Modern Gasworks Practice

WATER GAS 723 As is to be expected, the construction of blowing fans has undergone some considerable modification since the introduction of water gas. Some twenty or more years ago, when the manufacture of water gas was in its infancy, a blast pressure of from 8-inch to 15-inch water-gauge was considered suitable for the generators, and any good blowing fan would do the work. A few years later the output of gas plants was largely increased by ruiming the fans at greater pressures, and, as the speeds required for the new conditions were too high, for the fans then in use, the results in many cases were disastrous. In order to meet the conditions a special extra heavy fan was then introduced. This new type of fan, made exclusively for water-gas purposes, is capable of supplying air at a pressure of from 15-inch to 25-inch water-gauge, or even greater pressures if necessary. Bearing in mind the very important duty performed by the blowing fan in the manufacture of water gas, it cannot be emphasized too strongly that, in order to eliminate as far as possible all risk of breakdown, the blowing apparatus should be specially designed and suitable for long “ runs ” with a minimum of attention. The following table gives the approximate size of fan (Sturtevant type) required for a given size of water-gas plant:— Stan Size of Fan. Capacity of Water-gas Plant. Cubic feet per diem. B.H.P. absorbed by Fan. Speed of Fan. Revs, per minute. 4 150,000 10-25 3,400 5 200,000 14-50 2,920 6 300,000 15-50 2,475 7 500,000 •23-50 2,175 8 650,000 32 1,625 9 850,000 44 1,425 10 1,000,000 58 1,225 i In view of the faet that fans operating in conjunction with water-gas plants have to work intermittently, it would not seem advisable to employ blowers of tlie positive type. For instance, when no blast is required a positive blower must be stopped altogether, or a valve must be fitted on the blast main to release the air when the pressure rises above a predetermined figure. The latter, however, is a somewhat wasteful proposition. With this 'type of blower, moreover, the ques-tions of wear and tear and noise during working have to be considered. With a centrifugal fan it is possible to regulate the delivery of air by a valve placed in the blast main, the speed of the fan being kept constant. As the valve is closed or opened, less or more air is allowed to pass to the generator, and the power absorbed by the fan. rises or falls according to whether the valve is opened or closed. (3) Amount of Air Required. The amount of air which should be required during the period of the “ blow ” is most readily gauged upon the gas-producing capacity of the plant. As a general rule it may be taken that the quantity of air should not