- (01-31) -
4-5. VENTILATION DESIGN
4-5-1. QUANTITY, DYNAMIC PRESSURE / STATIC PRESSURE
The performance of a ventilation fan can be expressed
by “quantity” and “static pressure.” These two factors are
closely related to each other and constitute prerequisites for
considering “ventilation.” The first step to “ventilation design”
is to understand these two factors.
QUANTITY (AIRFLOW VOLUME)
This is the airflow volume exhausted (or taken in) by a ventilating
fan for a unit time, and generally expressed by m
3
/h or m
3
/min.
PRESSURE
This is the force applied by wind on a unit area, and generally
expressed by Pa. The pressure can be subdivided into the
following three categories.
Dynamic Pressure
Pressure derived from wind velocity is referred to as
dynamic pressure or velocity pressure. Windowpanes of
buildings bending under pressure of strong wind in storms
are typical indications of dynamic pressure.
Static Pressure
The pressure working on inflated balloons from inside is referred
to as static pressure, which functions even if air is not moving.
Total Pressure
The total pressure is the entire pressure of wind or air,
combining both dynamic and static pressures.
RELATIONSHIPS AMONG
DIFFERENT TYPES OF
PRESSURE
Airflow
Blower
Water-column
Manometer
Static
Pressure
Ps
Pv
Pt
Dynamic
Pressure
Total Pres-
sure
Fig. 6-1
The airflow inside a duct and each type of pressure there can
be illustrated as in Fig. 6-1.
Moving the certain amount of air in the duct will require “Static
Pressure (Ps)” to overcome the resistance within the duct.
“Dynamic Pressure (Pv)” represents the pressure applied
in the direction of the airflow and can be expressed as a
function of the wind velocity, can be used for measuring wind
velocity.
By adding the “Static Pressure” and the “Dynamic Pressure,”
“Total Pressure (Pt)” is generated. This relationship can be
expressed as follows.
V: Flow velocity (m/sec)
g: Gravity acceleration (m/sec
2
)
: Specific weight of air (kg/m
3
)
HOW TO INTERPRET STATIC
PRESSURE - QUANTITY
CHARACTERISTIC CURVE
(P-Q CURVE)
A graphic presentation of the relationship between the
quantity and the static pressure of a ventilating fan is referred
to as “P-Q Curve,” which can indicate the performance of the
fan. Fig. 6-2 illustrate a case in which a small intake opening
is installed on a wall, where the indoor pressure is a little
lower than the atmospheric pressure (the static pressure: B
[Pa] and the quantity: B’ [m
3
/h]), and the ventilator cannot
generate sufficient ventilation volume.
Fig. 6-3 illustrate a case in which a sufficiently large intake
opening is installed on a wall, where the indoor pressure is
almost equivalent to the atmospheric
pressure (the static pressure: O [Pa]
and the quantity: C’ [m
3
/h]), and the
ventilator can generate sufficient
ventilation volume.
Fig. 6-4 illustrate a case in which the intake
opening is large enough but the ventilating
fan has a certain resistant component, such
as pipe hood, etc. The
quantity (D’ [m
3
/h])
can be determined by
the intersecting point
made by P-Q curve
and the resistance loss
curve of the installed
component.
Small
intake
opening
A little lower
than
atmospheric
pressure
St
at
ic
p
re
ss
ur
e
(P
a)
* Static pressure:
Point B
* Quantity: Point
B’
B
→
Quantity (m
3
/h)
Fig. 6-2
Big
intake
opening
Big intake
opening
Almost
equivalent to
atmospheric
pressure
Almost
equivalent to
atmospheric
pressure
No
differ-
ence
* Static
pressure: Zero
* Quantity: Point
C’
B
C’
→
Quantity (m
3
/h)
Fig. 6-3
St
at
ic
p
re
ss
ur
e
(P
a)
No
* Static pressure:
Pont D
* Quantity: Point
D’
C
om
po
ne
nt
St
at
ic
pr
es
su
re
(P
a)
Resistance loss
curve
D
→
Quantity (m
3
/h)
Fig. 6-4
Big
intake
opening
Big intake
opening
Almost
equivalent to
atmospheric
pressure
Almost
equivalent to
atmospheric
pressure
No
differ-
ence
* Static
pressure: Zero
* Quantity: Point
C’
B
C’
→
Quantity (m
3
/h)
Fig. 6-3
S
ta
tic
p
re
ss
ur
e
(P
a)
No
* Static pressure:
Pont D
* Quantity: Point
D’
C
om
po
ne
nt
S
ta
tic
pr
es
su
re
(P
a)
Resistance loss
curve
D
→
Quantity (m
3
/h)
Fig. 6-4
