Modern Gasworks Practice

ELECTRICAL PLANT IN GASWORKS cc a maximum 234 amperes at a pressure of 220 volts. In practice the next largest commercial size would be ordered, say 55 or 56 kw. The efficiency of a dynamo rarely reaches more than 87 to 90 per cent., therefore the gas engine must supply sufficient power to allow for this loss. The power required at the coupling (assuming a dynamo efficiency of 88 per cent.) is— X 100 _ gg.g kw. or _ øg 600 watts. 88 But 1 h.p. L= 746 watts, therefore the required horse-power of the gas engine would be— 63,600 OK , , —’---- = 85 b.h.p. 746 The nearest convenient size would, therefore, be installed ; and it is important to note on what basis the b.h.p. of the engine is given by the makers. Frequently the figures given in catalogues refer to the maximum power obtainable calculated on a town’s gas having a calorific power of so much as 600 B.Th.U. Nominal or indicated horse-power need not be considered. With regard to the duplication of the plant it must be remembered that although the day load amounts to 55 h.p., at times it will be very much less than this and the engine will not have a high load factor. In the event of the large plant being used for night work the load factor would be lower still (at times only about 20 per cent.), with a corre-sponding increase in gas consumption. During the night there may be an average required horse-power of 20 at the gas engine, and assuming a consumption of 22 to 24 cubic feet per horse-power hour with the small plant, or, alternatively, 34 to 38 cubic feet per horse-power hour with the large set, the difference between the consumption of the two sets is 13 cubic feet per horse-power hour. It will be understood that this excess consumption of the larger engine is due to its working at about one-quarter load only. By using the larger engine on. the night load {i.e. for about 10 hours at a stretch) the waste of gas per night would be— 10 X 13 X 20 = 2,600 cubic feet; and if valued at 2s. 6t£. per 1,000 cubic feet the monetary loss will amount to about 6s. Qd. per day, or £110 per annum. Thus in balancing up the merits of a single installation or two of half the capacity, it will be borne in mind that a saving of £110 per annum would be equivalent to the interest, wear and tear, and depre-ciation (in all about 10 per cent.) on a Capital outlay of about £1,100. This sum would more than suffice to meet the extra cost of plant, switchboard, etc., for the duplicate plant. The amount of current necessary for electrically-driven machines (chargers and dischargers) may be taken at 0-2 to 0-4 units per ton of coal håndled. THE MOTOR The motor is the reverse of the dynamo, that is to say it transforms electrical into mechanical energy. The greater portion of the current supplied is absorbecl by the armature windings, whilst a small portion is drawn off by the fiele! windings.