![]() ![]() These proprietary flow tubes usually cost less than the classical and short-form venturis because of their short lay length. They all have short lay lengths, typically varying between 2 and 4 pipe diameters. The various flow tube designs vary in their contours, tap locations, generated d/p, and in their unrecovered head loss. The best known of these proprietary designs is the universal venturi (Figure 2-B). There are several propriety flow tube designs which provide even better pressure recovery than the classical venturi. ![]() In slurry service, the pipe taps can be purged or replaced with chemical seals, which can eliminate all dead-ended cavities. Piezometer rings can be used with large venturi tubes to compensate for velocity profile distortions. Pressure taps are located ¼ to ½ pipe diameter upstream of the inlet cone, and in the middle of the throat section. The short form venturi maintains many of the advantages of the classical venturi, but at a reduced initial cost, shorter length, and reduced weight. In the short form venturi, the entrance angle is increased, and the annular chambers are replaced by pipe taps (Figure 2-A). The classical venturi is limited in its application to clean, non-corrosive liquids and gases. The pressure taps feed into a common annular chamber, providing an average pressure reading over the entire circumference of the element. Inlet pressure is measured at the entrance, and static pressure in the throat section. The classical Herschel venturi has a very long flow element characterized by a tapered inlet and a diverging outlet. In spite of its high initial cost, the total cost of ownership of this type of instrumentation can still be favorable because of savings in installation and operating and maintenance costs. Their contoured nature, combined with the self-scouring action of the flow through the tube, makes the device immune to corrosion, erosion, and internal scale build up. Venturis are insensitive to velocity profile effects and therefore require less straight pipe run than an orifice. The initial cost of venturi tubes is high, so they are primarily used on larger flows or on more difficult or demanding flow applications. Furthermore, the total unrecovered head loss rarely exceeds 10% of measured d/p (Figure 1). And in this example also an advantage is that there is less pressure drop - for which again must be compensated - compared to venturi differential pressure flowmeters.Venturi tubes are available in sizes up to 72” and can pass 25 to 50% more flow than an orifice with the same pressure drop. By averaging the various point speeds, the uncertainty in the flow measurement is reduced. In very large pipes - such as in the air supply for burner stoves in power plants - a "network" of flow meters is placed, the so-called multipoint flow meter. Another way to solve the problem of a point measurement is a multipoint flowmeter. Fox Thermal supplies specifically for its thermal flowmeters a flow conditioner which gives a negligible pressure drop. Flow conditioning is actually recommended in all cases. ![]() But if this is not the case, there are greater uncertainties. This need not be a problem if the inlet and outlet lengths are long. This makes the flow meter highly sensitive to flow profiles. They measure the gas mass speed at a specific point in the pipeline. The disadvantage of insertion flowmeters is that they are point meters. The microprocessor then linearizes this data to provide a 4-20mA output signal. The instrument electronics measure the required power and thus the cooling effect of the gas flow. The electrical power required to keep the temperature difference constant is then directly proportional to the mass flow of the process gas. We supply Fox Thermal flowmeters which use technology that keeps the temperature difference between the two sensors constant. And the lower the temperature (also higher density) the stronger the cooling. But also the higher the density (due to higher pressure) the stronger the cooling. So the higher the speed of the gas, the stronger the cooling. The amount of cooling depends on the amount of molecules that dissipate kinetic energy. Because the heated sensor has a higher temperature than the environment it can cool down. One sensor measures the temperature of the gas, the other is heated by a built-in resistance wire. These flow meters have 2 thermal sensors that insert into the pipeline in the flowing gas. Insertion thermal flowmeters are also called "full bore" thermal flowmeters or industrial thermal flowmeters. ![]()
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