Thermal flow meters

Differential-Temperature Thermal Flow Meters. Meters of this type inject heat into the fluid and measure the resulting temperature rise or, alternatively, the amount of power required to maintain a constant temperature differential. The power required to raise the temperature of a flowing stream by an amount AT is given by the relation ... 

Fig. 18. (a) Differential-temperature thermal flow meter, (b) Tube length-temperature profiles for (—) 2ero flow and ( ) flow. AT is the temperature... 

There are do2ens of flow meters available for the measurement of fluid flow (30). The primary measurements used to determine flow include differential pressure, variable area, Hquid level, electromagnetic effects, thermal effects, and light scattering. Most of the devices discussed herein are those used commonly in the process industries a few for the measurement of turbulence are also described. 

The Wavebag (50 L volume) was placed on the temperature-controlled tray and completely inflated with air using a membrane pump. The airflow was adjusted to 2250 mL using a thermal mass flow meter. 

Figure 13.3 Flow outgassing with thermal conductivity detector, (a) a, Purge gas source b, needle valve c, cold trap Dj and D2, thermal conductivity detectors e, flow selector valve f, cell with powder g, flow meter, (b) Heated filaments 1 and 3 are D2,2 and 4 are. ...

Figure 1. Schematic of the apparatus (1) thermal conductivity cell detector, (2) column, (8) flow meter, (4) pressure regulator, (5) drying trap, (6) injection valve, (7) recording device (A) T.C. detector, (B) power supply, (C) recorder, (D) dc micro-voltmeter, (E) FM adaptor, (F) magnetic tape recorder...

Thermal /caloric flow meter 400 liquids 700 gases 50 1 0.5-0,8% 0.1-0,2% of span Only for very small flow rates [59,60]... 

Fig. 4.4-3 Thermal conductivity flow-meter for low flow-rates and a maximum pressure of 700 bar [60],...

ISO 8301, 1991. Thermal insulation - Determination of steady state thermal resistance and related properties - Heat flow meter apparatus. 

BS EN 12664, 2001. Thermal performance of building materials and products. Determination of thermal resistance by means of a guarded hot plate and heat flow meter methods. Dry and moist products of medium and low thermal resistance,... 

ASTM C518, 2004. Standard test method for steady state thermal transmission properties by means of the heat flow meter apparatus. 

A popular series of mass flow meters is available which is based on the use of a thermal sensor in a side stream (Fig. 7.13). Generally, these flow meters are... 

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. 

Fuel flow control in low-temperature cells is relatively simple. The control valves used are servomotor-driven quarter-turn valves, and the flow sensors can be the types used on gas furnaces, such as thermal dispersion type mass flow meters. 

Controlling the water feed to these boilers is a challenge in itself as the flow rates are low. For a 40 kW fuel cell system, the required water flow range is 0.25 to 10 g/s. Thermal dispersion flow meters can be considered. If conventional control valves do not prove adequate, pulsed solenoids can be used, with the flow being averaged to match the required ratio to the fuel gas. 

Velocity meters measure the velocity v of fluid flow in a pipe of known cross section, thus yielding a signal linearly proportional to the volume flow rate Q. Mass meters provide signals directly proportional to the mass flow rate m = pQ, where p is the mass density. Coriolis meters, which are true mass meters, can be used only for liquids. Thermal-type flow meters use a heating element and determine the rate of heat transfer, which is proportional to the mass flow rate. This type of device is used mostly for gas measurements, but liquid flow designs are also available. 

ASTM C 518-85 Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus, 17 pp (Comm C16)... 

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