EXHAUSTING MACHINERY
493
of blades ; and in one revolution of the exh.au.ster this volume is taken in and delivered four times. When considering a two- or tliree-blade machine the volume between the blades (for a given size) is, of course, greater than witli the four-blade ; but then it is only displaced two or three times in one revolution instead of four times.
For a four-blade exhauster the folio wing method of calculating capacity may be used, but it cannot be said to be closely accurate when very large or very small sizes are under consideration.
Let D == the inside diameter of outer drum in feet.
L = length of drum in feet.
N = Revolutions made per minute.
Q = Cubic feet of gas passed per hour.
Then Q=65D2LN.
This gives the gross capacity, from which 20 per cent, should be deductecl for “ slip.”
The capacity of modern exhausters lias been increased to a certain extent by the introduction of relief-port passages which. enable the blades to continue drawing for a longer period and provide a greater area for the issuing gas. It is questionable, however, whether these passages are perfectly satisfactory.
Driving Power Required.
The motive power is the largest item in the running costs of exhausters. As regards the horse-power required for any one installation it is almost impossible to lay down a hard and fast rule, for so much depends upon the conditions under which the plant is working. A mere table of powers is, therefore, apt to be misleading. The power varies considerably; for instance, an exhauster made to pass 100;000 cubic feet per hour at its standard speed would have about 65 to 70 per cent, efficiency if working against 30 indies to 60 inches back-pressure. If, however, the machine was put to work to pass 25,000 cubic feet per hour, the power required could not be taken in the same ratio to the gas passed, for the amount of power lost would bear a much larger ratio to the gas passed than in the first case. Hence the efficiency of the machine when dealing with small quantities will be much lower. There is no fixed rule as to what to allow for efficiency in such cases, and much depends on the type of machine under consideration. In practice, it is usual to treat each. case on its merits, to suit the prevailing conditions. Some idea of the probable horse-power required may, however, be obtainedfromthefollowingtable, compiled by E. B. Donkin :—