PRINCIPLES OF HEAT and

HEAT TRANSFER

JUNE 5TH, 2017 LECTURE 6 EPHREM M.

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

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.

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.

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.

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.

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.

• 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

Temperature Scales

Temperature Scales

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.

Latent HeatThe 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.

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.

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.

Specific HeatA 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.

MaterialSpecific Heat

(J/KgoC)

Specific Heat

(c/mloC)

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 14.184 joules = 1 calorie

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.

Specific HeatQ = 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

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.

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.

Heat transfer mechanisms:

the Bedouin at day and night

Transfer of Heat

Transfer of Heat

ConductionThe 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.

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

ConductionEx.3 If the outside temperature is T

o=10˚C and the inside is T

i= 22˚C, over a 10m

2 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 10m

2 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 6m

2with a U-value of

2.9W/m2˚C, both are exposed to T

o=20˚C and T

i=18˚C, which has higher heat flow rate?

Q a = U*A*ΔT

= 5W/m2˚C x 6m

2 x (20 - 18) ˚C

= 30W x 2

= 60W

Q b = U*A*ΔT

= 2.9W/m2˚C x 6m

2 x (20 - 18) ˚C

= 30W x 2

= 34.8W

Higher heat gain can be achieved with 6mm glazing

ConvectionIf 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.

ConvectionConvection 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.

RadiationRadiation 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.

Radiation

RadiationOpaque 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.

RadiationGlass 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.

Radiation

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+ Q

c+ Q

s+ Q

v+ Q

m+ Q

e= 0

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

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