Page 236 - Standard Handbook Of Petroleum & Natural Gas Engineering

Thermodynamics 209

Figure 2-30. Diagram for Example 2-21

THERMODY NAMlCS
Thermodynamics centers around the concept of “energy in transit,” but is con-
siderably more encompassing in its applications. The science of thermodynamics
deals very broadly with the concepts of how things work, why some things cannot
work, and why some things do not work as intended. Three “laws” of thermodynamics
have been formulated, which can be summarized as follows:
1. The First Law of Thermodynamics: A statement of the principle of
conservation of energy.
2. The Second Law of Thermodynamics: Deals with the concept of entropy, which
serves as a means of determining
whether or not a process is possible.
3. The Third Law of Thermodynamics: Defines the zero entropy state for any
substance in a single, pure quantum
state as the absolute zero of temperature.
Units of Energy

Measurements of energy are made in terms of absolute joules, but engineering
practice has persistently retained the thermochemical calorie as the unit of energy.
The two are related by the definition:
1 calorie = 4.1840 absolute joules (2-101)
which is often referred to as the mechanical equivalent of heat.
The following table expresses the relationship between several other useful energy
units and the calorie:

1 Btu = 252.16 cal
1 kWhr = 8.6056 x lo9 cal
1 hp-hr = 6.4162 x 10 cal
1 ft(lb,) = 0.324 cal
1 batm = 24.218 cal
A = work function (Helmholtz free energy), Btu/lb,,, or Btu
C = heat capacity, Btu/lbnloR
C, = heat capacity at constant pressure
C, = heat capacity at constant volume
F = (Gibbs) free energy, Btu/lb,” or Btu
g = acceleration due to gravity = 32.174 ft/s‘
g, = conversion factor between force and mass = 32.174 (lbr,,)(ft/s‘)/lb,
h, H = enthalpy or heat content, Btu/lb,,, or Btu
K = ratio CJC,
Mw = molecular weight, lbJlbm-mole
m, M = mass of fluid, lbm
m, M = mass flow rate, lb,/s