02 c&be-unit 3

AR 2204 CLIMATE & BUILT ENVIRONMENT

SEM-III, II YEAR B.ARCH MSAJAA, 2012-2013

AR 2204 CLIMATE & BUILT ENVIRONMENT

UNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTS TRANSFER OF HEAT THROUGH SOLIDS DEFINITIONS CONDUCTIVITY, RESISTIVITY, SPECIFIC HEAT, CONDUCTANCE, RESISTANCE AND THERMAL CAPACITY SURFACE RESISTANCE AND AIR CAVITIES AIR TO AIR TRANSMITTANCE (U VALUE) TIME LAG AND DECREMENT

UNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTS

TRANSFER OF HEAT THROUGH SOLIDS HEAT IS A FORM OF ENERGY. MEASURED IN JOULES (J). PREVIOUSLY USED UNITS:

BTU BRITISH THERMAL UNIT IT IS THE AMOUNT OF HEAT NECESSARY TO RAISE THE TEMPERATURE OF 1 lb OF WATER BY 1 degF.

kcal KILO CALORIE(S) IT IS THE AMOUNT OF HEAT NECESSARY TO RAISE THE TEMPERATURE OF 1 kg OF WATER BY 1 degC.

1 Btu = 1055.06 J 1 kcal = 4186.8 J

TRANSFER OF HEAT THROUGH SOLIDS

HEAT ENERGY FLOWS FROM A HIGHER TEMPERATURE BODY TO A LOWER TEMPERATURE BODY, UNTIL A PERFECTLY DIFFUSED UNIFORM THERMAL FIELD IS ACHIEVED. HEAT TRANSFER OCCURS THROUGH: CONDUCTION CONVECTION RADIATION.

MOTIVE FORCE BEHIND HEAT FLOW DIFFERENCE IN TEMPERATURE.


THERMAL QUANTITIES CONDUCTIVITY:

ALSO KNOWN AS k-VALUE. DEFINED AS THE RATE OF HEAT FLOW THROUGH UNIT AREA OF UNIT THICHNESS OF THE MATERIAL WHERE THERE IS UNIT TEMPERATURE DIFFERENCE BETWEEN THE TWO SIDES. UNIT OF MEASUREMENT W/m degC

LOWER THE CONDUCTIVITY VALUE, BETTER THE INSULATOR A MATERIAL IS.

VARIES BETWEEN 0.03 W/m degC FOR INSULATING MATERIALS AND UPTO 400 W/m degC FOR METALS.


THERMAL QUANTITIES

RESISTIVITY:

RECIPROCAL OF CONDUCTIVITY. UNIT OF MEASUREMENT m degC/W

CONDUCTANCE:

HEAT FLOW THROUGH UNIT AREA OF THE BODY WHEN THE TERMPERATURE DIFFERENCE BETWEEN THE TWO SURFACES IS 1 degC. UNIT OF MEASUREMENT W/m2 degC

NEGLIGIBLE MOVEMENT OF MOLECUES THROUGH CONTACT OF MOLECULES.



THERMAL QUANTITIES

CONDUCTANCE:

CONDUCTIVITY & RESTIVITY PROPERTIES OF MATERIAL CONDUCTANCE PROPERTY OF BODY OF GIVEN THICKNESS.

RECIPROCAL OF CONDUCTANCE RESISTANCE. R=1/C

RESISTANCE OF A BODY IS THE PRODUCT OF ITS THICKNESS & THE RESISTIIVITY OF ITS MATERIAL.

R = b x 1/k , b = THICKNESS OF MATERIALS IN M.

UNIT m2 degC/ W


THERMAL QUANTITIES

SURFACE CONDUCTANCE/ RESISTANCE:

IN ADDITION TO THE RESISTANCE OF A BODY TO THE FLOW OF HEAT, RESISTANCE WILL BE OFFERED BY ITS SURFACES THIN LAYER OF FILM SEPARATES THE BODY FROM SURROUNDING AIR.

DENOTED BY 1/f (m2 deg C/W) WHERE f = SURFACE/ FILM CONDUCTANCE.

SURFACE CONDUCTANCE INCLUDES: CONVECTIVE COMPONENT OF HEAT EXCHANGE RADIATIVE COMPONENT OF HEAT EXCHANGE.

OVERALL AIR-TO-AIR RESISTANCE = BODYS RESISTANCE + SURFACE RESISTANCE.


THERMAL QUANTITIES

TRANSMITTANCE:

DENOTED BY U-value.

MOST COMMONLY USED QUANTITY FOR BUILDING HEAT LOSS/ GAIN PROBLEMS.

IT IS THE RECIPROCAL OF AIR-TO-AIR RESISTANCE.

U = 1/ Ra

IT IS MEASURED AS W/ m2 DegC.


THERMAL QUANTITIES

CAVITIES:

BASICALLY IT IS AN AIR-SPACE ENCLOSED WITHIN A BODY.

BARRIERS TO PASSAGE OF HEAT.

MEASURED AS CAVITY RESISTANCE Rc

ITS RECIPROCAL IS THE CAVITY CONDUCTANCE.


HEAT ENERGY FLOWS FROM A HIGHER TEMPERATURE BODY TO A LOWER TEMPERATURE BODY, UNTIL A PERFECTLY DIFFUSED UNIFORM THERMAL FIELD IS ACHIEVED. HEAT TRANSFER OCCURS THROUGH: CONDUCTION CONVECTION RADIATION.

MOTIVE FORCE BEHIND HEAT FLOW DIFFERENCE IN TEMPERATURE.




CONVECTION: THE TRANSFER OF HEAT BY BODILY MOVEMENT OF A CARRYING MEDIUM.

MAYBE SELF GENERATING OR MAYB PROPELLED BY AN APPLIED FORCE.

RATE OF TRANSFER OF HEAT DEPENDS ON: TEMPERATURE DIFFERENCE RATE OF MOVEMENT OF THE CARRYING MEDIUM SPECIFIC HEAT OF THE CARRYING MEDIUM.



RADIATION: RADIATION RECEIVED BY A SUFACE MAYBE:

PARTLY ABSORBED PARTLY REFLECTED

THE ABOVE TWO IS REPRESENTED AS: CO-EFF OF ABSORPTION a CO-EFF OF REFLECTION r

ALWAYS, a + r = 1

BOTH LIGHT & DARK COLOURED SURFACES WILL REFLECT & ABSORB THE SAME AMOUNT OF HEAT; LIGHT COLOURED SURACES WIL RE-EMIT MUCH OF THE ABSORBED HEAT COMPARED TO THE DARK SURFACES.



IN BUILDING DESIGN PROCESS:

SOL-AIR TEMPERATURE (heat on walls/ exposed surfaces of buildings)

SOLAR GAIN FACTOR (heat flowing through windows & openings which are glazed or protected by some material)



SOL-AIR TEMPERATURE:

USED TO CALCULATE HEAT GAIN ON BUILDING SURFACES EXPOSED TO SOLAR RADIATION.

IT COMBINES THE HEATING EFFECTS OF RADIATION INCIDENT ON BUILDING WITH THE EFFECT OF WARM AIR.

TS = TO + I x a/ fo WHERE, To OUTSIDE TEMP IN degC I - RADIATION INTENSITY IN W/m2 a - ABSORBANCE OF THE SURFACE fo - SURFACE CONDUCTANCE (OUTSIDE) IN W/m2 degC



SOLAR GAIN FACTOR () :

DEFINED AS THE HEAT FLOW RATE THROUGH THE CONSTRUCTION DUE TO SOLAR RADIATION, EXPRESSED AS A FRACTION OF THE INCIDENT SOLAR RADIATION. q/I = a x U/ fo (non dimensional) WHERE,

q Extra heat flow rate per unit area caused by radiation U - Transmittance value in W/ m2 degC I - Radiation Intensity in W/ m2 a absorbance of the surface fo surface conductance (outside) in W/ m2 degC.

THE LESSER THE SOLAR GAIN FACTOR, THE LESSER IS THE HEAT TRANSFER THROUGH WINDOWS OR OPENINGS.


HEAT EXCHANGE PROCESS



CONDUCTION OF HEAT THROUGH WALLS EITHER INWARDS OR OUTWARDS (Qc) SOLAR RADIATION THROUGH SOL-AIR CONCEPT AND/ OR SOLAR HEAT GAIN THROUGH TRANSPARENT SURFACE (Qs) HEAT EXCHANGE IN EITHER DIRECTION WITH MOVEMENT OF AIR VENTILATION (Qv) INERNAL HEAT FROM HUMAN BODIES (OCCUPANTS) (Qi) INTERNAL HEAT GAIN/ LOSS FROM MECHANICAL CONTROLS (Qm) EVAPORATION ON THE SURFACE/ WITHIN THE BLDG (Qs)

THERMAL BALANCE: Qi + Qs Qc Qv Qm Qe = 0



CONDUCTION HEAT FLOW RATE Qc = A * U * T where, Qc Conduction heat flow rate A Surface area in Sqm. U Transmittance value, in W/ m2 degC T Temperature difference.

CONVECTION HEAT FLOW RATE Qv = 1300 * V * T where, Qv Convection heat flow rate, in W. 1300 volumetric specific heat of air, j/m3 degC V Ventilation rate in m3/s T Temperature difference.



RADIATION HEAT FLOW RATE Qs = A * I * where, Qs radiation heat flow rate A Surface area in Sqm.(area of window) I radiation heat flow density, in W/ m2 solar gain factor of window glass.

INTERNAL HEAT GAIN heat given out by occupants based on numbers & activity too.

EVAPORATION can be used in calculations, if rate of evaporation is known. usually ignored for calculation purposes, except for mechanical installations.



THERMAL MASS & THERMAL CAPACITY: Thermal mass - capacity of a house to store & regulate internal heat. Bldgs with high thermal mass take a long time to heat and long time to thus cool. Thus they have an almost steady internal temperature. Bldgs with low thermal mass heat up quickly and cool down quickly wide variety of internal temperature.

THERMAL CAPACITY: capacity of internal objects to absorb & store heat. Dense materials have high thermal capacity heat up slowly and lose heat slowly. Ex: brick, stone & concrete. Air has low thermal capacity.



TIME LAG () & DECREMENT FACTOR (): During the day, heat flow environment to bldg During night, heat flow bldg to environment. There is a time lag between the time the outdoor temperature has reached maximum and internal temperature reaches maximum. Time lag depends on specific heat of wall material. TIME DELAY DUE TO THERMAL MASS IS KNOWN AS TIME LAG ()

THE REDUCTION IN CYCLICAL TEMPERATURE OF A MATERIAL, ON THE INSIDE SURFACE COMPARED TO THE OUTSIDE SURFACE IS KNOWN AS THE DECREMENT ().



TIME LAG () & DECREMENT FACTOR (): As a rule of thumb for massive masonry, earth & concrete walls = 10 hrs for each 0.3 m thickness. decrement is very important in design of blgs in environments with very high diurnal range, ex: deserts, etc. ex: In deserts, if materials with time lag of 10 to 12 hrs are used, the low night temp will reach inside surface around the middle of the day & high day temperatures will reach inside surfaces late evenings or in the nights.

In constantly hot/ cold climates, time lag can be detrimental.

INSULATION can help increase time lag & decrement factor and hence help in heat balance of a building.

AR 2204CLIMATE & BUILT ENVIRONMENTAR 2204 CLIMATE & BUILT ENVIRONMENTUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTSUNIT 3 HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTS

02 c&be-unit 3

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