experiments with superheated steam - ideals
TRANSCRIPT
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EXPERIMENTS WITH SUPERHEATED STEAM
BY
Charles Ludwig Holl
THESIS FOR THE DEGREE OF BACHELOR OF SCIENCE
IN MECHANICAL ENGINEERING
IN THE
COLLEGE OF ENGINEERING
OF THE
UNIVERSITY OF ILLINOIS
PRESENTED JUNE, 1906 H
UNIVERSITY OF ILLINOIS
June , 1 190 G
THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY
CHARLES LUDWIG HOLL
entitled EXPERIMENTS WITH SUPERHEATED STEAM
IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE
of Bachelor of Science in Mechanical Engineering
head of department of Mechanical Engineering
UKX
CONTENTS
.
Page .
Superheated Steam 1
History and Early Application
Teste with Superheated Steam on Steam Engines 6
Tests with Superheated Steam on Steam Turbines
Reasons for Gain by Superheating 32
Test upon an Engine at the Meoh. Eng. Lab., Univ. of 111.. 34
Conclusions 39
SUPERHEATED STEAM
.
Superheatod steam is steam at any pressure at a temperature,
higher than the temperature of its evaporation frera water at that
pressure. Saturated steam may be superheated by imparting addition-
al heat to it at a constant pressure during which process its vol-
ume increases. Steam cannot be superheated in the presence of the
water from which it was evaporated, owing to the fact that the wat-
er takes up the heat and evaporates into further saturated steam.
The four principal methods of superheating steam to-day are,
first, heating by a furnace, independent of the boiler furnace:
second, heating by the hot gases passing from the boiler furnace to
the chimney: third, heating by the hot gases before these have
ceased to act on the boiler; and fourth, by a fluid which is it-
self heated by the hot gases to a higher temperature than the re-
quired temperature of the superheated steam
»
HISTORY AND EARLY APPLICATION.
Superheated a team is by no means a new thing. I may mention
a patent of Becker, a mechanic from Strassburg, dated 1827, and
two patents of Quillac of 1849. In June, 1850, Monoheui got a pat-
ent for a superheater composed of a bundle of tubes placed in the
interior of a flue into which the hot gases from the furnace
passed. Another of the earliest recorded attempts at superheating
was that reported in 1828 by Richard Trevithick, at the Birnie
Downs mine in Cornwall, on a condensing pumping engine making eight
strokes per minute, with a boiler pressure of 45 pounds, in which
the cylinders and steam pipes were surrounded with brickwork and
heated from a fire burning on a grate underneath. The results were
remarkable, for, while performing the same amount of work, 9,000
pounds of coal were used per 24 hours without the fire under the
cylinder, against 6,000 pounds when it was in use, the coal for
superheating included. This experience led Trevithick to the in-
vention of his tubular boiler and superheater, which was patented
in 1832, and was a remarkably modern-looking arrangement. The
boiler consisted of a series of vertical tubes, connected at the
top and bottom to a hollow ring. These tubes were placed so that
they formed a vertical flue in which the superheating pipes were
placed. The ste?„m from the boiler tubes passed through these sup-
erheating tubes on its way to the engine. Owing, no doubt, to the
difficulty in regulating the temperature of the steam obtained from
such an apparatus, little seems to have been done in the matter
during the next fifteen years, although in 1832, I. Howard, of Ber-
raondsey, produced a superheater which obtained an economy of 30
3.
per cent, and about 1335, Dr. Haycroft, of Greenwich, advocated sup-
erheated steam and found, experimentally, about the same saving.
From 1850 to 1865, the interest in this subject revived con-
siderably, and moderate degree of superheat was quite extensively
employed toward the latter part of that period. Hirn, in 1857,
published the results of experiments made by him at Colmar, which
were the most carefully conducted tests that had, up to that time,
been carried out, and showed that, on a simple engine, working with
a boiler pressure of 55 pounds, economies of 20 to 47 per cent
could be obtained with superheat of 100 degrees F. In 1859, John
Perm read a paper before the Institution of Mechanical Engineers
describing several applications to steamships, the superheater con-
sisting of a number of tubes about 2 inches in diameter placed in
the uptake just as it left the boiler, through which the steam
passed on its way to the engine. The superheating surface was about
15 per cent of the boiler heating surface, and with a boiler pres-
sure of 20 pounds on a condensing engine, about 20 per cent saving
in fuel was obtained with a superheat of 100 degrees F. John Ryder
in I860, in a paper before the same society described the Parson
& Pilgrim superheater, which consisted of two horse -shoe shaped
pipes placed in the internal flue of the boiler over the fire grate
and the Partridge, which was a cylinder filled with tubes through
which the gases passed, the steam being around them. Both of these
systems were stated to have given good results, which may appear
rather questionable in the case of the former; but a total of 5,000
horse -power had then been equipped. The superheating surface em-
ployed was 2 l/2 to 2 3/4 square feet per nominal horse -power, of
about the same as that described by Perm, and the economies ob-
tained wore practically equal.
About this time, high prensure 3team came into extensive use
and the suporheating of steam was abandoned. With the Introduction
of hydrocarbon lubricating oils, balanced valves and improved pack-
ing, the use of considerably higher temperaturos became possible,
and in 1890, interest in superheated steam was renewed by the at-
tention drawn to the results being accomplished in Germany by such
engineers as Gehre, Schwoeter, Uhler and others. Many roault3 were
published, all of which showed a gain from the practice and in
England, in addition to the introduction to their apparatus, sever-
al other types were developed along similar lines. Since that date
the progress of superheating in stationary practice has been com-
paratively steady, although slow, and it has in general developed
along the lines of moderate superheat, say from 50 degrees to 100
degrees, no special type of engine or boiler being usually employed
and its subsequent history, with one exception, concerns station-
ary and marine practice rather than locomotives. That exception is
the use of highly superheated steam, perfected after 14 years' ex-
perimenting by Mr. Wilhelm Schmidt, of Cassel, who succeeded about
1394 in producing a boiler and engine in which steam, superheated
to 700 degrees F. v<ras used at comparatively low pressures. Moder-
ate superheating, employing temperatures up to 150 degrees above
that of evaporation, first scientifically investigated by Him, in-
troduced extensively into early English marine practice by Penn and
others and then abandoned; continued in stationary practice in Al-
sace by Hirn's pupils and followers, has, in the last 10 or 15
years, become extensively used in stationary practice with in-
creasing steam pressures. High superheating, developod by Schmidt,
has been economical and satisfactory where employed, but so far, in
not in goreral use
.
TESTS WITH SUPERHEATED STEAM.
The following are results of tests with superheated steam,
which have been published in the various engineering magazines
from time to time, on various plants where superheated steam has
been employed.
ON STEAM ENGINES.
Test at Vienna on a horizontal, triple expansion, 4 cylinder,
condensing engine of 3,4000 H.P., 90 R.P. T '., showed a consumption
of 9.31 pounds of steam per horse-power hour and 1.32 pounds of
coal per horse-power hour. The steam temperature at the cylinder
was 572 degrees F. and the steam pressure was 180 pounds
.
Tests on a compound engine using superheated steam, located
at the Milbourne Mills in Philadelphia, gave some very interesting
results. The engine was of the Rice and Sargent horizontal, cross-
compound, condensing type, especially designed for using highly
superheated steam. A Schmidt superheater, separately fired, was
used for superheating the steam. The high pressure cylinder of the
engine was furnished with double beat poppet valves and the low
pressure cylinder with Corliss valves. A portion of the highly
superheated steam furnished to the engine passed through a coil in
the receiver between the high and the low pressure cylinders. By
this means, the steam was highly superheated for the low pressure
cylinder as well as for the high pressure cylinder. The exhaust
steam passed from the engine to a Worthington jet condenser con-
nected with a Worthington vacuum pump. Engine was • 16 x 28 x 42
inch stroke, 102 R.P.M. The steam used by the engine was furnished
by an Edge Moor water tube boiler, rated at 370 home -power.
In tests with saturated steam, the engine indicated 407 horse-
power, and with superheated steam, the engine indicated 420 horse-
power. On the latter tests, the temperature was 808 degrees P.,
which was 374 degrees above normal temperature of the steam.
On the test with superheated steam, the consumption of 3teara
was 9.56 pounds per indicated horse -power per hour, while on the
test with saturated steam, the consumption was 13.84 pounds. There
is a difference here of' 31 per cent in favor of the superheated
steam. No such difference, however, appeared in the coal consump-
tion, for it required a much larger quantity of coa-l to evaporate
the water and superheat the steam than it did to simply evaporate
the water. The actual evaporation per pound of dry coal without
the superheater was 8.59 pounds. YiJhen the superheater was in use,
however, the evaporation was only 7.09 pounds. Using these figures
of evaporation in connection with the steam consumption of the en-
gine, the actual amount of coal consumed was 1.35 pounds per indi-
cated horse-power hour, when the steam was superheated: and 1.61
pounds when it was saturated. The economy of superheating referred
to coal consumption, therefor, i3 reduced to about 16 per cent.
The superheat of the steam entering the low pressure cylinder was
145 degrees and entering the high pressure cylinder was 300 degrees.
Test at the Spring Creek Pumping Station of the Brooklyn Water
Works. The equipment of this station consisted of two horizontal
return tubular boilers, each 60 inches in diameter, by 15 feet long,
having sixty -six 3 inch tubes and 914 square feet of heating sur-
face. One of the boilers was equipped with a Foster superheater.
8
Each boiler had 25 square feet of grate and natural draft. The
pumping plant consisted of two Knowles Compound condensing direct-
acting duplex pumps, 6 l/2 and 11 l/2 x 16 x 24 inches, and one
Davidson Triple condensing direct-acting simplex pump, size 8 and
14 and 24 x 24 x 24. These three pumps were equipped with independ-
ent jet condensers, the exhaust steam from which was used to heat
the feed water to the boilers. The superheater was connected up to
the old steam main, making it possible to run a test on each boiler
under the same conditions.
Two tests Yfere run- one using the boiler containing the Foster
superheater and a second using the boiler without the superheater.
In both tests the main object was to determine the amount of work
which could be developed for 100 pounds of coal. Coal and feed
water supplied to the boiler during both tests were carefully
weighed. By comparison, it was shown that 25 l/2 per cent more
work was developed from 100 pounds of coal and 30.2 per cent more
work was developed from 1000 pounds of steam when superheated a-
bout 200 degrees than was developed when the steam was saturated.
Also that the same amount of work war accomplished with a saving
in coal of 20.2 per cent and a saving in feed water of 23.2 per
cent. The results of these tests are given in a table on page 16.
In a test of five days' run without the superheater the sta-
tion pumped 21,956,400 gallons of water with 31,760 pounds of coal
while with the superheater, the station records show that it
pumped 22,337,500 gallons of water and burned 23,750 pounds of
coal, showing a saving of about 25 per cent. No special arrange-
ments were made for the introduction of superheated steam and no
special oil or stuffing box packing nor steam pipe gaskets were
used
.
Test on a 400 H.P. Willans engine, located at Lancashire, Eng-
land which has been running for eight years. A superheater was in-
troduced into the downtake of the boiler. The amount of superheat
obtained in the engine was not definitely known, but, so far as
could be ascertained it was somewhere between 50 and 100 degrees,
F. The length of the steam pipe between the boiler and engine was
ISO feet and it was found that before the superheat was employed,
that there was 18 per cent of moisture present in the engine at
the stop valve. The comparative figures obtained from the test
were as follows: the water per indicated horse -power per hour was
13.7 pounds using superheated steam: 16.03 pounds with saturated
steam; showing a gain in favor of the superheater of 1.2S pounds,
or 14.8 per cent. The coal per I. H.P. per hour was 1.76 pounds
with the superheater and 1.89 pounds with the saturated steam,
showing a gain in favor of the superheater of 0.13 pounds, or 6.88
per cent.
Upon page 17 are given the results of a test by Professor
Schroeter on a 75 H.P. Schmidt engine and boiler. The engine was
a vertical tandem compound; 12 l/4 !' x 27 3/l6" x 19 l/2" . Profess-
or Schroeter also reports on a similar engine, a little larger
however, of 90 horse power, using superheated steam, which engine
used only 9.95 pounds of steam per horse power per hour and 1.43
pounds of coal, certainly a very remarkable result for such a
small engine
.
In 1896, Professor Ripper of England published the result of a
ay
10
series of tests of a small Schmidt engine using superheated steam.
The size of the engine was 7 l/l6 x 11 3/4; it made 180 revolutions
per minute. He used both saturated steam and stea-m of different
degrees of superheat in order to show the influence of the same.
The results are given in a table on page 17.
It will be apparent from this table how rapidly the consump-
tion of steam decreases with the increase of the superheat . With
338 degrees of superheat the number of pounds of steam used is not
quite one-half of that used for saturated steam. In itself this
performance of a single expansion, non-condensing engine, indicat->
ing only some 18 horse power and using less than 20 pounds of steam
per horse power per hour is very startling. »
In 1895, a large, triple expansion engine for superheated
steam was installed in Augsburg, Bavaria. The old boilers were to
be used and these could not carry more than 85 pounds pressure; but
in connection with these Schwoerer superheaters were installed.
Professor Schroeter conducted some very exhaustive tests on this
plant, using both superheated and saturated steam in order -to find
out whether the moderately superheated steam showed any gain.
The dimensions of the engine were as follows:
Diameter of high pressure cylinder, 27 l/2 inches.
Diameter of intermediate cylinder, 43 3/8 inches.
Diameter of each of the two low pressure cylinders, 45 3/8 inches
Stroke of all pistons, 63 inches.
The engine made 60 revolutions per minute.
The main results of tests are given in table on page 17. Although
tho degree of superheat here is very moderate and the steam pres-
sure vory low,yet a decided gain in economy is shown.
A small Schmidt steam engine and boiler installed at AsCher-
leben, Germany, was also tented by Professor Schroeter. The cyl-
inder bad a diameter' of 9 7/8 inches and a stroke of 15 3/4 inches
and was single acting; it made 150 revolutions per minute. A ser-
ies of tests were made to sbcw the influence of the cut-off and of
various degrees of superheat. The most important results were as
given in table on page 18.
The results conclusively shov; that the cut-off has very much
less influence on the economy of the steam engine when running with
superheated steam than with saturated, because there is very lit-
tle change in economy between a cut-off of 16 per cent and one of
48 per cent, whereas it is probable that if the engine had been
running with saturated steam this difference would probably have
been at least 25 per cent. It is also well known that the inter-
change of heat between the steam and the cylinder walls is very
rapidly reduced with increased superheat, so that, for instance,
the heal, absorbed by the wall during the admission period is almost
zero, when the steam is superheated from three to fcur hundred de-
grees. The gain in economy due to the use of superheated steam is
not alone due to the greatly reduced condensation in the cylinder,
but also and perhaps in just as high a degree, to the greatly re-
duced interchange of heat between the walls of the cylinder and the
steam while it is in the superheated state, this being due to the
absence of water, which is always adhering to the walls when sat-
urated steam is being used. From a number of carefully conducted
tests, it has been shown that the steam at the end of the expansion
in the high pressure cylinder of a compound engine is still slight-
ly superheated, if the superheat to begin with is from 200 to 300
degrees Fahrenheit. This is especially true if the cylinder is
jacketed with superheated steam, and under such circumstances it
would almost seem that it should be possible to eliminate all con-
dencation from the high pressure cylinder, and at least reduce the
heat interchange to such an extent that it hardly needs to be con-
sidered. In a single expansion cylinder or engine this would not
be true, at least with the degree of superheat Which is used at
present, as necessarily the change in temperature of the steam
would be very much greater. To prove these statements, the results
of two sets of tests made by Professor R. Dcerfel, are given on
page 19; one is for a compound and one for a triple expansion en-
gine .
In the case of the compound engine, the direct comparison be-
tween superheated and saturated steam is made, and as will be seen
the amount of heat given to the wall of the cylinder during the ad-
mission period is reduced to about one fourth of what it is for
saturated steam. For the triple expansion engine, it will be seen
that the loss of heat is so small that it hardly needs to be taken
into account in practical engineering. It should also be remem-
bered that in both cases the degree of superheat is comparatively
low, and that still better results can be expected and are in fact
realized with a higher degree of superheat.
That the consumption of steam in both cases is very low should
also be noticed, in fact a great deal lower than can be realized
when saturated steam is used, but obviously if the degree of super-
heat had "been greater, still better results would have been ob-
tained .
On page 21 arc given the results of sixteen tents on a 250 II. P.
compound engine: these tests were made by Professor Schroeter for
the purpose of determining the gain in economy due to the super-
heating of steam to various degrees and also to determine the in-
fluence of the variation in the load on the economy, using super-
heated steam. Six tests were made with saturated steam and dif-
ferent loads, then five tests with superheated steam and for the
same loads with exception of one, in which the engine was empty,
and finally, five tests were made for the same load but with dif-
ferent degrees of superheat.
The engine was a tandem compound, having poppet valves and
run condensing. The cylinders were 12 5/4 inches and 22 inches in
diameter, the stroke being 32 1/2 inches, and it made 127 revolut-
ions per minute. The steam pressure was kept as nearly as possible
equal to 145 pounds by the guage during all the tests.
By equivalent consumption of steam is meant the reduction of
the number of pounds of superheated steam used to the equivalent
number of pounds of saturated steam of the same pressure, the re-
duction being made on tho basis of heat units contained. The com-
parison of economy on this basis is consequently, perfectly equit-
able. The results as Will be seen, are certainly startling, as the
best results shew 11.48 pounds for saturated and 10.01 (equivalent)
for superheated steam.
On page 18 are the results of some tests made at the Universi-
ty of Wisconsin on a 50 II. P. cross compound Mordberg engine, pro-
vidod with pcppct valves. The steam used, was taken from the gen-
oral "boiler house of the University; but before being delivered to
the engine it was passed through a superheater of the Schwocrer
type installed near the engine. The superheater was, therefore, of
the independent type and was fired witb coke during the teste. By
means of discharging compressed air under the grate it was possible
to got any desired superheat up to 200 degrees Fahrenheit, and also
to keep it fairly constant. The engine was used exclusively for
experimental purposes, the power being absorbed by a Prony brake.
The tests were made for the purpose of determining the gain in
economy by the use of superheated steam at different degrees. In
all, eighteen tests were made; four with saturated steam, four with
steam superheated on the average of 50 degrees Fahrenheit, four of
100 degrees Fahrenheit, four of 150 degrees Fahrenheit, and two of
200 degrees Fahrenheit. In table on page 18 are given only the
averages for each of the tests
.
The cylinders were 8 inches and 13 inches in diameter and the
stroke was 20 inches. During these tests the engine xvas run at
92 revolutions per minute.
The table shows plainly the increasing gain in economy with
the increasin7 degrees of superheat. This increase is necessarily
not as large when hased on the number of B.T.U. used as when based
on the number of pounds of nteam used, but the gain is very large
in either case when the degree of superheat is 192 degrees F. as
in the last tests.
On pages 19 and 20 are given two tables, showing the steam
consumption of Sulzer engines with saturated and with superheated
steam. The tents were made by Professor Schrooter, of Munich; Pro-
fessor Weber, of Zurich, and other eminent engineers . The peculiar
features of the Sulzer engine are long stroke, high piston Bpeed,
reaching sometimes nearly 850 feet per minute, and balanced poppet
valves with small clearance. The poppet valuro is peculiarly adapt-
ed to the use of superheated steam. The type of engines in Wir-
themberg and Bavaria are tandem with intermediate superheaters.
T~ES T^S A T THE kSpK/A/O ChEEK PUMPING
TA TION Or THE BPOOKL YN Wa TEH WotiKS
WITH AND W/THOUT SUPERHEATED S TEA M.
With W/thout
Superheater Supetheater
H o urs run
Average Steam Pressuref/6s
Average Water- Hressos-e,tr/p/e
exp theac/ /r? fee t
Average Co/npouric/ He crc7 ft? Pee?.
Vac uurn opSac 7/0/7 for
tr/p/e erne/ co/rrpycu/ya' /rrc/ies oErrrerct/ty
To fa/ Head orr 77-fp/e./feet of water
' Cofnpoc/rc7 "
" Double ^Stt-o/ces "7ri/?/e
Cotrrpaorrc/
" 6a//orrs p?ampect as measured
hp we. f'r
Per cetrf >S/t/2
Pact pourtc/s,wetr
Pota/ Coa/ Coat3 osrrecf
Pe.r cetrit Pe fas e
To fa/ PeEu^se
Peect Water
Daty pet- /OO //?~s. Coo/
IOOO /6s SteamAverogre Temp of \5teat?i
fea v/rrg -3up>er/ieafer
Average Pern/?, of S team
H-rrTer/ncjr Soperheater
Average E)egr/-ee~s "Soper/heat.
2479.3
0.99
7/
22.9
29.OS
33.04
30, SS73 S, 3 9S
2479 4
7. OS7/
23,2f
29 46
33.33
34y/ /4
-32., /<5TG
4, 492, 66 O 4,549, 4QO
22.3 22. 2
/, 763)<Sf^S (3 7/S4
)933 S96
&<rto
23.7 /<5. 7
7,/<3S 7 20336,3 99 ^50,960
23 206)6°>6 /8,486,4&3
30.3 0G>49<3 e$2S3
J2f3
s-27 ° r
320° *r
207 c'/=
17.
TevS / on a V5 H P Schmidt /Lngtne<% J3o//er
Pressure. A56.2 /S9 .2K"V)Brake Horse Power. K 6 / .72. ^ 6276
Superheat, Degrees P. $835.0 ^23 /.6»
\S>reo/77 /n Lbs, per /HPper- hour: v tO. 9/ \ ZO./9
6tea/7? /// Lbs,per 3raAe H Pper hour -A /2.6/ g Zd.34
Coa/ tn Lbs, per /.HPper hour-. / .36 0) /.26
Coq/ tn Los, per PnoAe HPper hour. / 57 /.Set
Test on a 5>c/hm/c/f Engine 7jg * //£ /60P.PM
A/a 3team Pres- Temperature Degrees Lbs 3Teamof sure
, Lbs, of of per /H.P.
Test. Abso /ute. 3 tea/77, Deg P Superheat. /H.R per- hour:
/ 67.5 689.*
16.55 39.332 79.9 412. 66. /6.3a 34.66O 66/ 590 26 / Z6.43 2 / .48
<7 87.6 667 338 76.53 Z9 /3
S //s 6 /a 86 3 /7. 73 /6<34
6 It 6.0 653 306 Z7.60 Z7.58
Pe^st on a /OOO HP 7r/p/e TPxparr^/on PPa/he
.
No. S tearr? Pres- Temperati/re Decrees Lbs v5 teamof sure, Lbs, of of pen /.HP
Test Quage
,
^teo^ Defyp 6 uperTzeat. /.HP per- hoen
/ 64.34 4/4.y 67.9 //6 0.5 /3.062 6335 4/6/ S8.0 / 167.3 /a. 66v3 84.8/ 480./ 32.3 //2 7, 6 /2 05
64.77 327/ POO.
6
/4.3/5 63.35 326./ 976.9 Z3.556 8v3.v92 326.6" 993./ /s.aa.
10
Tesf on a Schmidt P/,9/ne * /5j /SORFM,
fvo. <5teom /sets-—j—/e/rfperc//<yre /Degrees
OT f * . r\f~f\~.ur - or j k3 U/~G. f. t*s,
rt/~Of O/ ^yep rf, t.
{-yocyqi (S-
,
. y /-\t t-rr / ) / /~ —* L/JLJt._ / f 1K f-/ / A f/ r) / / /
f 16.1 120.0 669 77.852 22.9 l/S. V 67/ 77.59
3 35.
o
III .4 673 v32<S 16. 034 41/ 98.
7
660 0^3 17.54
5 48.0 95.6 689 354 /3.03
6 22.n5" 1 13.6 667 321 17.45
7 15.7 IIS. 6 5 74 227 /6<36
8 23.4 115, O 568 241 16.52
S 36.5 /OS. 1 597 2 06 13.93
to £6.1 90S 49/ /60 2 3.0<3
II £4.3 98.7 40S 7/ 27.68
Test on a 750 HP Cro^s^s Compound /VorcfSero- Png/ne
.
Alo of Tes t. / 2 v3 4 5Ind/cafecf Horse. Power: 3234 3133 3f 44 37.73 3f6373raAe f-for&e Power 30 27 30.31 29.76 30.35 303If3o/7er Pre*$<sor'e.
tGoage,
.
901 93.9 920 976 95.6
fe/rrp. &f 5/e<7/T7, /Dec?. 5 33 / 390.5 431.2 4333 525.9/Decrees of 3uper/heat, 570 990 747.9 /920Z. As. 5teaff77per f'ff.fr'~hour. 22 76 20.(39 21 22 /9f 7705Ga/r> fr? fZcorrorny based or/
<Sfec?/T7 Corzsamp f/or> per cerf 3 7 16 2//3 7~U. per fff.Pper f?o(/r 416 39/ 402 373 338Ga/rr fr? /5cor?o/r/y /ft
per ctzrrf f&secf or* P. PL7, 6 3 fO 79
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ON STEAM TURBINES
,
Test on a 500 K.W., Curtin turbine at New Port. With a super-
heat of 150.5 degrees at the throttle valve and 175.2 decrees at
the superheater, the steam consumption at full load was 17.79
pounds per kilowatt hour, which war, 10.1 per cent less than the
19.78 pounds consumed with dry saturated steam. When the super-
heating was increased to 289.6 degrees at the throttle valve and
354.4 degrees at the superheater, the steam consumption at full
load was 15.91 pounds per kilowatt hour, which is 19.6 per cent
less than the consumption with dry saturated steam.
On page 29 is given a table of efficiencies of the De Laval
Steam Turbine. The initial pressure was 105 pounds absolute; rev-
olutions per minute were 20,000. -Ran non-condensing. The table
shows that superheating the steam decreases not only the steam con-
sumption, but also the heat consumption per unit of power developed.
The available power of the turbine is at the same tine consider-
ably increased. These results are produced mainly by the higher
temperature and velocity. A portion of the economy is, however, due
to the decreased internal friction of the turbine when superheated
steam is used.
The steam consumption of the 1,250 K.W. Parsons turbine, lo-
cated at the Interborough Rapid Transit Co. station in New York
was 18.48 pounds per kilowatt hour at full load using superheated
steam at 161 pounds pressure and temperature of 439 degrees F.
The table on page 30 was prepared by Mr. Gentsch. In this
table the effect of high superhert can be seen from the results of
the Frankfort turbine. Mr. Gentsch has also computed the probable
27.
results for turbines of the samo power using steam superheated to
540 degrees P., and having a vacuum of 95 per cent. If the Milan
turbine had been working with such steam and vacuum, the consump-
tion of steam per K.W. hour would probably have been only 13.5
pounds; the actual consumption war 16.1 pounds, certainly a very
excellent result.
From Dean & Main's test of a 400 K.W. parsons turbo-generator
it was found that 13.63 pounds of dry saturated steam was used per
brake horse per hour, at full load; when the steam was superheated
to 100 degrees F. this was reduced to 12.97 pounds per hour, and
with 180 degrees F. superheat, to 11.17 pounds. The conclusion of
Dean & Main is that a gain of one per cent in economy accompanies
a raise in the superheat of every 10 degrees
.
Mr. Gentsch gives the following results from a test of a GOO
K.W. Curtis turbine making 1,500 revolutions per minute. The steam
pressure was 139 pounds and the vacuum, 28.5 inches.
Power, K. W. 800 550 425 250 100 75
Lbs. steam per K.W. hour 18.96 19.20 19.47 20.97 24.44 25.44
Running with the same pressure and vacuum, but with steam sup-
erheated 117 degrees P., the results were as follows:
Power, K. W. 800 550 425 250 100 75
Lbs. steam per K.W. hour 17.27 16.76 17.07 18.37 21.45 22.70
A small turbine was tested with steam of 199 pounds pres-
sure, the vacuum being 18.5 inches and the superheat 117 degrees F.
with the following results:
Power, K. W. 700 500 400 200 150 100
Lbs. steam per K.W. hour 15.95 16.21 16.46 17.95 18.96 21.45
28.
For all three teste the results are very good, and the im-
provement in economy due to both increased pressure and superheat
is very evident.
A test of a Curtis turbine built by the British Thomson-
Houston Co. and rated at 500 K.W., gave the following results:
Steam pressure, 150.1 pounds; superheat, 115. 2 degrees F.; vac-
uum, .754 pounds per square inoh; R.P.M., 1,800; power, 6G0 K.Y7.
Steam consumption per K.W, per hour, 18.41 pounds.
29.
Table oi Efficiencies of De Lava/ Turb/ne
Ha/f Load ru/i L oadSaturated Super heated S aturctted ^Svperheated
In t tt of7 Temp. 327 S60 S27 932
ZD/schar<?e Temp 0/ S ream. 2 /2 5& <3 2/ 2 650Brake Horse flower: 2/4 245 44.1 3/9Lbs Srearr?per 3 HP-Hour 4 75 37 O 3S 9 2S.3
3. T U. per B H P- Hour-
Heat consumpt/or? . 56, /OO 44,6 00 46, OOO 37, 3 OOT^es/duct/ Super/hectt
.
3; 600 3,3 /O
Economy over *sa/c7 <stratn
when res/duct/ superheat /s reco vered
.
3/%
Test on a O.OOO /KM/. Brown, 3over/ Parsons Turbine, Heatedat /Tank tor r on the Aio/n . Steampressure 765 * Temp. <jr72°t~
i4. 74 Lbs Steam per rX.W Aot/r- ar fu// /oad. /Vot /nciud/ny -steam
75.63 " " /# " , for auxi/ictrtes
.
/6 3 " ^
on Q it pounds per If/./ /hoar
7esf on a 2,000 tX W. Curt/s Tunb/ne, 6,600 t/oits, 750PPP7.
Load m /< \rV.
Superheat, Dec? Pl/crcuam, /aches
Lbs steamperP W-hour
5Team pnessute
Test on a 400 A' W. Westinghouse-Parsons Turbine."Steam pressure at throttie 753pounds
jvacuum 2& /n exnaustp/pe .
Steamper B.H' P 72 7 72 47 72.66 74.62 77.45 iH7BraAe Loact^ H.P. 75C3.9 594 6 4453 239.9 5923 762 6
Superheat 104 t09 704 67 760 73O
w/th otry satarateof
Steamper B.HP 73 63 73.9/ 7446 76.06
Bnahe Loads HP 7c\S.4 593.2 44 <3 247.3
637 /OOO aOOO 22702 75 242 242 25
O
262 2S.9 2S.73 2<S 7
20.7 76.3 753 76 2750 /60 755 iOO
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5
REASONS FOR GAIN BY SUPERHEATING . - The object of superheating
is to secure dry steam in the cylinder, and the actual gain in
practice which follows the use of superheated steam is due to the
more or less complete removal of the loss by cylinder condensation;
for when the working fluid is saturated steam, no transfer of heat,
however small in amount, can take place from the steam to the metal
without accompanying deposition of water in the cylinder, which,
during the exhaust stroke, is evaporated at the expense of the heat
of the cylinder walls.
The result is, that the mean temperature of the cylinder-
walls with saturated steam is much below that of the steam on en-
tering the cylinder. On the other hand, when the steam is suffici-
ently highly superheated it is in a far more stable condition than
before the superheat was added, and can part with the whole of its
superheat to the cylinder-walls without undergoing any liquefact-
ion.
The drier the steam at cut-off, the more work is done per
pound of steam passing through the cylinder. The drier the steam
at release, the less demand upon the cylinder-walls during exhaust
for heat of re-evaporation, and the higher the mean temperature of
the cylinder-walls. A dry cylinder at release parts with little
heat to the comparatively non-conducting medium passing away during
exhaust, hence the smallness of the heat-exchange between the steam
and cylinder walls under such conditions.
Superheating thus removes the principal source of loss of heat
by the walls, namely, water in the cylinder at release, and reduces
also the amount of the heat-exchange between the steam and the
walls to a minimum. Superheating also reduces the loss by leakage
between the slide valve and the face of tho ports.
7A .
TEST UPON A 10 x 10 IDEAL ENGINE.
PRELIMINARY REMARKS
.
These testa were made upon a 10 x 10 Ideal, high speed, auto-
matic engine, located in the Mechanical Engineering laboratory of
the University of Illinois, by C. L. Holl.
PURPOSE OF TEST.
The purpose of the tests were to determine the difference in
steam consumption with and without superheated steam.
APPARATUS
.
The engine upon which the tests were made was a 10" x lo"
,
high speed automatic, Ideal engine, built at the A. L. Ide & Sons
Works, Springfield, 111. The engine was rated at 60 horse -power.
The power delivered by the engine was measured by means of a prony
brake attached to one of the fly-wheels. The engine was connected
to a Wheeler condenser so that all steam passing through the engine
had to pass through the condenser. The water condensed in the con-
denser was weighed in a tank resting on a pair of scales. A pres-
sure guage was located at the throttle valve. A thermometer cup
filled with mercury was inserted in the valve chest in order to
measure the temperature of the steam. All steam before entering
the cylinder of engine had to pass through a Cockrane separator.
The saturated ^team used was taken direct from the main in the
laboratory. The superheated steam was taken from a Poster inde-
pendently fired superheater. Steam was taken from the main in the
laboratory and had to pass through the superheater v/hen superheat-
ed steam was used. A sketch of the superheater used is shown on
page 35. The red arrows show the path of the gases from the grate
.
56.
The green arrows show the circulation of the otoar through the sup-
erheating tubes. It will be seen that the entering steam comes in
contact v. ith the coolest tubes first and that in leaving, it is in
contact with the hotest tubes. These tubes were 2" outside dia-
meter and were covered with cast iron rings. A thermometer cup was
also placed in the pipe leaving the superheater at A, so that the
temperature of the superheated steam on leaving the superheater
could be determined. The pipe conveying the steam from the super-
hoater to the engine consisted of a 3 inch pipe, uncovered and a-
bout 65 feet long and 11 feet of 4 inch pipe, uncovered, with six
elbows in the whole line.
MANNER OF CONDUCTING TESTS.
All scales and guages were calibrated before using. The en-
gine was run some before the actual test was started so that every-
thing was practically constant when the test began. All the tests
were made with the engine delivering about 45 horse-power at the
brake. This was kept practically constant by adjusting the brake
so that the scales were always balanced. The test war, run for two
hours. Temperature and pressure of steam, R.P.M. of engine, pres-
sure in the condenser, and indicator cards were taken every fifteen
minutes. Readings were also taken of the temperature of the steam
at the superheater to determine drop or loss of superheat in the
piping. The condenser pressure was kept practically the same for
all of the tes ts
.
SUMMARY
.
Upon page 38 are given the results of the tests. It will be
seen that the results with superheated steam vary quite a little.
In test No. 3 , 11 per cent lean water was used than with saturated
steam, while in test No. 4, 6.2 per cent less water per brake horse
power was used. These figures are some what high but can be ac-
counted for to some extent. The slide valve which had a pressure
plate, leaked, as very little vacuum was shown on the indicator
cards and furthermore, when the valve was set so that all ports
were closed and steam turned on, steam leaked into the cylinder for
when the indicator cocks were opened, steam blew out of both of
them
.
An interesting result in connection with the tests was the
loss of superheat between the superheater and the engine. This
ranged from 150 degrees to 170. degrees which goes to show that
pipes must be covered in order to derive the best economy of a
plant with a superheater.
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CONCLUSIONS.
From the tables of results given, it will be seen that tho
water consumption of engines and turbines is cut down from 10 to 20
per cent per 100 degrees of superheat. This certainly warrants its
application to the modern power plant and locomotives . From the
various magazines of to-day, one cannot help but notice the new
plants which are being built for the use of superheated steam.
Superheating the steam of locomotives is slowly coming into
practice in this country but it is sure to be a common practice be-
fore many years have gone by. One reason for this is that the lo-
comotive service is more favorable to the use of superheated steam
than any other in which it has been tried. In stationary or marine
practice, where the superheater is either a separate boiler-like
device, in which no water is carried, or is so combined with the
boiler as to always be in close communication with its furnace, the
circulation of steam within the superheater tubes ceases when the
engine or turbine throttle is closed, and the metal of the tubes,
together with the entrapped steam within them, remains exposed to
the undiminished intensity of the furnace action. The tubes are
thus often heated to very high temperatures, a result which, when
frequently repeated, leads necessarily to a failure of the super-
heater. Again, when after an interval of inactivity, the engine is
started, the steam which has been held back within the superheater
until it has been raised to an enormously high temperature, passes
on to the engine, oftentimes retaining enough of its heat to burn
the lubrication and sometimes to destroy the rod packings. But all
difficulties of this class which, to a greater or lesser degree,
40
have appeared in the operation of every stationary engine plant
using superheated steam, are doubtless avoided in the locomotive,
for in this machine the rate of combustion varies with the volume
of steam used. When the throttle is open, the fire burns brightly;
when it is closed, its activity is at one suppressed. When there
is no steam passing within the tubes of the superheater, the gases
circulating around them are comparatively low in temperature, and
when the conditions are so changed that the temperature of the gas-
es becomes maximum, the volume of steam passing through tho tubes
is greatest. Just as the draught, of a locomotive responds to the
varying demands which are made upon the boiler, so the volume of
heat which is available for superheating varies with the quantity
of steam which is to be superheated.
The system which the author thinks will be the best system for
stationary practice is what is known as the"Schmidt System" . The
highly superheated steam which comes from an independently fired
superheater is passed through a set of coils which superheat the
steam exhausted from a high pressure cylinder before it enters 'the
low pressure cylinder and whatever superheat is left in the live
steam goes to the high pressure cylinder. In this way, condensa-
tion in both cylinders is decreased to practically nothing. Re-
sults of tests show that it has the highest saving in water con-
sumption of any system so far applied.
Superheating will air/ays be applied in turbines as the high
temperatures can be employed because there are no packings or lu-
bricants with which the steam can come into contact with so as to
destroy them. Furthermore, superheated steam reduces the internal
1O3O0S of a turbine to a great extent. Thin is true because with
superheated steam thore is no water preeont on the blades of the
turbine, which would cause excessive friction if it were present.
Superheating is superior to any other known means of reduction
of the internal wastes of the steam engine. Jacketing ordinarily
suppresses but a fraction of that waste, and multiple cylinder en-
gines alro have their limitations; while superheating may not only
extinguish it but may also check other wastes due to resistance due
to flow of the wet, denser steam through ports, valves, etc.
Superheating even a few degrees improves the performance of
engines and in the average case superheating one hundred degrees
F. will entirely extinguish the internal waste.
The more wasteful the engine, the larger the promise of gain
by superheating, and small engines will profit more by it than
large, slow speed engines and simple engines more than the multiple
cylinder engines.
The extent of superheating should be adjustable -not only to
the particular size and type of engine in view, but also in the
same engine to the extent to which expansion is carried.
REFERENCES
Trans. A. S. M. E., 1901 - Volume XXII.
Trans . A
.
s. M. E.
,
1904 - Volume XXV.
Journal - W. o . Hi • , 1905 - December.
Journal - w. G TP 1905 - August
.
