CHAPTER II.

WORK AND EFFICIENCY.

Force, work, and energy.-Force is that which acts in producing or resisting motion in a body, and may be represented by a pressure or a pull. The British unit of force is the weight of one pound avoirdupois, and forces are therefore expressed generally as being equal to so many pounds weight.

A force is said to perform work when by its action resistance is overcome and motion produced. This union of force and motion is essential to the conception of work. However great the pressure applied, unless the body acted on be moved, no work is done. Energy is the term used to signify the capacity of a body for doing work. For example, if a force acts through a certain distance it is said to exert energy, while the resistance overcome through a certain distance by means of this exertion is the work done.

Measurement of work and energy.-The amount of work done is measured by multiplying the magnitude of the resistance-or, in other words, the force opposing the motion by the distance through which the resistance is overcome, estimated in the direction of the resistance. Energy is measured in a similar manner.

The British unit of work is the foot-pound, which is a very convenient term, implying the combination of force and motion, which is the essential condition for the performance of work. One footpound represents the amount of work done in raising a weight of one pound through a distance of one foot, or more generally the exertion of a pressure of one pound through the distance of one foot. If 20 pounds be raised 50 ft., the amount of work performed is represented by 20 × 50 =1,000 foot-pounds.

Sometimes for convenience other units of work are used, but they are all formed on the same basis and expressed in a similar manner. For example, the work performed in raising one ton one inch is sometimes called one inch-ton, and it is equal to 2,240 inch-pounds or 2240 foot-pounds. The work of lifting one ton one foot is one foot-ton, and so on. It will be seen that the different terms used are selfexplanatory and are convertible one to another. The foot-pound is, however, the general unit, the others only being employed for convenience in special cases.

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Power, horse-power.-In the conception of work and energy no question of time enters. When, however, we consider also the time taken to perform so much work, we are considering power. Just as

the term work necessarily involves distance, so does the term power

involve time as well as distance. Power may be defined as the rate at which work is done. The natural unit of power would be the power of doing work at the rate of one foot-pound per minute, but it is too small to be convenient in engineering. The unit of power adopted is the power of doing work at the rate of 33,000 foot-pounds per minute. This unit of power is termed a horse-power.

Efficiency. In every machine there are always certain causes acting that produce waste of work, so that the whole work done by the machine is not usefully employed, some of it being exerted in overcoming the friction of the mechanism, and some wasted in various other ways. The fraction representing the ratio that the useful work done bears to the total energy exerted on the machine is called the Efficiency of the machine; or

In the propelling apparatus of a vessel, in which the useful work is measured by its effect in giving speed to the ship, there are four successive stages, in each of which a portion of the initial energy is wasted, and these four stages all tend to decrease the total efficiency.

In the first place, only a portion of the heat of combustion of the coal is communicated to the water in the boiler, the remainder being wasted in various ways. The fraction of the total heat that is transmitted to the water in the boiler is, in ordinary cases, not more than 6 7

from to This fraction is called the Efficiency of the boiler. 10 10'

Secondly. The steam, after leaving the boiler, exerts energy on the piston of the engine; but, in consequence of the narrow limits of temperature between which the engine is worked, this energy represents only a small fraction of the total heat contained in the steam. The fraction varies very considerably, depending on the type of engine, its steam pressure, rate of expansion, &c.—say from to In large modern marine engines it may be taken as from to This fraction,

1
5 20'
1 1
5 9'

representing the ratio of the energy exerted by the steam to the total amount of heat expended on it, is called the Efficiency of the steam.

Thirdly. In the engine itself, a part of the energy exerted by the steam on the pistons is wasted in overcoming the friction of the work ing parts of the machinery, and in working the pumps, &c. The remainder appears as useful work in driving the propeller. The fraction, representing the ratio that this useful work bears to the total energy exerted by the pistons, is called the Efficiency of the mechanism. 8 8/3/

Its value is from to the former being more general.

10 10'

Fourthly. The propeller, in addition to driving the ship, expends some of the energy transmitted to it in agitating and churning the water in which it acts, and the work thus performed is wasted; the only useful work being that employed in overcoming the resistance of the ship and driving her through the water. The ratio of this useful work to the total energy expended by the propeller is called

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WORK AND EFFICIENCY

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the Efficiency of the propeller. It may be taken as averaging from 5 6 to

10 10°

The resultant Efficiency of the marine steam-engine or the whole propelling apparatus is made up of the four efficiencies just stated, and is given by the product of the four factors representing respectively the efficiencies of the boiler, the steam, the mechanism, and the propeller. Any improvement in the efficiency of the marine steam-engine, and, consequently, in the economy of its performance, is therefore due to an increase in one or more of these elements, and we shall deal with these several points, and in each case describe the efforts that have been made to increase the efficiency.

The efficiency of the marine steam-engine will be seen to be very low. Taking the best case indicated by our figures, viz. that of an engine which has the maximum efficiency in each of the four components of the resultant efficiency, the efficiency would be :

The highest efficiency now attainable is, therefore, a little over 7 per cent. with the marine steam-engine, and is generally less-say nearer 5 or 6 per cent.

Further information respecting this is given under the respective headings in greater detail.

CHAPTER III.

HEAT AND ITS EFFECT ON WATER.

In order to comprehend the principles on which the construction and performance of the steam-engine depend, and the object of the various improvements that have from time to time been introduced, it is necessary that the true nature and properties of heat should be known. We will, therefore, as concisely as possible, state the principal points relative to this subject, in order that the succeeding chapters may be clearly understood.

Temperature. The temperature of a body may be defined as the extent to which it may be capable of communicating sensible heat, or heat that may be felt, to other adjacent bodies.

When two bodies of different temperatures are placed in contact with each other, it is a well-known fact that the hotter body becomes cooler and the colder body hotter, till at length the two bodies become of the same temperature, after which no change in the temperature takes place. This is caused by the passage of heat from the hotter to the colder body, and shows clearly that heat is something that can be transferred from one body to another, so as to diminish the amount of heat in the former body and increase it in the latter.

Effect of and nature of heat. When heat is added to or abstracted from a body, one of the two following effects is produced: either the temperature of the body is altered, or its state is changed. For example, if heat be added to water under the atmospheric pressure, the temperature is increased until it reaches 212° Fahr. After this the addition of heat does not further increase the temperature, but causes the water to evaporate and become steam-that is, it changes the condition from that of a liquid to that of a gas. Again, if heat be abstracted from water, the temperature is reduced till it reaches 32° Fahr., after which the diminution of heat does not further decrease the temperature, but changes the condition of the water from the liquid to the solid state, forming ice. The quantities of heat passing from one body to another can thus be estimated by the effects produced, so that it is clear that heat is something that can be both transferred and measured.

The true nature of heat has been determined by experiments on friction. It is a matter of common observation that the work expended in friction is apparently lost-that is, it appears no longer in the form of mechanical work; but at every place where friction occurs, heat is developed, and the greater the friction the greater is the amount of heat produced. Experiments have shown that the amount of heat generated by friction is exactly equivalent to the

amount of work lost, and we therefore infer that heat is of the same nature as mechanical work that is, it is one of the forms of energy.

British thermal unit. The unit by which heat is measured is called a thermal unit, and in British measurements represents the quantity of heat necessary to raise one pound of water at its maximum density, which corresponds to a temperature of about 39° Fahr., through one degree Fahr.

Joule's equivalent. The honour of determining the exact relation between heat and mechanical work belongs to Mr. Joule, who proved, by an elaborate series of careful experiments on the friction of oil, water, mercury, and other substances, that one thermal unit is equal to 772 foot-pounds of mechanical work '--that is, that the quantity of heat necessary to raise the temperature of one pound of water at its maximum density, one degree Fahr., can be made to perform work equal to the raising of 772 lbs. one foot high. In honour of the discoverer this important constant, 772, expressing the relation between heat and mechanical work, is called Joule's equivalent, and is frequently denoted by the letter J.

The convertibility of heat and work, in a definite ratio, is expressed in the following statement, generally known as the mechanical theory of heat, viz. Heat and mechanical energy are mutually convertible, and heat requires for its production, and produces by its disappearance, mechanical energy in the proportion of 772 foot pounds for each unit of heat. This statement forms also the first law of the science of thermo-dynamics.

Communication of heat.-Heat may be communicated from one body to another in three different ways, viz., by radiation, conduction, and convection.

Radiation. Radiant heat is given off from hot bodies in straight lines, and the rays of heat are subject to the same laws as the rays of light. As far as the generation of steam is concerned the useful radiation is confined to the furnace, the crowns and sides of which, intercepting the rays of heat from the burning fuel, become themselves heated, and the heat passes through them to the water in the boiler. A considerable amount of heat is given off by radiation from burning coal, and it is very important, therefore, to intercept this, and to insure that as far as possible the whole of the heat diffused in this way should be transmitted, either directly or indirectly, to the water in the boiler, and not wasted on the external air or other bodies.

Radiation is an important item to be considered with reference to the economical employment of steam, for it always causes a certain loss of heat, and unless proper precautions are taken this loss may become very considerable.

The surfaces of the boilers, steam-pipes, cylinders, &c., when the engines are at work, are very much hotter than the surrounding bodies, and consequently, in order that loss of heat by radiation may be avoided as far as possible, all those surfaces should be clothed with some non-conducting material. Hair-felt has been largely employed for this purpose, and this is usually kept in its place by an outer

1 Subsequent experiment appears to show that the exact value is somewhat (nearly 1 per cent.) higher than this-viz., 778.