Refrigeration, air-conditioning and cooling technology - 2007.pdf

PLH_KKK_U2_31.QXP 25.05.2007 11:01 Uhr Seite 11FUNDAMENTALSPump efficiencyThe ratio of the delivered power – hydraulicpump capacity (flow x delivery head) – to theabsorbed power (drive power) is given by thepump efficiency. The efficiency changes alongthe pump curve.In building engineering, the pump efficiency isonly given indirect consideration when assessingthe pump. For this reason, this is often omittedfrom documentation. The power consumptionof the pump is the crucial factor.Delivery head H [m]Pump curve and efficiency curveηHOnly in larger aggregates, for example in processengineering or in large plant construction, wherethere is a differentiated consideration of thepump operation, these efficiency specificationsare mandatory.Flow Q [m³/h]Pump curve and efficiencycurve in the Q vs.H diagramThe pump efficiency is defined:η P =In the case of pumping in the customary temperaturerange for building engineering, thefollowing modified equation can also be used.η P =Q · H · ρ · gPQ · H367 · PSince the efficiency and power consumptionhave a direct relationship, a duty point withmaximum efficiency should be selected withregard to the operating costs.In general, the range of the best pump efficiencyis in the centre third of the pump curve. Pumpdimensioning in the first or last third of the pumpcurve always means operation in the worsepump efficiency range and should be avoided.For pumps where the drive motor is designedfor the entire curve, another thing that must beconsidered is that electromotors have their bestefficiencies only under full load, or at the maximumpermitted flow. This means, taking bothfactors into consideration, that the optimumduty point is shifted to the right of centre ofthe curve.Abbreviation DescriptionUnitη PPump efficiencyQ flow m 3 /sH delivery head mρ mass density of the fluid g [m/s 2 ] kg/m 3P power of the motor (shaft power) Wg local gravitational acceleration m/s 2367 3600 sec divided by 9.8665 = local gravitational accelerationWith pumps of the glandless series where thepump and motor form one encapsulated unit,instead of the pump efficiency η P customaryfor glanded pumps, the total efficiency η PGesis specified. They are coupled via the motorefficiency η M .The cause for this differentiated form of representationis the different construction form ofboth pump types.Wilo Planning Guide - Refrigeration, air-conditioning and cooling technology 08/2006 11