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Module 5: Flowcharts and Ventilation Systems - Hoods - Transport Velocity
Hoods
Transport Velocity
Contents
Lesson Material
Hoods Summary
Practice Problems
Objective
- Determine if the minimum transport velocity of a system is being maintained.
When the contaminant is captured by the hood system and enters the ductwork, a minimum transport velocity must be maintained to keep the contaminant from settling out of the gas flow stream and building up deposits in the ductwork. Declines in gas flow rates could indicate that the minimum transport velocity is not being maintained. As discussed earlier in this Module, lower flow rates will also lead to decreased hood capture efficiencies and increased fugitive emissions. The minimum transport velocity is particularly important when handling particulate-laden gas streams. Systems with heavy particulate-laden gas streams should have clean-out ports installed to remove any particulate that has settled out.
The following equation, which was introduced in Module 2, is used for determining velocity.
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Where:
The proper duct diameter is a key element when addressing minimum transport velocity. If a section of ductwork has a larger than necessary diameter, then settling out will most likely occur. If a section of ductwork is too small, the pressure drop will increase across this section, thus requiring the fan to handle more static pressure. Another concern when dealing with transport velocities is the abrasion of the ductwork, especially of the bends or elbows. The amount of abrasion that occurs depends on several factors: the gas velocity in the duct, the amount and type of particulate matter in the gas stream, and the construction of the ductwork.
Minimum transport velocities for different types of particulate matter can be obtained from standard tests concerning hoods and ventilation systems. Examples of transport velocities are listed below.
Example Problem 1.
Maintaining the Minimum Transport Velocity
A duct system transporting a very light dust requires a minimum transport velocity of 2,800 ft/min. The volumetric flow rate for the system is 978 ACFM. What is the necessary duct diameter in inches for this section of ductwork to maintain the minimum transport velocity?
Solution:
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Calculate the duct area.
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Calculate the duct diameter.
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Hoods are the first component of the air pollution control system and are of critical importance. If they fail to capture the pollutant, the overall collection efficiency of the system is reduced. Pollutants not captured by hoods become fugitive emissions. Many factors affect a hood's capture efficiency; however, one of the key factors is the distance between the pollutant source and the hood.
Maintaining an adequate airflow rate through the hood and ducts is important to achieve desired hood capture efficiencies and minimum transport velocities. Hood capture velocity equations can be used to determine the airflow rate sufficient to capture contaminated gas at a certain distance from the hood face (given a certain hood capture velocity). The gas flow rate through a hood-duct system can be monitored by using hood static pressure data. The hood static pressure can be used to estimate velocity pressure in the duct, which in turn can be used to estimate gas velocity and flow rate (based on the duct cross-sectional area).
The geometry of a hood opening influences the hood entry loss coefficient and the hood static pressure due to the formation of the vena contracta. Comparing the hood static pressure against baseline conditions provides a good indication if the system has developed any problems.
Maintaining a system's minimum transport velocity is necessary to ensure that all of the captured pollutant reaches the air pollution control device and to prevent build-up of the pollutants in the ductwork.
Practice Problems
Hoods - Transport Velocity
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.