Example 3.12: Heating of Moist Air: Moist air, saturated at 60 °F and pressure 29.921 in. Hg enters a heating coil at a rate of 10,000 cfm. Air leaves the coil at 110 °F. Find the required heat-addition rate.

Solution

C Locate state 1 on the saturation curve at 60 °F. Its properties are

Enthalpy of humid air, h₁ = 26.467 Btu/lb,

Humidity ratio, W₁ = 0.0111 lb_w/lb_a,

Volume of humid air, υ₁ = 13.329 ft³/lb_a

Heating process does not change humidity ratio, so humidity ratio remains the same.

Final temperature of air is 100 °F, so

C Locate state 2 at the intersection of t = 110 °F and W₂ = W₁

W₂ = 0.0111 lb_w/lb_a,

Saturation humidity ratio is W_s = 0.0595 lb_w/lb_a.

C The degree of saturation, μ = 0.0111/0.0595 = 0.1864

C Enthalpy of humid air at state 2, h₂ = 38.7 Btu/lb_a

Volumetric flow rate of air through the heating coil = 10000 ft³/min

C The mass flow of dry air = m_a = 10000(60)/13.329 = 45014.6 lb/h

C Heat addition = m_a(h₂ - h₁) = 550664 Btu/h