Heat Transmission

                           In refrigeration, the trasmission of heat may be accomplished in three ways;

1. By Conduction

2. By Convection

3. By Radiation

                                                      Conduction

                           This is the transference of heat by molecular impact from one particle to another in contact. For instance, if the end of a bar of iron is heated in the fire, some of the heat will pass through the bar to the cooler portion. Heat trevelling in a body, or from one body to another where the  two are in intimate contact, is termed conduction. Metal are usually splendid heat conductors. Each and every material has a conduction value, some good like the metals, other mediocre, and a few very poor. For instance, heat will quickly pass through a piece of copper but will have consideration difficulty in passing through a piece of cork. The material that have very low heat conductivities are termed heat insulators. Even the very poorest conductors, or insulation material, allow a certain amount of heat to pass through. There is no material which offers a perfect barrier or resistance to the passage of heat.

                                                      Convection

                              Convection is the principle used in hot-air heating. Air that is free to circulate, such as in any air body of appreciable size, will be set into motion where a difference of temperature occurs, for it will be absorb heat from the warmer wall, become heated, expand and become lighter The heated portion of the air eventually moves over to the colder wall, and the heat flows from the air to the colder object. Thus, any body of air capable of motion will transmit heat by convection. Hot-air and hot-water heating system work on the convection principle. They convey heat by bodily moving the heated substances from one place to another.

                                                      Radiation

                                Heat energy transmitted through air in the same way light is sent out by a lighted lamp, a radiant heater, or the sun, is called radiated energy. Large cold storage, warehouse, auditoriums, theaters, and homes are built with consideration of the heat evolved through radiant energy of the sun. Small household appliance rarely have to consider any radiant heat factor, for they are used in existing structures without any change in building design and are sheltered from direct heat.