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6. Total the number of elbows and flexible sections in
the exhaust system between the engine and the
exhaust system outlet. Compare the radius of the
bend (R) to the pipe diameter (D). Dia. is the
nominal pipe diameter in inches. Determine the
equivalent length in m (ft.) of straight pipe for the
elbows and flexible sections from the following:
Bend
Angle
Type
Close
90_
90_
Medium
90_
Sweep
45_
Close
Sweep
45_
Flex Sections
* Use the diameter of the silencer inlet in inches from
step 3 for the initial calculation. If the results from
step 9 indicate excessive back pressure drop, then
recalculate using the larger-diameter pipe size
selected.
[ Use the flexible exhaust adapter length from step 3
and add any additional flex sections in the exhaust
system expressed in inches.
Convert the equivalent pipe length calculated in
feet to meters using ft. x 0.305 = m, as needed.
Examples:
45_ sweep elbows:
9 x 6.0 in. / 12 = 4.5 equiv. ft. or 1.4 equiv. m
90_ close elbows:
32 x 6.0 in. / 12 = 16.0 equiv. ft. or 4.9 equiv. m
Flexible sections:
2 x 33.75 in. / 12 = 5.6 equiv. ft. or 1.7 equiv. m
Equivalent of straight pipe:
4.5 + 16.0 + 5.6 = 26.1 equiv. straight ft.
1.4 + 4.9 + 1.7 = 8.0 equiv. straight m
7. Determine the total length of straight pipe used in
the exhaust system. Add this calculation to the
equivalent length for elbows and flexible sections
obtained in step 6.
Example:
Straight pipe = 3.0 m (10 ft.).
Equivalent straight pipe from step 6: 8.0 m (26.1 ft.)
3.0 m + 8.0 m = 11.0 m or
10 ft. + 26.1 ft. = 36.1 ft. total
58 Appendix
Bend
Conversion
Radius
Factor
R = D
32 x D* / 12
R = 2D
10 x D* / 12
R = 4D
8 x D* / 12
R = D
15 x D* / 12
R = 4D
9 x D* / 12
2 x Length[ / 12
8. Refer to Figure 7-10 if the pipe size is 102 mm
(4 in.) or less or Figure 7-11 if the pipe size is
127 mm (5 in.) or larger.
Place a straight edge across the chart with the
edge in line with the pipe size in inches (D) on the
right column from step 3 and the engine exhaust
flow (Q) from step 2 on the left column.
Read back pressure kPa/m or in. of Hg/ft. (∆P) from
the center column. Calculate the total piping
system back pressure by multiplying the total
equivalent straight pipe in m (ft.) from step 7 by the
kPa/m or in. of Hg/ft. of pipe from this step.
Example:
11.0 equiv. m x 0.04 kPa/m =
0.4 total system back pressure in kPa
36.1 equiv. ft. x 0.004 in. Hg/ft. =
0.14 total system back pressure in inches of Hg.
9. Add the back pressure of the piping determined in
step 8 to the back pressure of the silencer
determined in step 5. The total should not exceed
the engine manufacturer's maximum allowable
system back pressure determined in step 2 or on
the generator set's specification sheet. If the total
exceeds the maximum, use a larger pipe size or
silencer or both. Repeat the calculation if new
components are selected to verify that the system
back pressure would not exceed the limit using the
larger component(s).
Example:
0.4 kPa (step 8) + 2.8 kPa (step 5) = 3.2 kPa
Maximum allowable back pressure = 10.2 kPa
3.2<10.2 back pressure drop is acceptable
0.14 in. Hg. (step 8) + 0.85 in. Hg. (step 5) =
0.99 in. Hg.
Maximum allowable back pressure = 3.0 in. of Hg.
0.99< 3.0 back pressure drop is acceptable
TP-6609 6/08
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