284 PHYSICAL LIMITATIONS HAVE BKKN OVLRCOML in density when heated, is an important consideration in air conditioning; for example, adequate provision should be made for the ventilation of the openings between the roof and the house proper so as to provide for the removal of hot air by convection during hot weather. 3. Radiation. Most of the heat energy which we receive is transferred to us by radiation. The warmth of a fire or of the sun is radiated heat. Air is such a poor conductor of heat that conduction would not account for this warmth. Convection currents in the air move upward from the earth and therefore could not conduct the heat of Furthermore, there is no material in inter- the sun down to the earth. stellar space by which heat could be conducted or in which convection currents could form. The presence of radiant energy can be recognized only when there is something present to absorb it. The nature of radiant energy will be discussed in Unit VL It will be sufficient at this point merely to mention that radiant energy can be transformed into heat and that the heat can be transformed back into radiant energy. A familiar illustration of the latter process is the illumination obtained from an electric-light bulb produced by heating a filament to a high temperature. It might be well to mention also that rough black bodies absorb and radiate heat better than smooth white bodies. The vacuum bottle, originally designed by Dewar as a liquid-air container, has found its place in nearly every home; it is a thin-walled glass flask wdth double w'alls, so constructed that the air can be removed by a vacuum pump and the air outlet can then be sealed off by melting the glass outlet. The removal of the air produces a partial vacuum which prevents conduction or convection. The walls are made thin to decrease conduction by the glass and to prevent it from cracking from unequal expansion or contraction. One of the walls is silvered. The mirror reflects radiant energy and thus cuts down to the minimum the transfer of heat by radiation. In planning a home to be air conditioned, attention must be given to radiation because most of the heat that comes through windows exposed to the sunlight enters the house by radiation. One way of controlling the temperature of the people in a house is to use pipes or panels in the floor, walls, or ceiling, covering all areas not occupied by such panels or pipes with reflecting non-heat-absorbing material. this method the people in the room are heated or cooled by radiation, although the walls, floors, ceilings, and the air in the room are not heated or cooled, with the exception of the radiating surfaces. People would feel cool in a room whose air temperature is 90° when cold water is circulated through the radiating panels, while people would feel By
AIR CONDITIONING 285 quite comfortable in a room at a temperature of 40° F. when the radiating panels are hot. This method of controlling the temperature should cost much less than systems which depend on heating or cooling the air. Fig. 97. A floor-warming pipe installation before laying the floor, (Courtesy of the Copper and Brass Research Association.) Radiant heating and cooling would save money in part because a house would not need to be heated or cooled when one was not in it. Cheap electricity would make it possible to provide comfort within a minute or two after turning a switch. STUDY QUESTIONS 1. What does complete air conditioning involve? 2. What is new about the recently advertised "air conditioned refrigerators"? 3. Would an "air conditioned" hat have any special sales appeal to you? 4. What is the most common cause of discomfort in crowded rooms? 5. Do you think that one should consider air conditioning in designing a new home today? 6. In what respects would air conditioning change the construction of a new home? 7. Where was air conditioning first introduced, and why? 8. What are the possible social consequences of air conditioning? 9. What is the best way to cool air? 10. What are the advantages of air conditioning? 11. What is meant by a reverse refrigeration cycle? 12. Discuss the advantages of radiant heating and cooling. 13. How may humidity be controlled? 14. Discuss the necessity of insulation in air conditioning. 15. How could radiant heat be kept out of a building? 16. Why should provisions be made for ventilating the space under roofs? 17. How could the direct rays of the sun be kept off window areas during the hot part of the day?