Overview of Dust Collection Systems - a PDH Online Course for Engineers

Overview of Dust Collection Systems

Course Outline

There have been numerous articles written over the years about the ill health affects of inhaling dust which may be toxic and carcinogenic. This is a serious problem in industrial environments. The combination of particulate and fume emissions can be harmful to operators, violate local safety codes, lead to combustion hazards, and jeopardize your considerable investment in equipment.

This 4- hour course provides a basic overview of the dust collection systems and is based entirely on the Occupational Safety and Health Administration (OSHA) manual on Dust Control Handbook for Mineral Processing and covers Chapter 3: "Dust Control Systems" ; Chapter 4: "Collecting and Disposing of Dust" and Chapter 6: "Estimating Costs for dust control Systems.

The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the reader will:

Understand the importance and necessity of dust collection systems;
Understand the different types of dust collectors;
Learn what information is required for selection of dust control system;
Understand the principles of airflow and the importance of shape and location of exhaust hoods;
Understand the fundamentals of ductwork design including pressure drop calculations;
Understand the principles of wet dust suppression system and learn the factors affecting surface wetting;
Learn the design of water spray system including finely atomized water sprays and electrostatically charged fogs;
Understand the principles of inertial separators and centrifugal cyclone collectors;
Understand the fundamentals of bag filters and their cleaning mechanisms (mechanical shaker, reverse air and reverse jet);
Understand the principles of electrostatic precipitators and the factors that affect the efficiency of dust capture;
Understand the techniques for efficient disposal of collected dust; and
Learn the cost estimating procedures and breakup of capital cost and operating cost comparisons for various type of dust collection system.

Intended Audience

This course is aimed at students, mechanical engineers, facility managers, environmentalists, and factory & workshop personnel involved in the operations of any plant where powder and bulk materials are processed.

Course Introduction

Dust collection is the process of removing or collecting solid particles from airflow to comply with stringent air pollution standards. There are many different types of dust collection systems in the industry using various methods to separate the dust from the air stream. Some Dust Collectors examples are:

1) Fabric Filter Dust Collectors (Bag houses and cartridge collectors) in which particulates are collected on filter membranes
2) Cyclone Collectors in which the particles are collected by centrifuge
3) Electrostatic Precipitators (ESP) in which particles are electrostatically collected on a high voltage plate
4) Wet Collectors in which the particles are collected by forming agglomerates on spraying water, foam and surfactants.

What constitutes a good dust collector is a system that-

1) Provides total design flow at pick-up points sufficient to control dust
2) Is low in maintenance
3) Is designed with worker and property safety in mind
4) Meets required dust emission standards
5) Uses the least electrical energy practical to accomplish the above
6) Has the flexibility to accommodate varying flows and pressures or future system requirements
7) Is price competitive on a first cost basis, while being the low cost choice over the life of the system

This course will review the above criteria and provides the framework of learning to various industries including adhesives, automotive, metal, mining & minerals, biotechnology, building and road construction, ceramics, chemical, food, pharmaceuticals, plastics, rubber, polymers, surface coatings, wood workshops etc..

Course Content

In this course, you are required to study Occupational Safety and Health Administration (OSHA) manual on Dust Control Handbook for Mineral Processing and covers

Chapter 3 Dust Control Systems http://www.osha.gov/dsg/topics/silicacrystalline/dust/chapter_3.html

Chapter 4 Collecting and Disposing of Dust http://www.osha.gov/dsg/topics/silicacrystalline/dust/chapter_4.html

Chapter 6 Estimating Costs for dust control Systems http://www.osha.gov/dsg/topics/silicacrystalline/dust/chapter_6.html
Course Summary

Dust collection is an on-line process for collecting process-generated dust from the source point on a continuous basis. Dust collectors may be of single unit construction, or a collection of devices used to separate particulate matter from the process air.

Factors affecting selection of dust collector

1) Type of dust - Consider the process generating the dust. In some cases the dust may be a fine powder and in another case, the material may be fume- for instance metal welding operation. This information is important to analyze the collector efficiency, temperature resistance and capacity.
2) Capture or conveying method - A fan below a grate that's located under a transfer point or other dust source can be used to capture heavier particles, while a capture hood above the source captures the fines. How a material is conveyed can also produce a different amount of fines - for instance open belt conveyer with several transfer points would produce more fines than a short enclosed belt conveyer. Knowing how dust is captured and conveyed can help determining what percentage of fines must be collected and how efficient the dust collector must be.
3) Particle size - The smaller the particle size, the more efficient the dust collects must be. For instance for particles 1 micron or smaller, a cartridge dust collector is typically the best.
4) Particle shape - Spherical particles typically don't stick to each other, can be collected by cartridge filter, but fibrous particles , which do stick to each other, can require a bag house dust collector with PTFE or other bag filters treated to release sticky particles.
5) Particle characteristics - If the dust is hygroscopic, electrostatically charged or sticks together because of other characteristics, baghouse is best alternative. If the dust is toxic, bag filters should be avoided and instead cartridge filter should be used as it can be changed quickly from outside without significant exposure to the worker.
6) Dust concentration or loading - If the dust is highly concentrated - for instance, in a transfer operation or pneumatic conveying system, use of cyclone before dust collector as pre-cleaner, than use either baghouse or cartridge filter is recommended.
7) Gas or odor removal - If the dust has an odor or emits gaseous pollutant, use a dust collector with an adsorption system such as activated carbon or chemical filtration.

Choice of appropriate Dust Collector

1) Bag type collectors is one of the most common air pollution control technologies that removes particulate matter from the air waste stream being emitted from a broad range of industrial processes. Bag filters are typically suitable for applications requiring
· Medium to high temperature operating conditions
· Hygroscopic, fibrous or other sticky dust
· Extremely heavy dust concentrations when only medium filtration efficiency is required.
2) The most common bag filter fabric is non-woven, such as needled polyester felt, which is best for applications with operating temperatures 300°F or less. Also popular is woven or felted fiberglass, which is inexpensive and handles operating temperatures to 500°F. Polytetrafluoroethylene (PTFE) fabrics withstand operating temperature to 500F and provide excellent acid resistance, but are much costlier than polyester or fiberglass.
3) The cartridge filter is made of pleated medium made of cellulose and other synthetic fibers. These are suitable to use with dry dusts and operating temperatures below 240°F. If the dust is fine and dry or when you want to recirculate the filtered air, a cartridge dust collector is the best. The unit can handle applications:
· With dry environments and heavy dust concentrations
· With high percentage of fines (particulates smaller than 1 micron) requiring high filtration efficiency
· Requiring process air recirculation (filtering the air so it can be returned to the workplace rather than exhausted outside the plant)
· With hazardous dusts (because cartridges are quicker and thus safer to change than bag filters)
· With low fan or blower horsepower requirements.
4) Cyclones operate most efficiently when used as primary product collectors, backed-up by bag house or cartridge filters for collection of 'super-fine' materials. Cyclones have no moving parts and have no replacement filters. Usually two smaller cyclones in parallel will have a higher efficiency than one larger cyclone with an equal pressure drop.
5) Historically, electrostatic precipitation devices have only been used to control emissions for high volume flue gas applications and are commonly used in combination with wet collectors where gaseous emissions must also be controlled.
6) Wet dust collectors are most suitable for explosive dusts such as titanium, magnesium, aluminum and zinc.

However, when several contradictory factors affect the selection, it is best to discuss with manufacturers and specialized consultants. These experts typically have testing facilities to help the customer analyze their dust characteristics and make an appropriate selection.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.