All About Engines

I4° All About Engines A diagrammatic section of an excellent air pump made by Messrs. Weir, Ltd., of Glasgow, is given in Fig. 86. It differs from a water pump mainly in the large size of the cylin- der and valves. The inlet valves, moreover, must be very light, or they will not lift easily enough to allow air to escape freely from the condenser. Nowadays engineers aim at not less than 28 inches of vacuum when the barometer stands at 30 inches.* This means that with an atmospheric pressure of 147 lb. on the square inch the pressure in the con- denser must not be more than 0-98 lb. on the square inch. Any air not expelled by the piston at the end of its stroke will exert 1— some pressure, which will Fig. 86.—Section through Weir ten(j to pæjj JoWll monotype air pump . valves, and with an ordin- ary piston pump this is unavoidable. Even with a vacuum of 28 inches, therefore, there will be much * Since the ordinary atmospheric pressure of 147 lb. on the square inch corresponds to a height of the mercury column in a barometer of 30 inches, pressure is sometimes spoken of in inches instead of in lb. per square inch.