THERMAL AND HEAT CONCEPTS: AN INTRODUCTION

CVNG 1008

Lecture objectives PART 1 ·     To define heat and temperature. ·     To introduce principal mechanisms of heat

flow/transfer. ·  To underline the importance of heat and

temperature measurement in the built environment.

PART 2 ·     To provide practical examples of heat transfer in

a construction context. Examples will be reinforced with relevant calculations.

·     To examine the process of structural and ventilation heat loss rates and their prediction.

Nature of Heat Heat is the form of energy called

thermal energy

. Thermal energy is the energy portion of a system that increases or decreases with its temperature.

What is Thermal energy??

Heat Transfer is the transfer of thermal energy from one body to another due to a temperature difference between the bodies

What is temperature??? On the microscopic scale, temperature

is defined as the average energy of microscopic motions of a single particle in the system per degree of freedom.

On the macroscopic scale, temperature is the unique physical property that determines the direction of heat flow between two objects placed in thermal contact.

Source of thermal energy in buildings

Source of energy: Coal Crude oil natural gas Petrol Wood chips and pellets sun

Three mechanisms by which thermal energy is transported

Conduction

Convection and

Radiation

Three mechanisms by which thermal energy is transported

 1.  Convection Some examples of natural convection are;

1.     The domestic hot water system inside the hot water cylinder.

2.     The draught up a chimney. 3.     The heating of rooms by convector heaters and

radiators. 4.     The draught under a window.

       

What is heat conduction

 Conduction   When heat is transferred through a solid

substance, the molecules are unable to move and start to vibrate. This vibration is passed to the next molecule by a chain reaction. In this way heat is transferred through a material and the process of CONDUCTION takes place  

 

Convection                                              

     

Latin:  com (together) + vehere (to carry); the bulk movement of thermal energy in fluids

Heat Conduction          

          H = kA (T2 - T1)/L                   (joules/second)

Example

Answer

Steel:  k = 14  J/s-m-C

How much energy is conducted in 40 seconds?--------------------------H= kA (T2 - T1)/L H= 14 (2)(475)/10    = 1330 J/s

Q= Ht = 1330 (40)   = 5.32 x 104 J

How many joules of thermal energy flow through the wall per second?

----------------------------------------------- k (insulation) = 0.20 J/(s-m-C)k (wood)      = 0.80 J/(s-m-C)

Solution Across insulation:

Hins = (0.20)(40)(25 - T)/0.076                 (1)     = 2631.6 -105.3 T                                 (2)Across wood:Hwood = (0.80)(40)(T - 4)/0.019      = 1684.2 T - 6736.8Heat is like a fluid:  whatever flows through the insulation must also flow through the wood:

Hwood  =  Hins   1684.2 T - 6736.8 = 2631.6 -105.3 T        (3)1789.5 T = 9368.4                                       (4)

            T = 5.235 C                                      (5)H= Hwood = Hins                                            (6)H= 1684.2 (5.235) - 6736.8 = 2080 J/s     (7)H= 2631.6 - 105.3 (5.235)   = 2080 J/s     (8)  

Radiation Heat   Radiation Heat can be transferred

through a vacuum from one body to another.

A good example of this is the heat from the sun, which passes through space to reach the earth.

This process is neither convection nor conduction.

 

Radiation Heat

The heat is carried in waveform in the same way as light is carried.

This is called electromagnetic radiation.

Other forms of electromagnetic radiation are infrared, ultra-violet, X-rays and gamma rays.

Important to know on Radiation Heat

Heat radiation is part of the same spectrum as light radiation, it obeys approximately the same rules, the most important ones being:

1.    It does not require a medium for transmission

2.    It travels in straight lines. 3.    It can be reflected and refracted in

the same way as light.

Radiation

Energy radiated per second:         H = esAT4

e = emissivity (0-1)s = Stefan-Boltzmann constant    = 5.67 x 10-8 J/(s-m2-K4)A = surface area of objectT = Kelvin temperature

Emissivity

Energy out = Energy in  Emitted energy/Incident energy = Emissivity = e.  

Radiation (Exercise)

How much energy is radiated by this  object in ten minutes?

---------------------------------------------------

      H = esAT4 

answer

 t = 10 x 60 seconds = 600 s  Q = radiant energy = H t  H =  esA T4  Q = (0.8)(5.67x10-8)(5)(500)4 x

(600)     = 8.5 x 106 J

THERMAL COMFORT

According to ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers)

Human thermal comfort is defined as the state of mind that expresses satisfaction with the surrounding environment

Human thermal comfort is governed by many physiological mechanisms variable for each individual

Based on Basal metabolic rate (BMR) (energy measure when completely at rest) Muscular metabolic Rate (MMR):

See you notes and available literature

LOSS of EXCESSIVE HEAT

Radiation LossesAbout 45% of

our Energy

Skin Surface Temperature

32C

Evaporation25%

Convection30% Conduction

Excessive body heat i) Conduction: A temperature

gradient thus occurs between the body core and the body surface.

ii) Radiation: About 45% of body heat is lost in this way.

iii) Convection: Some 30% of body heat is dissipated by convection.

iv)Evaporation: About 25% perspiration(breathing sweating etc)

Factors affecting thermal comfort

Personal variables (activity, clothing, age, gender)

and

Physical variables (air temperature, surface temperature, air movement and Humidity)

Factors influencing thermal comfort (personal variables)

Activity

Clothing

Age

sex

Factors influencing thermal comfort (physical variables)

The parameters that must be measured are those which affect energy loss such as:

Mean Radiant Temperature(MRT)

MRT = the average temperature affected by the radiation from surrounding surfaces.

MRT =

Sources of Humidity in the air The amount of water produced from normal household

activities can be quite considerable. using bottled gas and paraffin heaters add significant

amounts of water to the air Drying clothes over radiators will also significantly add

water vapour. The surface area of your lungs is in excess of 75 square

metres and warm air is passing over this wet surface as we breathe 15-20 times per minute; this is being breathed back into the environment!

Animals such as a large dog can give off even more water vapour than the average adult!

Source of heat into the building

Solar radiation is transmitted into buildings through;

windows, walls, roof, floor and by admitting external air into the

building.

Heat transfer on roofs

THERMAL: HEAT BALANCE

Thermal balance occurs when the sum of all the different types of heat flow into and out of a building is zero. That is, the building is losing as much heat as it gains so it can be said to be in equilibrium.

THERMAL: HEAT BALANCE

Effects of excessive temperature

If the core temperature varies from 37C;

·      Productivity and efficiency falls

·      Errors occur much more frequently

·      The ability to do work generally decreases.

Insulating materials

A thermal insulator is a material which opposes the transfer of heat between areas of different temperatures

Thermal insulator should have low density (why?)

Importance of Thermal Insulation within a Building

To maintain constant temperature by restriction on the exchange of heat energy

Conservation of energy and natural environment (Sustainability)

Reduce cooling/heating cost Unlike heating and cooling equipment,

insulation is permanent and does not require maintenance, upkeep, or adjustment.

Absorb noise and vibration, both coming from the outside and from other rooms inside the building, thus producing a more comfortable occupant environment.

insulators for Construction purposes

 Rigid preformed materials - aerated concrete blocks

Flexible materials - fibreglass quilts

Loose fill materials - expanded polystyrene granules

Site formed materials - foamed polyurethane

Reflective materials - aluminium foil

Ceramics and Insulative paints - which contain ceramic micro-spheres.

More information on micro spheres read: http://www.compositesworld.com/articles/microspheres-fillers-filled-with-possibilities

Thermal resistant must have

Strength and rigidity for the purposes Moisture resistant Fire resistant Resistance to pests and fungi Compatibility with adjacent materials Harmless to human and environment Must be sustainable

Solar Gain Through Fenestration

When solar radiation strikes on an unshaded window,

(a) part of the radiant energy is reflected back outdoors,

(b) part of the radiant energy is absorbed within the glass,

(c) the remainder is transmitted directly indoor, and

(d) the absorbed portion comes out again and flows either outward and inward.

Distribution of Solar Radiation Falling on 3mm Clear Plate

Type of Window Glasses

Clear Plate or Sheet Glasses

These are the types of glass which provide fine visual qualities and also a greater transmittance of solar radiation.

 Tinted Heat Absorbing Glasses These types of glass are manufactured

to have bronze, grey and blue-green colours. Tinted heat absorbing glasses absorb a greater amount of infra-red with some reduction of visible light.

Type of Window Glasses

Reflective Coated Glasses

These types of glasses have a microscopically thin metallic layer of ceramic layer coated on one of the surfaces

Insulating Glasses

These are made of two or three pieces of glasses separated by metal or rubber spacer around the edge and sealed in a stainless steel structure. The dehydrated airspace (6-12 mm) between the glass panes enhances the thermal insulation of the unit.

 

Glass windows VS Greenhouse Effect Window glass allows short-wave solar

radiation get into an interior space. This radiation is absorbed by the

interior of the building. The interior then radiates long-wave,

thermal radiation. Glass is opaque (not transparent) to

this long wave radiation. Thus heat energy is trapped in the

building and the indoor air temperature rises.

Radiation energy emitted by a human being The sun provides about 1000 watts per

square meter at the Earth's surface.  30 % is reflected by human skin.  

How much do the human body absorb the energy from the sun?

  If heat energy due to radiation is measured using H =

esA T4. Find irradiative energy emitted by healthy human being If estimated surface area of human being is 1.5 m2

Example:  How much does the human body radiate?

------------------------------------------------------------------------Body temperature = 37 C = 37 +273 = 310 K, Estimate surface area A = 1.5 m2        e = 0.70

H = esA T4

        = (0.70)(5.67 x 10-8)(1.5 m2)(310)4        = 550 watts (5 light bulbs)------------------------------------------------------------------------

Excercise

Example 1 :  Standing outdoors temperature 370C:

Example 2:  Standing outdoors temperature20 degrees centigrade:

Examples radiation

Example 1 :  Standing outdoors on hot day:

Body temperature: 37 C = 37 +273 = 310 K, Air temperature:     37 C = 310 K

   Hnet = esA [T4 - T04]  =  esA [3104 - 3104]          = 0

Example 2:  Standing outdoors on morning:

Body temperature = 37 C = 37 +273 = 310 K, Air temperature = 20 C = 293 K

A = 1.5 m2     e = 0.70

Hnet = esA [T4 - T04]          = (0.70)(5.67 x 10-8)(1.5 m2)(3104 - 2934)        = watts 

measures which can be adopted to reduce solar radiation in buildings. External and internal shading and by

careful building design. Natural vegetation such as tall trees can

also reduce solar heat gains. Window areas can be reduced although

natural day lighting is important in northern latitudes in winter so there is a limit to glass reduction.

Buildings can be orientated so that there is less window area facing west

References

Randall McMullan (2002) Environmental Science in Building fifth Edition PelgraveMacMillan

David M. Egan (1975) Concept in thermal comfort prentice Hall New Jersey

What you must know

Q1: If vacuum and air are the best insulators why are they not used as Thermal insulator?

Q2: Comment on the applications of Aluminum as an insulator while is a good conductor of heat.

Answers Vacuum provides perfect insulation against

conduction but is not practical for every purpose

Air is the mixture of gases with atom widely spaced therefore can provide good insulation against conduction but must be held still. The movement of air transfers heat by convection method.

Solution is to trap and hold air still used other materials i.e. fibres, aerated structures etc