heat - universiti sains malaysia · radiation is the transmission of electromagnetic rays through...

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HEAT

Sept. 2007, Yeoh OonSoon Architect/Lecturer Heat is a form of energy, appearing as molecular motion in substances or as radiation in space. It is measured in Joule. Temperature can be considered as presence of heat in a substance. The SI metric Celsius scale has the freezing point of water as 0 and the boiling point under normal atmospheric conditions as 100. Noted as deg. C. Body temp. 37 deg.C The USA still use the Fahrenheit scale, freezing point is 32 and boiling 212. Noted as deg. F. Body temp. 98.6 deg.F In scientific work, absolute temperature is used. Measured in Kelvin scale deg.K, starting point is absolute 0 i.e. –273.15 deg.C Thermodynamics is the science of the flow of heat and its relationship to mechanical work.

1. The first law of thermodynamics/energy is the principle of conservation of energy. Energy cannot be created or destroyed, only changed from one form to another.

2. The second law of thermodynamics, (Clausius 1850) states that heat (or energy), transfer can take place in one direction only i.e. from a hotter to a cooler body.

Heat is transferred from hot to cold in 3 basic ways;

1. Conduction 2. Convection 3. Radiation

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We study heat because we want to be comfortable inside buildings. This aspect is called Thermal Comfort. Heat is constantly produced by bodily processes and must be dissipated to keep the body temperature at its correct level. Normally, this is lost by radiation, convection and evaporation and to remain comfortable, appropriate quantity of heat must be lost and a proper balance maintained between the various modes of loss. Rate of heat loss in each aspect is governed by the surrounding environmental conditions.

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• The net heat loss by radiation is affected by the mean radiant conditions. • The rate of loss by convection is affected by the air temperature and rate of air

movement. • Evaporation losses by breathing and sweating depend on air temperature, relative

humidity and air movement.

BURBERRY, Peter. (1979). “Mitchell’s Building Series: Environment & Services”, B.T.Batsford, London BRE has in several studies shown that globe thermometer temperatures form a satisfactory measure of thermal environment in temperate regions.

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BURBERRY, Peter. (1979). “Mitchell’s Building Series: Environment & Services”, B.T.Batsford, London.

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“Clo” units represent different clothing levels.

BURBERRY, Peter. (1979). “Mitchell’s Building Series: Environment & Services”, B.T.Batsford, London

Abridged Web Extract from Innovative Insulation, Inc. 2004-2006;

HEAT GAIN / LOSS IN BUILDINGS

There are three modes of heat transfer: CONDUCTION, CONVECTION, and RADIATION (INFRARED). Of the three, radiation is the primary mode; conduction and convection are secondary and come into play only as matter interrupts or interferes with radiant heat transfer. As matter absorbs radiant energy, it is heated and a gradient temperature develops, which results in molecular motion (conduction in solids) or mass motion (convection in liquids and gas).

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All substances, including air spaces and building materials (such as wood, glass, plaster and insulation), obey the same laws of nature and TRANSFER heat. Solid materials differ only in the rate of heat transfer, which is mainly affected by differences in density, weight, shape, permeability and molecular structure. Materials which transfer heat slowly can be said to RESIST heat flow.

Direction of heat transfer is an important consideration. Heat is radiated and conducted in all directions, but convected primarily upward. Radiation is the dominant mode.

CONDUCTION is direct heat flow through matter (molecular motion). It results from actual PHYSICAL CONTACT of one part of the same body with another part, or of one body with another. For instance, if one end of an iron rod is heated, the heat travels by conduction through the metal to the other end; it also travels to the surface and is conducted to the surrounding air, which is another, but less dense, body. An example of conduction through contact between two solids is a cooking pot on the solid surface of a hot stove. The greatest flow of heat possible between materials is where there is a direct conduction between solids. Heat is always conducted from warm to cold, never from cold to warm, and always moves via the shortest and easiest route.

In general, the more dense a substance, the better conductor it is. Solid rock, glass and aluminum-being very dense-are good conductors of heat. Reduce their density by mixing air into the mass, and their conductivity is reduced. Because air has low density, the percentage of heat transferred by conduction through air is comparatively small. Two thin sheets of aluminum foil with about one inch of air space in between weigh less than one ounce per square foot. The ratio is approximately 1 of mass to 100 of air, most important in reducing heat flow by conduction. The less dense the mass, the less will be the flow of heat by conduction.

CONVECTION is the transport of heat within a gas or liquid, caused by the actual flow of the material itself (mass motion). In building spaces, natural convection heat flow is largely upward, somewhat sideways, not downward. This is called “free convection.”

For instance, a warm stove, person, floor, wall, etc., loses heat by conduction to the colder air in contact with it. This added heat activates (warms) the molecules of the air which expand, becoming less dense, and rise. Cooler, heavier air rushes in from the side and below to replace it. The popular expression “hot air rises” is exemplified by smoke rising from a chimney or a cigarette. The motion is turbulently upward, with a component of sideways motion. Convection may also be mechanically induced, as by a fan. This is called “forced convection.”

RADIATION is the transmission of electromagnetic rays through space. Radiation, like radio waves, is invisible. Infrared rays occur between light and radar waves (between the 3 -15 micron portion of the spectrum). Henceforth, when we speak of radiation, we refer only to infrared rays. Each material that has a temperature above absolute zero (-459.7 F.) emits infrared radiation, including the sun, icebergs, stoves or radiators, humans, animals, furniture, ceilings, walls, floors, etc.

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All objects radiate infrared rays from their surfaces in all directions, in a straight line, until they are reflected or absorbed by another object. Traveling at the speed of light, these rays are invisible, and they have NO TEMPERATURE, only ENERGY. Heating an object excites the surface molecules, causing them to give off infrared radiation. When these infrared rays strike the surface of another object, the rays are absorbed and only then is heat produced in the object. This heat spreads throughout the mass by conduction. The heated object then transmits infrared rays from exposed surfaces by radiation if these surfaces are exposed directly to an air space.

The amount of radiation emitted is a function of the EMISSIVITY factor of the source’s surface. EMISSIVITY is the rate at which radiation (EMISSION) is given off. Absorption of radiation by an object is proportional to the absorptivity factor of its surface which is reciprocal of its emissivity.

Although two objects may be identical, if the surface of one were covered with a material of 90% emissivity, and the surface of the other with a material of 5% emissivity, the result would be a drastic difference in the rate of radiation flow from these two objects. This is demonstrated by comparison of four identical, equally heated iron radiators covered with different materials. Paint one with aluminum paint and another with ordinary enamel. Cover the third with asbestos and the fourth with aluminum foil. Although all have the same temperature, the one covered with aluminum foil would radiate the least (lowest [5%] emissivity). The radiators covered with ordinary paint or asbestos would radiate most because they have the highest emissivity (even higher than the original iron). Painting over the aluminum paint or foil with ordinary paint changes the surface to 90% emissivity.

Materials whose surfaces do not appreciably reflect infrared rays, i.e.: paper, asphalt, wood, glass and rock, have absorption and emissivity rates ranging from 80% to 93%. Most materials used in building construction -- brick, stone, wood, paper, and so on -- regardless of their color, absorb infrared radiation at about 90%. It is interesting to note that a mirror of glass is an excellent reflector of light but a very poor reflector of infrared radiation. Mirrors have about the same reflectivity for infrared as a heavy coating of black paint.

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SZOKOLAY, S.V. (1980). “Environmental Science Handbook for Architects and Builders”, The Construction Press, UK

Qi = internal heat gain, heat from human bodies, lamps, appliances Qs = solar heat gain Qc = conduction heat gain Qv = ventilation heat gain Qe = evaporative cooling Qm = mechanical cooling Thermal balance exists when the sum of all heat flow terms is zero i.e.; Qi + Qs + Qc + Qv + Qm - Qe = 0 When this sum is greater than 0 (+), temperature indoor will heat up. When less than 0 (-), temperature indoor will cool down. The fabric of a building reacts to thermal changes and does so over a period of time. This contrasts with its behaviour in relation to wind and light where a wall gives a total and immediate barrier. Heat is not suddenly stopped but is delayed and passes through over a period. Thus it is necessary to take into account resistance of material to the passage of heat, the way in which they warm or cool and the time taken to do so. Heat from rays of the sun can easily raise inside temperatures way beyond comfort levels. Retarding Heat Flow – INSULATION 3 ways/forms of Insulation;

1. Reflective; shiny, low absorbance & low emittance 2. Resistive; bulk, vacumn or still air, low density & porous 3. Capacitive; massive, heat store

Types 1 & 2 have immediate effect, while type 3 operates over time.

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Sol-air temperature.

Sol-air temperature, pp 74, KOENIGSBERGER et.al “Manual of Tropical Housing & Building : Part 1 Climatic Design” Longmans, London 1974.

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Thermal Transmittance (U Value) The rate of heat transmission is known as thermal transmittance. This unit is in Watts that will be transferred through 1 sq.m of the construction when there is a difference of 1 deg C. between the temperature of air on the outside and that on the inside. This is called “U” value or “air to air” heat transmittance coefficient. These coefficients are calculated from the conductivity or “K” of the materials.

The following calculation example is for comparison purposes and uses a double leaf wall construction for temperate climates. For single leaf walls, common in the tropics, the calculation is much simplified.


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Presently, in Malaysia, there are no statutes/laws compelling designers/owners to keep buildings (designs) within certain sun shading, solar heat gain or thermal transmittance parameters. The considerations are advisory and mainly form the basis of good tropical design. However in the United Kingdom, thermal transmittance parameters are a requirement in their Building Regulations.

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The following solar heat gain example illustrate the intricate calculation/consideration done by engineers/scientists. Architects and designers are rarely expected to do such calculations in support of their design decisions.

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Active Thermal Controls 1. Ventilation/Air change; health reasons (Ventilation or Air Change Rate), warm

dirty inside air expelled & replaced by cleaner or fresher (sometimes cooler) air, use of exhaust fans, blowers, ducts/plenum. e.g. Ventilation ducts in basement carparks, exhaust fans in toilets/kitchens/food courts

2. Evaporative & Convection cooling; evaporation relies on latent heat loss when

water changes from liquid to vapour. (2400 kJ/kg of water). Fine spray of water. Successful when air is dry. For hot humid climate, only effective (partly psychological) outdoors with more air movement e.g. Misting fans @ Food & Beverage (F&B) outlets, kiosks, homes. Convective cooling (partly body evaporative) by ceiling/wall/stand fans blowing across comfort zone.

3. Mechanical Cooling / Air conditioning. Similar to the refrigerator. Commonly

referred to here as the air conditioner. Besides being cooled, the air is partially filtered, dehumidified, sometimes ionized or sanitized. Various systems available for domestic, commercial and industrial. E.g. Window unit, split-con, chilled water system. In temperate countries, where heating is required, the cycle can be reversed or positions interchanged – concept of a heat pump.

4. Blinds, curtains, screens, louvers. Usually inside but less effective than outside

installations. Manually or automatically manipulated to control insolation, glare, daylight or view. E.g. venetian blinds, blackout curtains, bamboo chits, mechanized or manual opaque louvers, etc.

Principle of a Heat Pump.

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BURBERRY, Peter. (1979). “Mitchell’s Building Series: Environment & Services”, B.T.Batsford, London.

heat - universiti sains malaysia · radiation is the transmission of electromagnetic rays through...

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