ECONOMY IN RUNNING A FARM ENGINE.--(CONT.)
PRACTICAL POINTS.
The first practical point in the direction of farm engine economy is
to note that the best work can be done only when every part of the
engine and boiler are in due proportion. If the power is in excess of
the work to be done there is loss; if the grate surface is too large
cold air gets through the fuel and prevents complete combustion, and
if the grate surface is too small, not enough air gets in; if the
steaming power of the boiler is too large, heat is radiated away that
otherwise could be saved, for every foot of exposed area in the boiler
is a source of loss; if the steaming power of the boiler is too low
for the work to be done, it requires extra fuel to force the boiler to
do its work, and any forcing means comparatively large loss or waste.
It will be seen that not only must the engine and boiler be built with
the proper proportions, but they must be bought with a nice sense
of proportion to the work expected of them. This requires excellent
judgment and some experience in measuring work in horsepowers.
GRATE SURFACE AND FUEL.
The grate surface in a firebox should be not less than two-thirds
of a square foot per horsepower, for average size traction engines.
If the horsepower of an engine is small, proportionately more grate
surface will be needed; if it is large, the grate surface may be
proportionately much smaller. An engine boiler 7×8×200 rev., with 100
lbs. pressure, should have a grate surface not less than six square
feet, and seven would be better. In a traction engine there is always
a tendency to make the grate surface as small as possible, so that the
engine will not be cumbersome.
Another reason why the grate surface should be sufficiently large is
that forced draft is a bad thing, since it has a tendency to carry the
products of combustion and hot gases through the smokestack and out
into space before they have time to complete combustion and especially
before the heat of the gases has time to be absorbed by the boiler
surface. A large grate surface, then, with a moderate draft, is the
most economical.
The draft depends on other things, however. If a great deal of fine
fuel is thrown on a fire, the air must be forced through, because it
cannot get through in the natural way. This results in waste. So a fire
should be as open as possible. Coal should be “thin” on the grates;
wood should be thrown in so that there will be plenty of air spaces;
straw should be fed in just so that it will burn up completely as it
goes in. Moderate size coal is better than small or fine. Dust in coal
checks the draft. A good engineer will choose his fuel and handle his
fire so that he can get along with as little forced draft as possible.
In a straw burning engine a good circulation of air can be obtained, if
the draft door is just below the straw funnel, by extending the funnel
into the furnace six inches or so. This keeps the straw from clogging
up the place where the air enters and enables it to get at the fuel so
much more freely that the combustion is much more complete.
We have already suggested that in firing with coal, the fresh fuel be
deposited in front, so that the smoke will have to pass over live coals
and so the combustion will be more complete. Then when the coal is well
lighted it can be poked back over the other portions of the grate. This
method has another advantage, in that the first heating is usually
sufficient to separate the pure coal from the mineral substances which
form clinkers, and most of the clinkers will be deposited at that one
point in the grate. Here they can easily be lifted out, and will not
seriously interfere with the burning of the coal as they would if
scattered all over the grate. Clinkers in front can easily be taken
out by hooking the poker over them toward the back of the firebox and
pulling them up and to the front. They often come out as one big mass
which can be easily lifted out.
The best time to clean the grate is when there is a good brisk fire.
Then it will not cause steam to go down. Stirring a fire does little
good. For one thing, it breaks up the clinkers and allows them to run
down on the grate bars when they stick and finally warp the bars.
If the fire is not stirred the clinkers can be lifted out in large
masses. Stirring a fire also creates a tendency to choke up or coke,
and interferes with the even and regular combustion of the coal at all
points.
The highest heat that can be produced is a yellow heat. When there is a
good yellow heat, forced draft will only carry off the heat and cause
waste. It will not cause still more rapid combustion. When the heat is
merely red, increased draft will raise the temperature. Combustion is
not complete until the flame shows yellow. However, if the draft is
slight and time is given, red heat will be nearly as effective, but it
will not carry the heated gases over so large a part of the heating
surface of the boiler. With a very large grate surface, red heat will
do very well. Certainly it will be better than a forced draft, or an
effort at heating beyond the yellow point.
BOILER HEATING SURFACE.
The heat of the furnace does its work only as the heated gases touch
the boiler surface. The iron conducts the heat through to the water,
which is raised to the boiling point and turned into steam.
Now the amount of heat that the boiler will take up is directly
in proportion to the amount of exposed surface and to the time of
exposure. If the boiler heating surface is small, and the draft is
forced so that the gases pass through rapidly, they do not have a
chance to communicate much heat.
Also if the heating surface is too large, so that it cannot all be
utilized, the part not used becomes a radiating surface, and the
efficiency of the boiler is impaired.
Practice has shown that the amount of heating surface practically
required by a boiler is 12 to 15 square feet per horsepower. In
reckoning heating surface, all area which the heated gases touch is
calculated.
Another point in regard to heating surface in the production of steam
is this, that only such surface as is exposed to a heat equal to
turning the water into steam is effective. If there is a pressure
of 150 lbs. the temperature at which the water would turn to steam
would be 357 degrees, and any gases whose temperature was below 357
degrees would have no effect on the heating surface except to prevent
radiation. Thus in a return flue boiler the heated gases become cooled
often to such an extent before they pass out at the smokestack that
they do not help the generation of steam. Yet a heat just below 357
degrees would turn water into steam under 149 lbs. pressure. Though it
has work in it, the heat is lost.
Another practical point as to economy in large heating surface is that
it costs money to make, and is cumbersome to move about. It may cost
more to move a traction engine with large boiler from place to place
than the saving in fuel would amount to. So the kind of roads and the
cost of fuel must be taken into account and nicely balanced.
However, it may be said that a boiler with certain outside dimensions
that will generate 20 horsepower will be more economical than one of
the same size that will generate only 10 horsepower. In selecting an
engine, the higher the horsepower for the given dimensions, the more
economical of both fuel and water.
The value of heating surface also depends on the material through which
the heat must penetrate, and the rapidity with which the heat will
pass. We have already pointed out that soot and lime scale permit heat
to pass but slowly and if they are allowed to accumulate will greatly
reduce the steaming power of a boiler for a given consumption of
fuel. Another point is that the thinner the iron or steel, the better
will the heat get through even that. So it follows that flues, being
thinner, are better conductors than the sides of the firebox. Long
flues are better than short ones in that the long ones allow less soot,
etc., to accumulate than the short ones do, and afford more time for
the boiler to absorb the heat of the gases.
Again, we have stated that heating surface is valuable only as it is
exposed to the gases at a sufficiently high temperature. Some boilers
have a tendency to draw the hot gases most rapidly through the upper
flues, while the lower flues do not get their proportion of the heat.
This results in a loss, for the heat to give its full benefit should be
equally distributed.
To prevent the heat being drawn too rapidly through upper flues, a
baffle plate may be placed in the smoke box just above the upper flues,
thus preventing them from getting so much of the draft.
Again, if the exhaust nozzle is too low down, the draft through the
lower flues may be greater than through the upper. This is remedied
by putting a piece of pipe on the exhaust to raise it higher in the
smokestack.
EXPANSION AND CONDENSATION.
We have already pointed out that economy results if we hook up
the reverse lever so that the expansive force of the steam has an
opportunity to work during half or three-quarters of the stroke.
One difficulty arising from this method is that the walls of the
cylinder cool more rapidly when not under the full boiler pressure.
Condensation in the cylinder is a practical difficulty which should be
met and overcome as far as possible.
High speed gives some advantage. A judicious use of cushion helps
condensation somewhat also, because when any gas like steam or air is
compressed, it gives off heat, and this heat in the cushion will keep
up the temperature of the cylinder. This cannot be carried very far,
however, for the back pressure of cushion will reduce the energy of the
engine movement.
LEAD AND CLEARANCE.
Too much clearance will detract from the power of an engine, as there
is just so much more waste space to be filled with hot steam. Too
little clearance will cause pounding.
Likewise there will be loss of power in an engine if the lead is too
great or too little. The proper amount of lead differs with conditions.
A high speed engine requires more than a low speed, and if an engine
is adjusted for a certain speed, it should be kept uniformly at that
speed, as variation causes loss. The more clearance an engine has the
more lead it needs. Also the quicker the valve motion, the less lead
required. Sometimes when a large engine is pulling only a light load
and there is no chance to shorten the cut-off, a turn of the eccentric
disk for a trifle more lead will effect some economy.
Cut-off should be as sharp as possible. A slow cut-off in reducing
pressure before cut-off is complete, causes a loss of power in the
engine.
THE EXHAUST.
If the exhaust from the cylinder does not begin before the piston
begins its return stroke, there will be back pressure due to the
slowness with which the valve opens. The exhaust should be earlier in
proportion to the slowness of the valve motion, and also, in proportion
to the speed of the engine, since the higher the speed the less time
there is for the steam to get out. It follows that an engine whose
exhaust is arranged for a low speed cannot be run at a high speed
without causing loss from back pressure.
In using steam expansively the relative proportion between the back
pressure and the force of the steam is of course greater. So in using
steam expansively the back pressure must be at a minimum, and this is
especially true in the compound engine. So many things affect this,
that it becomes one of the reasons why it is hard to use a compound
engine with as great economy as theory would indicate.
Another thing, the smallness of the exhaust nozzle in the smokestack
affects the back pressure. The smaller the nozzle, the greater the
draft a given amount of steam will create; but the more back pressure
there will be, due to the inability of the exhaust steam to get out
easily. So the exhaust nozzle should be as large as circumstances
will permit. It is a favorite trick with engineers testing the pulling
power of their engines to remove the exhaust nozzle entirely for a few
minutes when the fire is up. The back pressure saved will at once show
in the pulling power of the engine, and every one will be surprised. Of
course the fire couldn’t be kept going long without the nozzle on. We
have already pointed out that a natural draft is better than a forced
one. Here is another reason for it.
LEAKS.
Leaks always cause a waste of power. They may usually be seen when
about the boiler; but leaks in the piston and valve will often go
unnoticed.
It is to be observed that if a valve does not travel a short distance
beyond the end of its seat, it will wear the part it does travel on,
while the remaining part will not wear and will become a shoulder. Such
a shoulder will nearly always cause a leak in the valve, and besides
will add the friction, and otherwise destroy the economy of the engine.
Likewise the piston will wear part of the cylinder and leave a shoulder
at either end if it does not pass entirely beyond the steam-tight
portion of the inside of the cylinder. That it may always do this and
yet leave sufficient clearance, the counterbore has been devised. All
good engines are bored larger at each end so that the piston will pass
beyond the steam-tight portion a trifle at the end of each stroke. Of
course it must not pass far enough to allow any steam to get through.
Self-setting piston rings are now generally used. They are kept in
place by their own tension. There will always be a little leakage at
the lap. The best lap is probably a broken joint rather than a diagonal
one. Moreover, as the rings wear they will have a tendency to get loose
unless they are thickest at a point just opposite to the lap, since
this is the point at which it is necessary to make up for the tension
lost by the lapping.