Fig. 499 is a perspective view of a centrifugal vertical pump of _the submerged type_. This pump is used largely by contractors in excavations and coffer dam work and for keeping pits drained.
A double suction centrifugal pump, _driven directly by a steam engine_, is shown in Fig. 497; these are generally and very satisfactorily used for circulating water through surface condensers and the cooling pipes in refrigerating systems. The engine and pump thus combined occupy small floor space and consequently little masonry is required for a foundation.
_The Buffalo centrifugal pump_ is shown in Fig. 500. These are built by the Buffalo Forge Co. in two types, viz., the submerged and the suction; the latter is the one shown in the cut. The suction type is employed for pumping from mines, pits, etc., and all places where the supply will not allow of a horizontal pump to be used, or in others where the supply is either below the pump, _or sometimes above and at other times below_. This type possesses merit above the submerged design in that it will work equally well, when set either above or below the liquid to be pumped.
_Multi-stage centrifugal pumps._ Experience has demonstrated that by placing several pumps together and discharging from one into the other, water can be delivered to almost any height. For a long time after the introduction of centrifugal pumps, it was supposed that about sixty feet was the limit for their economical working, owing to the high speed at which they had to be run to accomplish the results desired.
It was a discovery of importance, that _by coupling two or more pumps in series_, so that each pump worked against only a part of the total delivery head, water could easily be raised to even two thousand feet or any reasonably high head with satisfactory efficiency. Pumps connected in this way will throw more water at a given speed than when operated separately, and are therefore attended by less wear and tear.
Pumps worked in series are built compound, triple or quadruple as required by service either belt driven or directly connected to engines. Owing to the fact that they have no valves or absolutely close-parts, they are able to pump muddy or gritty water with sand in suspension, and are, therefore, especially in the vertical type, the only ones that can be successfully used for draining deep mines.