Sierra Instruments, Inc.

Advanced thermal dispersion mass flowmeters

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Courtesy of Sierra Instruments, Inc.

Thermal dispersion (TD) mass flowmeters measure the mass flowrate of fluids (primarily gases) flowing through a closed conduit, such as a pipe. This article describes the operation and installation of TD mass flowmeters, and gives the reader information about what applications these meters are most suited for.

Background

The first general description of TD mass flowmeters is attributed to L.V. King who, in 1914 [1], published his famous King's Law revealing how a heated wire immersed in a fluid flow measures the mass velocity at a point in the flow. He called his instrument a 'hot-wire anemometer.' The first application of this technology was hot-wire and hot-film anemometers and other light-duty TD flow sensors used in fluid mechanics research and as light-duty mass flowmeters and point velocity instruments. This class of TD mass flowmeters is described in Ref. 2.

It was not until the 1960s and 1970s when industrial-grade TD mass flowmeters emerged that could solve the wide range of general industry's more rugged needs for directly measuring the mass flowrate of air, natural gas and other gases in pipes and ducts. That is the class of instruments described here.

TD mass flowmeters measure the heat convected into the boundary layer of the gas flowing over the surface of a heated velocity sensor immersed in the flow. Since it is the molecules of the gas, which bear its mass, that carry away the heat, TD mass flowmeters directly measure mass flowrate. Capillary-tube thermal mass flowmeters constitute a second type of thermal mass flow technology, but their principle of operation and their applications are sufficiently different from TD mass flowmeters that the American   Society  of Mechanical Engineers (ASME) has published separate national standards for each type [3, 4].

Typical gases monitored by industrial TD mass flowmeters include: air, methane, natural gas, carbon dioxide, nitrogen, oxygen, argon, helium, hydrogen, propane and stack gases, as well as mixtures of these gases and mixtures of hydrocarbon gases. Common applications are: combustion air; preheated air; compressed air; fluid power; boilers; electric power plants; cooling, heating, and mixing; drying of materials; food and beverage industries; natural gas distribution; aeration and digester gas monitoring in wastewater-treatment plants; co-generation with biogas; fuel gas; flare gas; semiconductor manufacturing; heating, ventilation and air conditioning; single and multipoint stack-gas monitoring; and chemical reactors.

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