Modern Gasworks Practice

THE HORIZONTAL RETORT BENCH 95 to a normal example where 2,000 Ib. of water is evaporated per hour from waste gases, and the liighest evaporative valne obtained from the rough breeze mixture used on gasworks boilers is assumed to be 7-5 Ib. of water per Ib. of fuel, then the saving of fuel efiected by employing waste heat will amount approximately to 1,000 tons of breeze per annum. With breeze at its present price this does not represent any very appreciable saving in money; but it must be remembered that the retort bench.es in a comparatively small works would yield 2,000 Ib. of steam per hour, so th.at in the largest works, were all waste heat recovered, th.e saving would be considerable. Again, there is the question of labour, and in this direction the economy effected is certainly equivalent to the saving in cost of fuel. From figures quoted earlier it will be gathered that by cooling th.e ch.imn.ey gases to 400° F. th.e quantity of steam (at 100 Ib. per square inch) which may be generated is equivalent to 40 Ib. for every 1,000 cubic feet of gas made. Accordingly, in the case of a comparatively large gasworks producing 1,000 million cubic feet of gas per annum, the steam which. would be generated. by making use of all waste heat from the retort settings would approxi-mate to 40 million Ib. per annum. Stated in terms of fuel, this represents an annual saving of nearly 2,500 tons of breeze. Effect of Sulphur.—When dealing with waste heat problem it is most important to bear in mind that the coke consumed in producers contains up to 1 per cent, of sulphur. In all cases, therefore, the waste products circulating in the boiler contain a certain. proportion of sulphur dioxide. From a series of tests tlie writer finds th.at the average content amounts to 38 grains per 100 cubic feet of chimney gas at n.t.p. Corrosion of tubes due to sulphur dioxide need not be feared so long as the gases in contact with. the metal are not reduced in temperature below 275° Ft-- If, how-ever, more complete extraction of the sensible heat is aimed at, then particular attention must be given to the tubes forming the feed water heater. These tubes, if of ordinary construction, will probably have only a comparatively short working life, and the expen.se of cleaning and renewal must be set ofi against any saving which. is shown by the principle of feed water heating. However, tubes may now be obtained which. atter treatment by a special process, are more or less proof against the ravages of sulphur compounds ; and, providing the initial cost is not excessive, such, treat-ment has probably provided a solution to th.e trouble. At any rate, th.e action of sulphur dioxide is a strong point in favour of effecting only partial appropriation of the sensible heat, so that the question of complete extraction is one for the future when. further experience has been gathered from prolonged practical acquaintance with. modern resistants. (e) Regeneration Regenerators and generators of various types may be primarily classified as follows :— Ordinary generators. (1) Primary and secondary air supply both. unheated. (2) Primary supply unheated. Secondary heated, but at expense of pro ducer.