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

PLH_KKK_U2_31.QXP 25.05.2007 11:01 Uhr Seite 14FUNDAMENTALSPressure behaviourPressure diagramp∆p L5∆p v ∆p L6 p 0 -∆p pEV∆p L4L∆p p∆p L3∆p L4∆p L5∆p vp 0p b∆p L6∆p L3∆p L1∆p L1∆p L2∆p L2Pressure curve in pipelines and fittingsPressure losses are reductions in the pressurebetween the component inlet and outlet.Among these components are pipelines, aggregatesand fittings. The losses occur due to turbulenceand friction. Every pipeline and fittinghas its own specific loss value, depending onthe material and surface roughness. The specificationscan be obtained from the manufacturer.An overview of the standard losses used by Wilocan be found in the appendix.AbbreviationEVp 0∆p P∆p v∆p rp b∆p LPressure surgeDescriptionGeneratorLoadMaximum pump pressurePressure loss in the pumpPressure drop at the valvePressure drop in the restof the systemReference pressure of the systemPressure loss in the systemIf a pipeline with flowing fluid is suddenly closedat one spot, the fluid mass inside it can only cometo rest with a time delay due to its inertia. Dueto this "negative" acceleration of the fluid mass,the forces applied to the pipe wall and shut-offdevice increase (F = m · a). Such types of pressuresurges must be observed in the dimensioningof pipeline systems (telescope lines, coolingwater circuits, etc.) as the maximum load. Airchambers are installed for damping the pressuresurge.Abbreviation DescriptionUnita Acceleration m/s 2Speed(Speed of sound for water ~ 1 400 m/s)m/sρ Density kg/m 3m Mass kgF Force NV˙ Volume flow m 3 /hEspecially endangered here are installationswhere lines are not laid continuously falling orrising. Since the water columns can break off atthe high points (vacuum formation) or increasedpressure is created when water columns meet,pipes could burst.The pressure increase when there is a suddenclosing of a throughflow fitting is simplified asfollows:∆p = ρ · V˙ · 14 Subject to change without prior notice 02/2007 WILO AG