Appendix B: Gas Flow Calculations - Siemens LDS 6 Operating Instructions Manual

Appendix B: Gas Flow Calculations

Appendix B: Gas Flow Calculations
CD 6C – Water vapor sensor for LDS 6
A5E00912721-01
Gas flow calculations are slightly complex because gases are
compressible fluids whose density changes with pressure. In this
application we are dealing with choked flow. The outlet pressure
is less than one half of the inlet pressure and the gas reaches
sonic velocity in the valve. A further decrease in outlet pressure
does not increase the flow.
The flow (q) of instrument air at p
(77°F) through the needle valve when it is fully opened would
then be a little more than 110 Nltr/min.
Figure 7.1 shows the flow (q) as a function of the up-stream
pressure (P ). There are two parameters in the diagram, the pro-
1
cess pressure (P ) and the temperature of the purge gas which is
p
assumed to be air. The diagram shows the flow through a system
with a total C of 0.1. The flow increases linearly with C
v
q = the flow rate on the low pressure side [Nltr/min.]
C = the flow coefficient (0,1 for the needle valve in our standard
v
sensor)
P = the inlet absolute pressure [hPa]
1
G = the gas specific gravity (air = 1.0)
g
T = the upstream temperature [°C]
1
Pp = process pressure (1023 hPa)
Figure 7.1 Purge air flow rate
= 6000 hPa and T = 25 °C
1 1
v
7
.
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