C l im a t o l o g yR 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 RM B S S P A 2 0 1 6

HEAT EXCHANGE IN BUILDINGS

HEAT EXCHANGE IN BUILDINGS

• Just like the human body, the building can also be consideredas a defined unit.

• Its heat exchange processes with the out‐door environmentcan be examined.

• The thermal balance, i.e. the existing thermal condition ismaintained if:

Qi + Qs ± Qc ± Qv ± Qm ‐ Qe = 0

If the sum of this equation is less than zero (negative), thebuilding will be cooling and

if it is more than zero, the temperature in the building willincrease.

Conduction

Conduction heat flow rate through a wall of a given area can bedescribed 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

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)  

ConvectionConvection 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 ventilationcan 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

ConvectionIf the number of air changes per hour (N) is given the ventilation rate can be 

found as: V = (N x room volume) / 3600where 3600 is the number of seconds in an hour.

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.

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. 

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)

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. Thewall 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?

SOLUTION

PERIODIC HEAT FLOWTIME‐LAGDECREMENT FACTOR

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 abovemethods 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 ofincreasing and decreasing temperatures.

The effect of this on a building is that in the hot period heat flows from theenvironment into the building, where some of it is stored, and at nightduring the cool period, the heat flow is reversed: from the building to theenvironment.

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 duringthe night when the indoor temp. is higher.

TIME‐LAG & DECREMENT FACTOR

The two quantities characterizing this periodic change are the time‐lag (or phaseshift θ) and the decrement factor (or amplitude attenuation µ).

The decrement factor is the ratio of the maximum outer and inner surfacetemperature amplitudes taken from the daily mean.

▪ MICRO‐CLIMATE CONTROL

▪ STRUCTURAL CONTROL

▪ MECHANICAL CONTROL

ControlsThe environment immediately outside and between buildings can beinfluenced by the design of a settlement and by the grouping of buildings to aminor extent.Structural (passive) means of control can provide a further leveling out of theclimatic variations, and often even comfort conditions can be achieved by suchmeans. (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 mechanicalcontrols is radically reduced and it becomes more economical. (ACs

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

Structural Controls

Heat absorbing glassOn opaque surfaces the incident radiation is partly absorbed and partlyreflected,a + r = 1with transparent bodies, it may be absorbed, reflected or transmitted.a + r + t = 1

An ordinary window glass transmits a large proportion of all radiationbetween 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. Itstransmittance is selective.

This selective transmittance can be modified by varying the composition ofthe glass to reduce substantially the infra‐red transmission, whilst onlyslightly affecting the light transmission. Such a product is referred to asheat absorbing glass.

Whilst the heat absorbing glasses achieve a selective transmittance by selectivityin absorption, the heat reflecting glass achieves a similar selectivetransmittance by selectivity in reflection.

The glass is coated by a thin film of metal (usually nickel or gold), applied byvacuum evaporation.

Such glasses absorb very little heat, therefore the improvement in reducing thetotal solar gain is far greater, but unfortunately they are still rather expensive.

Recently, several types of photo chromatic or light‐sensitive glasses have beendeveloped, containing submicroscopic halide crystals, which turn dark whenexposed to strong light and regain their transparency when the light source isremoved.

Their transmittance may thus vary between 74% and 1%. When the technique ismore developed and more economical, these glasses may have a future in solarcontrol.

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