Industrial Exhaust Duct Design

Ducts are low pressure pneumatic conveyors intended to convey dust, particles, shavings, fumes or chemically hazardous components from air in vicinity to a shop floor or any other specific locations like tanks, sanding machine, or laboratory hood. The hood captures the contaminated air right from its source of emission and conveys to cleaning devices like filters, scrubbers etc. and finally leaves through a chimny to atmoshere or rarely recirculated back.
 
IED are different from HVAC ducts in that the function of HVAC is to convey air for regulating the air temperature, humidity in a destination along with regulating indoor air quality where as IED system just takes role of a conveyer of air away to chimney through some collecting device. HVAC is primerly for residential buildings where as IEDs are exclusively for ensuring the air quality of industrial work
 

INTRODUCTION

Many industrial facilities have processes which produce hazardous  dusts, particles, shavings, mists, vapors or gases. An efficient exhaust system is required to capture, convey and collect/recover/purify the contaminants in air. This can be simply for maintaining air quality within specified limits or also can be for recovery of valuable material or

Dust/fumes/ gas/vapor control is needed for many reasons. Apart from achieving compliance with existing health and air pollution regulations, some of the advantages of installing control such device are

1) Prevention of dust explosion and fire

2) Increased visibility and reduced risk of accidents at works space

3) Prevention of unpleasant odors

4) reduction of equipment wear especially for bearings and pulleys on which fine abrasive particles can cause harm and

5) Ensuring work morale, productivity

6) Recovery of valuable products from air is also intended.

After dust-filled or fume/vapor/gas filled air has been captured by a dry dust collection system, it must be separated, collected, and disposed of.

The ultimate aim is to discharges cleaned air either into the atmosphere or back into the workplace.

MAJOR COMPONENTS OF EXHAUST SYSTEM

Five major components of an industrial exhaust system are hood, duct, fan, cleaners and stack. HOOD is a device that encloses, captures, or receives emitted contaminants along with the parent air. The hood converts duct static pressure to velocity pressure (discussed later) and hood entry losses. Needless to say, hood is the part of the exhaust system that situates closest to the emission or in a enclosing fashion around the emission.

Those part of the exhaust system, which convey the particles/fumes along with air to fan and then from fan to exhaust equipment are known as DUCTWORK or simply DUCT.

CLEANER is a devise to separate contaminant from the flowing air with which it is mingled. It is of course is an air-pollution control equipment. The design of the air cleaner depends on the degree of cleaning required. Different types of air cleaners are made to remove particulate (e.g., precipitators, cyclones, etc.) and gases and vapors (e.g., scrubbers). Regular maintenance of air cleaners increases their efficiency and minimizes worker exposure.

FAN is used to generate negative pressure at inlet side of the duct system and positive pressure at outlet side of the system and is solely responsible for adding energy to air causing air current.

STACK (chimney) disperse exhaust air into the ambient environment. A common arrangement of an industrial exhaust system would be as given below:

 

Source à Hood à Duct à Fan à Cleanerà Stack à Atmosphere

or

Source à Hood à Duct à Cleanerà Fan à Stackà Atmosphere

Detailed description of these components including some minor components is presented in relevant sections of this book.

 

FUMES / GAS / VAPOR Vs DUST EXHAUST SYSTEM

A typical industrial exhaust system can be classified in to two broad categories depending up on the contamination of air that need to be separated and exhausted. Fumes/gas/vapor exhaust system and dust exhaust system. Both are law-pressure pneumatic systems where atmospheric air is the parent fluid that conveys the contamination.

The distinguishing features of these two classes are,

1)       AIR VELOCITY: In the dust exhaust system, the air flow velocity past a dust particle should be sufficiently high for the particle to abandon the state of rest, to overcome the force due to gravity and to change to state of motion. Gases and fumes being floating in air require relatively law velocity to convey.

2)       DEVICES: Fume/gas/vapor exhaust system uses a chemical scrubber to trap the fume/gas and to send air with permitted level of chemicals to atmosphere usually through a chimney. The dust exhaust system uses a dust-collecting device for trapping the excess dust before throwing the air to atmosphere.

 

DESIGN OF EXHAUST SYSTEMS

Process design of exhaust system will include 1) Identification of contaminants, its density and size, 2) Deciding of air flow 3) Sizing of the ductwork, 4) Calculation of resistance, 5) Finalizing the capacity of blower etc. The aim is to keep contaminants out using minimum airflow. It is estimated that increase in an inch wg of static pressure can add a few thousands of dollars to the operation cost per annum.

Mechanical design of duct is then required to determine the loads on the duct and the required thickness to meet this. An extra thick duct system will add to expenses and overall weight of the structures. At the same time one can not reduce the thickness beyond a minimum as this can reduce the life of the system.

FIVE MAJOR COMPONENTS

Five major components are hood, duct, fan, cleaners and stack. HOOD is a device that encloses, captures, or receives emitted contaminants along with the parent air. Hood is the part of the exhaust system that situates closest to the emission or in an enclosing fashion around the emission. It captures the fumes or dust. Those part of the exhaust system, which convey the particles/fumes along with air to fan and then from fan to exhaust equipment are known as DUCTWORK or simply DUCT. CLEANER is a devise to separate contaminant from the flowing air with which it is mingled. Different types of air cleaners are used to remove particulate (e.g., precipitators, cyclones, etc.) and gases and vapors (e.g., scrubbers). Regular maintenance of air cleaners increases their efficiency and minimizes worker exposure. FAN is used to generate negative pressure at inlet side of the duct system and positive pressure at outlet side of the system and is solely responsible for adding energy to air causing air current. STACK (chimney) disperses exhaust air into the ambient environment. Air is the carrier in all industrial exhaust systems.

 
1) Duct Design Handbook, Anilkumar M, Optimisations INDIA. www.Colebrookequation.com ; http://www.frpdesigns.com/DUCT%20DESIGN%20HANDBOOK.htm