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  3. The use of heat pumps has been held back largely by skepticism about advertisers’ claims that heat pumps can provide as many as

The use of heat pumps has been held back largely by skepticism about advertisers’ claims that heat pumps can provide as many as

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问题     The use of heat pumps has been held back largely by skepticism about advertisers’ claims that heat pumps can provide as many as two units of thermal energy for each unit of electrical energy used, thus apparently contradicting the principle of energy conservation.
    Heat pumps circulate a fluid refrigerant that cycles alternatively from its liquid phase to its vapour phase in a closed loop. The refrigerant, starting as a low-temperature, low-pressure vapor, enters a compressor driven by an electric motor. The refrigerant leaves the compressor as a hot, dense vapour and flows through a heat exchanger called the condenser, which transfers heat from the refrigerant to a body of air. Now the refrigerant,as a high-pressure, cooled liquid, confronts a flow restriction which causes the pressure to drop. As the pressure falls, the refrigerant expands and partially vaporizes, becoming chilled. It then passes through a second heat exchanger, the evaporator, which transfers heat from the air to the refrigerant, reducing the temperature of this second body of air. Of the two heat exchangers, one is located inside, and the other one outside the house, so each is in contact with a different body air: room air and outside air, respectively.
    The flow direction of refrigerant through a heat pump is controlled by valves. When the refrigerant flow is reversed, the heat exchangers switch function. This flow-reversal capability allows heat pumps either to heat or cool room air.
    Now, if under certain conditions a heat pump puts out more thermal energy than it consumes in electrical energy, has the law of energy conservation been challenged? No, not even remotely: the additional input of thermal energy into the circulating refrigerant via the evaporator accounts for the difference in the energy equation.
    Unfortunately, there is one real problem. The heating capacity of a heat pump decreases as the outdoor temperature falls. The drop in capacity is caused by the lessening amount of refrigerant mass moved through the compressor at one time. The heating capacity is proportional to this mass flow rate: the less the mass of refrigerant being compressed, the less the thermal load it can transfer through the heat-pump cycle. The volume flow rate of refrigerant vapor through the singlespeed rotary compressor used in heat pumps is approximately constant. But cold refrigerant vapor entering a compressor is at lower pressure than warmer vapor. Therefore, the mass of cold refrigerant—and thus the thermal energy it carries is less if the refrigerant vapor were warmer before compression.
    Here, then, lies a genuine drawback of heat pumps: in extremely cold climates heat is needed—heat pumps are least able to supply enough heat. (441 words)
The role of the "flow restriction"(Line 6, Para. 2) in a heat pump is to____
选项 A、measure accurately the flow rate of the refrigerant mass at that point
B、reduce the pressure to bring about the evaporation and cooling of refrigerant
C、stop the refrigerant from further moving
D、reverse the direction of refrigerant flow when needed
答案B
解析 参考文章第二段第五句。
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