A. RELATION BETWEEN ENTHALPY CHANGES

Let a control volume be considered, enveloping a heat exchanger, through which there flows two streams, denoted by subscripts 1 and 2. Then the steady-flow version of the first law of thermodynamics leads to

\[\label{eq124_1} \dot {M}_1 \left( {h_{1,{\rm out}} - h_{1,{\rm in}} }\right)+\dot {M}_2\left( {h_{2,{\rm out}}-h_{2,{\rm in}}}\right)=0 \tag{1}\]

where Ṁ stands for the mass flow rate and h for the specific enthalpy.

Of course, it is here implied that kinetic energy, heat transfer to the outside, and shaft work input are all negligible, but anyone with elementary knowledge of thermodynamics will know how to include the relevant terms when necessary, by reference, for example, to Equation 94.3.