thshows the variation mass flow rate with suction pressure for
fixed condenser pressure. The mass flow rate through the capillary tube
increases as the evaporator pressure decreases. However at a pressure of p*
the
flow is choked. If the choking occurs at some interior point of the tube, the length
of the tube from this point to the exit will offer frictional resistance to the flow and
the pressure must decrease to overcome this. The pressure however cannot
decrease since the flow is choked. Hence, adjustment in the inlet conditions
occurs and the mass flow rate is reduced so that the flow will (always) be choked
at the exit of the tube with reduced mass flow rate. This is typical of compressible
sonic flow where upstream influence occurs; otherwise the downstream pressure
decides the mass flow rate