ENTHALPY OF MOIST AIR

The temperature-entropy diagram is shown in Fig. 20.2.

Enthalpy of moist air = Enthalpy of dry air + Enthalpy of water vapour associated with dry air

h = h_a + wh_v per kg of dry air

where h_a = c_pa t_d = 1.005 t_d kJ/kg

c_pa = 1.005 kJ/kg. K

t_d = DBT of air in °C.

h_v = c_pw t_dp +[(h_fg)_dp + c_pv (t_d −t_dp)]kJ/kg

= 4.1868t_dp + (h_fg)_dp +1.88(t_d −t_dp)

where c_pw = 4.1868 kJ/kg K is the specific heat of liquid water

(h_fg)_dp = latent heat of vaporization at DPT

c_pv = 1.88 kJ/kg.K is the specific heat of superheated water vapour

Figure 20.2 Temperature-entropy diagram

t_dp = DPT, °C

w = specific humidity

h = 1.005 t_d + w [4.1868 t_dp + (h_fg)_dp + 1.88 (t_d + t_dp)]