Heat exchangers usually operate for long periods of time with no change in
their operating conditions. Therefore, they can be modeled as steady-flow devices.
As such, the mass flow rate of each fluid remains constant, and the fluid
properties such as temperature and velocity at any inlet or outlet remain the
same. Also, the fluid streams experience little or no change in their velocities
and elevations, and thus the kinetic and potential energy changes are negligible.
The specific heat of a fluid, in general, changes with temperature. But, in