Level 2. Fluid mechanics CHAPTER 5. PUMPS We shall now turn our attention to a very common engineering appliance - and a straightforward use of the conservation laws - namely the study of pumps. Pumps are designed to transfer energy into a system, for example when raising a fluid from a low to a high level, as opposed to machines such as a turbine which remove energy from a system. As a practising engineer you are more likely to encounter pumps since they play a much larger role in our everyday lives, for example in refrigerators, cars etc. The conservation laws described in the previous chapter may be applied to the design of all aspects of a pumps performance, from blade length, angle and width, to the study of how a pump performs as part of a much larger system. The detailed design of the pump itself is rather complex and so we shall focus here only on how the pump performs as part of an overall system (e.g. a network of pipes, reservoirs etc.). We shall begin our discussion by looking at the different types of pump commonly available. 5.1 Pump Classification We shall concentrate here on rotodynamic pumps, i.e. pumps in which a rotating section, called an impeller, provides a continuous fluid flow. Pumps are generally classified according to the plane of rotation of the impeller. 110
Level 2. Fluid mechanics 5.1.1 Axial Flow A pump is classified as an axial flow pump if the flow approaches parallel to the axis of rotation of the impeller and exits the pump in the same direction (i.e. also parallel to the axis of rotation): IN OUT Rotation Axial Flow 5.1.2 Centrifugal Flow In a centrifugal pump, the flow again approaches parallel to the axis of rotation, but this time leaves the pump at an angle of 90° to the axis of rotation - i.e. in the plane of impeller rotation, or, if you like, in the "radial" direction: OUT IN Rotation Radial Flow OUT 111
Level 2. Fluid mechanics 5.1.3 Pump Selection Rotodynamic pumps generally are either of an axial or a centrifugal type, however on some occasions the two characteristics are combined to produce what is known as a mixed flow pump, which contains components of both radial and axial flow. Different types of pump (and there are many) will suit different types of application. As a very rough guide, their relative characteristics may be summarised as follows: (i) An axial pump produces a relatively small rise in pressure, but delivers a large volume flow rate. (ii) A centrifugal pump generates a higher rise in pressure but at a lower volume flow rate. (iii) A mixed flow machine is designed to combine the two characteristics above to provide a "compromise". It is not immediately obvious here why, under given circumstances, one should choose an axial/centrifugal or mixed flow pump. Of course the engineer may be forced, by considerations such as size, space and cost, to choose one particular type.