Example 3.15: Adiabatic mixing of air with water: Moist air at 70 °F dry-bulb and 45 °F wet-bulb temperatures is to be processed to a final dew point temperature of 55 °F by adiabatic injection of saturated steam at 230 °F. The feed rate of dry air is 200 lb/min. Find the final dry-bulb temperature of the moist air and the steam required in lb/h.

Solution

C Find enthalpy of the steam h_g = 1157 Btu

Δh/ΔW = h_g = 1157 Btu

C Use the protractor to join the origin with Δh/ΔW = 1157 Btu and generate a reference line.

C Locate State 1 (t = 70 °F, t^* = 45 °F) ® W₁ = 0.00070 lb_w/lb_a

C Draw a line parallel to the reference line passing through State 1 and generate a condition line.

C Draw a horizontal line from final dew point t_d = 55 °F.

C Locate intersection of these two lines that defines state 2.

C Read properties of state 2.

W₂ = 0.00920 lb_w/lb_a

C m_w = m_a(W₂ - W₁)

= 200´60´(0.00920 - 0.00070) = 102 lb/h