Why is it that a throttling device always has a valve leading to it? If in a closed system loop, a cooled gas exists right before the throttling device, shouldn't the sudden decrease in cross-sectional area reduce pressure without the valve, just as good as with it?
Maybe my misunderstanding lies in the operation of the valve. Is it for safety purposes and left open until emergency shutting is needed, or is it actually open and shut according to a timer or sensor in the circuitry so as to keep it "synced" with the position of the fluid circulating through the stages of the system's cycle (like evaporation, condensing, etc.)?
EDIT: Specifically, my question is related to the refrigeration cycle. Is the purpose of the valve to isolate the fluid at the stage leading to it for a while then letting it off to the next. For instance, the valve between condenser and evaporator. Is my understanding that it's shut starting when the compressor does its job until all the heat possible is dissipated in the condenser and ONLY THEN does the valve open so that the other half of the loop (that was sort of vacuumed out by the compressor beast)... i.e. from valve all the way to compressor, due to its low pressure creates the pressure DROP (compressor->valve loop leading to valve->compressor loop), correct? And that after the valve opens the compressor is shut down until all possible heat is taken from evaporator and it is ready to be compressed, when of corse, the valve shuts again and the compressor starts. Rinse, repeat ad infinitum!