Class 4.
—Large or small plant where carpet cleaning is not an important function of the cleaning system.
Renovators: Same as for Class 3.
Hose: 1¹⁄₂ in. or 1-³⁄₄ in.
Separators: One centrifugal, dry, with or without bag, according to type of exhauster adopted.
Vacuum Producer: Centrifugal fan or two-impeller rotary pump.
Capacity: 70 to 90 cu. ft. of free air per minute for each sweeper of plant capacity, with a vacuum of from 2 in. to 3 in. mercury.
Size of motor: 1 to 2 H. P. for each sweeper of plant capacity.
Control: With centrifugal fan, none; with pump, Cutler Hammer.
It is interesting to note that to produce the most efficient plant for all of the four cases named, all of the various types of vacuum cleaning systems which have come into general use have to be operated each under its most favorable conditions and the engineer should select his plant to best fulfill the conditions of the special case at hand, just as he would select his prime mover for an electric generating plant according to its size and location. There should be no more reason why any one of these systems should attempt to fulfill the requirements of every installation than there would be for a manufacturer of steam engines to attempt to use the same type of engine to drive a generator under all conditions. The writer believes that this condition will soon be realized by all manufacturers of vacuum cleaning systems and that they will endeavor to install apparatus of the type best suited to the conditions to be met in each case.