WATER PRESSURE ENGINES.
Water pressure engines are machines with a cylinder and piston or ram, in principle identical with the corresponding part of a steam engine; the water is alternately admitted to and discharged from the cylinder, causing a reciprocating action of the piston or ram. It is admitted at a high pressure and after doing its work on the piston is discharged.
The water in some of these machines acquires a high velocity; the useful work is due to the difference in the pressure of admission and discharge, whether that pressure is due to the weight of a column of water of more or less considerable height, or is artificially produced.
When an incompressible fluid such as water, is used to actuate piston engines, two special difficulties arise. One is that the lost work in friction is very great, if the water attains a considerable velocity; another is that there is over-straining action on the machinery. The violent straining action due to the more or less sudden arrest of the motion of water in machinery is termed hydraulic shock. For these reasons the maximum velocity of flow of water in reciprocating hydraulic machines should generally not exceed 5 to 10 feet per second.
Under high pressure, where there is less object in saving and it is very important to keep the dimensions of the machinery small, Mr. Anderson gives 24 feet per second as the limit of velocity. In large water-pressure engines used for pumping mines the average piston speed does not exceed 1⁄2 to 2 feet per second.
The suitability of water for the transmission of power has been fully recognized in recent years; the facility with which water under pressure is capable of being utilized, and the advantages that attend its use in motors have resulted in many practical difficulties being overcome, which were at first considered insurmountable.
At the outset of the employment of water pressure it was feared that the water in the pipes and machinery might freeze. This, however, has been found not to be a difficulty where well-known precautions are taken. The working parts should, where possible, be placed under ground, or should be cased in, if they are above ground. The water should be run out of all valves and cylinders which cannot be cased in, and protected as soon as the working of the machine ceases.
A very small gas jet or lamp placed near the unprotected parts will prevent freezing.
Experiments have also shown that a mixture of glycerine and water prevents the effects of frost at a temperature as low as 16° Fahr., provided the glycerine has a specific gravity of 1.125, and that it is mixed in the proportion of one part of glycerine by weight to four parts of water.