Heat transfer

Majority of Heat transfer takes place in buildings through building envelope. Building envelope consists of walls, roof, and fenestrations (openings). Heat transfers through walls and roof is by conduction and is through conduction and radiation in glazing materials. Heat loads are generated through convection which is termed as ventilation load. There are also internal

Heat transfer through Conduction:

Heat transfer through a material takes place by conduction from warm to cold side. The same process takes place in a building. Generally the thermal conductivity of the building materials will be much lower. In solid bodies including building components, thermal conduction takes place when one part of the component is subjected to higher temperature and the other part to a lower temperature condition. Most cases of thermal conduction are usually analysed and treated in their simplified form as one dimensional heat flow cases, i.e. heat flow in directions other than the main direction is neglected. Similarly, if the changes in atmospheric conditions (inside and I or outside) are assumed to be very slow, neglecting these changes, the process of heat transfer can be assumed to be “Steady State Heat Transfer” in its simplified form.

Heat transfer through Convection:

When heat is transported by a fluid, like air or water, this is called convection. The extent of convective heat transfer depends on a number of things, like the position of the surface (horizontal or vertical), but mainly on the speed of the passing air. The speed in outdoors is determined by wind speed and direction. When the air is driven by an outside wind force, this is called “forced convection”. When there is no wind, convection will occur by temperature or density differences. This is called “free convection”. The example of the hot air ascending above a radiator is an example of free convection. Room air is heated by the radiator and ascends because the density of the hot air next to the radiator is lower than the density of the cooler air in the rest of the room. This results in the warmer air rising, and being displaced by the cooler air. The heat transfer through forced convection is higher than that due to free convection, because of higher air speed.

Heat transfer through Radiation:

The phenomenon of thermal radiation is described as the transport of energy through electromagnetic waves. Unlike conduction and convection, radiation heat transfer is not bound with material, it can even occur through vacuum. Every body whose temperature is above absolute zero radiates energy in the form of electromagnetic radiation. The spectral distribution of the energy radiated from any body depends on the temperature of its surface. The higher the temperature of the body, the lower is the wavelength of the radiation that makes the major portion of the total emitted radiation. Since the temperature of most building components and their surroundings are much less as compared to heat of the sun, the spectral distribution of their emitted radiation has more share of larger wavelength. It means radiation emitted from such bodies is thermal radiation. In heat transfer due to radiation the rate of heat flow depends on the temperatures of the emitting and receiving surfaces and on certain qualities of these surfaces: the emittance and absorbance. Radiation received by a surface can partly be absorbed and partly be reflected. Heat is transferred into the building through radiation from the glass provided for fenestrations (openings)

Sensible heat:

Sensible Heat is defined as the heat energy stored in a substance as a result of an increase in its temperature. The heat is absorbed or transmitted by a substance during a change of temperature which is not accompanied by a change of state.

Latent heat:

Latent Heat is defined as the heat which flows to or from a material without a change to temperature. The heat will only change the structure or phase of the material. E.g. melting or boiling of pure materials. The heat is released or absorbed per unit mass by a system in a reversible isobaric-isothermal change of phase. In meteorology, the latent heats of evaporation (or condensation), fusion (melting), and sublimation of water substance are of importance.

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