Basic psychrometric processes

Let $\dot m_a$ be the mass flow of dry air, and 1 and 2 the inlet and outlet states.

(a) Sensible heating

Heat is added with no change in $\omega$. The state moves horizontally to the right; DBT, WBT and enthalpy rise, RH falls.

$$Q = \dot m_a (h_2 - h_1) = \dot m_a\,c_{pm}(t_2 - t_1)$$

with humid specific heat $c_{pm} = 1.005 + 1.88\,\omega$ kJ/kg K. Done by an electric or steam heating coil.

(b) Sensible cooling

Air is cooled by a coil whose surface temperature is above the dew point, so $\omega$ stays constant. The state moves horizontally to the left; RH rises. The same equation applies with the sign reversed.

(c) Cooling and dehumidification

The coil surface is below the dew point of the entering air, so vapour condenses. The state moves down and to the left.

$$Q = \dot m_a (h_1 - h_2) - \dot m_w h_w,\qquad \dot m_w = \dot m_a(\omega_1 - \omega_2)$$

The load splits into sensible and latent parts, $Q = Q_s + Q_l$. This is the main summer air-conditioning process.

(d) Heating and humidification

Used in winter: air is heated, then water or steam is added. Specific humidity and temperature both rise.

$$\dot m_w = \dot m_a(\omega_2 - \omega_1),\qquad Q = \dot m_a(h_2 - h_1) - \dot m_w h_w$$

 w ^
   |              o 2d  (d)
   |             /
   |  2b o<-----o 1 ---->o 2a
   |           /
   |      2c o            (c)
   +------------------------> DBT

(a) sensible heating, (b) sensible cooling, (c) cooling and dehumidification, (d) heating and humidification.