Cooling and Dehumidification

When moist air is cooled below its dew point by bringing it in contact with a cold surface, some of the water vapour in the air condenses and leaves the air stream as liquid, as a result both the temperature and humidity ratio of air decreases as shown. This is the process air undergoes in an air conditioning system. The actual process path depends on the type of cold surface, the surface temperature and flow conditions, but for simplicity the process line is assumed to be a straight line as shown in Figure 8.10.

Figure 8.10 Cooling and Dehumidification Process

The heat and mass transfer rates can be expressed in terms of the initial and final conditions by applying the conservation of mass and conservation of energy equations as given below:

By applying mass balance for the water:

 

m_a·ω_a = m_a·ω₂ + m_w

 

By applying energy balance:

 

m_a·h₁ = Q_r + m_w · h_w + m_a·h₂

 

Here, subscripts a and w are used for air and water, respectively.

From the above two equations, the load on the cooling coil, Q_t is given by,

 

Q_r = m_a (h₁ − h₂) − m_a (ω₁ − ω₂) h_w

 

The second term on the RHS of the above equation is normally small compared to the other terms, so it can be neglected. Hence,

 

Q_r = m_a (h₁ − h₂)

 

It can be observed that the cooling and de-humidification process involves both latent and sensible heat transfer processes, hence, the total latent and sensible heat transfer rates (Q_r, Q_l, and Q_s) can be written as