More or less of the water is lost by evaporation, so more must be added from time to time, and the radiator should be kept as full as possible to get the best results. If the water level falls too far below the return pipe at the top of the radiator, the area of the heating surface and the decreased quantity of water exposed to the cooling surface, are likely to cause undue heating, or vaporization.
The Pump.—A variety of pumps are used, but they are generally based on the principle of the turbine impelling system, or on centrifugal action. A type which utilizes both these principles is shown in Figs. 76 and 77, in which the former is a cross vertical section of 77 along line 1, and the latter is a central vertical section on line 2 of Fig. 76.
The device comprises a cylindrical shell A, with an inlet B, at one edge near the front wall, and an outlet C at the upper edge near the rear wall.
Pump Construction.—Within is a revoluble tubular hub D, with one end E projecting, to which power is applied. A disk partition G is secured to this hub, midway between its ends, and on each side of the partition is a pair of oppositely-projecting convolute blades, those on the inlet side, indicated by H, and the ones in the discharge side by I.
Fig. 76. Side View of Pump.
Fig. 77. Section.
It will be noticed that the blades H on the intake side are so disposed that their concave surfaces are on the advance sides while those in the discharge end of the shell have their convex faces in the retreating side.
Action of Pump.—The hub has inlet ports J below each blade, and discharge ports K between each of the blades I. When rotating the points of the blades H catch the water at the inlet and drive it inwardly through the ports J, from which it passes through the hub to the ports K, and is then violently thrown by centrifugal motion, and by the action of the blades I to the discharge opening C.