Liquid level factors

Process Measurements. The most commonly measured process variables are pressures, flows, levels, and temperatures (see Flow LffiASURELffiNT Liquid-levell asurel nt PressureLffiASURELffiNT Temperaturel asurel nt). When appropriate, other physical properties, chemical properties, and chemical compositions are also measured. The selection of the proper instmmentation for a particular appHcation is dependent on factors such as the type and nature of the fluid or soHd involved relevant process conditions rangeabiHty, accuracy, and repeatabiHty requited response time installed cost and maintainabiHty and reHabiHty. Various handbooks are available that can assist in selecting sensors (qv) for particular appHcations (14—16). 

Example 6 Losses with Fittings and Valves It is desired to calculate the liquid level in the vessel shown in Fig. 6-15 required to produce a discharge velocity of 2 m/s. The fluid is water at 20°C with p = 1,000 kg/m and i = 0.001 Pa - s, and the butterfly valve is at 6 = 10°. The pipe is 2-in Schedule 40, with an inner diameter of 0.0525 m. The pipe roughness is 0.046 mm. Assuming the flow is tiirhiilent and taking the velocity profile factor (X = 1, the engineering Bernoulli equation Eq. (6-16), written between surfaces 1 and 2, where the... 

The mass balance relationships for the feed plate, the plates in the stripping section, of the column and for the reboiler must, however, be modified, owing to the continuous feed to the column and the continuous withdrawal of bottom product from the reboiler. The feed is defined by its mass flow rate, F, its composition xp and the thermal quality or q-factor, q. The column bottom product is defined by its mass flow rate, W, and composition, xw and is controlled to maintain constant liquid level in the reboiler. 

The sum of the crest height plus the weir height equals the depth of liquid on the tray deck. One might now ask, Is not the liquid level on the inlet side of the tray higher than the liquid level near the outlet weir While the answer is Yes, water does flow downhill, we design the tray to make this factor small enough to neglect. 

Expert opinions differ somewhat on this factor. As a first approximation, the impeller can be placed at 1/6 the liquid level off the bottom. In some cases there is provision for changing the position of the impeller on the shaft. For off-bottom suspension of solids, an impeller location of 1/3 the impeller diameter off the bottom may be satisfactory. Criteria developed by Dickey (1984) are based on the viscosity of the liquid and the ratio of the liquid depth to the tank diameter, h/D,. Whether one or two impellers are needed and their distances above the bottom of the tank are identified in this table ... 

The gas pressure in the bulb equals the difference between the outside pressure and the manometer reading. The manometer reading indicates that the pressure of the gas in the bulb is less than atmospheric because the liquid level is higher on the side connected to the sample. Because mercury is more dense than mineral oil by a factor of 13.6/0.822, or 16.5, a given pressure will hold a column of mercury only 1/16.5 times the height of a column of mineral oil. 

Then the inventory loops are revisited. The liquid holdups in surge volumes are calculated so that the time constants of the liquid level loops (using proportional-only controllers) are a factor of 10 larger than the product-quality time constants. This separation in time constants permits independent tuning of the material-balance loops and the prod-... 

The power number and corresponding power of an anchor impeller are proportional to the height of the vertical arm. Thus, an anchor with a height H equal to 75 percent of the impeller diameter would have a power number equal to 75 percent of the typical values shown in Fig. 18-43. Similarly, a partially filled tank with a liquid level Z that covers only 75 percent of the vertical arm will also have a power number that is 75 percent of the typical correlation value. The addition of scrapers will increase the power requirement for an anchor impeller, but the effect depends on the clearance at the wall, design of the scrapers, processed material, and many other factors. Correlations are not practical or available. 

Column efficiency experimental data are usually compiled and saved and are used for estimating efficiencies of similar columns or for developing empirical correlations for predicting the efficiencies. When attempting to estimate a tray efficiency based on data available for another column, it is important to be aware of the factors that influence tray efficiency the most and take them into consideration. For instance, the tray dimensions should be comparable. Of particular importance are the tray diameter, tray spacing, number of passes, and liquid level on the tray. 

Feed to a falling film evaporator is usually introduced under the liquid level maintained at the top of the tubes, so that a reservoir of rather low velocity liquid is available for liquid distribution to the many vertical tubes. In falling film evaporator and re-boiler design, equal fluid distribution among the tubes and film initiation are very important factors. For this reason, a number of sophisticated and very effective hydraulic distributing devices have been developed to handle different types of process fluids.f In order... 

It is common practice to use geometric similarity in the scaleup of stirred tanks (butnot tubular reactors). Geometric similarity means that the production-scale reactor will have the same shape as the pilot-scale reactor. All linear dimensions such as reactor diameter, impeller diameter, and liquid height will change by the same factor, 5. Surface areas will scale as. Now, what happens to tmx upon scaleup A classic correlation by Norwood and Metzner (1960) for turbine impellers in baffled vessels can be used to estimate tm. The full correlation shows fmix to be a complex function of the Reynolds number, the Froude number, the ratio of tank to impeller diameter, and the ratio of tank diameter to liquid level. However, to a reasonable first approximation for geometrically similar vessels operating at high Reynolds numbers. 

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