Table of Contents
Advertisement
Exhaust back pressure limits engine power and
excessive back pressure causes serious engine
damage. Excessive back pressure usually results from
one or more of the following reasons:
D
The exhaust pipe diameter is too small.
D
The exhaust pipe is too long.
D
The exhaust system has too many sharp bends.
D
The exhaust silencer is too small.
D
The exhaust silencer is not the correct design for the
application.
Use the following procedure to verify that the installed
exhaust system does not exceed the engine's maximum
exhaust back pressure limit.
Exhaust System Back Pressure Calculation
Procedure
Determine the total back pressure by calculating the
effects of the individual exhaust system components
and adding the results. Make calculations using either
English or metric units. Exhaust pipe references are
nominal pipe NPT (in.) sizes. The procedure shows an
example with italic text. Calculations relate to end inlet
silencers.
Note: When calculating back pressure drop for side
inlet silencers, use the end inlet values shown
and add 0.75 kPa (0.25 in. of mercury or 3.4 in. of
water) to back pressure calculations.
1. Select
the
exhaust
application—hospital,
industrial. See the silencer specification sheet for
definitions for each exhaust silencer type. Confirm
silencer type availability for your generator set with
your authorized distributor/dealer, as some
generator sets do not use all four types.
Example: Determine the silencer back pressure for
the recommended critical silencer on a 230 kW,
60 Hz diesel generator set.
2. Refer to the generator set specification sheet for:
a. Engine exhaust flow at rated kW in m
(cfm)
Example: 57.5 m
b. Maximum allowable back pressure in kPa (in.
of
Example: 10.2 kPa (3.0 in. Hg)
TP-6609 6/08
Appendix B
silencer
type
for
critical,
residential,
3
/min. (2030 cfm)
Hg)
Exhaust System Back Pressure
3. Refer to the submittal catalog for:
a. The recommended critical silencer part number
Example: 343616
b. Silencer inlet diameter in mm (in.)
Example: 152 mm (6 in.)
c. Silencer inlet position (end or side)
Example: end inlet
d. The flexible exhaust adapter part number
Example: 343605
e. Flexible exhaust adapter, flexible section length
Example: 857 mm (33.75 in.)
4. Determine the exhaust gas velocity through the
silencer as follows:
a. Using the exhaust silencer inlet diameter
determined
corresponding inlet area using Figure 1.
Example: 0.0187m
b. Use this data to calculate the exhaust gas
velocity. Divide the engine exhaust flow from
step 2 in m
2
m
minute.
Example:
57.5 m
the
(2030 cfm / 0.201 sq. ft. = 10100 ft./min.)
or
Nominal Pipe
Size, in. NPT
1
1 1/4
1 1/2
2
2 1/2
3
4
5
6
3
/min.
8
10
12
14
16
18
Figure 1 Cross Sectional Area for Standard Silencer
Sizes
in
step 3,
2
(0.201 sq. ft.)
3
/min. (cfm) by the silencer inlet area
(sq. ft.) to get flow velocity in m (ft.) per
3
2
/min. / 0.0187 m
= 3075 m/min.
2
Inlet Area, m
Inlet Area, ft
0.00056
0.0060
0.00097
0.0104
0.00131
0.0141
0.00216
0.0233
0.00308
0.0332
0.00477
0.0513
0.00821
0.0884
0.0129
0.139
0.0187
0.201
0.0322
0.347
0.0509
0.548
0.0722
0.777
0.0872
0.939
0.1140
1.227
0.1442
1.553
determine
the
2
Appendix 55
Advertisement
Table of Contents
