A sonic nozzle is disclosed that is positioned between an evaporator and compressor in an air conditioning system. The sonic nozzle limits refrigerant flow rate to that allowed by sonic velocity at a throat area of the sonic nozzle. The sonic nozzle may have a by-pass flow area that allows for less pressure drop at lower flow rates where the by-pass flow area is sealed off at a predetermined pressure drop to force all flow through the sonic nozzle. Another embodiment that is disclosed features a thermally actuated suction throttling valve that is attached to a sonic nozzle. A power element of the throttling valve contains thermally active fluid, such as water, when frozen activates a piston member into the throat portion of the sonic nozzle to restrict fluid flow.