Determination of Silencer Pressure Drop

Determination of Silencer Pressure Drop
Information Required:  Exhaust flow rate
                                         Silencer inlet size
                                         Level of silencing

Procedure:                      Calculate exhaust gas velocity

Exhaust Gas Velocity  =      ExhaustFlow Rate (CFM)   
                                             Silencer Inlet Pipe Area (FT 2)

Using the Pressure Drop Graph determin the silencer pressure drop corresponding to exhaust gas velocity and silencing level.

Silencer recommendations are based on 1" Hg restriction and are estimated only.  As we have no control over piping system and other variables (i.e. tail pipe loss, flex, sudden expansion/contraction, etc.), we make the following recommendations:
Use a size larger silencer or a low back pressure unit* if the back pressure calculations exceed 1" Hg across the silencer or if the silencer back pressure is more then 50% of the allowable back pressure for the exhaust system (be sure to use the smaller of these two criteria in your determination).  If components of the exhaust system (without silencer included) exceed 50% of the allowable back pressure for the engine, adjust silencer size accordingly.

*Note:
When determining back pressure, velocity should not exceed 15,000 ft/min regardless of the allowable back pressure (10,000 ft/min for spark arresting silencers).

Example
Information Available:      Engine displacement (cubic inches)................638
.                                             Rate BPH.............................................................250
                                              Full load RPM...................................................2200
                                              Asperation type....................................................T
                                              Cycle........................................................................4
                                              Exhaust flow rate (CFM)*...............................1650
                                              @ (F)...................................................................1000
                                              Silencer inlet size (inch).........................................6
                                              Level of silencing.....................................200 Level

Exhaust Gas Velocity =  1650   = 8,405 ft/min
                                          .1963

The Pressure Drop = 9 inches of Water (using Pressure Drop Graph).

*If the exhaust flow rate is not available, it can be approximated by the following equation:

CFM  =  Engine Displ (cu in)  x  Full Load RPM  x  Eff  x  (Exh Temp F + 460)
                                                              C  x  941760

CFM = exhaust flow rate in cubic feet per minute
Efficiency  =  .85 for naturally aspirated engines
Efficiency  =  1.4 for turbo-charged engines
Efficiency  =  1.2 for engines with scavenging blower
C  =  1 for two-cycle engine    C  =  2 for four-cycle engine

If exhaust temperature is not available, use:
T  =  1200F for gasoline engine
T  =  900F for diesel engine

*Note:   Use the equation above only when the exhaust flow rate is not available.

Note:  All dimensions are in inches