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Cheat Sheet

Thermodynamics For Dummies

From Thermodynamics For Dummies by Mike Pauken

Thermodynamics sounds intimidating, and it can be. However, if you hone in on the most important thermodynamic formulas and equations, get comfortable converting from one unit of physical measurement to another, and become familiar with the physical constants related to thermodynamics, you’ll be at the head of the class.

Important Thermodynamic Equations and Formulas

Thermodynamics is filled with equations and formulas. Here’s a list of the most important ones you need to do the calculations necessary for solving thermodynamics problems.

  • Combustion equations:

    • Air-fuel ratio:

      image0.png
    • Hydrocarbon fuel combustion reaction:

      image1.png
  • Compressibility calculations:

    • Compressibility factor Z: Pv = ZRT

    • Reduced temperature:

      image2.png
    • Reduced pressure:

      image3.png
    • Pseudo-reduced specific volume:

      image4.png
  • Efficiency equations:

    • Thermal efficiency:

      image5.png image6.png
    • Coefficient of performance (refrigerator):

      image7.png
    • Coefficient of performance (heat pump):

      image8.png
  • Energy equations:

    image9.png image10.png image11.png
  • Entropy equations:

    • Entropy change for ideal gas, constant specific heat:

      image12.png image13.png
    • Entropy change for ideal gas, variable specific heat:

      image14.png
    • Irreversibility for a process:

      image15.png
  • Ideal-gas formulas:

    • Ideal-gas law: Pv = RT

    • Gas constant:

      image16.png
    • Ratio of specific heats:

      image17.png
    • Isentropic process for ideal gas:

      image18.png
  • Moist air properties:

    • Relative humidity:

      image19.png
    • Specific humidity:

      image20.png
  • Properties of mixtures:

    • Quality liquid-vapor mixture:

      image21.png
    • Saturated mixture property, y: y = yf + x · yfg

  • Work calculations:

    • Isobaric process: Wb = P0(V2 – V1)

    • Polytropic process:

      image22.png
    • Isothermal process of an ideal gas:

      image23.png
    • Shaft power:

      image24.png
    • Spring work:

      image25.png

Conversion Factors for Physical Measurements

Sooner or later, you’re going to have to convert from one unit of physical measurement to another. This list gives you some of the most common conversion factors you need in thermodynamics.

  • Acceleration: 1 m/s2 = 100 cm/s2

  • Area: 1 m2 = 104 cm2 = 106 mm2

  • Density: 1 g/cm3 = 1 kg/L = 1,000 kg/m3

  • Energy, heat, work, internal energy, enthalpy: 1 kJ = 1,000 J = 1,000 N·m = 1 kPa·m3

    1 kJ/kg = 1,000 m2/s2

  • Force: 1 N = 1 kg·m/s2

  • Length: 1 m = 100 cm = 1,000 mm

  • Mass: 1 kg = 1,000 g

  • Power, heat transfer rate: 1 W = 1 J/s, 1 kW = 1,000 W

  • Pressure: 1 Pa = 1 N/m2, 1 kPa = 1,000 kPa, 1 MPa = 1,000 kPa

    1 atmosphere (atm) = 101.325 kPa

  • Specific heat, entropy: 1 kJ/kg · °C = 1 kJ/kg · K = 1 J/g·°C

  • Specific volume: 1 m3/kg = 1,000 L/kg = 1,000 cm3/kg

  • Temperature: T(K) = T(°C) + 273.15, T(°C) = (5/9) T(°F) – 32, T(°F) = (9/5) T(°C) + 32

  • Velocity: 1m/s = 3.6 km/hr

  • Volume: 1 m3 = 1,000 L = 106 cm3

Physical Constants in Thermodynamics

You run across a few physical constants when working out thermodynamics problems. Following is a list of the constants you need when you’re working with potential energy, pressure, or the ideal gas law.

  • Acceleration of gravity: g = 9.81 m/s2

  • Standard atmospheric pressure: 1 atm = 101.3 kPa

  • Universal gas constant:

    image0.jpg
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