U factor & R value explained|Modes of Heat Transfer|Heat Load Calculation|HVAC

Why U-factor and R-value is important for a building?
What are the modes of heat transfer?
What is Heat Transfer calculation in HVAC?
What is Thermal Conductivity?
What is U-factor & R-value
There are three modes of heat transfer.1.Conduction2.Convection3.Radiation
Conduction :Heat transfer through the process of conduction occurs in substances which are in direct contact with each other. It generally takes place in solids.
Heat transfer takes place from the hotter part to the colder part in a body without any actual movement of the molecules of the body which is due to vibratory motion of the molecules
Convection of Heat
In this process, heat is transferred in the liquid and gases from a region of higher temperature to a region of lower temperature due to the actual movement of molecules or due to the mass transfer.
For example. Heating of milk in a pan.
Radiation of Heat
It is the process in which heat is transferred from one body to another body without involving the molecules of the medium. Radiation heat transfer does not depend on the medium.
For example: In a microwave, the substances are heated directly without any heating medium.
The thermal conductivity for various materials can be found in ASHRAE Handbook - Fundamentals.Thermal conductivity is a measure of how well a material conducts and promotes heat transfer
For example, metals are excellent conductors and thus have a high conductivity. Aluminum has a thermal conductivity of 128 Btu/(hr*ft*℉) and iron has a conductivity of ~30 Btu/(hr*ft*℉).
Poor conductors include materials like wood (Douglas fir -0.0833 Btu/(hr*ft*℉)) and insulation materials (Cellular Glass - 0.0275Btu/(hr*ft*℉) ; Glass Fiber - 0.0221Btu/(hr*ft*℉)).
Thermal conductivity is given in units of(Btu*in)/(hr*〖ft〗^2*℉). (This value is basically a thermal conductivity value per inch thickness of materials )
Insulation, masonry, plastering and wood materials often have thermal conductivity per inch of materials

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