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 ventilation systems. But be aware that due to the low specific heat capacity of air, the use of air for heating purposes is very limited. Large heat loads requires large volumes of air with huge oversized ducts and fans as results. Transporting huge volumes of air also requires a lot of energy.

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

L = Q / (cp ρ (th - tr)) (1)

where

L = air volume (m3/s)

Q = heat loss from the building (kW)

cp = specific heat capacity air - 1.005 (kJ/kgoC)

ρ = density of air - 1.2 (kg/m3)

th = heating air temperature (oC)

tr = room temperature (oC)

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

(1) expressed in imperial units:

L = Q / (1.08 (th - tr)) (2)

where

Q = heat (btu/hr)

L = air volume (cfm)

th = heating air temperature (oF)

tr = 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 deg C

Imperial units - Btu/h, cfm and deg F

- 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]

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

L = 20 / (1.005 1.2 (50 - 20))

= 0.55 m3/s

Labels: hvac system

Energy is the capacity or capability to do work and energy is used when work are done.

The unit for energy is joule J, where

1 J = 1 Nm

which is the same unit as for work.

Energy forms

There can be several forms of energy, including

* mechanical energy

* heat or thermal energy

* electrical energy

* chemical energy

* nuclear energy

* light energy

Energy Efficiency

Energy efficiency is the ratio between useful energy output and input energy, and can be expressed as

μ = Eo / Ei (1)

where

μ = energy efficiency

Eo = useful energy output

Ei = energy input

It is common to state efficiency as a percentage by multiplying (1) with 100.

Example - Energy Efficiency

A lift moves a mass 10 m up with a force of 100 N. The input energy to the lift is 1500 J. The energy efficiency of the lift can be calculated as

μ = 100 (N) 10 (m) / 1500 (J)

= 0.67 or

= 67 %

Labels: Thermodynamics