Thermal mass flowmeters

Thermal flowmeters are mostly used in gas services, but can also detect liquid flows. They can measure flows from 0 to 10,000 kg/h (22,000 lb/h) with 1 to 2% FS errors. Their rangeability is from 10 1 to 100 1, their operating temperature range is up to 500°C (950°F), and they can operate at pressures from atmospheric to 83 bar (1,200 psig). 

Thermal flowmeters can be divided into the following two categories (1) flowmeters that measure the rise in temperature of the fluid after a known amount of heat has been added to it, which can be called heat transfer flowmeters and (2) flowmeters that measure the effect of the flowing fluid on a hot body, which are sometimes called hot-wire probes, or heated-thermopile flowmeters. 

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Thermal flowmeter with external temperature sensors and heater. 

To facilitate measurement and control of larger flow rates, the heat-transfer-type flowmeters can also be placed in a small bypass around a capillary element in the process pipe. The laminar flow element serves to ensure laminar flow and also acts as a restriction forcing a portion of the flow into the sensor tube. The small-size bypass serves to minimize the electric power requirement and to increase the speed of response, but it requires upstream filters to protect it against plugging. 

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General Principles There are two main types of mass flowmeters (1) the so-called true mass flowmeter, which responds directly to mass flow rate, and (2) the inferential mass flowmeter, which commonly measures volume flow rate aud flmd density separately. A variety of types of true mass flowmeters have been developed, including the following (a) the Maguus-effect mass flowmeter, (b) the axial-flow, transverse-momentum mass flowmeter, (c) the radial-flow, transverse-momentum mass flowmeter, (d) the gyroscopic transverse-momentum mass flowmeter, aud (e) the thermal mass flowmeter. Type b is the basis for several commercial mass flowmeters, one version of which is briefly described here. 

Mass Meters Mass flowmeters measure the rate of mass flow through a conduit. Examples include Coriolis flowmeters and thermal mass flowmeters. Coriolis flowmeters can measure fluid density simultaneously with mass flow rate. This permits calculation of volumetric flow rate as well. Section 8 includes brief descriptions of Coriolis and thermal mass flowmeters. 

Figure 6.7. Thermal mass flowmeter and sensor element (Hewlett-Packard Co. available only on 5830/40 series chromatographs).

Thermal Mass Flowmeters The trend in the chemical process industries is toward increased usage of mass flowmeters that are independent of changes in pressure, temperature, viscosity, and density. Thermal mass meters are widely used in semiconductor manufacturing and in bioprocessing for control of low flow rates (called mass flow controllers, or MFCs). MFCs measure the heat loss from a heated element, which varies with flow rate, with an accuracy of 1 percent. Capacitance probes measure the dielectric constant of the fluid and are useful for flow measurements of slurries and other two-phase flows. 

Venturi-type thermal mass flowmeter. (Courtesy of TSI Inc.)... 

More flexibility (including the possibility of determining the desorption branch) is obtained, at the expense of stability, by the continuous gas-flow controlled procedure (Venero and Chiou, 1988), presented in Figure 3.9. Here, the flow of adsorptive is set at a pre-determined value and then controlled by a loop including the flowmeter and the leak-valve. With a thermal mass flowmeter of good quality, flow rates can be correctly controlled down to c. 5 cm3 h-1 With microporous adsorbents, and also when a low specific surface area necessitates the use of large amounts of sample, the flow rate may prove to be a limitation (i.e. not low enough to ensure the required quasi-equilibrium conditions). 

OTHER INSERTION METERS. Modified forms of magnetic meters, turbine meters, ultrasonic meters, thermal mass flowmeters, and other types are available as insertion meters. They all have advantages for certain services. Insertion meters are generally cheaper than full-bore meters and are usually the most cost-effective method of measuring flow in large pipes. 

TABLE III. COMPARISON OF DRY TEST METER AND THERMAL MASS FLOWMETER FLOWRATES IN THE RANGE OF 500-1500 seem... 

Test Gas Dry Test meter Flowrates, seem, measured by Thermal mass Flowmeter ... 

This is obviously a time-consuming method, and automatic collecting and measuring equipment is now available. In the automated gas burette developed by ICI (Figure 3.10), the liquid height is measured by a pressure transducer. After a preset volume of gas has been collected, a solenoid-controlled valve opens to empty the tube. The output from the transducer is recorded continuously and provides a measure of the gas evolved. Lambert and Amery describe a thermal mass flowmeter which records gas flow automatically, is simple to use and can handle most gases including those which are corrosive. 

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