Impinging streams, pressure drop

Gas-continuous impinging streams involve flows at high velocity, and so power consumption naturally becomes an important concern [62]. As is well known, the theoretical or minimum work per unit time for fluid transportation is equal to the product of the pressure drop and the volumetric flow rate of the fluid ... 

It is found from experimental measurements that the pressure drop caused by the impingement between the opposing streams is independent of the presence of particles. 

The pressure drops between Points A and C, between Points A and C, between Points A and B, and between Points A and B , denoted by -ApAC, -ApA C, -ApAB, and -A/Evb, respectively, are measured with inclined U-shape tubes filled with colored kerosene. The average of -ApAB and -ApA B- is taken as the pressure drop through the accelerating pipes, while that of -Ap v and -ApA c as the overall pressure drop across the impinging stream contactor, -ApT. Consequently, there should be... 

As described in Section 4.2, the sub-pressure drop measured between Points B and C or between Points B and C, -ApBC or -APb o is the sum of the pressure drop across the impingement zone, —Apim, and the structural resistance -A/ ds, but not any individual one of them. Such a measuring arrangement is not only because the pressure fluctuation frequently occurs round the impingement zone, making accurate measurement difficult, but also because both sub-pressure drops are too small (< 10 Pa) to be measured separately. To obtain the values for these sub-pressure drops, Eq. (4.17) can be specified for the impinging stream contactor studied here as... 

Combining the results of the theoretical analysis and the experimental investigation above, the overall pressure drop across the whole impinging stream contactor can be represented by... 

A comparison between the data measured for the overall pressure drop across the impinging stream contactor and the corresponding values calculated with Eq. (4.25) is illustrated in Fig. 4.6. Good agreement between the results measured and calculated can be observed clearly, suggesting that the total pressure drop model established is reasonable and feasible for application. In addition, it has the advantages of universal applicability and convenience in calculation. 

Bar and Tamir [63] studied the two-impinging stream dryer with two pairs of airfeeding tubes, as briefly shown in Fig. 6.7. The purpose of adding the lower two air streams is to increase the hold-up and the mean residence time of the particles in the dryer, and also aims to enhance the turbulence between phases in order to increase the drying intensity. However, the experiments did not show that the secondary air streams increased either the hold-up or the transfer coefficient. On the other hand, the induction of the two secondary air streams results in the greatly increased hydraulic resistance of the system. The pressure drop across the dryer, with two pairs of air-feeding tubes and with a volume treble that of the dryer shown in Fig. 6.6, is as high as 3800 Pa. 

The impinging stream gas-liquid reactor has low hydraulic resistance. In the range of operation conditions tested, the pressure drop across the reactor, Ap, is round 400 Pa only. 

Tamir, A., Luzzatto, K., Sartana, D. and Salomon, S. A. (1985). A correlation based on the physical properties of the solid particles for the evaluation of pressure drop in the two-impinging-streams gas-solid reactor. AIChE J., 31 1744. 

The pressure drop for single phase flow [AP in Eq. (5.13)] can be calculated from the following correlations derived for impinging streams with cylindrical impingement zones for H/D varying from 0.25 to 1.125 (Elperin et al., 1968) ... 

In the atomizing process, a stream of molten zinc is broken into tiny droplets by the force of a pressurized fluid impinging on the stream. The fluid can be any convenient material, although air is normally used. The atomized drops cool and soHdify rapidly in a coUection chamber. The powder is screened to specified sizes. Particulate zinc is also produced by other methods such as electrolytic deposition and spinning-cup techniques, but these are not of commercial importance. 

The particles in a pressurized gas jet do not necessarily move at the gas velocity. Measurement of gas entrained particle velocities is difficult. Therefore, another test method, involving moving the specimen through a suspended column of particles can also be used. The specimen or specimens are mounted on a rotating arm or wheel with set approach angles and rotated through a stream of particles dropped in their path or suspended in a fluidized bed. There is no ASTM standard for this however, recommended practice does exist for liquid particle impingement (see below). 

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