Book - School of Science and Technology

Pumps 209Pressure21kPa434032324(a)(b)Volume4.3 4.8 litre/sFigure 8.7 Centrifugal pumps. Application to systemssquare of the water velocity and hence of the quantity flowing. Taking any given systemtherefore, once the pressure loss has been determined for one flow quantity (see Chapter9), other values may be produced as the basis for plotting a curve which represents thepressure reaction of that system to varying rates of water throughput.Two characteristic curves produced in this manner are plotted in Figure 8.7(a), thatmarked `1' being for a system having a relatively low resistance in comparison with thatmarked `2'. The use of such curves is illustrated in Figure 8.7(b) where that having thehigher resistance is superimposed upon the pump volume/pressure characteristic reproducedfrom Figure 8.1(b). The point at which the system curve intersects with the pumpcurve represents the duty at which that particular combination would operate, i.e.4.8 litre/s against a pressure of 40 kPa. As an example of the type of adjustment to suchan intersection which may be required in practice, let us assume that a flow rate of 4.3 litre/sis critical to the operation of the system. There are two obvious ways in which this may beproduced, as shown in the diagram:. By adding resistance to the system (part closing a valve or some other similar action)such that a new system characteristic is developed, as `3', where 4.3 litre/s will bedelivered against a pressure of 43 kPa.or. By reducing the speed of the pump by 10% (changing a pulley or an impeller) such thata new pump characteristic is developed, as `4', where 4.3 litre/s will be delivered againsta pressure of 32 kPa.The second alternative would be preferred in this case since less energy will be expended.Pump applicationThe duties required of centrifugal pumps may be related to the heating capacity of asystem, these then being translated to litre/s. Mass flow of water is thus, in this respect, theenergy output in kW divided by the specific heat capacity of water and by the temperaturedifferential across the system, flow to return. For all practical purposes, therefore, over