Don't ya just love technology? Take the simpilicity of the cap tube I'm always impressed with the the human brain and it's marvilous achevements
The cap tube for example acts as a pressure reducing device. If air or water is pressurized at the inlet, there will be a linear or equal reduction of pressure for each foot of tubing. For example, if water enters a cap tube at 50 psig, the pressure decreases at a constant rate as the water passes through the tube. If refrigerant responded the same way as water, the cap tube could never be used as a refrigerant control. Why not? Because when the head pressure was low, the evaporator would be starved, and when the ambient temperature was high, the evaporator would be flooded with liquid (the flow rate would be greater than the rate of vaporization).
However, if there was a way to increase the flow rate when the head pressure was low, and decrease the flow rate at high ambient temperatures, you would have a refrigerant control. This modulating effect is exactly what happens when a cap tube is installed in a well-balanced system.