909 Naperville Road, Wheaton, Illinois 60187
When reading the July-August 1971 issue of The Iron-Men Album
Magazine, page 14, article ‘Let’s Keep ‘Em Puffing’
by Merlin Elrod, I felt compelled, being an engineer, to comment on
the article, in that there are some parts of it which are not
correct. I say this more in the interest of the younger generations
which may read it and in the interest of correct engineering, and
trust it will be taken as ‘constructive criticism.’
To give you a little of my background, I started working on
steam traction engines in 1916, finished two years of Agricultural
Engineering College in 1921, studied mechanical engineering by
Correspondence School to become a Registered Professional Engineer
in four States, and have been a consulting engineer since 1930
principally on steam power plants. I am 72 years old.
My interest in steam traction engines is a part of my hobby,
plus steam locomotives; both from a mechanical history standpoint.
So far it has been interesting.
Now on to the items mentioned in above article that do not agree
with present day engineering practice.
(1) a new fire built on only one side of the grate
would not raise steam very quickly due to the amount of cold air
passing through the other side. A kindling wood fire with a small
amount of coal thereon over the entire grate is a much better
starting and steaming fire. When the grate has incandescent coals
on the whole grate, then the alternate side to side firing is good
to reduce smoke and get quicker steaming. This respondent never
heard of a ‘thermo cap’ but appreciates the blower help to
quicker steaming.
(2) Anthracite or hard coal would be rather costly in
Illinois at $21 to $23 per ton and is much slower igniting than our
Midwest Bituminous coals which have higher volatile matter and
lower fixed carbon. The heat value of a pound of coal is seldom as
high as Mr. Elrod mentions since no coal as mined is 100% carbon,
however Pocahontas coal mined in the Virginia’s sometimes is as
high as 14,700 to 15,100 Btu., per lb., because it has a high fixed
carbon plus moderate hydrogen content. In Illinois bituminous coal
there is about 4.5 to 5.5% hydrogen, say 5% average, less the
hydrogen turned to water vapor during combustion which results in a
net hydrogen to burn to 62,000 Btu., per lb., or 4.113% of the
5.0%, so the hydrogen generates, .04113 x 62,000 = 2,550 Btu. The
fixed carbon content is about 65.75% producing .6575 x 14500 =
9,534 Btu., plus the sulphur content averages 3% or .03×4050 =1,215
Btu., The sum of the three items of heat value, 2,550 + 9,534 +
1,215 = 13,299 Btu., per lb., of dry coal. The coal ‘as
fired’ has moisture in it which is evaporated when the coal is
burned. Illinois coals have about 10% moisture so the ‘as
fired’ heat value is lowered by evaporating this moisture, or
100% minus 10% = 90% and .90 x 13,299 = 11,969 Btu., per lb.,
‘as fired’. We will use the latter figure later.
(3) It requires 970.3 Btu., to evaporate one pound of water from
and at 212OF., instead of Mr. Elrod’s 965.7 Btu. Now, one pound
of coal at 11,969 Btu., per lb., ‘as fired’ does not all go
into steam in the boiler. There is heat loss up the stack,
radiation loss from all steam and boiler water parts and loss due
to carbon out the stack and into the ashpan. All these losses add
up to quite a lot and the actual heat to steam is only about 62 to
68% of the heat value of the ‘as fired’ coal. Taking a
clean boiler, inside and fire side we may get 68% percent boiler
efficiency and I will use the 68%. So the heat of the coal into
steam is, lbs. water per lb. of coal = (11,969 x 0.68) divided by
970.3 = 8.39 lbs., ‘from and at’212oF. From Mr. Elrod’s
dimensions, there is actually 8.166 Sq. Ft. of grate surface, but
since he has used 3 Sq. Ft., as half the grate surface I will use
the same amount and the 10 lbs. of coal per Sq. Ft., he used which
results in the following amount of steam = 3.5 Sq. Ft. x 10 lbs.,
coal x 8.39 lbs. water = 293.65 lbs., steam ‘from and at’
212°F. This is considerable less than the Author or the article
calculated. One boiler horsepower equals 34.5 lbs., of steam
‘from and at’ 212oF., so 293.65 lbs., divided by 34.5 =
8.51 boiler horsepower. At 70 lbs., steam pressure and 100°F., feed
water to the boiler there is put into the steam 1116.7 Btu. derived
from 1184.1 Btu., heat in the steam at 70 lbs. pressure less the
feed water temperature of 100o-32oF., or 1184.1 less 68 = 1116.7
Btu., per lb. of steam at 70 lbs., pressure, since there is burned
35 lbs., of coal at 11,969 Btu., per lb., which equals (11,969 x 35
x .68) divided by 1116.7 = 255.1 lbs., of steam which at 29.98 lbs.
per boiler horsepower equals 8.51 horsepower. Mr. Elrod’s
figure of 30 lbs., steam per boiler horsepower is commonly used,
but his figure of 965.7 Btu., per lb., of steam ‘from and
at’ 212°F., has long since been changed to 970.3 as given in
Keenan and Keyes ‘Steam Tables’, 1st. Edition, Twenty-Fifth
printing 1953.
