Static pressure measurement

Disturbances upstream of the probe can cause large errors, in part because of the turbulence generated and its effect on the static-pressure measurement. A calming section of at least 50 pipe diameters is desirable. If this is not possible, the use of straightening vanes or a honeycomb is advisable. 

Gas flows are often determined by measuring the associated pressures. Figure 32-4 illustrates several different pressure measurements commonly made on systems carrying gases. Static pressure measurements are made to adjust the absolute pressure to standard conditions specified in the test procedure. 

TTie ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment ol thermal comfort. 

The pressure Pj, measured by the impact tube, is the stagnation pressure of the fluid given for ideal gases by Eq. (7.8). Then Eq. (7.9) applies, where p is the static pressure measured by tube b. Solving Eq. (7.9) for o gives... 

Pt - Psi = difference between total and static pressures measured with Pitot tube (in. H2O)... 

The solid symbols represent values of saturation vapor pressure, which correspond to the measured temperatures shown in parentheses. These temperatures ranged from 138.6° to 139.9 "R. The vapor pressures obtained from these temperature measurements agree with the local static pressures measured directly thus, thermodynamic equilibrium appears to exist within the cavity. The pressure within the cavity thus corresponds to the vapor... 

Figure 11.25 Side tube for static pressure measurement.

The fluid flows into the opening at point 2, pressure builds up, and then remains stationary at this point, called the stagnation point. The difference in the stagnation pressure at this point 2 and the static pressure measured by the static tube represents the pressure rise associated with the deceleration of the fluid. The manotheter measures this small pressure rise. If the fluid is incompressible, we can write the Bernoulli equation (2.7-32) between point 1, where the velocity y, is undisturbed before the fluid decelerates, and point 2, where the velocity yj is zero. 

EXAMPLE3.2-1 P IfIow Measurement Using a Pitot Tube A pitot tube similar to Fig. 3.2-la is used to measure the airflow in a circular duct 600 mm in diameter. The flowing air temperature is 65.6°C. The pitot tube is placed at the center of the duct and the reading Ah on the manometer is 10.7 mm of water. A static pressure measurement obtained at the pitot tube position is 205 mm of water above atmospheric. The pitot tube coefficient Cp = 0.98. 

This relationship was then used to calculate the tangential velocities from static pressure measurements in different places within hydrocyclones run with clean liquids. Driessen and many others following him thus deduced the general expression for tangential velocity profiles in the outer vortex given previously in equation 6.1, where n is an empirical exponent, usually from 0.6 to 0.9. Note that for a free vortex in inviscid flow n =, while in a forced vortex (solid body rotation) = 1. 

A Pitot tube is a simple device to produce a measure of abspeed. Pitot tubes are mounted on the outside of an abcraft s fuselage, pointing into the abflow. They measure the difference between the dynamic pressure of the air (the pressure measured when pointing into the direction of abflow), and the static pressure (measured perpendicular to the abflow). This pressure difference is proportional to the square of the airspeed (Fig. 4.1). 

Avoid "dynamic pressure" readings. Figure 52.3 illustrates the problem. The kinetic energy of the high-velocity inlet fluid will be converted to pressure as it impacts the vessel wall at Pi. This is called a dynamic pressure. The pressure at F2 will be lower than Pi. The correct pressure to read is the static pressure measured at P2. 

One way out of these difficulties lies in the observation that the static pressure at the wall is close to the cross-sectional mean of the static pressure plus the dynamic pressure stored in the swirl (Hoffmann et al., 1992). Or said in another way the static pressure measured at the wall is close to the static pressure that would be measured after an ideal rectifier (or pressure recovery diffuser ), which would convert all the swirl dynamic pressme into static pressme. We emphasize that this is not necessarily so, it only happens to be so because the static pressure in the vortex finder happens to be very nearly a linear function of the radius. Thus, in the absence of pressme recovery devices, the static pressure measured at the wall of the outlet tube minus the static pressme at the inlet gives the true dissipative loss between inlet and the measurement point in the outlet. One should be aware, though, that further dissipation of dynamic swirl pressure will take place in the downstream piping as the spin decays due to friction with the pipe wall, bends, etc. 

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