The following explanations that describe the flow of refrigerant were taken from member discussion in our A/C Forum. They were all good, and each describe the flow accurately with different emphasis.
Refrigerant flow as descibed by Prof.
(1)Compressor draws LOW PRESSURE HOT VAPOR refrigerant from the evaporator (suction port - large line), and compresses the vapor.
(2) HIGH PRESSURE HOT VAPOR from the compressor goes into the condensor. The vapor gives up its latent heat to the cooler outside air through the condensor and changes back to LIQUID (like steam back to water). Now its a HIGH PRESSURE LIQUID.
(3)The HIGH PRESSURE LIQUID now encounters an orifice (small opening) either an orifice tube or expansion valve. When the liquid squirts through this opening, its pressure is reduced and it gets very cold.
(4) The now LOW PRESSURE COLD LIQUID absorbs heat from the cab through the evaporator and the liquid inside turns into vapor (boils) (like water absorbing heat and turning to steam)except refrigerants boil at veeery low temperatures.
(4a) Note: If the orifice is an O-tube, under some conditions too much refrigerant might get into the evaporator and not all boil. An accumulator is hooked on the evaporator output to "accumulate" this excess liquid and prevent the liquid from reaching the compressor.
(5) This LOW PRESSURE HOT VAPOR from the evaporator is now drawn into the compressor. Back to step 1.
Refrigerant flow as descibed by Chris Bede
Hot compressed gas leaves the compressor via the small hose (aka high side hose) and enters the top of the condensor where it will cool off a little and "condense" from a hot gas to a high pressure liquid refrigerant.
Refrigerant exits the bottom of the condensor, and heads towards the evaporator as a high pressure liquid. To get the refrigerant to boil, and absorb the heat from the inside of the car, we need to turn that high pressure liquid into a low pressure boiling liquid. That refrigerant pressure drop happens right before the refrigerant enters the evaporator via an expansion valve or an orifice tube. This is the pressure split. The refrigerant enters the evaporator as low pressure mix of boiling liquid and vapor. Heat load on the evaporator changes the liquid to a gas vapor. This low pressure vapor then returns to the compressor (via the suction hose) to start the cycle over again.