Fig. 32.—Sturtevant Equipment for Office Cleaning.
Fig. 31 illustrates a stationary suction cleaning plant applied to cleaning railway carriage cushions, and Fig. 32 shows a similar installation in use in an office building.
Cleaning by Air Blast. By transferring the hose from the suction side to the discharge, a suction cleaner may be used to blow dust from machinery of all kinds and from places that are high up and cannot be cleaned economically by suction. For cleaning electric generators and motors by blast, these machines have many advantages, and on account of the large volume of air handled they are much to be preferred to the small-volume high pressure jet of the ordinary air compressor often used for this purpose. With the portable turbo-blower there is no danger of damage to the insulation through high pressure, or through the carrying of moisture and oil into the windings with the air jet.
Pumping by Compressed Air. Although, generally speaking, the raising of water by compressed air is not an economical method, it is frequently adopted in mining and tunnelling where the use of steam or electricity is objectionable. In these cases, cost of operation is a minor factor, and it may be interesting to give a few particulars of this form of pneumatic conveying.
The simplest form of compressed air pump consists of a closed chamber or tank immersed in the water, to be raised or fixed at such a level that the water will flow into the tank. An air pipe is connected to the top of the chamber, and the rising main is carried inside the tank to the bottom. On opening the air valve, pressure is exerted on the surface of the water in the tank, and the water is expelled through the lift pipe or rising main. On closing the air valve, water again fills up the tank, and the process is repeated.
A decided improvement on this pump is the return air pump, which consists of two closed chambers connected through valves with the rising main. The compressed air pipe passes through a two-way valve, either into one tank or the other, this valve being positively operated. The method of working is similar to that of the single acting pump, considering each chamber separately, but one tank is filling while the other is being emptied.
The air expelled from the filling tank, instead of being discharged to atmosphere, and part of its expansive power lost, is carried back through the pipe, which would be the air intake pipe when discharging, through a port in the two-way valve, and into the compressor intake pipe. The air leaving the filling tank is naturally above atmospheric pressure, and assists the piston on entering the compressor, thus reducing the power absorbed in driving the latter.
Air-lift Pumping. The air-lift pump is a common means of conveying by pneumatic means and should not be confused with the above methods of raising water by compressed air.
In the air-lift method of pumping air under pressure is admitted at the foot of a pipe already submerged in the well. The air does not merely bubble through the water, as might be supposed, but passes up the pipe as a mixture of air and water. The introduction of the air into the rising column of water makes the latter as a whole less dense than the water around the tube, and therefore we have a difference in head between the internal and external columns of water which will carry the internal column considerably higher than the external column.