Teledyne 9110TH - Nitrogen Oxides Analyzer - Manual (Page 288)

Model 9110TH NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 268
8.3.3.1. Critical Flow Orifice
The most important component of the flow control assemblies 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.
Figure 8-8: Flow Control Assembly & Critical Flow Orifice
The actual flow rate of gas through the orifice (volume of gas per unit of time), depends
on the size and shape of the aperture in the orifice. The larger the hole, the more gas
molecules, moving at the speed of sound, pass through the orifice.
With nominal pressures of 28 and 4 in-Hg-A for the sample and reaction cell pressures,
respectively the necessary ratio of sample to reaction cell pressure of 2:1 is largely
exceeded and accommodates a wide range of possible variability in atmospheric
pressure and pump degradation extending the useful life of the pump. Once the pump
does degrades to the point where the vacuum pressure exceeds 14 in-Hg-A so that the
ratio between sample and vacuum pressures is less than 2:1 a critical flow rate can no
longer be maintained. At this point, the instrument will display “XXXX" indicating an
invalid sample flow rate.