- (01-33) -
4-6. DUCT DESIGN
4-6-1. CALCULATION OF PRESSURE LOSSES DUE TO VENTILATION
THROUGH DUCT
Ventilation is always conducted through ducts in ventilation
fans for ducts and oven hoods (pressure type). To determine
the ventilation quantity, pressure losses due to the lengths,
the number of bends, and external installation components,
etc., should be accurately calculated, so that proper
ventilation quantities can be determined.
PROCEDURES FROM DUCT
CALCULATION TO MODEL
SELECTION
Equal Pressure Method
Simplified Method
Calculation of Required Ventilation Quantity
Duct Design (Duct Diameter, Duct Type, Piping Route,
Length, Bending)
S
im
pl
ifi
ed
M
et
ho
d
ba
se
d
on
“S
tra
ig
ht
D
uc
t
E
qu
iv
al
en
t L
en
gt
h”
M
et
ho
d
ba
se
d
on
“F
ric
tio
na
l R
es
is
ta
nc
e
D
ia
gr
am
”
To determine
ressure loss in the
straight duct with
“Frictional Resis-
ance Diagram.”
t
t
To determine the “straight
duct equivalent length” for
he entire duct by using the
straight duct equivalent
length table for the
component.
p
To determine the
local pressure loss
with the “local loss
coefficient.”
To determine the intersec-
tion by plotting the “loss
resistance curve” in the
“Static Curve - Quantity
Characteristic Curve.”
To determine the
pressure loss of the
entire duct system.
The required static pressure is determined by adding
10-20% tolerances to the obtained pressure loss.
To select a model satisfying the characteristics with the
“Static Curve - Quantity Characteristic Curve.”
Fig. 7-1
CALCULATION BASED ON
EQUAL PRESSURE METHOD
[1] Circular Duct
(1)
Duct resistance can be calculated from the following formula.
To determine the pressure loss caused in air flowing through
a straight duct (ΔP), the following formula is generally used.
Duct Resistance ΔP (Pa) =
: Friction Coefficient of Duct (0.01 - 0.25)
g: Gravitational Acceleration (9.8m/sec
2
)
: Air Density (kg/m
3
) 1.20kg/m
3
L: Duct Length (m)
d: Duct Diameter (m)
v: Wind Velocity in Duct (m/sec)
v =
3600
π
4
d
Q
2
×
Q: Quantity (Airflow) (m
3
/h)
Where,
= 0.02 (Galvanized Steel Pipe) = 9.8 = 1.2 is
substituted to produce the following.
P (Pa) = 0.02
9.80665
2
3600
π
4
2
d
Q
d
L
9.8
2
1.2
×
×
×
×
×
Friction coefficients of common ducts
(references)
Duct Materials
Aluminum flexible duct
0.03-0.04
PVC pipe
0.01-0.02
Galvanized steel pipe
0.016-0.025
(2) Method based on “Frictional Resistance Chart of Duct”
Frictional Loss Calculation Chart for Circular Ducts
(Part)
<Galvanized Steel Pipe, Internal Roughness ( ε ) = 0.18mm>
Quantity (
Q
, Airflow Volume) [m
3
/h
]
3
Friction Loss Rate ( ) [Pa/m]
Friction Loss Resistance Chart for Galvanized Steel Pipe
Fig. 7-2
<How to Use Chart>
(Conditions)
Ceiling-Mounted Ventilation Fan
Required Ventilation Quantity: 300m
3
/h
Duct Diameter: ø15cm
Duct Length: 5m
Quanti
ty
(Q
, Air
flo
w
Volume)
[m
3
/h
]
3
Fig. 7-3
Wind Velocity ( )
(m/s)
A
Friction Loss Ratio (Pa/m)
B
(1) To determine the intersection (A) of the duct diameter (d)
(15cm) and the quantity of wind running through the duct (Q:
airflow) (300m
3
/h).
(2)
To determine Point (B) by dropping (A) perpendicularly.
(3)
To determine the reading of (B) (2.2 Pa/m in this case), multiply
it with the duct length (5m) to produce 11 Pa.
