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MAN B&W 15.05
Calculation of Exhaust Gas Back�Pressure
The exhaust gas back pressure after the turbo�
charger(s) depends on the total pressure drop in
the exhaust gas piping system.
The components, exhaust gas boiler, silencer, and
spark arrester, if fitted, usually contribute with a
major part of the dynamic pressure drop through
the entire exhaust gas piping system.
The components mentioned are to be specified
so that the sum of the dynamic pressure drop
through the different components should, if possible,
approach 200 mm WC at an exhaust gas
flow volume corresponding to the specified MCR
at tropical ambient conditions. Then there will be
a pressure drop of 00 mm WC for d<strong>is</strong>tribution
among the remaining piping system.
Fig. 5.05.0 shows some guidelines regarding
res<strong>is</strong>tance coefficients and back�pressure loss
calculations which can be used, if the maker’s
data for back�pressure <strong>is</strong> not <strong>available</strong> at an early
stage of the project.
The pressure loss calculations have to be based
on the actual exhaust gas amount and temperature
valid for specified MCR. Some general formulas
and definitions are given in the following.
Exhaust gas data
M: exhaust gas amount at specified MCR in kg/sec.
T: exhaust gas temperature at specified MCR in °C
Please note that the actual exhaust gas temperature
<strong>is</strong> different before and after the boiler. The
exhaust gas data valid after the turbocharger may
be found in Chapter 6.
Mass density of exhaust gas (ρ)
ρ ≅ .293 x 273 ______ x .0 5 in kg/m3
273 + T
The factor .0 5 refers to the average back�pressure
of 50 mm WC (0.0 5 bar) in the exhaust gas
system.
Exhaust gas velocity (v)
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In a pipe with diameter D the exhaust gas velocity <strong>is</strong>:
v = M __
ρ
4
x _____
π x D2 in m/s
Pressure losses in pipes (∆p)
For a pipe element, like a bend etc., with the res<strong>is</strong>tance
coefficient ζ, the corresponding pressure
loss <strong>is</strong>:
∆p = ζ x ½ ρ v 2 x
MAN B&W MC/MC�C, ME/ME�C/ME�GI/ME-B engines 198 40 94�9.3
MAN Diesel
___
9.8
in mm WC
where the expression after ζ <strong>is</strong> the dynamic pressure
of the flow in the pipe.
The friction losses in the straight pipes may, as a
guidance, be estimated as :
mm WC per diameter length
whereas the positive influence of the up�draught
in the vertical pipe <strong>is</strong> normally negligible.
Pressure losses across components (∆p)
The pressure loss ∆p across silencer, exhaust
gas boiler, spark arrester, rain water trap, etc., to
be measured/ stated as shown in Fig. 5.05.0 (at
specified MCR) <strong>is</strong> normally given by the relevant
manufacturer.
Total back�pressure (∆p M )
The total back�pressure, measured/stated as the static
pressure in the pipe after the turbocharger, <strong>is</strong> then:
∆p M = Σ ∆p
where ∆p incorporates all pipe elements and
components etc. as described:
∆p M has to be lower than 350 mm WC.
(At design stage it <strong>is</strong> recommended to use max.
300 mm WC in order to have some margin for
fouling).