Gravity settling chamber

In gravity settling separation, the particle-laden gas is fed horizontally into a large expansion chamber. The enlargement of the cross section of the flow stream significantly reduces the gas velocity. The particles settle downward on the collection surface or hopper collectors at the bottom of the chamber. The decrease in the gas stream velocity reduces the reentrainment of collected particles and increases the residence time of the particles so that they may have sufficient time to settle by gravity. 

For a given type of gravity settler, the pick-up velocity and the minimum diameter of particles which can be collected must be defined. The pick-up velocity is the onset velocity for particle reentrainment and depends on the particle diameter. The gas flow rate is to be controlled in such a way that the average gas velocity in the settling chamber is much smaller than the pick-up velocity. The minimum diameter of collected particles can be evaluated by the following analysis. 

When the gas velocity in the settler is sufficiently high, the particles deposited on the collection plates can be picked up by the gas stream. This reentrainment can substantially reduce the collection efficiency. By neglecting interparticle friction, the pick-up velocity, f/pp, may be estimated as [Zenz and Othmer, 1960] 

In actual applications, the gas flow in a gravity settler is often nonuniform and turbulent the particles are polydispersed and the flow is beyond the Stokes regime. In this case, the particle settling behavior and hence the collection efficiency can be described by using the basic equations introduced in Chapter 5, which need to be solved numerically. One common approach is to use the Eulerian method to represent the gas flow and the Lagrangian method to characterize the particle trajectories. The random variations in the gas velocity due to turbulent fluctuations and the initial entering locations and sizes of the particles can be accounted for by using the Monte Carlo simulation. Examples of this approach were provided by Theodore and Buonicore (1976). 

Gravity settling chambers are the oldest and simplest means of removing suspended particles from a gas. In principle, pollutants are removed by reducing the velocity of the gas stream sufficiently to allow particles to settle out under the 

Before selecting or sizing a specific control device, a careful evaluation of all aspects of the process and contaminants must be made. Improper terminology can lead to poor design or operation of any type of device. 

The primary section of the chamber is characterized by its cross-sectional area (W H) and by its length (L). The 

For dilute systems, Stoke s law is applicable to particle settling. References cited at the end of this chapter provide design and sizing information. 

In specifying settling chamber dimensions, gas flow velocities must be maintained below the re-entrainment velocity (pick-up velocity) of deposited particulate. As a general guideline, linear gas velocities are kept below 10 ft/sec (600 ft/min = 304.8 cm/sec). This is satisfactory for most materials however, some low-density particulates are re-entrained at lower velocities. 

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Dilute This is a fully expanded condition in which the solids particles are so widely separated that they exert essentially no influence upon each other. Specifically, the solids phase is so fuUy dispersed in the gas that the den sity of the suspension is essentially that of the gas phase alone (Fig. 12-29). Commonly, this situation exists when the gas velocity at all points in the system exceeds the terminal setthng velocity of the solids and the particles can be lifted and continuously conveyed by the gas however, this is not always true. Gravity settling chambers such as prilling towers and countercurrent-flow spray diy-ers are two exceptions in which gas velocity is insufficient to entrain the sohds completely. 

U Length of gravity settling chamber in direction of gas flow m ft ... 

Flyash - Flyashes are finely divided matter generally entrained in flue gases that arise from combustion. Particles range from 1 /im in size on down. This is not within the operational range of gravity settling chambers. Wet... 

References 1 through 14 provide detailed information on gravity settling chambers. In addition, you can find some design case calculations for sizing chambers at http //WWW. chemeng. queensu. ca/chee481 /Downloads. htm. 

Within the range of their performance capabilities, cyclones are one of the least expensive dust-collection systems. Their major limitation is that, unless very small units are used, efficiency is low for particles smaller than five microns. Although cyclones may be used to collect particles larger than 200 microns, gravity-settling chambers or simple inertial separators are usually satisfactory and less subject to abrasion. 

Figure 5.1 is a simplified representation of a horizontal simple gravity settling chamber. It is a long duct fitted with hoppers on the floor to collect particulates. Physical dimensions are characterized by the ductwork above the collection hoppers length = L, width = W, and height = H. 

Figure 7.16. Typical gravity settling chambers (a) Horizontal flow settling chamber (b) Howard settling chamber.

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