Modern Gasworks Practice

ELECTRICAL PLANT IN GASWORKS CO co Very mucli depends upon tlie cost at which the public supply can be obtained, and the probable cost of generation at the works. Some gas undertakings are now taking the available supply and are giving up generating, but in the maj or i ty of instances it is preferable to generate the current on the spot. Where the current is taken from a public supply it is very frequently alternating, and most classes of gasworks machinery can be quite satisfactorily driven by alternating current motors. If, however, the supply is generated on the works, it is advisable to instal direct-current plant, and for small plants a suitable voltage is 220-250 volts. For a larger plant, where the power has to be transmitted a considerable distance about the works, 440-500 volts is advisable. One of the advantages of the 220-volt supply is that electric lighting can be taken direct from the generating plant for use in such places as is desirable, whereas in the case of the 440-volt supply either a balancer or a small motor generator set must be installed for lighting purposes. In gasworks, the current has usually to be transmitted only a very short distance ; consequently the most suitable system of supply—and that in common use—is the direct current. The advantages gained by the introduction of the multi-phase alternating system (i. (i. the transmission of high power through small cables and with little loss over a long distance) are, therefore, of no material account. Furthermore, the nature of the requirements of the various units to be driven does not call for the peculiarities of the alternating system. In faet, the starting torque of direct current is preferable with the majority of motors employed. The alternating current demands more skilied attention, and there is greater liability to personal danger than with the direct system. Once the system of supply and the motive power for the generators has been decided upon the type and size of plant should be considered. For small and medium-sized installations the standard horizontal single cylinder gas engine is to be recommended, the electric-Iigliting type being specially built for this dass of work. For very large sets the vertical high-speed engine with two or more cylinders is more effeetive. There are two methods of driving the dynamo :— (a) By belting. (&) Direct drive by means of a flexible coupling. Method (a) accounts for a small waste of power, but carries the advantage that the speed of the generator may be greater tlian that of the engine. Accordingly, for the same power a smaller generator may be used. The direct-coupled manhinp. will, however, last longer (owing to its slower running speed), is safer (owing to the absence of moving belts), and occupies less ground space. Taking all points into consideration, it will usually be found that a direct drive—in spite of the slight extra expense ineurred—is preferable, and of more lasting economy. The engine employed should be fitted with a magneto ignition and self-starter, except in the case of very small units. Generally speaking, for sets up to 50 or 60 kw. capacity a direct-coupled plant is the most satisfactory. Where direct-coupled plant is put down the best arrangement is to liave the engines mounted direct on the concrete, and to use a two-bearing type of dynamo coupled to the engine by means of a suitable flexible coupling.