Sierra Instruments, Inc.

Advances in Thermal Dispersion Mass Flow Meters - Part 1 Principle of Operation

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Courtesy of Sierra Instruments, Inc.

This paper describes an advanced microprocessor-based thermal dispersion mass flow meter that uses four temperature sensing elements in its flow sensor instead of the traditional two elements and has a dry velocity sensor that uses no potting materials instead of the traditional potted velocity sensor. This unique new mass flow meter automatically manages changes in gas selection, gas temperature, gas pressure, and outside temperature. This Part 1 describes the principle of operation of the four-temperature system. Part 2 of this two-part series describes advances in the installation and flow calibration of thermal dispersion mass flow meters.

Thermal dispersion mass flow meters measure the mass flow rate of fluids, primarily gases, flowing through a closed conduit. Their first general description 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 thermal dispersion flow sensors used in fluid mechanics research and as light-duty mass flow meters and point velocity instruments. This class of thermal dispersion mass flow meters is described in Reference 2.

It was not until the 1960s and 1970s that industrial-grade thermal dispersion mass flow meters emerged that could solve the wide range of general industry’s more ruggedized needs for directly measuring the mass flow rate of air, natural gas, and other gases in pipes and ducts. That is the class of instruments described here. Thermal dispersion mass flow meters 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, thermal dispersion mass flow meters directly measure mass flow rate. Capillary tube thermal mass flow meters constitute a second type of thermal mass flow technology, but their principle of operation and their applications are sufficiently different that the American Society of Mechanical Engineers (ASME) has published separate national standards for each type [3] [4].

Typical gases monitored by industrial thermal dispersion mass flow meters 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 waste water treatment plants; cogeneration with biogas; fuel gas; flare gas; semiconductor manufacturing; heating, ventilation, and air conditioning; single and multipoint stack gas monitoring; and chemical reactors.

General Description
Thermal dispersion mass flow meters directly measure the mass flow rate of single-phase pure gases and gas mixtures of known composition flowing through pipes or other flow conduits. As discussed in a later subsection, they also have limited application to single-phase liquids of known composition. In most of the following, we shall assume that the fluid is a gas, without the loss of applicability to liquids. Multivariable versions also provide an output for gas temperature and also, but less commonly, of gas pressure.

Thermal dispersion mass flow meters have two primary configurations: in-line and insertion. Figures 1(a) and 1(b), respectively, show these two configurations and their major components. Figure 1(c) shows the flow sensor that is common to both configurations, although in smaller in-line meters the flow sensor may not have a shield.

In-line flow meters are applied to pipes and ducts with diameters typically ranging from about 10 to 100 mm (0.25 to 4.0 inch pipe sizes), but some manufacturers offer sizes up to 300 mm (12.0 inch pipe size) . Process connections include flanges, pipe threads, and compression fittings. The built-in flow conditioner, described in Part 2 of this two-part series, reduces the length of upstream straight pipe required to achieve independence of upstream flow disturbances.

Insertion flow meters [5] usually are applied to larger pipes, ducts, and other flow conduits having equivalent diameters typically ranging from approximately 75 mm to 5 m. Because insertion meters are more economical than in-line meters, they also have found wide use as flow switches. Compression fittings and flanges are commonly used process connections. Insertion meters measure the mass velocity at a point in the conduit’s cross-sectional area, but for applications with smaller conduits, they may be flow calibrated to measure the total mass flow rate through the conduit. Multipoint insertion meters measure the mass velocities at the centroids of equal areas in the cross section of large pipes, ducts, and stacks. The total mass flow rate through the entire conduit is the average mass velocity of the several points multiplied by the total cross-sectional area and the standard mass density of the gas [6].

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