Encyclopedia of Environmental Science and Engineering, Volume I and II

836 PARTICULATE REMOVAL

The result is that in this application cyclone diameters

usually range from about 3½ to 5 ft. A large cat cracker

regenerator may have 20 pairs (primary and secondary) of

cyclones in parallel. The secondary cyclones discharge into

a common plenum head.

Operating Conditions Upper temperature limits for

cyclones are limited by materials consideration to about

1700 F in conventional designs. They can withstand pres-

sures of several psi with normal design, but for high pressure

operation enclosure or special fabrication is required. Only

fairly freely flowing material can normally be discharged,

and special design or operating techniques are required for

sticky or coking material. Low efficiency on fine particles

usually precludes use where a significant fraction of material

to be collected is below 10 or 15 m in dia.

Multicellular Cyclones

Examination of Eq. (1) in the discussion of cyclones shows

that d 50  becomes smaller and collection efficiency therefore

increases as cyclone diameter decreases. Advantage is taken

of this effect in multicellular cyclones, which have diameters

in the range of 6–10 in. as compared to several feet for con-

ventional cyclones. Because they are small, many cellular

cyclones must be operated in parallel, and they are designed

to facilitate manifolding into arrays which may number sev-

eral hundred. Figure 5 shows a typical cellular cyclone. For

ease of manifolding it is constructed with an axial inlet with

swirl vanes to impart spin to the gas instead of having the

tangential inlet of large cyclones. They are usually cast or

molded in standardized designs instead of being custom fab-

ricated as large cyclones are. An array of cellular cyclones is

shown in Figure 6. This type of collector is designed to oper-

ate at low pressure drop, typically less than 4in. of water.

This makes it attractive for use for fly ash recovery in power

plants, where only slight pressure drops can normally be

accommodated.

Arrays of cellular cyclones in parallel suffer an efficiency

debit compared to single cyclones of the same design. This

is usually attributed to unequal distribution of gas among

cyclones abetted by the low pressure drop, and to recircula-

tion patterns of gas down through one cyclone dust outlet

and up through the next. The gas upflow causes a loss of effi-

ciency. Many vendors will cite grade efficiency curves for

a single cyclone which are, however, not representative of

manifolded arrays. The magnitude of the performance debit

can be seen in Figure 7.

Electrostatic Precipitators

Electrostatic precipitators enjoy an image of being the ulti-

mate in high efficiency air pollution control equipment. There

is an element of reality in such a judgment, but precipitators

are extremely expensive in terms of investment, operation,

and space requirements, and they are often subject to obscure

maladies which may cause collection efficiency to be far

less than expected. Thus installation of a precipitator will be

costly and may not solve a particulate emission problem.

The strong points of precipitators are (1) very low pres-

sure drop (less than 1 in. of water, compared to about 3in.

for cloth filters, 5–30 in. for centrifugal collectors and up

to 60 in. for high energy scrubber); (2) sizes to handle very

high gas flow rates; (3) potential of high collection effi-

ciency on fine dust; (4) ability to operate at high tempera-

tures (800F is reached in commercial practice and pilot

units have operated at 1700F) and (5) dry collection of

particles avoiding water pollution problem. These factors

have been particularly suited to the requirement of coal-

fired power plants, which constitute probably the most

widespread users of precipitators or any other high effi-

ciency particle collectors.

The standard text on electrostatic precipitator prin-

ciples and practice is that by White.^29 A more recent work

by Oglesby and Nichols^30 also includes an extensive survey

of precipitator installations and performances. Other gen-

eral references include the monographs by Rose^31 and

Gottsclich.^32

DUST

DISCHARGE

DESIGN M105

CLEAN AIR

DISCHARGED

UPWARD

CLEAN

AIR

DUST

LADEN

AIR

FIGURE 5 Multicellular cyclone

with axial inlet vanes. (Courtesy:

Wheelabrator Air Pollution Control.)

C016_002_r03.indd 836C016_002_r03.indd 836 11/18/2005 1:06:46 PM11/18/2005 1:06:46 P