What is Vapour-compression cycle?

Almost every refrigerator, freezer, air conditioner and heat pump runs a refrigerant round four components: a compressor (receives the work, raises the pressure of the vapour), a condenser (the hot vapour liquefies at high pressure, releasing $|Q_h|$ to the hot side), an expansion valve (the liquid drops to low pressure without work or heat, and partly flashes to vapour) and an evaporator (the rest boils at low pressure, absorbing $Q_c$ from the cold side). The heat is carried as latent heat of vaporization, and the two pressures set the two boiling temperatures (Chapter 25).