Section BDirections: In this part there is a passage followed by some questions or unfinished statements. For each of them there are four choices marked A), B), C) and D). You should decide on the best choice.A heat pump is a single electric system that both heats and cools. In summer, a heat pump is a conventional air conditioner which removes heat from a building. In winter, the system reverses itself to use the natural heat that is always present in outdoor air. By taking advantage of the free heat, a heat pump is more economical to operate in winter than a system using electric resistance heat. Electric resistance heat uses electricity as a raw material to create heat where no heat existed before. Heat pump, on the other hand, uses electricity to move existing heat from where it already exists naturally (outdoors) to where it is wanted (indoors). Heat energy will naturally flow from areas of greater concentration to areas of lesser concentration. That is what happens when hot food grows cool. The heat from the food flows into the air in the room until the food stabilizes at room temperature. Blowing air over substance will speed up the process, which is why a winter day will seem cold when there is a strong wind than when the air is still. A heat pump uses this heat transfer process to move heat which is always in the air. The outdoor coil is made colder than the outdoors air. A fan blows outdoor air across the coil where heat is absorbed by the refrigerant. The refrigerant is then compressed and moved to the indoor coil and the heat is distributed in the building. During the summer, a heat pump reverses this process. The indoor coil is made cooler than the indoor air, and so room heat flows into the refrigerant which is compressed and moved to the outdoor coil. The outdoor coil, which is hotter than the outdoor air, will lose heat energy. 21. A heat pump is used_____.
A:to heat and cook
B:to heat but not cool
C:to cool but not heat
D:to heat and cool
答案: to heat and cool
相关试题