Duct sizing

Spray dryers may operate under positive, negative, or neutral pressures. In general, pressure drop in a complete system will range from 15 to 50 cm of water, depending on duct size and separation equipment employed. 

From Table 1 and solids rate, estimate duct size and flow in SCFM (standard cubic feet per minute). 

Balanced design method A method of duct sizing to ensure the correct airflow rate in all branches, also known as the static pressure balancing method. 

Plate (single blade) A hinged flap that, by virtue of its position relative to airflow, creates a flow variation. This simplest form of damper, only used on small duct sizes, does not provide accurate control. 

Nominal size of an ATD The nominal value of dimensions of the prepared opening (duct) into which the air terminal device is to be fitted. For an air diffuser, the nominal size is generally defined as the duct size into which the neck of the device is fitted. 

Point Hood Size Area Duct Size Area V.P. in.W.G. S.P. in.W.G. Velocity Volume Assessment of Control Remarks... 

The rough duct design method may result in premature dust deposition or excessive system erosion (due to incorrect duct sizes and velocities)... 

Hence, even if the rough duct design technique is selected, some form of balancing still should be performed so that the final duct sizes are close to optimal (in terms of pressure and velocity). 

Vj-mm appears to increase with duct diameter, I). This trend has been recognized in many traditional pneumatic conveyors (Wypych and Reed, 1990 Zenz, 1964 Cabrejos andKlinzing, 1994), but also appears to be relevant for dust extraction (Cable, 1994 Miletich, 1994), which can be considered as a form of low-concentration pneumatic conveying (e.g., Vj= 10 m s 1 may be suitable for a particular contaminant in a duct size ofD = 100 mm but may cause deposition in D = 300 mm). Further evidence of this can be seen in the work of Zenz,... 

Exit air usually is maintained far from saturated with moisture and at a high temperature in order to prevent recondensation of moisture in parallel current operation, with a consequent lowering of thermal efficiency. With steam heating of air the overall efficiency is about 40%. Direct fired dryers may have efficiencies of 80-85% with inlet temperatures of 500-550°C and outlet of 65-70°C. Steam consumption of spray dryers may be 1.2-1.81bsteam/lb evaporated, but the small unit of Table 9.19(b) is naturally less efficient. A 10% heat loss through the walls of the dryer often is taken for design purposes. Pressure drop in a dryer is 15-50 in. of water, depending on duct sizes and the kind of separation equipment used. 

Entrainment Separation Some kind of entrainment separation is necessary to reduce losses of glycerine entrained in the vapor stream from each evaporator. Control of the vapor velocity by proper evaporator and duct sizing will minimize the amount of entrainment. An entrainment separator should be installed at the outlet of each evaporator body, and the glycerol recovered should be returned... 

Table 16.2 is based on the properties of water. Suppose that the fluid is air at 1 atm. Calculate the Reynolds number and the characteristic diffusion time for the various duct sizes. The velocity in the 2-m pipe remains 1 m s . ... 

DCDA three catalyst passes preceding the IPAT and two passes following it. The DCDA process could be operated with a higher percentage SO2 in the converter inlet gases and thus reduced the gas volumes to be handled. The equipment and gas duct sizes could be reduced, resulting in lower capital cost for the plant. 

The last equation also can be used to estimate the loss in changing duct sizes by substitution of k, for k in the equation. The ratio given below is the ratio of the smaller flow area to the larger ... 

It is obvious that reduction of the size of the dryer will lead to a reduction in initial capital cost. Although this should not be a deciding factor in the selection of an individual dryer, since only 10 to 15 percent of the life-cycle cost of a direct dryer is due to the initial capital cost of the drying system, it is still an important consideration as it can reduce the space requirement, duct sizes, size of ancillary equipment, etc., as well. One must intensify the drying rates without adversely affecting product quality in order to make the equipment smaller. 

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