580 



GAS, COAL 



into the washing-lime apparatus, depends upon the temperature of the milk of lime, 

 and the quantity of gas generated in a certain time. 



1039 



It may be assumed as a determination sufficiently exact, that 10 square feet of 

 surface of the condenser can cool a cubic foot of gas per minute to the temperature 

 of the cooling water. For example, suppose a furnace or arch, with 5 retorts of 150 

 Ibs. of coal each, to produce in 5 hours 3,000 cubic feet of gas, or 10 cubic feet 

 per minute, there would be required, for the cooling surface of the condenser, 100 

 square feet= 10 x 10. Suppose 100,000 cubic feet of gas to be produced in 24 hours, 

 for which 8 or 9 such arches must be employed, the condensing surface must contain 

 from 800 to 900 square feet. 



After the action of the condenser, the gas still retains, chiefly in mechanical sus- 

 pension, a certain quantity of tarry matter, besides a slight percentage of ammonia. 

 To free it from these, it is passed through a scrubber D (fig. 1039), which consists of a 

 tall cylinder filled with bricks, paving stones, or coke, and having an arrangement by 

 which a stream of water can be admitted at top and removed at bottom. The chief 

 use of the water is to remove ammonia from the gas, but as it also dissolves some of 

 the luminiferous hydrocarbons, its use is objected to by Mr. Wright, and dry 

 scrubbers are now used at the Western Gas Works. It is also considered by the s.mir 

 gentleman, that the detention of a certain percentage of ammonia by the gas is 

 rather an advantage than otherwise, as it serves in part to neutralise the sulphurous 

 acid which is inevitably produced by the combustion even of the best gas. It must, 

 however, be borne in mind, that the presence of ammonia in gas gives rise to the 

 formation of nitric acid during its combustion. 



The Exhauster. The passage of the gas through the liquid of the hydraulic main, 

 and the other portions of apparatus between the retorts and gasholder, causes a very 

 considerable amount of pressure to be thrown back upon the retorts, an effect which 

 is productive of mischief in two ways : in the first place, if there be any fissure or 

 flaw in the retorts, or leakage in the joints, the escape and consequent loss of gas is 

 greatly augmented ; and in the second place, it has been ascertained by Mr. Grafton, 

 of Cambridge, that pressure in the retorts causes the decomposition of the illuminating 

 hydrocarbons with greatly-increased rapidity. It is, therefore, very desirable to 

 remove nearly the whole of this pressure by mechanical moans, and this is now done in 

 all well-arranged works, by the use of an apparatus termed an exhauster. Several 

 forms of exhausters are in use, but it will be necessary only to describe that of Mr. 

 J, T. Beale, which has been found by experience to bo very effective and economical. 



