![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Module 5: Flowcharts and
Ventilation Systems
Introduction
Contents
Lesson Material
Practice Problems
Objectives
-
Explain the importance of hood capture efficiency.
- Calculate fugitive and stack emission rates.
The pollutants generated or released in process equipment must be captured so that they can be transported to the air pollution control device. Hoods are inlets designed to capture contaminated air and direct it into the exhaust duct system.
Hoods are normally an integral part of the process equipment. The hood can consist of a simple, stationary plenum mounted above or to the side of the source, a large moveable plenum, or the process equipment itself.
Figure 1 
shows stationary hoods in an industrial setting.
If hoods do not capture pollutants generated by process equipment, the pollutants disperse directly into the plant air and eventually pass through roof vents and doors into the atmosphere. Evaluation of the ability of the hoods to capture pollutants at the point of generation is important in many inspections and engineering studies.
The EPA defines fugitive emissions as "emissions that (1) escape capture by process equipment exhaust hoods; (2) are emitted during material transfer; (3) are emitted from buildings housing material processing or handling equipment; and (4) are emitted directly from process equipment." (40 CFR Part 60, Method 22, July 1, 1997).
Stack emissions are emissions that have gone through the air pollution control device and are released from the stack.
-
Where:
The importance of hood performance is illustrated by Example Problems 1 and 2, which are based on the simplified industrial process shown in Figure 2. This system consists of a process unit that generates pollutants, a hood above the process equipment, the ductwork, an air pollution control device, a fan, and a stack.
Example Problem 1.
Calculating Fugitive and Stack Emission Rates
Calculate the fugitive emissions and the stack emissions if the process equipment generates 100 lbm/hr of volatile organic compounds (VOCs), the hood capture efficiency is 95%, and the collection efficiency of the air pollution control device is 95%.
Solution:
-
Calculate fugitive emissions.
.gif)
-
Calculate stack emissions.
a.gif)
-
Where:
b.gif)
c.gif)
- The capture of fugitive emissions is the key step in an air pollution control system. It is crucial that all the fugitive emissions are captured and transported to the air pollution control device. Example Problem 1 shows that even with hood capture efficiencies approaching 100%, fugitive emissions can be higher than emissions leaving the stack.
Example Problem 2.
Calculating Fugitive and Stack Emission Rates
Calculate the stack emissions and fugitive emissions if the process equipment generates 100 lbm/hr of VOCs, the hood capture efficiency is 90%, and the collection efficiency of the air pollution control device is 95%.
Solution:
These two Example Problems illustrate the importance of hoods. Slight changes in the ability of the hood to capture the pollutants can have a large impact on the total fugitive and stack emissions released into the atmosphere.
Unfortunately, it is not always possible to see the fugitive emissions. Gaseous and vapor emissions such as carbon monoxide, sulfur dioxide, hydrogen chloride, and nitric oxide are not visible. Even particulate emissions may be hard to see under the following circumstances:
-
If there are numerous small fugitive sites
-
If there is one major site that cannot be seen from normal areas accessible to personnel
- If the particulate matter is not in the size range that causes light scattering
Techniques for monitoring hood capture effectiveness are important because the quantities of fugitive emissions can be high, and these emissions are often hard to see.
Practice Problems
Hoods - Introduction
-
Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.