TOTAL KVA IS NOT A GOOD MEANS
EVALUATING ESP PERFORMANCE
by TRK Engineering Service, Inc
Over the years, many methods have been tried to determine ESP performance. The best method is stack testing, but this only provides a snap-shot in time. The most common, and most readily available method for continuous monitoring is use of an opacity monitor, which has become the industry standard. In recent years, people have been trying to utilize precipitator total power (KVA or KW) as a gauge of ESP performance, and some industries have had minimum KVA levels imposed in plant operating permits, similar to maximum opacity levels. KVA has been used in ESP modeling to predict performance, and can provide an accurate indication of performance under a specified (ideal) condition. However, with new ESP designs, that are often over-sized to meet a range of operating conditions, KVA can be a meaningless gauge to evaluate ESP performance. ESP manufacturers have often been required in recent years to design equipment to operate in compliance with 20% to 40% of the ESP out of service. In the field, KVA input at best can only predict the ability of the ESP to collect the material under a specific condition. Any changes in dust resistivity, particle size, particle concentration, gas distribution, total gas flow and internal buildups will effect the correlation between KVA and collection efficiency. Additionally, material reentrainment losses can and do bias any correlation that are made between KVA and ESP performance.
The primary problem with using KVA as a performance gauge is exemplified in the basic mathematical concepts used to model ESP efficiency. The basis for all ESP performance modeling starts with the Deutsch-Anderson equation. The Deutsch- Anderson equation describes the factors involved in the collection efficiency of a precipitator. Although the basic formula neglects a number of practical and empirical considerations that should be introduced for good design, it is useful to illustrate the effect the primary parameters have on precipitator efficiency. In its simplest form, the Deutsch-Anderson equation establishes a relationship between gas volume, collecting plate area and migration velocity:
|Collection Efficiency||N = 1 - e||x 100|
Where A = effective collecting surface area of the precipitator.
V = gas flow rate through the precipitator.
e = base of natural logarithim = 2.718.
w = migration velocity.
The exponent term w, known as the migration velocity, actually represents the speed of movement of the ash particle toward the collector surface under the influence of an electrical field. It should be considered more an indicator than an actual velocity, but it does have a finite value that can be used for comparison purposes. The migration velocity is comprised of:Migration Velocity W = a Eo Ep / 2 p q
Where a = particle radius, microns
EO = strength of field in which particles are charged
(represented by peak voltage)
EP = strength of field in which particles are collected
(normally the field close to the collecting plates)
q = viscosity, or frictional resistance coefficient of the gas
While the relationship established by the Deutsch equation is valid, there are a number of operating parameters that can cause the exponent to be in error by as much as a factor of two or more. It is well to remember that the basic Deutsch-Anderson equation can be used as an indicator or tool, but has limitations more often than not unless equated with some practical and empirical considerations by the designer. However, the equation clearly indicates the squaring effect voltage (V = E x d) has on the collection efficiency of a precipitator. It also emphasizes the effect particle size has on the overall ESP efficiency; as particle size becomes finer, higher voltages are required to maintain the same collection efficiency. This formula does not relate KVA to collection efficiency and more modern variations of this formal do not clearly relate KVA to ESP collection efficiency either. It is therefore important to understand the factors that affect the electrical characteristics of the precipitator to understand why good collection efficiency can be achieved at varying KVA levels.
Optimum power input to the precipitator varies among installations and can be subject to rapid change. There are seven (7) basic factors that directly affect the electrical characteristics. These are:
1. Design of the power supply.
2. Physical design of the precipitator.
3. Design of the electrode system.
4. Characteristics of the gas stream.
5. Characteristics of the particulate.
6. Maintenance factors.
7. Effect of process changes. The average precipitator can be sensitive to process changes in the following ways:
- Changes in gas temperature (affect on gas density and resistivity).
- Changes in gas pressure (affect on gas density).
- Changes in gas flow rate (effect on gas distribution and treatment time).
- Changes in gaseous composition.
- Changes in particulate chemical characteristics.
- Changes in particulate concentration or loading.
- Changes in the size distribution of particulate.
- Changes in the electrical conducting characteristics of the particulate (dust resistivity).
It is difficult to separate the effect of one process change on another. If the rate of process change is rapid, the readings can change almost instantaneously. On the other hand, rapid changes of temperature may not be seen readily on the meters because of the heat sink effect of the precipitator. Some changes in the process will cause large variations of voltage-current readings, while others will cause subtle affects.
Changes in dust resistivity can dramatically effect the KVA levels. The Deutsch-Anderson equation showed that the migration velocity contained the product of two precipitator voltage gradients without regard to the corona current component. The magnitude of the current flow will depend primarily on the resistivity conditions for any given voltage. Low resistivity levels will allow very high current levels with moderate voltage levels (high KVA), but the material collected can be easily reentrained, and in extreme cases material will be passed through the ESP and out the stack, significantly elevating opacity levels. By increasing the dust resistivity to a moderate level, voltage will increase in the ESP, the current will decrease and spark levels will rise (lowering total KVA). However, the increase in KV levels will improve material collection and the increase in resistivity will improve material bonding forces, reducing reentrainment losses and improving ESP performance at reduced KVA levels.
In reality, it is very easy to have the same opacity for many different KVA levels, or have high KVA and high opacity levels. This is most easily seen in oversized precipitators, where 40% or more of the ESP can often be operated at low KVA levels, or turned off completely, with little change in opacity level. Additionally, the selection of the fields that are turned off can dramatically effect the change in KVA levels. Since the majority of the collection occurs in the inlet fields of the ESP, the outlet fields have the least effect on efficiency, but require the highest power input. Therefore, turning off the outlet fields could reduce KVA levels by over 70% with little effect on overall collection efficiency.
The voltage and amperage components of KVA are important to evaluating precipitator performance however, their product (KVA) is misleading. ESP design and operation is very complex, which is why the industry has been supplying oversized precipitators. No one parameter can be used, or should be used, to evaluate performance of this complex system. Opacity levels remain the only true method available to monitor ESP performance on a continuous basis.
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Last updated: May 10, 2009.
Copyright © 1998 TRK Engineering Services, Inc. All rights reserved.
For more information contact: TRK Engineering Services - 95 Clarks Farm Road - Carlisle, MA 01741 - Telephone: 978-287-0550 - Fax: 978-287-0569 - email: email@example.com