488 MODERN GASWORKS PRACTICE
complete gas-driven exliausting plant with by-pass governor suitable for a small works.
There is little doubt that, when the size of the plant permits, exhauster govern-ing can be most effectively carried out by varying the speed of the engine employed for driving. This is simply cffected by employing a gasholder bell the interior of which is in communication with the vacuum side of the exhauster. Buoyancy is imparted by means of air-floats placed at the base of the bell, whilst the upward or downward movement of the bell is conveyed by link motion to the throttle valve of the engine. Briefly explained, the operation of the governor is as follows : Should. the quantity of gas passing to the inlet of the exhauster decrease, then the
intensity of vacuum in the inlet main will increase. But the service pipe (A, Fig. 299) transmits this increase of vacuum to the interior of the bell, which. accordingly; is ptilled downwards. The downward movement of the bell then operates through the link-motion to close the throttle valve (H) sliglitly so that the speed of the engine is reduced in accordance with the amount of gas coming along. In the same way, when the quantity of gas increases, the vacuum on the inlet falls, with the result that the bell ascends and opens the steam throttle to the required extent. By adjusting the cock placed at R, on the top of the bell, the rate at which the bell nses and falls may be regulated. In some instances steadiness of movement is assured by introducing the principle of the “ dash-pot.
In laying down exhauster installations it is frequently arranged that both