Lecture presentation of Climatology subject presented at the MBS School of Planning and Architecture.

1. Climatology
R O H I T K U M A R
A S S I S T A N T P R O F E S S O R
M B S S P A 2 0 1 6
HEAT EXCHANGE IN
BUILDINGS

3. HEAT EXCHANGE IN BUILDINGS
• Just like the human body, the building can also be considered
as a defined unit.
• Its heat exchange processes with the out‐door environment
can be examined.
• The thermal balance, i.e. the existing thermal condition is
maintained if:
Qi + Qs ± Qc ± Qv ± Qm ‐ Qe = 0
If the sum of this equation is less than zero (negative), the
building will be cooling and
if it is more than zero, the temperature in the building will
increase.

5. Conduction
Conduction heat flow rate through a wall of a given area can be
described by the equation:
Qc = A x U x ∆T
Where, Qc = conduction heat flow rate, in W,
A = surface area, in m²,
U = transmittance value in W/m² degC,
∆T = temperature difference in degC

6. Conduction
For a whole building enclosed by various elements with
temperature difference varying from side to side, the above
equation is sollved for each element and added.
If heat loss is being considered,
∆T= Ti‐ To
If heat gain is being considered,
∆T= To‐ Ti
If a surface is exposed to solar radiation,
∆T= Ts‐ Ti
(Ti is inside air temperature)

7. Convection
Convection heat flow rate between the interior of a building and the open air
depends on the rate of ventilation, i.e. air exchange. 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
Where, Qv = ventilation heat flow rate, in W,
1300 = volumetric specific heat of air, in J/m³ degC,
V = ventilation rate in m³/s,
∆T = temperature difference in degC

8. Convection
If the number of air changes per hour (N) is given the ventilation rate can be
found as:
V = (N x room volume) / 3600
where 3600 is the number of seconds in an hour.

9. Radiation through windows
The solar heat flow through windows is given by the equation:
Qs = A x l x θ,
Where, A = area of the window in m²,
l = radiation heat flow density in W/m²,
θ = solar gain factor of window glass.

10. Internal heat gain
Heat output rate of human bodies has been given in 2.1.2
(Koenigsberger)
95% of energy of incandescent lamps and 79% of energy of fluoroscent
lamps is emitted as heat.

11. Evaporation
Rate of cooling by evaporation can be calculated if the evaporation rate
is known. If evaporation rate is expressed in kg/h, the corresponding
heat loss rate can be found:
Qe = 666 x kg/h
Latent heat of evaporation of water at 20 deg Celsius is approx. 2400
KJ/kg.
2400000 J/h = 2400000/3600 J/s = 666W
(This is a general idea. Precise calculations are much more difficult to
obtain due to varying conditions)

12. PROBLEM
(Attempt the problems worked out in pages 78‐80 of Koenigsberger. The
following is an example)
A 4m x 5m wall consists of 3 glass windows of 1.5m x 1m dimensions. The
wall has thickness of 0.125 m and a thermal conductivity of 0.5 W/m.K, while the
glass windows are 6 mm thick with a thermal conductivity of 1.24 W/m.K. The
values of internal and external surface conductance for the wall (including glass)
are 8.3 W/m2.K and 34.4 W/m2.K, respectively. The internal and external
temperatures are 21oC and –30oC, respectively. Calculate the total heat transfer
rate through the wall. What percentage of heat transfer is through windows?

13. SOLUTION

14. PERIODIC HEAT FLOW
TIME‐LAG
DECREMENT FACTOR

15. PERIODIC HEAT FLOW
All the equations and calculation methods seen so far are valid if and only if,
both out‐door and indoor temperatures are constant.
As perfectly static conditions do not occur in nature, the basis of the above
methods is the assumption of steady state conditions.
In nature the variation of climatic conditions produces a non‐steady state.
Diurnal variations produce an approximately repetitive 24‐hour cycle of
increasing and decreasing temperatures.
The effect of this on a building is that in the hot period heat flows from the
environment into the building, where some of it is stored, and at night
during the cool period, the heat flow is reversed: from the building to the
environment.
As the cycle is repetitive, it can be described as periodic heat flow.
SIMPLE BUT NOT TECHNICALLY CORRECT:
The cycle in which heat flows from the environment to the building during the
day, when outdoor temp. is higher and from building to environment during
the night when the indoor temp. is higher.

16. TIME‐LAG & DECREMENT FACTOR
The two quantities characterizing this periodic change are the time‐lag (or phase
shift θ) and the decrement factor (or amplitude attenuation µ).
The decrement factor is the ratio of the maximum outer and inner surface
temperature amplitudes taken from the daily mean.

19. ▪ MICRO‐CLIMATE CONTROL
▪ STRUCTURAL CONTROL
▪ MECHANICAL CONTROL

20. Controls
The environment immediately outside and between buildings can be
influenced by the design of a settlement and by the grouping of buildings to a
minor extent.
Structural (passive) means of control can provide a further leveling out of the
climatic variations, and often even comfort conditions can be achieved by such
means. (passive solar heating/cooling techniques, etc.)
Precisely controlled indoor climate can only be achieved by mechanical
(active) controls (the straight line in the figure), but this may not be our aim,
and even if it is, with adequate structural controls, the task of mechanical
controls is radically reduced and it becomes more economical. (ACs

21. ▪ Reflected
▪ Absorbed then emitted
▪ Transmitted (transparent materials)

22. Structural Controls
Heat absorbing glass
On opaque surfaces the incident radiation is partly absorbed and partly
reflected,
a + r = 1
with transparent bodies, it may be absorbed, reflected or transmitted.
a + r + t = 1
An ordinary window glass transmits a large proportion of all radiation
between 300 and 3000 nm, i.e. both visible light and short‐wave infra‐
red, but very little around and outside the 300 to 3000 nm range. Its
transmittance is selective.
This selective transmittance can be modified by varying the composition of
the glass to reduce substantially the infra‐red transmission, whilst only
slightly affecting the light transmission. Such a product is referred to as
heat absorbing glass.

24. Whilst the heat absorbing glasses achieve a selective transmittance by selectivity
in absorption, the heat reflecting glass achieves a similar selective
transmittance by selectivity in reflection.
The glass is coated by a thin film of metal (usually nickel or gold), applied by
vacuum evaporation.
Such glasses absorb very little heat, therefore the improvement in reducing the
total solar gain is far greater, but unfortunately they are still rather expensive.
Recently, several types of photo chromatic or light‐sensitive glasses have been
developed, containing submicroscopic halide crystals, which turn dark when
exposed to strong light and regain their transparency when the light source is
removed.
Their transmittance may thus vary between 74% and 1%. When the technique is
more developed and more economical, these glasses may have a future in solar
control.

25. Koenigsberger, O. H., Manual of Tropical Housing and building, Orient Longman
private limited, 1973.