Methods (Modes) of Heat Transfer

Methods (Modes) of Heat Transfer Points : Methods (Modes) of Heat Transfer Heat Transfer Heat can flow from a material at higher temperature to a material at a lower temperature, just as water can flow only from a height to a lower point. Therefore, to cool (or refrigerate) a substance or space, cooling medium should be at lower temperature than the space or substance to be cooled. A liquid, which has a low boiling point can be made to boil inside a coil (evaporator) at a suitable temperature by maintaining a pressure. The latent heat of vapourisation of the liquid is taken from the surrounding space, thus producing refrigeration.

The gas formed in the evaporator can be condensed back into a liquid for re-use by raising its pressure. By raising the gas pressure, the, condensing point/temperature can be raised to such a level that the normal outside air or water can coal and take away the latent heat of vapourisation thus condensing the vapour back to a liquid.

In a vapour compression cycle, the compressor is used to compress the low pressure gas coming from the evaporator to high pressure for condensing in the condenser. Heat transfers accomplished by the following modes:

1. Conduction
2. Convection
3. Radiation

When a temperature gradient exists in a stationary medium which may be a solid or fluid, we use the term conduction to refer to the heat transfer that will occur across the medium. In contrast, the term convection refer to heat transfer that’ll occur between a surface and a moving fluid when they are at different temperatures. The third mode of heat transfer is termed as thermal radiation. All surface of finite temperatures unit energy in the form of electro magnetic waves. Hence in the absence of air intervening medium, there is net heat transfer by radiation between the surfaces at different temperatures.