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Module 2: Characteristics of Gases - Density - Practice Problems

Instructions:: Work these problems on a sheet of paper and check your answers against those provided below.
Helpful Calculators:: The following calculators may be useful in solving these problems. You can access them either from the "Calculators" button on the screen or from the links below.; Temperature Converter; Pressure Converter

#1: The gas temperature in a gas stream has increased by 250°F as it passes over an electrical strip heater. Has the gas density increased or decreased due to this temperature increase?

: Answer: The gas density has decreased.; Solution:; As indicated by the following equation, the gas density is inversely related to the absolute temperature.

#2: The gas pressure in a gas stream decreases as it passes through a high-pressure drop control system. Has the gas density increased or decreased due to this pressure decrease?

: Answer: The gas density has decreased.; Solution:; The gas density is directly related to the absolute pressure as indicated by the following equation:

#3: The air stream passing through an air pollution control device is at a temperature of 300°F. This stream is mixed with a second gas stream of equal flow rate that is 100% water vapor at 300°F. Is the average gas density of the mixed gas stream higher or lower than the inlet air stream? Use an average molecular weight of 29 lb_m/lb mole for air and a molecular weight of 18 lb_m/lb mole for water.

: Answer: The gas density of the mixed gas stream is lower than the density of the inlet air stream.; Solution:; As indicated by the equation below, the average gas density is related to the average molecular weight of the gas mixture. Water has a molecular weight of only 18 lb_m/lb mole and the addition of a large quantity of water vapor will reduce the average molecular weight of the gas.

#4: What is the average molecular weight of a gas mixture of 6% oxygen, 54% nitrogen, 7% carbon dioxide, and 33% water vapor?

: Answer: 26.06 lb_m/lb mole; Solution:

#5: What is the gas density of a combustion gas stream having an average molecular weight of 26.06 lb_m/lb mole, a temperature of 400°F, and an absolute pressure of 14.63 psia?

: Answer: 0.0413 lb_m/ft³; Solution:

#6: What is the gas density of a gas stream on the outlet side of a high energy venturi scrubber based on the following data?

: Answer: 0.0480 lb_m/ft³; Solution:

Convert the temperature and pressure data to absolute scales.
Convert pressure from in. W.C. to psia to conform with units of R, the universal gas constant.
Substitute the absolute temperature, absolute pressure, and molecular weight data into the equation used to calculate gas density.

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Density

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