Box 146, County Homes Road, Mt. Royal, New Jersey.
The injector is often thought of as a brass casting with
mysterious powers that somehow propels water from the feed tank
into the boiler. Often they fail to work at a very important moment
and often the common remedy of the bucket of cold water dousing
does little to alleviate the problem. Hauling the fire and shutting
down is then the only alternative, although sometimes a pressure
water hose applied to the injectors suction could save the
situation, but not always.
There are many kinds of them including the inspirator which is
only the same thing, except the jet is started first through the
overflow before opening the discharge valve to the feed check. I
think the make most associated with ‘Iron-Men’ is the
Penberthy, though the Metro and the Sellers may come onto the
scene. There were many others made by earlier companies that have
long since fallen by the wayside, but their product lives, on, and
often lay covered with dust perched up in the rafters of a barn. A
little understanding of their ‘mysterious inards’ could
possibly bring some of them back to work.
Our magical box was first invented by a French engineer named
Henri Giffard in 1858 and it appears to have been introduced into
this country by a William Sellers around 1860. In the original
Giffard the steam nozzle was made to advance and retard into the
combining cone and the jet was only established by fiddling with
the steam valve and water valve, plus adjusting the steam nozzle to
the correct depth. Sellers overcame this by his self-adjusting
injector in 1876 and later the fixed nozzle type. I have no history
of the Penberthy Company, but if someone has, I’m sure
‘Iron-Men’ would gladly print it.
Why does an injector feed water into a boiler? That is the
question! (Seems I’ve heard that before somewhere.) Well, from
an old 70-year handbook I have, may I quote: ‘An injector works
because the steam imparts sufficient velocity to the water to
overcome the boiler pressure.’ If we have a boiler running at
180 p.s.i. and we placed a thermometer inside, we would find that
the steam and the surface of the water were much the same
temperature, say 375 degrees F., but steam contains lots more work
units and can expand many times its volume. One pound of water by
weight, has far from the energy of one pound of steam because of
the heat stored within. This is why our injector will not work on
air pressure.
The whole secret to an injector is velocity, or feet per second
of the steam. Fixing a nozzle to our previously mentioned boiler of
180 p.s.i., we would find that the speed of the steam leaving the
nozzle would be around 3600 ft./sec. Now suppose we use water
instead of steam at the same 180 p.s.i. we would find only a
velocity, or speed, of 160 ft./sec. Divide 3600 by 160 and we get a
ratio of 22 to one.
ORIGINAL FORM OF THE GIFFARD
Whenever a jet of gas or liquid comes into contact with
whatever, it naturally drives it away, like blowing the chips off
your lathe with an air hose. This force is called the
‘momentum’ and is equal to the propelling jet in one sec.
by its velocity in one sec. To obtain this action in the injector,
steam leaves the nozzle and enters a funnel, or to give it its
correct name, ‘combining tube.’ This causes water to enter
all around the steam jet which cools the steam and causes it to
reduce in size. Now the magic box has been given its sucking
ability. At the same time this tremendous velocity of 3600 ft./sec.
is being given to the water. Each particle of water entering the
combining tube is driven faster and faster on by the atoms of steam
moving much more rapidly until the steam has totally condensed and
combined with the water into one fast and solid moving jet. This is
the reason for the funnel of combining cone, and its taper is equal
to the length of time it takes to condense all of the steam
jet.
PENBERTHY INJECTOR
A. STEAM CONE
B. COMBINING CONE
C. LOWER COMB & DELIVERY CONES
D. PLATE AIR RELEASE VALVE
E. OVER FLOW VALVE.
X. IT IS IMPORTANT THAT PARTS SEAL AT THESE LOCATIONS
Water now leaves the combining cone orifice, as we have
established, at a high velocity. O.K. now we nave to get it in the
boiler against our still 180 p.s.i. We know that our steam jet
travelled at 3600 ft./sec. Now if the suction created by the
condensing steam jet draws in say 10 lbs. of water per sec. at a
speed of 40 feet, its momentum is 10 times 40 which is 400. Add
this to our 3600 and we get 4000. We have also to remember that the
condensed steam jet is also an addition to the 10 lbs. of water we
sucked in, so with the weight of the jet per sec. it could be 12
lbs. Now if we take the figure of 4000 and divide it by 12, we get
333.3 ft./sec.
Going back to the jet of water discharging at 180 p.s.i. we
found that the velocity of the water was 160 ft./sec. So, all we
have to do is establish a jet of water over and above 160 ft./sec.
and it has got to overcome the p.s.i. of the boiler. Theoretically,
we have a jet moving at 333.3 ft./sec, but of course, like
everything else, we have losses and water friction within the cones
etc. The velocity would be more like 200 ft./sec, but still well
above the 160 ft./sec. required.
Having absorbed a little arithmetic, it should now be quite
evident how our injector works and simplify some of the mysteries
that surround them. In my sketch of the Giffard the parts are not
entirely complete but the main parts are shown so that its function
can be ascertained. To go over it simply, the steam entrance is
marked and its path is through the pet cock and down the opening
where it meets holes in the sliding tube. It now travels through
these into the inside of the sliding tube and by cranking open the
steam adjuster, it enters the steam cone. The remainder of its
action is the same as stated above. The only other movement not
mentioned is the cone adjuster which moves the steam cone back and
forth.
Getting to the Penberthy, you will notice that there is really
very little difference in principle than that of the Sellers,
except the steam cone is now fixed and a plate valve is fitted half
way up the lower section of the combining tube. This has to be a
good fit to make a fairly good air tight seal.
Previously I said that Penberthy was the type most widely used
by us ‘Steam Blokes’ so I have also submitted a drawing of
some of the steam nozzles I checked, and give their measurements.
Four inch size were taken apart, 2 of them from our roller 10541
and 2 supplied by Bob Hartzell. In each case the combining and
delivery cones were the same taper and the same throat size, this
being .118. The taper as near as I could figure, was 9 degrees in
the combining cone and about 5 degrees in the delivery.
For the steam cones, 2 were the same, hence only 3 drawn. For
the life of me I cannot remember which of the first two I put back
in the roller, although I believe it was the cone with a throat
size of .158. Cone #3, I did put back on the right hand side. It
works well with only 40 p.s.i. and the other works also, but does
not feed so low. By the way, the thread in the nozzle is not to
cause the steam to twist as I have heard, its merely for screwing
in a bolt so it can be withdrawn easily.
To make a few of these cones in the home shop would not be a
difficult task should anyone want to experiment, and by using the
formula I have given, the tapers would be easy to cut. To explain
this, big D is the diameter of the large end of the bore, small
‘d’ is the diameter of the small end, and ‘L’ is
the length of the taper. Looking at cone #1, big ‘D’ is
.296. Small ‘d’ is .158 and ‘L’ is .593. Subtract
i.e. .296 minus .158 is .138. Multiply L .593 by 2 and we get
1.186. Now divide .138 by 1.186 which equals .116. Most machinists
have a book containing natural trigonomical signs, so looking up
.116 we find it equals 6 degrees 40 mins. Swing the compound of
your lathe around to 6 degrees and you’re not far off.
STEAM CONES PENBERTHY
Looking at the Penberthy sketch you will find ‘X’ at
various point, and these are joints that must be air tight. The
screwed joint at the top ‘B’ tube usually fits fairly well,
so not much to worry here. The flapper ‘E’ valve is also
usually good unless a bit of rust gets under the seat and becomes
stuck. The plate valve ‘D’ should be a nice sliding fit up
and down the tube, but if worn a light lathe cut would clean it up.
By doing this though, you would have to make a new plate valve with
a smaller bore to suit the freshly turned tube. The two X’s at
the right of ‘C’ are the most critical locations and must
be air tight especially at the lower X which applies the pressure
to hold cone ‘C’ in place. I suppose one good way of
checking this would be by using a piece of lead wire between at
location lower X and screw in the cap. Even a pencil mark on the
edge would do. If a joint is made the pencil mark would disappear.
If it did not, then a couple of thousandths would have to be faced
off at Cap shoulder Y, but not the bottom of the injector.
Lastly, I would like to say that I am no expert on injectors,
but having had trouble with our roller, I thought I would carry out
a little research and seek out as much knowledge as I had books
available. Books used: ‘Machinery’s Handbook,’
‘Audells Power Plant Guide,’ ‘Maxims and Instructions
for the Boiler-room’ by N. Hawkins, and ‘Sellers Handbook
of Injectors.’ It was my enjoyment writing this article and
drawing the sketches, and I trust you chaps will find the
information interesting.
