Modern Gasworks Practice

712 MODERN GAS WORKS PRACTICE by permitting an endothermic reaction to take place during the period when maxi-mum heat expulsion is desirable. Moreover, by buming carbon to C02 about three times as much heat is evolved. as when it is burnt to CO only. It has to be borne in mind, however, that in many plants the case of succeeding apparatus comes in for consideration, although with the growing tendency for carburetted gas to disappear it becomes more and more important to retain as high a proportion of the liberated heat as possible in the generator. In the Lowe plant the generator is followed by two vessels which themselves must be raised to a high temperature ; whilst in the “ K. & A.” plant there is the regenerator to be heated up. Thus, the producer gas issuing from the generator (or generators, in the case of the twin plants) is burnt, by admitting a small quantity of secondary air from the biast main, in the succeeding vessels, maintaining these at the desired temperature. The average composition of the “ blow ” gases issuing from the generator when the fuel-bed of the latter is moderately deep is as follows :— Carbon monoxide . . . . • • • .17 per cent, by volume. Carbon dioxide 10 „ >> Nitrogen 73 „ » Traces of hydrogen and methane may also be found. Working with a fuel-bed of 6 feet 6 inches in the Lowe type plant, the author finds the following result:— “BLOW” GASES CO2. CO. End of Ist minute ___________________.... 18-6 per cent. 2-9 per cent. „ 2nd „ ___________________14-6 „ 9-4 „ 3rd „ ___________________12'0 „ 16-4 „ 4th „ . ■ • 7’6 „ 19-8 „ From the above it will be seen that theproportion of carbon monoxide in the producer gas gradually increases as the “ blow ’ ’ proceeds and the fuel gains in. temperature. The main overhead biast pipe, with connections to the carburettor and super-heater, is seen in Kg. 430. As regards apparatus for providing the biast, this in the early days of water gas invariably consisted of an. open fan driven by a liigh-speed reciprocating steam engine. The speed of the fan is usually in the neighbourhood of 2,000 revolutions per minute. For modern work the reciprocating engine has been almost entirely superseded by the steam turbine; or, in isolated cases, by electric motors. A common arrangement, namely, a steam turbine coupled to a Sturtevant fan, is shown in Fig. 442. In some cases the speed of the turbine is governed down in a ratio of about 10 to 1 so as to give the requisite fan speed, while in other instances low-speed turbines are used, the turbine motor running at the same speed as the fan. As regards the electric drive, this has -proved quite suitable, but it cannot be said that it affords the same elasticity as is obtained with the steam motor. Positive Blowers The ordinary water-gas fan is a high-speed machine with. a limited capacity in the way of producing a discharge pressure. On the other liand, the positive blower