Heat is a form of energy that is transferred between objects in contact with each other or at different temperatures. There are three main mechanisms of heat transfer: conduction, convection, and radiation. Conduction requires physical contact, convection occurs through the motion of fluids, and radiation can occur through empty space. Temperature is a measure of the average kinetic energy of molecular motion and is measured using thermometers on standardized scales like Celsius and Kelvin. The amount of heat required to change the temperature of a substance depends on its specific heat. Architectural design can influence heat transfer through a building's envelope and systems.

1. PRINCIPLES OF HEAT and
HEAT TRANSFER
JUNE 5TH, 2017 LECTURE 6 EPHREM M.

2. Temperature is an objective comparative measurement of hot or cold. It is
also can be defined as a measure of the average energy of molecular
motion in a substance. It is measured by a thermometer. It is a property
of a body.
Heat is a form of energy that can be transferred from one object to
another or even created at the expense of the loss of other forms of
energy. It is also the total energy of molecular motion in a substance.
Generally speaking, it is an energy flow to or from a body by virtue of
temperature difference

3. Science claims that the overall entropy of the universe is decreasing and eventually
ends up in what they call “thermal death” of the cosmos.
The heart warming truth is, however, for the coming few
billions years, the universe will be ”alive” with the
wondering heat energy from space to space.
During that state, every point will attain identical
temperature.

4. Heat energy, in physics, is transit; it flows from a substance at a higher
temperature that is placed in contact with a substance at a lower temperature,
raising the temperature of the latter and lowering that of the former provided
the volume of the bodies remains constant.
This reality accounts for the reason why sea breeze happens at night and how we
employ the temperature difference of air masses to scavenge our interior living
areas.
Architecture grows into a rational human servant as more and more raw facts of
science add up to the understanding of the current body of knowledge.

5. Temperature
The sensation of warmth or coldness on contact with a substance is
determined by temperature, by the ability of the substance to conduct
heat, and by other factors.
Although it is possible, with care, to compare the relative temperatures
of two substances by the sense of touch, it is impossible to evaluate
the absolute magnitude of the temperatures by subjective reactions.
Adding heat to a substance, however, not only raises its temperature,
causing it to impart a more acute sensation of warmth, but also
produces alterations in several physical properties, which may be
measured with precision.

6. Temperature
As the temperature varies,
• a substance expands or contracts,
• its electrical resistance changes, and,
• in the gaseous form, it exerts varying pressure.
The variation in a standard property usually serves as a basis for an
accurate numerical temperature scale.

7. Temperature
Temperature depends on the average kinetic energy of the molecules of a
substance, and according to kinetic theory.
This energy may exist in the form of rotational, vibrational, and translational
motions of the particles of a substance.
In thermal equilibrium, the average energy of each of these kinds of motion is
the same.
The temperature of the substance is proportional to this average energy.
Theoretically, the molecules of a substance would exhibit no activity at the
temperature termed absolute zero.

8. • Celsius scale - with a freezing point of 0° C and a boiling point of 100° C, is widely used throughout
the world, particularly for scientific work.
• Fahrenheit scale - used in English-speaking countries for purposes other than scientific work and based
on the mercury thermometer, the freezing point of water is defined as 32°F and the boiling point as
212° F.
• Kelvin scale - the most commonly used thermodynamic temperature scale; zero is defined as the
absolute zero of temperature, that is, -273.15° C, or -459.67° F. The size of the unit, called the kelvin
and symbolized K, is defined as equal to one Celsius degree.
• Rankine scale - another scale employing absolute zero as its lowest point is, in which each degree of
temperature is equal to one degree on the Fahrenheit scale. The freezing point of water on the Rankine
scale is 492° R, and the boiling point is 672° R
• International temperature scale - additional fixed temperature point is based on the Kelvin scale and
thermodynamic principles. The international scale is based on the property of electrical resistivity, with
platinum wire as the standard for temperature between -190° C and 660° C. From 660° C up to the
melting point of gold, 1063° C
Temperature Scales

9. Temperature Scales

10. Temperature Scales

11. Heat Units
In physical science, quantity of heat is expressed in the same
units as energy and work, namely joules. Another unit is the
calorie, defined as the amount of heat necessary to raise the
temperature of 1 gram of water at a pressure of 1 atmosphere
from 15° to 16° C.
heat is also measured in British thermal units, or Btu. One British
thermal unit is the quantity of heat required to raise the
temperature of 1 lb of water by 1° F and is equal to 252
calories.

12. Latent Heat
The amount of heat required to produce a change of phase is called latent heat.
Hence latent heats of sublimation, melting, and vaporization exist.
If water is boiled in an open vessel at a pressure of 1 atmosphere, the temperature
does not rise above 100° C (212°F), no matter how much heat is added.
The heat that is absorbed without changing the temperature of the water is the
latent heat; it is not lost but is expended in changing the water to steam and is
then stored as energy in the steam; it is again released when the steam is
condensed to form water.
Similarly, if a mixture of water and ice in a glass is heated, its temperature will not
change until all the ice is melted.
The latent heat absorbed is used up in overcoming the forces holding the particles
of ice together and is stored as energy in the water.

13. Specific Heat
The amount of heat required to raise the temperature of a unit mass of a substance
by one degree is known as specific heat.
If the heating occurs while the substance is maintained at a constant volume or is
subjected to a constant pressure it is referred to as a specific heat at constant
volume or at constant pressure.
The latter is always larger than, or at least equal to, the former for each substance.
The specific heat of water at 15° C is 4,184 joules per kilogram per degree
Celsius.

14. Specific Heat
The amount of heat per unit mass required to raise the temperature by one degree
Celsius. The relationship between heat and temperature change is usually expressed
in the form shown below where “c” is the specific heat. The relationship does not
apply if a phase change is encountered, because the heat added or removed during
a phase change does not change the temperature.

15. Specific Heat
A large specific heat means you have
to put in a lot of energy for each
degree increase in temperature.
Specific heat is measured in joules
per kilogram per degree Celsius
(joule/Kg°C) which means how many
joules it takes to raise 1kg of the
substance by 1°C. It can also be
measured in Calories (calorie/ml °C)
which means how many calories of
energy it takes to raise 1ml of a
substance by 1°C.
Material
Specific Heat
(J/Kgo
C)
Specific Heat
(c/mlo
C)
Air 21 0.005
Aluminium 897 0.21
Steel 490 0.12
Concrete 880 0.21
Copper 385 0.36
Gold 129 0.03
Iron 448 0.11
Lead 134 0.03
Oil 1900 0.45
Sea Water 3900 0.93
Silver 235 0.06
Water (Pure) 4184 1
4.184 joules = 1 calorie

16. Specific Heat
1. How much heat in joules would you need to raise the temperature of 1 kg of water by
a) 5°C?
b) 50°C?
2. How many calories would it take to raise the temperature of 1 ml of oil by
a) 1°C?
b) 5°C?
c) 100°C?
3. If 100 joules of energy were applied to all of the substances listed in the
table at the same time, which would heat up fastest? Explain your answer.
4. Is there more thermal energy in 1 kg of steel that is heated to melting,
or in all of the water in the ocean? Explain why you think so.

17. Specific Heat
Q = CMΔT ------ used to calculate the amount of heat energy added (heat input)
Qt = CMΔT ------------------------- Heat flow rate
t = CMΔT ÷ Q (heat input) ------- time taken to let the Heat flow
Ex.1 Given 0.5kg of water at 20˚C in an electric jug with an 800W immersion heater element,
how long will it take to bring it to boil?
t = CMΔT ÷ Q
= 4184 J/Kg˚C x 0.5Kg x (100-20)˚C ÷ 800J/sec
= 167,360J ÷ 800J/sec
= 167,360J x 1sec/800J
= 209.2sec  3.5min.
Thus it will take 3.5 minutes to bring a 0.5kg, 20˚C water to boil

18. Specific Heat
Ex.2 Given 0.5kg of Steel at 20˚C in an electric jug with an 800W immersion heater element,
how long will it take to bring it to 100 ˚C?
t = CMΔT ÷ Q
= 490 J/Kg˚C x 0.5Kg x (100-20)˚C ÷ 800J/sec
= 19,600J ÷ 800J/sec
= 19,600J x 1sec/800J
= 24.5sec
Thus it will take 24.5 sec to bring a 0.5kg, 20˚C Steel to reach 100 ˚C.

20. Transfer of Heat
The physical processes by which heat transfer occurs are conduction and radiation.
A third process, which also involves the motion of matter, is called convection.
Conduction requires physical contact between the bodies or portions of bodies
exchanging heat, but radiation does not require contact or the presence of any
matter between the bodies.
Convection occurs through the motion of a liquid or gas in contact with matter at a
different temperature.

21. Heat transfer mechanisms:
the Bedouin at day and night

22. Transfer of Heat

23. Transfer of Heat

24. Conduction
The only method of heat transfer in opaque solids is conduction. If the temperature
at one end of a metal rod is raised by heating, heat is conducted to the colder end.
The proportionality factor is called the thermal conductivity (U) of the material.
Materials such as gold, silver, and copper have high thermal conductivities and
conduct heat readily, but materials such as glass and asbestos have values of
thermal conductivity hundreds and thousands of times smaller, conduct heat poorly,
and are referred to as insulators.

25. Conduction
Q = U*A*ΔT
Q – Heat Flow Rate in Joules / Sec (W)
U – Thermal Conductivity in W/m2˚C
A – area in m2
ΔT – change in temperature

26. Conduction
Ex.3 If the outside temperature is To=10˚C and the inside is Ti= 22˚C, over a 10m2 brick wall with
a U value of 1.5W/m2˚C, what will be the heat flow rate?
Q = U*A*ΔT
= 1.5W/m2˚C x 10m2 x (22 - 10) ˚C
= 15W x 12
= 180W
Ex.4 Given a 6mm glass of 6m2 and U-value of 5W/m2˚C; and a double glazing 6m2 with a U-value of
2.9W/m2˚C, both are exposed to To =20˚C and Ti =18˚C, which has higher heat flow rate?
Q a = U*A*ΔT
= 5W/m2˚C x 6m2 x (20 - 18) ˚C
= 30W x 2
= 60W
Q b = U*A*ΔT
= 2.9W/m2˚C x 6m2 x (20 - 18) ˚C
= 30W x 2
= 34.8W
Higher heat gain can be achieved with 6mm glazing

27. Convection
If a temperature difference arises within a liquid or a gas, then fluid motion will
almost certainly occur, a process called convection. The motion of the fluid may be
natural or forced.
Natural convection is also responsible for the rising of the hot water and steam in
natural convection boilers and for the draught in a chimney.
Because of the tendency of hot air to
rise and of cool air to sink, radiators
should be placed near the floor and
air-conditioning outlets near the ceiling
for maximum efficiency.

28. Convection
Convection also determines the movement of large air masses above the Earth:
 the action of the winds,
 the formation of clouds,
 ocean currents, and
 the transfer of heat from the interior of the Sun to its surface.

29. Radiation
Radiation is fundamentally different from both conduction and convection in that the
substances exchanging heat need not be in contact with each other.
They can, in fact be separated by a vacuum.
Radiation is a term generally applied to all kinds of
electromagnetic-wave phenomena. The higher the
temperature, the greater the amount of energy emitted.
In addition to emitting, all substances are capable of
absorbing radiation. Thus, although an ice cube is
continuously emitting radiant energy, it will melt if an
incandescent lamp is focused on it because it will be
absorbing a greater amount of heat than it is emitting.

30. Radiation

31. Radiation
Opaque surfaces can absorb or reflect incident radiation.
Generally, dull, rough surfaces absorb more heat than bright, polished surfaces,
and bright surfaces reflect more radiant energy than dull surfaces.
In addition, good absorbers are also good emitters; &
good reflectors, or poor absorbers, are poor emitters.
Thus, cooking utensils generally have dull bottoms for good absorption and
polished sides for minimum emission thus maximizing the net heat transfer into the
contents of the pot.

32. Radiation
Glass transmits large amounts of short-wavelength
ultraviolet radiation, but is a poor transmitter of
long-wavelength infrared radiation.
Radiant energy from the Sun, predominantly of
visible wavelengths, is transmitted through the
glass and enters the greenhouse. The energy
emitted by the contents of the greenhouse,
however, which emit primarily at longer, infrared,
wavelengths, is not transmitted out through the
glass. Thus, although the air temperature outside
the greenhouse may be low, the temperature
inside the greenhouse will be much higher because
there is a sizeable net heat transfer into it.

33. Radiation

34. Heat flow is quantified by:
Qi = Internal heat gain
Qc = Conduction heat gain or heat loss
Qs = Solar heat gain
Qv = Ventilation heat gain or heat loss
Qm = Mechanical heating or cooling
Qe = Latent heat gain or loss
Calculating the Thermal System of a Building
Qi + Qc + Qs + Qv + Qm + Qe = 0

35. Qi = Internal heat gain
 Controlled in minor way by planning:
separate heat emitting functions from
occupied spaces or dissipate the
generated heat at the source
Qc = Conduction heat gain or heat loss
▪ shape of building
▪ surface-to-volume ratio
▪ thermal insulating qualities of the envelope
Controlling heat flow through architectural application
Qe = Latent heat gain or loss
▪ useful in hot-dry climates
▪ passive design includes pond or spray
▪ mechanical equipment

36. Qs = Solar heat gain
 Affected by shape and orientation of building
 Window size, orientation, glazing material and shading devices
 Vegetation and surrounding objects
 Thermal mass affects the retention and release
Controlling heat flow through architectural application
Qv = Ventilation heat gain or heat loss
▪ fenestration and their orientation to the wind direction, closing mechanism
▪ Air tightness of the envelope
▪ Building shape
▪ Fences, wing walls and vegetation