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Application, Capacity, Pressure Drop Data

Pitch line velocity (PLV) is the speed of the timing gear in feet per minute (FPM). For purposes of silencer application, PLV Is considered ''critical'' at about 3300 FPM for intake and about 2700 FPM for discharge.

Table 1. 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. Those in the critical range require more sophisticated, 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:

  • From Table 1., determine whether blower RPM is above or below the transition speed for critical PLV,
  • Consult Table 2. for recommended silencer models.

 
Table 1. Blower Transition Speed
BLOWER TRANSITION SPEED - R.P.M.
GEAR SIZE INLET DISCHARGE
2 6,300 5,155
21/2 5,040 4,125
3 4,200 3,435
4 3,150 2,575
5 2,520 2,060
6 2,100 1,720
7 1,800 1,470
8 1,575 1,290
10 1,260 1,030
12 1,050 860
14 900 735
16 785 645
18 700 570
20 630 515
22 570 470
24 525 430
 
Table 2. Silencer Model Recommendations
PITCH LINE VELOCITY
INLET
DISCHARGE (Single Silencer) DISCHARGE (Dual Silencers)
Below Transition UCI, URB URB, UCD, URD
WBM + UCD
Above Transition RIS SD, RD
 
Table 3. Silencer Capacity
  CAPACITY (Inlet CFM 14.7 psia & 70°F)
SIZE INLET SILENCER DISCHARGE SILENCER
    4 PSIG 6 PSIG 8 PSIG 10 PSIG 15 PSIG
1 30 35 40 40 40 45
1-1/2 70 80 85 90 95 105
2 120 140 150 160 165 185
2-1/2 190 220 235 245 255 285
3 270 320 335 355 370 415
3-1/2 370 430 455 480 505 560
4 480 560 600 630 660 735
5 750 880 935 985 1,030 1,150
6 1,080 1,260 1,340 1,410 1,480 1,650
8 1,920 2,250 2,390 2,510 2,630 2,940
10 3,000 3,520 3,730 3,930 4,110 4,590
12 4,300 5,070 5,370 5,660 5,920 6,600
14 5,900 6,890 7,310 7,700 8,060 8,990
16 7,700 9,000 9,550 10,000 10,500 11,800
18 9,700 11,400 12,100 12,700 13,300 14,900
20 12,000 14,000 14,900 15,700 16,400 18,400
22 14,500 17,000 18,100 19,000 19,900 22,200
24 17,300 20,200 21,500 22,600 23,700 26,400
26 20,300 23,800 25,200 26,600 27,800 31,000
28 23,500 27,600 29,300 30,800 32,200 36,000
30 27,000 31,700 33,600 35,400 37,000 41,300
EST.TEMP 70°F 115°F 140°F 165°F 190°F 240°F
 
Table 3. gives the nominal capacity of the various size silencers. ''Size'' in this table refers to the silencer nominal size, which is 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.
 
 
The following formulas may be used to calculate pressure drop through the silencers covered in this catalog.
Inlet: Δ P = ( V ) 2 C (assumes silencer inlet is open to atmosphere)
4005  
 
Discharge: Δ P = ( V ) 2 C x
P X 530  
14.7 T  
4005  
 
  • Δ P = Pressure drop through silencer, inches H 2 O.
  • V = Air velocity through silencer, feet per minute (1)
  • C = Individual silencer restriction coefficient - empirical constant (See Table 4)
  • P = Discharge pressure, PSIA (operating pressure in PSIG + 14.7)
  • T = Discharge temperature, °R. absolute (operating temperature in °F.
 
(1) To calculate velocity through silencer, divide flow in ACFM by cross sectional area of silencer inlet diameter in square feet.
 
SILENCER MODEL PRESSURE DROP COEFFICIENT - C
URB, URBY 4.2
UCI, UCIY, UCIH 4.2
RIS, RISY, RISH 4.2
UCD, UCDY 4.2
URD, URDY, URDH 4.2
SD, SDY, SDH 4.2
RD, RDY, RDH 4.2
WBM 2.1
RDS, SDS, URDS 7.0
 
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