Solenoid valves - Bürkert Fluid Control Systems

Specific values of certain media
Type of gas p 2 max.
Diatomic gases, e.g. N 2 , O 2 , and CO 1.400 0.528 0.484
Air 1.402 0.53 0.49
Triatom. gases, e.g. CO 2 ; superheat. steam 1.300 0.546 0.473
Sattdampf 1.135 0.577 0.45
Table 2
(Konstante)
( p 1
)
A corresponding relationship can also Outflow function
be stated for the volume flow.
If we reduce the pressure ratio p 2 /p 1
crit
3.3.3. Sub-critical and super-critical
outflow
On the basis of this ratio, a distinction
is made in regards to the outflow behavior
of gases in valves between a
sub-critical outflow and a super-critical
outflow:
p 2
>
p 1 (
p 2
p 1)
crit
3.3.2. Outflow function and critical
pressure ratio
Figure 9 shows as a function of the
pressure ratio p 2 /p 1 . In addition, is
slightly dependent on the adiabatic
exponent which must be entered as
a parameter. is a constant for a specific
ideal gas (see Table 2).
Figure 9
0,6
0,5
0,4
0,3
0,2
0,1
1,6
1,4
1,2
1,0
0,527
0,484
0,459
0,429
0
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
starting from value 1, i.e. reduce p 2
with constant pressure p 1 , and thus
M, initially rises. Starting at a specific
pressure ratio which is referred to as
the “critical pressure ratio”, the outflow
function and thus also the
mass flow remain constant ( max and
M max ). The critical pressure ratio is
calculated as follows:
p
(
2
p 1
) (
krit
In addition to the adiabatic exponent
, Table 2 shows the critical pressure
ratios and values of max for certain
gases. max may assume values between
0.5 and 0.6 for popular media.
The critical pressure ratios lie approximately
between 0.45 and 0.55. For
practical tasks, it suffices to use a critical
pressure ratio of
(p 2 = 1/2p 1 ).
=
p
(
2
≈ 0,5
p 1
)
crit
2 -1
·
+1)
Equation 6
= Sub-critical outflow,
p 2
≤
p 1 (
p 2
p 1)
crit
= Super-critical outflow.
The differences between sub-critical
and super-critical outflows are shown
While, in the case of sub-critical outflow,
the mass flow can be calculated
on the basis of equation 5, in the case
of super-critical outflow, this equation
can be specified more precisely as
follows:
·
M = · A · max 2 · 1 · p 1
Equation 7
By way of approximation, we can use
a value of 0.5 for max .
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