MODERN GASWORKS PRACTICE
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able to expect that no loss of gas what-ever need take place, and the most simple and safe means of providing against such a contingency is that of inserting a safety seal in the main leading from the retort house to the exhauster. This seal can be regulated to some definit? depth—usually about 6 to 8 inches—thus only a small pressure will be exerted upon the appar-atus before the gas blows away. The seal should be arranged at some point in the open where there will be no chance of the escaping gas igniting. Inmaking use of a seal of this kind it is necessary to ensure that the blow-off pipe is sealed to a greater depth than the maximum possible “ draw ” at this point ; otherwise air
Calculation of Capacity
Figures given by makers for the capacity of exhausters denote tlie amount of gas passed at normal temperature. The exhauster has, however. to deal with. an amount of gas vafying from 5 to 10 per cent, in excess of this rated quantity. In addition, “ slip ” past the blades is a fairly considerable item, for which further allow-ance has to be made. The amount of “ slip ” occurring is eütirely dependent upon the difference of pressure between the miet and outlet, of the exhauster, and is independent of speed. But it may be faken as a general rule that as speed increases the proportional “ slip ” undergoes reduction. In calculating the capacity of exhausters it is customary to make an allow-ance of 15 to 20 per cent, for “ slip,” according » *“
to the size of the machine.
The calculation of the capacity of an ex-liauster is not so simple as might at first sight be supposed. Considering an ordinary four-blade machine it must be remembered that the capacity is not the whole space swept out by each blade per revolution. The amount of gas carried forward by each pair of blades is the greatest volume which exists at any time between the two blades. By reference to Fig. 303 it can be seen that at each revolution a volume of gas having a cross-sectional area equal to the shaded portion of the figure is carried from inlet to outlet by each pair