LECTURE IX.-QUESTIONS.

1. If a pound of water at 212°F. be mixed with x pounds of water at 60°, what is the value of x when the resulting temperature is 120°? Again, if a pound of steam at 212° F. be mixed with y pounds of water at 60°, find y when the resulting temperature is 120°. Account for the difference between x and y. Ans. x = 1.53 lb. ; y = 17.5 lbs.

2. What is the latent heat of steam? If a quantity of steam weighing one pound, and at a temperature of 212° F., is condensed in 100 lbs. of water at 60° F., what is the resulting temperature? Ans. 71°06.

3. If 2 lbs. of steam at 212° F. are passed into 30 lbs. of water at 70° F., what is the temperature of the water at the end of the operation? Ans. 139° 2.

4. In a jet condenser the temperature of the condensing water is 60° F., and that of the entering steam is 193° F. Also the condenser remains at a temperature of 120°. Under these conditions find the weight of condensing water per pound of steam which enters the condenser. Ans. 17:53 lbs.

5. How many pounds of water at 50° F. must be mixed with 1 lb. of steam at atmospheric pressure to give a temperature of 105° F. to the mixture? Ans. 19.5 lbs.

6. If there pass at the same time into the condenser, and from thence into the hot-well, 2 tons of water at 55° F. and 1.5 cwt. of steam at atmospheric pressure, what will be the resulting temperature? Ans. 95°6 F.

7. Hot-well 105° F., injection 53°, and steam at atmospheric pressure. Required number of pounds of steam condensed by 4 cubic feet of the injection water. Ans. 12.1 lbs.

8. If there pass into the condenser at the same time 2 lbs. steam at atmospheric pressure and 50 lbs. water at 50° F., find the temperature of hot-well. Ans. 93°38 F.

9. Using only 12 lbs. water per lb. of steam at 212° F., find the temperature of the hot-well when injection water is at 60° F. Ans. 146° F.

10. From 1886 Steam Examination. Temperature of injection water 60° F., temperature of hot-well 100° F., latent heat of exhaust steam 1016 units. its temperature being 140° F.; find the pounds of injection water required per pound of steam condensed. Ans. 26'4 lbs.

11. What is the latent heat of steam at 212°F. expressed in foot-pounds? If I lb. of steam at 212° F. is mixed with 10 lbs. of water at 60° F., find the resulting temperature. (S. and A. Exam., 1889.) Ans. 161°·6 F.

LECTURE X.

CONTENTS.-The Successive Effects produced by the Continuous Application of Heat to a piece of very cold Ice until Dissociation takes place -Definition of Wet, Dry, and Saturated Steam-The Boiling Point of a Liquid-Experiment of Water boiling at Pressures less than One Atmosphere-Use of Large Air Pumps in connection with Condensers.

WE shall best understand the physical properties of steam by considering, in the first place, the several changes which take place in water from its solid condition, ice, until it becomes dissociated under the continuous application of heat.

These figures are purely imaginary, and not to scale.

ICE

WATER

SATURATED DRY SATURATED
STEAM

STEAM

Referring to the figure, suppose that we put 1 lb. of very cold ice in the bottom of an open-mouthed cylinder, and place a piston . on it, which, together with the pressure of the atmosphere, exerts a pressure of p lbs. on the square inch.

STAGE 1.-On the application of heat to the bottom of the cylinder, the ice is gradually heated until it arrives at 32° F.

STAGE 2.-The temperature now remains constant until all the ice melts and becomes converted into water. The bulk of the water being less than that of the ice from which it is formed, the piston descends a very little. As we have already noticed in

Lecture VIII., 143 units of heat must be communicated to the I lb. of ice at 32° F. before it is all melted into water at 32° F.

STAGE 3. Still applying heat, the water increases in temperature while the bulk diminishes, until 39° F. is reached (the maximum density point of water); thereafter, the volume gradually increases, but in a very slight degree, with the rise in temperature, until a little above 212° F. is reached, the limiting temperature of the water depending on the pressure p lbs. on the square inch. Had the pressure on the piston been nothing more than that due to the normal pressure of the atmosphere-viz., 14.7 lbs., corresponding to a barometric height of 29.9 inches, then the water would have been converted into steam at a temperature of 212° F.

STAGE 4.-The temperature remains stationary at that limit value, and the formation of steam commences, the piston rising as more and more of the water is evaporated. So long as any water remains at the bottom of the cylinder, we are producing what is called saturated steam, or wet steam. This is the condition of steam usually supplied to engines.

DEFINITION.-Wet Steam or Saturated Steam is steam in contact with the water from which it is generated. Its physical condition is such, that it is ready on the smallest increase of pressure, or decrease of temperature, to yield up or condense some portion into water, as we shall see afterwards; for a given pressure corresponds to one temperature and one volume.

STAGE 5.-When all the water in the bottom of the cylinder has been evaporated, and just when all the water or aqueous particles held in suspension with the steam have been converted into steam, we obtain dry steam, or what is sometimes termed dry saturated steam; then 966.6 units of heat must have passed into the contents of the cylinder, for, as we have already noticed, in Lecture VIII., 966.6 units of heat must be communicated to the 1 lb. of water before it is all converted into steam at 212° F.

DEFINITION.—Dry Steam or Dry Saturated Steam is that condition of steam just at the time when all aqueous or watery particles formerly held in suspension have been converted into

steam.

STAGE 6.—If more heat be added to the dry steam in the cylinder (the total pressure, P, on the piston remaining the same), the temperature will again begin to increase, and we get what is termed superheated steam. The more it is heated, the more nearly do its properties approach to those of a perfect gas. If the top of the cylinder had been closed from the commencement of stage 3, the pressure would have risen with the temperature until the commencement of stage 6, in accordance with the boiling points

given below; but during stage 6 we communicate more heat to the steam than its pressure would indicate by the tables. Superheated steam is not now much used for engines, on account of its destructive action on the packing of the glands, and working surfaces of the slide valve and cylinder.

DEFINITION.-Superheated Steam is that condition of steam in which, in addition to being dry, its temperature has been raised above that due to the corresponding pressure of saturated steam. (See Pressure and Temperature Tables, Lecture XII.)

STAGE 7.-Steam cannot be heated indefinitely without a molecular change taking place, termed dissociation, when it separates into constituent gases-hydrogen and oxygen. This action is practically carried out in the process of making "water gas," by blowing dry steam over very hot plates before carbonizing it, ready for illuminating purposes.

Thus, the successive effects produced by the continuous application of heat to a piece of very cold ice are:

1. Heating ice up to 32° F.

2. Melting ice, absorption of latent heat, 143 units per lb. 3. Heating water up to boiling point.

4. Formation of saturated steam, no increase of temperature. 5. Formation of dry steam, due to the complete absorption of the latent heat, or 966'6 units per lb. of water.

6. Superheated steam, increase of temperature above stage 3. 7. Dissociation or formation of hydrogen and oxygen. 8. Heating, no further alteration of the physical state. Boiling Point.-Before discussing the "Internal and External Work done during Evaporation," we shall digress a little to consider what is meant by the boiling point.

The boiling point of any liquid is that point on the temperature scale, when the tension throughout its mass just overcomes the surrounding pressure. The temperature of the boiling point, therefore, depends directly on the pressure under which the liquid is evaporated, and the greater the pressure the higher the temperature at which it boils.

The boiling points of fresh water at different pressures are approximately as follows:

It is thus clear that water will boil or give off steam far below, as well as far above, its normal boiling point, 212° F.

To illustrate this, take a glass flask half full of water with a thermometer in it, heat it over a spirit lamp or Bunsen burner until the water just begins to boil and the temperature, as registered by the thermometer, is 212° F. Now attach it, as shown, to an air-pump, AP, by a flexible india-rubber tube, and begin extracting the air. The water is observed to boil violently, although it may have cooled down to as low as 180° F. This plan of attaching it to the air-pump is much better than that of placing it under the glass-bell jar of the pump, as it permits the thermometer being easily seen after the moist steam has begun to rise.

If an air-pump be not at hand, the following simple experiment will illustrate the fact equally well to a class :--After heating the water in the flask to 212° F., and letting it boil freely for a minute to expel the air, cork it up quickly and tightly, leaving a thermometer inside. Now pour cold water on the outside of the flask, the water will at once begin to boil, although the temperature may be now below 200° F. It ceases to boil, however, if you stop