Electrical Sectionalization

Field Sectionalization
An electrostatic precipitator is divided into a series of independently energized bus
sections or fields (also called stages) in the direction of the gas flow. Precipitator performance
depends on the number of individual bus sections, or fields, installed. Figure
 shows an ESP consisting of four fields, each of which acts as an independent precipitator.

Field sectionalization

Each field has individual transformer-rectifier sets, voltage-stabilization controls, and

high-voltage conductors that energize the discharge electrodes within the field. This

design feature, called field electrical sectionalization, allows greater flexibility for

energizing individual fields to accommodate different conditions within the precipitator.

This is an important factor in promoting higher precipitator collection efficiency.

Most ESP vendors recommend that there be at least three or more fields in the precipitator.

However, to attain a collection efficiency of more than 99%, some ESPs have

been designed with as many as seven or more fields. Previous experience with a particular

industry is the best factor for determining how many fields are necessary to

meet the required emission limits.

The need for separate fields arises mainly because power input requirements differ at

various locations within a precipitator. The maximum voltage at which a given field

can be maintained depends on the properties of the gas and dust being collected. The

particulate matter concentration is generally high at the inlet fields of the precipitator.

High dust concentrations tend to suppress corona current, requiring a great deal of

power to generate corona discharge for optimum particle charging. In the downstream

fields of a precipitator, the dust loading is usually lighter, because most of the dust is

collected in the inlet fields. Consequently, corona current flows more freely in downstream

fields. Particle charging will more likely be limited by excessive sparking in

the downstream than in the inlet fields. If the precipitator had only one power set, the

excessive sparking would limit the power input to the entire precipitator, thus reducing

the overall collection efficiency. The rating of each power set in the ESP will vary

depending on the specific design of the ESP.

Modern precipitators have voltage control devices that automatically limit precipitator

power input. A well-designed automatic control system keeps the voltage level at

approximately the value needed for optimum particle charging by the discharge electrodes.

The voltage control device increases the primary voltage applied to the T-R set

to the maximum level. As the primary voltage applied to the transformer increases, the

secondary voltage applied to the discharge electrodes increases. As the secondary

voltage is increased, the intensity and number of corona discharges increase. The voltage

is increased until any of the set limits (primary voltage, primary current, secondary

voltage, secondary current, or spark rate limits) is reached. Occurrence of a spark

counteracts high ESP performance because it causes an immediate, short-term collapse

of the precipitator electric field. Consequently, power that is applied to capture particles is used less efficiently. There is, however, an optimum sparking rate where

the gains in particle charging are just offset by corona-current losses from sparkover.

Measurements on commercial precipitators have determined that the optimum sparking

rate is between 50 and 150 sparks per minute per electrical section. The objective

in power control is to maintain corona power input at this optimum sparking rate by

momentarily reducing precipitator power whenever excessive sparking occurs.

Besides allowing for independent voltage control, another major reason for having a

number of fields in an ESP is that electrical failure may occur in one or more fields.

Electrical failure may occur as a result of a number of events, such as over-filling hoppers,

discharge-wire breakage, or power supply failure. These failures are discussed in

more detail later in this course. ESPs having a greater number of fields are less dependent

on the operation of all fields to achieve a high collection efficiency.

Parallel Sectionalization

In field sectionalization, the precipitator is designed with a single series of independent

fields following one another consecutively. In parallel sectionalization, the

series of fields is electrically divided into two or more sections so that each field has

parallel components. Such divisions are referred to as chambers and each individual

unit is called a cell. A precipitator such as the one shown in Figure  has two parallel

sections (chambers), four fields, and eight cells. Each cell can be independently

energized by a bus line from its own separate transformer-rectifier set.

Parallel sectionalization (with two parallel

sections, eight cells, and four fields)

One important reason for providing sectionalization across the width of the ESP is to

provide a means of handling varying levels of flue gas temperature, dust concentration,

and problems with gas flow distribution.When treating flue gas from a boiler, an

ESP may experience gas temperatures that vary from one side of the ESP to the other,

especially if a rotary air preheater is used in the system. Since fly ash resistivity is a

function of the flue gas temperature, this temperature gradient may cause variations in

the electrical characteristics of the dust from one side of the ESP to the other. The gas

flow into the ESP may also be stratified, causing varying gas velocities and dust concentrations

that can also affect the electrical characteristics of the dust. Building

numerous fields and cells into an ESP design can provide a means of coping with variations in the flue gas. In addition, the more cells provided in an ESP, the greater the

chance that the unit will operate at its designed collection efficiency.