Rapping System

The collecting plates and discharge electrodes require periodic cleaning to remove collected particnlate. This collection process is continuous. As such, devices have been developed to clean both the discharge electrodes and the collecting plates on a 

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Recently, dry wire-pipe ESPs are being cleaned acoustically with sonic horns (Flynn, 1999). The horns, typically cast metal horn bells, are usually powered by compressed air, and acoustic vibration is introduced by a vibrating metal plate that periodically interrupts the airflow (AWMA, 1992). As with a rapping system, the collected particulate slides downward into the hopper. The hopper is evacuated periodically, as it becomes full. Dust is removed through a valve into a dust-handling system, such as a pneumatic conveyor, and is then disposed of in an appropriate marmer. 

Poor gas flow distribution High level of lines in hopper Improper control settings Rapping system degradation Process gas conditions PM fines electrical resistivity... 

Although the majority of the dust is deposited on the collectors, some is deposited on the discharge electrodes, and, to optimize corona production, the discharge electrodes should be kept deposit free. On a dry plant, this is achieved by rapping systems similar to those employed for the collectors, while on wet systems, water washing of the internals is usually effective in maintaining corona emission. 

REMOTE-AREA POWER-SUPPLY (RAPS) SYSTEMS AND THE VALVE-REGULATED LEAD-ACID BATTERY... 

As their name suggests, RAPS systems are expected to operate reliably in a variety of inhospitable environments. They can be confronted with the searing heat of the desert, the stifling humidity of the rainforests, or the intense cold of the tundra. The facilities may even be sited on oceans or atop mountains. The systems service a diverse range of applications with widely different levels of energy consumption, such as (i) houses and villages (ii) electric fences (iii) community dwellings and services ... 

RAPS systems capable of supplying continuous power must have either a diesel generator in constant operation and/or a battery bank. If renewable energy is abundant, RAPS systems can also include a photovoltaic (PV) array, a wind generator, and/or a hydro-generator. An inverter to convert direct current (d.c.) to... 

In terms of life-cycle costs, batteries are usually the most expensive component of a RAPS system and, therefore, it is advantageous to minimize the required capacity. The battery should, however, be sized to supply a significant portion of the anticipated daily load in the absence of diesel- or PV-generated power, e.g., from 20 to 50%. This would allow the diesel to remain idle for much of the day and to operate under relatively constant, high-load conditions for only a few hours each day. Further, the battery should be sized such that the daily depth-of-discharge (DoD) is limited in the interest of enhancing battery cycle-life. (The cycle-life of a battery is affected by several factors which include DoD, temperature, and charging procedure.)... 

Diesel generators are the energy source for many RAPS systems. Such units consist of an internal combustion engine (fuelled by diesel oil) and an alternator that produces a.c. power. Generators that deliver d.c. power have also been used, but they are no longer common as they are not as efficient as their a.c. counterparts. Petrol generators can also be employed for RAPS duty, but they have greater maintenance requirements and are more expensive to operate. 

Traditionally, inverters installed in RAPS systems have been based on either square-wave or modified square-wave technology. Although such units are relatively crude and offer a poor approximation of the a.c. sine wave, they are inexpensive and, hence, still find application in small RAPS systems. Such inverters can, however, give rise to background noise in hi-fi equipment, as well as cause operational problems with (i) electric motors (ii) fluorescent lights (iii) radios (iv) televisions ... 

Modem inverters installed in RAPS systems are called sine-wave inverters and, as this name suggests, produce sine-wave outputs. The devices produce high-quality, low-distortion power and can be used to operate even the most sensitive of electrical equipment. Sine-wave inverters are very efficient (> 95%) and have proven to be mgged and reliable. Bidirectional sine-wave inverters are also available. These devices also act as battery chargers, have inputs for renewable energy generators. 

Fig. 14.3. Schematic diagram of basic design for RAPS systems.

RAPS systems can be divided into two basic types, i.e., one that provides d.e. power and one that provides a.c. power. These configurations are described in more detail in Sections 14.3.1. and 14.3.2. There are also some RAPS systems that provide both a.c. and d.c. outputs. 

RAPS systems that incorporate a component of renewable energy generally require battery energy-storage. Examples of such systems, together with their application and associated service, are summarized in Table 14.1. 

Traditionally, flooded lead-acid batteries have been the technology of choice for use in RAPS systems. During the last 10 years, however, there has been a considerable shift towards VRLA batteries. The main driver for this change is the low maintenance requirement of the latter technology. This and other important issues are discussed in the following sections. 

Flooded batteries operated in RAPS systems require water maintenance on a regular basis. Unfortunately, it is not uncommon for owner/operators to neglect this tedious task. This situation can affect the performance of the system and can ultimately result in premature failure of the batteries. Problems can also arise if the water used for topping up contains excessive amounts of impurities. For example, the presence of chloride ion leads to enhanced corrosion of the positive grids [11]. Also, some RAPS systems are located in areas where access is very difficult, e.g., microwave repeater stations on the tops of mountains. Hence, the cost of maintaining flooded batteries can be very high. 

The use of VRLA batteries in RAPS systems is also very attractive where space is limited and/or real estate is expensive. The batteries can generally be operated on their sides and can be stacked together closely in racks as access is not required for water maintenance. This can result in a 50% decrease in both the footprint and the volume taken up by the battery bank. 

The reduced tendency of VRLA batteries to exhibit stratification (and, the consequent reduced need for overcharge), can represent a benefit in terms of a decrease in both positive-grid corrosion and charging time. This acts to increase the life of the RAPS system components and also improves the efficiency of the system, especially when a diesel generator is included. 

A major complication with the use of VRLA batteries in RAPS systems is that careful control of both charge and discharge is required to achieve maximum battery life. This is best accomplished by using microprocessor-based devices that can... 

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