Page 243 - Standard Handbook Of Petroleum & Natural Gas Engineering

2 16 General Engineering and Science

where SQ is the heat absorbed by unit mass of the system over the infinitesimal
temperature change, dT.
For constant pressure when only P-V work is involved, the first law yields
CP = (S) (2-117)

For constant volume
cv = (S) (2-1%)

C, is related to Cv by the following expression:
CP - cv = [ P + (3]( g)p (2-119)

For an ideal gas for which PV = RT/Mw, with Mw representing the molecular
weight, the enthalpy and internal energy are functions only of temperature and
Equation 2-1 19 becomes

c,-cv=P(g) =- R
(2-1 20)
P M,
The ratio of the heat capacity at constant pressure to that at constant volume is

(2-121)

Again, for an ideal gas, this ratio becomes
R
lc=l+- (2-1 22)
C”M,

Application of the Second Law
Heat Engines
The purpose of a heat engine is to remove heat Q, from a thermal reservoir at a
higher absolute temperature T,; extract useful work W; and reject heat Q, to a second
thermal reservoir at a lower absolute temperature T,. The device used to obtain the
useful work is the heat engine.
Considering an “ideal” heat engine as the system, the first law as applied to the
engine undergoing a series of reversible changes in a cyclical fashion becomes

CAUl = 0, or W = Q, - Q, (2-123)