Air only pressure drop

Wypych, P. W., and Pan, R., Determination of Air-Only Pressure Drop in Pneumatic Conveying Systems, Powder Handling Processing, 3(4) 303-309 (1991)... 

The pressure drop required to convey the air alone through the pipeline provides a datum for the conveying system. Only when the conveying line pressure drop exceeds this datum value will any material be conveyed, but then the greater the excess over this datum pressure drop the greater will be the material flow rate. This is illustrated in Figure 4.40, with the zero material flow rate line being the air only pressure drop for the pipeline. 

This air only pressure drop can be calculated reliably from basic fluid mechanics. The equation used here is that derived by Darcy ... 

This is an inverse law relationship and so for a given air supply pressure, and hence energy value, if the length of pipeline is doubled, for example, there will be an approximate halving of the material flow rate. If scaling is to a longer pipeline there will be an additional loss to take into account, because of the increase in air only pressure drop for the pipeline, for... 

Since pipeline bore comes in incremental sizes, fine tuning and spare capability need to be considered in terms of reserve pressure available. With first approximation values for pressure and pipeline bore, the available conveying data can be scaled more precisely to take account of differences between pipeline geometries. Conveying air velocities and the solids loading ratio can be evaluated so that differences between air only pressure drop and acceleration pressure drop values can also be taken into account. This is an iterative process, as there are many inter-dependent variables, and so in the initial stages approximations can be made. 

Using previously determined data on wheat (due to performance data on the actual material being unavailable), the following pressures were predicted using the Weber A4 Model [3]. ms = 01 h l Apf = 37 kPa (air-only pressure drop - too high for efficient venturi operation) ms = 41 h-l Apt = 66 kPa (close to foe specification) ... 

Dust Filter. The cloth or bag dust filter is the oldest and often the most reHable of the many methods for removing dusts from an air stream. Among their advantages are high (often 99+%) collection efficiency, moderate pressure drop and power consumption, recovery of the dust in a dry and often reusable form, and no water to saturate the exhaust gases as when a wet scmbber is used. There are also numerous disadvantages maintenance for bag replacement can be expensive as well as a sometimes unpleasant task these filters are suitable only for low to moderate temperature use they cannot be used where Hquid condensation may occur they may be hazardous with combustible and explosive dusts and they are bulky, requiring considerable installation space. 

The so-called hyperbar vacuum filtration is a combination of vacuum and pressure filtration in a pull—push arrangement, whereby a vacuum pump of a fan generates vacuum downstream of the filter medium, while a compressor maintains higher-than-atmospheric pressure upstream. If, for example, the vacuum produced is 80 kPa, ie, absolute pressure of 20 kPa, and the absolute pressure before the filter is 150 kPa, the total pressure drop of 130 kPa is created across the filter medium. This is a new idea in principle but in practice requires three primary movers a Hquid pump to pump in the suspension, a vacuum pump to produce the vacuum, and a compressor to supply the compressed air. The cost of having to provide, install, and maintain one additional primary mover has deterred the development of hyperbar vacuum filtration only Andrit2 in Austria offers a system commercially. 

Human evolution has taken place close to sea level, and humans are physiologically adjusted to the absolute partial pressure of the oxygen at that point, namely 21.2 kPa (159.2 mm Hg), ie, 20.946% of 101.325 kPa (760 mm Hg). However, humans may become acclimatized to life and work at altitudes as high as 2500—4000 m. At the 3000-m level, the atmospheric pressure drops to 70 kPa (523 mm Hg) and the oxygen partial pressure to 14.61 kPa (110 mm Hg), only slightly above the 13.73 kPa (102.9 mm Hg) for the normal oxygen pressure in alveolar air. To compensate, the individual is forced to breathe much more rapidly to increase the ratio of new air to old in the lung mixture. 

Vacuum pump capacity is conventionally based on the total cycle and expressed as mVh-m" (cfi7i/ft ) of filter area measured at pump inlet conditions. Thus, the gas volumes per unit area passing during each dry period in the cycle are totaled and divided by the cycle time to arrive at the design air rate. Since air rate measurements in the test program are based on pressure drop across the cake and filter medium only, allowance must be made For additional expansion due to pressure drop within the filter and auxiliary piping system in arriving at vacuum pump inlet conditions. 

For a special fan situation, a straight air duct of uniform cross-sectional area is used on the leaving side. The outgoing velocity Cj is the same as the fan leaving velocity C2. The only minor loss is the outgoing loss = pcT Another part of the pressure drop is the frictional pressure drop Ap, . Equation (9.127) gives... 

Pressure drop is the only factor that can be effectively controlled by an operator. Using the fan dampers, the operator can increase or decrease the cyclone s load by varying the velocity of the entering dirty air. 

The cross-flow fan generates only very slight pressure and its use is limited to appliances where the air pressure drop is low and predetermined. Its particular shape is very suitable for many kinds of air-handling devices such as fan coil units and fan convectors. 

The model is a significant improvement over the Lockhart and Martinelli correlations for pressure drop and holdup (discussed in Sec. 3.5.3). A severe limitation of the model, however, is the dependency on the empirical expression for Cf j [Eq. 3-126]. This expression is based on air-water data only, and has not been shown to apply to other systems. 

Figure 2. Transient pressure drop across the porous-medium micromodel of Figure 1 for foam pregenerated in an identical upstream medium. The foam frontal advance rate is 186 m/d. In the wet case, foam advanced into the downstream micromodel which was completely saturated with aqueous surfactant solution. In the dry case, the downstream micromodel contained only air.

When evaluating a material for the purpose of establishing dense-phase and long-distance suitability, it is important to undertake all the necessary tests (e.g., particle sizing, particle and bulk densities, fluidization and deaeration). Also, if possible, it is useful to compare such results with those obtained on previously conveyed similar materials (e.g., fly ash). However, it should be noted that such an evaluation only is a qualitative one and it is not possible to predict say, minimum air flows or pipeline pressure drop based on such data (i.e., pilot-scale tests normally are required to confirm minimum velocities, friction factors, etc., especially over long distances and for large-diameter pipes). 

The Carman-Kozeny equation relates the drop in pressure through a bed to the specific surface of the material and can therefore be used as a means of calculating S from measurements of the drop in pressure. This method is strictly only suitable for beds of uniformly packed particles and it is not a suitable method for measuring the size distribution of particles in the subsieve range. A convenient form of apparatus developed by Lea and Nurse 22 1 is shown diagrammatically in Figure 4.4. In this apparatus, air or another suitable gas flows through the bed contained in a cell (25 mm diameter, 87 mm deep), and the pressure drop is obtained from hi and the gas flowrate from h2. 

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