Modern Gasworks Practice

PLANNING AND LAYING OUT OF GAS WORKS 19 of a small deduction being made if there is a further long distance to the inlet of the exhauster. Various authorities recoinmend increasing surfaces up to nearly double this amount. As regards water-cooled condensers, there is considerable variation, according to type, amount and speed of water circulation, etc., but for general purposes about 3 square feet per 1,000 cubic feet per day should be allowed, i.e. about one-half of that necessary with the atmospheric type. Exhausters Except on very small works, say less than 10 or 12 millions, the exhausting plant is generally laid down in duplicate, to allow of repairs or adjustments to one set A useful combination for medium-sized works is one set equal to the maximum winter’s production and the second equal to that of the summer. It should be remembered that the cost price of exhausting machinery increases at an appre-ciably slower rate than capacity; hence it is often convenient, and in a growing place absolutely necessary, to lay down exhausters in excess of immediate require-ments. Slower running of an undertaxed exhauster will always make for smoother work and longer life. In a works of 4 to 6 millions per annum the saving to be effected in purchasing an exhauster is problematical, after paying interest on Capital and running costs. For works over 6 millions the buyer is on safe ground, and in such works (say less than 15—20 millions) it is usual to drive by gas engine, which provides power for operating tar, liquor and other pumps. If working to exact capacities, it is merely necessary to take a very small margin over the maximum day’s production, which is arrived at by dividing th.e yearly production by 24-0. The maximum day divided by 20 or 22 will give the maximum hour, which is the usual way of tabula ting exhauster capacities. The effective hourly capacity of an exhauster is guaranteed by the makers, and reference to catalogues gives all particulars relating to standard sizes, space occupied, etc. For the capacity of a four-blade exhauster, the following formula may be taken ; it is not, however, recommended when dealing with very large (over 4 feet) or very small exhausters:— Q = 65 D2 L.N. Where Q = cubic feet passed by exhauster per hour. D=inside diameter of outet drum in feet. L=length of drum in feet. N'=number of revolutions per minute. About 20 per cent, of the amount arrived at must th.en be deducted for “ slip.” The power necessary to drive exhausters is dependent on their capacity and the total back pressure to be overcome. (See Chapter XV.) The power required at the engine is slightly more than that necessary at the exhauster shaft, the extra being dependent on the method of driving, whether direct coupled or through shaftiug. Generally, however, the maker has a standard engine to a certain sized exhauster,