as already explained, allows the water to form a free vortex, and converts part of its kinetic energy into energy of pressure. The water moves in a free vortex in the volute-shaped pipe, the path of a partical being shown in the figure by a dotted line and arrows.

The difference of pressure produced by the pump depends upon the density of the fluid in the wheel as well as on the speed of rotation. Consequently, when there is only air in it, the pump is not able to produce a sufficient vacuum to make the water rise into it. In order to get over this difficulty, an ejector G, and a sluice C, are added. When the pump is to be started the sluice is closed and the air exhausted from the pump chamber by a jet of steam being passed through the ejector. The water then rises into the wheel and the sluice is gradually opened as the speed is increased. When the pump is fairly started the steam jet is shut off and the sluice fully opened. Sometimes a non-return valve is placed at the foot of the suction pipe to prevent the pump and pipe emptying when the wheel is stopped. In such a case, the pump is ready to start again at once.

Sometimes centrifugal pumps are made with radial blades. They then require a much larger whirlpool chamber to allow the kinetic energy to change into pressure energy without a serious loss in eddies.

LECTURE XXXVI.-QUESTIONS.

1. Sketch an undershot wheel. Explain why its efficiency is so low when it has radial blades, and show how the blades should be made to avoid this loss.

2. Water flows radially at 4 feet per second towards a part of a wheel of a centrifugal pump or turbine which is moving at 12 feet per second, find the angle of the vane that the water may enter without shock. If the vane were radial, at what angle ought the water to be guided so that it might enter without shock; its radial component of velocity being the same as before? (S. & A. Adv. Exam., 1898.)

3. Give outline sketches of the common types of water-wheel, and compare their relative advantage.

4. Distinguish between water-wheels and turbines, and explain the advantages of the latter.

5. Sketch the Pelton wheel and describe its action.

6. Sketch and describe a turbine of the Girard type, and mention its advantages and disadvantages.

7. Describe, with sketches, a Jonval turbine, and explain its relative advantages.

8. Sketch the wheel and case of an inward flow turbine for a fall of 50 feet; 8 cubic feet of water per second. Calculate the diameters and breadths of the wheel, the number of revolutions per minute, and the size of the shaft. (S. & A. Hons. Exam., 1897.)

454

LECTURE XXXVII.

REFRIGERATING MACHINERY.

CONTENTS.-Refrigeration-Preliminary Considerations-Carbon Dioxide as a Refrigerating Agent-Elementary Refrigerating Apparatus-Simple Refrigerating Machine-Carbon Dioxide Refrigerating Plant-Anhydrous Ammonia as a Refrigerating Agent-De La Vergne's Refrigerating Plant-De La Vergne's Double Acting Compressor-The Linde System of Refrigeration-Apparatus for Transmitting the cold produced to the Chambers requiring Refrigeration-Questions..

Refrigeration-Preliminary Considerations.-An interesting example of the conversion of heat into work is afforded by a refrigerating machine. The simplest form of machine consists of an air-compression pump driven by a steam engine, or other motive power, in which the pump is water jacketted and the air is cooled under pressure by being passed through a surface condenser where the water abstracts the sensible heat generated by the mechanical work of compression. The air thus cooled, but still under pressure, is conveyed to an air engine and allowed to perform work by expanding against some resistance. A large proportion of the work originally performed during the operation of compression is again given out, with a corresponding reduction in the air temperature. Α machine on this principle may be conveniently constructed by arranging the steam, compression and expanding engines to work on one crank-shaft. The expanding air thus assists in the work of compression. After deducting the necessary losses due to cooling, leakage, &c., the work done in the expansion cylinder amounts to about 65 per cent. of the power absorbed by the compression cylinder; the remaining 35 per cent. being supplied by the steam or other prime mover. The air having thus given up its heat, exhausts from the expansion cylinder at a very low temperature, reaching in one authenticated instance as low as 124° F.

The large coal consumption of machines of this class has, however, led to their being superseded, in almost all cases, by machines in which a more direct conversion of heat into work takes place.

If we take any liquid and commence to vaporise it, we find that, it is necessary to maintain a continual application of heat in order to bring about this physical change. The amount of heat necessary to convert a unit weight of a liquid to a unit

weight of gas at the same pressure, is always constant for the same liquid. For example, 1 lb. of water at a temperature of 212° F. and at atmospheric pressure, requires the application of 966-6 British Thermal Units to convert it into 1 lb. of steam at the same temperature and pressure. Conversely, to condense 1 lb. of steam to 1 lb. of water, both being at 212° F. and 14.7 lbs. pressure per square inch, we must abstract from the steam 966-6 thermal units by contact with a cold body. This principle holds good for any liquid.

A refrigerating machine with steam as a working medium, would not be practicable unless the temperature of everything in connection with it was maintained above 212° F., but there are many liquids which have, when compared with water, a very low boiling point; notably ether, sulphurous acid, ammonia, carbon dioxide, and ethylene. Each of these has been employed for purposes of refrigeration with more or less success; and all of them depend on the same principle-viz., the absorption or the giving out of their latent heat in converting the liquid to a gas, or vice versa.

It is not necessary here, to enter into a discussion as to the relative merits of different liquids as refrigerating agents, but in practice, anhydrous ammonia is the agent generally used, and in a lesser degree, carbon dioxide. The necessary machinery is of itself extremely simple, although the details afford scope for a great amount of elaboration and ingenuity.

Carbon Dioxide as a Refrigerating Agent.-Carbon dioxide or carbonic anhydride, which is commercially known as "carbonic acid," is a colourless gas, and quite without odour when pure. It is under all circumstances perfectly innocuous, and has practically no effect on animal tissues or other bodies. It will, however, produce asphyxiation in animals when present in the atmosphere in quantities exceeding 25 per cent. by excluding oxygen from the blood. This gas may be very readily liquefied, either by diminishing its temperature or by increasing its pressure. This fluid has a specific gravity of about 8, and can only remain in the liquid state when under considerable pressure, the pressure varying with the temperature of the liquid.* The moment the pressure is removed, the heat present in surrounding bodies, at once assists in the evaporation of the liquid carbon dioxide and the bodies themselves are consequently left in a colder condition than before the evaporation took place.

Elementary Refrigerating Apparatus.-Let us consider for a moment an elementary piece of apparatus in which refrigeration * Carbonic acid gas can only be liquefied by pressure when below 86° F. which is termed its critical temperature.