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Teledyne API Model T360/T360M Operation Manual Principles of Operation
201
7.3.2. Flow Rate Control
To maintain a constant flow rate of the sample gas through the instrument, the
Model T360 uses special flow control assemblies located in the purge gas line at
the entrance to the GFC wheel housing and in the exhaust gas line just before the
pump (Figure 7-8). These assemblies consists of:
a critical flow orifice.
two o-rings: Located just before and after the critical flow orifice, the o-rings
seal the gap between the walls of assembly housing and the critical flow
orifice.
a spring: Applies mechanical force needed to form the seal between the o-
rings, the critical flow orifice and the assembly housing.
7.3.2.1. CRITICAL FLOW ORIFICE
The most important component of this flow control assembly is the critical flow
orifice.
Critical flow orifices are a remarkably simple way to regulate stable gas flow
rates. They operate without moving parts by taking advantage of the laws of fluid
dynamics. By restricting the flow of gas though the orifice, a pressure differential
is created. This pressure differential combined with the action of the analyzer’s
pump draws the gas through the orifice.
As the pressure on the downstream side of the orifice (the pump side) continues
to drop, the speed that the gas flows though the orifice continues to rise. Once
the ratio of upstream pressure to downstream pressure is greater than 2:1, the
velocity of the gas through the orifice reaches the speed of sound. As long as that
ratio stays at least 2:1 the gas flow rate is unaffected by any fluctuations, surges,
or changes in downstream pressure because such variations only travel at the
speed of sound themselves and are therefore cancelled out by the sonic
shockwave at the downstream exit of the critical flow orifice.
SPRING
O-RINGS
FILTER
CRITICAL
FLOW
ORIFICE
A
REA OF
LOW
PRESSURE
AREA OF
HIGH
PRESSURE
Sonic
Shockwave
Figure 7-9: Flow Control Assembly & Critical Flow Orifice
07272B DCN6552