An insulated throttling valve receives saturated liquid R-134a. The exiting flow must have a pressure of 200 kPa.

Question

(a) Using your model of a throttling device, plot the exit quality of the R-134a as a function of inlet pressure, for pressures ranging from 250 kPa to 1000 kPa.

(b) Upon exiting the throttling valve, the refrigerant is to be used in a heat exchanger. In order to assure proper operation of the heat exchanger, the quality exiting the throttling valve must be no greater than 0.25. What range of inlet pressures to the throttling valve will satisfy this design constraint?

Given: P2 = 200 kPa; x1 = 0 (saturated liquid);
R-134a (Take State 1 as the inlet, State 2 as the outlet.)
Assume: Q?=W?=0. For a throttling valve, assume steady-state, steady-flow, single-inlet, single-outlet flow, with ?KE = ?PE = 0

Answer 1

Q?=W?=0. For a throttling valve, assume steady-state, steady-flow, single-inlet, single-outlet flow, with ?KE = ?PE = 0

Solution: (a) With these assumptions, the First Law for open systems reduces to h1=h2



The inlet state enthalpy depends on the inlet pressure:



At P2, hf = 36.84 kJ/kg; hg = 241.3 kJ/kg

Using x2=h2?hf/hg?hf , the following plot is made



(b) The constraint given is x2,max = 0.25

Inspection of the plot indicates that this °Ccurs around P2 = 700 kPa.

More accurate predictions can be found by solving for h2 when x2 = 0.25: h2 = 87.96 kJ/kg

Then, we want to find through interpolation on R-134a tables or R-134a property databases for when hf = 87.96 kJ/kg: P1 = 718 kPa

So, the allowed pressure range is 200 kPa < P1 < 718 kPa