were in an oil
bath. ( the hot junction of the thermocouple was
fastened
to the bulb of the thermometer)
This plot
was checked by running steam through the heating
tube when the boiler was empty and assuming the tube
wall temperature
to be that of the steam.
-47i
_
_
__
_
FIGUPE
_
v//
DETAILS OF THERMOCOUPLE INSTALLATION
·
.
.
-... 0,yC,
·
lj,I
j
I
,Z;s-
--. ,
I
Vj
It
i
I%,41
CROSS SECT/ON OF TUBE WALL AND
INSTALLED
THERMOCOUPLE
fo
/to?.jj
pofenfiome/er
I
''
_
i Ii
I
ii
-i= "Of
- - --- -- - - ·
__
_ _m
_
__m -_m
_~
_
_._
I
t
team /n
--_
II_-
-+--4./2" 4.0"
-- seam out
Ito3
II
__
.
I In
I
4.0 /---
/2./2"
.,
TOP VIEW OF HEA TING
L OCA T/ON
OF
TUBE SHOWING
THERAMOCOUPLES
R.H.B.
~ . . ......
.+........-,-.-*T. .---_
i............
.-.....
!-!
v,,
·
t~~~~~~~
...........---
'_
1
'y
r'_. -'.'l-:.'.:"i' , -'I...
..........
r: -F :L?
I'.'....'.'[':..:;.:..'.':.':;."'7....'"::,'7..-.....-.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~
~~~
~
.
t~~~~~~
...
~
l_,
'
~
,
'
, ;-
, . . *. j- 0, t-
....
" :
' 0 ;- "", [
-,- ,
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' Tl
~
.
.
. .
....
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.--l------'...
.'......, '7--:
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..!..........-~....
4,',!
..;i-.
i -:-T-. ~: ........ ~':'~':'!," '"':
*
r0
r
t-
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r.__.. .....--- ,- - ,
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'f<.
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; .. . . .. . ....
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:-. _i ....
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....-;:I-......-..+...
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t^as
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i, .
0
.
7~~~~~~~~~~~~~~~~~~~~~
. ! ' ',,' . ": .... , -t_:'__
..
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~~t....................,_...
. i: ' : ,: · ' _ i". ¢ ........
t
.... ......
,:,
,,,
j,:
'-.
!*1_-.,;:1
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._
i' .'-M--W--,
A...'....
tt
...
.............
-4.~-.
N'''*'"'*:''
.
.
~ . -, -- . , -,t .. --
_---
.
,1
" ~ ' -"
'
.......
*0T
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-
:. ......
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_.::., _ "~~ --~-.t
' ..... ''.'
...
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1_, , ,- :/t?
':-.'~--..
:',.-t
:-r ..
t-^4
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--
*'
--
......
_ _4. -
L
.
I
i-:-
-7~~7
.
7-
77-~~~~~~~~~~~..
! 7: :ii.~
,::::: t :E!~~:~::::?i:::i:~:i::
': :~':': !-:~
:-:i::
.
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'-':',L:;.
YN
~ ~ ~~~~"_.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.
......
'~~'':." ~'' ''
....
: ::t_
.':- :,-:. . . ... ~' .t~' :'
"!,i : "~ (:-!;~; i~
::! :!::i i !: i ~':1:i4 ;~
; :
-49-
C
NOMJNCLATURE
tube -
Ai
Inside area of heating
Ai)
Outside area of heating tube -
hi
Steam side coefficient of heat transfer -
sq.ft.
sq.ft.
B. t.u./(hr. ) (sq. ft.) (OF.)
Water side coefficient of heat transfer B. t.u./(hr.) (sq.ft. ) (OB1.)
Absolute steam pressure corrected from gage
calibration - lbs./sq.in.
P
Total heat transfer - 13.t.u./hr.
?
/\
q71
\\
Tc. w.
*1C. V*
heat flux - B.t.u./(hr.)(sq.ft.)
Temperature
of inlet cooling water -
cw. 2 Temperature of exit cooling water -
ATc..
Temperature rise of cooling water -
tL /
Temperatureoof boiling water -
C.
C.
F.
F.
Ito to Thermocouple tube wall temperatures (see Figure
°1
2 XVII) - OF.
C
t$
Oto
Temperature drop from tube wall to boiling
liquid (referred to as
this report) F.
t in the main body of
Atw
Temperature drop through the tube wall -F.
Ati
Temperature drop from steam to tube wall -
aTover
Temperature drop from steam to boiling liquid -F.
Ts
Inlet steam temperature - OF.
TS,
Exit steam temperature - OF.
F.
-150-
U0
Overall coefficient of heat transfer 0
B.t.u./(hr.) (sq.ft.) (°F.)
Vac.
Vacuum on the boiler - cms. of Lg.
W
Cooling water rate - lbs./hr,
The following
Table
symbols are used under ttREIVRKS" in
I.
*
Tube cleaned inside and outside prior to the run.
A
Fresh distilled water put in the boiler.
a
Steam well promoted with octyl thiocyanate.
-51-
D. SAIPLE
CALCUL.TI8ONS
an atmospheric check point from
Calculations fr
run 7.
Data
fr
(see pg.4
nomenclature)
lbs./sq.in.
P
=
27.7
Tp
=- 246°F.(steam tables)
W = 305 lbs./hr.
tL
=
Ao =
2
Tc w. =
1F.
t o ( ave.) = 233.5°F.
75 F.
222 sq.ft.
Ai = .165 sq.ft.
heat removed by the condenser
q = 305 x 75 = 22,900 B.t.u./hr.
Heat flux
q/A
- 22,900/.222 = 103,000 B.t.u./(hr.)(sq.ft.)
q/Ai = 103,000 x .222/.165
= 139,000
t
Overall temperature drop
ATover
246 - 212
34° F.
Temperature drop from tube wall to boiling liquid
btc) = 233.5 - 212 . 21.5°F.
Temperature drop through the tube wall
atw = . /Ao
=
k Aave.
21.,050
L A)
Temerature
t -
q/Ao
=
103,000
21,050
= 4.9°F.
drop from tube wall to steam
34 - (21.5 + 4.9) = 7.6°F.
Water side coefficient
h o - q/Ao
to
103,000
l.5
4,790 B.t.u./(hr.)(sq.ft.)(°F.)
-52-
Steamn side coefficient
h
hi
= 139,000
Ati
7.6
= 18,300
B.t.u./(hr.)(sq.ft.)(oF.)
Overall co effi ci ent
'U'
-=
/A
rover
=
3030 B.t.u./(hr.)(sq.ft.)(°F.)
DD.
SUI1Iv'ARY OF DATA AND CALCULATIONS
SUAMMARY OF OI/G/NAL
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-56-
E. PRECISION OF MI]EASUREMIENTS
The temperature rise of the condenser water was
obtained to within+..2F.
which is less than 1% of the
majority of values measured. The weight determination
of the water rate through the condenser was accurate to
about 2.
The heat losses from the boiler and condenser
are about 3
of the total heat transfer at low fluxes
and about 1-2% at higher fluxes. Therefore, the measurement of heat transfer is accurate to within 6% depending upon the magnitude of the heat flux.
The use of octyl thiocyanate as a steam promoter
resulted in uniform steam side coefficients. Thermocouple
readings,
except in the vicinity of the critical temp-
erature, were very nearly the same for a givenAt
( tube
wall to boiling liquid). The hot junctions were located
just under the surface of the copper tube and gave surface
temperatures within 2F.
to
.3F.
. The potentiometer was accurate
. Therefore, the measurement of At was accurate
to within 5"
In general, it was felt that the overall precision
of measurements should have been about A15.
The data
siow that this figure is correct ( Figures III and IV ).
-57-
F.BIBLIOGRAP'HUY
1.
akin, G.A.; "Heat Transfer to Boiling Liquids",
Sc.D. Thesis, Chem.
Eng., MV'.I.T.,
(1940)
2.
Akin, G.A., and McAdams, II.H.; Trans. Am. Inst.
Chem. Engrs., 35, 137, (1939)
3.
Cooper, H.B.H.; "heat Transfer to Boiling Liquids'
(1938)
S.M. Thesis, Chem. ng., M.[1.I.T.,
4.
Cryder, D.S., and
o.C., Trans.
inalborgo,
Inst. Chem. Engrs.,3J
m.
346, (1937)
5.
Cryder, D.S., and Gilliland, E.H.; Trans. Am.
Inst. Chem. ngrs., 24, 1382, (1932)
6.
Insigner, T.H., and Bliss, h.; Trans.
Chem.
7.
ngrs.36,
m. Inst.
491, (1940)
.T., Cooper, H.B.H.,
auer,
McAdams, W.H.; Mech. ng.,
Akin, G.A., and
60, 669, (1938)
8.
Sherman, L.M. and Kaulakis, A.F.,; Effect of
Pressure on Heat Transfer to Boiling Liquids",
S.B. Thesis, Chem. Eng., M.I.T., (1938)
9.
¥Walker, K.W.; "Heat Transfer to Boiling Liquids
Under
(1940)
Vacuum"',
S.M. Thesis, Chem. Eng., TM.I.T.