THE CONDENSATION OF COAL GAS 441
Many otter rules exist for computing the capacity of condensers. One based on the quantity of coal states that 100 to 120 superficial feet should be allowed for ■every ton carbonized per diem. Anotlier rule, based on thoroughly sound principles, but difficult of application in practice, says that 4 square feet of surface should be allowed per gallon of water yielded per ton of coal. In order to preclude the too sudden condensation of gas there was in past years a generally recognized formula stipulating that of the total lengtli of foul main 20 feet of length per inch diameter of the pipe should be under cover in the retort house. The rule, however, has lost its significance in present-day practice. As regards the dissipation of heat it may be assumed that this will amount, approximately, to 1 B.Th.U. per minute for each superficial foot of cooling surface exposecl to the air and for each degree (Fahr.) above atmospheric temperature.
WATER CONDENSERS
Water condensers as now employed are almost solely constructed from riveted mild-steel plates (which form the outer stell) and Steel or wrought-iron tubes. There are two distinet types at present in use, namely :—
(«) Multitubular condensers.
(6) Water-tube condensers.
Although. both types possess their respective advantages, it is now generally recognized that, except in cases where the condensing water is exceptionally clean,
the water-tube condenser is to be preferred. The outstanding distinetion between the multitubular and water-tube coadenser is that in the former the water passes outside and around the tubes which carry the hot gas, whereas in the latter type the opposite is the case. The difference is clearly seen by reference to Figs. 270 and 271. It will be realized that when any deposit is likely to take place it is far preferable to have this within the tubes, which can be readily cleaned, ratter tlian m the chamber surrounding them. Thus when only muddy water pumped from nvers or canals is obtainable there should be no hesitation in adoptmg the water-tube system. Oh the other hånd, if the incoming gas is particularly dirty and