:0

Projection of Turbine Blades. In all the above cases we have constructed the vanes for a turbine of infinite radius, sometimes known as a "turbine rod." We shall now proceed to give a construction for bending the rod round to a turbine of small radius.

The blades for the straight turbine being given, draw a series of lines across as shown; in this case only one is shown for sake of clearness, viz. ab, which cuts the blade in the point c. Project this point on to the base-line, viz. d. From the centre o describe a circle a'b' touch

a'

ing the line ab. Join od, cutting the circle d'b' in the point f. This point f on the circular turbine blade corresponds to the point on the straight blade. Other points are found in the same manner, and a smooth curve is drawn through them.

The blades for the straight turbine are shown dotted, and those for the circular turbine in full lines.

Efficiency of Turbines.-The following figures are taken from some curves given by Professor Unwin in a lecture delivered at the Institution of Civil Engineers in the Hydromechanics course in 1884-5 :—

Losses in Turbines.-The various losses in turbines of course depend largely on the care with which they are designed and manufactured, but the following values taken from the

source mentioned above will give a good idea of the magnitude of the losses.

CHAPTER XVIII.

PUMPS.

NEARLY all water-motors, when suitably arranged, can be made reversible-that is to say, that if sufficient power be supplied to drive a water-motor backwards, it will raise water from the tail-race and deliver it into the

head-race, or, in other words, it will act as a

pump.

The only type of motor that cannot, for practical purposes, be reversed is an impulse motor, which derives its energy from a free jet or stream of water, such as a Pelton wheel.

We shall consider one or two typical cases of reversed motors.

Reversed Gravity Motors: Bucket Pumps, Chain Pump, Dredgers, Scoop Wheels, etc.-The two gravity motors shown in Figs. 549, 550, will act perfectly as pumps if reversed; an example of a chain pump is shown in Fig. 583. The floats are usually spaced about 10 feet apart, and the slip or the leakage past the floats is about 20 per cent. They are suitable for lifts up to 60 feet. The chain speed varies from 200 to 300 feet per minute, and the efficiency is about 63 per cent.

The ordinary dredger is also another pump of the same type.

Reversed overshot water-wheels have been used as pumps, but they do not readily

lend themselves to such work.

FIG. 583.

A pump very similar to the reversed undershot or breast wheel is largely used for low lifts, and gives remarkably good

results; such a pump is known as a "scoop wheel" (see Fig. 584).

The circumferential speed is from 6 to 10 feet per second. The slip varies from 5 per

cent. in well-fitted wheels to 20 per cent. in badly fitted wheels.

The diameter varies from 20 to 50 feet, and the width from 1 to 5 feet; the paddles are pitched at about 18 inches. The total efficiency, including the engine, varies from 50 to 70 per cent.

FIG. 584.

Reversed Pressure Motors, or Reciprocating Pumps. -If a pressure motor be driven from some external source the feed pipe becomes a suction pipe, and the exhaust a delivery pipe; such a reversed motor is termed a plunger, bucket, or

piston pump. They are termed single or double acting according as they deliver water at every or at alternate strokes of the piston or plunger. In Figs. 585, 586, 587, and 588 we show typical examples of various forms of reciprocating pumps.

Fig. 585 is a bucket pump, single acting, and gives an intermittent discharge. It is only suitable for low lifts. Sometimes this form of pump is modified as shown in dotted lines, when it is required to force water to a height.

Fig. 586 is a double-acting force or piston pump. When such pumps are made single acting, the upper set of valves are dispensed with. They can be used for high lifts. The manner in which the flow fluctuates will be dealt with in a future paragraph.

Fig. 587 is a plunger pump. It is single acting, and is the

form usually adopted for very high pressures. The flow is intermittent.

Fig. 588 is a combined bucket-and-plunger pump. It is double acting, but has only one inlet and one delivery valve. The flow is similar to that of Fig. 586.

There is no need to enter into a detailed description of the manner in which these pumps work; it will be obvious from the diagrams. It may, however, be well to point out that if the velocity past the valves be excessive, the frictional resistance becomes very great, and the work done by the pump greatly exceeds the work done in simply lifting the water. Provided a pump is dealing with water only, and not air and water, the amount of clearance at each end of the stroke is a matter of no importance.

Fluctuation of Delivery. In all forms of reciprocating