TYPES OF STEAM ENGINES
There are various ways of classifying steam engines according to their
construction, the most common, perhaps, being according to speed. If
this classification is employed, they may be grouped under three general
headings: High-speed, from 300 to 400 revolutions per minute;
moderate-speed, from 100 to 200 revolutions; and slow-speed, from 60 to
90 revolutions; all depending, however, upon the length of stroke. This
classification is again sub-divided according to valve mechanism,
horizontal and vertical, simple and compound, etc. The different forms
of engines shown in the following illustrations show representative
types in common use for different purposes.
The Ball engine, as shown in Fig. 42, is a typical horizontal single
valve high-speed engine with a direct-connected dynamo. It is very rigid
in design and especially compact for the power developed. The valve is
of the double-ported type shown in Fig. 2, having a cover plate for
removing the steam pressure from the back of the valve. The piston is
hollow with internal ribs similar to that shown in Fig. 29, and is
provided with spring packing rings carefully fitted in place. The
governor is of the shaft type, having only one weight instead of two, as
shown in Fig. 37.
[Illustration: Fig. 42. The Ball Engine]
The Sturtevant engine shown in Fig. 43 is a vertical high-speed engine
of a form especially adapted to electrical work. Engines of this general
design are made in a variety of sizes, and are often used on account of
the small floor space required. In the matter of detail, such as valves,
governors, etc., they do not differ materially from the high-speed
horizontal engine.
Fig. 44 illustrates a moderate-speed engine of the four-valve type.
These engines are built either with flat valves, or with positively
driven rotary or Corliss valves, the latter being used in the engine
shown. It will be noticed that the drop-lever and dash-pot arrangement
is omitted, the valves being both opened and closed by means of the
wrist-plate and its connecting rods. This arrangement is used on account
of the higher speed at which the engine is run, the regular Corliss
valve gear being limited to comparatively low speeds. All engines of
this make are provided with an automatic system of lubrication. The oil
is pumped through a filter to a central reservoir, seen above the center
of the engine, and from here delivered to all bearings by gravity. The
pump is attached to the rocker arm, and therefore easily accessible for
repairs.
The standard Harris Corliss engine shown in Fig. 45, is typical of its
class. It is provided with the girder type of frame, and with an
outboard bearing mounted upon a stone foundation. The valve gear is of
the regular Corliss type, driven by a single eccentric and wrist-plate.
The dash pots are mounted on cast-iron plates set in the floor at the
side of the engine, where they may be easily inspected. The governor is
similar in construction to the one already described, and shown in Fig.
27. The four engines so far described are simple engines, the expansion
taking place in a single cylinder. Figs. 46 to 48 show three different
types of the compound engine.
[Illustration: Fig. 43. The Sturtevant Vertical Engine]
The engine shown in Fig. 46 is of a type known as the tandem compound.
In this design the cylinders are in line, the low-pressure cylinder in
front of the high-pressure, as shown. There is only one piston rod, the
high-pressure and low-pressure pistons being mounted on the same rod.
The general appearance of an engine of this design is the same as a
simple engine, except for the addition of the high-pressure cylinder.
The governor is of the shaft type and operates by changing the cut-off
in the high-pressure cylinder. The cut-off in the low pressure cylinder
is adjusted by hand to divide the load equally between the two
cylinders for the normal load which the engine is to carry.
[Illustration: Fig. 44. Moderate Speed Engine of the Four-valve Type]
The engine shown in Fig. 47 is known as a duplex compound. In this
design the high-pressure cylinder is placed directly below the
low-pressure cylinder, as indicated, and both piston rods are attached
to the same cross-head. The remainder of the engine is practically the
same as a simple engine of the same type.
[Illustration: Fig. 45. The Harris Corliss Engine]
Fig. 48 shows a cross-compound engine of heavy design, built especially
for rolling mill work. In this arrangement two complete engines are
used, except for the main shaft and flywheel, which are common to both.
The engine is so piped that the high-pressure cylinder exhausts into the
low-pressure, through a receiver, the connection being under the floor
and not shown in the illustration. One of the advantages of the
cross-compound engine over other forms is that the cranks may be set 90
degrees apart, so that when one is on a dead center the other is
approximately at its position of greatest effort.
Selection of an Engine
The selection of an engine depends upon a number of conditions which
vary to a considerable extent in different cases. Among these may be
mentioned first cost, size and character of plant, available space,
steam economy, and utilization of the exhaust steam. The question of
first cost is usually considered in connection with that of operation,
and items such as interest and depreciation are compared with the saving
made through the saving in steam with high priced engines.
[Illustration: Fig. 46. The Skinner Tandem Engine]
[Illustration: Fig. 47. American Ball Duplex Compound Engine]
The principal use of the stationary engine is confined to the driving of
electric generators and the furnishing of motive power in shops and
factories. For the first of these uses, in cases where floor space is
limited, as in office buildings, and where the power does not exceed
about 100 I. H. P., the simple non-condensing high-speed engine is
probably employed more than any other type. For larger installations, a
saving may usually be made by the substitution of the moderate-speed
four-valve engine. The question of simple and compound engines in this
class of work depends largely upon the use made of the exhaust steam. In
winter time the exhaust is nearly always utilized in the heating system,
hence steam economy is not of great importance, and the simple engine
answers all purposes at a smaller first cost. In localities where the
heating season is comparatively short and fuel high, there is a decided
advantage in using compound engines on account of their greater steam
economy when operated within their economical range as regards load.
[Illustration: Fig. 48. The Monarch Corliss Engine]
In large central plants where low cost of operation is always of first
importance, it is common practice to use the best class of compound
condensing engines of moderate or low speed. Those equipped with some
form of Corliss valve gear are frequently found in this class of work.
In the generation of power for shops and factories, where there is
plenty of floor space, low-speed engines of the Corliss type are most
commonly used. When space is limited, very satisfactory results may be
obtained by using the moderate-speed four-valve engine. In deciding upon
an engine for any particular case, the problem must be studied from all
sides, and one be chosen which best answers the greatest number of
requirements.