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

PLH_KKK_U2_31.QXP 25.05.2007 11:01 Uhr Seite 17FUNDAMENTALSRequired NPSHR visThe viscosity of the fluid has a two-fold influenceon the NPSHR value. With increasing viscosity,the friction increases, which in turn leadsto an increase in the NPSHR value. At the sametime, a higher viscosity leads to reduced diffusionof air and vapour particles in the fluid.Thus, bubble formation is slowed down andthere is also a thermodynamic effect, whichleads to a slight reduction in the NPSHR value.The effect of viscosity on the NPSHR value isbasically a function of the Reynolds number.However, this effect cannot be expressed usinga single relationship for all the different pumpdesigns and models. As a general rule: Pumpswith larger dimensions and consistent and wideimpeller inlet openings are less susceptible whenthere are changes in the viscosity of the fluid.Conversion to new delivery data by means ofEDP supportThe Wilo Select program is very recommendablefor converting from water to other viscosities.A relatively exact calculation is made with thestored data. However, it must be observed thatthe known calculation method according toISO/TR 17766 and the Hydraulic Institute etc. involvea tolerance. Exact specifications can onlybe determined by individually testing the pumpswith the actual fluid at concrete operating conditions.To do this, a special job order must begiven to the pump manufacturer.Gas dissolved in the fluid and gas entrained bythe fluid in the form of dispersed bubbles impairthe NPSHR value in a manner different from thatof large gas bubbles. When the flowrate at theinlet opening of the pump is high enough, smallamounts of the entrained gas are not separatedand usually have no or only a small effect on theNPSHR value. If, however, there are larger gasbuild-ups, this has a major effect on the pumpsuction. Then the NPSHR curves of the totaldelivery head change their shape from a welldefined"knee" to a step-like incline of thedelivery head. This increases the point of thedelivery head loss of 3 %, or in other words:The NPSHR value increases.The equations here are used for calculating thecorrection factor for adjusting the NPSHR valuefor the pump water performance, based on a standarddelivery head drop of 3 % on the NPSHR visvalue with the corresponding viscous liquid:1NPSHR BEP-WC NPSH = 1 + A · − 1 · 274 000 ·{ (C (Q BEP-W ) 0.667 · N 1.33H) ] }A = 0.5 for lateral suction portA = 0.1 for axial inlet[Wilo Planning Guide - Refrigeration, air-conditioning and cooling technology 02/2007 17