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Ronald P. Vallort, P.E
Ron Vallort and Associates
502Forest Mews Drive, Oak Brook, IL, USA
ABSTRACT
Engineers, technicians, and food scientists have continually sought to safeguard food’s wholesomeness. While technology has made significant contributions to this end, it will have to make even greater strides in the future. As the world’s population increases, he need for a worldwide cold chain to preserve the food supply will be crucial.
Through the ages man has sought to remedy the problem of perishable food. From the ancient Egyptians use of evaporative cooling with moistened porous clay vessels placed in the hot, dry breezes to the most common medium-ice. Technology began to look past ice utilizing various mean of mechanical refrigeration: a vapor-compression machine, with sulfuric (ethy1) ether as a refrigerant, a absorption machine used in 1870, until by 1914, almost all meat packing plants in the United States cooled their product with ammonia refrigeration based upon the same basic principles used in current refrigeration systems. These early refrigerants were replaced by the highly efficient.
Chlorofluorocarbons(CFC’s) with most of the earlier refrigerants being retired. But…
When the connection between the depleted ozone layer and CFC’s was identified, CFC’s were replaced. Soon thereafter, another attack on the planet was noted-Global warming due to the ＂Greenhouse Effect.＂ Refrigerants with long atmospheric lifetimes and high numbers of carbon-fluorine bonds contribute to global warming, but their impact is extremely small when compared with the carbon dioxide produced by burning fossil fuels to produce the power to drive the refrigeration units.
Concerns for the environment, energy conservation and food safety now drive the technical community. Refrigeration systems using natural refrigerants such as carbon dioxide and ammonia are once more in demand, even propane is viable refrigerant again. The absorption cycle is again popular for applications in co-generation plants.
Earth friendly refrigerants, increased efficiency and reliability of refrigeration systems and equipment in all phases of the cold chain must continue to be priority among the technical community. Energy efficient systems will benefit the world by reducing the amount of CO2 produced which reduces the potential for climate change; and by reducing the amount of energy consumed forestalling the depletion of the earth’s energy reserves; and reducing the cost of running equipment making refrigeration more manageable for developing countries to further raise their quality of life.
Summary
Engineers, technicians, and food scientists have continually sought to safeguard food’s wholesomeness. While technology has made significant contributions to this end, it will have to make even greater strides in the future. Concerns for the environment, energy conservation and food safety now drive the technical community. The people of the future need: refrigeration equipment that is earth friendly both in emissions and energy consumption, cogeneration applications that will provide additional energy and lower cost refrigerated storage facilities that have both a low initial cost and a technological match with the location’s infrastructure. Engineers, scientists and technicians have a pressing mission ahead of them to advance the arts of refrigeration and HVAC in a way that will benefit both the earth and its inhabitants.
Introduction
The food chain stretches from the fields to the table: starting with snowing, breeding or seeking the product then proceeding through the various steps of harvest, transportation, processing, storage, and finally consumption. It is obvious that most of nature’s largess cannot be kept at ambient conditions and survive the journey to the table. Engineers, technicians, and food scientists have continually sought to discover safer the re effective ways to chill or freeze food, maintain a constant temperature, control the humidity and possibly even modify the atmosphere to safeguard food’s wholesomeness. Storing the world’s perishable food supply is an awesome task that the technical societies and research institutes of the world can advance for the future. As the world’s population increases, the need for a worldwide cold chain to preserve the food supply will be crucial.
The Problem
The problem, as predicted by Thomas Malthus in 1798, was that the population would continue to grow at an exponential rate, while the food supplies would expand merely in a linear fashion. The hunger, disease, and famine foretold by this simple fact did not materialize. Mankind’s ingenuity and technology shaped the quality of life for the men and women of his future. The power of science and technology not only boosted the land’s productivity, but provided the means to effectively process, transport and store crops, meat, eggs, dairy products and other goods once thought too perishable for anything but immediate, proximate use.
Current Conditions
The world’s population has doubled over the last 40 years from three billion in 1960 to six billion in 2000 (IFA 2002). Nevertheless, individual consumption rates, in Kcal/person/day, have risen. Further, the percent of the world’s population deemed to be undernourished has been steadily dropping since the late 1960’s (FAO2000). There is also considerable evidence that food consumption patterns throughout much of the world are becoming increasingly more diverse resulting in more varied and nutritious diets (Dyson, 2000). With the population currently increasing at a rate of 77 million people per year (Haaga,2003 ), mostly in areas where food deficits already exist, the burden will continue to be on technology to ensure that the uninterrupted flow of food items from production to consumption continues.
Historical Perspective
Through the ages man has sought to remedy the problem of perishable food. Ancient Egyptians used evaporative cooling by way of moistened porous clay vessels placed in the shade where the hot, dry breezes would evaporate the moisture. But for centuries, the most common method of food preservation was no more complex than a cool, dark hole in the ground such as a cave or cellar, or for some lucky people an icehouse. These methods were far from effective and reliable. Accordingly, it was not unusual for people to die of “summer complaint”as late as the 1800s due to spoiled food during warm weather.
It was noted that ice was the most efficacious method; consequently ice became a sought after commodity. The ice industry continued to grow; in 1970, approximately 15 million tons of ice were produced in the United States (Krasner-Khait, 2003). By this time, however, ice’s allure was on the wane as it had become a health hazard due to pollution and sanitary runoff. Technology was able to provide a temporary solution to the refrigeration crisis with mechanically manufactured ice.
Refrigeration Technology
It was during this era that technology began to look past the obvious medium –ice. Although the vapor-compression machine, using sulfuric (ethy1) ether as a refrigerant, was invented by Jacob Perkins in the 1830’s, (Perkins, 1834), one of the first commercial applications of mechanical refrigeration was an absorption machine used by S. Liebman’s Sons Brewing Company in Brooklyn, New York in 1870 (Krasner-Khait). In 1992, well before the frozen fish epiphany of Clarence Birdseye, a Sandusky, Ohio processor was freezing fish using mechanical refrigeration. This particular process was known as “sharp freezing ”wherein the fish were deposited on metal plates which in turn rested upon tubes into which refrigerant was directly expanded or brine was circulated (Thevenot, 1979). By 1914, almost all meat packing plants in the United States cooled their product with ammonia refrigeration based upon the same basic principles used in current refrigeration systems.
Let’s fast-forward through the next 75 years or so. The odorous, toxic and corrosive compounds such as sulfur dioxide, carbon tetrachloride, ethy1ether, and methyl chloride were quickly replaced by fluorinated refrigerants first developed by Thomas Midgley Jr. in 1930. commercial production of R-12 began in 1931, follow by R-11 in 1932. other chlorofluorocarbons (CFC’s) were introduced in this period with most of the earlier refrigerants being retired, with the notable exception of R717 (ammonia). The CFC
S were doing a great job. But …the ozone layer was being depleted and CFC’s identified as a source of the chlorine in the stratosphere that caused ozone depletion. The Montreal Protocol (originally signed in 1987 and substantially amended in 1990 and 1992) mandated the phase out of chlorinated and brominated refrigerants.
Soon thereafter, another attack on the planet was noted-Global warming due to the “Greenhouse Effect”.The temperature at the surface of our planet results from an equilibrium, between incoming solar energy and heat back into space. The heat radiated back into space is in the infrared range of emissions therefore, gases that absorb these infrared rays enhance the greenhouse effect. Refrigerants with long atmospheric lifetimes and high numbers of carbon-fluorine bonds contribute to global warming, but their impact is extremely small when compared with the carbon dioxide produced by burning fossil fuels to produce the power to drive the refrigeration units. The Kyoto Protocol of 1997 addressed the reduction of greenhouse gas emissions and signaled the phase out of more refrigerants.
Figure 1: Ozone Depleting Potential(ODP)vs. Global Warming Potential
As seen in figure 1 (Calm and Didion 1997)these two mandates have most of the “new refrigerants”out of contention as the refrigerant of the future. So the refrigeration community is again touting the praises of natural refrigerants: carbon dioxide, ammonia, or even air and water. Similarly, the need to use less power by perfecting more efficient refrigeration systems is foremost in researcher’s minds. Looking at Figure 2 (Calm and Didion 1997)it is easy to note that the energy related GDP emissions are much greater than the refrigerant related GDP.
Figure 2:Total equivalent warming impact for the best available chillers.
“Everything Old is New Again”
“Everything Old is New Again”is the title of a song that seems to fit the present state of refrigeration and refrigeration research. Refrigeration systems using natural refrigerants such as carbon dioxide and ammonia must now earnestly extort as much effective cooling from their systems as possible.
Obvious areas for improvement are: the standard vapor-compression cycle, heat exchangers, frequency controlled fans, adjustable speed drives, automatic air purgers and the use of computers and electronics for system design, monitoring and control. Some of the ways to improve efficiency are so apparent as to sometimes be overlooked: improved operation and maintenance, hot-gas instead of electrical resistance for defrost, use of desiccant dehumidification to supplement conventional cooling systems and better insulation. Efficiency combined with good operation and maintenance cannot be underestimated: a 2,000 square foot restaurant will decrease its power usage by approximately 30 percent annually and not allowing cold items to warm up before loading them into the refrigerator. Even as little as a 1℅ improvement in the energy efficiency of Canada’s installed refrigeration capacity would save 4 million gigajoules (GJ) of energy a year, the equivalent of 650,000 barrels of oil(CanMet 2003)
Other “Mature” Technologies
The Absorption Cycle was invented in 1846 by Ferdinand Carre for the purpose of producing ice using heat input. The absorption cycle enjoyed widespread use from the 1920’s as gas powered refrigerators/ice-makers. Maybe some of you can remember the Servel refrigerators from the 1950’soperated using natural gas.
The cycle is based on the principle that absorbing ammonia in water causes the vapor –pressure to decrease. Absorption cycles produce cooling and/or heating with the use of thermal input and minimal electric input, by using heat and mass exchangers, pumps and valves.
Figure 3:Typical Absorption Cycle
An absorption cycle can be viewed as a mechanical vapor-compression cycle, with the compressor replaced by a generator, absorber and liquid pump. The absorption cycle enjoys the benefits of requiring a fraction of the electrical input, plus uses the natural substances ammonia and water, instead of ozone depleting halocarbons. An absorption cycle can use a variety of working pairs. The working pair is made up of a refrigerant, typically ammonia or water; TriDroxide-water; and alkitrate-water working pairs with specialty applications in industry.
Absorption cycle can operate at high efficiency by utilizing advanced cycles, using generator- absorber heat exchange, and multiple pressures. These cycle use extensive internal heat recovery thereby requiring less prime fuel input to produce the same thermal output. High efficiency operation, plus benefits of environmentally friendly refrigerants, clean-burning fuels, and few moving parts requiring maintenance make absorption a very viable choice; especially where waste heat is available and cooling is required.
Co-generation is interesting aspect of absorption. Exhaust heat from prime movers can be used for the absorption generator; then by capturing the heat given off in the absorber and/or condenser segment(s) additional power can be generated. Rejected heat can also be used for other applications such as hot water heating or supplying the low-level heat to regenerate a desiccant system. A small town in Alaska has solved the problem of creating ice for the summer salmon catch by using a vapor exchange cycle. Waste heat, provided by the jacket cooling water absorption (75℃) from the 2 diesel engines supplying the town’s electricity, is used to produce 10 tons of flake ice per day. The project supported by state and local energy companies has been in operation since 1993.
Figure 4：Waste Heat Fired Icemaker in Kotzebue, AK
More Possibilities
Other technologies can be used in specific niches. Heat pump either air or ground-coupled can be used in different localities where the ambient temperatures and ground characteristics permit. Heat pump and/or under ground thermal energy storage can improve both the cooling and heating without fear of ozone depletion or global warming. Sold energy can be used as a heat source for ice production.
Challenges
Our planet earth has plenty of room for the people of the future. Right now about 90℅ of the earth’s population lives on 10℅ of the land (Rosenberg, 1999). As this idle land is utilized, we will need to refine and design refrigeration and HVAC equipment so that is reliable and earth friendly no matter where it is constructed. e.g. This year on a fact finding tour to, the now independent country, Moldova, I noted a startling situation. Moldova has 95 cold storage facilities however, 85 of them are not operable for one reason or another. The country’s main resources are farm produce and wine reliable and earth friendly no matter where it is constructed. e.g. This year on a fact finding; they desperately need sustainable refrigeration to survive, and to become an economically sound nation.
The population continues to grow at about 1 or 2 percent per year, but our food production increases 3 or 4 percent per year so that more people can gain better nutrition over time. But, according the World Health Report of 2002, the number one cause of death for people around the world is malnourishment. We need to find ways to construct energy efficient, how cost refrigerated transport and storage facilities in countries ravaged by hunger. These cold links in the food chain need not be high tech, but they must be reliable, easy to use, and transportable.
The cold chain is also necessary for the medical needs of the world. Refrigeration is necessary for the transportation of vaccine. According to UNICEF more than 30 million children are unimmunized. Measles, a viral respiratory infection, killed over 770,000 children in 2001, more than any other vaccine-preventable disease. Portable solar refrigerators are used to transport vaccines, but until refrigeration is a reality in the developing nations the young will continue to suffer.
In lesser developed countries, from 10℅ to 50℅ of the crops fail to make it to the first phase of the after-harvest food chain (Saulnier, 2000). Similarly, the waste heat given off by refrigeration and power generating systems dissipates uselessly into space. Further research and development in cogeneration can enable refrigeration, heating and power generating plants to work harmoniously together to provide comfort, refrigeration and power at the lowest cost to developing countries. Creativity must be employed to utilize alternative means for HVAC and refrigeration when the usual means won’t do the job because of locate.
Conclusion
Earth friendly refrigerants, increased efficiency and reliability of refrigeration systems and equipment in all phases of the cold chain must continue to be a priority among the technical community. Energy efficient systems will benefit the world by reducing the amount of CO2 produced which reduces the potential for climate change; and by reducing the amount of energy consumed forestalling the depletion of the earth’s energy reserves; and reducing the cost of running equipment making refrigeration more manageable for developing countries to further raise their quality of life.
The art of refrigeration and HVAC has contributed to the comfort, safety, and health of the world. It has enable people to live in inhospitable climates, explore the reaches of space, and combat diseases throughout the world. We know the world will be a very different place in 50, 100 or 200 years. Will there be any fossil fuels left? Will cold fusion be a reality? Will the United Nations be welcomed to an inter-galactic alliance? We can guarantee a few things: that no child will go hungry, that preventable diseases will be eradicated, that the earth’s atmosphere will be intact, that tropical rain forests will only exist in the proper latitudes, and that people will continue to be comfortable enough to dream of the future.
REFERNCES
1) Calm, James M. and Didion, David A.(1997)Trade-Offs in Refrigerant Selections: Past, Present, and Future. Paper-ASHRAE/NIST Refrigerants Conference, October 1997.
2) CanMet Energy Technology Centre-Ottawa, 2003
3) Dyson, Tim (2000) Rrcent Global Trends in Population and Food . Paper-ESRC Global Environmental Change Programme.
4) Food and Agriculture Organization (FAO) of the United Nations, Economic and social Department. (2000) Food and Environment. Agriculture: Towards 2015/30, Technical Interim Report, April 2000
5) Haaga, John. (2003)UN Projects Slower Population Grown. Population Reference Bureau.
6) Impact-Krasner-Khait, Barbara. (2003) The Impact of Refrigeration. History Magazine (February-March)
7) Newton-Myron, Harold. (2002) Human Life Span. Newton BBS Argonne National Laboratory, Division of Educational Programs. ( )
8) Rosenberg, Matt. (1999) Population Density. About Geography. ( )
9) Saulnier, John M. (2000) Working in common Cause to Serve the Global food Industry. IARW Yearbook.
10) TSW-Tennessee Solid Waste Education Project. (2002) The History of Municipal Solid. ( )
11) Wisconsin Focus on Energy- Public Bulletin 2003. (focusonenergy.com)
未来的制冷
文摘
工程师、技师和食品科学家们不断寻求保障健康的食品。当技术已经做出了重大贡献,为此,就必须要在未来更大的进步。随着世界人口的增加,他需要全球冷链保存食物的供应是很重要的。
古往今来人类试图解决目前的问题易腐食品。从古埃及人利用蒸发冷却多孔粘土容器滋润放在炎热干燥的微风的冰的媒介最常见。技术开始看一下过去的冰利用各种意味着机械制冷:蒸汽压缩机,采用硫酸(ethy1)醚作为一种清凉剂,一个吸收器,直到1870年到1912年,几乎所有的肉类包装工厂在美国氨制冷产品凉快下来的相同的基本原则的基础上,在当前的制冷系统使用。这些早期的制冷剂取代了高效的。
氯氟化碳的大部分的早期制冷剂被退休。但是…
当之间的连接臭氧层耗竭氯氟化碳被代替氯氟化碳。那以后不久,另一个攻击这颗行星被著名的全球变暖由于“温室效应。“制冷剂和较长的大气寿命和大量的氟债券导致全球变暖,但它们的影响是非常小的二氧化碳相比,矿物燃料燃烧产生的生产动力来驱动制冷单位。
对环境的关注、节能和食品安全现在送技术社区。制冷系统采用天然制冷剂二氧化碳及氨更需要一次,甚至丙烷是可行的冷冻了。吸收循环再次流行热电厂应用于植物。
地球友好制冷剂、提高效率和可靠性的制冷系统和设备的所有阶段中冷链必须继续受到的先后顺序技术社区。节能系统将世界受益减少二氧化碳的数量减少了潜在的气候变化问题;以及降低消耗的能量损耗由于地球能量储备;,并降低生产成本运行设备制造制冷更易于处理,为发展中国家,进一步提高他们的生活质量。
摘要
工程师、技师和食品科学家们不断寻求保障健康的食品。当技术已经做出了重大贡献,为此,就必须要在未来更大的进步。对环境的关注、节能和食品安全现在送技术社区。未来的人需要:制冷设备都是地球友好的排放和能耗、热电联产系统的应用,将提供额外的能量和更低的成本冷藏储运设备,都较低的初始购置成本,而技术配合位置的基础设施。工程师、科学家和技术人员已是一项紧迫的任务在他们前面促进暖通空调制冷与艺术,会更有利于地球和它的居民。
介绍
食物链的领域中延伸到桌上:从下雪,养殖和寻求产品然后通过各种各样的步骤进行收割的时候、运输、处理、存储和最后的消费。很明显,自然界的大部分不能保存在外乡人的环境条件和生存旅行到桌子上。工程师、技师和食品科学家们不断地试图发现安全有效的方法来回应寒冷或冻结食物、净化恒温,控制湿度,甚至很有可能修改大气,保障健康的食品。储存世界最易腐烂的食物供应任务是个可怕的技术社会和研究机构,能有效地提高学生的世界的未来。随着世界人口的增加,需要一个世界性的冷链保存食物的供应是很重要的。
这个问题
这个问题,预言,在1798年马尔萨斯的人口将会持续增长速度达到了指数级,而仅仅将扩大的食物供应以线性的方式。饥饿、疾病、饥荒藉这个简单的事实并没有具体化。人类的聪明才智和技术形成了患者的生活质量,男女他的未来。科技的力量不仅提升了土地生产率,但提供有效的手段过程、运输、储存农作物、肉、蛋、奶和其他物品一旦认为太易逝性如果不是迫在眉睫,紧邻的使用。
现状
世界人口过去40年中翻了三个亿2000年1960年到60亿(IFA 2002)。然而,个人消耗速率,在千卡/人/每天,却上升了。另外,占世界人口总数的百分之被视为处于营养不良的状态一直在稳步下降的从1960年以来,FAO2000)。也有相当多的证据显示,食品的消费模式在整个大部份的世界正变得越来越更多元化的造成更多的多样化和营养的饮食(戴森牌,2000年)。随着人口的目前的速度在增加77亿人(Haaga每年,2003年),大部分食品的地区,这个负担已经存在的赤字将会继续被技术,以确保不间断流程从生产到消费的食物还在继续。
历史的角度
古往今来人类试图解决目前的问题易腐食品。古埃及人利用蒸发冷却的多孔粘土容器滋润置于阴凉在热、干燥的微风将水分蒸发。但几个世纪以来,最常见的方法没有食品保存复杂多凉爽阴暗的在地坑如洞穴或的地窖,或对于一些比较幸运的人一个冰屋。这些方法还远没有有效并且可靠的。因此,它是不寻常的人们死于“夏日抱怨:“晚到十九世纪由于变坏的食品在温暖的天气。
指出,冰是最有效的方法,因此冰成为一个受欢迎的商品。冰工业仍然在继续生长;在1970年,大约15万吨冰了在美国(Krasner-Khait,2003)。然而,此时洲际交易所的魅力凋零,因为它已经成为一种危害身体健康由于污染、卫生流失。技术是能提供临时的解决方法制造机械制冷危机冰。
制冷技术
正是在这个时代开始看一下过去,技术-冰明显的介质。虽然蒸汽压缩机,采用硫酸(ethy1)醚作为一种清凉剂,雅各发明在1830年珀金斯,(柏金斯1834年),第一个商业应用机械制冷是吸收器由s . Liebman的儿子酿造有限公司在纽约的布鲁克林1870年(Krasner-Khait)。1992年,在冷冻鱼的主显节,Birdseye劳伦斯俄亥俄州Sandusky处理器是寒冷鱼使用机械冷藏。这个特殊的过程被称为“尖锐的麽冰冷其中鱼是沉积于金属板从而都寄托在管,直接扩张或冷盐水,Thevenot分发了该(1979年)。到1914年,几乎所有的肉类包装工厂在美国氨制冷产品凉快下来的相同的基本原则的基础上,在当前的制冷系统使用。
让我们对未来75年左右。恶臭的,中毒和腐蚀性物质,如二氧化硫四氯化碳乙基醚,和甲基氯氟取代很快就被由托马斯·Midgley制冷剂最早出现在小1930.商业生产R-12开始跟随R-11 1931年,在1932.其他氯氟化碳的时期,介绍了最早期的制冷剂被退休,除了著名的汉(氨)。党
硫干得很好。但是…臭氧层正在枯竭和氯氟化碳源已识别出的氯的臭氧层造成平流层。蒙特利尔议定书签署(原来在1987和本质上在1990和1992年修订的阶段)授权和溴化制冷剂氯。
那以后不久,另一个攻击这颗行星被全球变暖变暖由于“温室效应”。温度在细胞表面的我们的星球之间的平衡,结果进来的太阳能和热回宇宙中。热辐射回空间在红外范围内的气体排放因此,吸收这些红外线提高温室效应。制冷剂和较长的大气寿命和大量的氟债券导致全球变暖,但它们的影响是非常小的二氧化碳相比,矿物燃料燃烧产生的生产动力来驱动制冷单位。1997年的京都议定书对减少温室气体排放,标志着更多的制冷剂淘汰出局。
图1:破坏臭氧潜力(ODP与。全球变暖潜在
曾经出现在图1(平静和Didion 1997)这两个命令有大部分的“新制冷剂“继续冷冻未来。所以制冷社区的赞美是再吹捧自然制冷剂:二氧化碳,氨,甚至空气和水。同样,需要耗电量较少完善更有效率的制冷系统中重要的是研究人员的思想。看着图2(平静和Didion 1997)很容易注意到能源问题关系国内生产总值(GDP)的温室气体排放比冷相关的国内生产总值。
图2:全面等效变暖影响机组的最好的。
“一切旧是新的了”
“一切旧是新的了”的书名是一首歌似乎符合目前的制冷和制冷的研究。制冷系统采用天然制冷剂二氧化碳及氨现在必须认真一样勒索从他们的系统有效的冷却是不可能的。
明显区域了改进:标准蒸汽压缩周期、热交换器、变频控制的球迷,调速驱动、空气自动清洗,长时间使用电脑,以及电子系统设计、监测和控制。一些方法来提高效率非常明显的,有时被忽视:强化操作和维修、气体高温而不是电气电阻使用除霜,干燥剂除湿来补充传统的冷却系统和更好的绝缘材料。效率结合了好的操作和维护:不可低估2000平方英尺的饭馆将会减少用电,每年约有30%不允许冷前的热身项目下载它们放入冰箱。正如小1℅的能源效率改善加拿大的安装制冷能力可以节省400万千焦耳 06一年的能量,相当于65万桶油(CanMet 2003)
其他技术“成熟”
吸收周期在1846年发明了由费迪南德Carre为了生产冰使用热量。吸收周期从1920年享有广泛使用的冰箱/ 冰制造者动力气体。也许你们中有些人可以记住第一台可使用的冰箱从50年代'工作运转使用天然气。
这个循环的原则基础上,吸收氨会使水中产生蒸汽-压力减少。吸收周期产生冷却和/或加热用热输入和最低限度的电输入,利用热质交换机、泵和阀门。
典型的吸收周期图三:
吸收的周期可以被看作是一种机械蒸汽压缩周期,伴随着压缩机被一种发电机、吸收塔和液体泵。吸收周期享受的好处的一小部分需要电输入,再加上采用天然物质氨和水,而是根据破坏臭氧的。吸收循环可以使用多种工作对。工作两人是由冷冻、典型的氨和喝水,TriDroxide-water;alkitrate-water专业应用工作对产业。
吸收周期可以操作的高效利用先进的周期,使用发生器-吸收热量交换和多重压力。这些循环使用大量的内部热回收从而首相要求相对较低燃料输入才能产生同样的热输出。高效率的工作,再加上福利的环保制冷剂、清洁燃烧燃料,一些运动部件需要维护使得吸收很可行的选择,特别是在余热是可得到的,冷却是必需的。
是有趣的方面热电厂吸收。排气热从原动力可用于吸收发生器;然后通过占领热放出吸收体和/或冷凝器段(s)可以产生额外的电源。拒绝了热也可以用于其他的应用,如热水供暖、供应低层热,以重建除湿系统。在阿拉斯加的一个小镇,解决了问题创造冰夏天捉到鲑鱼使用蒸汽交换周期。废水,提供的外衣冷却水吸收(75℃)的2柴油发动机提供镇上的电力,用于生产1000吨的鳞片冰每天。计划赞助国家和地方能源公司自1993年以来已运行。
图四:在冰制造器余热解雇,AK Kotzebue
带来了更多的
其他技术可用于特定位。热泵或空气或压力共轭可适用于不同地区在环境温度和地面特性允许。热泵和/或地下热储能能促进企业的制冷和加热,而不必担心臭氧层破坏或全球变暖。出售可以用来作为一种能量的热源制作冰块。
挑战
我们的地球已经有足够的空间为未来。大约90℅现在地球上的人口居住在10℅的土地(罗森伯格(1999)。当这闲置土地使用,我们都需要改进,并设计制冷与空调设备是可靠和地球友好无论身在何处,它是建筑.例句。这一场比赛中呈现出事实发现旅游,现在独立的国家、摩尔多瓦,我提到了一个令人吃惊的情况。摩尔多瓦有95冷藏设备然而,85都不具有可操作性的由于这样或那样的原因。这个国家的主要资源是农产品和酒可靠、地球友好无论身在何处,它是建筑.例句。今年的事实发现,他们急需的可持续制冷继续活下去,并成为一个经济噪音上的国家。
随着老年人口的大约增长每年1%或2%,但我们的食物产量增加3个或者4个1%的一年,让更多的人可以获得更好的营养。但是,根据2002年世界卫生报告》,排名第一的死亡原因一直是世界各地的人们。我们需要找到方法构建高效节能,怎么成本啊冷藏运输和仓储设施饥饿蹂躏的国家。这些冷漠的链接,在食物链中不需要高科技,但它们必须可靠、使用方便,并可移动。
寒冷的链还需要医疗需求的世界。制冷的运输所需的疫苗。根据联合国儿童基金会超过30万儿童被没有接受免疫麻疹、病毒性呼吸道感染杀死了超过交换机在2001年的孩子,比任何其他可预防疾病。便携式太阳能冰箱是用来运送疫苗,但是直到制冷是一个现实发展中国家年轻的将继续遭殃。
在较小的发达国家,从10℅ 50℅的作物欠收的第一阶段的Saulnier 产量后的粮食供应链,2000年)。同样,余热放出制冷与发电系统地消散到太空中。进一步研究和发展能使热电制冷、供暖和发电植物工作和谐相处、制冷和力量安慰以最低的成本在发展中国家。创意必须采用替代方法利用HVAC和制冷当通常意味着不会做那份工作,因为当地的。
结论
地球友好制冷剂、提高效率和可靠性的制冷系统和设备的所有阶段中冷链必须继续优先考虑技术社群中。节能系统将世界受益减少二氧化碳的数量减少了潜在的气候变化问题;以及降低消耗的能量损耗由于地球能量储备;,并降低生产成本运行设备制造制冷更易于处理,为发展中国家,进一步提高他们的生活质量。
暖通空调制冷、的艺术也导致了这些安慰、安全和健康的世界。它使人们生活在不适合居住的气候条件下,达到空间的探讨,以及战斗疾病等世界各地。我们知道的世界会是一个十分不同的地方,在50 100 - 200个年。会有什么化石燃料留下吗?将冷聚变不是现实吗?将联合国欢迎埋葬银河的联盟的吗? 我们不能保证几件事:“不让一个孩子就要挨饿,可预防的疾病将被根除,地球的大气层将完整的,热带雨林将只存在于适当的纬度,所以人们将继续自在地空想着未来。
REFERNCES
1)冷静,詹姆斯·m .和Didion,大卫答:(1997)必须有所取舍冷冻选择:过去、现在和将来。Paper-ASHRAE /美国国家标准与技术研究院制冷剂发布会上,1997年10月。
2)Centre-Ottawa canmet能源技术,2003年
3)戴森牌,提姆(2000)Rrcent全球趋势在人口和食物。Paper-ESRC全球环境改变计划。
4)粮食及农业组织(FAO)联合国、经济和社会的部门。(2000) 食品和环境。农业:对2015/30、技术的临时报告2000年6月9日
5)haaga,约翰。 (2003) 联合国项目减缓人口增长。人口参考局。
6)impact-krasner-khait,芭芭拉。 (2003) 制冷的影响。历史杂志(February-March)
7)newton-myron,哈罗德。 (2002) 人类的寿命。牛顿BBS划分伊利诺伊州阿格涅国家实验室的教育活动。()
8)罗森伯格,马特。(1999) 人口密度。地理。()
9)saulnier,约翰米。 (2000) 工作在普通的原因为全球食品行业。IARW年鉴》。
10)tsw-tennessee固体废物教育项目。 (2002) 城市生活垃圾的历史。()
11)威斯康辛州的重点集中在能源,公众公告2003年。(focusonenergy.com)
示例二：电子专业外文翻译
Sinusoidal oscillator
Definition
Sine wave oscillator is not able to automatically control the input signal is converted to direct current specific frequency and amplitude of the sinusoidal alternating voltage (current) circuit.It consists of four components: amplifier, frequency selective network, feedback network and the steady increase circuit.Commonly used sine wave oscillator capacitor and inductor feedback oscillator feedback oscillator two.The latter output power is small, low frequency; while the former can be output power, frequency is higher.
Categories
Sinusoidal oscillator can be divided into two categories: one is the principle of using the feedback form feedback oscillator, which is one of the most widely used class of oscillator; the other is the negative resistance oscillator, it will be a direct negative impedance elementsconnected to the resonant circuit, the use of negative resistance device to offset the negative impedance effect of the loss of the loop, resulting in a sine wave oscillation.
Applications
Sinusoidal oscillator is widely used in various electronic devices.Such applications, the request of the oscillator is the oscillation frequency and oscillation amplitude accuracy and stability.Another use of sine wave oscillator as a high frequency heating equipment and medical electrical stimulation devices in sinusoidal alternating energy.Such applications, requests for the main oscillator is large enough to efficiently generate the sinusoidal alternating power, while the oscillation frequency accuracy and stability requirements are generally not as demanding.Edit this paragraph feedback oscillatorBlock diagram of feedback oscillator
Principle Analysis
Feedback oscillator amplifier and feedback network is composed of a closed loop.Amplifier and feedback network which consists of two parts.A frequency selective amplifier is usually the network (such as the oscillation circuit) as the load is a tuned amplifier; feedback networks are generally composed of passive components by linear network.Start ------> ------> stable oscillation with non-linear process
Equilibrium
Hutchison closed-loop voltage amplification factor Ku (s), open-loop voltage amplification factor K (s), voltage feedback factor F (s), loop gain T (s), the feedback factor F '(jω) =- F (jω).The condition is self-oscillation loop gain is 1, ie T (jω) = K (jω) F (jω) = 1, often called the equilibrium condition of the oscillator.Oscillator amplitude equilibrium conditions can be divided into equilibrium (| T (jω) | = 1) and phase equilibrium conditions (ψ (T) = ψ (K) + ψ (F) = ± 2nπ, n = 0,1,2, ....)Worthy of note are: 1. When | T (jω) |> 1, increases the formation of oscillation circuit; as T | (jω) | <1, the formation of reduction in oscillation.2. Equilibrium energy equal to the loop power supply energy consumption; 3. The usual loop only just met at a particular phase conditions.
Starting conditions
Oscillation in order to increase the output amplitude should be returned to the feedback signal to the amplifier input signal than the big oscillations should begin to increase the oscillation, that is, T > 1, called the self-oscillation of the starting conditions.Corresponding with the equilibrium condition, oscillator start-up conditions can be divided into the amplitude of vibration from the conditions of (| T (jω) |> 1) and phase conditions (ψ (T) = ψ (K) + ψ (F)+ ψ (F ') = ± 2nπ, n = 0,1,2 ...), which is the onset of the phase condition of positive feedback conditions.
Stability condition
Oscillator stability conditions corresponding stability conditions can be divided into amplitude and phase stability conditions.(1) For stable conditions the amplitude of the amplitude stability of oscillators in the equilibrium point must have the ability to prevent the amplitude.Specifically, it is the equilibrium point, so that when the instability amplitude increases, the loop gain will be reduced, so that the amplitude decreases.(2) Similarly, the phase stability conditions, to make the phase stability of the oscillator in its equilibrium phase change must have the ability to stop.
Frequency stability
Oscillator frequency stability is due to changes in external conditions, causing the actual operating frequency of the oscillator degree of deviation from the nominal frequency, it is a very important oscillator indicators.Frequency stability can be divided into: long-term frequency stability (generally refers to more than one day even a few months of relative changes in the frequency interval), short-term frequency stability (generally refers to one days, in hours, minutes, or seconds in mindtime interval relative changes in frequency) and instantaneous frequency stability (generally refers to the time interval of seconds or milliseconds relative change in frequency).Generally called short-term frequency stability is the stability.General shortwave, FM transmitter frequency stability of 10-4 ~ 10-5, television transmitters in the frequency stability of about 5 × 10-7.Improve the frequency stability of the measures are: 1. To improve the standard of the oscillation circuit (refer to the standard circuit components and capacitors, temperature is the main factors) 2. To reduce the impact of the transistor 3. Improve the circuit quality factor 4. Reduce the power, load and other effects
The basic principles of LC oscillator
Basic LC oscillator circuit, known as three-terminal type (also known as three-point) of the oscillator, the LC circuit of the three endpoints of the three electrodes are connected to the transistor from the circuit.According to the nature of the resonant circuit, resonance was purely resistive circuit should be, so the three reactive elements can not be the same character components.In general, a high-Q circuit, the loop current is much larger than the transistor base current İb, the collector current and emitter current İc İe.
LC oscillator design
The principle can be seen by the oscillator, the oscillator is a feedback of the actual nonlinear system, it is very difficult to calculate accurately, but also unnecessary.Therefore, the oscillator design is usually a series of design considerations and approximate estimates, choose the right line and operating point, determine the component values, and work status and the exact number of components needed in the adjustment, commissioning fully finalized.First, select the LC oscillator oscillator circuit generally work in a few hundred kHz to several hundreds of megabytes Hz range.Oscillator circuits primarily for the frequency range and band width to choose.In the short-wave range: feedback oscillator inductor, capacitor feedback oscillator can be used.If the requirements of a wide output frequency adjustment range: Select the inductor feedback oscillator; if the requirements of high frequency: often used carats splash, Syracuse circuit.In the medium and short wave radio, a transformer in order to simplify the feedback oscillator circuit to do the local oscillator.Second, the transistor selected from the perspective of frequency stabilization, high fT transistors should be chosen so that the internal phase shift transistors smaller.Usually choose fT> (3 ~ 10) f1max.Also hope that the current amplification factor β bigger, it is easy oscillations, but also easy to reduce the coupling between the transistors and circuits III, the choice of the DC feed lines to ensure the amplitude of the oscillator start-up conditions, the initial operating point should be set up onlineof the enlarged area;starting from the frequency stabilized, steady state should be the cut-off area, and not in the saturated zone (because of saturation of the output impedance is small), or loop-load quality factor QL will be reduced.Therefore, the transistor should normally be set in a small static bias point of the current area, the circuit should be self-bias.Fourth, oscillation frequency stabilization circuit component selection starting from the oscillation loop capacitor C should be as large as possible, but C is too large, not conducive to the band work; inductance L should be as large as possible, but after L large, bulky, large distributed capacitance, L too small, the circuit quality factor is too small and should therefore be a reasonable choice of loop C, L.In the short-wave range, C generally take tens to hundreds of picofarads, L generally take 0.1 to tens of micro-Henry.Fifth, the feedback loop component selection by the above we can see, in order to ensure a certain degree of stability of the oscillator amplitude and easy start-up, usually in a static operating point should be chosen: Y (f) R (L) F '= 3 ~ 5 when the quiescent pointdetermined, Y (f) the value to be, for low-power transistors can be approximated as: Y (f) = g (m) the size of the feedback factor should be selected in the following range: 0.1 ~ 0.5
In all walks of life test applications, the signal source plays an extremely important role. But with many different types of sources, different types of sources in function and characteristics respectively on each are not identical, be applied to many different applications. At present, the most common source types including arbitrary waveform generator, function generator, RF signal source and basic analog output module. Used in signal DDS technology in the current test measures the industry has gradually called a mainstream approach. DDS is A digital frequency synthesizer, by phase accumulators, waveform ROM, D/A converter and low-pass filter composition. After the clock frequency given the signal frequency, output depends on frequency control word, the frequency resolution depends on accumulators digits, phase resolution take ?Waveform generator is a common source, widely used in electronic circuits, automatic control system and teaching experiment, etc. This course design use the generator AT89S51 constitute produces sawtooth wave, triangle wave, sine wave, waveform of the cycle can use the program change, and can choose according to need single polarity output or dual polarity output, has the line is simple, compact structure, etc. In this design basis, plus button control and LED display, can be set by button needed waveform frequency and LED display on the frequency, amplitude voltage, waveform usable oscilloscope display.
SCM automatically endow it task process, is also the microcontroller program execution process, namely a process of the execution of an instruction is required, the so-called instructions executed with various operations microcontroller write down the order form, this is the design personnel giving it determined the command system, an instruction corresponds to a fundamental operation; SCM can all the instructions executed, is the single-chip microcomputer instruction system, different kinds of microcontroller, its instruction system is also different. In order to make SCM can automatically accomplish a specific task, must take to solve problem into a series of instructions (these instructions must be selected microcontroller can identify and executed instructions), this series of set of instructions will become program that need to be stored ?
Program is usually order, so the procedure in the execution of a sequence of instructions is deposited during the execution procedures, microcontroller can these instructions to be executed extract and ionization, must have a component can track instruction location, the components that are contained in the program counter PC (CPU), in start executing the program, to PC attached to program in the first instruction obtained address, then to perform every one of the content in order, the PC will automatically increase, amount of this directive length, possibly 1, decided 2 or 3 to point to the starting address of the next instruction, ensure instruction sequence execution.
Introduced the single chip computer Microcontroller as is typical of MCU embedded Microcontroller controller (Microcontroller Unit), common English letters is the abbreviation of MCU single-chip microcomputer, it said was first used in industrial control field. By chip microcontroller only CPU dedicated processor within. The earliest design concept is through a CPU and peripheral devices will be integrated in a chip, make the computer system smaller, more easily integrated into complex and to volume demanding control equipment of. Intel-based Z80 is the earliest designed according to this kind of thought the processor, henceforth, SCM and dedicated processor development parted.
Early microcontroller is 8 bits or four. One of the most successful is because intel-based 8031, simple, reliable and performance good got a lot of praise. Then developed in the microcomputer 8031 singlechip system MCS51 series. Based on this system in SCM system until now still widely used. With the improvement of the industrial control field requirement, begin to emerge the 16-bit single chip, but because the price is not ideal did not get the very widely. After the 1990s as consumer electronics products development, microcontroller technology got great improvement. With i960 series especially INTEL later ARM series widely, 32-bit SCM 16-bit single chip rapidly replacing the high-end status, and into the mainstream market. While the traditional 8-bit microcontroller performance also obtained the rapid increase, processing power than in the 1980s. ?
Single chip microcontroller and said, it is not complete micro controller one logical functions chip, but put a computer system integration to a chip. The equivalent of a miniature computers, and computers, microcontroller lack only compared the I/O devices. General speaking: a chip became a computer. Its small volume, light quality, price cheap, for learning, application and development provides a convenient conditions. Meanwhile, learn to use the MCU is knowledge of computer principle and structure of the best choice.
Single-chip microcomputer and computer functions with interior also similar module, such as CPU, memory, and hard drive, and parallel bus same storage device, the role of different is it these components performance all relative our home computer, but the price is also a lot weaker low, usually less than 10 yuan can... Use it to do some control electric appliance category is not very complicated work is sufficient. We now have roller washing machine, smoke exhaust hood, VCD and so on household appliance inside can see the figure! ... It is used as the core component of the control part.
Single-chip microcomputer chip
The single chip computer is running on program, and can be modified. Through the different program realization of different function, especially special unique some function, this is another device needs to take a lot of energy to do it, some are great effort also very hard to do. A not very complicated function if use the development of the United States during the 1950s, or the 1960s 74 series CD4000 series of pure hardware to fix these words, circuit must be a big PCB board! But if if use American 1970s success into the market series microcontroller, and the results will be difference! Just because MCU through your programming can achieve high intelligence, high efficiency and high reliability.
Due to cost is sensitive microcontroller, so right now, the dominant software or minimum level assembly language, it is besides the binary machine code above the lowest language, since such a low-level why use? Many senior language has reached the level of visual programming is why not? The reason is very simple, that is no home computer that microcontroller as the CPU, no hard drive mass storage device like that. A visual small programs written in high-level languages in only one button, and even the size will reach the dozens of K! For household PC hard disk speaking nothing, but for single-chip microcomputer speaking is unacceptable. Microcomputer in the hardware resources utilization must high to just go, so assembly although original but ?
正弦波振荡器
正弦波振荡器是指不需要输入信号控制就能自动地将直流电转换为特定频率和振幅的正弦交变电压（电流）的电路。它由四部分组成：放大电路，选频网络，反馈网络和稳幅电路。常用的正弦波振荡器有电容反馈振荡器和电感反馈振荡器两种。后者输出功率小，频率较低；而前者可以输出大功率，频率也较高。
正弦波振荡器可分为两大类：一类是利用反馈原理构成的反馈振荡器，它是目前应用最广的一类振荡器；另一类是负阻振荡器，它将负阻抗元件直接连接到谐振回路中，利用负阻器件的负阻抗效应去抵消回路中的损耗，从而产生出正弦波振荡。
应用
正弦波振荡器广泛用于各种电子设备中。此类应用中，对振荡器提出的要求是振荡频率和振荡振幅的准确性和稳定性。正弦波振荡器的另一类用途是作为高频加热设备和医用电疗仪器中的正弦交变能源。这类应用中，对振荡器提出的要求主要是高效率地产生足够大的正弦交变功率，而对振荡频率的准确性和稳定性的要求一般不作苛求。
反馈型振荡器是由放大器和反馈网络组成的一个闭合环路。它由放大器和反馈网络两大部分组成。放大器通常以某种选频网络（如振荡回路）作负载, 是一种调谐放大器；反馈网络一般是由无源器件组成的线性网络。起振------>非线性过程------>稳幅振荡 记 闭环电压放大倍数 Ku(s)，开环电压放大倍数 K(s)，电压反馈系数 F(s)，环路增益 T(s)，反馈系数 F′(jω)=-F(jω)。 自激振荡的条件就是环路增益为1，即T(jω)=K(jω)F(jω)=1，通常又称为振荡器的平衡条件。振荡器的平衡条件又可细分为振幅平衡条件（|T(jω)|=1）和相位平衡条件（ψ(T)=ψ(K)+ψ(F)=±2nπ, n=0,1,2…）。值得说明的是：1. 当|T(jω)|>1，形成增幅电路振荡；当T|(jω)|<1时，形成减幅振荡。2. 平衡时电源供给的能量等于环路消耗的能量；3. 通常的环路只在某一特定才满足相
为使振荡过程中输出幅度不断增加，应使反馈回来的信号比输入到放大器的信号大，即振荡开始时应为增幅振荡，即T(jω)>1，称为自激振荡的起振条件。与平衡条件相应的，振荡器的起振条件又可细分为起振的振幅条件（|T(jω)|>1）和相位条件（ψ(T)=ψ(K)+ψ(F)+ψ(F')=±2nπ, n=0,1,2…），其中起振的相位条件即为正反馈条件。
稳定条件
振荡器的稳定条件相应地可分为振幅稳定条件和相位稳定条件。 (1) 振幅稳定条件要使振幅稳定，振荡器在其平衡点必须具有阻止振幅变化的能力。具体来说，就是在平衡点附近，当不稳定因素使振幅增大时，环路增益将减小，从而使振幅减小。相位稳定条件。同理，要使相位稳定，振荡器在其平衡点必须具有阻止相位变化的能力。
频率稳定度
振荡器的频率稳定度是指由于外界条件的变化, 引起振荡器的实际工作频率偏离标称频率的程度, 它是振荡器的一个很重要的指标。频率稳定度又可分为：长期频率稳定度（一般是指一天以上甚至几个月的时间间隔内频率的相对变化）、短期频率稳定度（一般是指一天以内，以小时、分钟或秒记的时间间隔内频率的相对变化）和瞬时频率稳定度（一般是指秒或毫秒的时间间隔内频率的相对变化）。一般所说的频率稳定度是指短期稳定度。一般短波、超短波发射机的的频率稳定度为10-4～10-5，电视发射台的频率稳定度为5×10-7左右。提高频率稳定度的措施有：提高振荡回路的标准性（指回路元件和电容的标准性，温度是影响的主要因素）减少晶体管的影响、提高回路的品质因数。 减少电源、负载等的影响
LC 振荡器的基本原则
LC振荡基本电路，就是通常所说的三端式(又称三点式)的振荡器，即LC回路的三个端点与晶体管的三个电极分别连接而成的电路。根据谐振回路的性质, 谐振时回路应呈纯电阻性，因此三个电抗元件不能是同性质元件。一般情况下，回路Q值很高，因此回路电流远大于晶体管的基极电流İb 、集电极电流İc以及发射极电流İe。
LC振荡器的设计方法
由振荡器的原理可以看出，振荡器实际为一个具有反馈的非线性系统，要精确计算是很困难的，而且也不必要。因此，振荡器的设计通常是进行一系列设计考虑和近似估算，选择合理的线路和工作点，确定元件的数值，而工作状态和元件的准确数字需要在调整、调试综最后确定。一、振荡器电路选LC振荡器一般工作在几百千赫兹至几百兆赫兹范围。振荡器线路主要根据工作的频率范围及波段宽度来选择。在短波范围：电感反馈振荡器、电容反馈振荡器都可以采用。 若要求输出频率调节范围较宽：选择电感反馈振荡器；若要求频率较高：常采用克拉泼、西勒电路。在中、短波收音机中,为简化电路常用变压器反馈振荡器做本地振荡器。二、晶体管选择从稳频的角度出发,应选择fT较高的晶体管,这样晶体管内部相移较小。通常选择fT >(3～10)f1max。同时希望电流放大系数β大些,这既容易振荡,也便于减小晶体管和回路之间的耦合三、直流馈电线路的选择为保证振荡器起振的振幅条件,起始工作点应设置在线性放大区;从稳频出发,稳定状态应在截止区,而不应在饱和区（因为饱和区的输出阻抗较小）,否则回路的有载品质因数QL将降低。所以,通常应将晶体管的静态偏置点设置在小电流区,电路应采用自偏压。四、振荡回路元件选择从稳频出发，振荡回路中电容C应尽可能大，但C过大，不利于波段工作；电感L也应尽可能大，但L大后，体积大，分布电容大，L过小，回路的品质因数过小，因此应合理地选择回路的C、L。在短波范围，C一般取几十至几百皮法，L一般取0.1至几十微亨。五、反馈回路元件选择由前述可知,为了保证振荡器有一定的稳定振幅以及容易起振,在静态工作点通常应选择：Y(f)R(L)F'=3~5 当静态工作点确定后,Y(f)的值就一定,对于小功率晶体管可以近似为：Y(f)=g(m) 反馈系数的大小应在下列范围选择：0.1~0.5
单片机自动完成赋予它的任务的过程，也就是单片机执行程序的过程，即一条条执行的指令的过程，所谓指令就是把要求单片机执行的各种操作用的命令的形式写下来，这是在设计人员赋予它的指令系统所决定的，一条指令对应着一种基本操作；单片机所能执行的全部指令，就是该单片机的指令系统，不同种类的单片机，其指令系统亦不同。为使单片机能自动完成某一特定任务，必须把要解决的问题编成一系列指令（这些指令必须是选定单片机能识别和执行的指令），这一系列指令的集合就成为程序，程序需要预先存放在具有存储功能的部件——存储器中。存储器由许多存储单元（最小的存储单位）组成，就像大楼房有许多房间组成一样，指令就存放在这些单元里，单元里的指令取出并执行就像大楼房的每个房间的被分配到了唯一一个房间号一样，每一个存储单元也必须被分配到唯一的地址号，该地址号称为存储单元的地址，这样只要知道了存储单元的地址，就可以找到这个存储单元，其中存储的指令就可以被取出，然后再被执行。
程序通常是顺序执行的，所以程序中的指令也是一条条顺序存放的，单片机在执行程序时要能把这些指令一条条取出并加以执行，必须有一个部件能追踪指令所在的地址，这一部件就是程序计数器PC（包含在CPU中），在开始执行程序时，给PC赋以程序中第一条指令所在的地址，然后取得每一条要执行的命令，PC在中的内容就会自动增加，增加量由本条指令长度决定，可能是1、2或3，以指向下一条指令的起始地址，保证指令顺序执行。
单片机介绍
单片微型计算机简称单片机，是典型的嵌入式微控制器（Microcontroller Unit），常用英文字母的缩写MCU表示单片机，它最早是被用在工业控制领域。单片机由芯片内仅有CPU的专用处理器发展而来。最早的设计理念是通过将大量外围设备和CPU集成在一个芯片中，使计算机系统更小，更容易集成进复杂的而对体积要求严格的控制设备当中。INTEL的Z80是最早按照这种思想设计出的处理器，从此以后，单片机和专用处理器的发展便分道扬镳。
早期的单片机都是8位或4位的。其中最成功的是INTEL的8031，因为简单可靠而性能不错获得了很大的好评。此后在8031上发展出了MCS51系列单片机系统。基于这一系统的单片机系统直到现在还在广泛使用。随着工业控制领域要求的提高，开始出现了16位单片机，但因为性价比不理想并未得到很广泛的应用。90年代后随着消费电子产品大发展，单片机技术得到了巨大提高。随着INTEL i960系列特别是后来的ARM系列的广泛应用，32位单片机迅速取代16位单片机的高端地位，并且进入主流市场。而传统的8位单片机的性能也得到了飞速提高，处理能力比起80年代提高了数百倍。目前，高端的32位单片机主频已经超过300MHz，性能直追90年代中期的专用处理器，而普通的型号出厂价格跌落至1美元，最高端[1]的型号也只有10美元。当代单片机系统已经不再只在裸机环境下开发和使用，大量专用的嵌入式操作系统被广泛应用在全系列的单片机上。而在作为掌上电脑和手机核心处理的高端单片机甚至可以直接使用专用的Windows和Linux操作系统。
单片机比专用处理器更适合应用于嵌入式系统，因此它得到了最多的应用。事实上单片机是世界上数量最多的计算机。现代人类生活中所用的几乎每件电子和机械产品中都会集成有单片机。手机、电话、计算器、家用电器、电子玩具、掌上电脑以及鼠标等电脑配件中都配有1-2部单片机。而个人电脑中也会有为数不少的单片机在工作。汽车上一般配备40多部单片机，复杂的工业控制系统上甚至可能有数百台单片机在同时工作！单片机的数量不仅远超过PC机和其他计算的总和，甚至比人类的数量还要多。
单片机又称单片微控制器,它不是完成某一个逻辑功能的芯片,而是把一个计算机系统集成到一个芯片上。相当于一个微型的计算机，和计算机相比，单片机只缺少了I/O设备。概括的讲：一块芯片就成了一台计算机。它的体积小、质量轻、价格便宜、为学习、应用和开发提供了便利条件。同时，学习使用单片机是了解计算机原理与结构的最佳选择。
单片机内部也用和电脑功能类似的模块，比如CPU，内存，并行总线，还有和硬盘作用相同的存储器件，不同的是它的这些部件性能都相对我们的家用电脑弱很多，不过价钱也是低的，一般不超过10元即可......用它来做一些控制电器一类不是很复杂的工作足矣了。我们现在用的全自动滚筒洗衣机、排烟罩、VCD等等的家电里面都可以看到它的身影！......它主要是作为控制部分的核心部件。
它是一种在线式实时控制计算机，在线式就是现场控制，需要的是有较强的抗干扰能力，较低的成本，这也是和离线式计算机的（比如家用PC）的主要区别。
单片机是靠程序运行的，并且可以修改。通过不同的程序实现不同的功能，尤其是特殊的独特的一些功能，这是别的器件需要费很大力气才能做到的，有些则是花大力气也很难做到的。一个不是很复杂的功能要是用美国50年代开发的74系列，或者60年代的CD4000系列这些纯硬件来搞定的话，电路一定是一块大PCB板！但是如果要是用美国70年代成功投放市场的系列单片机，结果就会有天壤之别！只因为单片机的通过你编写的程序可以实现高智能，高效率，以及高可靠性！
由于单片机对成本是敏感的，所以目前占统治地位的软件还是最低级汇编语言，它是除了二进制机器码以上最低级的语言了，既然这么低级为什么还要用呢？很多高级的语言已经达到了可视化编程的水平为什么不用呢？原因很简单，就是单片机没有家用计算机那样的CPU，也没有像硬盘那样的海量存储设备。一个可视化高级语言编写的小程序里面即使只有一个按钮，也会达到几十K的尺寸！对于家用PC的硬盘来讲没什么，可是对于单片机来讲是不能接受的。 单片机在硬件资源方面的利用率必须很高才行，所以汇编虽然原始却还是在大量使用。一样的道理，如果把巨型计算机上的操作系统和应用软件拿到家用PC上来运行，家用PC的也是承受不了的。
