Thermal mass flow meters are used in the monitoring and/or controlling of mass-related processes – such as chemical reactions – that depend on the relative masses of unreacted ingredients. In detecting the mass flow of compressible vapors and gases, the measurement is unaffected by changes in pressure and/or temperature. One of the capabilities of thermal mass flow meters is to accurately measure low gas flow rates or low gas velocities (under 25 ft. per minute) – much lower than can be detected with any other device.
Thermal mass flow meters are available in high-pressure and high-temperature designs – and in special materials including glass, Monel®, and PRA. Flow-through designs are used to measure small flows of pure substances (heat capacity is constant if a gas is pure), while bypass and probe-type designs can detect large flows in ducts, flare stacks, and dryers.
Theory of OperationThermal mass flow sensors operate either by introducing a known amount of heat into the flowing stream and measuring an associated temperature change or by maintain a probe at a constant temperature and measuring the energy required to do so.
The components of a basic thermal mass flow meter include two temperature sensors and an electric heater between them. The heater can protrude into the fluid stream (Figure 1), or can be external to the pipe (Figure 2).
In the direct-heat version, a fixed amount of heat (q) is added by an electric heater. As the process fluid flows through the pipe, heat is drawn, and resistance temperature detectors (RTDs) measure the temperature rise while the amount of electric heat introduced is held constant. Usually, Pt100 type RTDs are used for this temperature measurement. Once the fluid starts to circulate in the measuring tube, the heated temperature sensor is cooled by the fluid's motion, with the speed of the flow determining the degree of cooling. As a result, the electric current needed to keep the temperature difference constant is a direct indication of mass flow.
The mass flow (m) is calculated on the basis of the measured temperature difference (T2-T1), the meter coefficient (K), the electric heat rate (q), and the specific heat of the fluid (Cp), as follows: m = Kq/(Cp(T2-T1))