有机无机化学化工热力学教学课件流体传质

1、1 2 Content 8.1 PIPE, FITTINGS, AND VALVES 8.2 PUMPS 8.3 Centrifugal Pumps* 8.4 FANS, BLOWERS, AND COMPRESSORS 8.5 MEASUREMENT OF FLOWING FLUIDS 3 8.1 PIPE, FITTINGS, AND VALVES 1. Pipe and tubing 2. Joints and fittings 3. Allowances for expansion膨胀余量 4. Prevention of leakage around moving parts 5.

2、Valves 6. Recommended practice 4 1. Pipe and tubing Pipe: heavy walled and large diameter and comes in moderate lengths. Tubing: thin walled and coils 5 (a) Material Pipe and tubing can be made from: Metals and alloys: low carbon steel， copper Wood Ceramics（陶瓷） Glass Various plastics 6 (b) Size 1 -i

3、n. Schedule 40 pipe Appendix 3 25x2.5mm 19x2mm 7 8 (c) Selection of pipe sizes = optimum velocity,m/s 2 4 4 s s VVAd V V d V opt V 9 10 2. Joints and fittings Joints : Screwed fittings螺纹连接件,flanges 法兰 welding焊接 Soldering焊接（锡焊，软焊） , compression， flared fitting扩口式管接头 Flanges or bell and spigot joints法

4、兰或卡 套连接 11 Fittings 12 13 3. Allowances for expansion膨胀 余量 4. Prevention of leakage around moving parts (a) Stuffing boxes 14 FIGURE 8.1 Stuffing boxes: (a) simple form; (b) with lantern gland(液封环). 15 (b) Mechanical seals 16 5. Valves Gate valves 闸阀 Globe valves 截止阀 Plug cocks and ball valves 旋塞阀和球

5、阀 Check valves 止回阀 17 Gate valves 闸阀 Handwheel:手轮 Stem:阀杆 Follower or gland：压 盖 Packing ：填料 Nut:螺母 Bonnet：阀盖 Gate:闸板 18 Globe valves 截止阀 19 Ball valves 球阀 20 (a) lift check升降式止回阀; (b) ball check球形止回阀; (c) swing check旋启式止回阀. Check valves 止回阀 21 6. Recommended practice The detail issues in designing a

6、nd installing a piping system. 22 8.2 PUMPS 1. Developed head 2. Power requirement 3. Suction lift 吸上(or安装)高度 and cavitation 气蚀 4. Positive-Displacement Pumps 5. (8.3)Centrifugal Pumps * 23 Energy needed to transport fluid f aa a a p bb b b h V gZ p W V gZ p 22 22 22 22 aa a a f bb b b p V gZ p h V

7、gZ p W g V Z g p g h g V Z g p g W aa a a f bb b b p 22 22 22 22 aa aaf bb bbp V gZph V gZpW J/kg J/N, m J/m3 ,P a 24 The devices used to transport fluid (1). Liquid: pump Centrifugal pump离心泵 Positive-displacement pump正位移泵 (2). Gas: Fan 通风机 Blower 鼓风机 Compressor 压缩机 Vacuum pump 真空泵 25 Pump 泵 26 Cent

8、rifugal pump 2-1 27 1. Developed head A typical pump application is shown diagrammatically in Fig.8.5. 28 Write Bernoulli equation between station a and b. 22 22 aa a abb b b P V gZ PV gZ p W The friction in the pump itself is accounted for by the mechanical efficiency ,so friction loss hf=0 29 2 2

9、V gZ P H The total head (压头)is defined as: g V Z g P g H 2 2 w o r k / m a s s J/kg or m2/s2 Static pressure head potential head velocity head work/weight J/N or m 30 The total suction head is Ha g V Z g P g H aa a aa 2 2 g V Z g P g H bb b bb 2 2 The total discharge head is Hb HHHW abp 31 2. Power

10、requirement HHH W ab p (8.4) The work supplied to the pump. 轴功 The power supplied to the pump is denoted by PB (W)轴功率. Hm mWP PB (W) 32 The power delivered to the fluid is denoted by Pf (W)有效功率 HmPf f B P P 33 3. Suction lift 吸上（or安装）高度 and cavitation气蚀 Suction lift 34 (a) Cavitation:（气蚀现象） The pump

11、 must be installed at proper p o s i t i o n t o a v o i d cavitation. 35 (b) Net positive suction head(NPSH)气蚀 余量 To avoid cavitation, the pressure at the pump inlet must exceed the vapor pressure by a certain value. Net positive suction head(NPSH) is defined as: g p g p g V NPSH va a a 2 2 (m) 36

12、Write B-eq. between station a and station a a fs aaaa Z g h g p g V g p 2 2 Substituting a fs aaaa Z g h g V g p g p 2 2 Therefore, g p g p g V NPSH va a a 2 2 37 afs va Zh pp g NPSH 1 Pa-absolute pressure at surface of reservoir Pv -vapor pressure at the operating condition hfs -friction loss in su

13、ction line. (8.9a) 38 (c) Suction lift 允许吸上高度(安装高度) NPSHh pp g Z fs va a 1 Za - the suction lift. That is, the pump must be installed lower than Za to avoid cavitation. 39 1. Minimum required net positive suction head (NPSHR)必需气蚀余量, which specified by the pump manufacturers, is given in the manufact

14、urers catalogs. Actual NPSH must large NPSHR more than 0.5m. 2. NPSHR increase with the increase of flow rate, therefore, suction lift should be calculated according to the highest flow rate in the operation. EXAMPLE 8.1 40 (d) 允许吸上真空度（离心泵的 另一抗气蚀性能指标 ） 泵入口处的允许最高真空度，习惯上以输送液体的 液柱高度来计量，称之为允许吸上真空度，以Hs来

15、表示，单位为m液柱。 g pp H a s 1 41 Writing Bernoulli equation from a to a, g h g V HZ fs a a sa 2 2 a fs aaaa Z g h g V g p g p 2 2 42 Hs值由泵的制造厂家在98.1kPa(10mH2O)下，用20清水实 验测得。实验值列在泵样本或说明书的性能表中。若操作条 件与上述实验条件不一致或输送其它液体时，可按下式对Hs 值进行换算， 3 1000 24. 0 1081. 9 10 v ass p HHH Hs -操作条件下输送液体时的允许吸上真空度，m液柱； Hs -实验条件下送水时

16、的允许吸上真空度，mH2O； Ha -泵安装地区的大气压强，mH2O， pv -操作条件下液体的饱和蒸气压，Pa； 10 -实验条件下的大气压，mH2O； 0.24 -20下水的饱和蒸气压，mH2O； 1000 -实验条件下水的密度，kg/m3； -操作条件下液体的密度，kg/m3。 43 (e) How to avoid cavitation! 1. Move pump closer to source tank. 2. Decrease the head loss in the suction line.(Enlarge the diameter, reduce the length, r

17、educe the number of fittings and valves, fully open suction line valve etc) NPSHh pp g Z fs va a 1 44 4. Positive-Displacement Pumps Reciprocating pump往复泵 Rotary pump旋转泵 45 1. Cylinder泵缸 2. Piston or plunger活塞 3. Piston rod活塞 杆 4. Inlet check valve 吸入阀 5. Discharge check valve排出阀 (a) Structure and w

18、orking process of a Reciprocating pumps 46 Working principles of reciprocating pump2-5 Double-acting pump2-6 47 Single-acting pump 48 double-acting reciprocating pump 49 Volumetric flow rate q (m3/s)流量 (b)Performance characteristics特性参 数 A: cross section area of piston, m2 S: stroke of piston, m nr:

19、 reciprocating number per min, time/min or time/s. a: cross-section area of piston rod, m2 50 Single-acting pump: q =Asnr m3/min or m3/s Double-acting pump: q=(2A-a)Snr m3/min or m3/s Volumetric efficiency.容积效率 volumetric efficiency = (the volume of fluid discharged) / (the volume swept by the pisto

20、n or plunger) 51 There is no relationship between volumetric flow rate and the developed head for positive- displacement pump. Developed head H （J/kg） or H(J/N or m) 压头 52 The mechanical efficiency of reciprocating pumps varies from 40 to 50 percent for small pumps to 70 to 90 percent for large ones. Mechanical efficiency 机械效率 53 Power input, shaft power PB轴功率 The power supplied to the pump by an external source is denoted by PB (W). )(W HqHm mWP PB In chine