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Module 2: Characteristics of Gases - Density
Lesson MaterialPractice Problems
Objective
- Determine the gas density as a function of the absolute gas temperature.
Gas density is important primarily because it affects the flow characteristics of the moving gas streams. Gas density affects the velocities of gas through ductwork and air pollution control equipment. It determines the ability to move the gas stream using a fan. Gas density affects the velocities of gases emitted from the stack and thereby influences the dispersion of the pollutants remaining in the stack gases. It affects the ability of particles to move through gases. It also affects emission testing. Gas density data is needed in many of the calculations involved in air pollution control equipment evaluation, emission testing, and other air pollution control-related studies.
As discussed earlier, the volume of a gas increases as the temperature increases due to the motion of the gas molecules. As the volume occupied by the gas increases, its density decreases. Density is the mass per unit volume as indicated in Equation 1. Throughout these modules, the Greek symbol 
will be used as the symbol for density.
Note: Gas density is expressed as the mass per unit of volume of gas. The gas volume is always expressed at actual conditions. The gas volume is not corrected for temperature, pressure, moisture, or oxygen levels.
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#1
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Which of the following factors affect gas density?
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Pressure
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Temperature
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Gas viscosity
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Molecular weight
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Molecular structure (i.e. aromatic, straight chain)
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Pressure
The following equation is very useful in calculating gas density and can be derived by substituting the algebraic expression for volume (from the ideal gas law) into Equation 1 above for density. For a more detailed derivation, see the answer to Question #1 above.
The ideal gas law can be arranged as shown below.
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Density can be expressed by the following:
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By combining these two expressions, the following convenient equation can be derived:
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#2
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The gas stream passing through a high energy venturi scrubber shown in the figure below has a static pressure drop of 50 in. W.C. Is the gas density higher, lower, or unchanged on the outlet side of the venturi scrubber?
Gas temperature and pressure variations cause substantial variations in the gas density in air pollution control equipment and in emission testing systems. The extent of these variations is shown in Tables 1 and 2.
The data shown in Tables 1 and 2 apply strictly to air. However, the data are reasonably accurate for gas mixtures having an average molecular weight similar to air (i.e. 29 lbm/lb mole). Many gas streams generated by combustion processes have average molecular weights of 28 to 30 lbm/lb mole. Due to this similarity, the data in Tables 1 and 2 are sometimes applied to combustion gases as well as to ambient air.
Example Problem 1.
Calculating Average Molecular Weights of Gases
What are the average molecular weights of air and combustion gas having the following compositions?
Solution:
Average molecular weights are calculated using the volume percent concentration data.
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Example Problem 2.
Calculating Density of Air
What is the density of air at a temperature of 200°F and an absolute pressure of 14.7 psia? Use an average molecular weight of 29 lbm/lb mole for air.
Solution:
Example Problem 3.
Calculating Density of Gas
What is the density of combustion gas at a temperature of 200°F and an absolute pressure of 14.7 psia? Use an average molecular weight of 27 lbm/lb mole for combustion gas.
Solution:
Practice Problems
Density
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.