制冷原理双语版第十一章.doc

Chapter 11 Condensing Equipment第11章 冷凝设备11-1 Function and Classification of Condensers 李增扬11-1 冷凝器的作用和分类1. Function of Condensers1. 冷凝器的功能A condenser is a heat exchanger which condenses a refrigerant from its vapor to its liquid state. In so doing, the latent heat is given up by the refrigerant and will transfer to the condensing medium. The condenser is one of the principal components in a vapor compression refrigeration system.冷凝器是将制冷剂从蒸气冷凝到液体状态的换热器。这样，制冷剂释放潜热，热量被冷凝媒介带走。在蒸气压缩制冷系统中冷凝器是一个主要的设备。As shown in Fig.11-1, the purpose of the condenser is to remove heat from the refrigerant vapor leaving the compressor, so that the refrigerant will condense to its liquid state at the condensing pressure. Liquid refrigerant is often subcooled the condensing process.如图11-1所示，冷凝器的目的是将离开压缩机的制冷剂蒸气热量移走，所以制冷剂在冷凝压力下将被冷凝到液体状态。在冷凝过程中，液体制冷剂被冷却到过冷。The heat-transfer process in a condenser occurs in three stages as implicated in Fig.11-1.在冷凝器中发生的传热过程发生在图11-1所包含的三部分。1) Desuperheating of the hot vapor.1) 热蒸气的过热降低2) Condensing of the vapor to liquid state and release of the latent heat.2) 蒸气被冷凝为液体并放出潜热。3) Subcooling of the liquid refrigerant.3) 液体制冷剂过冷Though the surface heat-transfer coefficient is lower on the hot vapor side during desuperheating of the vapor, it does not affect too much to the heat transfer as there is a greater temperature difference between the vapor and the cooling medium. The subcooling only takes place in a small portion of the condensers surface area. Therefore, for simplification, an average heat-transfer coefficient is used for the whole condensers surface area, and the condensation of refrigerant is assumed to occur at the condensing temperature.虽然在热蒸气过热降低的过程中，蒸气侧的表面换热系数很低，但由于热制冷剂蒸气与冷凝媒介间的温差很大，所以它的影响并不大。在冷凝器表面，过冷值占了很少的一部分区域。因此，为了简化，整个冷凝器表面都用假定的平均换热系数，并且假定制冷剂在冷凝温度下液化。2. Classification of Condensers2. 冷凝器的分类Based on the diverse applications, condensers are manufactured in a wide variety of types, shapes, and sizes. Condensers can be classified in a number of ways according to their structure, condensing medium and arrangement of air flow, etc. Fig.11-2 lists the ways of classification for condensers.对于不同的应用，冷凝器加工为各种不同的形式，形状和大小。根据冷凝器的结构，冷凝方式和气流布置等许多方式可以将冷凝器分为许多不同类型。图11-12列出了冷凝器的分类。Although brine or direct expansion refrigerants are sometimes used as condensing medium in low temperature applications, great majority of the cooling medium employed for condensers is either air or water, or a combination of them.虽然在低温应用时，盐水或直接蒸发制冷有时也用做冷却介质，但大部分冷却介质还是用空气或水，或它们的混合。Fig.11-2 Classification of condensers 1图11-2 冷凝器分类Condensing mediumInletOutletFlow through condenserRefrigerantCondensing mediumTemperature / CCondensingSubcoolingDesuperheating5528Fig.11-1 The state and temperature variation inside a condenser图11-1 冷凝器内的状态和温度变化11-2 Water-Cooled Condensers 韩志明11-2 水冷式冷凝器In a water-cooled condenser, cooling water is used to remove condensing heat from the refrigerant. The water supply can be either a once-through or recirculated arrangement. In once-through use the water comes from a permanent supply in sufficient quantity and is wasted after passing through the condenser. In most situations an adequate water supply is not available, and the water must be recirculated.在水冷冷凝器中，用冷却水带走制冷剂中的冷凝热，冷却水供给可以是直流式也可以是循环式的。在直流式冷却水系统中，冷却水来自于水量充足的长久水源，当水流过冷凝器后就被浪费掉了。在大部分情况中，都没有充足的水源，所以冷却水必须循环使用。Types of water-cooled condensers can be classified according to their construction (Fig.11-2). Each has features that make it suitable for certain application.水冷式冷凝器按其结构可分为多种形式，如图11-2。每一种都有各自的特点以满足某种特定情况下的应用。1. Double-pipe condenser1. 套管冷凝器The double-pipe condenser consists of two pipes or tubes so arranged that one is inside the other (Fig.11-3). Water is piped through the inner tube while the refrigerant flows in the opposite direction in the space between the inner and outer tubes. With this arrangement air-cooling of the refrigerant is provided through the outside of the tube in addition to the water-cooling, reducing the required size. Counter-flowing results in the highest rate of heat transfer.套管式冷凝器含有两根管子，并且其中一根管子在另一根管子里面，如图11-3。内部一根管子输送冷却水，制冷剂以相反的流动方向在内管和外管之间流动。这样在水冷的同时，制冷剂管的外面与空气接触，可以同时进行空气冷却，这样就减小了冷凝器的尺寸。逆流拥有最高的传热速率。Fig.11-3 Double-pipe water-cooled condenser图11-3，套管式水冷冷凝器A double tube circular arrangement is a compact construction (Fig.11-4). The chemical cleaning is required for the tube because mechanical cleaning is very difficult for this arrangement.双管圆形设计结构非常紧凑(图11-4)，这种安排的管道必须进行化学清洗，因为机械清洗十分困难。Fig.11-4 Circular and trombone double-tube condenser 11图11-4，圆形和U型双管冷凝器Another type of double-pipe condenser has cleanable tubes (Fig.11-5). It is constructed of straight lengths of tube with headers and removable end plates. This makes the inner tubes possible to be cleaned mechanically by removing the end-plates. 另外一种双管冷凝器管道可以进行清洗(图11-5)，它由带端板和可移动尾板的直长管组成。这样的设计就可以通过移动尾板来对内管进行机械清洗。Fig.11-5 Double pipe condenser with cleanable tubes图11-5，带可清洗管的双管冷凝器2. Shell-and-coil condenser2. 壳盘管式冷凝器The shell-and-coil condenser is made up of one or more bare-tube or finned-tube coils enclosed in a shell (Fig.11-6). The condensing water circulates through the coils while the refrigerant is contained in the shell surrounding the coils. Hot refrigerant vapor from the compressor enters the shell and condenses on the surface of the coils. The condensed liquid drains off the coils into the bottom of the shell. Since an excessive accumulation of liquid in the condenser will cover too much of the condensing surface and cause an increase in the discharge temperature and pressure, the system cannot be overcharged with refrigerant.壳盘管式冷凝器由壳体内的裸管或肋片管组成(图11-6)。壳体内的制冷剂包裹住盘管，冷凝水在盘管里循环，由压缩机排出的热制冷剂蒸气进入壳体并且在盘管表面被冷凝，被冷凝后的液体制冷剂脱离盘管流至壳体底部。由于随着冷凝器内聚集的制冷剂液体会覆盖一部分冷凝面积会提高制冷剂排出液体的温度和压力，因此系统内部的制冷剂量不能过多。The shell-and-coil condensers are cleaned by circulating an approved chemical through the water coils. As a general rule, shell-and-coil condensers are used only for small installations up to approximately 10 tons capacity. Both vertical and horizontal types are available. The vertical arrangement is convenient when floor apace is limited. The shell-and-coil condensers have the advantage of being compact in size and serve the dual function of condenser and receiver.壳盘管式冷凝器通过盘管内规定的化学清洗剂的循环进行清洗，按一般的惯例来说，壳盘管式冷凝器只用于大约小于10吨制冷能力的小型装置，可以用立式的也可以用卧式的壳盘管式冷凝器，对于面积受限的地方用立式壳盘管式冷凝器显得很方便，壳盘管式冷凝器有尺寸较小、结构紧凑的优势，并且拥有冷凝器和回收器的双重作用。Fig.11-6 Shell-and-coil condenser (vertical type) 1图11-6，壳盘管式冷凝器(立式)3. Shell-and-tube condenser3. 壳管式冷凝器The shell-and-tube condenser consists of a cylindrical metal shell in which a number of straight tubes are arranged in parallel and held in place at the ends by tube sheets. There are two main types of shell-and-tube condensers: horizontal and vertical.壳管式冷凝器含有一个圆柱状的金属壳，金属壳内部为大量的固定在末端管板的平行直管。壳管式冷凝器主要有两种型式：卧式和立式。(1) Horizontal shell-and-tube condenser (1) 卧式壳管式冷凝器The principal form of water cooled condenser in large sizes is the horizontal shell-and-tube condenser. The condensing water is circulated through the tubes that are bare or finned surface. The refrigerant flows outside the tubes but inside the shell and condenses on the surface of tubes through which water flows. Heat is transferred from refrigerant vapor to water through the tube walls. The condensed refrigerant drains off the tubes and collects at the bottom of the shell where it drains to a receiver. The water enters the tubes from the end covers, also called headers or water boxes, and makes one or more passes through the length of the condenser. The headers are removable so that the tubes may be mechanically caleaned.卧式壳管式冷凝器是大型水冷冷凝器的主要形式，冷凝水在裸管或肋片管内循环。制冷剂在壳体内部管子外部流动并且在水管表面处被冷凝。热量在管壁处由制冷剂蒸气传给冷凝水。被冷凝的制冷剂液体脱离水管流至壳体底部并排向回收器。水从端盖或水箱进入水管，并且在冷凝器长度方向上流过单程或多程。顶盖可以移动，因此管道可以进行机械清洗。There can be many variations on the horizontal shell and tube condenser design. The tubes may be straight or bent in the shape of a U, called U-tubes. Most shell-and-tube condensers are either 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell. In a single pass condenser, the fluid goes in one end of each tube and out the other. Two and four pass designs are common because the fluid can enter and exit on the same side. This makes construction much simpler. Fig.11-7 gives an example of a two pass shell and tube condenser.卧式壳管式冷凝器有很多种不同的类型，壳体内的管子可以是直的也可以弯曲成U型，称为U型管。大部分壳管式冷凝器的管路都设计成单程、双程或4程的，这就涉及到管内流体在壳体内流体内部流动时间长短。在单程冷凝器中，流体从管子的一段流进并且从另一端流出。双程和四程的设计最为常见，因为流体可以在同一侧流进流出，这样使得冷凝器的结构更为简单。图11-7为一个双程壳管式冷凝器的例子。Fig.11-7 Two-pass shell and tube condenser图11-7双程壳管式冷凝器For the same total number of tubes and the same water quantity, the heat transfer coefficient will be higher for the four-pass condenser than the two or single pass condenser. However, because of the high pressure drop, the power required to circulate the water will be greater. High water velocities are desirable for good heat transfer and also to inhibit the deposited of dirt on the interior walls of the tubes. Unfortunately, with soft metals like copper velocities higher than about 2m/s tend to cause erosion with some water. Higher values can be used with steel, depending on the acceptable pressure drop. 对于相同的总管数和冷凝水质量，四程冷凝器的传热效率比单程或双程的都要高，但由于水的巨大压降，四程冷凝器水循环所消耗的能量也会更多。较高的水速对传热效果及抑制管内壁杂质的沉淀都有好处。但是对于铜类的软金属，高于2m/s的较高流速会造成水腐蚀现象。在可以接受的压降前提下的较高流速则可以运用于钢金属管。Shell-and-tube condensers are available in capacities ranging from 2 tons up to several hundred tons of refrigeration or more. Shell diameters range from approximately 0.1m up to several meters, whereas tube length varies from approximately 1m to near 100m. The number and the diameter of the tubes depend on the diameter of the shell. Tubes diameters of 16mm through 50mm are common.壳管式冷凝器可以应用于制冷能力在2冷吨到数百冷吨或者更高范围内的制冷系统。壳体的直径在在大约0.1m到几米的范围内，因此管长大约在1m到100m范围内。内管的数量及长度取决于壳体的直径，直径在16mm到50mm间的内管最为常见。(2) Vertical shell-and-tube condensers (2) 立式壳管式冷凝器Vertical shell-and-tube condensers are used for dirty water and employed on large ammonia installations. The vertical shell-and-tube condensers are mechanically cleanable and are readily cleaned while the system is still in operation. So this type of condenser is ideal for installations where poor water quality and/or other operating conditions causing high scaling rates.立式壳管式冷凝器可以用劣质的水以及用于大型氨制冷装置中，立式壳管式冷凝器可以进行机械方式清洗并且当系统正在运行时也可以轻易地进行清洗。因此对于那些由于水质差以及其他运行条件所引起的高结垢率的制冷系统，该种类型冷凝器是较为理想的冷凝器。As shown in Fig.11-10, the vertical condenser is equipped with a water box at the top to distribute the water to the tubes and a drain at the bottom to carry the water away. Cooling water is pumped to the top and then it flows by gravity through the vertical tubes to a collecting sump. Refrigerant gas enters the side of the shell in the middle or near the top and the condenser liquid is drained from the bottom. 如图11-10所示，在立式冷凝器的顶部安装了一个水箱以将水分配到各个管中，在底部布置了排水管以将水排出。冷却水被泵打到顶部并且在重力作用下从立管流到集水坑。制冷剂气体从壳体的中部或接近于顶部处流入，冷凝后的制冷剂液体从底部排出。Fig.11-10 Vertical shell-and-tube condenser图11-10，立式壳管式冷凝器The heat transfer coefficient of vertical condenser is not so high as in horizontal condensers because of the lower water velocity and the thicker condensate film which forms on the long vertical tubes. The diameter of the tubes is usually bigger than those in horizontal condensers because the velocity of the water inside is determined by gravity and the height of the tubes. Hence a cheaper construction with fewer tubes, which are also more easily cleaned, can be used 3.立式冷凝器的换热效率没有卧式冷凝器的高，因为立式冷凝器水速较低，而且在长立管上凝结有较厚的被冷凝的制冷剂液体膜层。立式冷凝器的管径通常大于卧式冷凝器的管径，因为管内的水流流速取决于重力以及管高，因此立式冷凝器结构更为简单，管数也更少，而且更容易进行清洗。4. Heat transfer in water cooled condenser4. 水冷冷凝器内的热交换A condenser is a heat transfer surface to transfer heat gained in a refrigeration system to some cooling medium. All of the basic principles in Heat Transfer can be applied to the condenser.冷凝器就是将从制冷剂系统中获得的热量传递给一些冷却媒介的热交换表面。所有的热量交换的基本原理都可以用于冷凝器。The following equation is used to calculate heat transfer in water cooled condenser.下公式就是用来计算水冷冷凝器中的换热量的。 (11-1)Where此处的: Heat transfer rate, kW：热交换率，kW: overall heat transfer coefficient, kW/m2 K:全热交换效率，kW/m2 K: Heat transfer area, m2:传热面积，m2: mean effective temperature difference (METD), K:平均有效温差(METD)，KThe overall heat transfer coefficient:全热交换效率： (11-2)Where此处的: thermal resistance in refrigerant side :制冷剂侧的热阻: thermal resistance of tube wall:管壁热阻: thermal resistance of water fouling :水污垢热阻: thermal resistance in water side: 水侧热阻: ratio of outside to inside tube surface area :管外面积与管内面积之比The temperature of cooling media is constantly changing in a condenser, and therefor the temperature difference between the the refrigerant and the cooling media is not constant, as seen in Fig.11-11. A mean temperature difference must be determined for use in Equation 11-1. It has been found that the Mean Effective Temperature Difference (METD) is the best way to calculate this mean temperature difference, is在冷凝器中，冷却媒介的温度始终是变化着的，因此制冷剂和冷却媒介间的温差也不是恒定不变的，如图11-11。式11-1中的温差就是平均温差，我们发现平均有效温差(METD)是计算这个平均温差的最好方法。平均有效温差为： (11-3)Where此处: TDA= temperature difference at one end of the condenser, K: TDA=冷凝器一端的温差，K: temperature difference at other end of the condenser, K: 冷凝器另一端的温差，KFig. 11-11 Temperature profile in a condenser图11-11，冷凝器内的温度图Increasing is one of the methods to enhance heat transfer of the condensers. Counterflow and parallel flow are two common arrangements applied to shell-and-tube heat exchangers. For a given set of required conditions, counterflow will results in a greater METD than parallel flow. When a refrigerant is condensed in the condenser from saturated vapor to saturated liquid, the METD will not be affected by counterflow and parallel flow because the the temperature of the refrigerant does not change (Fig.11-11).增加是增强冷凝器换热效果的一种方法，逆流和顺流是壳管式换热器的两种常见形式。若一些必要条件已经给出，逆流的平均有效传热温差METD将会比顺流的大。如果制冷剂在冷凝器中从饱和蒸气冷凝至饱和液体，那么平均有效温差不会受逆流或顺流的影响，因为制冷剂的温度不变。In order to increase the heat transfer performance of condenser, the overall thermal resistance, should be as small as possible. Of the four elements involved in Equation 11-2, the thermal resistance of metal tube wall resistance Rw is the least significant . The amount of resistance to heat flow offered by the metal is so small that it is of no consequence. Thus, the k of the condenser is determined primarily by the other three factors and usually the fouling resistance and liquid film resistance take a larger portion of the total heat transfer resistance. Table 11-1 compares the thermal resistances in a water cooled condenser with the ammonia as refrigerant.为了增强冷凝器的传热性能，总热阻应该尽可能的小。在11-2涉及的四个元素中，金属管壁的热阻Rw是最为不重要的一个。因此冷凝器的传热系数k主要取决于其他三个因素，并且水垢热阻和制冷剂液体薄层热阻占了全部热阻的大部分。表11-1比较了一下以氨为制冷剂的水冷冷凝器的热阻。Table 11-1 Thermal resistances in a water cooled condenser表11-1，水冷冷凝器中的热阻Thermal resistance type热阻类型Heat transfer coefficient ()Thermal resistance()Condensation of refrigerant106029.410-5Lubrication oil film* 250033333410-4Thermal conduction(steel tube)181825.510-5Fouling of water26743.7410-4Convection of water47592.1010-4Total923.510.8310-4*:This is special for ammonia. Some of the refrigerants do not have this type of resistance because the lubrication oil can dissolve in these refrigerants.*:对氨来说较为特殊，一些制冷剂没有没有这种类型的热阻，因为润滑油在这些制冷剂中能够分解。11-3 Cooling Tower 吕汪敏11-3 冷却塔1. Types of Cooling Towers1.冷却塔的类型Cooling tower is a practical device used to cool the warm water from condenser for the water to be reused as cooling medium. In a cooling tower as shown in Fig.11-12, the warm water from a condenser is pumped to the top of the cooling tower from where the water falls or is sprayed down to the tower basin. The temperature of the water is reduced by evaporation of some water into the air circulating through the tower. Cooling towers are essentially water conservation or recovery devices. 冷却塔是一种非常实用的装置，用来冷却从冷凝器出来的热水，对于做冷却介质的水可以循环使用。如图11-12，在一个冷却塔中，从冷凝器出来的热水，被泵抽到塔的顶部，然后从上面倒下或是喷下，最后流到塔的底盘。一些水蒸发到空气中从而使剩余的水的温度下降。冷却塔本质上是一种节水装置或是回收装置。The cooling effect of a cooling tower results almost entirely from the part of evaporated water. As the water falls through the tower, the latent heat required to evaporate the water is drawn from the rest of the water so the remaining water is cooled down. The evaporated water is carried away by the air flowing through the tower. 冷却塔的冷却效率和所蒸发的水有很大的关系，当水从顶部被倒下去时部分水吸收了潜热而蒸发从而使剩余的水得到冷却。蒸发的水则被流过塔的空气带走。The main factors that influence the performance of cooling towers are ：the wet bulb (WB) temperature of the entering air, the amount of exposed water surface and the length of the exposure, the velocity of the air passing through the tower, and the arrangement of flows beween the air and the water (parallel, transverse, or counter).影响冷却塔性能的主要因素有：进入的空气的湿球温度，水的接触面的大小，接触时间，通过塔的空气的速度，水和空气的布置方式（平行的，横向的，反向的）The exposed water surface includes the surface of the water in the tower basin, all wetted surfaces in the tower, and the combined surface of the water droplets falling through the tower.水的接触面包括： 塔地盘水的表面， 塔的所有湿表面， 水滴的并合面Fig.11-12 A schematic diagram and a picture of a natural draft cooling tower 12According to the methods of air circulation, cooling towers are classified as either natural draft or mechanical draft. 根据空气环流的方式冷却塔可以分为自然通风和强制通风。The natural draft towers do not use fans to move air through the tower. Air circulates in the towers by gravity and density difference between the warm and cold airs Fig.11-12. Since the amount of exposed water surface depends primarily on the spray pattern, a good spray pattern