HEAT EXCHANGERS: Everything, and everyone, is a heat exchanger, a solid body that takes in heat on one side and gives it up on the other. An exchanger that is designed for that purpose will probably be part of a "closed" loop, meaning that the same charge of fluid is constantly re-circulated within the loop. On the other side, the exchanger may be in another closed loop, or in an open, un-pressurized supply of fluid. In that situation, the loop is closed so that the fluid inside can give up, or absorb, heat through the walls of the exchanger and go back to repeat the cycle, without contaminating the material on the other side. Also, if the heat-exchange loop uses water, there is another important reason why it must be a closed loop. Open water contains quite a bit of dissolved air, which is 18% oxygen. When that oxygen-laden water contacts some metals, an oxide forms. In a closed loop, when the water is first put into it, the oxygen that came in with the water will cause a little rusting in iron-body pumps and engine blocks. After that, when the oxygen in the original charge is used up, there will be no more rusting, provided that the loop remains closed. Conversely, any open supply of water will form an oxide on certain metals. With aluminum, for example, a constant flow of fresh water will oxidize the aluminum slightly, wash the oxide away, and eventually eat holes in the exchanger. There are exchangers in all space-heating and -cooling systems. There are less obvious ones in every computer, and every refrigerator, and there are quite a few on every truck and auto. Metals are the best conductors of heat and, therefore, the most efficient heat exchangers. The efficiency with which a metal will transfer heat from one side to the other is shown by the value, Coefficient of Conductivity. The coefficients of some of the metals are shown below. Since we did not understand the units that went with the table, please look upon them as relative values, with silver the best. Metal Heat Conductivity Silver -------------- 406 |