Question

A piston/cylinder assembly  has 1 kg of R-134a at state 1 with 110°C, 600 kPa, and is then brought to saturated vapor, state 2, by cooling while the piston is locked with a pin. Now the piston is balanced with an additional constant force and the pin is removed. The cooling continues to a state 3 where the R-134a is saturated liquid. Show the processes in a P-V diagram and find the work in each of the two steps, 1 to 2 and 2 to 3.

Answer 1

CV R-134a This is a control mass.

Properties

State 1:  (T,P) B.5.2 => v = 0.04943 m³/kg

State 2:  given by fixed volume v₂ = v₁ and x₂ = 1.0

v₂ = v₁ = v_g = 0.04943 m³/kg => T = 10°C

State 3 reached at constant P (F = constant) v₃ = v_f = 0.000794 m³/kg

Since no volume change from 1 to 2 => ₁W₂ = 0

₂W₃ = ∫P dV = P(V₃ -V₂) = mP(v₃ -v₂) Constant pressure

= 415.8 (0.000794 - 0.04943) 1 = -20.22 kJ