Terminal-tube velocity

Terminal-tube velocity. Have you ever seen a heat-exchanger tube bundle pulled Perhaps many of the tubes were bent and twisted like partly cooked spaghetti. This distortion could not be very good for the... 

To summarize, some of the tubes in the bundle continue to foul until they plug off. These tubes get hotter until they reach the temperature of the hot oil circulating through the shell. Other tubes continue to receive more and more flow. The velocity in these tubes increases to the point where the rate of fouling becomes inconsequential. This velocity is called the terminal-tube velocity. These tubes may run 50 to 150°F cooler than the plugged tubes. 

Figure 19.5 Effect of terminal-tube velocity on exchanger performance.

This is an exchanger that was designed to operate with a low tube-side velocity. Note how there is a rapid loss in the heat-transfer coefficient, as the tubes foul and plug, as a result of low velocity. The loss in heat-transfer coefficient U stops only when the terminal-tube velocity is reached in the unplugged tubes. 

The effects of excessively low tube-side velocity will be to damage the tube bundle due to terminal tube velocity problems. 

Sedimentation of particles follows the principle outlined above [Eq. (1)] in which particles in the Stokes regime of flow have attained terminal settling velocity. In the airways this phenomenon occurs under the influence of gravity. The angle of inclination, t /, of the tube of radius R, on which particles might impact, must be considered in any theoretical assessment of sedimentation [14,19]. Landahl s expression for the probability, S, of deposition by sedimentation took the form ... 

Results are shown graphically in Figure 4 for a brine temperature of 220°F., condenser tube velocity of 5 feet per second, blowdown temperature of 90°F., and brine concentration of twice sea water. As can be seen, a minimum water cost for these conditions is obtained with a 50-stage plant operating with a terminal temperature difference of about 4°F. Similar calculations were made for a blowdown concentration of 1.5 times sea water and for a once-through system. By cross plotting, it was then possible to determine the optimum blowdown salt concentration for the plant. It was about 1.7 times sea water. However, the curve is almost flat in the range of 1.5 to 2.0 times sea water. 

The walls of the vessel containing the liquid exert an extra retarding effect on the terminal falling velocity of the particle. The upward flow of the displaced liquid, not only influences the relative velocity, but also sets up a velocity profile in the confined geometry of the tube. This effect may be quantified by introducing a wall factor, /, which is defined as the ratio of the terminal falling velocity of a sphere in a tube, V, to that in an imconfined liquid, V, viz.. 

Different nozzle-shaped tubes are available, which are pressed onto the exhaust terminal. The air passes through the tube and a one-point velocity measurement is carried out in the throat of the device. The flow rate is determined from the calibration curve. 

When the static pressure in a moving fluid is to be determined, the measuring surface must be parallel to the direction of flow so that no kinetic energy is converted into pressure energy at the surface. If the fluid is flowing in a circular pipe the measuring surface must be perpendicular to the radial direction at any point. The pressure connection, which is known as a piezometer tube, should terminate flush with the wall of the pipe so that the flow is not disturbed the pressure is then measured near the walls where the velocity is a minimum and the reading would be subject only to a small error if the surface were not quite parallel to the direction of flow. A piezometer tube of narrow diameter is used for accurate measurements. 

In the riser tube, the gas velocity of chlorine, is greater than both of the terminal velocities of the slag particle and the petrocoke particle, makes the particles to be at a pneumatic transport state. No agglomeration occurs in the riser tube. At the top of the riser tube, a... 

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