Book - School of Science and Technology

Chapter 8Pumps and other auxiliary equipmentThe principal component parts which must be brought together to make up an indirectheating system are the energy source, the distribution pipework and the heat emittingequipment. There are, however, a number of important auxiliary items which each make acontribution to the functioning of the whole. The subject matter of the present chapter isdevoted to consideration of these enabling items, some understanding of which is fundamentalto appreciation of system operation.PumpsThere are two basic categories of pump used in connection with indirect heating systems,positive displacement and centrifugal. Of the former, the rotary gear type is used exclusively(in the present context) for liquid fuel handling, as described in Chapter 11. Directacting positive displacement pumps were applied to early heating systems, but they arenow rarely used, even for those boiler feed duties for which they were once popular as aresult of their robust reliability. The wide use of the centrifugal type for other applicationsis such that no further introduction is required.Reciprocating pumpsAs the name implies, pumps of this type produce a discharge as a result of the axialreciprocating movement of a plunger within a cylinder, displacing fluid from suction todelivery. Pumps may be arranged vertically or horizontally and be either single actingwith drive from a piston rod and crank or double acting with a direct drive through ashaft which is common to the plunger and to a steam piston and cylinder. A twin cylinderbalanced action is usual where a pump is single acting. Pumps of this type operate at lowspeeds and are particularly suited to providing output against high pressures in circumstanceswhere an inherent minor intermittency in delivery is not of consequence.Rotary gear pumpsThese take the form of two interlinked and contra-rotating gears, set with close clearanceswithin a single casing. On the suction side, as the gears disengage, fluid fills the spaces betweenthe teeth and is conveyed round the periphery of the casing. It is then discharged as the gearsre-engage, to provide a practically constant level of delivery against any chosen pressure.