instruction and operation manual ZHK NANO
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V04-18
8.1.1 Airflow measurement by differential pressure measurement at the fan
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
9ռ
Nominal airflow rate
m³/h
9ռ
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
)
ǻS
w
Measured diff. pressure
Pa
ǻS
w
Measured diff. pressure
Pa
ȡ
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.
:LWKWKHIROORZLQJIRUPXODVWKHVHWSRLQWǻS
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
¨S
w.set
Target differential pressure
Pa
ǻ
p
w.set
Target differential pressure
Pa
9ռ
Target airflow rate
m³/h
9ռ
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
)
ȡ
V
Air density at the fan (Nomi-
nal value)
kg/m³
Table 5:
Formulas for airflow rate measurement
The corresponding K-factors of the fan are shown in the fan-motor data sheet or on the technical
data sheet for the air handling unit. The data on the data sheet always refers to one fan.
The air density at the measuring point is to set up manually, depending on sea level, temperature
and humidity. In most cases, 1.2 kg/m³ is a suitable value.
For AHUs supplied by EUROCLIMA with control, the airflow rate sensors are already preset.
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