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Module 2: Characteristics of Gases - Velocity
Gas VelocityAnimation #1:
Gas Velocity Comparisons
Velocity Pressure
Objectives
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Calculate the gas velocity based on the gas flow rate and cross-sectional area.
- Calculate the average gas velocity in a duct based on the measured velocity pressure.
Gas velocity is one of the fundamental design variables for ventilation systems and air pollution control equipment. Gas streams containing particulate are usually maintained at velocities of 3,000 to 4,500 ft/min in ductwork leading to particulate collectors to minimize particle deposition. The velocity of gas streams without particulate matter is often in the range of 1,500 to 3,000 ft/min. The gas velocities in air pollution control equipment are usually low to allow for sufficient time to remove the contaminants. For example, gas velocities through electrostatic precipitators are usually in the range of 2.5 to 6 ft/sec. The filtration velocities through pulse jet fabric filters are usually in the range of 2 to 10 ft/min. Variations in the gas velocity can have a direct impact on the contaminant removal efficiency.
The average velocity of a gas stream in an emission testing probe, an industrial duct, or an air pollution control device is a function of the actual gas flow rate and the cross-sectional flow area.
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This is illustrated in Figure 1.
Units of Measure
The units of measure for velocity are provided below. These velocity units of measure are not corrected to standard oxygen or moisture levels because velocity is calculated based on actual conditions.
This animation presents several different situations with ducts of different sizes and gas flow rates. Compare the gas velocity for each set of ducts and explain how duct size and shape affect gas velocity.
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Animation #1
This animation illustrates (No audio)
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Example Problem 1.
Gas Velocity Through a Duct
What is the velocity of a 24,000 ACFM gas stream moving through a duct having a width of 4 ft and a height of 2 ft? See Figure 2.
Solution:
Calculate the gas velocity (v) using the following equation:
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Calculate the duct area.
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Calculate the gas velocity.
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Example Problem 2.
Gas Velocity Through a Duct (Cgs Units)
What is the velocity of a 500 m3/min gas stream moving through a duct having dimensions of 0.5 m by 1.2 m?
Answer: 833 m/min
Solution:
Calculate the gas velocity (v) using the following equation:
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Calculate the duct area.
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Calculate the gas velocity.
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Example Problem 3.
Gas Velocity Through a Baghouse
What is the velocity of a 9,000 ACFM gas stream moving through a pulse jet fabric filters with 100 bags, each having a diameter of 6 in. and a height of 12 ft?
Solution:
Calculate the gas velocity (v) using the following equation:
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Calculate the total area of 100 bags, ATotal
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Calculate the gas velocity.
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Note: It is important to remember to use the gas flow rate in units of ACFM or m3/min when calculating velocities.
The gas velocity is usually determined by measuring the velocity pressure, which is the additional pressure caused by the motion of the gas stream.
You experience "velocity pressure" when you walk outside on a windy day. The force of the wind exerted over the area of your body is felt as a pressure. It is obviously significant even when the wind speeds are only in the range of 15 to 30 mph. The typical velocities of gas streams in industrial ductwork and in emission testing equipment range from approximately 15 mph to more than 50 mph.
The velocity pressure increases as the gas velocity increases. Gas velocity pressure increases when a greater volume of air travels through a cross-sectional area or when the volume of air remains constant but the cross-sectional area, through which the air travels, decreases.
The velocity pressure is exerted only in the direction of flow. A pitot tube, such as the one shown in Figure 4, measures velocity pressure.
One sensor of the pitot is pointed into the direction of gas flow (Point 1) so it can measure the combined static pressure and the velocity pressure. The other sensor of the pitot tube (Point 2) measures the static pressure of the moving gas stream. By connecting the two sensors together, the velocity pressure is measured as indicated by Equation 2.
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The velocity pressure is used in emission testing strictly to evaluate the gas flow rate in a duct or stack of known diameter. Once the average velocity across the measurement location is known, the gas flow rate can be measured. These measurements are conducted using U.S. EPA Reference Methods 1 and 2.
A number of measurement ports are required because the gas stream velocities are rarely uniform in ducts and stacks. Some of the typical velocity profiles are illustrated in Figure 5.
The average velocity of a gas stream is calculated based on the velocity pressures measured at each of the points shown in Figure 5. The data is tabulated as shown in the Table below.
The average gas temperature and the average square root of the velocity pressures (see columns 2 and 4 above) are used in the following equation to calculate the average velocity.
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Note: In the above equation, the velocity pressure has units of inches W.C. and the average pressure has units of inches Hg.
Example Problem 4.
Calculating the Average Gas Velocity
What is the average gas velocity based on the measured data provided below and in Table 3?
Solution:
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Practice Problems
Gas Velocity
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.