Nozzles and Venturi

Table 1. Required Straight Pipe Lengths for Orifice Plates, Nozzles, and Venturis ...

FIG. 10-16 Values of expansion factor Y for orifices, nozzles, and venturis. 

Ejector Performance The performance of any ejec tor is a function of the area of the motive-gas nozzle and venturi throat, pressure of the motive gas, suction and discharge pressures, and ratios of specific heats, molecular weights, and temperatures. Figure 10-102, based on the assumption of constant-area mixing, is useful in evaluating single-stage-ejector performance for compression ratios up to 10 and area ratios up to 100 (see Fig. 10-103 for notation). 

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

ISO 5167-1 1991. Measurement of Fluid Flow by Means of Pressure Differential Devices-Fart 1 Orifice Plates, Nozzles and Venturi Tubes Inserted in Circular Cross-section Conduits Running Full. International Organisation for Standardisation, 1991. 

These conditions are similar to flow through orifices, nozzles, and venturi tubes. Flow through nozzles and venturi devices is limited by the critical pressure ratio, r,. = downstream pressure/upstream pressure at sonic conditions (see Figure 2-38C). 

For nozzles and venturi meters, the flow is limited by critical pressure ratio and the minimum value of Y to be used. 

Figure 2-38C. Critical Pressure Ratio, r, for compressible flow through nozzles and venturi tubes. By permission, Crane Co., Technical Paper 410, 1957. Also see 1976 edition. See note at Figure 2-18 explaining details of data source for chart. Note P = psia p= ratio of small-to-large diameter in orifices and nozzles, and contractions or enlargements in pipes.

Figure 10-6 International standard shapes for nozzle and venturi meter. (From White, 1994.)...

Figure 10-11 Expansion factor for orifice, nozzle, and venturi meter, (a) k = 1.3 (b) k = 1.4. (From Crane Co., 1978.)...

In the presence of flow pulsations, the indications of head meters such as orifices, nozzles, and venturis will often be undependable for several reasons. First, the measured pressure differential will tend to be high, since the pressure differential is proportional to the square of flow rate for a head meter, and the square root of the mean differential pressure is always greater than the mean of the square roots of the differential pressures. Second, there is a phase shift as the wave passes through... 

International Organization for Standards Report DIS 5167, Geneva, 1976). Similar equations are given for other kinds of orifice taps and for nozzles and Venturi meters. 

FIGURE 5.18 International standard shapes for nozzle and venturi meters. (From White, F. M., Fluid Mechanics, 3rd ed., McGraw-Hill, New York (1994).)... 

The bnttons at the end of the lines in Figure 5.23 for the nozzle and venturi correspond to critical (choked) flow. Althongh there are no buttons shown on the lines for the orifice, choked flow does occnr in orifices as well. However, the choked flow is not as reproducible for orifices because of the variable effects of the vena contracta. 

Measurement of Eluid Elow in Pipes Using Orifice, Nozzle, and Venturi," ASME MFC-3M-1989, The American Society of Mechanical Engineers, 345 East 47th Street, New York. 

ASME recently approved a Standard covering Measurement of Fluid Flow in Closed Conduits with Orifice, Nozzle and Venturi. This has been proposed to ANSI. 

Measurement of fluid flow in pipes, udng oti e, nozzle, and Venturi... 

Ya = adiabatic-expansion factor for flow nozzles and venturi tubes... 

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