2.1Rotary PositiveBlowerSilencersBlowerSilencersGeneralInformationBlowerSilencersIn a closed blower discharge system,structure-borne noise—such as that radiatedby pipe wall and silencer shell—may be aconsideration, particularly where a stringent,close-proximity noise specification applies.For these applications, various means areavailable to treat the pipe and shell radiatednoise, such that most reasonablespecifications may be met.For instance, it is possible to lag the silencershell externally and reduce any shell noisecontribution to below the casing and mechanicalnoise of the blower and driving machinery.Universal Silencer invites your inquiriesconcerning special applications where EPA,OSHA or other noise specifications apply.Special applications are handled on anindividual basis and recommendations aremade according to specific requirements ofthe installation.See pages 1.1–1.3 for ordering information | www.universalsilencer.com 1 2 3 4 5 6 7 8 9 10Rotary Positive BlowersThe Rotary Positive Blower is a two impellercompressor that delivers a large quantity ofgas or air relative to the individual pulses.Blower capacities are expressed in CFM atinlet conditions (ICFM). Blower size is usuallyexpressed as gear diameter by rotor length.Pitch Line Velocity (PLV) is the peripheralvelocity of the timing gear—equal to theproduct of the gear circumference and therotative speed of the blower, usuallyexpressed in feet per minute (FPM).The blower presents two problems:1) pulsation within the piping system and,2) noise radiation in the vicinity of the blowerand piping.The importance of these relative to each otheris a function of blower size and speed; bothincrease proportionately to the blower sizeand the square of the speed.Pulsation is more pronounced on thedischarge side. Peak pulse pressures are quitesevere and can result in unsilenced dischargesound power levels up to 140–145 dB. Theinlet, although producing less severe pulsationand noise, receives equal attention since theinlet is usually open to atmosphere and thenoise much more apparent.SilencersThere is little question that silencers are anecessity on any blower installation.Regardless of the size or speed of the blower,silencers of some type are nearly always used.In the selection of blower silencers, there aretwo basic considerations: 1) the silencer mustbe the correct size (i.e., sufficient capacity forthe volume flow) and, 2) the silencer must bethe proper type for the application. Thenominal silencer size need only be based onthe gas volume, (i.e., the CFM of the gas orair at the operating conditions). However, thesilencer (design) must be selected withconsideration of the blower size andoperating speed. Complete application andcapacity information is given on page 2.3.There are two types of silencers commonlyused on positive blowers: a reactive typesilencer which consists of a series ofexpansion chambers having interconnectingtubes, a more sophisticated silencer design.is the combination chamber-absorptive type.This combination silencer is similar to thereactive type with the exception that anacoustically-packed, sound absorbingsection is included, comprising an extensionof the silencer connection closest to theblower. The inlet of a discharge silencer andthe outlet of an inlet silencer are the endshaving the packed section.A third basic type of silencer—the simple,straight-through packed type—is occasionallyused on blowers. This type of silencer is usuallyused on small, high speed machines whichcharacteristically produce significant highfrequency noise and relatively mild pulsations.The PLV is normally the criterion forsilencer type selection. If the blower isoperating in the critical PLV range, it willgenerate objectionable high frequency noisewhich may cause shell ring or tank hammerin the piping and silencer. These criticalPLV conditions will always require acombination chamber-absorptive silencerfor satisfactory results.Inlet SilencersFor inlet service, a PLV of 3,300 ft/min orgreater is considered critical. This transitionspeed is empirically established and issomewhat arbitrary, however, it is commonlyaccepted that blowers operating at or above3,300 ft/min are considered critical for thepurpose of inlet silencer application. Thoseoperating below 3,300 ft/min are consideredsubcritical. Subcritical PLV applications canusually be silenced adequately with achamber-type silencer, such as Universal URBor UCI Series. Blowers operating above thecritical PLV of 3,300 ft/min will invariablyrequire the RIS Series combination chamberabsorptivetype silencer. Inlet Filters or FilterSilencers are commonly used on blowerinlets, either individually or in series with aseparate inlet silencer. Please reference theFilters and Filter Silencers section of thiscatalog for further information.Discharge SilencersFor the more severe discharge conditions oftypical blower installations, a PLV of2,700 ft/min is accepted as the criticaltransition speed. Blowers operating below2,700 ft/min are considered subcritical andcan usually be adequately silenced on thedischarge side by use of a chamber-typesilencer UCD or URD Series. Machinesoperating above the 2700 ft/min transitionspeed will require combination chamberabsorptivesilencers such as SD or RD Series.In some larger blower installations, pipingrequirements or space restrictions maypreclude the use of a large, single dischargesilencer such as the SD or RD Series.Where two or more blowers discharge into acommon header, individual silencers upstreamof the header are required to subdue theindividual blower pulsations. Otherwise, thepulsations tend to beat with each other andcan be extremely objectionable.Note: Silencers should be mounted as closeto the blower as possible since any pipingbetween the blower and silencer will radiatenoise. Standard silencer connections are notdesigned to carry external piping or valveloads, so good piping support practicesshould be used to prevent stresses thatcause fatigue and eventual fracture of thesilencer or piping. It is also good practice toisolate the blower from the silencer with aflexible expansion joint. Contact UniversalSilencer for special design considerationswhere loading is a factor.Attenuation CurvesNoise attenuation curves are given for thevarious models within this section.The curves represent insertion loss ofairborne noise for typical applications underaverage conditions. It is not feasible to chartthe expected performance of a silencer overa wide range of applications and conditions,therefore, the curves must be used withdiscretion. Structure-borne noise (seeabove) may be a consideration and willrequire separate analysis, since it is notairborne noise and not used for silencerperformance rating.