A combination of Venturi tube and orifice plate. This tool is generally used for applications with large flow rates. This is limiting type flowmeter, which causes a slight reduction in flow velocity by construction. Differential pressure produced by a dall tube higher than the venturi tube and the nozzle, all with the same throat diameters.
It has the same conical entrance portion of a venturi tube but has a “shoulder” similar to the exit portion of the orifice plate to create a strong pressure drop. The pressure differential is typically measured using diaphragm pressure transducers with digital readout.
Construction:
The construction of Dall tube is much simpler than venturi meter, which needs complex machinery and it is much shorter which makes the insertion to the flow channel easier.
The tube consists of a short and straight entry section followed by an abrupt decrease in the inner diameter of the tube. This section, called the input shoulder, is followed by the convergent input cone and a divergent output cone.
The two cones are separated by a slot or space between the two cones. The low pressure is measured in the grooved throat (the area between the two cones). The high pressure is measured at the edge upstream of the input shoulder.
The dall flow tube is available in medium to very large sizes. In large sizes, the cost is usually less than that of a venturi flow tube. This type of flow tube has a pressure loss of around 5%. . Flow and pressure drop are related as shown in the equation:
Advantages of Dall flowmeter:
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Short length
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Lower pressure loss
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Dall Flow Tube has a developed pressure ratio higher than the pressure loss than the venturi flow tube.
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Accurate flow metering of clean gases, liquid and steam
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It is more compact and is commonly used in large flow applications.
Disadvantages:
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More complex to manufacture
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Accuracy based on flow data
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Sensitive to turbulance