Teledyne 9110T instruction manual Flow Rate Control Critical Flow Orifices

Model 9110T NOx Analyzer				Principles of Operation
				Wait period (AZERO dwell time). Ensures reaction cell
Auto	Open to	Open to	0 – 4 s	has been flushed of sample gas and chemi-	Figure
	Auto Zero	vacuum		luminescence reaction is stopped.
Zero	valve	manifold			13-4
			4 - 6 s	Analyzer measures background noise without sample
				gas
			Cycle repeats every minute

13.2.2. FLOW RATE CONTROL - CRITICAL FLOW ORIFICES

Sample gas flow in the 9110T analyzer is created via the use of several flow control assemblies (see Figure 13-8 for an example) located in various places in the gas streams of the instrument. These assemblies consist 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 sintered filter

∙a spring (applies mechanical force needed to form the seal between the o-rings, the critical flow orifice and the assembly housing)

Figure 13-8: Flow Control Assembly & Critical Flow Orifice

13.2.2.1. Critical Flow Orifice

The most important component of each flow control assembly is the critical flow orifice. Critical flow orifices are a simple means 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 through the orifice, a pressure differential is created. This pressure differential, created by the analyzer’s external 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.

Teledyne Analytical Instruments 330