Question

It is required to design an air-conditioning plant for a small office room for following winter conditions : 

Outdoor conditions ...... 14ºC DBT and 10ºC WBT 

Required conditions ...... 20ºC DBT and 60% R.H. 

mount of air circulation ...... 0.30 m³/min./person. 

Seating capacity of office ...... 60. 

The required condition is achieved first by heating and then by adiabatic humidifying. 

Determine the following : 

(i) Heating capacity of the coil in kW and the surface temperature required if the by pass factor of coil is 0.4. 

(ii) The capacity of the humidifier. 

Solve the problem by using psychrometric chart.

Answer 1

• Locate the points ‘1’ and ‘3’ on the psychrometric chart. 
• Draw a constant enthalpy line through ‘3’ and constant specific humidity line through ‘1’.
• Locate the point ‘2’ where the above two lines intersect.

From the psychrometric chart : 

h₁ = 29.3 kJ/kg, h₂ = h₃ = 42.3 kJ/kg

t_db2 = 24.5ºC, v_s1 = 0.817 m³/kg

The mass of air circulated per minute,

m_a = \(\cfrac{0.30\times60}{0.817}\) = 22.03 kg/min

(i) Heating capacity of the heating coil

= m_a(h₂ – h₁) = 22.03 (42.3 – 29.3) = 286.4 kJ/min. 

= 4.77 kJ/s or 4.77 kW.

The by-pass factor (BF) of heating coil is given by :

t_db4 (coil surface temperature) = 32.8ºC. 

(ii) The capacity of the humidifier