首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
考研
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
admin
2009-04-27
26
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 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 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 or 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.
选项
A、explain the differences in the working of a heat pump when the outdoor temperature changes
B、contrast the heating and the cooling modes of heat pumps
C、describe heat pumps, their use, and factors affecting their use
D、advocate the more widespread use of heat pumps
答案
C
解析
本文主要叙述空调系统中的热泵的原理、使用和问题。
转载请注明原文地址:https://kaotiyun.com/show/Bta4777K
0
考研英语一
相关试题推荐
Globalwarmingcausedbyhumanemissionsofgreenhousegasesishavingcleareffectsinthephysicalworld:moreheatwaves,hea
UTOPIANISMinpoliticsgetsabadpress.Thecaseagainstthegrand-scale,state-directedkindiswellknownandoverwhelming.U
Aneweconomicspaperhassomeold-fashionedadviceforpeoplenavigatingthestressesoflife:Findaspousewhoisalsoyourb
Strategyandcultureareamongtheprimaryleversattopleaders’disposalintheirnever-endingquesttomaintainorganizationa
Readthefollowingtextandanswerquestionsbyfindinginformationfromtherightcolumnthatcorrespondstoeachofthemarked
ReadthefollowingtextandanswerthequestionsbychoosingthemostsuitablesubheadingfromthelistA-Gforeachnumberedpa
InSeptember1919,theyearaftertheendofWorldWarI,aGermancaptainnamedKarlMayr,whoranapropagandaunitincharge
Readthefollowingtextandanswerquestionsbyfindinginformationfromtherightcolumnthatcorrespondstoeachofthemarked
SupposeyouwanttoapplyforanEnglisheditorprovidedbyanewspaper.Writeanemailtothepersonincharge,JohnSmithto
Shortstoriesareduearevival.Inrecentyears,therehavebeencritically【C1】________collectionsbyAmericanwriterssuchas
随机试题
对于双色调模式的图像可以设定单色调、双色调、三色调和四色调,在通道调板中,它们分别包含几个通道?( )
下列哪一个试验不属于沉淀反应
不属于水溶性维生素特点的是()。
慢性盘状红斑狼疮毛细血管病理改变不包括
你一到长沙就给我打个电话好吗?
劳动教养戒毒工作的对象为经过公安强制戒毒后,又吸食、注射毒品成瘾,被劳动教养机关依法决定劳动教养的人员。()
食盐长征
在窗体上画一个命令按钮,然后编写如下事件过程:PrivateSubCommand1_Click()DimI,NumRandomizeDoForI=1To100
Theworld’sgreatestsnow-cappedpeaks,whichruninachainfromtheHimalayastotheTianshanMountainsontheborderofChina
Whatistherelationshipbetweenthemanandwoman?
最新回复
(
0
)