Air Heating Systems

Using air to heat buildings - temperature rise diagram

It is often convenient to heat buildings with air. Air heating systems may be cost effective if they can be made simple or if they can be combined with a ventilation system. Be aware that due to the low specific heat capacity of air the use of air for heating purposes is very limited. Larger heat loads requires larger volumes of air resulting in huge oversized ducts and fans. Transport of huge volumes of air requires a lot of energy.    

Required Air Volume in Air Heating Systems

Required air volume in an air heating system can be calculated as

L = Q / (c_p ρ (t_h - t_r))          (1)

where

L = air volume (m³/s)

Q = heat loss from the building (kW)

c_p = specific heat capacity air - 1.005 (kJ/kg^oC)

ρ = density of air - 1.2 (kg/m³)

t_h = heating air temperature (^oC)

t_r = room temperature (^oC)

As a rule of thumb the heating supply temperature should be in the range 40-50^oC. The air flow should be in the range 1-3 times the room volume. 

Equation (1) expressed in imperial units:

 L = Q / (1.08 (t_h - t_r))          (2)

where

Q = heat (btu/hr)

L = air volume (cfm)

t_h = heating air temperature (^oF)

t_r = room temperature (^oF)

Air Heating - Temperature Rise Diagram

The diagrams below can be used to estimate heat required to rise temperature in air flows. 

SI units - kW, m³/s and ^oC

Imperial units - Btu/h, cfm and ^oF

• 1 m³/s = 3,600 m3/h = 35.32 ft³/s = 2,118.9 ft³/min (cfm)
• 1 kW (kJ/s) = 859.9 kcal/h = 3,413 Btu/h
• T(^oC) = 5/9[T(^oF) - 32]

Example - Heating a single room with air

A building with a large room with heat loss 20 kW is heated with air with a maximum temperature 50 ^oC. The room temperature is 20 ^oC. The required air volume can be calculated as

L = (20 kW) / ( (1.005 kJ/kg^oC) (1.2 kg/m³) ((50 ^oC) - (20 ^oC)) )

    = 0.55 m³/s

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