cooling it. Why? Because the tension of the vapour generated equals that of the natural tension of the water; but condense this vapour by a second application of cold water, and again it begins to boil, even with the temperature below 180 ̊F. A knowledge of these facts is most important to the engineer, for it shows him that in the condensers of large engines, he must provide air-pumps of sufficient capacity to carry off the steam vapour generated at even low temperatures. It was but recently that an acquaintance of the author's, overlooking this point, put in a set of very small air

pumps to a pair of marine engines which he was constructing, under the impression that, all that was necessary was to lift the condensed water, and that marine engineers generally, were putting on air-pumps out of all proportion to the work to be done! He soon discovered his mistake, for, on the day of the trial trip, he could not keep up a vacuum above a few inches. In addition to the steam vapour which is generated at pressures below the atmospheric pressure, any air which may have come over with the steam at once expands on a reduction of pressure, and has to be sucked away at every stroke, otherwise it will spoil the vacuum. The experiment of raising the boiling point by raising the pressure is easily done. Procure a flask, as in the former experiment, with a tight-fitting stop cock. Half fill the flask with water, heat it with the cock open until the water boils and all the air has been expelled, then shut the stop cock. The steam now generated rises in pressure and temperature. The increasing pressure raises the boiling point and thus stops the violent ebullition, unless heat is applied very rapidly. Allow the temperature to rise, say to 240° F., then slightly open the cock, ebullition is at once observed, although the pressure is equal to two atmospheres above a perfect vacuum. The presence of salt or dirt in water raises the boiling point for any particular pressure.

LECTURE X.-QUESTIONS.

1. Describe in your own words the several effects which take place in succession on applying heat to a lump of ice enclosed in a cylinder.

2. Distinguish between (1) wet or saturated steam, (2) dry or dry saturated steam, (3) superheated steam.

3. How many units of heat are absorbed in converting 1 lb. of water at 212° F. into 1 lb. of dry saturated steam? Suppose the I lb. of water were only converted into wet or saturated steam, what then? Why?

4. What is meant by the Boiling Point of a liquid? State the ordinary boiling point of fresh water open to the atmosphere, also when subjected to pressures of 30, 45, and 60 lbs. respectively.

5. Sketch and describe how you would illustrate that water can be made to boil below as well as above 212° F.

LECTURE Xa.

CONTENTS.-Work Done during the Conversion of Water into Dry SteamDefinitions of Internal and External Work-Efficiency of SteamEfficiency of High Pressure Steam-General Expressions for External and Internal Work during Evaporation-Example I.-Heat Rejected to Condenser-Example II.-Partial Evaporation-Example III.— Generation of Steam in a Closed Vessel-Questions.

Work Done during the Conversion of Water into Dry Steam. We can now give a more definite account of the distribution of heat expended during the conversion of water into steam, and thus prepare the way for a more thorough understanding of the economical use of steam in a steam engine.

An ordinary steam engine consists essentially of

1. A boiler wherein the steam at a given pressure is generated from water at a given temperature.

2. A cylinder containing a movable, steam-tight piston, on which the steam acts and does useful work.

3. Frequently, another part, called the condenser, is added. The function of the condenser is exactly the opposite of that of the boiler. For in it, the steam is converted back again into water after passing through the working cylinder. Engines having only the first two essential parts are called non-condensing, whilst those consisting of the three parts are called condensing engines. These three organs are usually quite distinct and separate from each other, the connections being made by pipes, valves, &c. For our present purposes it will be best to leave out of account all connections such as pipes and valves. We shall therefore suppose the boiler, working cylinder and condenser to be one and the same vessel. Also, we shall neglect all losses of heat, such as that due to radiation, conduction, &c. Further, we shall, in the meantime, consider the case of 1 lb. of water at an initial temperature of 32° F., raised into dry steam at 212° F. The pressure of the steam is, therefore, that due to atmospheric pressure-viz., about 14° 7 lbs. per square inch.

Take a cylindrical vessel fitted with a weightless, frictionless, and steam-tight piston, and place between the piston and the bottom of the vessel 1 lb. of water at 32° F. The cylinder being

I

open at the top the pressure on the piston will be constantly that due to the atmosphere. For convenience, suppose the cross

Р

-26-36ft

-016!

ILLUSTRATING EXTERNAL WORK DONE DURING EVAPORATION OF I LB. OF WATER FROM AND AT 212° F.

sectional area of the cylinder to be one square foot (or 144 square inches). Then,

Total pressure on piston = P=144 × 14°7 = 2116.8 lbs. Since 62.5 lbs. of fresh water occupy a volume of 1 cubic foot,

The cross area of the cylinder being 1 square foot, it follows that the under surface of the piston will be '016 foot above the base of the vessel.

By applying heat to the bottom of the vessel the temperature of the water will be ultimately raised to 212° F. The heat expended in this operation is (212-32) = 180 B. T. U. Now, the volume of the 1 lb. of water at the end of this operation is slightly greater than 016 cubic foot, as shown by the graphic figure on page 67. The piston has, therefore, been raised by a small amount, and consequently work has been done in overcoming the atmospheric resistance. We thus see that rather less than 180 B. T. U. are employed in increasing the molecular kinetic energy of the water. This increase in the volume of the water between 32° F. and 212° F. is so small (being only •016 × 043 = '000688 cubic foot (see Fig. page 67)* that it may

* The volume at 32° F. of a certain quantity of water is (as shown by the figure and text at page 67)=1000127, and at 212° F. =1043. The difference is practically 043. Consequently if a certain weight of water occupies about unit volume at 32° F. and increases by 043 unit when its

=

safely be neglected. The piston therefore remains almost stationary between these two temperatures.

Continuing the application of heat to the water at 212° F., the water becomes evaporated and the piston rises rapidly, whilst the temperature remains constant. Suppose the source of heat to be withdrawn just when the last particle of the 1 lb. of water has been converted into dry steam. Then we know that 966.6 B. T. U. have been spent in bringing about this change. The piston will now be at a considerable height above the base of the vessel, and, consequently, a certain fraction of the latent heat will have been employed in doing work against atmospheric pressure. Referring to column 5 of the "Table of the Properties of Saturated Steam" on page 107, we notice that I lb. of dry steam at atmospheric pressure (temperature 212° F.) occupies a volume of 26.36 cubic feet. Hence the piston will now stand at a height of 26.36 feet above the base of the vessel. The vertical displacement of the piston is, therefore, 26'36-'016=26.35 feet approximately.

Thu', of the 966'6 B. T. U. of latent heat, 72°25 B. T. U. are employed in doing mechanical work external to the substance (water) which is undergoing a change of state; while the remainder (894 35 B. T. U.) is spent in bringing about internal changes.

DEFINITION.—The energy spent in bringing about internal or molecular changes in a substance is called Internal Work, and that spent on bodies external to the substance is called External Work.

The student must carefully distinguish between internal and external work. The former represents energy in the substance itself, whether in the form of molecular kinetic energy or that due to change of state; the latter represents energy which has passed out of the substance to external bodies.

temperature is raised to 212° F., what will be the increase in volume of 016 cubic foot of water under the same circumstances?