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

PLH_KKK_U2_31.QXP 25.05.2007 11:02 Uhr Seite 23NOISES – AIRBORNE SOUND – STRUCTURE-BORNE SOUNDThe expansion joint with elastic length limitationis the "reasonable" compromise between noiseabsorption and connection forces in many applications.When absorbing elements are used,their limited service life and sensitivity to hotwater are to be observed.Expansion jointsExpansion joint without lengthlimitationLength limitersExpansion joint with lengthlimitation without elastic elements(lateral expansion joint)Elastic elementsExpansion joint with lengthlimitation with elasticelementsThe effectiveness of the absorbing measurescan be seen in the figures on page 24, whichshow oscillograms of structure-borne noisemeasurements on a pipeline made to vibrate bya heating circulation pump. Depicted are threedifferent cases of structure-borne noise, whichinclude the unfiltered measuring signal and thefiltered-out low- and high-frequency portions,i.e. their blade frequency of 150 Hz (4-pole electromotor,impeller with six blades) or the electromagneticfrequency of 600 Hz.In the first case, the state is shown with thepipeline connected with the pump. In the secondcase, the state is shown after the installationof rubber/metal pipe connectors on the inletand outlet sides. As can be seen, thehigh-frequency portions are considerably reduced.By installing rubber expansion joints(third case), both the high-frequency and lowfrequencyportions are greatly reduced.Whether the absorbing measures in cases 2 and3 are appropriate for the individual case dependson the frequency of the dominant system noise.The absorbing measures described using the exampleof pumps having in-line construction canbe also applied sensibly for pumps set up on thefloor.Wilo Planning Guide - Refrigeration, air-conditioning and cooling technology 02/2007 23