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
60
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
LONDON—SecurityexpertsarewarningthattheglobalcyberattackthatbeganonFridayislikelytobemagnifiedinthenewworkwe
LONDON—SecurityexpertsarewarningthattheglobalcyberattackthatbeganonFridayislikelytobemagnifiedinthenewworkwe
Joyandsadnessareexperiencedbypeopleinallculturesaroundtheworld,buthowcanwetellwhenotherpeoplearehappyord
Runningmaybethesinglemosteffectiveexercisetoincreaselifeexpectancy,accordingtoanewreviewandanalysisofpastre
Writeanessaybasedonthefollowingbarchart.Inyourwriting,youshould1)interpretthechart,and2)giveyourcomm
Strategyandcultureareamongtheprimaryleversattopleaders’disposalintheirnever-endingquesttomaintainorganizationa
InSeptember1919,theyearaftertheendofWorldWarI,aGermancaptainnamedKarlMayr,whoranapropagandaunitincharge
FrankSinatraknewhewasgettingarawdeal.Hecouldsingbuthewasnotmuchofasongwriter,soheneversawacentwhenmos
Thedeclineofcivilityandgoodmannersmaybeworryingpeoplemorethancrime,accordingtoGentilityRecalled,editedbyDigb
Itmaybesummertime,butthelivingisn’teasy—notforwearyworkerswhoselastvacationisadistantmemory.Accordingtoone
随机试题
青少年好发的肿瘤为()。
Farmersareallowedtogrowsmallgardensoftheirownandtheyselltheirvegetables______theblackmarket.
如果取精液检查,应在检查前至少几天内不排精。
华支睾吸虫对人的危害主要是
关于胰岛素治疗,下列不妥的是下列哪一部位不可注射胰岛素
治疗成人呼吸窘迫综合征最有效的措施为()
《中华人民共和国广告法》规定,药品、医疗器械广告不得有的内容是()
设齐次线性方程组当方程组有非零解时,k值为:
某工业企业仅生产甲产品,采用品种法计算产品成本。3月初在产品直接材料成本130万元,直接人工成本18万元,制造费用10万元。3月份发生直接材料成本80万元,直接人工成本4871元,制造费用6万元。3月末甲产品完工100件,在产品200件。月末计算完工产品成
Translatingisacomplexandfascinatingtask.Infact,A.Richardshasclaimedthatitisprobablythemostcomplextypeofeve
最新回复
(
0
)