Saturated moist air at T1 5 oC enters an insulated chamber

Saturated moist air at T₁ = 5 ^oC, enters an insulated chamber operating at steady state with a mass flow rate of 0.5 kg/s and mixes with moist air at T₂ and f₂ entering the chamber with a mass flow rate of 0.3 kg/s. The mixture leaves the chamber at T₃ = 14 ^oC, f₃ = 80 %. P₁= P₂ = P₃ = P = 1bar.

(a)   Starting with the most general equations, simplify the equations for the conservation of mass, the first law, and the second law of thermodynamics as applied to the mixing chamber. Expand each term in the resulting equations in terms of properties of dry air and water vapor. Neglect kinetic and potential energy effects.

(b) Determine the temperature, T₂, in ^oC, and the relative humidity, f₂. If the solution requires an iteration process, use IT to solve the problem. Include a copy of the IT program listing and the results with your solution.

(c) Evaluate the rate of entropy production, in kW/K.

Solution

T1=278K,m1=0.5,R.H(f1)=0.54(from properties table)

T2=?,m2=0.3,R.H(f2)=?

T3=287K,m3=0.8 and RH(f3)=0.8

Heat transfer takes place at constant pressure

The Moisture Balance when Mixing Air
The moisture balance for when mixing air can be expressed as:

m1 * f₁ + m2 * f2 = (m1 + m2) * f3   

0.5*0.54+0.3*f2=0.8*0.8

f2=0.37