An insulated mixing box in an airconditioning system is used

An insulated mixing box in an air-conditioning system is used to mix two streams of refrigerant R134a to produce a single stream. The pressure of each stream is 6 bar. Inlet 1 is a saturated vapor with a flow rate of 0.371 kg/s, and inlet 2 is at 44.9oC with a flow rate of 0.123 kg/s. Determine the rate of entropy production in the mixing box (kW/K). Assume steady state operating conditions prevail. Answer is 0.000253 use first and second law of thermodynamics

Solution

At Inlet 2

h₂ = 433.262 kJ/kg

s₂ = 1.795 kJ/kgK

m₂ = 0.123 kg/s

At Inlet 1

h₁ = 410.485 kJ/kg

s₁ = 1.717 kJ/kgK

m₁ = 0.371 kg/s

Using the First Law Of Thermodynamics

m₁h₁ + m₂h₂ = (m₁+m₂)h₃

h₃ = 416.156 kJ/kg

As the pressure remains Constant and the enthalpy is graeter than that at saturation ,hence the box has superheated R-134a vapor

From this we get the Entropy s₃= 1.736933198 kJ/kgK

Rate of Entropy Generation = (m₁+m₂)s₃ - m₁s₁ - m₂s₂ = 0.00253 kW/K