Principles of

Thermal

Design

CHAPTER 3ANJANAA DEVI , LAKSHYA WADHWANI , HEENA MEVADA , KASHYAP MEWADA , SHIVANI BHANDHARI ,

DEVANSHI SARAYIA

Heat Flow

Heat energy tends to flow from high temperature

to lower temperature zones by the following

ways:-

Conduction

Convection

Radiation

Conduction

When two bodies are in direct contact,heat flows

through molecular movement.

The rate of the molecular movement in a body is

the thermal conductivity of the material.

Factors Of Resistance

Density of the material.

Surface conductance – In addition to the resistance of a body to the flow of heat , a resistance will be offered by its surface.

Air – to – Air resistance is the sum of the body’s resistance and the surface resistance.

Transmittance is the reciprocal of air – to – air resistance.

Cavity Resistance.

Convection

The heat is transferred by a carrying medium

between the bodies.

Radiation

Energy waves travel through vacuum or media.

Sol-air Temperature

An imaginary temperature of a layer of air

adjacent to the surface of the material.

It is used to calculate cooling load of the building and determine the total heat gain through the

exterior surfaces.

Solar Gain Factor

It is the increase in temperature in a structure due

to solar radiation.

It is defined as the heat flow rate through the construction due to solar radiation expressed as a

fraction of the incident solar radiation.

Internal Heat Gain

Heat output of human bodies , lamps , motors and appliances.

Mechanical Controls

Deliberate introduction or removal of heat using

external heat supply.

Ventilation

Heat exchange in either direction may take place

with the movement of air.

Evaporation

It produces a cooling effect when the vapours

are removed.

Thermal Balance Equation

𝑄𝑖 + 𝑄𝑠 ± 𝑄𝑐 ± 𝑄𝑣 ± 𝑄𝑚 − 𝑄𝑒 = 0

If the sum of this equation is less than zero,the

building will be COOLING.

If it is more than zero,the TEMPERATURE IN THE

BUILDING WILL INCREASE.

Applications

Heat loss calculation

Heat gain calculation

Cooling by air

Heating by air

Transmittance

Thermal Gradient – Calculating the temperature

at any point within the wall.

Condensation

Thermal design

Assumption of steady

state

The equation and calculation methods are valid

only if both outdoor and indoor temperatures are

constant.

This does not occur in nature.

Hence periodic calculation is graphically

represented – Timelag and Decrement factor.

Applications

Insulation

Unventilated cavity

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