Editor’s choice
FMT400 thermal mass flowmeter helps maximise biogas production
Volume flow is the most common metric used to determine the flow of liquids; however, it is rarely used for gas due to the vast volume changes that can occur with a change in either the temperature or pressure of the gas. To determine an accurate flow rate for gases a mass flow is preferable. In our Q&A article David Bowers, Pressure and Process Flow product manager UK&IE for ABB Measurement & Analytics, explains how ABB thermal mass flowmeters have increased in accuracy and flexibility.
Describe the FMt400 therMal Mass FlowMeter?
The FMT400 is a highly accurate ATEX-rated thermal mass flowmeter for gases that operates across the widest flow range, at low pressures and/or flowrates.
what are the key beneFits to custoMers?
Utility companies and sewage treatment works are using anaerobic digestion to treat biodegradable waste and sewage sludge to produce biogas. Biogas is a mixture of methane, carbon dioxide and other contaminant gases which is a by-product of the microorganisms breaking down the organic matter contained in the sewage sludge. The methane and any carbon monoxide can be combusted to produce energy. In addition, the sewage sludge is stabilised and its dry matter content is reduced. Today a much of the biogas produced in anaerobic digestion plants is from those on municipal wastewater treatment sites, but they are also used for the break down and recycling of food waste. To run an efficient anaerobic digestion process, the biogas produced must be measured. The rate of digestion can alter dependent on the weather, the time of year and the amount of biomass added. Thermal mass flowmeters can be used to accurately measure the mass and composition of the biogas produced in the process, but also used to help control the process by adding oxygen or air to the digester to ensure the bacteria thrive. Biogas production is one of many uses for thermal mass flowmeters. Perhaps the most common use for these sensors is in determining the efficiency of any combustion engine. The mass flow is a critical measurement when it
Gas flow ṁ
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comes to determining the flow of gases and controlling combustion. Mass flow can also be crucial for the control of many industrial processes, including in the water industry the amount of ozone delivered to water to clean the bacterial from it, or carbonise the water to adjust its pH. In industry they are used to measure the use of compressed air, or in the food and beverage industry the amount of nitrogen used as a preservative in packaged foods. Thermal mass flowmeters are therefore commonly used for measuring flow of gases.
how Do they work?
The thermal mass flowmeter works on the principal of heat energy exchanges. As the diagram below shows, the heated sensor is placed in a fluid stream, the heat will transfer from the heated sensor to the fluid. The type and quantity of the fluid molecules impinging the surface will determine the amount of heat lost and as such the mass of that fluid flow can be determined.
The amount of heat exchanged will also depend on the temperature of the fluid itself and so two sensors are included in the flowmeter and a constant temperature difference is maintained between the two sensors. The platinum sensing element is part of a circuit and as its heat is reduced by the flow of fluid passed it, the resistance also reduces. The
Gas temperature sensor
sensor is designed to maintain a constant temperature difference between the two probes and therefore a higher heating current is required to maintain that temperature. The mass flowrate is proportional to the heating current. The principle of thermal mass flowmeter operation was founded in 1914 and known as the “hot-wire anemometer”.
how accurate are these sensors?
Thermal mass flowmeters work from very low pressures and have an excellent turndown performance of typically 100:1. It is incredibly important to have this low turndown particularly when working with flammable gases
Regulation of a constant temperature difference between
both sensors (e.g. 40°C) with heating current Ih Type and quantity of the gas molecules impinging on the heated surface determine the amount of heat.
Electric heat able platinum sensor
Heating current Ih ~ Gas flow m April 2021 Instrumentation Monthly
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