|Mass Thermal Flowmeters - How they work
Thermal flowmeters were born on the West Coast of the United States—the result of independent development by first two, then three separate companies. One company began by developing thermal flow switches that were used in the oil patch. The switches detected the movement of oil in oil well pipes, but they didn’t actually evolve into actual flowmeters until 1981.
The second strain of early development in the flowmeter marketplace was a result of the collaboration of John Olin, Ph.D., and Jerry Kurz, Ph.D. They developed hot-wire and hot-film anemometers for research applications in gas dynamics, turbulence, and air flows. These instruments are based on thermal dispersion technology and have sensors consisting of heated tungsten wires or a thin platinum film deposited on a quartz rod. The small diameter of these sensors gave the fast time response needed for fluid mechanics research, but made them too fragile for general industrial applications.
How they work
Thermal flowmeters are used almost exclusively to measure gas flow. They introduce heat into the flowstream, and measure the rate of heat dissipation. The heat dissipation is measured by one or more temperature sensors. This method works best with gas flow because the much greater heat absorption capacity of liquids rapidly saturates the signal, leading to a loss of measurement resolution.
While all thermal flowmeters inject heat into the flowstream, there are two different methods used to measure the rate of heat dissipation. One method is called constant temperature differential. Thermal flowmeters using this method utilize a heated RTD as a velocity sensor and another sensor that measures the temperature of the gas. The flowmeter attempts to maintain a constant difference in temperature between the two sensors. Mass flowrate is computed based on the amount of electrical power added to heat the velocity sensor to maintain this constant difference in temperature.
The second method is called constant current. It also uses a heated RTD as a velocity sensor and another temperature sensor to measure the temperature of the flowstream. With this method, the power to the heated sensor is kept constant. Mass flow is computed based on the difference in temperature between the heated velocity sensor and the temperature of the flowstream. Both methods make use of the principle that higher velocity flows produce a greater cooling effect. And both methods measure mass flow based on measuring the amount of cooling that occurs in the flowstream.
Environmental Applications Drive the Market
Thermal flowmeters are used for a wide variety of applications. However, there are two periods in the history of thermal meters when environmental applications have boosted sales. The first period was in the early 1990s, when the need for continuous emissions monitoring (CEMS) required measurement of sulfur dioxide (SO2) and nitrous oxides (NOX). Thermal flowmeters were ideal for this purpose. By combining a measurement of the concentration of SO2 and NOX with a measurement of flowrate, it can be determined how much of these gases are released into the atmosphere. This is important since they have been identified as the primary causes of acid rain. At that time, thermal flowmeters competed with averaging Pitot tubes and ultrasonic flowmeters to make this measurement.
The broad acceptance of global warming as scientific fact rather than as mere theory has presented a second opportunity for thermal flowmeters. Beginning with the election of the Obama administration in 2008, the U.S. government has made the identification and reduction of greenhouse gases a major priority. For example, the Obama administration has the stated goal of reducing greenhouse gas emissions by 80 percent by 2050.
Other countries have also joined in the effort to reduce greenhouse gases. The Kyoto Accord has resulted in the creation of several mechanisms for measurement of greenhouses gases internationally. These include Certified Emission Reductions (CER), a credit system designed to help European countries achieve reduced emission targets. A second program is the Clean Development Mechanism, which allows countries to help reduce emissions in developing countries by investing in sustainable development programs that have that result.
Some of the applications that have opened up as a result of efforts to reduce greenhouse gases include the following:
Ethanol distillation and refining
Recovery of methane from coal mines
Measuring emissions from boilers, process heaters, and steam generators
Measurement of recovery of landfill gases
Measurement and monitoring of flue gases and flare gases
More Growth & More Feature-Rich Products
The thermal flowmeter market is continuing to get a boost from the drive to measure greenhouse gas emissions, as well as other energy management applications. As a result, suppliers are responding with more feature-rich products that have enhanced performance levels. While the total market size still remains small relative to some other technologies, this market is poised for some very rapid growth. While thermal meters may never be as widely used as Coriolis meters, they have their own niche and set of applications. Competition in the market is increasing as suppliers introduce better performing products. This is a situation where end-users win. Look for more growth and more high-performance products to continue to emerge in this market.