Teledyne GFC 7001EM Flow Rate Control, Critical Flow Orifice, Spring Rings Filter

Theory of Operation Model GFC7001E Carbon Dioxide Analyzer

11.4.FLOW RATE CONTROL

To maintain a constant flow rate of the sample gas through the instrument, the GFC 7001E/EM uses a special flow control assembly located in the exhaust gas line just before the pump. In instruments with the O2 sensor installed, a second flow control assembly is located between the O2 sensor assembly and the pump. 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 spring: Applies mechanical force needed to form the seal between the o-rings, the critical flow orifice and the assembly housing.

11.4.1.1.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 through 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.

CRITICAL

FLOW

ORIFICE

AREA OF	AREA OF
HIGH	LOW
PRESSURE	PRESSURE

	Sonic
	Shockwave
SPRING	O-RINGS
	FILTER

Figure 11-8: Flow Control Assembly & Critical Flow Orifice

Teledyne Analytical Instruments 235