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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.
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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 |
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Table 2. Silencer Model Recommendations |
PITCH LINE VELOCITY |
INLET |
DISCHARGE (Single Silencer) |
DISCHARGE (Dual Silencers) |
Below Transition |
UCI, URB |
URB, UCD, URD |
WBM + UCD
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Above Transition |
RIS |
SD, RD |
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Table 3. Silencer Capacity |
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CAPACITY (Inlet CFM 14.7 psia & 70°F) |
SIZE |
INLET SILENCER |
DISCHARGE SILENCER |
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|
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 |
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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. |
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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 |
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Discharge: |
Δ P = |
( |
V |
) |
2 |
C x |
|
4005 |
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- 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.
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(1) To calculate velocity through silencer, divide flow in ACFM by cross sectional area of silencer inlet diameter in square feet. |
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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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