外文翻译 装备

本科毕业设计（论文）外文翻译译文

学生姓名： 院 （系）： 专业班级： 指导教师： 完成日期： 2014年 5月26日

冷 却 塔

如果冷却塔设备被用来向建筑空调提供冷却水，在这个过程中冷却水吸收的热能必须释放掉。释放蒸汽压缩过程产生的热能的两个最常用的方法是直接用空气冷却和用冷却塔。在冷却塔内，水在被不断循环蒸发中与周围空气进行热量交换。这种冷却水能够被用来吸收释放冷却设备冷凝器的热能。暖通空调应用中使用最广的冷却塔是机械通风冷却塔（如图4.2.13）。机械通风塔用一个或多个风机推动空气在塔内的流动，用一个换热器或填料层使循环水与空气充分接触，用一个水箱来收集物质循环水，和一个配水系统来确保水分散在塔的填料层中。

图4.2.14表示循环水和空气在逆流式冷却塔内相互作用的关系。蒸发冷却过程即水和空气充分接触，包括了同时进行的热质的交换。理想状态下，水通过配水系统形成飞溅或分裂成较小的水滴，增加了热交换中水的换热面积。通常用湿球来表示塔的大小和工作情况。它被定义为出塔冷却水和进塔空气湿球温度之间的差值。理论上，水在塔内循环可以到达湿球温度，但在实际工作中是不可能实现的。

图4.2.14逆流式冷却塔空气与水温度的关系

制冷设备与冷却塔组成的工作范围是由冷凝器热负荷和冷却水流程决定的，而并不是由冷却塔的容量决定的。冷却塔的工作范围用进出冷却塔的水温差表示。冷却塔的运行动力是其周围的湿球温度。平均湿球温度越低，冷却截越容易达到其设计运行参数。暖通空调应用的典型温度为6℃(10℉)。因此，要相同的热负荷条件下，炎热干燥气候中的冷却塔要比炎热潮湿环境中的冷却塔小得多。

正是因为冷却塔可以通过多种途径释放热量，这就允许设计者不考虑一些通常的问题，因而被广泛采用。机械通风冷却塔的主要优点是能把水冷却到周围3－6℃（5－10℉）的湿球温度，这就意谓着冷却塔能提供较低温度的冷凝水，改善（降低）了工作压头，从而提高制冷设备的工作效率。

冷却塔的设计

冷却塔的设计《ASHRAE系统和设备手册》（1996）提供了10多种冷却塔的设

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计方案。其中三种基本的冷却塔设计方案被常应用于暖通空调。根据于空气和水的流程方向和风机的安装位置，又可分为逆流诱导通风型、顺流诱导通风型和逆流强制通风型三种冷却塔。

冷却塔的通常组成部分是热交换设备或是填料层，它们安装在配水系统下方和风道中。最常见的填料层有微粒状和膜状两种。微粒填充通过把水分裂成较小的水珠，使水的可用换热面积趋于最大，并在空气流中保持较长一段时间。水是通过连续设置的微粒填料层被处理的。膜状填料层是通过迫使水流过薄填料层来达到上述的效果的。这些薄填料层被覆盖在密集分布的薄片上，以有利于水的垂直通过。如果冷却塔的空间受到限制，在一定热负荷条件下，通常选用膜状填料层会更加简便。微粒填料层对空气和水的比例问题表现得不够灵敏，而且在水质较差的环境中具有较好的运行性能，会使微粒填料层的沉淀物增多，这又是一个问题。

逆流诱导通风型冷却塔

逆流诱导通风型冷却塔内的空气被设置在塔顶的一个或多个风机抽出。空气通过塔底的基板进入冷却塔，然后与从塔顶基板进入的水进行接触。因此，在这个结构中，流体的流动方向是相反的（水由上往下流动，空气由下往上流动），图4.2.15表示了这个布局。在某些方面这个机械中，随着水通过逆流的空气，温度得到下降。同时，空气被加热加湿。进入气流的小水滴在挡水板处被阻挡下来，并流回到水箱中。空气和一些残留的小水滴通过风机从塔顶排出。然后水箱收集的冷却水被抽回冷凝器。

图4.2.15逆流诱导通风型冷却塔

由于空气均匀地分布在冷却塔的填料层里，通常逆流式冷却塔的运行要比顺流式冷却塔好得多。虽然这种前者要比后者高，需要较高的冷凝水泵压头，但有较高的排气速度，可以减速少废气在塔内的循环而出现的问题。

顺流式诱导通风型冷却塔

和逆流式冷却塔一样，顺流式冷却塔的风机也安装在冷却塔的塔顶（如图4.2.16）。空气从塔侧或塔底基板进入塔内，水平通过填料层。水从塔顶注入塔内的填

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料层中，在热交换器中与顺流的空气进行换热冷却（空气水平流动，水向下流动）。然后冷却水被收集到水箱中，并抽回到制设备的冷凝器。顺流式冷却塔改善了空气流动，因而要求的塔高相对较低，因此与逆流式冷却塔相比，顺流式冷却塔所需的冷凝器水泵压头较低。随着塔的高度的降低，有利于废气从塔顶到塔侧或塔底的循环，将影响到冷却塔的工作效率。

逆流式强制通风冷却塔

逆流式强制通风冷却塔的风机安装在塔底空气吸入口或靠近吸入口的位置（如图4.2.17）。和其他冷却塔相同，水也是向下通过塔的填料层，通过直接与大气进行换热而被冷却。与逆流诱导通风冷却塔的换热相似。对设备来讲，与诱导通风型相比，风机的振动也较小。同时由于风机排出的空气直接通过水箱，对水进行了进一步冷却，所以它又存在额外蒸发冷却的优点。但这种冷却塔也存在一些不足之处。首先，空气不均匀地通过填料层，降低了冷却塔的效率。其次，由于风机吸入口的风速较高，存在废气再循环的可能性，这将降低冷却塔的效率。这种冷却塔适用于中小型制冷系统。

材料

冷却塔为了能满足长期在潮湿环境中运行，需要一种满足要求的标准结构材料。除了湿环境以外，循环水在蒸发过程中，会使水具有较高的无机盐浓度。制造冷却塔各部件的材料要具有最好的耐腐蚀性和较低廉的价格。木材是过去冷却塔常用的材料。红木和冷杉通常通过化学防腐剂的处理后，被常用于制造冷却塔部件。像铬酸砷铜或铬酸铜的化学材料有助于防止真菌腐蚀和白蚁的破坏。

图4.2.16顺流式诱导通风型冷却塔

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图4.2.17逆流式强制通风冷却塔

中小型冷却塔的结构通常使用镀锌钢板。金属部件一般用黄铜或青铜制成。像驱动轴、有棱角的金属部件可由302号或304号不锈钢制成。铸造铁可在基础部件、电动机外壳和风机壳裤找到。在一些特殊部件上常采用带有塑料涂层的材料。

许多制造商在冷却塔的结构、管道、风机叶片、容器、档板的嵌入物和连接部件用玻璃纤维加强塑料（FRP）制成。填充材料、排风设备和挡板由聚乙烯氯化物（PVC）制成。填料层外壳和流体流通口通常要注入聚丙烯和丁二烯丙烯酸苯合成的材料。 混凝土通常用于直立式冷却塔水盆或水池。磁砖、砖一般应用于对美观方面有特殊要求的冷却塔。

性能

冷却塔的主要任务是释放制冷设备产生的热负荷。这种热能释放可以由一个完善系统完成。它可以把制冷设备的总压缩功和像风机和冷凝水泵产生的冷却塔负荷降到最低。在设计者完成整个冷却塔分析之前，包括冷却塔的选择范围、水和空气的比例、塔的形式、填充类型和结构、配水系统等必须被确定。表4.2.6收录了一些常用设计标准和冷却塔正常工作范围。

大多数暖通空调应用要求冷却塔使用冷却塔制造商提供的“零散的结构”部件。制造商代理人往往宣称他们的产品和适用标准都相当好。当根据表4.2.6要求，制定出设计程序方案后，就可以联系一个或多个冷却塔代理人，选择他们供应的合适的冷却塔。

制冷设备的控制方案 大多数冷却塔在通常的运行中受到负荷和周围环境湿球温度变化带来的影响。对一个典型的冷却塔，风机所用的电能相当于制冷压缩机所用电能的10%，冷凝水泵耗电量大约是压缩机的2－5%。当耗能量降到最小时，能够控制冷却塔向冷凝器提供足够的冷却水，是一较为理想的操作方案。大概大多数为暖通空调负荷提供冷却塔系统控制方案，通常把排出的混合水的温度维持在27℃(80℉)。风机的交替工作是达到这

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种冷却塔控制效果和适应多单位、多单元冷却塔安装的一种常用方法。但这种控制方案并不能把制设备与冷却塔组成的系统各部件消耗的总能量降到最小。

降低冷凝水的温度，能提高制冷设备的效率。当蒸发温度一定时，降低冷凝温度，将会产生一个介于冷凝压力和蒸发压力的低压，从而降低了压缩机的负荷。然而，最重要的是必须认识到降低冷凝温度会限制制冷效率的提高。被提高的制冷设备的工作效率可被冷却塔和水泵的投资所抵消。最理想的状态是，冷凝温度不降到制冷设备的最小参数以下，且又能把温度维持在接近最低温度的一定值。

随着两档或三档调速风机在大多数现代冷却塔中的应用，一种接近理想的控制方案被如下发展起来了（Braun和Diderrich,1990）：

·为了使制冷设备－冷却塔组合系统的能耗降到最低，冷却塔风机在部分负荷工作时必须有序地保持恒定运行。

·冷却塔的工作范围、冷凝水的流速和热能的释放等状态来决定冷却塔风机的有序运行。

·发展一种冷却塔的生产能力和冷却塔风机有序运行情况的简单关系。

De saulles和Pearson(1997)发现冷却塔的设定值控制和近理想控制在节能方面很相似。这些控制方案要求冷却塔的冷却水生产能力近可能地控制在最低设定值，但不能低于制冷设备制造商允许的最低值，并与上诉采用近理想控制操作获得的节能效果相比较。他们发现达到的节能水平取决于负荷情况和最优化方法。他们的模拟实验显示，单设定值方法能够节能2.5%到6.5%。而近理想控制方法能节能3%到8%。使用变速风机只可能在多数冷却塔中提高节能水平。采用多个冷却塔风机同时以相同速度运行，比采用在开启下一个风机前，使前一个风机达到最大速度的方法更加经济。当冷却塔有必要时可采用变速风机。

系统设计者必须确保新安装的冷却塔以ASME标准PTC23(AWME 1986)或CTI标准ATC-105为依据进行调试。这些调试能确保冷却塔的运行符合设计要求，并能符合制冷设备或冷负荷的要求。

选择标准

设计者通常会优先考虑表4.2.6列举的标准。如果不是很确定时，一般会采用公认能的标准。这些标准被用来决定冷却塔的设计条件下释放负荷热所需的冷却量。除了考虑冷却量以外，还要在经济、维护、环保和美观方面加以考虑。这里的很多因素都是相互关联的，但在选择特殊的冷却塔设计方案时，应尽可能地全面考虑。选择过程中经济性是一个很重要的部分，因此一般有使用周期性投资风析和投资回收分析两种方法。这些方法与设备的本身固有成本、运行成本和维护费用为基础进行比较。其他影响冷却塔设计方案最后选择的标准还有：建筑安全法规、建筑结构、适用性、可获得的合格维修人员以及应对变化的负荷的机动操作能力。另外冷却塔发出的噪声是一个敏感的环境问题。如果当地建筑法规对噪音有限制，那么在空气的吸入口和风机

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出口必须考虑设置消声器。在现代建筑和在土地空间受到限制的场所美观是一个值得考虑的问题。许多冷却塔生产商能建立一个消费者组织，使他们能完全参与冷却塔的设计和生产的全过程。

应用

冷却塔不同于冷却器，水泵以及空气处理器，它一定是安装在一个合适的、空旷的空间，并且需要认真考虑可能引起再循环(相同的冷却塔里回收部分暖和的和潮湿的排出气体)或限制空气流程的因素。冷却塔的位置安装不好可能会导致再循环，然而这个问题不适用于那些湿的冷却塔。类似的再循环也同样会发生在用空气冷却的冷凝器中。由于冷却塔的再循环，计入湿球温度计的温度不断的升高，性能也受到了影响。循环的主要因素是不良的线路设置，使塔靠近建筑物，或排气的速度不合适，或者是循环塔的进出口间的间隔不充分。

当从一个循环塔出来的空气被吸到一个位于其顺风口的位置时，若干个这样的循环塔的安装就易于发生冲突。这种迹象类似于再循环的现象，再传递给其余顺风处的循环塔。对于再循环、冲突或者物理上地对循环塔气流的阻断，都会增大冷却器中冷凝压力的波动范围。然而再循环和冲突，只要经过仔细的计划和设计都可以得到避免。

在安装一个冷却塔时要仔细考虑雾的影响，或卷入或转移。雾常出现于较冷的天气里，当潮湿的暖空气接触到从冷却塔里喷射出的冷空气时，经过冷凝后就形成了雾。来自冷却塔的雾能限制可见度，并且可能引起建筑上的障碍。当输送气流中的小水滴没有被漂浮物清除器所吸附，并且也不随排出气体一起流动时，才会发生转移。这些小水滴从排出的空气中沉淀下来，然后掉落到地上，就象起雾或下小雨一样(在非常的情况下)。转移物或漂流物中含有来自经冷却塔处理过的矿物和化学物质，能引起其表面原颜色的着色或变色。如同对待再循环一样，为了减少来自转移物或尘雾的问题，设计者应该考虑附近的交通格局，停车区域，盛行的风向，大的玻璃区域或考虑其他的建筑。

操作和维护

冬天操作

如果在寒冷的天气里使用冷却器或制冷仪器，要考虑冻结保护，避免在冷却塔里面或外面形成冰的冻结。容量控制是一种可用来控制冷却塔里水温和它的组成成份的方法。电加热器通常被安装在塔的集水坑中，提供附加的结冰保护。由于在吸入口的冰冻对冷却搭的性能是不利的，可以利用通风机来解除这些地方的冰冻。如果在非常寒冷的天气下操作风机，在风机叶片的刀口边缘会产生结冰，能引起通风机系统的严重不平衡。在转换负载时，在风机上要安装检测震动或偏心越限的仪器。和其它操作

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设备一样，在极端的天气期间要时检查系统的运行状况。 水处理

在冷却塔里的循环冷冻水必须要有足够的水质保证，来保持循环塔的冷却效率并使设备维持在正常的运行状况。在塔中的水里所掺杂的杂质以及易于溶解的物质，由于连续的蒸发过程，和塔水一样在冷却塔里循环。污垢，灰尘和气体也能通过它们各自的途径进入塔中的循环水里面，都会被循环水带入循环系统或者在塔的集水坑里沉积下来。为了减少这些污染物的集中，循环水的百分比也被减少了。在比较小的蒸发式冷却系统里，这一个程序叫做排放，并且是连续的。减少量通常占总水循环是0.8%到1.2%，使之来维持减少杂质的集中和控制比例的形成。如果循环塔中的水质很差，就需要附加的化学处理来抑制腐蚀控制生物制品的生长，而且限制残渣的集中。 军团病

军团病的发病已经与蒸发式凝结器、冷却塔以及其他的建筑物循环加热的成份相关联。研究人员已经发现，保养好并且使用良好的水质的冷却水塔不易出现那些军团菌病毒。在一份关于军团病的文件中，冷却塔学会(CTI，1996)阐述由于军团病菌在冷却塔的分布比较集中，这样做会滋生或散布带菌飞沫。据研究，细菌生长的最适宜温度大概是在37℃（99℉），而在冷却塔里很容易达到这个温度。冷却塔学会建议采用冷却塔的设计和操作规范来最大限度的减少军团病菌的出现。他们不推荐经常对冷却塔进行军团病菌的检测，因为很难解释检测出来的结果。一个干净的塔很快可能被再传染，而且一个被污染的塔也不意味一个疾病的发作将会发生。 维护

冷却塔制造者通常会为一个新塔的安装提供一本操作和维护手册。这些手册必须包括冷却塔中所使用部件的一份详细列表和塔中可替换的部件，并且详细介绍对冷却塔的日常维护工作。至少，下列几个部分在塔的安装和维护工作中是不可缺少的。

要定期对冷却塔的整体和部件进行检查，确保其维修良好。

对塔中全部变湿的表面要进行定期的、完全的排污和清洁工作。这样就可以定期的对沉积下来的污垢、粘土、废渣以及对藻类和病菌可能生长的区域进行清理。 周期性的对水进行生化处理。

塔的操作和维护工作中要进行连续的日志记录。这为将来的维护和检修工作准备一定的资料，并且对采取正确的维护方法这点也是很重要的。 4.2.4组装的设备

对一个特殊的冷、热负荷来说，采用集中的采暖、通风和空调系统不一定是最好的方法。集中系统的初投资要比那些分体式或组装的系统明显贵很多。当然也许在建筑物的安装过程中由于机械设备组成部件自身尺寸的缘故而受到限制。从制造厂里装配出来的单体式或组装式的系统是单冷型或者是热泵型这些系统是按照设计者根据一般的情况而制造出来的各种结构。对一个简单的装置来说，例如橱柜式或撬装式，

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典型的设备通常由一个蒸发器，风机，压缩机，凝结器，如果是一个组合的系统，还有一个加热环节。单体式的负荷能力大约大概从5千瓦到460千瓦(1.5到130吨)。典型的、单一的系统有整体式系统(窗户单位，屋顶单位)，分体式系统，热泵型系统和水源热泵系统。单体式系统的寿命不如集中式系统(只有8到15年)，而且通常效率比较低。

整体式系统一般可以应用在低于八层的建筑物中，但是它们通常在冷负荷较小的一、二、层的建筑中。它们大多数应用于分散的空间、小的办公建筑以及教室。整体式设备在已经设置好容量性能特性的系统中是可行的，例如通过单体空调处理设备的总风量L/s（立方英尺/分）一些设计者结合集中采暖、通风和空调的组装设备，应用在周边建筑区域。这种技术的合成能解决湿度和空间的温度要求，并且要优于单一的组装设备。这种组合也可以在那些不适合用组装设备来服务于内部区域的建筑物中。

表4.2.7列举了一些组装设备和单一设备的各自优缺点。 缺点 由于尺寸上因素的限制其性能的发挥。 普通的个体系统对于闭式室内湿度的调节是不利的。 空间温度的控制是通过调节两个极限温度来实现的。 组装设备的使用寿命相对较短。 由于集中生产的增加和设备的过大化，使得它的耗能要高于中央系统； 通常来说，要完整的实现经济型的循环是不可能的。 由于在各个不同的房间安置设备，限制了气流的分区控制。 设备所产生的噪音级别会让人生厌。 优点 可以在不同的房间进行分别控制 供冷和供暖在任何情况下是可行的，并且在不同的地方操作是比较独立的。 在任何情况下进行通风操作都是可行的。 单体的容量在制造时都是确定的。 在一些空闲的空间可以很容易的关闭设备，比较容易实现能源的节约。 单体设备的操作通常是比较简单的。 组装设备所占用的空间要少于集中设备。 初投资较少。 设备可以安置在一些节省管道的地方或者可以减少管路的占用空间。 安装相对比较简单；不要求是专业的人员操其个体的形态在美学上来说是很难接受的。 作的。 由于过滤器的原因，空气流过单体设备时，气流受到了限制。 来自设备的冷凝水会让人讨厌。 由于个体的数目、安装位置以及入门的困 8

难，设备的维护工作将会是一个议题。 通常来说，冷负荷超过18kw(5 吨)的商业建筑物常使用整体式系统。然而，在一些情况下，由于空间的需要，设备尺寸的限制或者其它方面的原故，也可以采用户用集中式系统。如果一个整体式系统使用10年或更久，那么通过用合适的类型替换整体式系统，能源的储蓄可以接近一个能效的模型。

a 电动风冷或水冷，并且容量超过19kw的分体式系统和整体组装的系统也包括在这里。

b 能效比是指通过在标况下送风温度为35°C(95°F)的风冷设备和送水温度在29°C(85°F)的水冷模型所各自得到的冷负荷除以消耗的标准电能（瓦）（空调与制冷学会）

c 基于空调与制冷学会210/240测试成果。

d 季节性的能效比是指系统设备在普通情况下的输出的总的冷负荷kw与在同段时期内消耗的电能（瓦）的比值。

E 制冷量在19kw以下的分体式系统和整体式组装系统也同样包括在这里。这一项分析不包括窗式和组装末端的设备。

对于任何一个采暖、通风和空调系统，适当的维护和操作能确保的系统的性能良好，并且同时能提高它的使用寿命。分体式空气调节装置和热泵系统大多数是应用在户用和小型商场所。如同分离的构件一样，这些部件被固定在建筑物的某个地方，待安装好冷凝器（室外机）和蒸发器（室内机）后，通过制冷剂的流通管道将它们连接起来。空调技术人员必须要确保机器的合适地制冷剂充注量和适当的检查操作。如果系统制冷剂的充注量过多或过少，它的性能表现上就会起不利的作用。(1996)发现采用毛细管节流的空调系统，如果制冷剂的充注量不合适会直接影响系统的性能。(图4.2.18)。

图4.2.18在室外95°F(35°C)情况下改变制冷剂的冲注量，采用膨胀阀节流和毛细管节流对系统制冷能力的影响。(1996)

在图4.2.18中明确的展示了在采用毛细管节流中，当制冷剂的充注量减少为20%时，系统的制冷能力也减少了30%。在对制冷剂泄露的检测中也可以得到相同的结论。对一个新安装的系统，通常的问题是在扩压器、(散热器)护栅和回压空气附近的管道连接处的设置不适宜密封。当泄露数量达到回风量的5%时，则对于高湿度气候的制冷量和效率就将近减少了20%。对于低湿度的气候来说，这个值将会减少7%。对制冷剂充填和泄露的研究表明要根据安装承包商、维护承包商和系统拥有者的需要来确定合适的空调安装系统。

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图4.2.19被包装暖气和空调部件的屋顶布置图。(适用于开利公司)

组装的部件

组装的部件是完整的采暖、通风和空调构件，通常是被安装在建筑物（屋顶或墙壁）的外部，这些设备向室内地板空间释放有价值的物质。它们也可以安装在底层的混凝土构架垫层。因为它们是独立的构件，全部是制造的单位，并且安装费用也要比总计的采暖、通风和空调便宜。

整体式系统包括一个风机，过滤器，蒸发器盘管，并且至少还有一个压缩机（大的系统可能还要多的），还有一个空气冷却式冷凝器。系统可能还要配置一个加热区域。加热采用天然气或电的能源。在一些以电能为唯一能源的地区就可以采用热泵系统。单一的热泵由于尺寸原因被限制只能达到70个千瓦(20吨)。

组装设备随着使用时间的变长而变的性能变差，在对系统进行维护的同时，它的效率通常也随着下降。提高现有的组装设备，使之成为高效率的模型，这样能实质性的实现长期的能源储蓄。在过去的10到15年中，制造业者对组装系统的性能提高已经做了重大的改进。在蒸发器和凝结器盘管的能源传递效率都已经被改良了， 高效率马达是现在一流标准， 并且在高效率的组装系统中风机和压缩机的设计也得到了改进。螺旋式压缩机在中型系统(70到210个千瓦；20到60吨)中是很普通的。新系统的能效比是有季节性的，一般在9.50到13.0之间浮动。对于老的系统来说，它的效率接近6.0，并且大部分要低于9.0是很常见的。很典型的，在采用煤气加热的地方，年度燃料利用效率大概在80%。所有装置在户外屋顶的系统都有出厂安装配备的微处理器控制的。这些设备的配置使得维护工作更加简便，并且可以提高单体的能效比乃至整个采暖、通风和空调系统的能效比。控制特征包括温度调节和开停机的时间安排。较大的系统还可以实现变风量的传递。当然，大多数的单体有一个与能源管理控制系统相联的、具有选择性的连通接口。

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立式的组装单位

立式的组装单位被典型地设计用来做户用或是穿墙安装的。这些部件常应用在旅馆或公寓中。某些设计产品还采用冷却塔或城市给水作为冷煤来冷却冷凝器。另外大多数一般采用空气冷却式冷凝器。这两种系统的其它组成部件都已经组装在其内部。如果有需要，可以配置管道系统连接到单体并进行风量的调节。

图4.2.20分体式系统的简图

分离-系统组装单位

分体式系统可以使冷凝器安置在室外的设备间或屋顶上。压缩机区段通过制冷剂管路与室内的操作个体，以及蒸发器盘管相连接。如果它们不是热泵类型的，那么就无法向室内提供热量。加热盘管可以安装在空气处理区域，尤其是那种有热水或蒸汽锅炉加热的系统中。换句话说，室内的部件可以通过煤气加热来提供热量。 空冷式热泵

空冷式热泵系统是一种很典型的在屋顶放置设备，完全整体的或采用分离的系统。分散包装的热泵系统是在室内安装一个空气处理装置，而压缩机和冷凝器都被安放在室外设备间或屋顶的室外机里。在制冷模式，热泵系统按空气调节器运转。在制热模式下，系统逆转运行，从室外大气中吸收能量并且送到室内的空间。这些系统的运转示意图分别在4.2.21和4.2.22中表示出来了。这种单体式热泵系统的负荷范围大概在5到7kw(11/2到20吨)。在一些情况下，现存的采用电阻加热的包装的制冷单位可以改良成热泵系统来提高能效比。

热泵系统最好应用在温和的气候，就如在美国的东南部地区，和那些采用天然气加热比较困难的地方。在极其寒冷的天气，所需的加热空间可能要超过热泵系统的容量。这是因为制冷设备尺寸的大小大多数是按照冷负荷的需要而确定的。随着室外气温的下降，热泵系统的性能系数也不断的减小。一个放置在屋顶、制冷量为26kw(71/2

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吨)的热泵系统，在较高温度(8.3°C)的能效系数为3.0，而在较低温度(–8.3°C)的能效比只有2.0或更低。因为随着室外温度的变换，制冷能力也降低，热泵系统需要补充电阻加热来维持建筑物的温度。图4.2.23展现以空气为媒介的热泵系统随着室外温度的改变，效能和制冷能力的变化趋势。第4.2章讨论热泵的特性。

4.2.21空冷或水冷热泵的制冷模式。

图4．2．22热泵系统的制热模式图

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图4.2.23 系统的制热能力随室外温度变化图

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Cooling Towers

If a chiller is used to provide chilled water for building air conditioning, then the heat energy that is absorbed through that process must be rejected. The two most common ways to reject thermal energy from the vapor compression process are either directly to the air or through a cooling tower. In a cooling tower, water is recirculated and evaporatively cooled through direct contact heat transfer with the ambient air. This cooled water can then be used to absorb and reject the thermal energy from the condenser of the chiller. The most common cooling tower used for HVAC applications is the mechanical draft cooling tower (Figure 4.2.13). The mechanical draft tower uses one or more fans to force air through the tower, a heat transfer media or fill that brings the recirculated water into contact with the air, a water basin (sump) to collect the recirculated water, and a water distribution system to ensure even dispersal of the water into the tower fill.

Figure 4.2.14 shows the relationship between the recirculating water and air as they interact in a counterflow cooling tower. The evaporative cooling process involves simultaneous heat and mass transfer as the water comes into contact with the atmospheric air. Ideally, the water distribution system causes the water to splash or atomize into smaller droplets, increasing the surface area of water available for heat transfer. The approach to the wet-bulb is a commonly used indicator of tower size and performance. It is defined as the temperature difference between the cooling water leaving the tower and the wet-bulb of the air entering the tower. Theoretically, the water being recirculated in a tower could reach the wetbulb temperature, but this does not occur in actual tower operations.

FIGURE 4.2.14 Air/water temperature relationship in a counterflow cooling tower.

The range for a chiller/tower combination is determined by the condenser thermal load and the cooling water flow rate, not by the capacity of the cooling tower. The range is defined as the temperature difference between the water entering the cooling tower and that leaving. The driver of tower performance is the ambient wet-bulb temperature. The

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lower the average wet-bulb temperature, the ―easier‖ it is for the tower to attain the desired range, typically 6°C (10°F) for HVAC applications. Thus, in a hot, dry climate towers can be sized smaller than those in a hot and humid area for a given heat load.

Cooling towers are widely used because they allow designers to avoid some common problems with rejection of heat from different processes. The primary advantage of the mechanical draft cooling tower is its ability to cool water to within 3–6°C (5–10°F) of the ambient wet-bulb temperature. This means more efficient operation of the connected chilling equipment because of improved (lower) head pressure operation which is a result of the lower condensing water temperatures supplied from the tower. Cooling Tower Designs

The ASHRAE Systems and Equipment Handbook (1996) describes over 10 types of cooling tower designs.Three basic cooling tower designs are used for most common HVAC applications. Based upon air and water flow direction and location of the fans, these towers can be classified as counterflow induced draft, crossflow induced draft, and counterflow forced draft.

One component common to all cooling towers is the heat transfer packing material, or fill, installed below the water distribution system and in the air path. The two most common fills are splash and film.Splash fill tends to maximize the surface area of water available for heat transfer by forcing water to break apart into smaller droplets and remain entrained in the air stream for a longer time. Successive layers of staggered splash bars are arranged through which the water is directed. Film fill achieves this effect byforcing water to flow in thin layers over densely packed fill sheets that are arranged for vertical flow. Towers using film type fill are usually more compact for a given thermal load, an advantage if space for the tower site is limited. Splash fill is not as sensitive to air or water distribution problems and performs better where water quality is so poor that excessive deposits in the fill material are a problem. Counterflow Induced Draft

Air in a counterflow induced draft cooling tower is drawn through the tower by a fan or fans located at the top of the tower. The air enters the tower at louvers in the base and then comes into contact with water that is distributed from basins at the top of the tower. Thus, the relative directions are counter (down for the water, up for the air) in this configuration. This arrangement is shown in Figure 4.2.15. In this configuration, the temperature of the water decreases as it falls down through the counterflowing air, and the air is heated and humidified. Droplets of water that might have been entrained in the air stream are caught at the drift eliminators and returned to the sump. Air and some carryover

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droplets are ejected through the fans and out the top of the tower. The water that has been cooled collects in the sump and is pumped back to the condenser.

FIGURE 4.2.15 Counterflow induced draft cooling tower.

Counterflow towers generally have better performance than crossflow types because of the even air distribution through the tower fill material. These towers also eject air at higher velocities which reduces problems with exhaust air recirculation into the tower. However, these towers are also somewhat taller than crossflow types and thus require more condenser pump head. Crossflow Induced Draft

As in the counterflow cooling tower, the fan in the crossflow tower is located at the top of the unit (Figure 4.2.16). Air enters the tower at side or end louvers and moves horizontally through the tower fill. Water is distributed from the top of the tower where it is directed into the fill and is cooled by direct contact heat transfer with the air in crossflow (air horizontal and water down). Water collected in the sump is pumped back to the chiller condenser. The increased airflow possible with the crossflow tower allows these towers to have a much lower overall height. This results in lower pump head required on the condenser water pump compared to the counterflow tower. The reduced height also increases the possibility of recirculating the exhaust air from the top of the tower back into the side or end air intakes which can reduce the tower’s effectiveness. Counterflow Forced Draft

Counterflow forced draft cooling towers have the fan mounted at or near the bottom of the unit near the air intakes (Figure 4.2.17). As in the other towers, water is distributed down through the tower and its fill, and through direct contact with atmospheric air it is cooled. Thermal operation of this tower is similar to the counterflow induced draft cooling

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tower. Fan vibration is not as severe for this arrangement compared to induced draft towers. There is also some additional evaporative cooling benefit because the fan discharges air directly across the sump which further cools the water.There are some disadvantages to this tower. First, the air distribution through the fill is uneven, which reduces tower effectiveness. Second, there is risk of exhaust air recirculation because of the high suction velocity at the fan inlets, which can reduce tower effectiveness. These towers find applications in smalland medium-sized systems. Materials

Cooling towers operate in a continuously wet condition that requires construction materials to meet challenging criteria. Besides the wet conditions, recirculating water could have a high concentration of mineral salts due to the evaporation process. Cooling tower manufacturers build their units from a combination of materials that provide the best combination of corrosion resistance and cost. Wood is a traditional material used in cooling tower construction. Redwood or fir are often used and are usually pressure treated with preservative chemicals. Chemicals such as chromated copper arsenate or acid copper chromate help prevent decay due to fungi or destruction by termites.

FIGURE 4.2.16 Crossflow induced draft cooling tower.

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FIGURE 4.2.17 Counterflow forced draft cooling tower.

Galvanized steel is commonly used for small- to mid-sized cooling tower structures. Hardware is usually made of brass or bronze. Critical components, such as drive shafts, hardware mounting points, etc., may be made from 302 or 304 stainless steel. Cast iron can be found in base castings, motor housings, and fan hubs. Metals coated with plastics are finding application for special components.

Many manufacturers make extensive use of fiberglass-reinforced plastic (FRP) in their structure, pipe, fan blades, casing, inlet louvers, and connection components. Polyvinyl chloride (PVC) is used for fill media, drift eliminators, and louvers. Fill bars and flow orifices are commonly injection molded from polypropylene and acrylonitrile butadiene styrene (ABS).

Concrete is normally used for the water basin or sump of field erected towers. Tiles or masonry are used in specialty towers when aesthetics are important.

Performance

Rejection of the heat load produced at the chilling equipment is the primary goal of a cooling tower system. This heat rejection can be accomplished with an optimized system that minimizes the total compressor power requirements of the chiller and the tower loads such as the fans and condenser pumps. Several criteria must be determined before the designer can complete a thorough cooling tower analysis, including selection of tower range, water-to-air ratio, approach, fill type and configuration, and water distribution system. Table 4.2.6 lists some of the common design criteria and normally accepted ranges for cooling towers.

Most common HVAC applications requiring a cooling tower will use an ―off the shelf‖ unit from a cooling tower manufacturer. Manufacturer representatives are usually well informed about their products and their proper application. After the project design process has produced the information called for in Table 4.2.6, it is time to contact one or more cooling tower representatives and seek their input on correct tower selection. Control Scheme with Chillers

Most cooling towers are subject to large changes in load and ambient wet-bulb temperature during normal operations. For a typical cooling tower, the tower fan energy consumption is approximately 10% of the electric power used by the chiller compressor. The condenser pumps are about 2–5% of the compressor power. Controlling the capacity of a tower to supply adequately cooled water to the condenser while minimizing energy

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use is a desirable operational scheme. Probably the most common control scheme employed for towers serving an HVAC load is to maintain a fixed leaving water temperature, usually 27°C (80°F). Fan cycling is a common method to achieve this cooling tower control strategy and is applicable to multiunit and multicell tower installations. However, this control method does not minimize total energy consumed by the chiller/cooling tower system components.

Lowering the condensing water temperature increases a chiller’s efficiency. As long as the evaporator temperature is constant, a reduced condenser temperature will yield a lower pressure difference between the evaporator and condenser and reduce the load on the compressor. However, it is important to recognize that the efficiency improvements initially gained through lower condenser temperatures are limited. Improved chiller efficiency may be offset by increased tower fan and pumping costs. Maintaining a constant approach at some minimum temperature is desirable as long as the condensing temperature does not fall below the chiller manufacturer’s recommendations. Since most modern towers use two- or three-speed fans, a near optimal control scheme can be developed as follows (Braun and Diderrich, 1990):

? Tower fans should be sequenced to maintain a constant approach during part load operation to minimize chiller/cooling tower energy use.

? The product of range and condensing water flow rate, or the heat energy rejected, should be used to determine the sequencing of the tower fans.

? Develop a simple relationship between tower capacity and tower fan sequencing.

De Saulles and Pearson (1997) found that savings for a setpoint control versus the near optimal control for a cooling tower were very similar. Their control scheme called for the tower to produce water at the lowest setpoint possible, but not less than the chiller manufacturer would allow, and to compare that operation to the savings obtained using near optimal control as described above. They found that the level of savings that could be achieved was dependent on the load profile and the method of optimization. Their simulations showed 2.5 to 6.5% energy savings for the single setpoint method while the near optimal control yielded savings of 3 to 8%. Use of variable speed fans would increase the savings only in most tower installations. It is more economical to operate multiple cooling tower fans at the same speed than to operate one at maximum before starting the next fan. Variable speed fans should be used when possible in cooling towers.

The system designer should ensure that any newly installed cooling tower is tested according to ASME Standard PTC 23 (ASME 1986) or CTI Standard ATC-105. These field tests ensure that the tower is performing as designed and can meet the heat rejection

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requirements for the connected chiller or refrigeration load. Selection Criteria

The criteria listed in Table 4.2.6 are usually known a priori by the designer. If not known explicitly, then commonly accepted values can be used. These criteria are used to determine the tower capacity needed to reject the heat load at design conditions. Other considerations besides the tower’s capacity include economics, servicing, environmental considerations, and aesthetics. Many of these factors are interrelated, but, if possible, they should all be evaluated when selecting a particular tower design.

Because economics is an important part of the selection process, two methods are commonly used — life-cycle costing and payback analysis. These procedures compare equipment on the basis of owning, operation, and maintenance costs. Other criteria can also affect final selection of a cooling tower design: building codes, structural considerations, serviceability, availability of qualified service personnel, and operational flexibility for changing loads. In addition, noise from towers can become a sensitive environmental issue. If local building code sound limits are an issue, sound attenuators at the air intakes and the tower fan exit should be considered. Aesthetics can be a problem with modern architectural buildings or on sites with limited land space. Several tower manufacturers can erect custom units that can completely mask the cooling tower and its operation. Applications[1]

Unlike chillers, pumps, and air handlers, the cooling tower must be installed in an open space with careful consideration of factors that might cause recirculation (recapture of a portion of warm and humid exhaust air by the same tower) or restrict air flow. A poor tower siting situation might lead to recirculation, a problem not restricted to wet cooling towers. Similar recirculation can occur with air-cooled condensing equipment as well. With cooling tower recirculation, performance is adversely affected by the increase in entering wet-bulb temperature. The primary causes of recirculation are poor siting of the tower adjacent to structures, inadequate exhaust air velocity, or insufficient separation between the exhaust and intake of the tower.

Multiple tower installations are susceptible to interference — when the exhaust air from one tower is drawn into a tower located downwind. Symptoms similar to the recirculation phenomenon then plague the downwind tower. For recirculation, interference,

[1]节选自James B. Bradford et al. ―HVAC Equipment and Systems‖.Handbook of Heating, Ventilation, and Air-Conditioning.Ed. Jan F. Kreider.Boca Raton, CRC Press LLC. 2001

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or physically blocking air-flow to the tower the result is larger approach and range which contribute to higher condensing pressure at the chiller. Both recirculation and interference can be avoided through careful planning and layout.

Another important consideration when siting a cooling tower installation is the effect of fogging, or plume, and carryover. Fogging occurs during cooler weather when moist warm air ejected from the tower comes into contact with the cold ambient air, condenses, and forms fog. Fog from cooling towers can limit visibility and can be an architectural nuisance. Carryover is when small droplets of entrained water in the air stream are not caught by the drift eliminators and are ejected in the exhaust air stream. These droplets then precipitate out from the exhaust air and fall to the ground like a light mist or rain (in extreme cases). Carryover or drift contains minerals and chemicals from the water treatment in the tower and can cause staining or discoloration of the surfaces it settles upon. To mitigate problems with fog or carryover, as with recirculation, the designer should consider nearby traffic patterns, parking areas, prevailing wind direction, large glass areas, or other architectural considerations. Operation and Maintenance Winter Operation

If chillers or refrigeration equipment are being used in cold weather, freeze protection should be considered to avoid formation of ice on or in the cooling tower. Capacity control is one method that can be used to control water temperature in the tower and its components. Electric immersion heaters are usually installed in the tower sump to provide additional freeze protection. Since icing of the air intakes can be especially detrimental to tower performance, the fans can be reversed to de-ice these areas. If the fans are operating in extremely cold weather, ice can accumulate on the leading edges of the fan blades, which can cause serious imbalance in the fan system. Instrumentation to detect out-of-limits vibration or eccentricity in rotational loads should be installed. As with any operational equipment, frequent visual inspections during extreme weather are recommended. Water Treatment

The water circulating in a cooling tower must be at an adequate quality level to help maintain tower effectiveness and prevent maintenance problems from occurring. Impurities and dissolved solids are concentrated in tower water because of the continuous evaporation process as the water is circulated through the tower. Dirt, dust, and gases can also find their way into the tower water and either become entrained in the circulating water or settle into the tower sump. To reduce the concentration of these contaminants, a percentage of the

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circulating water is drained or blown-down. In smaller evaporatively cooled systems, this process is called a bleed-off and is continuous. Blow-down is usually 0.8 to 1.2% of the total water circulation rate and helps to maintain reduced impurity concentrations and to control scale formation. If the tower is served with very poor water quality, additional chemical treatments might be needed to inhibit corrosion, control biological growth, and limit the collection of silt. If the tower installation presents continuing water quality problems, a water treatment specialist should be consulted. Legionellosis

Legionellosis has been connected with evaporative condensers, cooling towers, and other building hydronic components. Researchers have found that well-maintained towers with good water quality control were not usually associated with contamination by Legionella pneumophila bacteria. In a position paper concerning Legionellosis, the Cooling Tower Institute (CTI, 1996) stated that cooling towers are prone to colonization by Legionella and have the potential to create and distribute aerosol droplets. Optimum growth of the bacteria was found to be at about 37°C (99°F) which is an easily attained temperature in a cooling tower.

The CTI proposed recommendations regarding cooling tower design and operation to minimize the presence of Legionella. They do not recommend frequent or routine testing for Legionella pneumophila bacteria because there is difficulty interpreting test results. A clean tower can quickly be reinfected, and a contaminated tower does not mean an outbreak of the disease will occur. Maintenance

The cooling tower manufacturer usually provides operating and maintenance (O&M) manuals with a new tower installation. These manuals should include a complete list of all parts used and replaceable in the tower and also details on the routine maintenance required for the cooling tower. At a minimum, the following should also be included as part of the maintenance program for a cooling tower installation.

? Periodic inspection of the entire unit to ensure it is in good repair.

? Complete periodic draining and cleaning of all wetted surfaces in the tower. This gives the opportunity to remove accumulations of dirt, slime, scale, and areas where algae or bacteria might develop.

? Periodic water treatment for biological and corrosion control.

? Continuous documentation on operation and maintenance of the tower. This develops the baseline for future O&M decisions and is very important for a proper maintenance policy.

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4.2.4 Packaged Equipment

Central HVAC systems are not always the best application for a particular cooling or heating load. Initial costs for central systems are usually much higher than unitary or packaged systems. There may also be physical constraints on the size of the mechanical components that can be installed in the building. Unitary or packaged systems come factory assembled and provide only cooling or combined heating and cooling. These systems are manufactured in a variety of configurations that allow the designer to meet almost any application. Cabinet or skid-mounted for easy installation, typical units generally consist of an evaporator, blower, compressor, condenser, and, if a combined system, a heating section. The capacities of the units ranges from approximately 5 kW to 460 kW (1.5 to 130 tons). Typical unitary systems are single-packaged units (window units, rooftop units), split-system packaged units, heat pump systems, and water source heat pump systems. Unitary systems do not last as long (only 8 to 15 years) as central HVAC equipment and are often less efficient.

Unitary systems find application in buildings up to eight stories in height, but they are more generally used in one-, two-, or three-story buildings that have smaller cooling loads. They are most often used for retail spaces, small office buildings, and classrooms. Unitary

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equipment is available only in preestablished capacity increments with set performance characteristics, such as total L/s (cfm) delivered by the unit’s air handler. Some designers combine central HVAC systems with packaged equipment used on perimeter building zones. This composite can solve humidity and space temperature requirements better than packaged units alone. This also works well in buildings where it is impractical for packaged units to serve interior spaces.

Table 4.2.7 lists some of the advantages and disadvantages of packaged and unitary HVAC equipment.

Table 4.2.8 lists energy efficiency ratings (EERs) for typical electric air- and water-cooled split and single package units with capacity greater than 19 kW (65,000 Btuh).

Typically, commercial buildings use unitary systems with cooling capacities greater than 18 kW (5 tons). In some cases, however, due to space requirements, physical limitations, or small additions, residential-sized unitary systems are used. If a unitary system is 10 years or older, energy savings can be achieved by replacing unitary systems with properly sized, energy-efficient models.

a Electric air- and water-cooled split system and single package units with capacity over 19 kW(65,000 Btuh) are covered here.

b EER, or energy efficiency ratio, is the cooling capacity in kW (Btu/h) of the unit divided by its electrical input (in watts) at standard (ARI) conditions of 35°C (95°F) for air-cooled equipment, and 29°C (85°F) entering water for water-cooled models.

c Based on ARI 210/240 test procedure.

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d SEER (seasonal energy efficiency ratio) is the total cooling output kW (Btu) provided by the unitduring its normal annual usage period for cooling divided by the total energy input (in Wh)during the same period.

e Split system and single package units with total capacity under 19 kW (65,000 Btuh) are covered here. This analysis excludes window units and packaged terminal units.

FIGURE 4.2.18 Comparison between TXV and short-tube orifice systems capacity for a range of charging conditions and 95°F (35°C) outdoor temperature. (From Rodriquez et al., 1996).

As with any HVAC equipment, proper maintenance and operation will ensure optimum performance and life for a system. Split-system air conditioners and heat pumps are the most common units applied in residential and small commercial applications. These units are typically shipped to the construction site as separate components; after the condenser (outdoor unit) and the evaporator (indoor unit) are mounted, the refrigerant piping is connected between them. The air conditioning technician must ensure that the unit is properly charged with refrigerant and check for proper operation. If the system is under- or overcharged, performance can be adversely affected. Rodriquez et al. (1996) found that performance of an air conditioning system equipped with a short tube orifice was affected by improper charge (Figure 4.2.18).

The plot in Figure 4.2.18 clearly shows that for a 20% under-charge in refrigerant, a unit with a short tube orifice suffers a 30% decrease in cooling capacity. This same study also investigated the effects of return-air leakage. A common problem with new installations is improper sealing of duct connections at the diffusers and grills as well as around the return-air plenum. Leakage amounts as low as 5% in the return air ducts resulted in capacity and efficiency reductions of almost 20% for high humidity climates.

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These reductions dropped to about 7% for low humidity climates. The results of the charging and leakage studies suggest the need for the installation contractor, maintenance contractor, and system owner to ensure the proper installation of the air conditioning system.

FIGURE 4.2.19 Rooftop packaged heating and air conditioning unit. (Adapted from Carrier Corporation). Packaged Units

Packaged units are complete HVAC units that are usually mounted on the exterior of a structure (roof or wall) freeing up valuable indoor floor space (Figure 4.2.19). They can also be installed on a concrete housekeeping pad at ground level. Because they are self-contained, complete manufactured units, installation costs are usually lower than for a site-built HVAC system.

Single-package units consist of a blower section, filter bank, evaporator coil, at least one compressor (larger units may have more than one), and an air-cooled condensing section. Units may also come equipped with a heating section. Heating is accomplished using either natural gas or electricity. Heat pump systems can be used in situations where electricity is the only source of energy. Unitary heat pumps are restricted in size to no more than 70 kW (20 tons).

As packaged units age and deteriorate, their efficiency often decreases while the need for maintenance increases. Upgrading existing packaged units to high-efficiency models will result in substantial longterm energy savings. In the last 10 to 15 years, manufacturers have made significant improvements in the efficiency of packaged units. The efficiency of energy transfer at both the evaporator and condenser coils has been improved, high-efficiency motors are now standard, and blower and compressor designs have

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improved in high-efficiency packaged units. Scroll compressors are now commonplace on mediumsized (70 to 210 kW; 20 to 60 ton) rooftop units. Energy efficiencies of newer units have a SEER in the range of 9.50 to 13.0. It is not uncommon to find older units operating at efficiencies as low as 6.0, and most operate at less than 9.0. Gas-fired heating sections typically have an annual fuel utilization efficiency

(AFUE) of about 80%. All newer packaged rooftop units are equipped with factory-installed microprocessor controls. These controls make maintaining equipment easier and improve energy efficiency of both the unit and the overall HVAC system. Control features include temperature setback and on/off scheduling. Larger systems can be delivered with variable air volume capability. Also, most units have an optional communication interface for connection to an energy management control system. Vertical Packaged Units

Vertical packaged units are typically designed for indoor or through-the-wall installation. These units are applied in hotels and apartments. Some designs have a water-cooled condenser, which can be fed from a cooling tower and/or city water. Many others use standard air-cooled condensers. Both style units have all other components mounted inside the package. Ductwork, if needed, can be connected to the unit to distribute the air.

FIGURE 4.2.20 Split system diagram (courtesy of the Trane Co.).

Split-System Packaged Units

Split-system packaged units can have the condenser mounted on an outdoor housekeeping pad or on a rooftop. Refrigerant piping connects the compressor section to an indoor air handling unit and evaporator coil. Unless they are heat pump type units, they cannot provide heat to the space. Heating coils can be installed in the air handling section,

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particularly if there is a central source of heat such as hot water or steam from a boiler. Alternatively, the indoor unit can be coupled to a gas-fired furnace section to provide heating.

Air Source Heat Pumps

Air source heat pump (ASHP) systems are typically rooftop units, either packaged complete or as split systems. Split-package heat pumps are designed with an air handling unit located inside the conditioned space, while the condenser and compressor are packaged in units for outdoor installation on a housekeeping pad or on the roof. During cooling mode, the heat pump operates an air conditioner. During heating mode, the system is reversed and extracts energy from the outside air and provides it to the space. Each of these cycles is shown schematically in Figures 4.2.21 and 4.2.22, respectively. The size of unitary heat pump systems ranges from approximately 5 to 70 kW (11/2 to 20 tons). In some cases, existing packaged cooling units with electric resistance heat can be upgraded to heat pumps for improved energy efficiency.

Heat pump applications are best suited to mild climates, such as the southeastern portion of the U.S., and to areas where natural gas for heating is less available. Space heating needs may exceed the capacity of the heat pump during extremely cold weather. This is because the units are most often sized to satisfy the cooling load requirements. As the outdoor temperature drops, the coefficient of performance (COP) of the heat pump decreases. A 26 kW (71/2 ton) rooftop heat pump unit that has a high temperature (8.3°C) COP of 3.0 can have a low temperature (–8.3°C) COP of 2.0 or less. Because the capacity also drops with outdoor temperature, heat pumps require supplemental electric resistance heat to maintain temperature in the building. Figure 4.2.23 shows typical trends in capacity and COP for an air source heat pump. Chapter 4.2 discusses the characteristics of heat pumps.

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FIGURE 4.2.21 Air or water source heat pump in cooling mode (courtesy of the Trane Co.).

FIGURE 4.2.22 Heat pump schematic showing heating cycle (courtesy of the Trane Co.).

FIGURE 4.2.23 System heating capacity as a function of outdoor air temperature.

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