Print this page Sizing Ventilation Equipment - Hoods, Ductwork & Stack
A. Bhatia, B.E.
Course Outline
The enormous inefficiencies of most industrial ventilation systems can be reduced through proper system design. Systems can be more effective by good hood design and appropriate distribution of airflow. Exhaust air requirements can often be reduced drastically by judicious design of exhaust hoods and proper placement of capturing hoods. Fan static pressure requirements can be reduced by correct choice of inlets, elbows and duct sizes. As a participant you will learn how to evaluate these choices and how to properly select air cleaning devices using experience-proven methods.
This 3-hour course
discusses the design criteria and cost aspects of selecting a right ventilation
system equipment including hoods, ductwork and stacks. This course is based
entirely on the guidelines recommended by Environmental Protection Agency (EPA)
and is available on their website link www.epa.gov/ttn/catc/dir1/cs2ch1.pdf.
This course includes
a multiple-choice quiz at the end, which is designed to enhance the understanding
of course materials.
Learning Objective
At the conclusion of this course, the reader will:
- Understand the function and application of various types of hoods;
- Understand the hood sizing factors including hood shape, airflow rate, capture velocity and friction loss;
- Understand the methods and design approaches for optimal sizing of ductwork;
- Learn about the appropriate duct shapes, materials and type of fittings;
- Understand the principles of fluid flow and Bernoulli's equation;
- Understand the steps to perform pressure loss calculations;
- Understand the regulatory requirements and method for sizing stack heights and diameter;
- Learn to perform temperature and pressure adjustments; and
- Learn to perform
cost analysis for ventilation equipment.
Intended Audience
This course is aimed at facility managers, environmentalists, H &S professionals, O & M personal, safety engineers, industrial hygienists, and engineers responsible for the design, purchase or upkeep of an industrial ventilation system.
Course Introduction
HVAC systems for
industrial facilities are significantly different from those for commercial
and institutional buildings. High-bay areas; special ductwork to convey exhaust
or product; and equipment that emits fumes, vapors, oil mists, and heat pose
unique requirements for HVAC design in industrial facilities. Many of the industrial
equipments and processes emit fumes, vapors, oil mists and heat that pose unique
requirements for HVAC design in industrial facilities. Local exhaust system,
which involves trapping airborne contaminants at their source before they contaminate
the air which is breathed, is the most common ventilation system. A hood, ductwork,
and a stack are key elements in any local exhaust ventilation system. Because
each of these elements is different, both in appearance and function, each must
be designed separately. But at the same time, these elements comprise a system,
which is governed by basic ventilation principles and the laws of fluid mechanics.
This course will provide you a basic understanding for evaluating and sizing
local exhaust ventilation equipment for industrial applications. The fans and
drives shall be covered in a separate course.
Course Content
Sizing Ventilation Equipment - Hoods, Ductwork & Stack
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A local exhaust system consists of a hood to capture the contaminants, ducts to transport them to the outside, an exhaust fan to move the air, and sometimes air cleaners to remove particulates from the air.
A hood is designed
to confine or capture the contaminant at its source. The air velocity at the
hood opening and inside the hood must be sufficient to capture and carry the
air contaminants. The hood should enclose the source of contaminant as much
as possible or be placed as close to the source as possible. There are four
types of hoods: (1) enclosures, (2) booths, (3) captor (capture) hoods, and
(4) receptor (receiving) hoods, each used for particular application.
- Total enclosure hoods surround an emitting process completely, so that all emissions are captured. Example: extracts involving solvents and fumes involving volatile organic compounds (VOC). The design must ensure that the vapor concentration in air shall be kept to less than 1/4 of the lower explosive limit (LEL).
- Booths are like enclosures, in that they surround the emission source. Example: spray painting, portable grinding, polishing, and buffing operations.
- Captor hoods (also termed active or external hoods) do not enclose the source at all. Example: overhead canopy hoods over kilns, furnaces or welding operations.
- A receptor hood
typically is located above or beside a source, to collect the emissions, which
are given momentum by the source. Example: extract from an open tank containing
a hot liquid (a buoyant source).
Ducts carry contaminated
air away from the work area. Ducting should be as short as possible, as increased
in length means an increase in duct resistance, and may require more powerful
fans or additional booster fans. In general round duct should be used rather
than rectangular ones to minimize turbulence. Duct turns and bends should always
as gradual as possible. The angle of duct bends should be greater than 90º
so as to reduce air flow resistance. Ducts should be inspected regularly for
leaks, damages, and routinely maintained and cleaned.
Exhaust stacks
also need to be designed and located properly for the most efficient operation
of a local exhaust system. A common mistake is to locate them too close to building
fresh air intakes. Generally they should be located no closer than 50 feet to
prevent re-circulation of contaminants. Stacks work best when they are tall,
usually at least 10 feet above the roof line. Air velocity out of the stack
should be at least 3000 feet per minute to overcome the effects of downdrafts
from wind blowing over the building.
All components
of ventilation systems must be designed properly in order for the systems to
operate safely and efficiently. Failure to provide effective ventilation systems
in the design stage can create costly obstacles to safe and efficient operation
and maintenance, which ultimately presents a threat to personnel safety and
health.
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.
