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To set it much lower than this would create a possibility of its leaking when the turbine was making a non-condensing run, and when the pressure at the turbine exhaust end is often above that of the atmosphere.
It is to be noticed as a feature of the plant, in common with many of later construction, that it was placed well away from the water's edge, and equipped with non-condensing engines; whereas the modern plant invariably seeks the bank of a river or lake for the purpose of a generous supply of water for its condensing engines or steam-turbines.
That is, speaking roughly, 1 lb. of gas gives about ten times as much power as 1 lb. of coal does in a good non-condensing engine. But at 18s. 8d. a ton we get 10 lb. of coal for 1d.; so that with these figures the cheapness of the coal would just compensate for the efficiency of the gas.
For non-condensing engines: Multiply the square of the diameter of the cylinder in inches by 7 and divide the product by 80. For condensing engines: Multiply the square of the diameter of the cylinder in inches by 7 and divide the product by 200.
Although good coal may yield 14,000 units by its combustion, only about 11,000 of these reach the boiler; so that the ratio of the useful heat is 11/19. The thermal efficiency of the best non-condensing engine to that of the gas-engine is in the ratio 4/22. Multiplying together these two ratios, we get
Should it become necessary, while operating, to shut down the condenser and change over to non-condensing operation, particular care should be observed that the change is not made too suddenly to non-condensing, as all the low-pressure sections of the turbine must be raised to a much higher temperature.
An absolutely pure mineral oil must be supplied, of a non-foaming character, and it should be kept free through filtering from any impurities. The above refers particularly to Allis-Chalmers turbines of the type ordinarily used for power service. For turbines built to be run non-condensing, the part relating to vacuum does not, of course, apply.
This low temperature, however, has never been reached, and in all practical cases we are confined within much narrower limits. Taking the case of the condensing engine, the limits were 312° to 102°, or 773° and 563° absolute, respectively. With non-condensing engines, the temperatures may be taken as 312° and 212°, or 773° and 673° absolute respectively.
This would reduce the efficiency to about 6 per cent. for the best, and 3 per cent. for the ordinary non-condensing engines; and if to this we add the inefficiency of some boilers, it is certain that many small engines do not convert into power more than 2 per cent. of the potential energy contained in the coal.
While this may not cause an accident, it is well to avoid the stresses which necessarily result from the sudden change of temperature. The same reasons, of course, do not hold good in changing from non-condensing to condensing. Shutting Down
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