Page 186 - Modelling in Transport Phenomena A Conceptual Approach

166 CHAPTER 6. STEADY-STATE MACROSCOPIC BALANCES

Analysis

System: Compressor and heat exchanger

Conservation of total mass, Eq. (6.2-1), reduces to

Therefore, Eq. (6.3-15) becomes

in which the enthalpy change of the air and the interphase heat transfer rate are
given by
(fil - a2)air = (eP)air(Tl- ~2)oir (3)

-
Qint = (~CP)H~O(Tout Tin)HzO (4)
Substitution of Eqs. (3) and (4) into Eq. (2) and rearrangement gives

Comment: Defining of a system plays a crucial role in the solution ofthe problem.
Note that there is no need to find out the temperature and pressure at the exit of the
compressor. If, however, one chooses the compressor and heat exchanger as two
separate systems, then the pressure and temperature at the exit of the compressor
mut be calculated.

6.3.2 Energy Equation With Chemical Reaction
6.3.2.1 Thermochemistry

Thermochemistry deals with the changes of energy in chemical reactions. The
difference between the enthalpy of one mole of a pure compound and the total
enthalpy of the elements from which it is composed is called the heat of formation,
AHf, of the compound. The standard heat of formation, AH;, is the heat of
formation when both the compound and its elements are at standard conditions as
shown in Figure 6.4. The superscript implies the standard state. Since enthalpy
is a state function, it is immaterial whether or not the reaction could take place at
standard conditions.