Transfer of Heat

Basic Methods of Heat Transfer:
The prime purpose of refrigeration is to produce desired temperatures within a spesific area by transferring unwanted heat to a location where it is not objectionable. To understand how these processes are carried on,it is necessery to have a working knowledge of heat flow, how heat may be transferred, and how heat enters a refrigerated space.

There are three basic methods of heat transfer: conduction,,convection and radiation. Each of these methods is described and illustrated in this unit.

Next, the materials and operating mechanisms that are either protection against or part of control devices for heat transfer are reviewed in terms of each basic method. A few formulas and typical problems are included. These show some of the places where the servicman, designer or manufacturer must be able to use simle mathematical calculations in the testing, servicing, design or production of refrigerating equipment.

Consept of Heat flow and Heat Transfer :
The natural tendency of heat is always to flow from a warm body to a cooler one. Heat is thus said to flow "down hill". In this respect, heat flow may be compared to water flow. When water in two separate containers is at the same level, no water flow from one the other. When the water level is either raised or lowered in one of the containers, the water flow from the higher level to the lower one.

In a like manner, if two materials or substances are at the same temperature, no heat flows betwen them. However, if there is a temperature difference between them, heat flows from the warm body to the colder one. In the following illustration, (A) shows two liquids at the same temperature. If the temperature of one liquid is lowered to 32 degrees F., (B).The temperaure of the liquid at (C) has been increased to 212 degrees F. The heat now flows from this higher temperature of 212 degrees to 70 degrees F.

The heat that must be transfered enters a refrigerated space in three main ways:
1. The heat leaks through the walls surrounding the space to be cooled.
2. The heat rushes into the refrigerated area as a door is opened.
3. The heat is introduced into the refrigerated area in the material that is to be
cooled, frozen or stored.

Several steps are involved in the transfer of heat energy from the time it is picked up in the evaporator until it is rejected at the condenser. The schematic drawing shows that in the transfer of heat, different temperatures are involved as the heat travels through diffrent material. In addition, the refrigerant must be moved through the system, water or air must be circulated in the condenser , and air or brine must be moved in the evaporator. All of these steps or processes depend on an understanding of the three basic ways that heat moves or is transferred.

The flow of heat energy is always from hot to cold or from a high intensity to a low intensity. This flow continues until the speed of the molecules at the cold end increases with the absorption of heat and it reaches the same temperature.
At the same temperature, the molecules in all parts have the same average activity.
This method of trnsferring heat from molecule is known as "conduction".

The conduction process may be illustrated with a heated steel part. As such a part is placed in cold water, the fast-moving molecules on the outer surface of the steel transmit the heat to the slower moving molecules of water and increase their speed. In turn, as the steel cools to the temperature of water, the speed of its moleciles decreases.

While the outer surface of the steel is cooled, the center is still hot. The rapid motion of the molecules is transmitted to the outer surface by conduction. The collision of a fast-moving molecule at the center with a slow moving one from the outer surface causes the to lose speed while the other gains speed. This process continues until a uniform temperature is reached throughout the steel part.