Condensation of water from engine exhaust for airship ballasting

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File D 52.83/38
AIR SERVICE INFORMATION CIRCULAR
(AEROSTATION )
Vol. I
( .
CONDENSATION OF WATER FROM ENGINE
EXHAUST FOR AIRSHIP BALLASTING
By Mr. Robert F. Kohr
Bureau of Standards
Washington, D. C.
WASHINGTON
GOVERNMENT PRINTING OFFICE
1924
CERTIFICATE: By direction of the Secretary of War the matter contained
herein is published as administrative information and is required for the proper
transaction of the public business.
(n)
V
·CONDENSATION OF WATER FROM ENGINE EXHAUST FOR
AIRSHIP BALLASTING
PART !.- GENERAL
INTRODUCTION
The maintenance of a constant total lift in lighter­than-
air craft falls naturally into two distinct divisions:
(1) Recovery of ballast as the consumption of fuel
tends to lighten the ship. (2) Control of the temper­ature
and thus the density of the lifting gas in order
to compensate for changes of lift due to changes of
atmospheric density or superheating of the lifting
gas by solar radiation upon the envelope.
The problem is of great significance when helium
- is us~d as a lifting gas. It is generally conceded by
those familiar with the operation of airships that
valving of lifting gas must be nearly eliminated
before a gas so rare and expensive as helium can be
adopted for general use. Inasmuch as the known
worlct supply of helium is largely found in this country,
it constitutes a national monopoly for military use,
and its conservation is therefore imperative.
In long flights both phases of the ballasting problem
assume considerable importance, but, of the two, the
ballast recovery appears to be the more vital.
Although the development of both projects was for a
time carried on along parallel lines, · it was thought
advisable to postpone all work on temperature control
until ballast recovery reached a point where satis­factory
apparatus was available. This paper, there­fore,
deals only with the work on ·ballast recovery
apparatus.
The desirability of making up ballast during flight
at a ra.te equivalent by weight to the fuel consumption
has been realized since the beginning of airship
navigation. A number of means of maintaining
equilibr~um have been proposed, including the ab­sorption
of water vapor from the air by hygroscopic
substances, compression of the lifting gas into rigid
containers, and condensation of the water vapor
which forms a part of the engine exhaust gas.
The first of these proposals seems impracticable on
account of the small quantity of water vapor present
in the air under · ordinary flying conditions (about
0.0005 pound per cubic foot) and the immense exposed
area which would be required. The second method
is eliminated by the great weight of compressing
machinery and high-pressure gas containers. The
recovery of water of combustion from engine exhaust
gas by cooling and condensation, however, is feasible
as a method for the solution o( the problem.
When the usual gasoline fuel is .used , about 1.4
pounds of water vapor is formed for each pound of
fuel burned. This affords an ample margin for
inefficiency in the process of cooling the gas and
separating the entrained moisture, so that extreme
refinements are unnecessary.
PREVIOUS WORK
The recovery of water ballast from engine exhaust
gas is the subject of a publication of the (British)
Advisory Committee for Aeronautics, by Guy Barr,
elated September, 1915. This report discusses the
principles involved and describes the operation of
experimental apparatus installed on No. 1 Naval
Airship.
The apparatus embodied a water-cooled muffler at
the engine and a single air-cooled pipe of 4-inch diame­ter,
about 400 feet long. Separation of entrained
moisture at the end of this pipe was accomplished by
making the cooled gas bubble through water. The
engine served by this apparatus had an output of
180-200 horsepower.
The test quoted lasted 35 minutes and yielded water
in the amoimt of 52.2 per cent of the weight of the
fuel burned. During this period there was a marked
increase of temperature, both in the exhaust gas and
the cooling water, indicating a decrease in the per cent
recovery with further operation. In view of the crude
apparatus used, the result might well be regarded as
having given promise of future success.
When the work described in this paper was initiated
at the Bureau of Standards, the Navy Department was
testing an experimental water-recovery apparatus
developed by Mr. H. S. McDewell, engineer in charge
of the Aeronautic Engine Laboratory. The Navy
Department, through Mr. McDewell, was kind enough
to give every possible assistance to this bureau in
determining the lines which might most profitably
be fo llowed in further work.
The Navy apparatus embodied a length of about
50 feet of 5-inch finned sheet-iron pipe, for initial air
cooling, from which the exhaust gas passed to a jet
condenser, and thence through a baffle type separator
to the atmosphere. From the separator and the jet
condenser the water drained through settling basins
to radiators, and was then p11mped back through the
condenser jets. The excess water, representing the
recovery from the exhaust gas, was drained to buckets
and weighed.
This apparatus produced, 11nder favorable conditions
(cold weather), 1.2 pounds of water per pound of fuel
burned . Mr. McDewell must be credited with haviJ1g
( 1)
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•
I
demonstrated the entire feasibility of collecting
exhaust water in sufficient quantities to fully com­pensate
for the increased lift due to fuel consumption .
The apparatus used in this work was constructed
with little regard for weight and as a result, was far
too heavy to be used on an airship. Mr. McDewell
estimated that, by exercising the greatest care in
design, the weight of such an apparatus could be
reduced to about 6 pounds per brake horsepower.
SUMMARY OF WORK AT THE BUREAU OF
STANDARDS
It is to be noted that both the systems already
outlined involve t he use of both water and direct air
cooling. The same general scheme was at first fol­lowed
by the Bureau of Standards in laying out a
proposed experimental design .
2
This first design differ ed only in detail from the
Navy apparatus already described . The settling
basins were replaced by a centrifugal separator and
the whole apparatus was made as compact as possible.
Computations based on this arrangement indicated
that the apparatus, exclusive of radiators and cooling
water, could be built within weight limits of about 2
pounds per brake horsepower.
While collecting preliminary data necessary to design
such an apparatus within acceptable weight limits, a
critical analysis was made of the fundamentals enter­ing
this heat transfer problem. The primary require­ment
to be met was that all the heat abstracted from
the engine exhaust must be delivered fairly promptly
to the air through which the ship moves. No large
heat reservoirs could be used, nor did the cooling prob­lem
offer any similarity to that of a marine or sta­tionary
gas engine, where an unlimited quantity of
cold water is available. The cycle is parallel to t he
cooling of a motor car or aviation engine, where, if
water is used to take heat from the engine, it must in
t urn give tip all this heat to the air, and return to its
original temperature before again abstracting heat
from the engine. In such cases, a water system, as
opposed to direct' transfer of heat from t he hot body
to the air, is introduced solely for the p urpose of
utilizing much more effectiv ely some factors concerned
in the heat transfer process. Unless t here is such a
compensating featme, no ju t ification for the extra
weight and inconven ience of the water syst em exists.
The present problem seems to differ from that of an
engine cylind er, in that there seems to be no reason
why, in the direct transfer of heat from the hot gas to
the air, every unit of cooling surface and every degree
of temperature head can not be used to maximum
advantage. Accordingly, the information regarding
a ircraft radiators, obtained in several years of work
upon this subj ect at the Bureau was brought to bear
upon t he question of securing the most effective direct
heat exchanger to take heat from a hot gas and deliver
it to a stream of air.
By the time the proposed design and weight estimate
for a water-cooling syst em were available, the invest i­gation
of the direct air cooling method was well under
way and seemed to give promise of somewhat lower
weights. The jet condenser system was held up
pending the completion of the preliminary air-cooled
design, and was then definitely abandoned in favor of
the latter which showed a designed weight of about
1.5 pounds per brake horsepower, complete, in addi­tion
to avoiding t he mechanical complications of
water cooling and handling apparatus.
While t he detai led d esign of the test apparatus was
being carried forward, it was thought advisable to
check the theory involved by a laboratory test of a
sma ll section similar to the proposed full -size apparatus.
With this end in Yiew a wind tunnel of 9 inches
sq uare section was constructed, served by a centrifugal
blower fan driven by a small automobile engine which
also furni sh ed the exhaust gas . A condenser was made
up of standard 1-inch iron pipe, with three 20-foot
banks of t hree pipes in parallel, having return bends
connecting the successive banks. This was installed
in the tunnel and connected at one end to the engine
exhaust manifold, and at t he other to a small baffie
type separator .
Measurements were made of gasoline consumpt ion,
exhaust water collected, air velocity and temperature,
and final exhaust temperature. The resvlts were found
to closely check those expected from theoretical con­siderations,
and the work of construction of the •full­size
units was begun without further delay.
The full air-cooled system allows of extensive use of
aluminum in the form of thin seamless t ubes and cast
headers. The design and detailed description of t he
two condensers built are discussed in Part III.
In general, the method is to pass the exhaust gas at
high velocity through parallel lengths of about 60 feet
of 1-inch aluminum tube, the outer surface of which is
swept by air at a rate determined by the flying speed
of the ship. For convenience, the 60 feet is arranged
in three banks connected by return headers, so that the
overall length is about 21 feet. To handle the quan­t.
ity of exhaust gas produced by the two 150-horsepower
engines to be used , with due consideration for the
desired velocities and back pressures, there are 50 -such
cooling t ubes in parallel. The who]e ·unit presents t he
appearance of 150 parallel 1-inch tubes in one bank
whose overall length is 21 feet. Both Models I and II
have t his general form, although t hey differ consider­abl,
v in details of construction and arrangement of the
hot and cold banks of t ubes.
Both of t hese units are intended for suspension be­tween
the envelope and car on Class C and D airships
of the original design, and a re normally slanted about
6° downward to t he rear to assist in draining the tubes
of condensed water as well as to provide some cross
flow of air between them, t hus avoiding the loss of
effi ciency clue to heated a ir coming from a hot tube
and striking a cooler tube fart her along. In each case
a baffle type separator is provided to collect the en­trained
moisture from the gaseous produ cts leaving
the condenser.
The first of these condensers, Model I, was set up for
a ground test early in NoYember, 1921, using a stock
400-horsepower Liberty engine to provide both the
exhaust gas and air stream. The propeller used was
furni shed by the Engineering Division, Air Service, and
a calibration had shown that. it would absorb 307 horse­power
at 1,600 revolutions per min.ute.
/'
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3
The complete set-up is shown in Figure 1, from which The endurance run was begun on March 11, 1922,
it will be seen that the suspension of t he condensing and was terminated at the end of 90 hours, April 3,
unit is taken care of by six diagonal cables from the because of failure of the engine. It was thought that
supporting posts, simulating as closely as possible the the endurance of t he condenser had been sufficiently
proposed installation on an a irship. It may be men- demonstrated. The r esults are indicated in a later
tioned here that the apparatus shown, without sus- I section of the paper.
FIG. 1
pensions, separator , or exhaust conducting pipe,
weighed just 400 pounds. When installed on a ship
the total loss of lift would probably not exceed 450
pounds. The results of t his test showed the perform­ance
of the apparatus to be satisfactory, and it was
decided that an endurance test of 100 hours should be
run to ascertain the effects of continued vibration and
After the completion of the endurance test the ap-
. paratus was taken down and packed for shipment to
Langley Field. Another Liberty engine was set up
and the Model II condenser swung in place for an en­durance
test. It was thought that 50 hours would be
enough to show up any distinct weakness of construc­tion,
and would allow of a considerable saving of
Fw. 2
possible carbon deposition. ,vhile no pro v1s10n was
made to secu re a uniform ai r blast over the condenser,
thus permitting of securing suffi cient p hysical data to
check the theory of its design, it nevertheless seemed
advisable to make such meaurements of temperature
as could be easily secured. Accordingly, another set­up
was made in which were incorporated a number of
thermocouples at various points in the exhaust gas
stream as well as the necessary weighing apparatus for
gasoline and collected water .
~ ------------- - - - - - - - - -
money and t ime as well as avoiding many of t he delays
incident to engine trouble.
The Model I condenser was shipped to Langley Field
early in the spring (1922) and authorization was
secured for the use of Airship D- 3 for two months in
order to carry on flight tests. The work of installation
was begun about July 21 and t he first fligh t was made
on the morning of August 17.
The condenser is shown in position on the a irship in
Figures 2 and 3, which were taken just prior to the
4
FIG. 3
initial flight. A comparison of size with the whole some heat from the surfaces of the warmer second half.
ship, car, engines, etc., may be had in Figure 2. Figure The schematic diagram, Figure 4, illustrates the flow.
3 shows some of the detail looking from the left side. The detailed arrangement of tubes and return
The separator is mounted at the upper left in Figure 3 headers is shown in the photograph, Figure 5, in
on the side opposite the observer. which the exhaust enters at the upper left while the
During the first flight the exhaust piping developed air flow is from right to left.
faults and water leaks were discovered at the first It wiU be noted from the photograph of component
drains from the condenser. These were repaired the parts, Figure 6, that an attempt \Yas made to stream­next
clay, and the ship was again flown on the after­noon
of August 18. No further mechanical trouble
appeared and, after one short flight on the morning of
August 19, the ship was flown to Aberdeen Proving
Grounds, where further flight work was conducted.
The D- 3, equipped with the Model I condenser, was
flown for a total of 53 hours, and during the greater
portion of that time the apparatus was found to func­6AS
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FIG. 4
tion satisfactorily. A detailed account of the results line both the headers and header manifolds in order lo
appears in another section of this paper.
DESCRIPTION OF CONDENSERS
!Ji the Model I condenser the tubes are arranged so
that two-thirds of the total cooling area is in counter­flow;
that is, having the direction of flow of the exhaust
gas opposite to the flow of the air stream. In addition,
if the unit be considered as two halves, front and rear,
the forward half is the cooler one, thus providing a
secondary overall counterflow effect, since the air pass­ing
over the fir st half will still be cool enough to abstract
secure the best possible conditions of air flow around
the castings and between the tubes, as well as to keep
down the head resistance of the apparatus as a whole.
The result is shown in the encl view, Figure 7, lookiPg
in the direction of the air stream.
The Model II condenser, figures 8, 9, and 10, differs
from the Model I chiefly in respect to arrangement of
tubing and type of header. The flow of ' exhaust gas
and air is two-thirds counterflow as in the case of
Model I, but no attempt was made to secure a corre­sponding
over-all counterflow effect. The schematic
1
5
FIG. 5
F 1G. 6. - l. Tube. 2. B ender manifold. 3. End heade r. 4. Return header.
FIG. 7
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FIG. 8
FIG.)
FIG.LO
diagram, Figure 11, indicates the direction of gas and
air flow.
The construction is somewhat similar to that of
Model I , a lthough t he headers are of an entirely
different type. The same ma terials are employed,
with the exception of the first 10-foot section which,
in this case, is of Benedict nickel, a commercial a lloy
containing 16- 18 per cent nickel, balance copper.
This material is used to avoid warping of the tubes
due to the high temperat ures in t his section.
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In Figure 10 it will be observed that in the middle
layer of the upper bank many of the tubes have taken a
decided permanent set. This is due to the poor gas
distribution between the tlll'ee layers of the bank in
question. The exha ust gas tends to flow through the
central layer on acco~mt of the more direct path as
opposed to the elbow nipples of the outer tubes. For
this reason the center tubes are hotter, and their ex­pansion
greater, with the r esult that they fail as long
columns. These tubes are of aluminum.
It is noted that some of the upper tubes at the far
end (Benedict nickel) are also bent. This, however,
is a result of accidental mechanical stresses and not of
any temperature effect .
The number of parallel tubes per bank in this con­denser
is 50, and ea ch bank is composed of two 10-foot
lengths, end to end, so that a total of 3,000 feet of
tubing is used. For better mechanical stiffness and
strength the aluminum t ubing used has a wall thickness
of 0.022 inch in st ead of 0.016 inch. This is the thin­nest
walled tube made as a stock product by manufac­turers
of aluminum and is far cheaper than the thinner
material used in Model I. The Benedict nickel,'
which is very easily worked, was drawn to. a thickness
of 0.010- 0.012 inch , making it about 40 per cent
heavier than the 0.022 inch aluminum tube. The
actual weights per foot are as follows :
Pound
0.010- 0.012-inch Benedict nickel_ __ ___ ___ ____ 0. 1208
0.022-inch aluminum __ __________ ____ ______ _ 0. 0857
This makes the total weight of tubing 274.6 pounds,
which is about 50 per cent higher than that of Model
I. The increase in tube weight, however, is offset by
a decrease in the casting weight, to such an extent
that the Model II condenser is only about 9. per cent
heavier than Model I. The actual weights without
suspensions, exhaust pipe connections, or separators
7
linear expansion, and a header design providing good
gas distribution, the added weight of Benedict nickel
tubing would not be justifiable.
The importance of the separators for the collection
of entrained moisture can not be overemphasized. No
matter how low a temperature is reached in cooling the
exhaust gases, much of the water condensed remains in
the form of ~pray and is carried along by the gases until
the separator is reached. In the absence of such a
device this represents a loss of about 50 per cent of the
water condensed.
Since the functioning of the separators is the same
for both condensers, only the Model II will be described
in detail. This separator was bolted up directly to
the exit elbow shown in Figure 9 and extended back
under the condenser, where its free end was supported
by wires extending diagonally upward between the
tubes to the header casting above.
The separator is shown in some detail in Figures 12
and 13. Its action depends upon the drops of water
being thrown against the baffles shown in Figure 12
and guided by the small vertical gutters to holes
punched through the bottom to a fal se bottom, from
which the collected water is drained to the ballast
tanks.
The exhaust gas enters at the upper right, Figur~ 13,
passes between the baffles, and escapes at the left. It
will be observed that there is a lip on the outlet end to
prevent t he separated water washing along the bottom
past the holes and so escaping with the gas.
The separator is built of tinn_ed sheet iron, and is
soldered together. This material was selected purely
because of its availability and the ease with which it
can be worked. Some noncorrosive material .such as
aluminum or a copper-nickel alloy might be a decided
improvement.
The Model I separator, differing only in outer shape,
may be seen at the upper right, Figure 5, and upper
left, Figure 7.
TEST RESULTS
The results of the 90-hour ground test of the Model I
condenser are shown graphically on Fignre 14. A
summary of the results follows :
Average air speed entering condenser __ 48 m. p. h.
Average air temperature ________ _____ 1.5° C.
Total weight fuel used ________ _____ ___ 15, 075 lb.
Total weight water collected ___ _______ 13, 943 lb.
·water collected 2 _ _ _ _ _ _ _ _ _ ______ _____ 92. 5 per cent
The recovery is somewhat low, but t his is not inher­ently
a fault of the condenser. , Trouble was experi­enced
during the test, with misfiring of the engine,
cracked exhaust ducts, small water leaks at drain con­are
as follows:
Pounds nections and thermocouple bushings, ,and clogged
Model!_ _____ ______ _______ ___ ____ ______ _____ 400 drain pipes. Inprovements in construction should
Model IL ____ _________ __ ___ ___ ____ ____ __ ____ 434 obviate these difficulties and materially increase the
It is proba ble t hat wi th exhaust pipe connections of
ample flexibility, freely moving slip joints to t ake up
1 Benedict nickel is a copper nickel alloy used largely in the manu·
facture of plumbing fixtures on account of its noncorrosive character.
It is easily worked and has a bigber tensile strength tban aluminum.
96997- 24t--2
recovery.
2 The water recoverable, water condensed, and water collected, when
expressed as per cent, are based upon the weight of the gasoline or other
fuel burned. Thus, a recovery or collection or 100 per cent exactly com­pensates
for the weight of fu el burned.
I
8
42-''
Fir.. 12
'Ten roi,vs or bo/'rles equa//y
spaced as shown-end prov/ded
wir.17 f "r/aps ,ror so/den~ T()
rop and bofron1 o.f" veparalor .
.5ee baf'f'!e A-A/ far details
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ror clra/n connect/on.
FrG. 13.-Moctel II separator elevat ion
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FIG. 14
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!n ·conducting this test it was intended that a
·"helium heater," for regulating the temperature of the
lifting gas, should be tested at the same time. This
apparatus was connected into the exhaust line by a by­pass
valve which allowed of passing the exhaust gas at
will through the heater or direct to the condenser.
The valve plate was of brass, clamped to a steel oper­ating
stem by set screws. Shortly after completion
of 50 hours of operation the set screws loosened, drop­ping
the valve to the position necessary to cut in the
heater. As the valve was very inaccessible, and
inasmuch as the test was for the purpose of demon­strating
the mechanical strength of the condenser unit,
the run was continued under the new conditions.
The point at which the helium heater was cut in is
readily observed on the chart, Figure 14, from which
it will be seen that the change came during the fifty­second
hour of operation. For this reason the recovery
header. The tubes were in a similar condition, with
the addition of white corrosion spots at frequent
intervals. These spots were not deep and gave no
indication that trouble might be expected from this
source. It will be noted also in Figure 15 that the
machined surfaces of the nipples show no signs of
exhaust-gas leakage.
With regard to carbon formation it should be noted
here that the engines used, both in this test and in the
50-hour test of Model II, were equipped with the Navy
type oil scraper pistons which were known to materially
reduce the oil consumption. It would seem that every
effort should be made to minimize oil consumption
when exhaust-water-recovery apparatus is to be
employed.
Some of the castings used in the construction of the
Model I condenser had been found porous and were
plugged with aluminum solder in order to stand the
FIG. 15
and fuel consumption have also been computed sepa­rately
for the first 50 hours of the run. The results
follow :
Average air temperature ______ ______ __ 16.5° C.
Total gasoline burned _________________ 8,126 lbs.
Total water collected _________________ 7,662 lbs.
Water collected __ __ ___ ________ _____ __ 94.4 per cent.
These results indicate that the decrease of the exhaust­gas
temperature at the entrance to the condenser was
more than offset by the increase in the losses, already
mentioned, during the last 40 hours of the run.
The carbon or soot in the water collected took the
form of a light foamy scum or a mushy sludge. The
latter gave some trouble by clogging drain tubes
wherever a contraction or shoulder allowed an accumu­lation
to build up. Substitution of large,' smooth
drains should prove an effective remedy. The deposit
of carbon in the tubes and castings of the condenser
proper proved to be practically negligible . . A light soot
accumulated during periods devoted to warming up the
engine under idling conditions, but immediately upon
opening the throttle this soot was blown out through
the separator with the appearance of dense black smoke.
After completion of the 90-hour run one of the header
castings and the adjacent tubes were sawed into
sections in order to permit examination of the carbon
deposit. Figure 15 shows parts of this sectioned
leakage test. This solder melted and oozed from the
pores of the exhaust entrance header manifold during
the preliminary ground test but the leakage of gas was
thought to be so small as not to warrant replacement.
In the light of this experience, however, it would be
well to specify that castings for this purpose be required
to stand a leakage test without repair of any kind.
No mechanical failure of any kind occurred in the
Model I unit during all test work to which it was
subjected, and the structural design is therefore con­cluded
to be sound, although it is realized that many
modifications may prove desirable in laying out appa­ratus
for service use. ·
Ground test of the Model II condenser is charted
in Figure 16. Summarized results follow:
Average air speed entering condenser_ ___ 35 m. p. h.
Average air temperature_______________ 20° C.
Total weight fuel used __ _____ _____ ____ 8,195 lbs.
Total weight water collected ___ __ ______ 5,324 lbs.
Water collected ___ ____ ____ ______ _____ 65.0percent.
The postponement of this test for several months
resu1ted in serious rusting of the flexible leads from the
exhaust manifolds of the engine to the exhaust pipe
line to the condenser, and as a consequence the re­sumption
of test work was marked by persistent
occurrence of breaks in the flexible piping with attend­ant
leakage of exhaust gas. Trouble was also en-
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12
countered wi th clogging of t he drain from the sepa­rator,
owing to rust and sed iment deposits which would
not be expected during continuous or semicontinuous
operation.
In Figure 16 the line of water condensed is obtained
by referring poin ts from t he line of final exhaust
temperature to l<' igure 21 , assuming the mixture ratio
to have been 15 : 1. It will be noted that the line of
water condensed is at least in the same general range
as that of water collected during the few runs made in
August, whereas with the runs of October these lines
at once diverge so that there is an average difference
of perhaps 20 per cent. This is believed to result from
exh aust-gas leaks and \Yaste of water clue· to persistent
stoppage or the separator drain. This stoppage
causes the falsjl bottom of the separator to fill and over­flow
, so that the water separated by t he baffles from
the· issuing exhaust gas can not drain off and is swept
out by t he gas.
The only mechan ical failure \Yas occasioned by .dif­ferential
expansion of the entrance header elbol\· and
the header bolted to it. The entrance elbow casting
was, of course, hotter t han the header proper, and the
upper fl ange of the elbow warped ·upward, pulling off
the lugs of t he corrcsponcli11g flange of the header.
This occurred early in t he test and \Yas remedied by
passing a bolt through the elbo1Y castiug, thu~ drawing
down the upper edge and holdiug it permanently in
place. The bolt may be seen in Figure 8.
A condensed log of the flight tests of i\Iodel I con­denser
is shown in Table 1. Totals and averages
follow:
Air temperature ____ _____ ___ __ ____ ___ -22° C.
Total weight fu el burned ___ ___ _______ 2,799 lbs.
Total weight water collected ______ __ __ 2,170 lbs.
Water collect ed ____ ___ _ _____ _____ ___ 79.4 per cent.
In rn.aking up t he separator drain from whate.ver
ma t erials and fittings were a,·a ilable, a shoulder was
left at the p oint of connection of the drain pipe and
separator, and this resulted in the loss of a co nsiderable
qua ntity of wa ter, clue to clogging of the drain by
carbon.
01\"ing to lack of data as regards engine power and
final exhaust temperature, a nd to the variation in air
speed , no comparison can be made between the t heo­retical
and act ual res ults on this test. It is t hough t,
hO\,·ever, that the leakage of exhaust water due to
stoppage of the separator drain and sticking of t he
ballast dumping valve may easily account for the differ­ence
in " ·eight between the water collected and the
fuel burned.
It is to be noted that condensers designed for a:n air
temperature of 15° C. can not be expected to condense
sufficient water to completcl_,. compensate for t he
weight of fuel burned when higher t emperatures are
encou ntered, as in summer flying. Figure 17 indicates
the variation of water condensed with change of a ir
t emperature based on the design of the Model II con­denser.
In computing the points for this curYe, all
other conditi ons are assumed as constant.
In the flight test , as in t he ground test of t he same
apparatus, no t ro'uble of anr kind was experienced with
the condenser proper . Some difficulties were en­countered
\\·ith a ccessories, such as exhaust piping and
drain tnbes, but these parts were of a makeshift nature
and should give no trouble in a service unit whose
parts are well designed and constructed.
It is worthy of comment that the installation of the
Model I condenser on the D- 3 produced no appreciable
decrease in the ship's flying speed at a given propeller
speed. Th.is was contrary to expectations but may be
readily credit ed when t he immense projected fron tal
area of t he ship is taken into account.
TABLE 1.- S ummary of flight tests on Nlodel I condenser
'"l'ime
Fligbt No. \
-- Hrs_:_1;\l:
L --·--·-·--· - ·- · •) ___ ____ ____ ___ _ _
3 __ _____________ _
4_ - . - -· -----·---·
56 •_._ _·_-_- -_-_-_-__-_-_-_·_· -_-_ _
7 - - • ---- - ---· - ·--
8 . -- · ---- - ----· - -
910- _·-__·-_-__-_-_-_- ·__--_· ·__· ·_
11 . _. -------- - - -- -- --
12 ___ - ----- - --- - -
3 j
~I 2
3
4
13
18
52
47
9
2.J
59
18
11
1
0
0
:1
I I
13_ __ _____ ____ ___ 4 I o
:\lean ___________ '. ------!------
1 This rnlue not included.
Mean
air
temper­ature
o C.
20. 5
24. J
23. 5
24. 4
20. 5
22. 7
21. 5
18. 5
24. 5
22. 3
Air
speed
M . p. h.
39
37
46
34
38
44
48
Altitude
Feet
1, 000
1,000
800
900
I. 000
950
J, 250
800
50
900
Engine
R. p . m .
900
J. 100
1, 000
I , 200
J,000
900
1, 100
900
900
J, 100
J. 200
Water
Pounds
206
254
210
212
232. 5
227
180
278
371
G r Water I aso inc collected
Pou'rirls
24 2. 5
3:31
285
332
266
302
260. 5
309. 5
4il
Per cent
84. 9
76. 7
73. 7
1 63.8
87. 3
75. 2
69.1
89.S
T otaL _.. 2. 1 70. 5 2, 799. 5
78. 8
79.4
Remarks
"Separator dra iu clogged.
Gasoline record uncertain.
\Yater spilled in draining.
\\' ater leaked through emergenry release
v?Jve.
Release valve stuck-water lost.
Flight abandoned due to oil leak on left .
motor.
Mean .
)i°OTE .- Six preliminary flights were made (total time 12 hours, 46 minutes) in order to eliminate minor mecbaoical defects. The average water
collected for these flights was 72.4 pe,r cent .
13
JO
-10
70 80 9 0 / 00 //0 /20 ·
% CoNOEN,5AT/ON
FIG. Ji
CONCLUSIONS
Al1 hough no claim is made for the present general
design as repre~eniing the ultimate form of ballast
recovery apparatus-, the feasibility of condeusation of
water from the engine exhaust as a means of com­pensating
for the increased lift, due to the loss of the
fuel burned, has been demonstrated.
The weight of the apparatus is not excessiYe and
can be wholly rompe11satecl for IJ.v a correspoucling
decrease in the \Yeight of the water ballast usually
carried.
Owing to the large frontal area of an airship, the
head r('sistance of a condenser of the type thus far
used represeuts such a small portion of the whole as
to be nearly negligible.
Condensers of the general t_vpe of Models I and II,
ha\·ing no moving parts, require little attention and
eompare favorably, as regards fly ing li fe , with most
aircraft engines 1101T in service use.
In cold weather, 1\·ith a couclensat iou in excess of
the fuel equilibrium requirements, the exhaust ,rnter
may also be used to at least partly compensalc for the
change in l ift clue to ('hanging barometric pressure or
temperature of the lifting gas.
Although some assumptions used in the design work
a,ppear to be only rough approximations, none of the
test results seem to indicate any great error within
the range of conditions thus far encountered.
...
PART II- THEORY
In reviewing the theory involved in the process of
obtaining water from the engine exhaust gas, it may be
well, in order to avoid confusion, to first discuss ·the
nomenclature used.
Reference is made to the chart, Figure 18, in which
ordinates represent weights of the quantities indi­cated.
The fuel burned is, of course, the basis upon which
the other quantities are determined. The total water
!
W'ATEH e1VreRIN6
W'ITH AIH
i
,
perature itself, it at once becomes apparent that the
recoverable water is the total minus the vapor neces­sary
to saturate the gaseo us exhau8t products at a ir
temperature.
The designed recovery is, of cou rse, limited by the
recoverable water as determined by an assumed air
temperature. It may be arbitrarily placed at any
value less than that of the recoverable water, but
would not ordinarily be less than the fuel burned.
~ ,....
·, ~
~ q~ ~
~ ! ~
~ " ~ ~ ~ ~ ~ - R
~ ~ ~ ~ l
~ ~~ Q: ~ "' ....., ~ 8 ~
a\ j ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
~
V
,
FIG. 18
in the exhaust gas is the sum of the water of combustion
and the moisture entering with the carburetor a ir.
This quantity depends upon the kind of fuel and to a
small extent upon the mixture ratio. When using gaso­line
there is present more than 1.4 pounds of water for
every pound of fu el burned .
Since the amount of water lost as vapor depends
upon the exhaust-gas final temperature, and since, in an
air-cooled system, the limit of cooling is the air tern-
Since the designed r ecovery is based upon a designed
final exhaust ·temperature, it is evident that error in
design assumptions or changes in flying speed or air
temperature will change this final temperature and
therefore the water actually condensed. The water
condensed may be either greater or less t han the
designed ·recovery, but is represented as showing a loss.
In the collection of t he water condensed , leaks of any
kind, as well as faulty separation of entrained moisture,
(14)
will lower the quantity actually available as ballast.
The water collected is therefore a lmost certain to be
less than that actually condensed.
Any of the quantities shown, when expressed as per
cent, are based upon the weight of fuel burned.
The design of the condensing a pparatus is dependent
upon a large number of variables 'which may best be
taken up as they appear.
It is at once apparent that the \\·ater condensed is
dependent upon exhaust-gas composition and its fina l
temperature. The first of these is readily determined
when the chemical composition of the fuel, ratio of air
.ss
15
The air composition is approximately oxygen 23 per
c.ent and nitrogen 77 per cent by weight. The air
temperature was assumed to be 15° C. lower than that
of the exhaust products leaving .the condenser. It is
obvious that the exhaust gas wiU never be cooled to
the temperature of the air. The relative humidity was
assumed as 60 per cent, which is an approximate aver­age
for all parts of this country for the whole year for
altitudes less than 5,000 feet. As a basis of comparison
of the different fuels their chemically correct air-fuel
ratios were used.
ff'o /0 eo 30 'fo SO 60 70 80 90 100 110 !ZO / 30 /40 ISO
7o CCWOf NMTION
FIG. 19
to fu el, and relative humidity and temperature of t he
air are known . The fina l exhaust temperature; how­e,
·er, is a fun ction of the length, d iameter, spacing and
number of condenser tubes, mass flow, and initial tem­pera.
tu re of exhaust gas, and the velocity and tempera­t
ure of the air flowing past the tubes.
Taking first the effect of different fuels, Figure 19
,;hows t he water condensed in per cent by weight of the
fuel burned, plotted against the final exhaust tempera­ture,
for a number of liqu id fuels.
In order to arrive at the curves shown it was neces­sarv
to make assumptions for carbureter air composi­t
io1:, temperature and humidity , and n1ixture ratio, or
pounds air per pound fuel.
96997-24t--q
The condensed water per pound of fuel is, of course,
the water of combustion plus the moistme entering
with the air, minus the water issuing as vapor with
the cooled exhaust products. It may be well to fol­low
through the computation with one of the fue'.s
involved.
Taking pentane as an example, we have from its
composition (C5H 12) the hydrogen and carbon content
per pound 0.167 and 0.833 pound, respectively (see
Table 2). Then from the combining proportion of
hydrogen and oxygen (1: 8) the water of combustion
will be nine times th!;! wei~ht of the hydrogen, or 1.500
pounds.
16
TABLE 2.- Combustion data for liquid fuels assuming correct mixtiire ratio for complete combustion
Content per pound
of fuel
Product per pound
of fu el Air content Ratio Exhaust
Molecular molecular gas
Fuel 1-----~---1----.----1---- ---1 Mixture
ratio
weight we,gt~t weight
of au per
exhaust molecular pound
A ir
factor
Hz C 02 H,O co, 0 2 ;;~';!\ of fuel
--- ---- --- -------- --- - ------ -----
Hexane C,Ht< ---- -- --- --­Pentane
C,H12------- ---­Dodecane
C12H20 - - ---- - --
Alcohol C,H,OH _____ ___ _
Beuzol C,H, ____________ _
'l'oluol C,Hs--------------1
XEtyhleorl CCs,HHI1O0-0- _-_-_-_-_- -__-_-_-_--_-_
0.163 I
. 167
.153
. ] 30
.077
. 087
. 094
. 135
0. 837
. 833
. 847
. 522 . 348
. 923
. 913
. 906
. 649 ---.216- 1
1. 467 3. 07 3. 54
l. 500 3. 05 3. 55
l. 375 3. J05 3. 48
l. 173 l. 915 2. 08
. 692 3. 38 3. 08
. 782 3. 35 3. 13
. 850 3. 32 3.17
l. 215 2. 38 2. 394
11. 86 15. 4 30. 28 0. 952 14. 93 14. 2:3
ll. 89 15.44 30. 28 . 952 14. 94 14. 24
11. 66 15.14 30. 28 . 952 14. 76 14. 07
6. 96 9.04 30. 39 . 958 8. 875 8. 42
10. 32 13.4 30.72 . 939 13. 70 12. 87
10. 49
13. 62 I 30. 68 . 940 13. 84 13. 02
10. 62 13. 79 30. 65 . 941 13. 94 13.12
8. 68 11. 27 30. 40 . 949 11. 06 10. 50
Similarly, we know that the relation of the carbon The total exhaust gas weight, exclusive of water, per
d
. "d t b . 44 b . . pound of fuel is t he sum of the carbon dioxide and
10x1 e o car on 1s 12, . nngmg the carbon dioxide
nitrogen present, or
44
produced to 12 X 0.833 or 3.050 pounds . The oxygen 3.050 + 11.890 = 14.940 pounds
r equired for combustion is then the difference between
the weight of t he fuel and t hat of t he products of
combustion,
l.500 +3.050-1.00 =3.550 pounds
Since the air is 77 per cent nitrogen by weight, the
nitrogen content of the exhaust gas will be
;~ X3.550 =U.890 pounds.
and the total weight of a ir per pound of fuel \\·ill be the
sum of the oxygen and nitrogen, or
3.500 +11.890=15.440 pounds.
Expressed as a ratio of weight, t his is the correct com-_
bining air-fuel ratio.
The weight of water vapor which will be taken up
by a pound of dry gas at any temperat ure is propor­t
ional to the specific volume of the gas or inversely
proportional to its densit y . · For convenience, t he
molecular weight is substituted for density.
For air, the molecular weight is
100
77 + 23 = 28.84
28 32
where 28 and 32 are the molecular weights of nitrogen
and oxygen, respectively.
Similarly, the molecular weight of the exhaust gas
products, exclusive of water, is
3.050+11.890
3.050 + 11.890 =30.28.
44 28
From this we obtain t he ratio of molecular weights,
and hence of the densities of air and exhaust gas
28.84 = 0 952
30.28 ·
which is also the in verse ratio of specific volumes, and
hence the direct ratio of specific volumes of exhaust gas
to air.
and th·e product of this with t he ratio above gives
the a ir factor shown in the last column of Ta ble 2
14.940 X0.952 = 14.240.
Since the air factor is the product of t he rat io of
specific volumes of exhaust gas and air and t he weight
of the exhaust gas (other than water vapor) per pound
of fuel burned, t he vapor content of the exhaust prod­ucts
leaving the condenser may be readily obtained
by multiplying this factor by the weight of water
necessary to saturate a p ound of dry a ir at the same
temperature aud pressure. A physical r epresentation
of the air factor is the number of pounds of dry air
which would be saturated by the quantity of water
leaving the apparatus with the other exhaust products
from 1 p ound of fu el, · at t he same temperature and
pressure. For a constant fuel composition and mixture
ratio the air factor does not change.
Referring now to Table 3, the first line is the tem­perature
of the exhaust gases issuing from the condenser.
T he second line is the moisture content of 1 pound of
dry air saturated, at this temperature and atmospheric
pressure (Goodenough's tables). These values mult i­plied
by t he air factor from Table 2 give the water
content of the outgoing gases per pound of fuel in each
case.
The a ir temperatures in Table 4 are shown as 15° C.
lower than those of the final exhaust in Table 3, com­plying
with .the assumption made. To ascertain the
moisture entering with the air for each pound of fuel it
is only necessary to multiply the water per pound of
a ir saturated by 0.60 (assumed relative humidity) and
again multiply by the mixture ratio.
To find the water condensed (Table 5) at any tem­perature
considered, t he water co ntent of tll°e enteriug
air (Table 4) is added to the water of combustion
(Table 2) and the water content of the exhaust -gas at
the corresponding temperature is subtracted from this
sum. The curves in Figure 17 are plotted from values
recorded in Table 5.
17
TABLE 3.- Pounds of waler vapor leaving with exhaust gases per pound of Juel. Relative humidity, 100 per cent
Exhaust temperature, °C 10 15 w I 25 w u ~ ~ - w ~
c. 00764 0. 01068 o. 01476 1 0. ~2017 0. 02729 0. 03662 0. 04899 0. 0654 0. 0873 0. 1156
Pounds moisture per pound
saturated air_ _______ __ _____ _ 0. 00542
-----------1------------------------- ------ ---- - - ------
Hexane ___ __ __ _____ __________ _
Pentane ____ __________________ _
Dodecane ___ _______ ______ -----
Alcohol __ - - - - - ------------ ___ _
Benzol __ -- ---- - - - - - --- _______ _
ToluoJ ___ ___ ___ ______________ _
Xylol _______ _________________ _
Ether -- - - --- -------------··-·-
. 0772
. 0772
. 0762
. 0456
. 0697
. 0706
. 0711
. 0569
.1087
.1088
.1074
.0644
.0984
.0995
.1002
.0803
. 1522
. 1523
.lW5
. 0901
. 1370
.1392
. 1403
. 11 22
.W85
.W87
. 2062
.1242
.1 886
. Hl09
. 1923
. 15W
. 2872
. 2874
. 2838
. 1699
. 2596
. 2627
. 2647
. 2117
. 3913
. 3918
. 3868
. 2297
. 3540
. 3580
. 3608
. 2918
. 5210
. 5215
. 51W
. 3080
. 4712
.4764
. 4800
. 3847
. 6974
. 6977
. 6890
. 4125
. 6305
. 6378
. 6432
. 5147
. 9315
. 9321
. 9210
. 5510
. 8324
. 8524
. 8585
. 6872
1. 240
1. 2415
1. 2265
. 7345
L 122
L 1350
L 1435
. 9168
l. 645
1. 647
1.627
. 974
1. 487
L 506
L 517
L 214
TABLE 4.- Po1mds waler vapor entering with air per pound of fuel. Relative humidity , 60 per cent
Air temperature, 0 0 - 10 -5 0 10 15 20 25 30 35 40
-----------,---------------- ----- ------- -----
P ounds moistme per pound
saturated air _______________ O. 001643 O. 002553 O. 003782 0. 00542 · 0. 00764 0. 01068 0. 01476 O. OW17 O. 02729 0. 03662 0. 04899 ---- ·~1- - ---- Hexane ____ ______ _____________ . 0152 . 0236 .0350 . OWl .0706 .0987 .1364 .186.1 . 25W . 3383 . 4527
Pentane___ ___ __ _______________ . 0152 . 0237 . 0350 . OW2 . 0708 0990 . 1367 . 1868 . 2528 . 3392 . 4539
Dodecane_ ______ ______________ . 0149 . 0232 . 0344 . 0493 . 0694 ·I . 0971 .1341 .1833 . 2480 . 3327 . 4453
Alcohol __ _____ __ ______________ . 0089 . 0139 . 0205 . 0294 . 0414 I . 0579 . 080l . 1094 .1480 . 1986 . 26,57
,Benzo]__ ___ ___________________ . 0132 . OW5 . 0304 . 0436 . 0614 . 0859 . 1186 . 1621 . 2193 . 2944 . 393~
roluoL ___ ____________________ . 0134 . 0209 . 0309 . 0443 . 0624. . 087a - . 1206 .1648 . 2230 . 2992 . 4003
XyloL ______________________ __ . 0136 . 02ll . 0313 . 0449 . 0632 . 0884 .1222 .1669 . 2258 . 3031 . 4054
E ther_ ___ ___ ________ __________ . 0111 . 0173 . 0256 . 0366 . 0516 . 0722 . 0998 .1363 . 1845 . 2476 . 33 11
I
TABLE 5.-Pouncls waler condensed per pound of f uel, assuming complete.combustion. Relative humidity, 60 per cent
Exhaust tempera- Water II
___tu _,·e, oc_ __, _ _g _~_~z_otn- --5--·- -10-- _ _ 15 _ __2_0 _ __:__ __3_0 ___3_ 5 __4_ 0 ___45 _ 1__:__ __55 _ _
Hexane ___________ _
Pentane ______ ___ __ Dodecane ____ _____ Alcohol__ ______ ___ _ _
BenzoL ____ __ _____ _
'l'oluoL ____ _______ _
Xylol. _ ---------- -­Ether
__ - - - - - - ---- --
1. 467
1. 500
1. 375
l. 173
. 692
. 782
. SW
l. 215 1
I. 405
I. 438
l. 314
l. 136
.636
. 725
. 792
1.169
I. 382
L 415
1. 291
1.122
. 614
. 703
. 769
1. 152
1. 350
1. 383
1. 259
l. 103
. 585
. 674
. 741
L 128
1. 309
1. 342
1. 218
I. 078
. 547
. 635
. 703
I. 097
At the lowest assumed temperature the water of
combustion from · pentane is 1.50 pounds (Ta ble 2).
Adding to this the water entering with the air (Ta ble 4)
we have
1.500+ 0.015= 1.515 pounds
Subtracting the water lost as vapor with the exha ust
gas leaving the appa ratus we have
1.515 - 0.077= 1.438 pounds
which is the water conden.sed .
Since it is improbable that any fu el having exactly
the composition of one of those shown in Figure 19
will ever be actually put into service use, thfa figme is
supplemented by Figure 20, showing the final t em­perature
to which the exhaust gas must be cooled in
order to condense 1 p ound of water per pound of any
hydrocarbon fuel within the range of fuel composition
apt to be encountered. This curve was derived by
plotting the points at which the curves of Figure 19
cross the 100 per cent condensation line. In order to
secure the points for the lower end of the curve, fuels
having lower h ydrogen content than t he gasoline group
were assumed. To find the required final t emperat ure
for 100 per cent condensation from any blend of hydro­carbon
fuels it is only necessary to k now the h ydrogen
content of the mixt ure. The lower limit of the curve
I. 250
I. 283
1. 161
1. 044
. 494
. 582
. 649
1. 055
1.174
1.W7
l.085
l. 001
. 424
. 511
. 578
. 995
I. 082
l. 115
. 994
. 945
. 339
.426
. 492
. 930
0. 956
. 989
. 869
. 870
. 224
. 309
. 374
. 837
0. 788 1 0. 565
. 821 . 598 . 307
. 702 1 . 481 . 193
770 . 637 . 465
. 079 - - -- - --- --
0. 275
.153 - --- - - -- -- - - --------
. 217 1 .010
. 712 . 546 . 332
of Figure 20 represents a fuel blend of about 41 per
cent benzol with a gasoline of 15 per cent hydrogen
content.
All the curves shown thus far are based on the
theoretical combining mixture ratios of air to fuel,
which correspond to extremely lean carburetor set­tings
and give the economy so vitally important in
the operation of airships. Some multi-cylinder en­gines
are unable to operate on mixtures quite as lean
as those shown, but since the leaner mixtures are less
advantageous as regards water recovery, the curves
are based on the worst expected condition and are on
the safe side.
The effect of mixture ratio upon water condensed at
various temperatLires is shown in Figure 21. These
curves were computed in the same way as those of
Figure 19, as represented by the computation sheets,
Tables 6, 7, 8, and 9. The fuel in this case was assumed
to be gasoline having a 15 per cent hydrogen content,
which closely approximates the fuel now in service use.
The final temperatures required for 100 per cent
condensation (1 pound of water per pound of fuel)
using this fuel and the indicated mixture ratios, are
shown in Figure 22, which is obtained. by plotting the
points where the curves of Figure 21 cross the 100 per
cent line. •
<· ...
.18
I
~ n ff / M ~ ~ 2 ~ ~ - Z ~ 3 38 'f'O
/7/VAL CXHAU.5T ffMPC:RATURe- °C
FIG. 20
so / O zo ~o -fO so 60 7o ~o 9o 100 110 120 /30 #O ,so
% Cb.lYL7CIY..l4T/ON
FJG. 21
19
Since the humidity is uncontrolled, it may be that
for special cases some figure other t han the average
one of 60 per cent may be applied. The curves of
Figure 23 indicate the limits of t he humidity range
between zero a 1Jd 100 per cent when using gasoline
having a 15 per cent hydrogen content with a 13 : 1 ai r­fu
el ratio. The data for t hese curves (Table 10) are
largely obtained from the mixture ratio computations .
~ Jix ture ratio ~
fu el
Oxygen
in air
TABLE 6
Exhaust gas content \ Moleru- Weightof
Oxygen l\1Iole9u- lar we_ight exhaust
in air ,----~--~----,tar weight ra~10 gas per
less 1.2 of ex- air pound
CO2 N 2 Oz lrn.ust gas exhaust of fuel
Air
factor
-------·--- - --------·I---- - - - - - --- ---------------- --------
12_
13 ___ _____ __ ____ _____ _______ _____ _____ ____ _____ ____ _
14 ____ ___ ___ ___ . ----- -- - · _ --- - - - ------- --- -- - - - - --
IA __ ---- - --- - ---- - - - -- - -- - - --- - ----- - - - - - - -- -- - ---- -
16 ___ ____ ____ ______ _____ __ - - --- --- - - - - - - - - - -- - - --- --
Gasoline assumed l5 per cent hydrogen.
Water produced= .15X9= l. 35 #/# fuel.
Necessc1ry oxygen= I. 35-. 15= 1. 2 pound~.
::VIolecular weigh t of ai r =28. R4 .
. \ ir assumed 2.1 per eent oxygen, 77 per ct>nt nit.ru~en.
2. 76
2. ll9
:1. 22
3. 45
3. 68
l. 56
l. 79
2. 02
2. 25
2. 48
2.14
2. 46 '
2. 78
3. 09
3. 11
9. 24 29. 7 0.972
10. 01 30. 18 . 956
10. 78 30. 27 . 953
JI. 55 30. 32 . 952
12. 32 0. 22 30. 25 1 . 954
TABLE 7.-Poirnds Wlller vapor losl wilh saluralecl exhcwsl gases per pownd of gasoline
11. 38 11. 05
12. 47 11. 91
13. 56 12. 91
14. 64 13. 93
15. 65 14. 92
Exhaust temperature, °C. 5 ; IO 1;; . 20 I 25 30 :!5 40 · 45 50 55
:lloist ure per ~y~nct saturated O 00542 1 o 00764 O 0 1068 o 01476 I O 02017 O 02729 0. o:l662 0 04899 0 065.J O 0873 0. 1156
1'.lixture ratio: I--- - --------1--------------------------------
. 12 __________ ___ ____ ___ . 060 084 118 163 .223 302 405 542 .723 .965 1 278
13 ___ _________________ .065 .091 . 127 . 176 .240 . 326 .436 .584 .779 1.039 J. 377 .
!4 . . ------- - -------- - - - - - -- . 070 . 099 . 13~ . 190 . 260 . 352 . 47;3 . 633 . 845 1.1 27 I. 492
15 ______ _______ ______ .07f5 .106 . 149 .206 .281 .380 .5 10 .683 .912 1.215 J. 612.
]6 _ _ _ _ _ _ _ ___ __ _ ____ ______ __ .081 .114 . 159 . . 220 .301 .407 .546 .730 . 977 1.301 1.727
TABLE 8. - Pounds waler vavor entering with air per pound of gasoline. Relative lwmidily, 60 per cent
Air lemperatore, °C. - 10 -5 o 5 :o 15 20 25 30 35 I 40 --- - - - - - --·--- - - !-
:lloisture per pound satu rated
1
_ 001643 air . 002553 . 003782 I • 00542 . 00764 . 01068 . 01476 . 02017 . 02729 . 03662 . 04899
Mixfte ratio: ______________ _ \
13. ____ _____ ______ -·--- - --
14 . ___ _____ __ ___ _____ __ -- - - :t :::::::::::::::::::::::\
. 012
. 014
. 015
. 016
. om
1
. 018
. 020
. 021
. 023
. 024
I
. C27
. o:io
. 032
. 0:34
. 036
. 039
. 042
. 046
. 049
. 052
. 055
. 060
. 064
. 069
. 073
. 077
. 083
. 090
. 096
. 103
. 106
.. 1l 2145 1
. 133
. 142
. 145
. 157
. 169
. 181
. 194
.196
. 213
. 230
. 246
262 1
. 263
. 286
. 307
. 329
. 351
. 353
. 382
. 411
. 441
. 470
TABLE 9.-Poimds waler condensed per poimd of gasoline. Relative humidity, 60 per cent
1vl ixture rat io: .
12 ____________________ ___ _ 1143 ______________ _-_--__-_-_- _-_- _- -_-_-_-__-_- -__- -_
15 _ ______ ______ ___ ___ ____ _
]6 ____ ___ __ __ _______ _____ __ .
10 15 I 20 25 30
1
35 I 40 45 50 I 55
J. 302 J. 284 I. 259 1 I. 226 1.182 I. 125 , l. 05 L . 953 . 823 ---.:-----:
1. 298 1. 219 1. 2s:i I 1. 216 1. 170 1. 101 1. 029 . 923 . 784 . 597 . 35.,
I. 294 I. 272 I. 244 I. 206 I. 154 J. 088 I. 001 . 886 . 735 . 530 . 269
1. 290 1. 251 1. 235 1 1. 193 1. 138 1. oori o. 013 . 848 I . 684 . 464 . 1,0
J. 285 J. 260 J. 227 L 182 I. 122 J. 046 0. 946 . 814 . 635 . 400 . 093
I
l 'iual exhaust temperature, °C.
TABLI,; 10.- Pounds water condensed per po·und of gasoline wilh dry and 8alti-rnled air, mixliire ralio, 13:1; ga:;o­line,
15 per cenl hydrogen
Final exhaust temperature, °C. 10 15 20 25 30 35 40 45 50 55
Air temperature, °C. -10 -5 10 15 20 25 30 35 40
Water entering with sat urated
air __ -- -- - - ---- - - - --- --- -- -- -
Water lost with exhaust gas __ . 024 . 033 . 049 . 070 . 099 . 139 . 192 . 262 . 355 . 476 . 637
.065 .091 .127 . 176 . 240 . 326 . 436 . 584 . 779 I. 039 1. 377
Water condensed:
Air saturated __ ____ _ l. 309 l. 292 J. 272 l. 244 I. 209 I. 163 I. 106 1.028 . 926 . 787 . 610
Air dry ______ ______ __ ____ I. 285 1.259 I. 223 I. 174 I. J 10 I. 024 . 9JA . 766 . 571 . 311
96997-24-....J4
•
20
/6
/.
JO 31 :JZ 33 J,f- 3S ~6 :57 38 J9 -?O "11 42 43 -?
riNAL .C%/7AUST «MPeli'ATURe - °C
FIG. 22
So l
$0
~1.;
"' ~10
K
~
~.15
~~ 3o
....
':>
:i
~ZS
~
l(J 20
"I' .
~ ;:,-
/0
"o / 0 2o Jo 0 .,.0 60 70 6'0 0 /00 / / 0 /ZO /30 /. 0 /~O
% C'ONDEIV.SATION
F IG. 23
21
The effect of humidity upon the final temperature I The equations are developed from the simple as-
'.rnce~sary for 100 ~er cent water condensed is shown j s.u~ption th~t . in a given len~th of pipe for equi-
111 Figure 24. This figure was obtamed by plottmg a hbnum cond1twns, the heat given up by t he passing
series of curves similar to those of Figure 21 for several exhaust gas is equal to that taken up by the pipe wall,
conditions of humidity, and picking off the points of which is equal to that dissipated by the pipe wall and
intersection with the 100 per cent condensing line as equal in turn to that taken up by the external air
before. stream. These conditions give three equations in
The points plotted on Figure 23 represent a verage four variables, of which one is readily eliminated,
values of the water collected in the endurance t est of giving a pair of simultaneous differential equations to
Model I condenser. It will be noted that most of express 0, the t emperature ,of the exhaust gas, and cf,,
these points lie within the range indicated. Any the t emperature of the air st.ream, as fun ctions of x,
/.
90
/0
0 ZS JO 3~ ,f-O .P.~
17/VAL fYltAUST TCMPfRATURc- °C
FIG. 24
leakage of exhaust gas or condensed water would-tend I t he coordinate along the pipe
to lower the apparent condensation, so that the as- ' direction of exhaust gas flow.
sumption of 100 per cent separation is felt to be cor - simultaneous equations is
length, posit ive in the
The solut ion of these
r ect within reasonable limits.
It is _obvious from the foregoing p aragraphs and
curves tha t it is a relatively simple matt er to deter­mine
a desirable fin al exhaust temperature, ha ving
ass_umed the kind of fuel, mixture ratio, a ir temperature,
and relative humidit y . From t h is point on the task
is so to select a nd arrange the cooling system as to
a rrive at the desired final t emperature.
The equations for heat transfer which are the basis
of the design of the exhaust cooling syst em ha ve been
derived under the following assumptions : (a) The
t emperature of the pipe surfaces is the same inside and
out at any given point in the length of t he pipe and is
of course uniform around the circumference; (b) the
a ir flowing over the external pipe surface is of tem­perat
ure, density, pressure, and humidity characteriz­ing
normal atmospheric conditions, and the velocity of
this air is uniform and perfectly turbulent.
O =AeN +B (1)
(2)
Where
N =-p -qh+qc ( H1 + H1 ) X .
qh qC h C
e = base of natural logarithms.
p = perimet er of pipe - cm.
q" = coeffici ent of surface heat transfer between ex­haust
gas and pipe wall -cal/secX°C. Xcm2•
q0 = t he same coefficient, air st ream and pipe wall,
cal/secX°C. X cm2•
Hb = heat capacity of the mass of exhaust gas which
passes any point in a unit time-cal/°C.Xsec.
H0 =heat capacity of the mass of air stream -
cal/°C.Xsec.
22
A and B a re constants of integration .
To eliminate these constants the following end
ditions are applied:
For co unterflow of e),haust gas aed air-fJ
=T.}
</> =lb
when x = 0.
fJ=1\}
</> =l.
when x_· L.
And for parallel flow-fJ=
T • } w:rnn .c = 0.
</> =l.
o--T b }w hen x = L.
</> =lb .
Where T,.=initial exhaust temperature, °C.
Tb =final exhaust temperature, °C.
l,. = initial air temperature, °C.
lb =final air temperature, °C.
L =length of pipe, cm.
con-
The res ulting equations arranged in the form most
convenient for design work are :
For counter flow-
(3)
And for parallel flow-
(4)
The terms Hh and H 0 are obtained from the relation
in which
Mh mass flow of ·exhaust in one tube, g/sec
Ci, = specific heat, at constant pressure of exhaust
i\1 0 = mass flow of air around or.e tube, g/sec
C0 = specific heat of air.
The terms qh and q0 are obtained from the Lan­chester-
Stanton relation 1, 2.
q = c"M [ o.0765 Cvf D)
0
·"
5
+ 0.0009 J (5)
in ,,·hich
C" specific heat at co nstant pressure
μ viscosity, c. g. s. units
~I mass flow, g/sec X cm2
D = diameter of t11 be, cm
In " ·orking to,Yard a satisfactory heat dissipating
apparat us t he first determination 11ecessary is the
total rate of mass flow of exhaust gas coming from the
engin e. This \\·ill , of course, be the product of the
brake horsepower, the fuel economy, expr~ssed in
p ounds per brake horsep ower-hour, and the weight of
mixture taken by the ellgine per pound of fuel , ·giving
the mass flo"· in pounds per hour.
From consideration of equations 3 and 4, for maxi­mum
efficiency of heat transfer, it is desirable that the
velocities in side and outside of the tube be somewhere
near the same. Hence having a known flying speed
1 Great Britain Committee for Aeronautics, Tech. Rep. 1912-13, p. 45.
' B. S. Tech. P aper No. 211, page 320.
it is an easy matter to set this equal to the mean
velocity of flow inside the pipe, and from the mean
density over the temperature range within the pipes
compute the mass flow. There is no point in folJow­ing
through this process in this paper. Suffice it to
say that for an air speed of 45 miles per hour the
desirable mass flow inside the pipes works out to be
0.017 pound per second per square inch of cross
sectional area (1.195 g/sec. X cm2), provided the tem­perature
of the exhaust gas lies in the neighborhood
of 650° C. at the hot end and 32° at the cold end.
This figure (0.017) may be increased or decreased in
proportion to the air speed, but since little gain is to
be expected from any but broad approximations in
this respect, it is probably as well to use this mass
flow as a constant when working ,Yith airships having
the cruising speeds of any now in serv ice or likely to
be fo r some time to come.
The selection· of the most advantageous size of tubes,
must of necessity be a compromise between heat
transfer efficiency and mechanical consid eratioH s.
The effectiveness of the apparatus will be found
(equation 5) to be increased as the d iameter of the
t ubes is . decreased, whereas the smaller the t ubes,
t he greater will be the exhaust back press ure and
liability· to clogging, and the less the mechanical
strength of the tubes both as self-supporting beams
and as long columns.
An indication of the relative resistance to flow wit h
increase or decrease of pipe sizes may easily be ol::­tained,
but for the purposes of this work it is safe to
let the limiting factor in redu cing tube diameter l;e
the mechanical strength necessary in a structure of
the type contemplated. Soon after the beginnillg of
design " ·ork on the full ai r-cooled type of " ·ater re­covery
apparatus a tube of 1 inch diameter ,,·as
assumed and the strength of thin tube of this size
was computed. This choice of dimensions has, since
been vindicated by the ground and fl ying tests, ~ince
tubes of this diameter and the thinnest walls com­mercially
obtainable have proven amply strong, while
the back pressure is nearJ~, at the allowable limit. In
the theory and design sections of this paper, therefore ,
only tubing of circular section, 1 inch in diameter, is
considered .
Having the assumed tube size, t he known total
mass flow, and adYantageous mass flow per unit area,
the total number of tubes and mass flow per tube is
easily computed.
In solving equation 5 for q, the mass flow of ai r i\l
is determined by the air dcnsit.v, flying speed, nnd
spacing and munber of the tubes, " ·bile D has a Yalue
a lso determi11ed by t he tube spacing. The diameter
of a circle equivalent in area to the space include:l by
four tubes was used as an approximat ion.
D=-J4:B-d2
in which
D=equivalent diameter used in equation 5.
d=diameter of tube.
~} =vertical and transverse spacing of tubes, respec­tiyely.
23
The specific heat of the exhaust gas is approximately
constant for the range from 600° C. down to 53°, the
temperature at which condensation starts. Assuming
t he gas 90 per cent air, with Cp=0.24 and 10 per cent.
steam, with Cμ=0.45, the equivalent C11 is 0.26.
For the range below 53°, the effective specific heat
is very high due to the latent heat of condensation.
This was approx imately determined by computing
the total heat of 1 gram of the gas mixture at frequent
temperatu re intervals below 53° and finding the rate
of change of total heat "·ith temperature. This rate
of change was taken as the effective specific heat with
a mean value of 2.83 in the range 53° to 45° and 1.92
in the range 45° to 35° or 2.20 for the ent ire range 53°
to 35° . The figure 45° C. for the exhaust temperature
at the end of the second condenser bank \Yas secured
b~· successive approximations, and is used only to
determine the effective specific heats over the ranges
indicated. These values were. checked by a more
rigorous mathematical determination which can well
be omitted here.
The viscosity of the gas changes considerably with
ten:iperature but an average value of μ over the range
600° to 50° was taken as 2.4 X 10-, c. g. s. units, t he
viscosity of air at 150° C. For the range below 53°
it was assumed the effect of the condensing vapor was
negligible and the value 2.0 X 10-, c. g. s. units, for
the viscosity of air ,Yas used . For the air flowing out­side
t he tubesμ = l.8.5 X 10-•.
Thus we have constant values for the variables in
equation (5) so that qb and q0 an'cl consequently qh+q"
. qh qc
may be taken as constant for each range above and
below the condensation point.
For a t hree-bank condenser of the general form of
Model II it is impossible to directly compute the
necessary L on account of the changing functions of
t he variables during parallel and counterflow as ·well
as t he change of CP with temperature of the e·xhaust
gas. The most convenient method is to assume an
over-all length for the condenser and work through using
the equations given. It is of course necessary to
measure or assume the temperature of the exhaust gas
received by the condenser, as well as the air tempera­ture.
It will be found that there will be left two variables
in each case, but that one of them can be eliminated
by solving simultaneo11 sl_1· the eq uation expressing t he
fact that the heat lost by the exhaust is equal to the
heat taken up by the air
Design computations will be taken up and examples
given in another section of this paper.
Inasmuch as the heat to be abstracted from t he
exhaust gas varies 1Yith change in composition of the
fu el this total heat has been computed for th ree fuels,
gasoline and two half-and-half mixtures of gasoline
with benzol and alcohol, r espectively. These latter
represent two classes of nondetonating fuel P!L rt icu­la
rly suited to the requirements of h igh compression
engines. The change in total heat thus computed is
equivalent to the heat abstracted in a constant pressure
process between 650° C. and 32° C. The ex haust
gas is assumed to have been cooled to 650° C. by the
piping from the engines.
Under these conditions, assuming each fu el burned
with a chemically correct mixture rad io, t he heat
abstracted is as follows :
B. t. u.
Gasoline __ _______ ______ ___________________ 5,226
Gasoline-benzoL ___________________________ 4, 564
Gasolin e-alcohol_ ___ ______ . __ __________ _____ 4, 2211
By using Figures 19 and 21 it will be found tliat at
the fi nal temperature of 32° C. the water condensed
from these three fu els will be 103.0 per cent, 71.3 per
cent, and 98.0 per cent, respectively. Owing to the
differnnee in mixture ratio, the same engine would
burn about 25 per cent more of the alcohol mixture,
so that the heat to be abstracted would be brought to
about the same figure as that for gasoline alone. For
100 per cent condensation the exhaust from this mix­ture
must be cooled to 30° C.
The benzol blend, while having a low exhaust heat
value, must be brought below 0° C. before 100 per cent
condensation can be obtained. It is indicated, t here­fore,
that blending of antiknock fuels for airship
engines, especially for mixtures containing large
proportions of the antiknocking component, should
be accomplished b~· the use of alcohol rather than
benzol.
It is to be noted that exhaust water produced b?
the combustion of commercial benzol is very apt to
contain sulph ur compounds having a corrosive action
on some of t he metal parts of the condensing apparatus.
PART III- DESIGN AND CONSTRUCTION
For_purposes of illustration the design of a condenser
for a class " D " airship is taken up, assuming
Maximum B. H. P. equals 300.
Fuel equa ls aviation gasoline.
Cruising speed equa ls 45 miles per ho ur.
Mixt ure ratio equals 15 :1.
Referring now to Figure 21, the 15 :1 mixture ratio
curve crosses t he 100 per cent condensation line at a
temperature of 33.5° C. This is the final temperature
to which t he exhaust gas must be cooled.
The total mass flow, based on the assumption of
0.50 pound fuel per brake horsepower hour, is
0.50X (15.0 + 1.0) X 300 =2,400 lb ./hr.
For a sp eed of 45 miles per hour, a mass flow of
exhaust gas of 0.017 pound per second per square inch
cross-sectiona l area is about right (see p. 22).
Assuming round t ubes of 1 inch diameter (p. 22)
t he cross-sectional area of each is -i square inch.
The mass flow per tube is
0.017X-i =0.0133 lb. /sec.
=6.0 g/sec.
The total mass flow is
2,400
3,6oo=0.667 lb./sec.
Hence, the n umber of tubes necessary is
The r ise in temperatu re of the cooling air must be
kept small if the final temperature of the cooled ex­ha
ust is to be low. The mass flow of a ir, t herefore,
must be much greater than that of the exhaust gas.
For this reason the spacing of the Model II con­denser
was chosen as 2~-inch center to center of t11bes
in both directions. The cross-sectional a rea of the a ir
Rpare under this co ndit ion is
2~ X2X-i'=4.277 sq . in. per t.1 1be.
The mass fl ow of a ir is
pVA=Mc
in whi ch-p=
density = 0.754 lb ./cu. ft. (750 mm. Hg. 15° C.).
V =velocity =45 m . p. h. =66 ft. /sec.
A= cross-sectional area= 4.277 sq. in . = 0.0297 sq. ft.
M0 =0.0754 X66 X0.0297=0.148 lb. /sec. =67 g/scc.
From page 23-
C0 =0.24.
Ch =0.26 above 53° C.
Ch =2.83 between 53° C. and 45° C.
C1, = 1.92 between 45° C. and 35° C.
H ence
H c=Mc Cc=67X0.24 = 16.08
and
Hh=6X0.26=1.56 above 53° C.
= 6 X 2.83 = 17 .0 between 53° C. and 45° C.
=6Xl.92=11.5 between 45° C. and 35° C.
The t hree values of Hh apply , r espectively, to the
first, and first part of the second section , t he last part
of the second, and all of the third sect ion, in a t hree­bank
condenser, since these general temperature ranges
prevail as noted. Small departures from the indi­cated
temperature ranges will not affect the values of
Hh to an appreciable extent, so that they may with
reasonable accuracy be considered as constants.
Let us consider the condenser as d ivided into four
sectons of which 1 is the first bank, 2 is the first part
of t he second , 3 the last part of t he second , and 4 the
whole of the third bank. Then, since t he first bank is
in counterflow, t he function of H 0 and H h is HI - H1
,
h C
while the sign changes to plus for the second bank and
back again to minus for the third. Calling this fun c­t
ion of H 0 and Rh, H0 we may compute the following
values:
Section
1
2
3
4
Ho
0. 580
. 703
. 121
. 0247
The coefficients of heat transfer qh and q0 for t his
size pipe and spacing and the corresponding mass flow
of gas and air per unit area are computed from equation
(5). From pages 22, 23, and 24 the values Cv, M, μ ,
and D are as follows:
c. ______ ____ _________ _______ ____ _
M sec.; cm2 ---- - --- - - -- ----- - -~-
D \,t~:-~~'.~s_:: ::::::::::::::::::
Exhaust
gas above
53° C.
0. 26
L 19
2. 4X l0-'
2. 54
Exhaust
gas below
53° C.
2. 2
1.19
2.0X JQ-!
2.54
S11bstituting t hese values in equation (5)
qh = 0.0012 above 53° C.
q1, =0.0090 below 53° C.
g. = 0.0014.
Outside
air.
0. 24
2. 43
J. 85X l H
5. 93
Hence the values of qh+q. for t he sections numbered 1 to
qh qc
4 above are as follows, Q0 representing the fun ction of
q1, and q0 :
Section Q.
1 and 2 0. 000647
3 and 4 0. 001211
.(24)
25
We may now combine all the fun ctions of q1, , q0 ,
H1,, H0 , and pin a single list of constants by multiplying
together H0 , Q0 , and p. Call t his constant K . The
res ults follow:
Section K
1 0. 00299
2 0. 00362
3 0. 001169
4 0. OOOZJ9
With t he, above values equat ions 3 and 4 take t i1e fo rm
T - t
log. - • - "= KL for counterflow
Tb- t,
and
T -t
log0 T: - l:= KL for parallel fl ow.
For Ta, t he initial exha ust temperat ure at ent rance
to t he condenser, 650° C. may well be taken . This
approximates t he melting p oint of aluminum. For
t,., t he ini t ia l a ir temperature, 15° C. is probably a
fairly high fi gure. With t hese assumpt ions only two
temperatures and t he length remain to be determined .
T he only way to arrive at the desired temperature of
33.5° C. for Tb is to assume a length and apply t he
equations for each section, finding t he final T., for t he
last section. The length of t he condeuser may t hen
be increased or decreased according as t he final t em­perature
of exhaust is too high or too low.
For t his purpose an effective over-a ll leugth of 20
feet has been assumed , making a total length of 60
feet in t hree banks, of which t he second is parallel flow.
Keeping to t he same designa ti on of t he four sections
involved in t he calculation , we ha ve for section 1
L =20 ft. =610 cm.
T . =650° C . .
t. = 15° C.
K =0.00299
log. i:·= ~:=KL =0.00299 X610 = 1.824 = log. 6.20
650-tb=6 20
T1, -15 ·
tb = 650- 6.20 T b+93.0 =734.0-6.20 1\ .
Simultan eously
(T 0 -T1,) H1, = (t1,-t,) H e
or
Therefore
(650 -1\) 1.56 = (tb - 15) 16.08
lb =15 + 63.1-0.09711\
= 78.1-0.0971 1\ .
743.0 -6.201\ = 78.1 - 0.09711\,
6.103 1\ = 664 .9
Tb= 108.9° C.
T he condit ions at section 2 are thus determined:
T . =108.9°C.
t. =l5°C.
Tb = 53° C.
K =0.00362.
Since parallel flow now prevails, the main equation
becomes
T -t
log. T • t"=KL = 0.00362L
1,- b
log.
10
;/_~b
15
=0.00362L
93.9 . L
log. 53 -l" =0.00362
a nd al ·o (T . - 1\) l:-Ii, = (l0 -t") He
(108.9 -53) 1.56 = (t1, -15) 16.08
ti,= 15+1
1
/0
6
8 (108.9-53)
=15+5.42 =20.42
sustituting above for tb
93.9 L 93.9
log. 53 _ 20_42 = O. 00362 = log.32.58
loge 2.88 = 1.058
1.058
L=_00362=292 cm.
The le11gth for section 3 will be the d iffe rence be­brnen
610 and 292 centimeters. The condit ions at
this section are then
Ta=5:{° C.
t. =20.42
L=610-292=31S cm.
K =0.0011 69.
T he fl u,,· is still parallel as in sectiuu 2 so t hat
l T.-t, YL
ug. 1\-tb = "-
53-20.42
loge T t 0.001169X318 =.3717 =loge 1.-!50
b - b
32.58 = l -! -o
T.,-tb . ::,
t" = rI\ -22.46.
Simultaueously
(Ta - T.,) H11 (t., -l,) H0
(53 - 1\) 17 = (lb -20.42) 16.08
l1, =20.42 +56 - 1.056rl\
Droppiug t.,
rl\-22.46 = 76.-!2 - 1.056T.,
1.056 T., =98.88
Tb =48.1° C.
The condit ions for section 4 arc now determined ati
T.=48.1
t. = 15° C.
L,;610 cm.
E =0.000239
Tlic flu,,· i~ 110 ,,· changed again tu counterflow so t hat
T ,. -tb 1-1,
loge rr -t = \.
b a
loge 4-T8. 1 -lt' b= 0.000239X 6 10 = 0 .1 4 5,s
b- ;J
= loge 1.157
tb =48.1 -1.1571\ + 17 .35
=65.-!5 - 1.157Tb
26
Simultaneously
Heuce
(1'. -T,,J H11 = (t1,-t.)Hc
(48. 1 -'1\) ll.5 =(t,,- 15) 16.08
tb = 15 +34.4-0.716T,,
=49.4 - 0.7161\
65.45-l.157T" =±9.J -0.716T,,
0.4411\ = 16.05
Tb =36.4° C.
This temperature is some1d1at higher t lian that
required for 100 per cent condensation under the
14
zz
10
ra
/6
14
~
"~' fl
~
~
~ /0
i::
~
~ 8 "
"
4-
,l
0
-z
--+
17 flO
It may be found advisable on account of drainage to
cha nge t he position so t hat the long dimension of
cross-section is vert ical, as if, for example, it were
p laced agaiust the side wall of the car of a D or C
class ship.
It is by no means certain t hat the use of the same
number of t ubes in each bank is desirable for t he best
condit ion of heat exchange. It is suggested that
valuable data m ight be made available by a mathe­matical
comparison of t he present type exemplified by
Model II, and one in which the number of tubes in the
CoNoJ<"NSE"R L,rNr;rH-/'iu:r
F1G. 25
assumed condit ions, but tl1e d ifference is no greater
than the probable error in lhe as8111ned values used in
comput ing the heat trausfer. The fact that the cool ­ing
surface of the cast aluminum headers has not been
taken iuto consideration a lso makes probable a some­what
better performance than that indicated by t he
computations.
It is not assumed that a three-bank condenser of t he
same type as Models I and II is the ultimate solution
of the p roblem of exhaust heat transfer. ,vhen t he
construction and arrangement of the a irship permit,
it may be possible to secure better res ul ts by using
fewer banks and a greater overall length or vice versa.
first bank is increased by one-third or one-half and the
number in the last bank co rrespondingly deereased.
This would give a more uniform velocity of exhaust
gas and might on this account increase t he overall
efficiency of the unit. For the p urpose of designing
apparatus similar to the two condensers already b uilt,
t he computations have been worked out between wide
limits of a ir speed, condenser length, and initial
exhaust temperature. The final temperature of the
exhaust necessary for 100 per cent condensation was
taken as 32° C., which allows a ma rgin of safety of
1.5° C. The curves a re shown on Figure 25, which
requires little explanation.
27
Assuming the same conditions used in the design
computations just preceding, that is, an entrance ex­haust
gas temperature of 650° C., air temperature 15°
and air speed 45 miles per hour, the required length is
readily determined. It is only necessary to project
the horizontal line of 15° air temperature to its inter­section
with the second curve of the lowest group and
then to read the required length 011 the scale at the
bottom of the sheet. The value thus obtained is
approximately 25 feet, which is somewhat longer than
the value computed. It is to be remembered that the
final exhaust temperature in this case (32°) is some 5°
lower than that computed for the 20-foot condenser,
and that the cooling over the ra11ge of temperature
covered by this difference is extremely ineffective,
owing to the small temperature head between the
exhaust gas a11d the cooling air stream.
CONSTRUCTION-MODEL I
The parts of the Model I condenser are few in kind
if not in number, and the machine work necessary is
not excessive after the necessary tools and fix tures are
at hand. The materials used are a commercially
pure aluminum for the tubes, and F lynite, 3 a light
alumi11um alloy, for the castings.
The aluminum tubes used are seamless, 1 inch in
outside diamet er, and have a wall 0.016 inch thick.
The manufacture of these tubes is a process susceptible
of being held within close limits and 'no trouble what­ever
was experienced with variation i11 diameter. The
weight of the tubing as it comes from the mill is 0.0591
pound per foot. There are 51 parallel tubes in each of
the three banks of this unit, of which two require a tub­ing
length of 20 feet and the other 20 feet 10 inches.
This gives an aggregate of 3,102.5 feet and a total
weight of 183.4 pounds for the tubing alone.
The principal castings are those used as return bends
and entrance and exit headers. The pattern was made
with removable lugs and flanges, thereby serving for
four different castings as indicated in Figure 26. The
use of the lugs for anchorage of horizontal and vertical
tie rods is best illustrated by Figures 5 and 7. The
castings made with the flange in place were sawed
t hrough with a narrow milling cutter, thus provjding
wi th one operation two entrance or exit headers pro­vided
\Yith flanges having a finished surface for recep­tion
of ~ gasket when attached to the header manifold,
as indicated in Figure 6.
The only other castings are the three-hole tube con­nectors,
Figure 27, and the header manifold 8hown in
Figures 28 and 29. The former serves the purpose of
joining the fore and aft tubes of the lowest bank of the
condenser, while the latter distributes the exhaust gas
from the line to the individual headers at the entering
encl, and collects it again at the exit for conveyance into
the separator.
The problem of joining the tubes with the headers
was the subject of much consideration before the
3 F lynite is a copper-a.Juminum alloy containing about 10 per cent
copper, balance aluminum . It closely corresponds to alloy No. 2 as .
described under Specification for A)uminum Alloy Castings, Interna­tional
Aircraft Standards, 3 N 11; October, 1917.
method used was tried and found to be satisfactory .
This joint is made by forcing the thin tube into a hole
whose sides a re first parallel and then tapered until a
shoulder is reached. Details of the holes into which
the tubes are pushed are shown in Figure 30.
A leakage test of a joint thus made was conducted ~s
follows: A 10-foot tube was driven into one of the nip­ples
of a return bend header casting (Figure 6), using
a mixture of litharge and glycerine as a cement. The
other holes of the casting were plugged with corks, and
the whole assembly filled with water and left with the
tube extending upward for a period of 24 hours. No
leakage was discoverable.
On attempting to remove the tube from the nipple,
it was found necessary to drive the header off the tube
by means of a heavy hammer.
The process of machining the header nipples and
three-hole connectors for r eception of the tubes is rela­tively
simple, but req uires care and can not be hurried
greatly with the tools thus far developed. The header
casting or tube connector is first clamped to a fixture
on the bed of a vertical spindle mill with the nipples
pointing straight up. A rough boring tool is then car­ried
right through the nipple, leaving it approximately
the diameter of the inside of the tube. This tool is re­moved
and a finishing cut is made with a piloted reamer
which leaves a 1-inch hole having parallel sides for a
distance of one-half inch and tapering 0.006 inch in
the next three-eighths inch, ending in a shoulder at a
depth of seven-eighths inch. The headers used to con­nect
the tubes with the header ma nifold a re machined
for reception of the tubes before being cut through the
flange.
The assembly is accomplished by driving the headers
on the tubes after first smearing a t hin mixture of
litharge and glycerin _in the holes. The condenser is
first built up in sections as shown in Figure 31, and
17 of these are bolted together with -A--in ch chrome
vanadium st_eel rods. Short pieces of aluminum tube
strung on the rods keep the headers properly spaced.
The entrance and exit headers are then secured to the
machined face of the header manifold with filli ster
head machine screws. An asbestos gasket in terposed
between headers and manifold sec ures gas tightness
at t his point.
The vertical tie rods a lready mentiouecl are made
up of !--inch stock in order to provide s ufficien t flexi­bility
to allow for differential expansion clue to tem­perature
differences between the upper and lower sec­t.
ions. The front and rear upper banks are mechani­cally
connected by stiff steel straps strung on the trans­verse
tie rods between the headers as shown at A in
Figure 32.
The collect ion of condensed water is taken care of
by drains from the low poi nts as shown in Figure 5.
The drain at t he extreme left is made up of i -inch
copper t ubes running from fittings screwed into the
header castings to a i -inch iron pipe which conducts
the collected water to the ballast tank. The other
drain from the condenser proper is shown in Figure .5
emerging from the center of the apparatus. On ac­count
of limited space, short tee fittings are screwed
into these headers, and these are connected straight
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t hrough by short alumin um tu bes, t hus bringing all
t he collected wa t er out to one point from which it
d rains to t he t an k . The other d rain is from t he false
bott om of t he baffl e-type sepa rator Rh own a t upper
ri gh t in Figure 5.
In orrler to counteract a nY t endency of the tubes to
creep out of t he headers, a nd to t ake care of t he pos­s
ible end t hrust d ue to explosions of unburned gas in
t he condenser , ~o. 22 steel p iano wires wi t l1 t urn­buckles
were strnng between and paral lel to the tubes,
ty ing t h.e headers together and t ru ssing t he stru cture
d iago na lly in the vert ical pl a ne in o rder to gua rd aga inst
st resses imposed b.,· un equal tension of support ing
cables . These wires may be seen in Figure 7, and are
indica t ed by t he lines BC, CD, E F , a nd GR in F ig­ure
32.
MODEL II
The castings of the Model II conde nser a rc of t he
same mater ial (F lyn ite) as t hose of Model I. Fig­ures
33 and 34 show the deta il of t he ma in header
casting, nsed for both en t rance a nd exit a nd return
bends. The header cover , Figures ·35 and 36, is 1he
return bend used to co nnect a dj acen t banks. The
e ntrance und ex it elbows a rc idcnt icul, a nrl a re i11u s­trated
b)' F igure 37. The condenser tube suppor t s,
Figure 38, serve to connect I he 10-foot lengt hs of
t ubing in ea ch bunk.
T he joints between t u bes and headers a re made as
in t he Model I conde nser b.v pressing the t ubes into
t he tapered h oles in t he header nipples and t he bush­ings
of t he tube supports.
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40
The assembly of this condenser is as follows :
Fifty tubes are pushed by hand into the nipples of
one header, while a grid or frame near the center of
their length keeps the tubes properly spaced. A tube
support is then placed against the projecting tube
ends, and each tube is withdrawn a slight amount
from its position in the header so that it just enters
t he corresponding hole in the central tube support.
The header and support are then backed up by 6-
inch st eel H beams connected by six long !-inch steel
rods having nuts for forcing the header and support
together. These nuts are taken up until no further
movement of the tubes into the nipples can be de­tected.
The beam back of the tube snpport is then removed,
more tubes inserted on this side, and the process
repeated with another header on the end. It was
found that simply extending the rods through from
end to end of the two sections would not serve to force
on the second header because of the tendency of the
whole bank to buckle. For this reason steel bars
were inserted between the tubes back of the centra\
tube support and the rods were threaded into them
thus leaving the first end unstressed during the as~
sembly of the second.
To keep the headers from moving off the tube ends,
five 11-gauge spring steel wires were run straight
through each section from end to end and secured to
eye-bolts passing through bosses between the tubes
on the face of the headers. These bolts were drawn
up with nuts inside the headers before installing the
header covers. The assembly proved to be so stiff
t hat the addition of diagonal braces was unneces­sary.
Differential expansion is taken care of by slip­joints
between the banks as indicated by Figure 39.
An attempt was made in this condenser to drain all
the water condensed in the tubes through the two
bosses which may be seen at the bottom of the header
bends at the rear end,· Figure 8.
It was found that the greater part of the water
condensed in the first two sections swept past these
drain holes and collected in the lower tubes and el­bows
or was swept on and out into the separator,
overloading the separator drain and causing a con­siderable
loss of water. Sheet metal gutters inside
the header casting and leading to the drain hoies
remedied the condition only to a slight extent and
these drains were finally replaced by a series of tubes
attached by fittings to the lowest elbows on the rear
lower header. The fi t tings may be seen at the bottom
of Figure 8.
Operation with the new drains proved fairly satis­factory,
although from inspection of the drainage
systems of Models I and II it will be seen that the
form er has the better chance since the last drains are
only 10 feet from the outlet end and hence are in a
position to t ake care of the water C'omlensed in the
first fi ve-sixths of the condenser.
SUGGESTIONS
It has been thought advisable to assemble some of t he
general points bearing on furt her work. These apply,
of course, only to apparatus of the same general type
as that exemplified by Models I and II.
Probably the most obvious fault of either condenser
has been clogging of separator drains. It is of the ut­most
importance that these be made Jarge and smooth
with no shoulders or contractions of any kind whicl;
might catch and retain any clogging ma terial. It would
doubtless be advisable to make these drains of a di­ameter
of not less t.han ! inch. A noncorrosive ma te­rial,
such as Benedict nickel or Mone! metal could
readily be drawn thin and light for this purp~se and
should serve admirably for making joints.
In laying out a separator it is well to keep toward
the lower limit of gas velocity. The limiting values a re
about 900 and 500 feet per minute, but 600 should
prove an acceptable figure. Below 500 feet per minute
the velocity is too low to throw out the water, while
above 900 the drops deposited on the baffles are again
torn off and return to the gas stream.
In assembling a condenser care must be taken to
avoid stressing any tube so highly tha t it is distorted at
the entrance to the header nipple. Such a distortion
is a_ weak point which is apt to crack in service, just as
a kmked wire is weakened.
The wires used to connect adjacent headers must be
designed with regard not only to strength but to elas­ticity,
since the linear expansion of the t ubes is enough
to stress them severely as long columns if this is not
taken care of by stretching of the wires.
It is quite possible that rearrangement of the tubes
in order to decrease the velocity in the first section and
increase it in the last section would result not only in
decreased back pressure but also in increased effici~ncy
of heat transfer with a corresponding decrease in
weight.
If a condenser of the type of Model II could be so
mounted that the long dimension of cross section were
vertical it might simplify the draining problem to
some extent since only four drain connections would be
necessary on the lowest nipples of the second and last
section.
The greatest care must be employed in coupling the
condenser to the engine of an airship in order to avoid
transmission of the engine vibration s, as · well as to
eliminate excessive stresses on the flexible connections.
It is al so essential that a condenser be mounted so that
only the end where the exhaust enters is rigidly an­chored
to the car in a fore-and-aft direct ion. The
other end must be free to allow over-all expa nsion of t he
unit.
The most promising fi eld for decreasing the weight of
condensers of this t ype appears to be r educt.ion of t he
w~ight of the h eaders . Castings can· doubtless be
designed which will weigh fa r less t han t hose used and
the possibility of building up headers of Rheet ~1.ock
should be worth investigating.
'T
111:THOC OF CONNECtlN&
CONOl':NSl:R BI/IYJf~
FIG. 39
0
T
DETIIILS OF CONDENSER 1tAwrw• NO.
.SUP-JOINTS .JK·/O-f