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HEAT EXCHANGE PROCESS AND U-VALUE, 2nd Law of Thermodynamics, Solar heat…
HEAT EXCHANGE PROCESS AND U-VALUE
Building Purpose
Maintain a comfortable internal temp.
Provide shelter
Security & privacy
Protection from extreme weather
Passive design
Architectural design
Building Configuration
Material choice
U-VALUE
Prevent heat from transmitting between the interior and exterior of a building
High U-value
= High heat loss
Low U-value
= Good insulator
FACTORS AFFECTING U-VALUE
THERMAL CONDUCTANCE
Deals with material thickness
Time rate od steady state heat flow through a unit of a material induced by a u it temp. Difference between the body
Unit: W/m2K or. W/m2 oC
THERMAL CONDUCTIVITY
Heat transmitted through a unit area and thickness of material
Expressed as k=b/R
Unit: W/mk or W/m oC
All materials have different conductivity
THERMAL RESISTANCE
Heat property and a measurement of a temperature difference by which an object resist a heat flown
Expressed as : R=b/K
Reciprocal of thermal conductance
THERMAL RESISTIVITY
Time required for wine unit of heat to pass through the material surface area
U-VALUE CALCULATIONS
Unit: W/m2k or W/m2 oC
U= 1/ (Rsi + Rso + Ra + R(n)
2nd Law of Thermodynamics
Heat moves to achieve equilibrium
Building respond to environment by gaining or losing heat
Solar heat gain = Window area
x
Radiation heat flow
x
solar gain factor