Modern Gasworks Practice

346 MODERN GASWORKS PRACTICE will run perfectly in parallel on a variable load. The makers’ instructions regarding Connections must, of course, be carefully carried out. The switchboard should be arranged so that all generators are connected on to a common set of bus-bars, the supply to the motors in the works being taken from this common set. By this means the whole of the generating plant can be made available for any part of the works, and by putting all the motors together on to the generating plant they balance out more evenly tlian if they were separated, consequently allowing smaller generating plants to be used. In connection with running generators in parallel it is absolutely imperative that each generator should be protected on the switchboard by a circuit breaker, provided with an overload release and fitted with a reverse-current trip. This ensures that if for any reason a gas engine should fail and the dynamo should cease to generate, it will be disconnected from the main and will not be driven by the other dynamos which may be running. In calculating the size of gas engine for a given dynamo it should be remembered that it is the usual practice to specify a dynamo output as the maximum load which it will carry continuously without a temperature rise exceeding a certain figure. On the other hånd, it is usual with a gas engine to specify in the catalogue the maximum output which it will develop if firing on every stroke, but a margin of at least 10 per cent, must be allowed on the gas engine power for continuous normal duty. This margin need not be allowed on the dynamo. A Typical Example Consider a works carbonizing about 200 tons of coal per diem, and ha,ving from 130 to 150 retorts. The larger number would represent the normal maximum work of a combined charger-discharger, or two-thirds the work of separate macliines. For the purpose of the example it can be assumed that the maximum power required by the machine is 18 h.p. The necessary motors would, therefore, be on lines such as the following :— B.H.P. Charging machinery . . . . . . . . . .18 Breaker, say 20 tons per hour ........ 11 Elevator ............ 9 Coal conveyor . . . . . . . . . .7 Coke conveyor ........... 10 Thus the maximum day load is 55 b.h.p. and the minimum night load 28 b.h.p. Motors of this type should run at an average efficiency of from 80 to 85 per cent. Assuming the lower figure, so as to allow for small losses between generators and motors, the following current would have to be supplied :— KK 100 , 55 X — = 69 h.p. 80 F i.e. 69 X 0-746 = 51-5 kilowatts. The latter, then, must be the power of the dynamo, i.e. it must generate as