2.6 Key Equations

2. Thermodynamic Properties of a Pure Substance

Pressure, temperature, and specific volume

Pressure

P = F / A

Absolute and gauge pressures

P_{g a u g e} = P_{a b s} - P_{a t m}

Absolute and vacuum pressures

P_{v a c} = P_{a t m} - P_{a b s}

Density

ρ = m / V

Specific volume

v = V / m = 1 / ρ

Conversion of temperatures in Kelvin and Celsius degrees

T (K) = T (^{\circ} C) + 273.15

Energy, enthalpy, and entropy

Total stored energy in a system

\begin{aligned} E & = U + K E + P E \\ = m u + \frac{1}{2} m V^{2} + m g z (V : v e l o c i t y) \end{aligned}

Total stored specific energy in a system

e = \frac{E}{m} = u + \frac{1}{2} V^{2} + g z (V : v e l o c i t y)

Enthalpy

H = U + P V

Specific internal energy

u = U / m

Specific enthalpy

h = H / m

and

h = u + P v

Specific entropy

s = S / m

Saturated liquid-vapour two-phase mixtures

Quality

x = \frac{m_{g}}{m_{m i x}}

Specific volume

v = v_{f} + x (v_{g} - v_{f}) = (1 - x) v_{f} + x v_{g}

Specific internal energy

u = u_{f} + x (u_{g} - u_{f}) = (1 - x) u_{f} + x u_{g}

Specific enthalpy

h = h_{f} + x (h_{g} - h_{f}) = (1 - x) h_{f} + x h_{g}

Specific entropy

s = s_{f} + x (s_{g} - s_{f}) = (1 - x) s_{f} + x s_{g}

Compressed liquid (when the compressed liquid tables are not available)

Specific volume

v \approx v_{f @ T}

Specific internal energy

u \approx u_{f @ T}

Specific enthalpy

h \approx h_{f @ T}

Specific entropy

s \approx s_{f @ T}

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