Burkert 8006 Series, Operating Instructions Manual, Page: 14 / 148

14 - MFC/MFM
english
Depending on the version of the device, the setpoint is set either via the standard
signal input as an analog signal, or digitally via the field bus interface. Independent
of the control status, the flow rate measured by the sensor is sent as an analog
signal via the standard signal input or digitally via the field bus interface.
In order to obtain a dynamic or a more sluggish process value output signal, the
degree of filtering of the output signal can be adjusted with the software
"MassFlowCommunicator".
In all MFC series, the servo elements used are linear-armature proportional
valves from the Bürkert valve range. Design measures, especially with the valves
in the MFC for low flow rate (Types 8710 / 8711 / 8712 / 8713), assure low-friction
guidance of the moving armature. In combination with the PWM drive, this assures
a continuous, largely linear characteristic curve as well as high response
sensitivity. Both are important for optimal functioning in the closed control loop of
the MFC.
The nominal diameters of the valves are determined from the required nominal
flow rate Q
nom
, the pressure conditions in the application and the density of the
operating gas.
The manufacturer selects a proportional valve on the basis of these data whose
flow coefficient k
Vs
enables a maximum flow rate of at least the required nominal
flow rate under the specified pressure conditions, according to flow equations:
a) for subcritical flow (p
2
> p
1
/2) :
Q
max
= 514 *
1
2
*
*
T
p p
N
ρ
∆
* k
Vs
> Q
nom
(1)
or
b) for supercritical flow (p
2
< p
1
/2) :
Q
max
= 257 *
1
*
1
T
N
ρ
* p1* k
Vs
> Q
nom
(2)
Proportional valve
Where the pressures p
1
and p
2
in equations (Gl.) (1) and (2) refer to
measurement points directly before and after the MFC.
The pressures before and after the MFC are often unknown, only the inlet and
outlet pressures p
1
*
and p
2
*
for the overall system. A part of the pressure drop p
1
*
-
p
2
*
occurs over other flow resistances (isolation valves, nozzles, piping, filters,
etc.) whose flow coefficient may be collected together in a variable k
Va
.