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Permanent Pressure Loss

By Rajaram K. Prabhudesai
From Chemical Engineering License Review, Second Edition

Permanent Pressure Loss

Venturi Permanent pressure loss through venturis depends upon the diameter ratio ? and the discharge cone angle ?. The pressure loss for smaller angles (5 to 7 ) is 10 to 15 percent of the total pressure differential ( P 1 ? P 2). It is 10 to 30 percent for large angles (> 15 ).

Subsonic Flow Nozzles For subsonic flow nozzles, the pressure loss is given by

where ? is the nozzle throat diameter divided by the pipe diameter.

Concentric Circular Nozzles In the case of the concentric circular nozzle,

Example 2-2

A venturimeter is to be installed in a schedule 40, 6-in line to measure the flow of water. The maximum rate is expected to be 800 gpm at 86 F. A 50-inHg manometer is to be used. Specify the throat diameter of the venturi and calculate the power required to operate it. The discharge cone angle is 5 .

Solution. From Eq. (2-14),

C = 0.98 for venturis if N Re > 10,000 (i.e., fully turbulent flow). Assume fully turbulent flow. Then

where D P is the inside diameter of pipe, ft. Squaring both sides, one obtains

Simplifying and solving for D T gives

Use 2.375 in = 0.1979 ft (a standard size). Check the Reynolds number through the throat of the venturi.

This N Re is greater than 10,000 and therefore the flow is in the fully turbulent region. Therefore, the size of...

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