Untitled Quiz

Gas Constant

• The gas constant (R) is different for each gas and is determined by the universal gas constant (Ru) and molar mass (M)
• Ru (universal gas constant) is the same for all substances and its value is 8314 J/kg.K
• R = Ru/M
• Molar mass is the mass of one mole (gmol) of a substance in grams, or the mass of one kmol (kgmol) in kilograms

Ideal Gas Equation of State

• The ideal gas equation of state is Pv = RT
• P is pressure, v is specific volume or volume per unit mass, R is the gas constant and T is temperature.
• There are two other equations of state for ideal gas:
  • PV = mR*T (m is the mass of the substance), and
  • PV = NRu*T (N is the mole number)
• In a closed system, the ideal gas equation of state can be used to find the relationship between two states:
  • P1V1/T1 = P2V2/T2
• This relation is applicable when the working fluid is an ideal gas and the mass is constant.

What is an Ideal Gas?

• An ideal gas is a model substance of infinitely small particles (point masses) that completely fill a defined space and do not interact
• Air, nitrogen, oxygen, carbon dioxide and argon are common examples of ideal gases

What is the Equation of State?

• An equation of state is an equation that relates the pressure, temperature, and specific volume of a substance
• The ideal-gas equation of state predicts the P-v-T behavior of a gas quite accurately within some properly selected region

Compressibility Factor [Z}

• The compressibility factor (Z) is a correction factor that measures the deviation from the ideal gas behavior.
• Z = 1 for an ideal gas, and Z can be greater than or less than 1 for real gases.
• The farther away Z is from unity, the more the gas deviates from ideal-gas behavior.
• Z can be determined using reduced temperature (TR) and reduced pressure (PR)
  • TR = T/Tcr, where Tcr is the critical temperature
  • PR = P/Pcr, where Pcr is the critical pressure

At what pressures/temperatures can water vapor be approximated as an ideal gas?

• Water vapor can be approximated as an ideal gas at pressures below 10 kPa regardless of its temperature.

Compressibility Factor Chart

• There is a compressibility factor chart available that can be used to determine Z
• At low pressures (PR < 2), ideal-gas behavior can be assumed with good accuracy regardless of the pressure

Ideal Gas Behavior Deviation in Water Vapor

• The deviation of water vapor from ideal-gas behavior is greatest in the vicinity of the critical point.