Settling Chambers

Device Type

One of the simplest (and oldest) air pollution control devices is the settling chamber. These are also sometimes called knock-out boxes or drop-out boxes. The equipment is in the form of a large chamber, which allows reduction of the gas velocity to a point where the particulate it carries simply drops out.

Today, settling chambers are used for coarse removal of large particulate in advance of higher efficiency particulate control equipment.

They are rarely, if ever, used as the final gas cleaning device.

Typical Applications and Uses

Settling chambers are primarily used to reduce the loading of particulate from sources such as kilns, calciners, and mills or grinders that inherently produce high-particulate concentrations. If the particulate is valuable in a dry form, the settling chamber usually is designed to settle out the smallest size particle that can economically be separated. If the product is not valuable or further downstream particulate separation is to be used (such as a cyclone, scrubber, or fabric filter collector), the chamber is usually sized to afford some basic separation at low cost.

They are often followed by product recovery cyclones which are, in turn, followed by collectors designed for high-efficiency collection of the fine particulate that pass through the upstream devices.

Operating Principles

A settling chamber operates on the principle that if you slow a gas stream down sufficiently, the solid particulate contained within that gas stream will settle out by gravity. In general, the larger the particle, the faster the settling rate. In addition, larger particles will settle out faster in each moving gas stream than smaller particles.

The settling velocity for particulate was explored extensively in the mid- 1800s by a scientist named Stokes. His equation for the terminal settling velocity of particulate is used to this day. It is called Stokes' law:

where:

V_g = terminal settling velocity (ft/s)

D = particle diameter in feet

d_p = density of particle, lbsm/ft³

d_g = density of gas, lbsm/ft³ (pounds [mass])

g = acceleration of gravity, ft/s²

v = gas viscosity, lbm/ft/s

The settling relationship is accurately applied only for particles of about 2 pm and greater aerodynamic diameter. Usually, for calculations involving air at ambient conditions, the density of the gas is ignored because it is minor when compared with the particle density.

What this equation shows is that the greater the particle diameter and density, the higher the particle's settling velocity. Resisting this settling, the higher the viscosity of the gas, the lower the particle's terminal settling velocity.

Settling chambers are therefore designed to allow the mean gas stream velocity to slow down to a point at or below the target particle's settling velocity so that the particle drops out within the confines of the chamber. Because the particle settles at a given rate (i.e., distance per unit time) as predicted by Stokes' law, the chamber must be sufficiently long to allow this settling to be completed before the gas reaches the device's gas outlet. Settling chambers are therefore large in cross-sectional area, to slow the gas stream down, and long, to allow sufficient time for settling.

What about particles under approximately 2 pm diameter? Unfortunately, these particles (about %5^th the diameter of a human hair) are so small that they are influenced greatly by surrounding gas molecules and do not follow Stokes' law. They do not even follow a trajectory as such. They are buoyed and buffeted by surrounding gas molecules. A correction for Stokes' law was derived by a researcher named Cunningham. Thus, we have Cunningham's correction factor for non-Stokes-sized particles. Sometimes called a slip- correction factor, it is a multiplier applied to Stokes' equation to adjust for the particle size and its actions below 2 pm aerodynamic diameter.

Experience has shown that settling chambers are of practical value only for reducing the loading (concentration) of large (above 100 pm aerodynamic diameter) particulate and possibly for the recovery of large, valuable product. They are used in various design configurations on devices such as kilns and calciners, waste solid fuel boilers, or similar devices. They are almost invariably followed by more efficient gas cleaning equipment.

Dust chambers are often used at the feed end of lightweight aggregate kilns. Similar knock-out chambers are used on mineral lime, cement, and lime sludge kilns and sometimes on dryers. Large particulate that is air conveyed out of the rotating portion of the kiln/dryer is encouraged to drop out in the knock-out chamber and be recovered. Sometimes, a vertical baffle is used in the chamber to direct the gas stream in a pattern that makes the gas perform a 90° or even 180° turn to enhance separation. The larger particulate cannot make this turn and therefore drops out.

Primary Mechanisms Used

The primary mechanism used is the drag force applied on the particle by the viscosity of the carrier gas. As the gas stream slows down, the influence of the viscous force of the gas on the particle is reduced and the particle begins to settle by primarily gravitational forces.

Design Basics

Settling chamber design is predicated on the particle size, its density, the gas viscosity and velocity, and space considerations. An infinitely large settling chamber would, in theory at least, settle out all particulate. Economics, however, limits the size of the chamber. Stokes, in turn, limits the size of the particle that can be economically separated.

If the chamber is used for valuable product recovery, the smallest particle that would be worthwhile collecting dry is the common target. The design focus then needs to answer the question, Is there enough space? An iterative design then follows. As mentioned earlier, Stokes' law defines the settling velocity, and the velocity dictates the size of the equipment. This usually results in a design particle size more than 50-100 pm, otherwise the chamber becomes excessively large. If the 50-100 pm particle is not worth collecting, the designer would size the chamber to capture much larger particles, thereby at least economically lowering the loading of particles requiring further control but letting the smaller particles pass through.

Chamber (or can) velocities of 5-7 ft/s or lower are common. Baffles can sometimes be used to provide beneficial changes of direction if the particles do not stick to the baffles. Curtains of chains can be used to in effect divert the gas flow but allow some measure of self-cleaning. Given the low gas velocity, the pressure drop is usually under 1-inch water column.

Figure 13.1 from Fan Engineering (Howden North America, Inc.) shows a general diagram of a crossflow settling chamber. Note the hoppers used

FIGURE 13.1

Settling chamber.

to remove the collected solids. Gas flow is left to right. The vector diagram depicts the primary forces on the particle, which influences the trajectory and, therefore, the length of the settling chamber.

Even given a dispersion of particulate above 100 pm, the efficiency of a settling chamber is quite low. Typically, only 25%-50% of the particulate of that range or larger drops out. Settling chambers are often, therefore, called rock boxes in the industry because they remove only the "boulders." In doing so, however, they can serve a valuable purpose in reducing the total loading of particulate that must be removed by downstream devices.

Operating/Application Suggestions

Most often, the designer of the process equipment includes a settling chamber in his design as an integral part of the device. The settling function may be just a minor one. The primary purpose may be to allow material to be fed into the device or to allow for seals and so on to function properly. It is therefore best to use the design provided by or recommended by the process equipment vendor.

If a settling chamber is used, care should be taken to design a suitable solids discharge system so that the particulate does not build up to a point where it entrains into the gas stream. Access doors should be provided for service access and cleaning. If the gas stream contains acids and its temperature and humidity pass through the acid dewpoint, the chamber should be suitably insulated and even heated to reduce corrosive effects.

The structural support for a settling chamber should be sufficient to support the filled weight of the device. This can be a significant factor, since these devices are inherently large.

Settling chambers should not be used where the particulate is sticky or can bridge or build up. In those cases, quite the opposite design is used. The ductwork is sized to be above the conveying velocity of the target particulate, and that velocity is maintained until the particulate reaches a suitable gas cleaning device.

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Spray Towers/Scrubbers

Device Type

Spray tower scrubbers use spray nozzles to extend the surface area of the scrubbing liquid to enhance mass transfer of contaminant gas(es) into the liquid. They are primarily used for gas absorption.

Spray scrubbers include designs that use spray nozzles (hydraulically or air or steam atomized) to absorb gases and control larger diameter (+10 g) particulate.

Typical Applications and Uses

Spray tower scrubbers are often used on wet flue gas desulfurization (FGD) systems at public and industrial power generation facilities. These FGD systems use lime or limestone slurries as the scrubbing liquid. Their open vessel design is an advantage where plugging or scaling may occur. The simplicity of the design makes them a lower-cost alternative for high gas volume scrubbing applications (over 100,000 acfm).

They are also used as part of quenching and gas conditioning systems wherein the gas must be brought to saturation or near saturation with water.

Most spray towers are countercurrent in design wherein the gas flows vertically upward and the liquid falls downward through the ascending gases. Some units, used for odor control, are horizontally oriented using a multiplicity of concurrent spray sections in series. Often, a series of baffles is incorporated into the scrubber vessel to change the direction of the gas stream and to provide intimate contact of the spray and the contaminant gas.

Some spray tower designs have been modified to act as direct contact condensers in applications wherein packed devices may plug with solids. Though not as mechanically efficient in that application as pure countercurrent designs (such as a packed tower), the ability to resist plugging and therefore provide greater on-line availability can make the spray tower condenser attractive.

Spray scrubbers cover a wider variety of designs. These vary from devices as simple as a spray header in a duct to cyclonic-type devices (often called preformed spray scrubbers).