Burkert 8626 - Page 15

MFC/MFM - 15
english
In this case, in analogy to equations (1) and (2), one first determines from the
desired nominal flow rate Q
nom
and the pressures p
1
*
and p
2
*
, the minimum flow
coefficient of the overall system k
Vges
. Via the relationship
(3)
22
2
111
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
VaVsVges
kkk
which describes series connection of the resistances of the MFC (k
Vs
) and the
system (k
Va
), one can determine, with known k
Va
, the required k
Vs
value of the
MFC or the nominal diameter of the servo element. This will be greater than if the
other flow resistances were not present.
The so-called valve authority
()
()
[]
22
2
0
0
VsVa
VsV
kk
k
p
p
+
=
∆
∆
=
ψ
(4)
is important for the control characteristics of the MFC in the system. It should not
be less than 0.3 ... 0.5.
Meaning of the symbols in the equations:
k
Vges
flow coefficient of the system with MFC installed
k
Va
flow coefficient of the system with MFC not installed (to be determined by
"short-circuiting" the piping at the point of installation)
k
Vs
flow coefficient of the MFC with fully opened servo element in [m³/h]
ρ
N
density of the medium in [kg/m
3
] under standard conditions (1013 mbar,
273 K)
T
1
temperature of the gas in K
p
1
, p
2
absolute pressures in [bar] before and after the MFC
∆p = p
1
- p
2
Q
max
maximum flow rate of the valve in [l
N
/min]
Q
nenn
maximum flow rate of the MFC in [l
N
/min] when correction to 100 % of
the setpoint has been made
(∆p)
0
pressure drop over the entire system
(∆p)
V0
fraction of the pressure drop occurring over the MFC with the valve fully
open