—The Garden City rotary pump is a good example of the single-impeller type of pump and is or has been used to some extent by at least two makers of vacuum cleaning systems. Its interior arrangement is shown in [Fig. 84]. A solid cylindrical impeller, A, is mounted eccentrically in the cylindrical outer casing, the impeller being fitted with four sliding vanes which are provided with distance pieces, E, and wearing faces, B. The oil reservoir is provided with a needle valve which is automatically opened as soon as there is any vacuum produced and closes automatically when the machine is shut down. The rate of feed of oil is adjusted by the screw I. This type of pump offers a large surface in rubbing contact with the case and becomes very hot when in operation. It requires liberal lubrication in order to prevent heating and cutting of the surface of the casing. End wear in these pumps causes leakage, and, as usually constructed, there are no means provided for taking up this wear. It can be provided for, however, by using metal shims on the ends of the cylindrical casing.
FIG. 84. INTERIOR ARRANGEMENT OF THE GARDEN CITY ROTARY PUMP.
FIG. 85. POWER REQUIRED TO OPERATE GARDEN CITY TYPE OF ROTARY PUMP.
The power required to operate this type of pump (Curve a-b, [Fig. 85]), is nearly the same as that required to operate a piston pump for vacuum less than 12 in. mercury, but when the vacuum becomes higher, the power required becomes much greater than that required by the piston pump. The efficiency (Curve c-e, [Fig. 85]), is identical with that obtained with the light-weight poppet valve pump (Curve c-e, [Fig. 80]) from 0 to 11 in. vacuum, but for higher vacuum the efficiency of this type of pump falls off, while the efficiency of the piston pump becomes greater as the vacuum becomes higher. This difference in the characteristics of the two types of pumps is due to the presence of valves in one case and their absence in the other. With the piston pump the atmospheric pressure reaches the cylinder only while air is being discharged, the eduction valves being closed at other times and a partial vacuum exists on both sides of the piston. The higher the vacuum produced, the less time there is atmospheric pressure on the piston until, when no air is discharged, the air contained in the clearance space of the cylinder is compressed and expanded, the compression and expansion lines being coincident. The indicator card will have no area, and the only power expended is that required to overcome the friction in the moving parts. With the rotary pump there are no discharge valves to hold the atmospheric pressure from the discharge side of the impeller and the compression of the rarified air is accomplished by the atmospheric pressure admitting air through the eduction port into the chamber. As it comes opposite the eduction port there is no difference in the time during which the impeller is subject to atmospheric pressure, no matter what the quantity of air being discharged. The higher the vacuum in the spaces containing rarified air, the greater the difference in pressure on the opposite sides of the sliding vane and, therefore, the greater total power required to turn the rotor.
FIG. 86. ARRANGEMENT OF DOUBLE-IMPELLER ROOT TYPE ROTARY PUMP FOR VACUUM CLEANING WORK.
Another type of rotary pump which is fast becoming the most popular is the double-impeller type. This is generally known as the Root blower, as the firm of this name was the first to manufacture same. They have been in use for many years as blowers for gas works, and as vacuum producers for various purposes, mainly the operation of pneumatic tube systems.
Why this form of vacuum producer was not earlier adopted in vacuum cleaning systems, instead of the sliding-vane type, is hard to understand. This pump contains two impellers or cams which are mounted on shafts geared together and revolve in opposite directions inside of a case, always being in close proximity to the case and to each other, but never touching. They are, therefore, frictionless in operation and the introduction of a small amount of water renders them practically air tight. There being no metallic contact between the moving parts, internal lubrication is unnecessary and there is no wear on either the impellers or the casing and no means of taking up wear are necessary.