Key Applications of Thermal Mass Flowmeters

Thermal mass flowmeters work on the principle that when you place a heated object in the midst of a fluid flow stream and measure how much heat the fluid transfers away from the heated object, then you can be able to determine the mass flow rate. Industrial thermal mass flowmeters consists of a specially designed flow tube with two temperature sensors inside: One that is heated and one that is unheated. The heated sensor acts as the mass flow sensor (cooling down as flow rate increases) while the unheated sensor serves to compensate for the “ambient” temperature of the process fluid.  The following diagram shows an example of a thermal mass flowmeter from Magnetrol.

Magnetrol Thermal Flowmeter

An important factor in the calibration of a thermal mass flowmeter is the specific heat of the process fluid. Fluid with high specific heat values make good coolants because they are able to remove much heat energy from hot objects without experiencing great increases in temperature. Since thermal mass flowmeters work on the principle of convective cooling, this means a fluid having a high specific heat value will elicit a greater response from thermal mass flowmeter than exact same mass flow rate of a fluid having a lesser specific heat value. Therefore it is paramount that you know the specific heat value of the fluid you plan to measure with a thermal mass flowmeter and be assured that its specific heat value will remain constant. For this reason, thermal mass flowmeters are not suitable for measuring the flow rates of fluid streams whose chemical composition is likely to change over time.

Another potential limitation of thermal flowmeters is the sensitivity of some designs to changes in flow regime. Since the measurement principle is based on heat transfer by fluid convection, any factor influencing the convective heat-transfer efficiency will translate into a perceived difference in mass flow rate. Turbulent flows are more efficient at heat convection than laminar flows. Therefore, a change in flow regime from turbulent to laminar will cause a calibration shift for this design of thermal mass flowmeters.

So what are some of the applications where thermal flowmeters are used?

Generally thermal flowmeters are used in applications where the composition of the fluid is known especially in purified gases. Having said that, lets look at some of the areas where thermal flowmeters are commonly used:

Natural Gas

Flow measurement of natural gas fuel usage is important for combustion efficiency as well as general energy management projects for both industrial and commercial facilities. Thermal mass flowmeters will monitor the flow to individual combustion sources.

Air Efficiency

In combustion applications, thermal mass flow meters can ensure repeatability of air flow measurements to obtain an efficient air-to-fuel ratio. On compressed air, it is common to measure for plant allocation or to determine leaks.

Tank Blanketing

Nitrogen is frequently used to maintain an inert environment in the vapor space of a tank. Thermal meters are ideally suited for measuring the flow of nitrogen in such applications because they support mass measurement, they are easily installed into the pipe & are excellent at measuring low flow rates.

You can also read: 

• The Ultimate Guide to Industrial Flow Instruments
• How Pitot Tubes are used In Flow Measurement

Flare & Vent Gas

Thermal mass meters are a particularly good fit for flare measurement. Flares can range from vent gases at atmospheric pressure to high flow applications needing extended turn down. Oil and Gas is a common industry here.

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Biogas

Both the landfills and anaerobic digesters at wastewater plants produce a mixture primarily composed of methane and carbon dioxide. Excellent low flow sensitivity, hot tap capabilities and mixed gas calibration make thermal mass flow measurement a popular technology.

Source: Magnetrol