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In Situ Non Contact Type Flue Gas Flow Measurement : Week 2/15/2013

In our last article, we discussed about available techniques of measurement & also about new upcoming techniques. In this article, we will discuss about new techniques of measurement.

The In-Situ Non-Contact type measurement

The non-contact measurement of gas velocity is usually done using a time-delay correlation of flue gas infrared emissions. The emissions are received by two detectors placed fixed distance apart. This technique is suitable for all combustion gases including hot gases and gases with a high dust burden. Two robust infrared detectors are used for prime sensing. These detectors are mounted on the stack or duct, typically one meter apart in the direction of flow. High efficiency air curtains are also fitted, which extend the time between maintenance periods and window cleaning.

Principle of operation

The method used is similar in principle to the technique of flow measurement by the injection of chemical dye or radioactive tracers, where the velocity is derived from the transport time of the tracer between two measuring points that are known distance apart.

Instead of an artificial tracer being added, the naturally occurring turbulence of the gas stream is used as the tracer. This flow turbulence causes fluctuations to occur in infrared radiation emitted by the gas. This continuously variable turbulent pattern is monitored by two infrared detectors mounted typically one meter apart along the direction of gas flow.

An electronic correlation technique is used to continuously compare the two detector signals to determine the time delay between these signals imposed by the gas velocity. Typical signals from the sensor A and B are shown here.

The signal from sensor B shows a strong similarity to that from sensor A. This signal from sensor B is delayed by a time t, which is the time taken for the gas to flow from point A to point B. Continuous determination of the detector signal time delay by the signal processor unit produces a continuous measurement of gas velocity since Velocity V = L/t, where L is the distance between two sensors.

Advantages of an In-Situ Non-Contact type measurement

  • Since it is a Non contact measurement, it is suitable for hot, dusty and aggressive gases
  • As there are no moving components, it’s easy to maintain
  • Low maintenance ensures high measurement availability
  • These days instruments with automated span checks for Hi and Lo span are available. This further enhances reliability of measurement
  • Because of local digital display and control unit, results are available locally

In our next article, we will discuss some interesting facts about compressed air, which is no more cheap resource now. We’ll also learn how to handle the compressed air in the industrial environment & various methods of reaping benefits from compressed air management.