- The building can be considered as a defined unit and its heat exchange processes with the out- door environment can be examined:
Qi + Qs ± Qc ± Qv ± Qm – Qe = 0
- Qi = Internal heat gain
- Qs= Radiation through windows
- Qc= Heat gain or loss due to conduction
- Qv= Heat gain or loss due to covection
- Qm= Heat gain or loss due to mechanical equipment
- Qe= Heat loss due to evaporation
- Conduction of heat may occur through the walls either inwards or outwards, the rate of which will be denoted as Qc (convective and radiant components in the transfer of the same heat at the surfaces are included in the term: transmittance)
- Conduction heat flow through a wall of given area can be described by equation:
Qc = A x U x ∆T
- Qc = conduction heat flow rate, in W
- A = surface area, in m2
- U = transmittance value, in W/m2degC
- ∆T = temperature difference
- The effects of solar radiation on opaque surfaces can be included in the above by using the sol-air temperature concept, but through transparent surfaces (windows) the solar heat gain must be considered separately. It may be denoted as Qs
2) Radiation through windows
- The effects of solar radiation through transparent surfaces (windows) the solar heat gain may be denoted as Qs
- If the intensity of solar radiation (I) incident on the plane of the window is known — this itself being a value denoting a density of energy flow rate (W/m2) — it will have to be multiplied by the area of the aperture only (m2) to get the heat flow rate in watts. This would be the heat flow rate through an unglazed
- For glazed windows this value will be reduced by a solar gain factor (θ) which depends on the quality of the glass and on the angle of incidence.
- The solar heat flow equation can therefore be established as:
Qs = A x l x θ
- A area of window, in m2
- I = radiation heat flow density, in W/m2
- θ = solar gain factor of window glass
- Heat exchange in either direction may take place with the movement of air, e. ventilation, and the rate of this will be denoted as Qv
- Convection heat flow rate between the interior of a building and the open air, depends on the rate of ventilation, i.e., air exchange.
- This may be unintentional air infiltration or may be deliberate ventilation, The rate of ventilation can be given in m3/s.:
The’ rate of ventilation heat flow is described by the equation: Qv = 1300 x V x ∆T
- Qv = ventilation heat flow rate, in W
- 1300 = volumetric specific heat of air, J/m3 degC
- ∆T = temperature difference, deg C
- If the number of air changes per hour (N) is given the ventilation rate can be found as: (3600 is the number of seconds in an hour)
3) Internal heat gain
- An internal heat gain Qi may result from the heat output of human bodies, lamps, motors and
- Heat output from a body (inside the building) is a heat gain for the building. Thus, the heat output rate appropriate to the activity must be selected and multiplied by the number of occupants. The result, in watts, will be a component of Qi
- The total rate of energy emission of electric lamps can be taken as internal heat gain. The larger part of this energy is emitted as heat (95% for incandescent lamps and 79% for fluorescent lamps) and the part emitted as light when incident on surfaces, will be converted into heat. Consequently, the total wattage of all lamps in the building (if and when in use) must be added to the Qi
- If an electric motor and the machine driven by it are both located (and operating) in the same space, the total wattage of the motor must be taken as Qi .
4) Heating and cooling
- There may be a deliberate introduction or removal of heat (heating or cooling), using some form of outside energy supply. The heat flow rate of such mechanical controls may be denoted as Qm.
- Heating and cooling, i.e., mechanical controls, the heat flow rate of these systems is subject to the designer’s intentions, and it is deliberately controllable.
- It can thus be taken as a dependent variable in the equation, i.e., it can be adjusted according to the balance of the other factors.
- Finally, if evaporation takes place on the surface of the building (e.g. a roof pool) or within the building (human sweat or water in a fountain) and the vapours are removed, this will produce a cooling effect, the rate of which will be denoted as Qv
- The rate of cooling by evaporation can only be calculated if the rate of evaporation itself is known. If the evaporation rate is expressed in kg/h, the corresponding heat loss rate can be found:
Qe = 666 x kg/h
- As the latent heat of evaporation of water around 20°C is approximately 2400 kJ/kg, this gives: 2400000 J/h = 2400000/3600 J/s = 666W
- The estimation of evaporation rate is a more difficult task, and it can rarely be done with any degree of accuracy (except under mechanically controlled conditions), as it depends on many variables, such as: available moisture, humidity of the air, temperature of the moisture itself and of the air and velocity of the air movement.
- It can be measured indirectly, e.g., by measuring the reduction in the quantity of water in an open vessel, or it can be estimated from the number of people in the room, their activity and thus their likely sweat rate (a value between 20 g/h and 2 kg/h).
- Usually, evaporation heat loss is either ignored for the purposes of calculations (except in mechanical installations), or it is handled qualitatively only: evaporative cooling will be utilized to reduce air temperature ‘as far as possible’.