3.0 Introduction
The original purpose of a building is to
provide shelter and to maintain a comfortable or at least live able internal
temperature. Other purposes include security, privacy and protection from wind
and weather. To feel comfortable in a thermal sense, a human has to be able to
release a well-defined amount of heat. If this gets difficult, a person will either feel
cold or hot.
3.1
THERMAL
TRANSMISSION IN A BUILDING.
The human body operates as a chemical reactor
that converts chemical energy of food and respiratory oxygen into mechanical
work and heat. Heat output can vary from about 100 W for a sedentary person to
1000 W for an exercising person [ASHRAE Fundamentals (1993)].
To maintain body temperature
within a narrow band, the heat produced by an occupant must be released to the
indoor environment. If too much heat is lost, room temperature should be
increased or warmer clothes be worn. The heat transfer on the human skin, the
indoor temperature and the heat transfer through the building envelope are
factors that influence thermal comfort [Mayer (1991)].
Figure 3.1 shows
schematically the ranges of temperature variations of the human body, of the
room air and outdoor air. The adjustment of heat transfer around the human
alone (by variation of clothing or sweating) is not normally sufficient to
control body heat release at large outdoor temperature variations without the
thermal protection of the building envelope and heating or cooling. The dynamic
storage of heat in building components is important to control indoor
temperature variations.
Figure 3.1 : Temperature ranges in a building. Heat transfer at
the building envelope and on the human body determine thermal comfort. High
thermal resistance of insulating layers reduces temperature amplitudes felt on
the human skin.
Heat transfer is only possible in the direction from a higher
temperatures to a lower one. It becomes zero if temperatures are equal. The
heat loss through an envelope should therefore be proportional to the
difference Tinside – Toutside, or to a
positive power of it for small differences. For a simple formula, a linear
dependence on temperature difference is sufficient. Accepting further that heat
loss grows linearly with surface area A, one finds:
(1)
Q = AU (Tinside – Toutside)
Example 3.1
The constant of proportionality, U, is the Overall Heat Transfer Coefficient in W/(m2K). refer to
figure 3.2 a building is represented by a cube of 5m × 5m × 5m. If no heat is
lost into the solid and with U = 0.4 W/(m2K).
Solution 3.1
The total heat loss is:
Q = 5 x 52 x 0.4 x 20w
= 1000w
From figure 3.2 at 20 K
temperature difference, this cube loses 1000 W to the atmosphere at an assumed
overall heat transfer coefficient of 0.4 W/(m2K), and if heat loss
to the ground is neglected.
Figure 3.2 : Estimation of the heat loss of a two-storey
building.
3.1.1
Heat
exchange processes.
Heat
exchangers are devices used to transfer heat energy from one fluid to another.
Typical heat exchangers experienced by us in our daily lives include condensers
and evaporators used in air conditioning units and refrigerators.
Boilers
and condensers in thermal power plants are examples of large industrial heat
exchangers. There are heat exchangers in our automobiles in the form of
radiators and oil coolers. Heat exchangers are also abundant in chemical and
process industries.
There
is a wide variety of heat exchangers for diverse kinds of uses, hence the
construction also would differ widely. However, in spite of the variety, most
heat exchangers can be classified into some common types based on some
fundamental design concepts. We will consider only the more common types here
for discussing some analysis and design methodologies. The Kelvin is a formal SI unit of temperature. Degree Celsius use in common practice.
T
= Ѳ
+ 273
T
= Thermodynamic temperature (K)
Ѳ = Celsius temperature (°C)
For
example: 20°C + 273 = 293°K
3.1.2
Heat
Capacity
Heat
capacity ( c ) is a characteristic of a substance, the amount of heat required
to change its temperature by one degree and has units of energy per degree. The SI unit for heat capacity is J/K (joule per kelvin). In
the English system, the units are British thermal units per pound per degree
Fahrenheit (Btu/oF).
Heat capacity can be expressed like:
Q = C dt
where
Q = amount of heat supplied (J, Btu)
C = heat capacity of the system or object (J/K, Btu/lb oF)
dt = temperature rise (oK, oF)
The specific heat capacity (c) of a material is
quantity of heat energy required to rise the temperature of 1kg of that
material by 1°c. For example, water must be supply with more heat than the oil in order to
produce the same rise in temperature. The unit is j/kg °c.
The Specific Heat is the amount of heat required to change a unit
mass of a substance by one degree in temperature. The heat supplied to a unit
mass can be expressed as
dQ = m c dt
where
dQ = heat supplied (kJ, Btu)
m = unit mass (kg, lb)
c = specific heat (kJ/kg oC, kJ/kg oK, Btu/lb oF)
dt = temperature change (K, oC, oF)
Expressing Specific Heat using (1)
c = dQ / m dt
Table 3.1 : specific heat
capacity of material
Material
|
Specific heat capacity J/kg°C
|
Water
|
4190
|
Concrete and brickwork
|
3300
|
Ice
|
2100
|
Paraffin oil
|
2100
|
Wood
|
1700
|
Aluminum
|
910
|
Marble
|
880
|
Glass
|
700
|
Steel
|
450
|
Copper
|
390
|
Converting
between Common Units
1 Btu/lbmoF =
4186.8 J/kg K = 1 kcal/kgoC
Example 3.2- Heating Aluminum
2 kg of aluminum is
heated from 20 oC to 100 oC. Specific heat of aluminum is 0.91 kJ/kg0C and the heat required can be
calculated as:
dQ = (2 kg) (0.91 kJ/kg0C) ((100 oC) - (20 oC))
= 145.6 (kJ)
Example 3.3 - Heating Water
One litre of water is
heated from 0 oC to boiling 100 oC. Specific heat of water is 4.19 kJ/kg0C and the heat required can be
calculated as:
dQ = (1 litre) (1 kg/litre) (4.19 kJ/kg0C) ((100 oC) - (0 oC))
= 419 (kJ)
a) Density
The
heat capacities of different materials are compare on the basis equal masses. The
same mass of different materials may occupy different volume of space depend on
density. Its unit is kilogram per cubic meter (kg/m³).
b)
Change of State
In science, change in the
physical state (solid, liquid, or gas) of a material. For instance, melting,
boiling, and evaporation and their opposites, solidification and condensation,
are changes of state. The former set of changes are brought about by heating or
decreased pressure (except for the melting of ice, which is favoured by
pressure); the latter by cooling or increased pressure.
These changes involve the absorption or release of heat energy, called latent heat, even though the temperature of the material does not change during the transition between states. Forms (solid, liquid, or gas) in which material can exist. Whether a material is solid, liquid, or gaseous depends on its temperature and pressure. The transition between states takes place at definite temperatures, called the melting point and boiling point.
In the unusual change of
state called sublimation, a solid changes
directly to a gas without passing through the liquid state. For example, solid
carbon dioxide (dry ice) sublimes to carbon dioxide gas.
Solid
state is when the molecules are held
together in fixed positions, the volume and shape are fixed. Liquid state is
when the molecules are held together but
have freedom of movement, the volume is fix but shape is not fix. Gas state is
when the molecules move rapidly and have complete freedom, the volume and shape
are not fixed. The absorption of heat by a solid or a liquid can produce the change of state. (Refer to figure 3.3)
Figure 3.3: Type of physical
state of material
Figure 3.4 : change of
state
c) c) Sensible Heat
Sensible heat is the energy absorbed or
released from a substance during a change in temperature. Sensible heat is in a
single stage of ice, water or steam when the temperature rises uniformly as
heat is supply.
d) Latent Heat
Latent heat is heat energy absorbed or
released from a substance during a change of state, with no change in
temperature. Latent heat is given back when the steam changes to water, or the
water change to ice.
Figure 3.5 : change of
water
e) Enthalpy
Enthalpy is the total heat content of the sample.
The steam at 100°c has higher total heat content then liquid water at 100°c.
Steam at high temperature and pressure has very high enthalpy.
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