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Module 2: Characteristics of Gases - Density - Answers

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Q icon #1
Which of the following factors affect gas density?

  1. Pressure
  2. Temperature
  3. Gas viscosity
  4. Molecular weight
  5. Molecular structure (i.e. aromatic, straight chain)

A icon
Answer:
  1. Pressure
  2. Temperature
  3. Molecular weight

Solution:
Pressure, temperature, and molecular weight all affect the gas density. The factors that affect the gas density can be determined directly from the ideal gas law discussed earlier in this Module. As a starting point, it is necessary to remember that the definition of density is as follows:

equation 1(a)

equation 1(b)

For example, if there are 0.05 lb moles of gas with a molecular weight of 26 lbm/lb mole in a volume of 20 ft3, the density is calculated as follows:

equation 1(c)

The term n/V in this calculation can be obtained by rearranging the ideal gas law equation:
equation 1(d)

Therefore, by substituting P/RT into the density equation, the following expression is derived:
equation 1(e)

The molecular weight, the pressure, and the temperature of the gas stream affect density.

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Q icon #2
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?

Figure 1

A icon
Answer: The outlet gas density is considerably lower. The relationship between density and pressure is described by the following equation:

equation 2

This equation shows that gas density is directly related to pressure. If the pressure decreases, the gas density decreases. This change must be taken into account to achieve adequate gas velocities in the ductwork and in sizing the fan to handle the outlet gas stream.

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