Silencer Sizing

Positive Displacement blowers have two impellers that trap a “pocket” of air each time they sweep by the inlet and discharge connection. A pressure pulse is created each time this occurs. This pressure pulse creates noise and vibration. Blower silencers can be used to reduce this noise and vibration.

Silencers are typically used on the inlet and discharge of a positive displacement blower used for pressure applications. A discharge silencer is typically used on a vacuum application.

There are three basic types of silencers used to reduce the noise and vibration generated by the pressure pulse: Chamber Type, Chamber Absorptive Type, and Absorptive Type. Click to read more about the differences between these silencer types.

 

 

The normal criterion for silencer type selection is the Pitch Line Veloctiy (PLV) of your blower system. PLV is the peripheral velocity of the timing gear, equal to the product of the gear circumference and the rotative speed of the blower, usually expressed in feet per minute (FPM).  For purposes of silencer application, PLV is considered “critical” at 3,300 ft/min for intake and 2,700 ft/min for discharge.

The Blower Transition Speed table gives transition speeds in RPM. Blowers running at these speeds or greater will have critical PLV. Operating speeds below transition will be in the sub-critical range. Blowers operating in the sub-critical speed range usually require only simple chamber-type silencers while those in the critical range require combination chamber-absorptive type silencers. If there is doubt, it is best to use the combination-type silencers. When gear size and operating speeds are known, the proper type silencer is easily selected.

The Silencer Capacity table below gives the nominal capacity of the various size silencers. “Size” in this table refers to the silencer “nominal size” or its “inlet size”. Capacities are expressed in inlet CFM (ICFM), thus discharge silencers are rated at higher capacities than inlet silencers since the air is compressed to reduced volume at the discharge operating pressure.

Silencer Capacity based on Inlet CFM 14.7 PSIA at 70°F


SizeINLET SILENCERDISCHARGE SIL.
4 psig
DISCHARGE SIL.
6 psig
DISCHARGE SIL.
8 psig
DISCHARGE SIL.
10 psig
DISCHARGE SIL.
15 psig
1303540404045
1.57080859095105
2120140150160165185
2.5190220235245255285
3270320335355370415
3.5370430455480505560
4480560600630660735
57508809359851,0301,150
61,0801,2601,3401,4101,4801,650
81,9202,2502,3902,5102,6302,940
103,0003,5203,7303,9304,1104,590
124,3005,0705,3705,6605,9206,600
145,9006,8907,3107,7008,0608,990
167,7009,0009,55010,00010,50011,800
189,70011,40012,10012,70013,30014,900
2012,00014,00014,90015,70016,40018,400
2214,50017,00018,10019,00019,90022,200
2417,30020,20021,50022,60023,70026,400
2620,30023,80025,20026,60027,80031,000
2823,50027,60029,30030,80032,20036,000
3027,00031,70033,60035,40037,00041,300
Est. Temp.70°F115°F140°F165°F190°F240°F


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