Reducing Emissions
Reducing Emissions

Reducing Emissions

With spreading consciousness about the need to preserve the environment, there has been a renewed interest in bag houses, which help to arrest pollution.

As the country is progressing, education is spreading and people are becoming more aware about their health requirements and more forceful about their rights. Environmental laws at our end and across the world are becoming more stringent. In the last couple of years, the cement industry has been facing a tough challenge of meeting the revised environment regulations. With this backdrop, we take a fresh look at process filters.

A bag house, as it is commonly known as, is essential equipment for a cement plant. The attention paid and the involvement of top management in every aspect of the pollution-arresting device is an indication of the importance of the subject. A bag house is nothing but a dust-arresting device in the cement plant. Though it appears very simple, it a perfected engineering product. There is always a comparison between an Electrostatic Precipitator (ESP) and a bag house since both are dust-control equipment. There are locations in the plant where a bag house is preferred over ESP and there are locations where ESP will be preferred over a bag house.

Bag House Operation
Bag houses consist of filter media (bags) suspended inside a huge casing. Fans on the outside of the housing blow the dirty or polluted air through the filters, capturing the suspended particulate matter and solids on the bags and pushing clean air through the outlet. While filtering, a bag house allows the formation of a layer of particulate matter on its surface, called a dust cake. This dust cake continues to build until the thickness reaches a level where flow is sufficiently restricted; at this point, the bags are cleaned. Cleaning can be done during operation or offline, depending on the type of bag house.

As air is filtered through the bag house, (Ref to Pic. 1) the dust cake on the bag filters continually thickens. For most bag fabrics (those without a membrane coating), the cake is what does most of the filtering of the particulate matter in the air stream. A thicker dust cake increases both collection efficiency and pressure drop as the pathways through the bag become finer and also more restrictive. Cleaning mechanisms must find the right balance for this trade-off - too thorough or frequent cleaning results in a lower collection efficiency and possibly reduces bag life, but insufficient cleaning will cause excessive energy requirements for blower fans (i.e., high-pressure drops).

Bag House Design
Although the design of bag houses is typically the responsibility of the manufacturer, an understanding of the most important design criteria is helpful for making an informed selection.

The air-to-cloth ratio, also known as the superficial filtering velocity (in units of ft/min), is the most important criterion for bag house design. It is defined as the amount of air entering the bag house divided by the total surface area of the filter fabric in the bag house. This ratio determines the airflow capacity of the bag house, and must be optimised to balance the size of the bag house (capital costs) with the pressure drop (operating costs).

The differential pressure, or pressure drop, is a measure of the resistance to gas flow in the system. Bag houses with higher pressure drops require higher-powered fans to move air through the system, resulting in increased energy costs. The total differential pressure is the sum of individual pressure drops due to the fabric, particulate layer (dust cake), and bag house structure. An abnormally high pressure drop in a bag house can be caused by a number of factors relating to poor design or setup, including:

Bag Materials
The bag material is an important part of bag house design and selection, as it determines the life and effectiveness of the filter bag. Fabric filter media must be compatible both physically and chemically with the gas stream and system conditions. Selection of the correct bag material incorporates factors like particle size, operating temperature of the bag house, compatibility with gas stream chemistry, including moisture levels, acidity or alkalinity, electrostatic nature of the particles, abrasiveness of the particles, air-to-cloth ratio, fabric cost, etc. In addition to the material type, whether the fabric or material is woven (or otherwise) will affect what systems the bag is suitable for.

Non-woven materials consist of randomly placed fibres supported and attached to a woven backing. This strong construction is required for high-energy cleaning techniques like pulse jets and aggressive shakers.

Woven materials have fibres wound in uniform, repeating patterns. This construction is used for low-energy cleaning methods such as reverse air and lower-intensity shakers. The weave space affects the strength of the fabric and the permeability/capture efficiency of the filter.

Cleaning Mechanism
Bag houses are primarily classified based on the methods they use for bag cleaning. There are three different types of bag house cleaning mechanisms; each offers its own advantages for different applications. Reverse air (R/A) bag houses use continuous streams of low pressure air to remove collected solids. Bags are cleaned by back washing (reversing the air flow) within a chamber after shutting off the dirty gas flow and isolating the compartment. The recommended air-to-cloth ratio for these bag houses is between 1.75:1 and 2.5:1.

R/A bag houses are typically compartmentalised, allowing sections to be cleaned without shutting off the whole system. The cleaning action is very gentle, which lengthens bag life. ItGC´s preferred for high temperatures due to its gentle cleaning action.

Cleaning air must be filtered. It provides no effective means for removing residual dust build-up. It also requires more maintenance than other types due to dust re-entrainment on the bags.

Bag house filters selection guide, shaker
Shaker bag houses use mechanical shaking or vibrating actions to dislodge the filter cake. Bag bottoms are secured to a plate and their tops are connected to horizontal beams. These beams, driven manually or by a motor, vibrate to produce waves in the bags which shake off particulate matter. The recommended air-to-cloth ratio for these bag houses is between 2.0:1 and 2.5:1.

It has design and operation simplicity. They can be compartmentalised to allow sections to be cleaned without shutting off the whole system.

Cannot operate in high temperatures. It is also more energy- and time-intensive than other cleaning methods. Small amounts of positive pressure inside the bag can significantly reduce collection efficiency. Large footprint and space requirements, and requires a large number of bags.

Pulse-jet (P/J) or reverse-jet bag houses use compressed streams of high pressure air to remove particulate matter. During cleaning, brief (0.1 second) pulses of air are pushed through the bag, dislodging solids which collect in a hopper below. The recommended air-to-cloth ratio for these bag houses is between 3.25:1 and 4.0:1.

Cleaning mechanism allows P/J bag houses to be cleaned while the system is online. More complete cleaning than shaker or reverse air bag houses, lengthening bag life. It also operates at lower pressure drops and with lower space requirements.

These bag houses require the use of dry compressed air. They also require special fabrics for higher temperatures. They cannot tolerate high moisture levels or humidity in exhaust gases.

Some others depend on sonic horn technology, which uses high-intensity sound waves to provide additional vibrational energy for dislodging particles.

Cleaning Sequence
When considering a bag house´s cleaning mechanism, the cleaning sequence is a particularly important factor. It determines when and how often the cleaning takes place in the system.

Intermittent cleaning requires the fan/process to be stopped at intervals while the bags are cleaned. This sequence is used for single-compartment bag houses, usually shaker types. Continuous offline cleaning involves taking individual compartments offline in turn to clean, meaning the overall process is not shut down during cleaning. This sequence is used with multiple-compartment reverse air or pulse-jet bag houses. Continuous online cleaning allows the process flow to continue during cleaning. This fully-automated sequence is typically used for pulse-jet bag houses.

The other challenge in the operations of bag house is spotting the punctured bag where use of instrumentation is common.

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