Pressure-Enthalpy Diagram

The pressure-enthalpy diagram opposite
is for a typical refrigerant. The two-phase
zone is shaded orange, and a typical cycle
is indicated with the numbers corresponding
to those used in this experiment. Isotherms
in the vapor region (right of the two phase zone)
are shown as orange arcs. In the two phase
zone, isotherms and isobars coincide, and in
the saturated liquid region the isotherms are vertical.

From: Moran & Shapiro, "Fundamentals of Engineering Thermodynamics," Wiley (1992)

The path (1- 2s) is for an ideal isentropic compressor, the path (1 - 2) is for a real compressor. The path (2 - 3) takes place in the condensing tank, and the path (4 - 1) is in the evaporating tank. Process (3 - 4) represents the expansion valve.