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标题: 阅读NO题求指点！！！ [打印本页]
作者: tv加菲    时间: 2012-4-10 09:55
标题: 阅读NO题求指点！！！
原文：
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 vapor 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 vapor 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 of 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 single-speed 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 than if the refrigerant vapor were warmer before compression.
Here, then, lies a genuine drawback of heat pumps: in extremely cold climates—where the most heat is needed—heat pumps are least able to supply enough heat.


19.    It can be inferred from the passage that, in the course of a heating season, the heating capacity of a heat pump is greatest when
(A) heating is least essential
(B) electricity rates are lowest
(C) its compressor runs the fastest
(D) outdoor temperatures hold steady
(E) the heating demand surges

21.    The passage suggests that heat pumps would be used more widely if
(A) they could also be used as air conditioners
(B) they could be moved around to supply heat where it is most needed
(C) their heat output could be thermostatically controlled
(D) models with truly superior cooling capacity were advertised more effectively
(E) people appreciated the role of the evaporator in the energy equation
想不明白啊不明白，
答案是A和E，，，求指点！
作者: 竹林中人    时间: 2012-4-12 13:07
顶上去，做阅读的同志帮个忙啊
作者: songxiaoshi    时间: 2012-4-14 22:33
21题
The heating capacity of a heat pump decreases as the outdoor temperature falls这句说这个capacity随着温度的降低而降低，后面解释了一大堆它为什么降低。大概就是Mass 和这个热能转换有关，虽然V（体积）是一定的，但是P（压强）温度低的时候小了，所以M小了。Therefore, the mass of cold refrigerant—and thus the thermal energy it carries—is less than if the refrigerant vapor were warmer before compression.
也就是说温度低的时候M小，导致thermal energy it carries 少，导致capacity小。
合理推断，反之，温度高的时候，capacity大。
19题问什么时候capacity最大，就是温度最高的时候呗。而温度最高的时候恰巧根本不需要heat pump，所以选A
21题
第一段第一句说人们对heat pump是skeptical的，因为它好像违法了energy守恒。
第二段最后一句说其实没有违反能量守恒，因为evaporator填补了缺少的能量。
推理就是，如果人们知道evaporator就不怀疑heat pump 违法能量守恒了，这样heat pump就会更widely 的使用了。

以上仅个人意见。。。仅供参考。我也在做No题，有空可以一起讨论。
作者: tv加菲    时间: 2012-4-18 22:16
21题
The heating capacity of a heat pump decreases as the outdoor temperature falls这句说这个capacity随着温度的降低而降低，后面解释了一大堆它为什么降低。大概就是Mass 和这个热能转换有关，虽然V（体积）是一定的，但是P（压强）温度低的时候小了，所以M小了。Therefore, the mass of cold refrigerant—and thus the thermal energy it carries—is less than if the refrigerant vapor were warmer before compression.
也就是说温度低的时候M小，导致thermal energy it carries 少，导致capacity小。
合理推断，反之，温度高的时候，capacity大。
19题问什么时候capacity最大，就是温度最高的时候呗。而温度最高的时候恰巧根本不需要heat pump，所以选A
21题
第一段第一句说人们对heat pump是skeptical的，因为它好像违法了energy守恒。
第二段最后一句说其实没有违反能量守恒，因为evaporator填补了缺少的能量。
推理就是，如果人们知道evaporator就不怀疑heat pump 违法能量守恒了，这样heat pump就会更widely 的使用了。

以上仅个人意见。。。仅供参考。我也在做No题，有空可以一起讨论。
-- by 会员 songxiaoshi (2012/4/14 22:33:06)



Soga~~谢谢~~~



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