TOPIC 2: PRINCIPLES OF

HEAT
NATURE OF HEAT
Nature of Heat – Heat Energy
 Heat is a form of energy expressed
in unit Joule (J).
 Other units are calories (1 cal =
4.187 J), kilowatt hour
(1kWh=3.6MJ) and British Thermal
Unit (BTU)(1 BTU = 1.055kJ)
 Heat energy is an internal molecular
property of a material.
Nature of Heat – Heat Capacity
The same mass of different materials can “hold”
Material Specific heat capacity (J/kg
different quantities of heat. K)
Water has a greater heat capacity than oil. Water 4190
Hence, water must be supplied with more heat
Concrete and brickwork 3300
than the oil in order to produce the same rise in
temperature. Ice 2100

The Specific Heat Capacity (c) of a material is Paraffin oil 2100

the quantity of heat energy required to raise the Wood 1700
temperature of 1 kg of that material by 1 degree Aluminium 910
kelvin (or 1 degree Celsius). Unit: J/kg K (or
Marble 880
J/kg0C)
Glass 700
Steel 450
Copper 390
Nature of Heat – Change of State
In the normal ranges of temperature and pressure, there are three
All matters is possible states of matter with their basic characteristic.
made from Solid state: the molecules are held together in fixed position; the
small
volume and shape are fixed.
particles
called atoms. Liquid state: The molecules are held together but have freedom of
movement; the volume is fixed but the shape is not fixed.
A molecule is  Gas state: The molecules move rapidly and have complete
a group of freedom; the volume and shape are not fixed.
atom which
are combined.
Nature of Heat – Change of State
 The state of substance depends upon the conditions of temperature and pressure
which act on the substance.
 The absorption of heat by a solid or a liquid can produce the following changes of
state.
Liquefaction Vaporisation
SOLID Melting LIQUID Boiling, Evaporation GAS
 The release of heat from a gas or a liquid can produce the following change of
state.
Condensation Solidification
GAS LIQUID SOLID
(Fusion)
 Nature of Heat – Sensible & Latent Heat
Sensible Heat: the heat
energy absorbed or
released from a
substance during a
change in temperature.

Latent Heat: the heat

energy absorbed or
released from a
substance during a
change of state, with no
Latent heat Sensible Heat Latent heat
change in temperature.
335 kJ 420 kJ 2260 kJ

Change of state for water

Heat transfer
 Heat energy always tends to transfer from high
temperature to low temperature regions.
 If several bodies at different temperatures are close
together, the heat will be exchanged between them
until they are at the same temperature.
 Process of Heat Transfer
 There are three basic processes of heat transfer.
1. Conduction
2. Convection
3. Radiation
 Heat may also be transferred by the process of
evaporation when latent heat is absorbed by a vapour
in one place and released elsewhere.
 Process of Heat Transfer
 Conduction
• Is the transfer of heat energy through a material without the molecules of
the material changing their basic position.
• Can occur in solids.
• The speed at which it occurs will vary depending on the types of materials.
• Different materials conduct heat at different rates.
• Metal is the best conductor of heat.
• Good conductors have many applications for the efficient transfer of heat,
such as in boilers and heating panels.
• Poor conductors are called insulators and include most liquids and gases.
 Process of Heat Transfer
 Convection
• Is the transfer of heat energy through a material by the
bodily movement of particles.
• Can occur in fluids (liquids and gases) but never in solids.
• Natural convection occurs when a sample of fluid, such as
air is heated and then expands.
• Air is a poor conductor of heat, yet still possible to heat all
the air in a room from a single heating panel by the process
of convection.
Process of Heat Transfer
 Radiation
• Is the transfer of heat energy by electromagnetic waves.
• Occurs when the thermal energy of surface atoms in a material generates electromagnetic
waves in the infra-red range of wavelengths.
• The rate at which the body emits or absorbs radiant heat depends upon the nature and
temperature of its surface.
• Rough surfaces present a larger total area and absorb or emit more heat than polished
surfaces.
• Dark surface – absorb more heat.
• Good absorbers – good emitters
• Poor absorbers – poor emitters.
• General Rules:
 Dull black surfaces have the highest absorption and emission of radiant heat.
 Shiny silver surfaces have the lowest absorption and emission of radiant heat
 Process of Heat transfer

Source: http://www.ces.fau.edu

Source:
http://www.slideshare.net/quillshare/heat-
load-calc
Source: http://www.atticareusa.com
Simple Heat Transfers Calculation
 Simple Heat Transfers Calculation:
Principle of Thermal Quantities
 Thermal conductivity, k Different techniques of measurement are needed for
different types of material. The General formula is:
• In order to calculate heat transfer and to
compare different materials it is necessary to
quantity just how well a material conducts
heat. Q/t = kA(θ1 – θ2)
• Thermal conductivity (k) is a measure of rate d
at which heat is conducted through a Where,
particular material under specified conditions k = thermal conductivity of that material
Unit : W/m°C (W/m°C)
• This coefficient of thermal conductivity, or ‘k- Q/t = rate of heat flow between the faces
value’, is measured as the heat flow in watts (J/s = W) / Heat transfer
across a thickness of 1m for a temperature A = cross-sectional area of the sample (m²)
different of 1°C and a surface area of 1m². θ1 – θ2= temperature difference between the
• It is important to remember that the thermal faces (°C)
conductivity of many building materials d = distance between the faces (m)
varies with moisture content.
Simple Heat Transfers Calculation:
Principle of Thermal Quantities
 Thermal resistivity, r  Thermal Resistance, R
• Thermal resistivity (r) is an alternative • Thermal resistance (R) is a measure of the
index of conduction in materials and is opposition to heat flow given by a
reciprocal of thermal conductivity :   particular component in a building
r = 1/k element.
• Unit: m²C/W
 Thermal conductance, C • The idea of thermal resistance is
• Conductance (C) is sometimes used to comparable to electrical resistance and a
express the reciprocal of thermal high thermal reduces heat flow. So, for
resistance : good thermal insulation high values of
C = 1/R thermal resistance are required.
   •  
 

 
 Simple Heat Transfers Calculation:
Principle of Thermal Quantities
 Thermal transmittance , U-value  U-Value = 1
• A U-Value is a measure of the overall rate at which heat is
transmitted through a particular thickness of wall, roof or Rsi + R1 + R2 + ….+ Ra + Rso
floor.
• Unit : W/m²°C
Or
• U-Value is measured as the rate of heat flow in watts through U = 1/ΣR
1m² of a structure when there is a temperature difference
across the structure of 1°C. Where,
• The lower the U-Value then the better the insulation.
U = U-value
 
 

ΣR = sum of all component thermal resistance
 

Rsi = standard inside surface resistance



  
  

R1 , R2 = resistance of that particular material

  
  


  



 
 
Ra = standard resistance of any airspace
Rso = standard outside surface resistance
  


  
 Simple Heat Transfers Calculation:
Principle of Thermal Quantities
 Average U-value
The general formula is as follows:

U Average = A1 U1 + A2U2+……..
A1 + A2 + …..   
Where ,
A1 + A2 + ….. are the areas with the U-values U1 , U2…….. Etc.
 Simple Heat Transfers Calculation:
Principle of Thermal Quantities
 Temperature Gradients   Temperature Boundary
• A temperature difference between the inside
and outside of a wall or roof causes a  
progressive change in temperature from the
warm side to the cold side. • The boundary temperature between
• This temperature gradient changes uniformly layers in a structural element can be
through each component.  determined from the thermal
• A structure made up of different materials will resistances which make up the U-
have varying temperature gradients between
inside and outside. value of that element.
• The layers with the highest thermal resistances
will have the steepest gradients. This is because
the best insulators must have the greatest
temperature differences between their surfaces.
 
 
Simple Heat Transfers Calculation:
Principle of Thermal Quantities
Formula:
ΔΘ = R
ΘT ΣR
Where ,
ΔΘ = Temperature difference across a
particular layer
Θ T = Total temperature difference
across the structure
R = Resistance of that layer
∑R = Total resistance of the structure.
Layer/Thickness  
Temperature gradient through a wall
 Worked Example 1
Calculate the u-value of a cavity wall with a 105
mm thick brick outer leaf, a 50 mm unventilated Solution:
cavity, then a 100 mm aerated concrete block inner
leaf with a 15mm layer of lightweight plaster.

Thermal conductivity in W/moC are:

Brickwork = 0.84
Aerated Concrete block = 0.19
Lightweight plaster = 0.16
Standard thermal resistance (R) in m C /W are :
20

Rsi = 0.123
Rso = 0.055
R cavity =0.18
U-value = 1/Total R
= 1/1.103
= 0.907 W/m2oC
 Worked Example 2
An external cavity wall of a garage is constructed of two-layer brick each of 102.5 mm thick. The wall is finished
externally with 5 mm thick slate tiles on 18 mm thick plaster and 30 mm wood finished internally. The wall is 10m x
3.5m consists of a steel door 5.0 m x 3.0 m x 4 mm thick and a window 1.5m x 1.5m x 5mm thick glass.
Data:
Outside surface air resistance 0.045 m20C/ W
Inside surface 0.130 m20C/ W
Thermal resistance for cavity 0.155 m20C/ W
Outside temperature 34oC
Inside temperature 24oC
k-value of brick 0.75 W/moC
k-value of plaster 0.45 W/moC
k-value of wood 0.079 W/moC
k-value of slate tile 1.2 W/moC
k-value of steel 210 W/moC
k-value of glass 1.056 W/moC
Calculate the heat transfer through the wall , door and window.
 
Worked Example 2
Solution : Wall
Worked Example 2
  U-value wall = 1/∑R
= 1/ 1.028
= 0.973 W/m2o C

Heat Transfer = k/d x A x (θ 1 - θ 2)

Area of wall = 10m x 3.5m
= 35 m2
Actual wall area = 35 – area of door – area of window
= 35 – [ (5 x 3) + (1.5 x 1.5 ) ]
= 35 – 17.25
= 17.75 m2

Heat transfer through wall, Q wall = 0.973 x 17.75 x 10

= 172.7 watts
 Worked Example 2
Door

U-value steel door = 1/∑R

= 1/ 0.175
= 5.714 W/m2oC
 
Heat transfer through door, Q steel door = k/d x A x (θ 1 - θ 2)
= 5.714 x 15 x 10
= 857.1 watts
 
 
 Worked Example 2
Window

U-value glass = 1/∑R

= 1/ 0.180
  = 5.56 W/m2oC
 
Heat transfer through door, Q window = k/d x A x (θ 1 - θ 2)
  = 5.56 x 2.25 x 10
= 125.1 watts
 
 Worked Example 3
A brickwall has a total area of 8 m2 of which 2 m2 are windows. The u-values are 0.95
W/m2oC for the brickwork and 2.8 W/m2oC for the windows. Calculate the average u-
value for the wall.
 
Solution,
U1 = 0.95 A1 = 8m2 – 2m2 = 6m2
U2 = 2.8 A2 = 2m2
 U Average = A1 U1 + A2U2+……..
A1 + A2 + …..
= ( 6 x 0.95 ) + (2 x 2.8 )
6+2
= 1.41 W/m2oC
References
 McMullan, R., 1993, Environmental science in Building 3rd Ed., The
Macmillan Press Ltd., London.
 Koenigsberger O.H., Ingersoll T. G., Mayhew A. & Szokolay, S. V., 1980,
Manual of Tropical Housing and Building, Longman, London.
 http://www.aplusphysics.com/courses/honors/thermo/phase_changes.html
 http://www.ces.fau.edu
 http://www.atticareusa.com
 http://www.slideshare.net/quillshare/heat-load-calc
End of
Lecture