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17 Pumps
Positive-Displacement Pumps
In the first major class of pumps, a definite volume of liquid is trapped in a chamber, which is alternately filled from the inlet and emptied at a higher pressure through the discharge. There are two subclasses of positive-displacement pumps. In reciprocating pumps, the chamber is a stationary cylinder that contains a piston or plunger; in rotary pumps, the chamber moves from inlet to discharge and back to the inlet.
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Reciprocating pumps
Piston pumps, plunger pumps, and diaphragm pumps are examples of reciprocating pumps. In a piston pump, liquid is drawn through an inlet check valve into the cylinder by the withdrawal of a piston and then is forced out through a discharge check valve on the return stroke.\u2460 Most piston pumps are double-acting with liquid admitted alternately on each side of the piston so that one part of the cylinder is being filled while the other is being emptied. \u2461Often two or more cylinders are used in parallel with common suction and discharge headers, and the configuration of the pistons is adjusted to minimize fluctuations in the discharge rate. The piston may be motor-driven through reducing gears, or a steam cylinder may be used to drive the piston rod directly. The maximum discharge pressure for commercial piston pumps is about 50 atm.
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For higher pressures, plunger pumps are used. A heavy-walled cylinder of small diameter contains a close-fitting reciprocating plunger, which is merely an extension of the piston rod. At the limit of its stroke the plunger fills nearly all the space in the cylinder. Plunger pumps are single-acting and usually are motor-driven. They can discharge against a pressure of 1,500atm or more.
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In a diaphragm pump, the reciprocating member is a flexible diaphragm of metal, plastic, or rubber. \u2462This eliminates the need for packing or seals exposed to the liquid being pumped, a great advantage when handling toxic or corrosive liquids. A typical unit is shown in Figure 17.1. Diaphragm pumps handle small to moderate amounts of liquid, up to about 100gal/min, and can develop pressures in excess of 100atm.
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14 Equipment for Heat Exchange14个热交换设备
A wide variety of equipment is available for conducting heat exchange.一个是各种各样的设备进行热交换。
①Commercial units range in size form very small, double-pipe heat exchangers, with less than 1 ft2 of heat transfer surface, to large, air-cooled units called fin-fan heat exchangers because they consist of tubes with external peripheral fins and fans to force air past the tubes.
①商业单位的尺寸范围的形式非常小,双管换热器,小于1传热表面平方英尺到大,风冷机组称为鳍帆式换热器,因为它们与外部周边鳍片和风扇来管组成部队的空军过去的管。
Finned area in a single unit is as large as 20,000 ft2. By far the most common units are shell-and-tube heat exchangers, which come in a variety of configurations in sizes from 50 to 12,000 ft2.
翅片在一个单位面积高达二点〇万平方英尺大。目前为止,最常见的单位是壳管式换热器,这在不同的配置,尺寸从50到一点二〇〇万平方英尺来。
For specialized applications, compact heat exchangers are challenging shell-and-tube units.
对于专门的应用程序,紧凑式换热器是具有挑战性的壳管式单位。
Double-Pipe Heat Exchangers双管式换热器
Figure 14.1 Double-pipe heat exchangers: (a) single unit;
图14.1双管换热器:(一)单一单位;
(b) hairpin unit(二)发卡单位
A typical double-pipe units is shown in Figure 14.1a.② In its simplest form, it consists of an inner straight pipe of circular cross section, concentric to and supported within an outer straight pipe by means of packing glands. One stream flows through the inner pipe, while the other stream flows countercurrently through the annular passage between the outer wall of the inner pipe and the inner wall of the outer pipe. When more heat transfer area is need, return bends and heads are used with additional pipes to build a hairpin unit, as shown in Figure 14.1b. Hairpin units are available in sizes up to about 200 ft2 of heat transfer area, and are competitive with shell-and-tube exchangers to at least 100 ft2. To prevent sagging of the inner pipe with a resulting distortion of annular cross section, pipe length is limited to 20 ft. Therefore, a 200-ft2 unit of 3-in.-diameter inner pipes requires 10 hairpin connections. When one stream is at high temperature and/or high pressure, and/or is corrosive, it is passed through the inner pipe. ③If the other stream is a gas, longitudinal fins can be added to the outside surface of the inner pipe to help balance the inner and outer heat transfer resistances. If crystallization occurs from a liquid stream flowing through the inner pipe, scrapers can be added inside that pipe to prevent buildup of crystals on the inner wall. Double-pipe exchangers are not recommended for use in boiling or vaporization services.
一个典型的double-pipe单位被显示在图14.1a。②它最简单的形式,它由一个内在的直线管路圆形截面和支持,同心外层直管内采用填料密封垫。一个流流经内管流的流动,而另countercurrently通过环形通道之间的内管外壁及内部墙上的外管。当更多的热交换面积是需要的,返回弯曲并头采用附加管道单位建立一个发夹,如图14.1b。发夹单位可在尺寸约200平方尺的热交换面积,也很有竞争力与shell-and-tube换热器,至少100平方尺。防止下垂的内管产生的扭曲与环形横截面上管长度被限制到20英尺。因此,200-ft2 3-in.-diameter单位内部管道需要10个发夹固定连接。当一个人流则是在高温和/或高压,和/或颇具腐蚀性,它是通过内管。③如果其他流是一种气体,纵向翅可以加内管的外表面,有助于平衡的内部和外部的换热电阻。如果结晶发生从液体流流经内管,铲运机可补充内,管线上,以杜绝积聚晶体内墙上。Double-pipe器不推荐用滚水内或汽化服务。
14 Equipment for Heat Exchange
14设备换热
A wide variety of equipment is available for conducting heat exchange. ①Commercial units range in size form very small, double-pipe heat exchangers, with less than 1 ft2 of heat transfer surface, to large, air-cooled units called fin-fan heat exchangers because they consist of tubes with external peripheral fins and fans to force air past the tubes. Finned area in a single unit is as large as 20,000 ft2. By far the most common units are shell-and-tube heat exchangers, which come in a variety of configurations in sizes from 50 to 12,000 ft2. For specialized applications, compact heat exchangers are challenging shell-and-tube units.
各种各样的设备能够进行热交换。①商业单位的范围大小形态非常小,double-pipe热交换器,并要求有少于1平方尺的传热面,大、风冷机组称为fin-fan热交换器,因为它们是由管与外部周边鳍和球迷们迫使空气过去管。翅片地区一个单一的单位是20,000平方尺一样大。到目前为止最常见的单位是shell-and-tube热交换器,它有多种不同的配置,面积从50到12000平方尺。为专业的领域中,紧凑热交换器是具有挑战性的shell-and-tube单位。
Double-Pipe Heat Exchangers
Double-Pipe热交换器
Figure 14.1 Double-pipe heat exchangers: (a) single unit; (b) hairpin unit
数字保存Double-pipe换热器:(一)单一的单位;(b)发夹单位
A typical double-pipe units is shown in Figure 14.1a.② In its simplest form, it consists of an inner straight pipe of circular cross section, concentric to and supported within an outer straight pipe by means of packing glands. One stream flows through the inner pipe, while the other stream flows countercurrently through the annular passage between the outer wall of the inner pipe and the inner wall of the outer pipe. When more heat transfer area is need, return bends and heads are used with additional pipes to build a hairpin unit, as shown in Figure 14.1b. Hairpin units are available in sizes up to about 200 ft2 of heat transfer area, and are competitive with shell-and-tube exchangers to at least 100 ft2. To prevent sagging of the inner pipe with a resulting distortion of annular cross section, pipe length is limited to 20 ft. Therefore, a 200-ft2 unit of 3-in.-diameter inner pipes requires 10 hairpin connections. When one stream is at high temperature and/or high pressure, and/or is corrosive, it is passed through the inner pipe. ③If the other stream is a gas, longitudinal fins can be added to the outside surface of the inner pipe to help balance the inner and outer heat transfer resistances. If crystallization occurs from a liquid stream flowing through the inner pipe, scrapers can be added inside that pipe to prevent buildup of crystals on the inner wall. Double-pipe exchangers are not recommended for use in boiling or vaporization services.
一个典型的double-pipe单位被显示在图14.1a。②它最简单的形式,它由一个内在的直线管路圆形截面和支持,同心外层直管内采用填料密封垫。一个流流经内管流的流动,而另countercurrently通过环形通道之间的内管外壁及内部墙上的外管。当更多的热交换面积是需要的,返回弯曲并头采用附加管道单位建立一个发夹,如图14.1b。发夹单位可在尺寸约200平方尺的热交换面积,也很有竞争力与shell-and-tube换热器,至少100平方尺。防止下垂的内管产生的扭曲与环形横截面上管长度被限制到20英尺。因此,200-ft2 3-in.-diameter单位内部管道需要10个发夹固定连接。当一个人流则是在高温和/或高压,和/或颇具腐蚀性,它是通过内管。③如果其他流是一种气体,纵向翅可以加内管的外表面,有助于平衡的内部和外部的换热电阻。如果结晶发生从液体流流经内管,铲运机可补充内,管线上,以杜绝积聚晶体内墙上。Double-pipe器不推荐用滚水内或汽化服务。
14设备换热
各种各样的设备能够进行热交换。①商业单位的范围大小形态非常小,double-pipe热交换器,并要求有少于1平方尺的传热面,大、风冷机组称为fin-fan热交换器,因为它们是由管与外部周边鳍和球迷们迫使空气过去管。翅片地区一个单一的单位是20,000平方尺一样大。到目前为止最常见的单位是shell-and-tube热交换器,它有多种不同的配置,面积从50到12000平方尺。为专业的领域中,紧凑热交换器是具有挑战性的shell-and-tube单位。
Double-Pipe热交换器
数字保存Double-pipe换热器:(一)单一的单位;(b)发夹单位
一个典型的double-pipe单位被显示在图14.1a。②它最简单的形式,它由一个内在的直线管路圆形截面和支持,同心外层直管内采用填料密封垫。一个流流经内管流的流动,而另countercurrently通过环形通道之间的内管外壁及内部墙上的外管。当更多的热交换面积是需要的,返回弯曲并头采用附加管道单位建立一个发夹,如图14.1b。发夹单位可在尺寸约200平方尺的热交换面积,也很有竞争力与shell-and-tube换热器,至少100平方尺。防止下垂的内管产生的扭曲与环形横截面上管长度被限制到20英尺。因此,200-ft2 3-in.-diameter单位内部管道需要10个发夹固定连接。当一个人流则是在高温和/或高压,和/或颇具腐蚀性,它是通过内管。③如果其他流是一种气体,纵向翅可以加内管的外表面,有助于平衡的内部和外部的换热电阻。如果结晶发生从液体流流经内管,铲运机可补充内,管线上,以杜绝积聚晶体内墙上。Double-pipe器不推荐用滚水内或汽化服务。
14热交换设备
各种各样的设备可以进行热交换。①大量装置的尺寸范围非常小,套管式热交换器只有不到1平方英尺的传热面,大的风冷装置称为翅扇式热交换器,因为它们是由外部边缘带有翅与扇的管组成,由此迫使空气越过管道。一个单一装置翅片区有20,000平方尺大。到目前为止最常见的装置是管壳式热交换器,它有多种不同的配置,面积从50到12000平方尺不等。在专业的领域应用中,紧凑式热交换器对管壳式热交换器形成挑战
套管式热交换器
图14.1套管式热交换器(a)单一结构 (b)U形管结构
图14.1a.为典型的套管式热交换器。②在此最简单的形式中包含由直径不同的直管制成的同心套管, 一种流体走管内,另一种流体走环隙, 而其他流体通过环形管道的内管和环隙流向相反方向。当传热较大时,可以使用逆流的液压制成U形管结构。如图14.1b,U形管结构可以在200平方尺内的热交换区域使用,与管壳式热交换器的至少100平方尺相比具有很有优势。为防止下垂的内管产生的扭曲,管长度被限制到20英尺。因此,在一个200平方英尺的区域内要求由10个直径为3英尺的内管连接。当一种流体在高温高压并且有害的情况下,它会流经内管。③如果其他流体为气体时,纵向翅可以增加内管的外表面积去平衡的内部和外部的换热电阻。如果液体流流经内管时发生结晶,可以在内管上加装刮刀以杜绝晶体积聚于内壁上。沸腾或汽化设备不推荐使用套管式热交换器。
14 热交换设备
热交换设备多种多样。商业领域中,有小至1-2ft大小的双管热交换器,大至百叶窗翅片式热交换器,它独特的外装翅片扇叶推动空气通过导管。翅片扇叶单位大小可达2万平方英尺。现在,最常用的是管壳式热交换机,机体大小有50平方英尺到1.2万平方英尺。有特殊程序控制,小型热交换机正在挑战传统管壳式热交换机。
图14.1 双管热交换器:(a)单口单元 (b)串口单元
图14.1a是典型的双管热交换器,在其最简单结构中,含一个圆形截面的内管,与一个填函料外管相连。一条液体流路经内管,而其它流路则在内管外壁和外管内壁间逆向流动。需要更多热交换空间时,回归管将与更多的导管共同使用,如图14.1(b)。串口单元可供高达200英尺热交换面积,在100平方英尺面积内可与管壳式热交换机媲美。为避免内管环形截面下垂,管长不得高于20英尺。因此,一个200平方英尺的3口径内管要求10条管束。当(无论是否高温、是否有腐蚀性)液体必流经内管,若其它流体为气态,可在内农牧管外壁安装翅片以平衡内外阻抗。若流经内管时发生液体凝固,安装相应刮片防止内管壁晶体沉积。双管交换器不适合运用于加热或蒸发。
绛旓細with less than 1 ft2 of heat transfer surface, to large, air-cooled units called fin-fan heat exchangers because they consist of tubes with external peripheral fins and fans to force air past the tubes.鈶
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