Miele PDR 908 EL Operating And Installation Instructions page 66

Installing the ducting
Calculating the total ducting
length
The entire ducting system (including
bends and components) generates air
resistance that pushes against the
exhaust air. The relative pipe lengths
and total ducting length have to be
calculated for this reason:
– The  relative pipe length (table I)
indicates how much greater the air
resistance of a bend is, for example,
when compared to 3' 3" (1 meter) of
a straight plastic waste-water pipe.
– Adding all of these relative pipe
lengths together gives the  total
ducting length. This is not the actual
length but a calculated figure. The
total ducting length (table II)
expresses the air resistance of the
entire exhaust-air system.
Because a larger  pipe diameter
(table II) reduces the air resistance, a
longer total duct length requires a
greater pipe diameter.
Procedure
 Measure the length needed for the
straight sections of ducting. Multiply
the length by the corresponding
relative pipe length from table I.
 Calculate the number of bends and
elbows needed. Use table I to help
you add together their relative pipe
lengths.
 Calculate the total duct length: Add
together all of the relative pipe
lengths calculated under  and .
 Refer to table II for the pipe diameter
needed for the total duct length.
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Table I
Components
Exhaust-air hose (highly
flexible)*
3' 3 3/8" (1 m), laid straight
45° bend (radius of bend =
9 13/16" (0.25 m))
90° bend (radius of bend =
9 13/16" (0.25 m))
Exhaust-air hose (flexible
or flexible aluminum)*/
plastic waste-water pipe
3' 3 3/8" (1 m), laid straight/
straight pipe
45° bend (radius of bend =
9 13/16" (0.25 m))
90° bend (radius of bend =
9 13/16" (0.25 m))
Wall pipe* or window
connection*
With inserted grille
With non-return flap
(swinging flap)
Non-return flap* for
integration into the ducting
(see "Shared exhaust-air
duct")
Flat ducting (for stacked
installation)*
* Optional accessories
Table II
 Total duct length
Maximum
65' 7 3/8" (20 m)
Maximum
131' 2 13/16" (40 m)
Maximum
262' 5 5/8" (80 m)
 Relative pipe
length
5' 10 7/8" (1.8 m)
4' 11" (1.5 m)
8' 2 7/16" (2.5 m)
3' 3 3/8" (1.0 m)
1' 11 5/8" (0.6 m)
2' 7 1/2" (0.8 m)
12' 5 5/8" (3.8 m)
4' 11" (1.5 m)
46' 11" (14.3 m)
36' 1" (11.0 m)
 Pipe diameter
4" (100 mm)
5" (125 mm)
5 7/8" (150 mm)