Modern Gasworks Practice

458 MODERN GASWORKS PRACTICE of the tar extractors of ordinary construction showed from eight to nineteen milli-grammes of tar per cubic foot.j ESTIMATION OF TAR FOG IN GAS It is only with considerable difficulty that the tar fog present in gas can be estimated. The conditions prevailing in tiie main conveying the gas almost preclude a really accurate determination of th.e tar present; but, from the practical standpoint, a test which has a comparative valne is of considerable utility to the gas engineer. In practice, the only suitable method of estimating tar-fog in gas is by processes of filtration, the cmdest application of which is to hold a piece of blotting paper or white linen over the end of a lautem cock connected to the gas main. For a visual comparison of the kind will usually suffice to show how the ex-traction plant is performing its duty, and by elaboration of the same idea a roughly quantitative test may be devised. The most important work on the subject has been carried out by A. Edwards,1 who employed a method whereby the gas is filtered through a disc of filter paper (Fig. 285) cut to a diameter of 1’8 cm. and exposing 2 sq. cm. of filtering area. The volume of gas which has to be passed through, the filter paper to give a stain within measurable limits varies from 80 to 9,000 c.c., according to the proportion of tar fog present. The method employed is a visual one, which has been made quantitative as well as comparative. If the area of the stain (A) on the filter paper is known, also its density (D), the weight of the tar to give unit density on unit area (T), and the volume of the gas (V), then the quantity of tar fog present in the gas is :— A X D X T V The result may be expressed in grains per cubic foot, or grains per cubic metre, according to the particular units taken. T is determined by suspending the tar, collected. at the point considered, in solvent naplitha or similar colourless liquid, and measuring the density in a glass cell with parallel sides. If tlien W is the weight of tar taken, V the volume of solvent in which. W is dispersed, Vc the volume of the cell, A the area of the side of the cell, and D the density of tar and liquid, in the cell, the weight of tar to give unit density if spread over unit area is 1 Journal Soc. Chem. Ind., 1918, 35 T.