HEAT TRANSFER AND THERMAL DESIGN
Learning outcomes or objectives
By the end of this section, the students should be able to:
(i) define heat and heat transfer
(ii) explain mechanisms of heat transfer
(iii) explain the fundamental properties or concepts influencing heat transfer through the building
envelope
(iv) explain variables influencing fundamental properties or concepts.
(v) explain the effects of heat transfer
(vi) Solve problems using the formulas for thermal heat transfer, conduction, convection and
radiation
Introduction
Heat is an interesting form of energy. Not only does it sustain life, make us comfortable and help
us prepare our food, but understanding its properties is key to many fields of scientific research.
For example, knowing how heat is transfer and the degree to which different materials can
exchange thermal energy governs everything from building heaters and understanding seasonal
change to sending ships into space.
What is Heat?
Heat is a type of thermal energy that moves from a hot to a cold thing. The amount of energy
used to vibrate the molecules in a system is referred to as heat. Examples of heat are (i) When we
contact something cold (ii) When we fill a container with water and place it on the stove (iii) Our
palms feel cold when we hold an ice cube in our hands. When we contact something cold, heat
from our bodies is transferred to the cold item. The passage of heat ceases when the two objects
reach the same temperature.
The standard unit of heat is the Joule (J). 1 joule is the amount of energy supplied by 1 watt (W)
in 1 second (s). Other units of heat include British thermal unit (1Btu = 1.055 kJ), calorie (1 cal =
4.187 J), kilowatt hour (1kWh = 3.6 MJ) and therm (1 therm = 105.5 MJ).
Heat flow can be a transient or a steady process. In the transient process, temperature and/or
heat flow vary with time. In steady process heat flow assumes that temperatures on both sides of
a building envelope element (while different) are constant for a sufficient period of time that heat
flow on both sides of the assembly is steady.
Sensible Heat
The perceptible heat is referred to as "sensible heat." The energy that moves from one system to
another alters the temperature instead of changing its phase. In other words, it's the heat one
experiences when one is over a fire or outside on a hot day. The SI system uses the Joule (J) as
the measure of sensible heat, while the FPS system uses Btu.
Examples where the concept of Sensible Heat is observed, are:
(i) The most common example we have seen of sensible heat is when we get out of our house in
the summer, or, we can say, on a hot sunny day, we feel something different from the winter or a
cold day. This is because of the change in temperature in the surrounding area.
(ii) When we go camping and do bonfires, we observe the sensation of heat when we get close to
the bonfire, which is sensible heat. This is because of the change in the temperature of items.
(iii) When we boil normal water in a pan by using a gas stove, we light the fire first, and then the
process starts as the fire continues to heat the water pan and the water’s temperature starts to
increase. Sensible heat is what causes the water’s temperature to change (from room temperature
to boiling point).
(iv) For instance, if one starts heating a container of water, one can feel the variations in
temperature by dipping one’s fingers in the water. This procedure is referred to as sensible
heating because one can feel the temperature grow over time.
The formula required for calculating the Sensible Heat is
Qsensible = 1.10 × Cfm× (to -ti) (1)
where,
Qsensible is External heat gain in Btu/h, 1.10 is the product of the air’s heat capacity (0.018
Btu/oF), Cfm is airflow rate moving in from outside, to is Outside temperature and ti is the Inside
,temperature
Example 1
When the indoor air temperature (ti) is 68oF and the outside air temperature (to) is 78oF,
determine the sensible heat gain with a ventilation flow rate of 20,000 Cfm.
Solution:
Given: Cfm = 20,000, to= 78oF, ti = 68oF
Using the formula of sensible heat
Qsensible = 1.10 × Cfm × (to -ti)
Qsensible = 1.10 × 20000 × (78 – 68)
= 1.10 × 20000 × 10
Qsensible = 220,000 Btu/h
Latent Heat
The heat required to shift from one form of matter to another without changing temperature or
the heat or energy that is absorbed or released during a phase change of substance. It could be
either be from a gas to a liquid or liquid to a solid and vice vera. Latent heat is related to a heat
property called enthalpy
Latent Heat of Fusion
The latent heat of fusion is the heat consumed or discharged when matter melts, changing state
from solid to liquid structure at a constant temperature.
Latent Heat of Vaporization
Latent heat of Vaporization is the amount of heat that must be absorbed at a fixed
temperature to evaporate a specific amount of liquid or the heat consumed or discharged
when matter disintegrates, changing state from liquid to gas state at a constant
temperature.
Difference between Sensible heat and latent heat
(i) Condensation occurs in a cooling system when latent heat is released and the refrigerant
(cooling liquid) changes temperature as a result of sensible heat.
(ii) While the latent heat capacity is used to explain the ability to remove moisture from the air,
the sensible heat capacity is used to explain the ability to lower the temperature.
(iii) A change of sensible heat is characterized by a change of temperature while a change
of latent heat is associated with some mass altering its phase. Phases are gaseous, liquid, solid.
(iv) Sensible heat results only in the shift in the temperature of a gas or material, not the phase
shift causes sensible heat to be of concern. The phase transition between solid, liquid, and gas is
relevant to latent heat.
Temperature
Temperature is the measure of the hotness and coldness of an object. The SI unit of Temperature
is Kelvin. Other units of temperature are °C and °F. Temperature change intervals of 1oC on
thermodynamic temperature (K) = temperature in oC + 273.15. For example, water at 30oC =
303.15 K on
Difference between Heat and Temperature
(1) Heat is a form of energy that is transferred between two bodies because of a temperature
difference existing between them. Temperature is defined as the measure of the degree of hotness
or coolness of any object.
(2) Heat is the total kinetic energy and potential energy obtained by molecules in an
object. Temperature is the average kinetic energy of molecules in a substance.
(3) Heat has a working capacity. Temperature doesn’t have a working capacity.
(4) Heat can flow from a hot to a cold object. Temperature can rise when heated and decreases
when an object is cooled down.
(5) The SI unit of heat is Joule (J) and can also be measured in Calories (Cal). SI unit of
temperature is Kelvin (K). Other units of temperature are °C and °F.
(6) Heat can be denoted by the symbol Q. Temperature can be denoted by the symbol T.
(7) The measure of Heat can be done with Calorimeter. The measure of Temperature can be done
with Thermometer.
Heat Capacity
Heat capacity or thermal capacity is an extensive property of matter, that defines its physical
property. It is the amount of heat required to increase the temperature of a given mass of a
substance by one unit without causing a phase change. It describes a substance’s ability to absorb
thermal energy. The heat capacity of various substances will determine how much their
temperatures will rise. It is measured in Joules per Kelvin (J/K). Mathematically, Heat Capacity
, for the unit mass of the substance is defined as:
C= (2)
where: ΔQ is the amount of heat supplied to the substance
ΔT is the change in temperature
Specific Heat Capacity
Specific heat capacity is defined as the number of heat changes i.e., heat absorbed or rejected by
a substance per unit mass in order to change its temperature by one unit. Specific Heat Capacity
(SHC) is the amount of heat energy required to raise 1 kilogram (kg) of a substance by 1 K. The
values of SHC will vary slightly with temperature and pressure. The given substance is
undergoing no phase change during temperature change. Unit: Jkg–1K–1.
Water 4180J/kgK Aluminium 910J/kg K
Ice 2100 Cast iron 500
Nylon 1700 Copper/Zinc 385
Air 1010 Lead 126
If Q is the amount of heat absorbed or extracted or rejected i.e., ΔQ by a substance of mass m
when it experiences a temperature change ΔT, then the specific heat capacity, of that substance is
given by
SHC = (3)
Example 2
88.3g sample of metal at 95.24oC is added to 35.10 g of water that is initially at 17.27°C. The
final temperature of both the water and the metal is 29.20°C. The specific heat of water is 4.184
J/(g°C). Calculate the specific heat of the metal.
Solution:
Given, mass of metal is 88.3g, the initial temperature of the metal is 95.24°C, mass of water:
35.10g, the initial temperature of the water is 17.27°C, the final temperature of the water and the
metal is 29.20°C, the specific heat of water is 4.184 J/(g°C).
Therefore, the expression where the energy from the hotter metal transfers to the cooler water is
moCoΔTo = mwCwΔTw
where:
mo = mass of a metal object, ΔTo = temperature change of metal object, Co = specific heat
capacity of metal object, mw = mass of water, ΔTw = temperature change of water, Cw = specific
heat capacity of water
Rearrange the above expression,
Co = (mwCwΔTw)/(moΔTo)
Substitute the values in the above expression,
Co = [35.10 4.184(29.20−17.27)]/[88.3(29.20-95.24)]
Co = 0.301 J/g°C
What is Heat Transfer
Heat transfer is the movement of heat across the border of the system due to a difference in
temperature between the system and its surroundings. It occurs when energy from a higher-
temperature body transfers to a lower-temperature body. The amount of energy used to vibrate
the molecules in a system denoted by Q is given as
Q = c × m ×ΔT (4)
where:
Q = heat, c = specific heat capacity, m = mass of the body, ΔT = temperature difference
Example 3
Determine the heat needed to raise a 1 kg of iron from 250° C to 600° C?
Solution
Mass, m = 1 Kg, Specific heat of iron, C = 0.45 Jg−1°C. Also, temperature difference,
ΔT=700°C–250°C =450°C
Now applying the heat formula,
C =Q/m ×ΔT
Rearranging the formula
Q=mcΔT
Q=1×0.45×103×450
= 20.25 J
Example 4
Determine how much heat energy is lost if 50 Kg water is cooled from 600oC to 200oC. Specific
Learning outcomes or objectives
By the end of this section, the students should be able to:
(i) define heat and heat transfer
(ii) explain mechanisms of heat transfer
(iii) explain the fundamental properties or concepts influencing heat transfer through the building
envelope
(iv) explain variables influencing fundamental properties or concepts.
(v) explain the effects of heat transfer
(vi) Solve problems using the formulas for thermal heat transfer, conduction, convection and
radiation
Introduction
Heat is an interesting form of energy. Not only does it sustain life, make us comfortable and help
us prepare our food, but understanding its properties is key to many fields of scientific research.
For example, knowing how heat is transfer and the degree to which different materials can
exchange thermal energy governs everything from building heaters and understanding seasonal
change to sending ships into space.
What is Heat?
Heat is a type of thermal energy that moves from a hot to a cold thing. The amount of energy
used to vibrate the molecules in a system is referred to as heat. Examples of heat are (i) When we
contact something cold (ii) When we fill a container with water and place it on the stove (iii) Our
palms feel cold when we hold an ice cube in our hands. When we contact something cold, heat
from our bodies is transferred to the cold item. The passage of heat ceases when the two objects
reach the same temperature.
The standard unit of heat is the Joule (J). 1 joule is the amount of energy supplied by 1 watt (W)
in 1 second (s). Other units of heat include British thermal unit (1Btu = 1.055 kJ), calorie (1 cal =
4.187 J), kilowatt hour (1kWh = 3.6 MJ) and therm (1 therm = 105.5 MJ).
Heat flow can be a transient or a steady process. In the transient process, temperature and/or
heat flow vary with time. In steady process heat flow assumes that temperatures on both sides of
a building envelope element (while different) are constant for a sufficient period of time that heat
flow on both sides of the assembly is steady.
Sensible Heat
The perceptible heat is referred to as "sensible heat." The energy that moves from one system to
another alters the temperature instead of changing its phase. In other words, it's the heat one
experiences when one is over a fire or outside on a hot day. The SI system uses the Joule (J) as
the measure of sensible heat, while the FPS system uses Btu.
Examples where the concept of Sensible Heat is observed, are:
(i) The most common example we have seen of sensible heat is when we get out of our house in
the summer, or, we can say, on a hot sunny day, we feel something different from the winter or a
cold day. This is because of the change in temperature in the surrounding area.
(ii) When we go camping and do bonfires, we observe the sensation of heat when we get close to
the bonfire, which is sensible heat. This is because of the change in the temperature of items.
(iii) When we boil normal water in a pan by using a gas stove, we light the fire first, and then the
process starts as the fire continues to heat the water pan and the water’s temperature starts to
increase. Sensible heat is what causes the water’s temperature to change (from room temperature
to boiling point).
(iv) For instance, if one starts heating a container of water, one can feel the variations in
temperature by dipping one’s fingers in the water. This procedure is referred to as sensible
heating because one can feel the temperature grow over time.
The formula required for calculating the Sensible Heat is
Qsensible = 1.10 × Cfm× (to -ti) (1)
where,
Qsensible is External heat gain in Btu/h, 1.10 is the product of the air’s heat capacity (0.018
Btu/oF), Cfm is airflow rate moving in from outside, to is Outside temperature and ti is the Inside
,temperature
Example 1
When the indoor air temperature (ti) is 68oF and the outside air temperature (to) is 78oF,
determine the sensible heat gain with a ventilation flow rate of 20,000 Cfm.
Solution:
Given: Cfm = 20,000, to= 78oF, ti = 68oF
Using the formula of sensible heat
Qsensible = 1.10 × Cfm × (to -ti)
Qsensible = 1.10 × 20000 × (78 – 68)
= 1.10 × 20000 × 10
Qsensible = 220,000 Btu/h
Latent Heat
The heat required to shift from one form of matter to another without changing temperature or
the heat or energy that is absorbed or released during a phase change of substance. It could be
either be from a gas to a liquid or liquid to a solid and vice vera. Latent heat is related to a heat
property called enthalpy
Latent Heat of Fusion
The latent heat of fusion is the heat consumed or discharged when matter melts, changing state
from solid to liquid structure at a constant temperature.
Latent Heat of Vaporization
Latent heat of Vaporization is the amount of heat that must be absorbed at a fixed
temperature to evaporate a specific amount of liquid or the heat consumed or discharged
when matter disintegrates, changing state from liquid to gas state at a constant
temperature.
Difference between Sensible heat and latent heat
(i) Condensation occurs in a cooling system when latent heat is released and the refrigerant
(cooling liquid) changes temperature as a result of sensible heat.
(ii) While the latent heat capacity is used to explain the ability to remove moisture from the air,
the sensible heat capacity is used to explain the ability to lower the temperature.
(iii) A change of sensible heat is characterized by a change of temperature while a change
of latent heat is associated with some mass altering its phase. Phases are gaseous, liquid, solid.
(iv) Sensible heat results only in the shift in the temperature of a gas or material, not the phase
shift causes sensible heat to be of concern. The phase transition between solid, liquid, and gas is
relevant to latent heat.
Temperature
Temperature is the measure of the hotness and coldness of an object. The SI unit of Temperature
is Kelvin. Other units of temperature are °C and °F. Temperature change intervals of 1oC on
thermodynamic temperature (K) = temperature in oC + 273.15. For example, water at 30oC =
303.15 K on
Difference between Heat and Temperature
(1) Heat is a form of energy that is transferred between two bodies because of a temperature
difference existing between them. Temperature is defined as the measure of the degree of hotness
or coolness of any object.
(2) Heat is the total kinetic energy and potential energy obtained by molecules in an
object. Temperature is the average kinetic energy of molecules in a substance.
(3) Heat has a working capacity. Temperature doesn’t have a working capacity.
(4) Heat can flow from a hot to a cold object. Temperature can rise when heated and decreases
when an object is cooled down.
(5) The SI unit of heat is Joule (J) and can also be measured in Calories (Cal). SI unit of
temperature is Kelvin (K). Other units of temperature are °C and °F.
(6) Heat can be denoted by the symbol Q. Temperature can be denoted by the symbol T.
(7) The measure of Heat can be done with Calorimeter. The measure of Temperature can be done
with Thermometer.
Heat Capacity
Heat capacity or thermal capacity is an extensive property of matter, that defines its physical
property. It is the amount of heat required to increase the temperature of a given mass of a
substance by one unit without causing a phase change. It describes a substance’s ability to absorb
thermal energy. The heat capacity of various substances will determine how much their
temperatures will rise. It is measured in Joules per Kelvin (J/K). Mathematically, Heat Capacity
, for the unit mass of the substance is defined as:
C= (2)
where: ΔQ is the amount of heat supplied to the substance
ΔT is the change in temperature
Specific Heat Capacity
Specific heat capacity is defined as the number of heat changes i.e., heat absorbed or rejected by
a substance per unit mass in order to change its temperature by one unit. Specific Heat Capacity
(SHC) is the amount of heat energy required to raise 1 kilogram (kg) of a substance by 1 K. The
values of SHC will vary slightly with temperature and pressure. The given substance is
undergoing no phase change during temperature change. Unit: Jkg–1K–1.
Water 4180J/kgK Aluminium 910J/kg K
Ice 2100 Cast iron 500
Nylon 1700 Copper/Zinc 385
Air 1010 Lead 126
If Q is the amount of heat absorbed or extracted or rejected i.e., ΔQ by a substance of mass m
when it experiences a temperature change ΔT, then the specific heat capacity, of that substance is
given by
SHC = (3)
Example 2
88.3g sample of metal at 95.24oC is added to 35.10 g of water that is initially at 17.27°C. The
final temperature of both the water and the metal is 29.20°C. The specific heat of water is 4.184
J/(g°C). Calculate the specific heat of the metal.
Solution:
Given, mass of metal is 88.3g, the initial temperature of the metal is 95.24°C, mass of water:
35.10g, the initial temperature of the water is 17.27°C, the final temperature of the water and the
metal is 29.20°C, the specific heat of water is 4.184 J/(g°C).
Therefore, the expression where the energy from the hotter metal transfers to the cooler water is
moCoΔTo = mwCwΔTw
where:
mo = mass of a metal object, ΔTo = temperature change of metal object, Co = specific heat
capacity of metal object, mw = mass of water, ΔTw = temperature change of water, Cw = specific
heat capacity of water
Rearrange the above expression,
Co = (mwCwΔTw)/(moΔTo)
Substitute the values in the above expression,
Co = [35.10 4.184(29.20−17.27)]/[88.3(29.20-95.24)]
Co = 0.301 J/g°C
What is Heat Transfer
Heat transfer is the movement of heat across the border of the system due to a difference in
temperature between the system and its surroundings. It occurs when energy from a higher-
temperature body transfers to a lower-temperature body. The amount of energy used to vibrate
the molecules in a system denoted by Q is given as
Q = c × m ×ΔT (4)
where:
Q = heat, c = specific heat capacity, m = mass of the body, ΔT = temperature difference
Example 3
Determine the heat needed to raise a 1 kg of iron from 250° C to 600° C?
Solution
Mass, m = 1 Kg, Specific heat of iron, C = 0.45 Jg−1°C. Also, temperature difference,
ΔT=700°C–250°C =450°C
Now applying the heat formula,
C =Q/m ×ΔT
Rearranging the formula
Q=mcΔT
Q=1×0.45×103×450
= 20.25 J
Example 4
Determine how much heat energy is lost if 50 Kg water is cooled from 600oC to 200oC. Specific
