Sierra Instruments, Inc.

New Developments in Thermal Dispersion Mass Flow Meters

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Courtesy of Sierra Instruments, Inc.

Introduction
This paper describes a newly developed microprocessor-based thermal dispersion mass flow meter that uses four temperature sensing elements in its flow sensor instead of the traditional two elements and automatically manages changes in gas selection, gas temperature, gas pressure, and outside temperature. This new mass flow meter is ideally suited for apportionment applications in natural gas distribution systems.

Applications
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 natural gas, air, 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].

Figure 1 In-line and insertion meter configurations of thermal dispersion mass flow meters.

Typical gases monitored by industrial thermal dispersion mass flow meters include: natural gas, air, methane, carbon dioxide, nitrogen, oxygen, argon, helium, hydrogen, propane, natural gas, and stack gases, as well as mixtures of these gases and mixtures of hydrocarbon gases. Common applications are: natural gas distribution systems; combustion air; preheated air; compressed air; fluid power; boilers; electric power plants; cooling, heating, and mixing; drying of materials; food and beverage industries; 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.

Table 1 Calculation of output variables for in-line flow meters

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