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Instruction Manual ZHK
V07-21.0 101/140
8.1.2 Airflow measurement by differential pressure measurement at the fan
If the fan is provided with pressure test points for airflow measurement and black test points are
provided on the outside of the fan unit (accessories are indicated on the technical data sheet), then
a differential pressure signal can be taken.
The delivered airflow rate can be calculated or displayed from the measured differential pressure.
A so-called K-value and an associated formula are used for calculation or for input into display or
control devices.
Usually, two different formulas and thus two different K-values are in use:
Formula A Formula B
In this formula, the respective air density at
the fan is taken into account. In this formula, a variable air density is not taken into
account.
The air density must be determined as a
function of air temperature, air humidity, sea
level and atmospheric pressure. Instead, a "fixed" air density of 1.20 kg/m 3 is as-
sumed.
With the following formulas, the airflow rate can be determined from the pressure signal:
-
Airflow rate calculation
-
Airflow rate calculation
V̇
Nominal airflow rate m³/h
V̇
Nominal airflow rate m³/h
K
A
K
A
–
factor for formula A m²*s/h k
B
k
B
–
factor for formula B m³/(h*Pa
0,5
)
Δp
w
Measured diff. pressure Pa
Δp
w
Measured diff. pressure Pa
ρ
V
Air density at the fan kg/m³
If several fans in a fan section are operated in parallel with the same speed, then the total airflow
rate is accordingly a multiple of the calculated individual airflow rate.
With the following formulas, the setpoint Δp
w
can be determined for a certain airflow rate (e.g. for di-
mensioning a pressure sensor, for constant airflow rate control):
-
Target differential pressure calculation
-
Target differential pressure calculation
∆p
w.set
Target differential pressure Pa
Δp
w.set
Target differential pressure Pa
V̇
Target airflow rate m³/h
V̇
Target airflow rate m³/h
K
A
K
A
–
factor for formula A (s.a.) m²*s/h k
B
k
B
–
factor for formula B (s.a.) m³/(h*Pa
0,5
)
ρ
V
Air density at the fan (Nomi-
nal value) kg/m³
Table 14:
Formulas for airflow rate measurement
For entry into a display or control unit, please check whether or not this is programmed according
to formula A or formula B and enter the corresponding value K
A
or k
B
.
𝑉̇ = 𝐾
𝐴
∙ √
2 ∙ ∆𝑝
𝑤
𝜌
𝑉
𝑉̇ = 𝑘
𝐵
∙ √∆𝑝
𝑊
∆𝑝
𝑤.𝑠𝑒𝑡
=
𝑉̇
2
∙ 𝜌
𝑉
𝐾
𝐴
2
∙ 2
∆𝑝
𝑤.𝑠𝑒𝑡
=
𝑉̇
2
𝑘
𝐵
2