BUILDING SCIENCE & ENVIRONMENT (BGN273) - Principle of Heat

BUILDING SCIENCE & ENVIRONMENT (BGN273) - Principle of Heat

Dzuliqyan Jasni

MAIN TOPIC;

1- NATURE OF HEAT

2- HEAT TRANSFER

3- HEAT TRANSFER CALCULATION

1- NATURE OF HEAT - Heat Energy

HEAT (Q) is form of ENERGY.
 Measured in Joule (J); the same unit as for measuring any form of energy.
Human + Buildings --> respond to the heat that is around them, and also contribute to this heat.
HEAT ENERGY is an internal molecular property of a material. 

NATURE OF HEAT – Change of State

In the normal ranges of temperature and pressure, there are three possible states of matter with their basic characteristic.

SOLID STATE : the molecules are held together in fixed position; the volume and shape are fixed.
LIQUID STATE : The molecules are held together but have freedom of movement; the volume is fixed but the shape is not fixed.
GAS STATE : The molecules move rapidly and have complete freedom; the volume and shape are not fixed. 

***All matter is made form small particles called atoms. A molecule is a group of atom which are combined.

 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. 

 The release of heat from a gas or a liquid can produce the following change of state. 

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 change in temperature.

2- 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 (3) basic processes of heat transfer.

a. Conduction
b. Convection
c. 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. 

(a) 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, LIQUIDS and GASES.
• 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 - 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.

(b) 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.

(c) 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.
• Rough surfaces present a larger total area and absorb or emit more heat than polished surfaces.
DARK SURFACEabsorb more heat.
GOOD ABSORBES = Good Emitters.
POOR ABSORBES = 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

3- HEAT TRANSFER CALCULATION

SIMPLE HEAT TRANSFERS CALCULATION - Principle of Thermal Quantities

(i) Thermal Conductivity (k)

Thermal conductivity (k) is a measure of rate at which heat is conducted through a particular material under specified conditions
• UNIT : W/mºC
• This coefficient of thermal conductivity, or ‘k-value’, is measured as the heat flow in watts across a thickness of 1m for a temperature different of 1°C and a surface area of 1m².
• It is important to remember that the thermal conductivity of many building materials varies with moisture content.

Different techniques of measurement are needed for different types of material. The GENERAL FORMULA is: 

(ii) Thermal Resistivity (r)

Thermal resistivity (r) is an alternative index of conduction in materials and is reciprocal of thermal conductivity : 

(iii) Thermal Conductance (C)

• Conductance (C) is sometimes used to express the reciprocal of thermal resistance :

(iv) Thermal Resistance (R)

Thermal resistance (R) is a measure of the opposition to heat flow given by a particular component in a building element.
Unit: m²C/W.
• The idea of thermal resistance is comparable to electrical resistance and a high thermal reduces heat flow. So, for good thermal insulation high values of thermal resistance are required.

(v) Thermal Transmittance (U-value)

• A U-Value is a measure of the overall rate at which heat is transmitted through a particular thickness of wall, roof or floor.
• Unit : W/m²ºC.
• U-Value is measured as the rate of heat flow in watts through 1m² of a structure when there is a temperature difference across the structure of 1°C.
• The lower the U-Value then the better the insulation. 

(vi) Average U-Value

The general formula is as follows: 

(vii) Temperature Gradients

• 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.
• This temperature gradient changes uniformly through each component.
• A structure made up of different materials will have varying temperature gradients between inside and outside.
• 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.

(viii) Temperature Boundary

• The boundary temperature between layers in a structural element can be determined from the thermal resistances which make up the U-value of that element.

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