—When water-power is employed, a reservoir has to be formed by damming some near-by mountain stream at a point as high as practicable above the tunnel. The provision of a reservoir, instead of drawing the water directly from the stream, serves two important purposes. It insures a continuous supply and constant head of water in case of drought, and also permits the water to deposit its sediment before it is delivered to the turbines. The construction of these reservoirs may be of a temporary character, or they may be made permanent structures, and utilized after construction is completed to supply power for ventilation and other necessary purposes. In the first case they are usually destroyed after construction is finished. In either case, it is almost unnecessary to say, they should be built amply safe and strong according to good engineering practice in such works, for the duration of time which they are expected to exist.
Canals and Pipe Lines.
—For conveying the water from the reservoirs to the turbines, canals or pipe lines are employed. The latter form of conduit is generally preferable, it being both less expensive and more easily constructed than the former. It is advisable also to have duplicate lines of pipe to reduce the possibility of delay by accident or while necessary repairs are being made to one of the pipes. The pipe lines terminate in a penstock leading into the turbine chamber, and provided with the necessary valves for controlling the admission of water to the turbines.
Turbines.
—There are numerous forms of turbines on the market, but they may all be classed either as impulse turbines or as reaction turbines. Impulse turbines are those in which the whole available energy of the water is converted into kinetic energy before the water acts on the moving part of the turbine. Reaction turbines are those in which only a part of the available energy of the water is converted into kinetic energy before the water acts on the moving vanes. Impulse turbines give efficient results with any head and quantity of water, but they give better results when the quantity of water varies and the head remains constant. Reaction turbines, on the contrary, give better results when the quantity of water remains constant and the head varies. These observations indicate in a general way the form of turbine which will best meet the particular conditions in each case. The number of turbines required, and their dimensions, will be determined in each case by the number of horse-power required and the quantity of water available. The power of the turbines is transmitted to the air compressors or pumps by shafting and gearing.
Air Compressors.
—An air compressor is a machine—usually driven by steam, although any other power may be used—by which air is compressed into a receiver from which it may be piped for use. For a detailed description of the various forms of air compressors the reader should consult the catalogues of the several makers and the various text-books relating to air compression and compressed air. Air compressors, like other machines, suffer a loss of power by friction. The greatest loss of power, however, results from the heat of compression. When air is compressed, it is heated, and its relative volume is increased. Therefore, a cubic foot of hot air in the compressor cylinder, at say, 60 lbs. pressure, does not make a cubic foot of air at 60 lbs. pressure after cooling in the receiver. In other words, assuming pressure to be constant, a loss of volume results due to the extraction of the heat of compression after the air leaves the compressor cylinder. To reduce the amount of this loss, air compressors are designed with means to extract the heat from the air before it leaves the compressor cylinder. Air compressors may first be divided into two classes, according to the means employed for cooling the air, as follows: (1) Wet compressors, and (2) dry compressors. A wet compressor is one which introduces water directly into the cylinder during compression, and a dry compressor is one which admits no water to the air during compression. Wet compressors may be subdivided into two classes: (1) Those which inject water in the form of spray into the cylinder during compression, and (2) those which use a water piston for forcing the air into confinement.
The following brief discussion of these various types of compressors is based on the concise practical discussion of Mr. W. L. Saunders, M. Am. Soc. C. E., in “Compressed Air Production.” The highest isothermal results are obtained by the injection of water into the cylinders, since it is plain that the injection of cold water, in the shape of a finely divided spray, directly into the air during compression will lower the temperature to a greater degree than simply to surround the cylinder and parts by water jackets which is the means of cooling adopted with dry compressors. A serious obstacle to water injection, and that which condemns this type of compressor, is the influence of the injected water upon the air cylinder and parts. Even when pure water is used, the cylinders wear to such an extent as to produce leakage and to require reboring. The limitation to the speed of a compressor is also an important objection. The chief claim for the water piston compressor is that its piston is also its cooling device, and that the heat of compression is absorbed by the water. Water is so poor a conductor of heat, however, that without the addition of sprays it is safe to say that this compressor has scarcely any cooling advantages at all so far as the cooling of the air during compression is concerned. The water piston compressor operates at slow speed and is expensive. Its only advantage is that it has no dead spaces. In the dry compressor a sacrifice is made in the efficiency of the cooling device to obtain low first cost, economy in space, light weight, higher speed, greater durability, and greater general availability.
Air compressors are also distinguished as double acting and simple acting. They are simple acting when the cylinder is arranged to take in air at one stroke and force it out at the next, and they are double acting when they take in and force out air at each stroke. In form compressors may be simple or duplex. They are simple when they have but one cylinder, and duplex when they have two cylinders. A straight line or direct acting compressor is one in which the steam and air cylinders are set tandem. An indirect acting compressor is one in which the power is applied indirectly to the piston rod of the air cylinder through the medium of a crank. Mr. W. L. Saunders writes in regard to direct and indirect compression as follows:—
“The experience of American manufacturers, which has been more extensive than that of others, has proved the value of direct compression as distinguished from indirect. By direct compression is meant the application of power to resistance through a single straight rod. The steam and air cylinders are placed tandem. Such machines naturally show a low friction loss because of the direct application of power to resistance. This friction loss has been recorded as low as 5%, while the best practice is about 10% with the type which conveys the power through the angle of a crank shaft to a cylinder connected to the shaft through an additional rod.”