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How Much Purge Air Does My Opacity Monitor Need?

Thursday, February 11, 2021 | Derek Stuart
Categories : Products

Opacity monitors are used to measure emissions of smoke and dust from combustion sources. It is a non-contact measurement, with only a light beam penetrating into the combustion gases in the stack or duct, so the delicate optical and electronic components can be mounted in the ambient conditions. 

Air Purge Blog - DerekHowever the instrument needs to be mounted directly on the stack, and so there is a risk of damage if the hot, dusty and corrosive flue gases make contact with the instrument’s optical surfaces. 

That is why it is important to provide a continuous flow of clean, filtered purge air through the instrument’s mounting and into the stack. The purge air must be flowing any time there are combustion gases in the flue. It is especially important to ensure the air purge system is installed and functioning correctly before the instrument is first installed on the stack.

The AMETEK Land 4400 and 4500 MkIII opacity monitors are designed to operate with a flow velocity of 5 m/s across the purge air outlet. This means the minimum flow is 30.5 m3/h or 18 ft3/min each for the transceiver and retroreflector. A higher flow rate is not generally harmful, but a low flow can allow a vortex to develop which pulls flue gases back into the purge nozzle, potentially contaminating the optical surfaces.

AMETEK Land recommends the use of a side-channel blower to provide the purge air. This is a high-volume pump with only one moving part, supplied with an inlet filter and a weather cover. In most cases, a single blower can provide sufficient air for both transceiver and retroreflector.

Many users have asked whether they can use compressed air to purge the opacity monitor. In general, this is not something we recommend because it is very expensive to maintain such a high flow of clean, dry, oil-free air. The payback time for a purge blower is usually in the region of 6 months.

Click here to learn more about the opacity monitors we have to offer.

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