A throttling valve in a refrigerator receives saturated liquid R-134a at 800 kPa, and exhausts the R-134a as a saturated mixture at 180 kPa.

Question

(a) Assuming no changes in kinetic energy of the R-134a, determine the quality of the exiting mixture.
(b) For the same inlet conditions, use your throttling device model and plot the exit quality of the R-134a as the exit pressure is varied between 100 kPa and 700 kPa.

Given: P1 = 800 kPa; x1 = 0 (saturated liquid); P2 = 180 kPa
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

With these assumptions, the First Law for open systems reduces to h1=h2

(a) For R-134a: h1 = 93.42 kJ/kg

At P2, hf = 33.45 kJ/kg; hg = 239.71 kJ/kg

With h2 = h1: x2=h2?hfhg?hf = 0.291

(b) Plotting the exit qualities as a function of exit pressure yields:



The relationship is not linear.