Steam Engines Part 4

[Ill.u.s.tration: Fig. 41. Decreasing Steam Consumption by Condensing]

The _thermal efficiency_ of an engine is the ratio of the heat transformed into work to the total heat supplied to the engine. In order to determine this, the _absolute_ temperature of the steam at admission and exhaust pressures must be known. These pressures can be measured by a gage, and the corresponding temperatures taken from a steam table, or better, the temperatures can be measured direct by a thermometer. The absolute temperature is obtained by adding 461 to the reading in degrees Fahrenheit (F.). The formula for thermal efficiency is:

_T__{1} - _T__{2} ----------------- _T__{1}

in which

_T__{1} = absolute temperature of steam at initial pressure.

_T__{2} = absolute temperature of steam at exhaust pressure.

_Example_:--The temperature of the steam admitted to the cylinder of an engine is 340 degrees F., and that of the exhaust steam 220 degrees F.

What is the thermal efficiency of the engine?

(340 + 461) - (220 + 461) Thermal efficiency = ------------------------- = 0.15 (340 + 461)

The _mechanical efficiency_ is the ratio of the delivered or brake horsepower to the indicated horsepower, and is represented by the equation:

B. H. P.

Mechanical efficiency = -------- I. H. P.

in which B. H. P. = brake horsepower, I. H. P. = indicated horsepower.

All engines are designed to give the best economy at a certain developed indicated horsepower called full load. There must, of course, be more or less fluctuation in the load under practical working conditions, especially in certain cases, such as electric railway and rolling mill work. The losses, however, within a certain range on either side of the normal load, are not great in a well designed engine. The effect of increasing the load is to raise the initial pressure or lengthen the cut-off, depending upon the type of governor. This, in turn, raises the terminal pressure at the end of expansion, and allows the exhaust to escape at a higher temperature than before, thus lowering the thermal efficiency.

The effect of reducing the load is to lower the mean effective pressure.

(See Figs. 38 and 39.) This, in throttling engines, is due to a reduction of initial pressure, and in the automatic engine to a shortening of the cut-off. The result in each case is an increase in cylinder condensation, and as the load becomes lighter, the friction of the engine itself becomes a more important part of the total indicated horsepower; that is, as the load becomes lighter, the mechanical efficiency is reduced.

Effect of Condensing

So far as the design of the engine itself it concerned, there is no difference between a condensing and a non-condensing engine. The only difference is that in the first case the exhaust pipe from the engine is connected with a condenser instead of discharging into the atmosphere.

A condenser is a device for condensing the exhaust steam as fast as it comes from the engine, thus forming a partial vacuum and reducing the back pressure. The attaching of a condenser to an engine may be made to produce two results, as shown by the work diagrams ill.u.s.trated in Figs.

40 and 41. In the first case the full line represents the diagram of the engine when running non-condensing, and the area of the diagram gives a measure of the work done. The effect of adding a condenser is to reduce the back pressure on an average of 10 to 12 pounds per square inch, which is equivalent to adding the same amount to the mean effective pressure. The effect of this on the diagram, when the cut-off remains the same, is shown by the dotted line in Fig. 40. The power of the engine per stroke is increased by an amount represented by the area enclosed by the dotted line and the bottom of the original diagram.

a.s.suming the reduction in back pressure to be 10 pounds, which is often exceeded in the best practice, the gain in power by running condensing will be proportional to the increase in mean effective pressure under these conditions. For example, if the mean effective pressure is 40 pounds when running non-condensing, it will be increased to 40 + 10 = 50 pounds when running condensing, that is, it is 50/40 = 1.25 times as great as before. Therefore, if the engine develops 100 I. H. P. under the first condition, its final power will be increased to 100 1.25 = 125 I. H. P. under the second condition.

Fig. 41 shows the effect of adding a condenser and shortening the cut-off to keep the area of the diagram the same as before. The result in this case is a reduction in the quant.i.ty of steam required to develop the same indicated horsepower. The theoretical gain in economy under these conditions will run from about 28 to 30 per cent for simple, and from 20 to 22 per cent for compound engines. The actual gain will depend upon the cost and operation of the condenser which varies greatly in different localities.

CHAPTER V

TYPES OF STEAM ENGINES

There are various ways of cla.s.sifying steam engines according to their construction, the most common, perhaps, being according to speed. If this cla.s.sification 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 cla.s.sification is again sub-divided according to valve mechanism, horizontal and vertical, simple and compound, etc. The different forms of engines shown in the following ill.u.s.trations 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.

[Ill.u.s.tration: 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 s.p.a.ce required. In the matter of detail, such as valves, governors, etc., they do not differ materially from the high-speed horizontal engine.

Fig. 44 ill.u.s.trates 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 cla.s.s. 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 ill.u.s.tration. 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 s.p.a.ce, 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.

[Ill.u.s.tration: Fig. 46. The Skinner Tandem Engine]

[Ill.u.s.tration: Fig. 47. American Ball Duplex Compound Engine]

The princ.i.p.al use of the stationary engine is confined to the driving of electric generators and the furnis.h.i.+ng of motive power in shops and factories. For the first of these uses, in cases where floor s.p.a.ce 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 subst.i.tution of the moderate-speed four-valve engine. The question of simple and compound engines in this cla.s.s 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.

[Ill.u.s.tration: 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 cla.s.s of compound condensing engines of moderate or low speed. Those equipped with some form of Corliss valve gear are frequently found in this cla.s.s of work.

In the generation of power for shops and factories, where there is plenty of floor s.p.a.ce, low-speed engines of the Corliss type are most commonly used. When s.p.a.ce 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.

CHAPTER VI