There are still a few furnace plants which are without an electric supply, these are lit at night by oil torches, colloquially known by the descriptive name of "smoke pots," whose light is extremely poor and inconvenience very great, but within a few years the furnace which does not have an electric plant at least for lighting purposes will be a thing of the past. At practically all modern plants so much of the work is done by electricity that an electric plant of considerable size must be provided. In many cases the electric stock hoist and electric bell hoist are used, and pumping plants, lights, and many other minor but necessary auxiliaries of the furnace are operated by electricity.

Where the furnace plant is connected with steel works the electric plant of the combined works becomes enormous. That of a steel works of two or three thousand tons daily capacity is probably as large as that of the average city of half a million people. The electric plant for the blast-furnace does not differ essentially from electric plants for other uses and must be designed primarily with reference to its size and the service to be expected of it, never forgetting that on account of its convenience other duties will be hung on to it from time to time, and that the plant which does not start with a hundred per cent. reserve capacity will probably soon be too small to do what is required of it.

Where very large electric plants are to be installed it is still a question as to whether the gas engine or the steam turbine is the more economical. The gas engine is undoubtedly the more economical by twenty-five to thirty-three per cent. on the heat unit basis, but its operating cost is decidedly higher, and its fixed charges are very much greater than those of the steam turbine. It is therefore very largely a matter of local conditions. If coal be high, intelligent labor plentiful, and capital not too dear, the gas engine is to be preferred. When these conditions are reversed undoubtedly the steam turbine is the better. The intermediate conditions must be decided on their merits after careful examination of all the factors.

The steam turbine has one great advantage in that it may be used as a low-pressure machine exclusively, and in two of the most modern plants which combine both blast-furnaces and steel works the blowing engines are steam-driven, and exhaust their steam about at atmospheric pressure into turbines which drive the electric generators. The turbine is by far the most economical motor in existence for low pressures, while it is inferior to the piston engine at higher pressures. This arrangement places each type of machine under the conditions for which it is best adapted, and makes a type of plant which has everything to commend it when the electrical requirements are sufficient to use up the steam from the blowing engines.

In smaller sizes the electrical plant may be either steam engine or turbine-driven, the steam engine probably being preferable in small sizes and the turbine having an increasing advantage as the size of the plant increases.

Voltage And Kind Of Current To Be Selected

The voltage and kind of current, direct or alternating, to be preferred depends on the local conditions. For small plants two hundred and twenty volt direct current gives excellent satisfaction. This voltage is high enough to keep the cost of copper in transmission lines within reason, and is low enough to give good service with incandescent lamps, while for electrical machinery which requires much control, such as cranes, ore bridges and the like, direct current still seems to have the advantage over the alternating on account of the ease of reversal, speed control, dynamic breaking, etc. At large plants where the power is transmitted in great quantities the direct current is out of the question except as an auxiliary. The alternating current must be used so that it may be stepped up to any voltage desired for transmission and stepped down at the point of utilization. If direct current be required for crane service or the like, it is produced by motor generator sets or rotary converters. When alternating current is used different voltages may be used in different services to suit the individual conditions. For mill motors and the like where men are likely to come in close contact with them four hundred and forty volts or less are used, whereas on large motors such as those which drive rolling mills direct, voltages from twenty-two hundred up to sixty-six hundred are used.