DOUBLE-PIPE HEAT EXCHANGER

Transcription

1 DOUBLE-PIPE HEAT EXCHANGER by Jeffrey B. Wllams Project No. 1H Laboratory Manual Assgned: August 26, 2002 Due: September 18, 2002 Submtted: September 18, 2002 Project Team Members for Group B: Thomas Walters, Group Leader Dong-Hoon Han Jeffrey B. Wllams Jeffrey Wllams

2 TABLE OF CONTENTS SUMMARY v I. INTRODUCTION 1 II. THEORY 3 III. EQUPMENT SPECIFICATIONS 6 IV. SYSTEM OVERVIEW 11 V. PROCEDURE 13 A. OPTO B. USING DATA IN EXCEL 15 C. SYSTEM STARTUP 17 D. TURNING ON THE STEAM 17 E. CALIBRATION OF THE FLOW METERS 18 F. MAKING MEASUREMENTS 18 G. SAFETY 19 TABLE OF NOMENCLATURE 21 REFERENCES 23

3 LIST OF FIGURES No. Ttle Page 1. Pump 6 2. Gate Valve 7 3. Dsk Globe Valve 7 4. Ball Valve 8 5. Control Valve 8 6. Flow Meter 9 7. Process Flow Dagram of Double-Ppe Heat Exchanger Photo of Double-Ppe Heat Exchanger Loadng the Heat Exchanger Software Selectng the Double-Ppe Heat Exchanger Opto-22 Interface Openng the Double-Ppe Data Text Wzard Step One Text Wzard Step Two Text Wzard Step Three Pump Power Swtch 19

4 SUMMARY Double-Ppe Heat Exchanger, Project No. 1H Group B Jeffrey B. Wllams (report author), Thomas Walters, Dong-Hoon Han Report Date: September 18, 2002 A Procedures Manual was created for the double-ppe heat exchanger. The theores of transent heat transfer n double-ppe heat exchangers were explaned and followed by lterature correlatons. All of the nstrument specfcatons were defned. A procedure for use of the equpment and the software was outlned. Safety and other concerns durng operaton were dscussed. Ths manual wll serve to drect anyone n how to start up and run the double-ppe heat exchanger. It s recommended that ths Procedures Manual be fled wth Robert Cox n MEB It s recommended that students be gven access to the followng manual, to ad them n ther understandng of the use of the equpment. It s also recommended that students begn wth a calbraton of the flow meters and possbly the thermocouples before begnnng use of the equpment. All calbraton data, where possble, should be coordnated wth the computer-generated data. v

5 I. INTRODUCTION Temperature can be defned as the amount of energy that a substance has. Heat exchangers are used to transfer that energy from one substance to another. In process unts t s necessary to control the temperature of ncomng and outgong streams. These streams can ether be gases or lquds. Heat exchangers rase or lower the temperature of these streams by transferrng heat to or from the stream. Heat exchangers are a devce that exchange the heat between two fluds of dfferent temperatures that are separated by a sold wall. The temperature gradent, or the dfferences n temperature facltate ths transfer of heat. Transfer of heat happens by three prncple means: radaton, conducton and convecton. In the use of heat exchangers radaton does take place. However, n comparson to conducton and convecton, radaton does not play a major role. Conducton occurs as the heat from the hgher temperature flud passes through the sold wall. To maxmze the heat transfer, the wall should be thn and made of a very conductve materal. The bggest contrbuton to heat transfer n a heat exchanger s made through convecton. In a heat exchanger forced convecton allows for the transfer of heat of one movng stream to another movng stream. Wth convecton as heat s transferred through the ppe wall t s mxed nto the stream and the flow of the stream removes the transferred heat. Ths mantans a temperature gradent between the two fluds. The double-ppe heat exchanger s one of the smplest types of heat exchangers. It s called a double-ppe exchanger because one flud flows nsde a ppe and the other flud flows between that ppe and another ppe that surrounds the frst. Ths s a concentrc tube constructon. Flow n a double-ppe heat exchanger can be co-current or counter-current. There are two flow confguratons: co-current s when the flow of the two streams s n the same drecton, counter current s when the flow of the streams s n opposte drectons. As condtons n the ppes change: nlet temperatures, flow rates, flud propertes, flud composton, etc., the amount of heat transferred also changes. Ths transent behavor leads to 1

6 change n process temperatures, whch wll lead to a pont where the temperature dstrbuton becomes steady. When heat s begnnng to be transferred, ths changes the temperature of the fluds. Untl these temperatures reach a steady state ther behavor s dependent on tme. In ths double-ppe heat exchanger a hot process flud flowng through the nner ppe transfers ts heat to coolng water flowng n the outer ppe. The system s n steady state untl condtons change, such as flow rate or nlet temperature. These changes n condtons cause the temperature dstrbuton to change wth tme untl a new steady state s reached. The new steady state wll be observed once the nlet and outlet temperatures for the process and coolant flud become stable. In realty, the temperatures wll never be completely stable, but wth large enough changes n nlet temperatures or flow rates a relatve steady state can be expermentally observed. 2

7 II. THEORY The theory behnd the operaton of a double-ppe heat exchanger s covered n Incropera and Dewtt (1996). Also n ths same textbook s the dervaton of how transent behavor s treated wth respect to heat transfer. As wth any process the analyss of a heat exchanger begns wth an energy and materal balance. Before dong a complete energy balance a few assumptons can be made. The frst assumpton s that the energy lost to the surroundngs from the coolng water or from the U- bends n the nner ppe to the surroundngs s neglgble. We also assume neglgble potental or knetc energy changes and constant physcal propertes such as specfc heats and densty. These assumptons also smplfy the basc heat-exchanger equatons. The determnaton of the overall heat-transfer coeffcent s necessary n order to determne the heat transferred from the nner ppe to the outer ppe. Ths coeffcent takes nto account all of the conductve and convectve resstances (k and h, respectvely) between fluds separated by the nner ppe, and also takes nto account thermal resstances caused by foulng (rust, scalng,.e.) on both sdes of the nner ppe. For a double-ppe heat exchanger the overall heat transfer coeffcent, U, can be expressed as 1 1 R fo 1 d, = + + ln U A Ao ho Ao 2 k l π d, o R f + A 1 +. h A ( 1) In a heat exchanger the log-mean temperature dfference s the approprate average temperature dfference to use n heat transfer calculatons. The equaton for the log-mean temperature dfference s T LM = ( T T ) ( T T ), o o, T ln T, o, T, T o, o, o o, o. ( 2) 3

8 Flud propertes such as densty, vscosty and heat capacty are evaluated at the average temperatures. The average s found by usng the nlet and outlet values T, a T o, a = = T + T, o, 2 T + T o, o o, 2. (3). (4) Thermal conductvty, k, can be evaluated at the average of the average temperatures. In a double-ppe heat exchanger the nner ppe s made of a conductve metal and s thn. The problem can be further smplfed f the equpment s assumed to be clean, whch means that no scalng exsts. Ths s a poor smplfcaton wth the double-ppe heat exchanger n the laboratory, because t s many years old. The foulng factors R fo and R f can be looked up from varous sources, ncludng Standards of the Tubular Exchange Manufacturers Assocaton, or lumped together and determned expermentally. The only part of the overall heat-transfer coeffcent that needs to be determned s the convectve heat-transfer coeffcents. Correlatons are used to relate the Reynolds number to the heat-transfer coeffcent. The Reynolds number s a dmensonless rato of the nertal and vscous forces n flow. d m, Re =. (5) µ a In the nner ppe f the Reynolds s less than 2000 ths s consdered to be lamnar flow and the Nusselt number s equal to If the Reynolds number s greater than 10,000, the Nusselt number s gven by for turbulent, fully developed flow where Nu = 0.023Re 4 5 Pr n l Re 10,000, 10, 0.6 Pr 160, d, n = 0.4 for T > T or n = 0.3 for T > T. s m m s ( 6) Cp µ Pr =. k ( 7) 4

9 Ths gves a Nusselt number that can then be use to fnd h Nu = h d k,. ( 8) The convectve heat transfer coeffcent n the annulus s more dffcult to determne. The hydraulc dameter s used to fnd the Reynolds number. The hydraulc dameter s defned as the cross-sectonal area perpendcular to flow dvded by the wetted permeter. Wth the Reynolds number calculated the same correlatons apply and wth these h o can be determned. Once all the separate heat-transfer coeffcents are calculated an overall heat transfer coeffcent s calculated. Now everythng that was necessary for an energy balance s avalable. Wth the prevous assumptons made earler the dynamc equatons would be dt, a m Cp = q ρ Cp dt dt 0, a mo Cpo = qo ρ o Cp dt ( T, T, o o ( To, To, o ) U A T LM ) + U A T. LM. ( 9) ( 10) Wth the transent data taken expermentally an overall heat-transfer coeffcent can be determned at each tme step. Ths can be solved numercally. 5

10 III. EQUIPMENT SPECIFICATIONS The followng s a lst of all peces of equpment and ther specfcatons for the double-ppe heat exchanger. 1) Pump Manufactured by: Dayton Electrc Manufacturng Model: Teel Industral Seres (see Fgure 1) Horsepower 2 RPM: 3485 Effcency 80 Incomng ppe dameter: 2 n, Schedule 40 stanless steel Outlet ppe dameter: 1 1/2 n, Schedule 40 stanless steel Fgure 1 Ths Pump s used to pump the flud from the tank to double-ppe exchanger. 2) Double-Ppe Heat Exchanger Materal: Schedule 40 stanless steel Length: 14 ft Insde Ppe Dameter: 1 1/4 n Outsde Ppe Dameter: 2 n Steam Pass 1 Coolng Water Pass 4 6

11 3) Valves Gate Valves Manufactured by: Stockham Locaton: Steam Valves (see Fgure 2) Fgure 2 Ths valve allows steam to enter the steam ppe n the annulus of the double-ppe heat exchanger. Dsk Globe Valve Manufactured by: Nbco Locaton: Cold Water Valves (see Fgure 3) Fgure 3 When ths valve s open the cold water can enter the double-ppe heat exchanger. Ball Valves Manufactured by: Watts Regulator or Apollo 7

12 Locaton: Process Valves, Tank Valve, Dran Valve (see Fgure 4) Fgure 4 When the valve (Apollo) on the left s open t allows the coolng water to travel to the dran. When the valve (Watts Regulator) on the rght s open, the process flud can travel to the dran. Computer Controlled Valves Manufactured by: Scott Johnson Model: Valtek (see Fgure 5) Operatng Temperature: 0 to 55 C Maxmum Ar Pressure: 30 psg Fgure 5- Control valve used to control amount of coolant flow to the heat exchanger. 8

13 4) Flow meters Manufactured by: Brooks Instruments Model: MT 3810 (see fgure 6) Accuracy: ±5% full scale from 100% to 10% of scale readng Repeatablty: 0.25% full scale Operatng Temperature: -39 to 215 C Flow Range: 4.1 to 41.6 gpm for nner-ppe flow meter 2.6 to 26.4 gpm for outer-ppe flow meter 5) Thermocouples Fgure 6 Meter measures the flow of process flud comng from the pump. Manufactured by: Model: Sheath Materal: Sheath Length: Omega Type T 304 Stanless Steel 12 n Temperature Range: -60 to 100 C Accuracy: 1.0 C or 0.75% above 0 C (whchever s greater) 1.0 C or 1.5% below 0 C (whchever s greater) 9

14 6) Low Pressure Steam Pressure: 27 psa Temperature: 118 C 7) Computer Manufactured by: Operatng System: Software: Dell Systems Wndows NT Opto-22 electroncs and computer based software, Verson R3.16. Copyrght Opto

15 IV. SYSTEM OVERVIEW The double-ppe heat exchanger used n expermentaton s located n MEB Fgure 7 descrbes the setup of double-ppe heat exchanger. Flud from the tank s frst heated n the by steam that s condensng n the annulus and s then cooled by the four coolng-water passes. In all nstances low-pressure steam s used to heat the flud and water s used to cool the flud. Once cooled the flud s then returned to the tank. There are sx thermocouples that record temperature at sx dfferent ponts that can be seen n the followng fgure. The frst records the temperature of the nlet process flud, the second records the process flud temperature after heatng wth steam, the thrd records the temperature after coolng wth the water, the fourth records the coolng-water temperature at the nlet, the ffth records at the outlet and the sxth records the steam temperature at the nlet. There s a control valve that controls the steam nlet, the process flud nlet and the coolng-water outlet. There are manual valves that also need to be opened before the process could begn, even f the control valves were open to 100%. Once the proper valves are opened the pump can be manually actvated. Fgure 7- Process flow dagram for the double-ppe heat exchanger 11

16 Fgure 8 s a full overvew of the double-ppe heat exchanger taken from the south sde. The dsk ncluded wth ths manual ncludes ths pcture, as well as other pctures. Fgure 8- Pcture of the double-ppe heat exchanger from the southwest corner 12

17 V. PROCEDURE A. OPTO-22 Software The valves on the double-ppe heat exchanger are electroncally controlled, and the data from the thermocouples and the flow meters are taken va computer. The followng steps wll explan how to start the software, and what the varous sectons of the software mean. 1. Turn on the computer. 2. Clck on the con readng Shortcut to Heat Exchanger MMI. Ths wll load the Opto-22 software. 3. The frst screen that appears wll look lke ths: Fgure 9- Loadng the heat-exchanger software. 4. Clck on the Heat-Exchanger Menu button. At ths tme, another menu wll come up. Fgure 10- Selectng the double-ppe heat exchanger 5. Select the Double Ppe. 6. Fgure 11 opens up next. Ths s the Opto-22 nterface screen. All the work that s done whle the heat exchanger s operatng wll be done here. For both the coolng and the water (process) flow the manpulated varable (MV) s the 13

18 percent openng of the respectve control valves. 100% means that the valve s fully opened and 0% means that the valve s completely closed. The valve settng s changed by openng up the green MV and nputtng a value of Fgure 11- Opto-22 nterface and control screen 7. The lower porton of Fgure 11 shows values for the sx dfferent thermocouple readngs, for the coolant and process flow meters and also for the control valves themselves. The colors n these boxes correspond wth the colors of the lnes n the graphs. Ths Opto screen provdes numercal values and plots the numercal values to the graph. A new readng s taken and recorded at least every 5 seconds. Old data are saved to a fle and are accessble n ths screen. 14

19 B. USING DATA IN EXCEL 1. Open the fle n Excel. It s found n the C drve n double-ppe data. Fgure 12 Openng the double-ppe data 2. The data are saved n comma-delmted form. So Excel has to convert ths to rows and columns. Fgure 13 Text wzard Step one ndcates that the data are n delmted form 15

20 Fgure 14 The data are delmted usng commas, ths s apparent from the prevew Fgure 15 The last step n the mport wzard s to specfy the data format of the columns and create the spreadsheet 3. Once n Excel the data can then be studed and used n any necessary calculatons. All the data (thermocouple readngs, flow measurements and % control valve openng) gven on the Opto-22 nterface are recorded n ths style for later use. Data such as ths are very useful n the study of transent behavor. 16

21 C. SYSTEM STARTUP Between runs and at startup, the heat exchanger has to be cleaned n order to remove any rust or scale that has bult up n the ppes. 1. Make sure the computer s on, and the Opto-22 software s runnng on the double-ppe heat exchanger. Open both the valves to 100%. 2. Open the globe valve to the cold water. Close valve that allows water to the reach the tank. Allow tme for the coolng water to flush the coolant ppe. 3. Close the ball valve to the recycle, the tank and to the dran. Open the ball valve that allows mxng of coolng water and process flud. Ths wll allow the coolng water to run through the nner ppe and clean out any debrs or deposts. 4. Once the tank s about half way full open the tank valve and the dran valve and allow tank and nner ppe to dran, repeat ths same process untl the process flud looks clean. 5. Once the flud s clean close the tank valve, close the dran valve and proceed to fll the tank. 6. When fnshed, close the valve that allows the coolng water to mx wth the process flud. Ths wll close the loop to the process. Open the dran valve to allow coolng water to cycle. D. TURNING ON THE STEAM 1. Get a ladder, put on thermal resstant gloves and open the steam valve above the heat exchanger. 2. Open the nlet steam valve to the exchanger. 3. Open the outlet steam (gate) valve. 4. The steam trap wll capture the steam and condense t. Ths wll control the flow of steam to the exchanger. The steam wll condense as t transfers t s heat to the process flud. The steam trap wll allow only lqud to the steam dran. 17

22 E. CALIBRATION OF THE FLOW METERS Smple calbratons can be made for the flow meters. Calbrated data should be compared not only wth the nstrument tself, but when and where possble also wth Opto-22. The thermocouples should not be removed from the heat exchanger, however the thermocouples stll need to be calbrated regularly, f and when, necessary calbraton of the thermocouples should take place 1. Use a bucket or other contaner that can hold water. The contaner should have a volume of at least 3 gallons, but not exceed 5 gallons. Obtan a scale that can measure mass up to 10 kg. Obtan a stopwatch. 2. Determne what range of flow rates that are necessary to gve the requred condtons. At least three separate flow rates should be used. 3. Open or close the control valve to gve the needed flow rates. 4. Record the flow rate both on the computer and on the flow meter; these should be the same. 5. Have one person record tme, one person hold the bucket and one person watch the flow meter to look for varaton n the flow. 6. Wegh the empty bucket. 7. Fll the bucket from the tank nlet, record the tme t takes to fll the bucket. 8. Record the temperature of the outgong stream. 9. Wegh the bucket wth water. Fnd the weght of the water. 10. Determne the flow rate usng densty of the flud at the recorded temperature 11. Repeat the calbraton two to four tmes. F. MAKING MEASUREMENTS Once the flow meters have been calbrated and the system s flushed of any loose scalng, measurements can be made. Dependng on the nature of the experments to be performed, whether they be steady- or unsteady-state, the followng procedure mght vary. It s recommended however to frst open the control valves to a settng that allows the type of flow that s needed, whether t be lamnar or 18

23 turbulent. Second s to actvate the pump to gve the needed flow n the process flud. Fgure 16 shows the pump control on the west wall that needs to be used. The pump on the operators left s the one that s used for the double-ppe heat exchanger. Once the pump s actvated, wat and see when the system reaches steady state and then make any necessary changes to the system. Fgure 16 The pump power swtch s the swtch on the left. These power swtches are located on the wall behnd the heat exchanger. G. SAFETY Safety precauton s of utmost mportance wth any process. At all tmes operators of the equpment should wear safety glasses and hardhats, especally when the steam s turned on, or people are workng on ladders (wth the steam valves). When openng or closng the steam valves, always wear heat-resstant gloves. After the steam s turned on, care must be exercsed as the water and ppes wll become warm. Avod touchng the warm metal. Most of the crtcal areas are nsulated; however, there are several exposed ppes that can become qute warm. The level of water n the tank should be kept at least one-thrd full to decrease the amount of splashng, especally when the water s hot. As water splls may occur, t s also mportant to have a mop and bucket on hand whenever the heat exchanger s used. Clean up any splls mmedately to avod damagng any electroncs, especally on the computer 19

24 controllng the equpment or the pump. It s mportant to keep the area surroundng the pump clear. Whle t s enclosed and mounted, the pump does requre ventlaton. Also, never run the pump dry or run t wth the process and wth the bypass valve shut. Not only does t wear out the valves and seals, but the pump can also overheat. 20

25 TABLE OF NOMENCLATURE Symbols A =Area for heat transfer, ft 2 A, =Surface area of the nsde of the nner ppe, ft 2 A,o =Surface area of the outsde of the nner ppe, ft 2 a =Cross sectonal area of nner ppe, ft 2 Cp =Heat capacty of the flud n the nner ppe, Btu/lbm. o F Cp o =Heat capacty of the flud n the outer ppe, Btu/lbm.o F D,, d o,, d,,o d o,,o h h o k l m m o Nu Nu Pr Pr o Re Re o T, T o, T,o T o,o =Inner dameter of nner ppe, ft =Inner dameter of outer ppe, ft =Outer dameter of nner ppe, ft =Outer dameter of Outer ppe, ft =Convectve heat-transfer coeffcent of flud n nner ppe, Btu/hr. ft 2.o F =Convectve heat-transfer coeffcent of flud n outer ppe, Btu/hr. ft 2.o F =Thermal conductvty of nner ppe materal, Btu/hr. ft.o F =Total ppe length, ft =Mass of flud n nner ppe, lbm =Mass of flud n outer ppe, lbm =Nusselt number of flud n nner ppe =Nusselt number of flud n outer ppe =Prandtl number of flud n nner ppe =Prandtl number of flud n outer ppe =Reynolds number of flud n nner ppe =Reynolds number of flud n outer ppe =Inlet temperature of flud n nner ppe, o F =Inlet temperature of flud n outer ppe, o F =Outlet temperature of flud n nner ppe, o F =Outlet temperature of flud n outer ppe, o F DT LM =Log-mean temperature dfference T,a T o,a =Average temperature of flud n nner ppe, o F =Average temperature of flud n nner ppe, o F 21

26 Rf =Foulng factor nsde the nner ppe, Btu/hr. ft 2.o F Rf o =Foulng factor outsde the nner ppe, Btu/hr. ft 2.o F u u o =Velocty of the flud n the nner ppe, ft/s =Velocty of the flud n the outer ppe, ft/s q =Volumetrc flow rate of the flud n the nner ppe, ft 3 /s q o =Volumetrc flow rate of the flud n the nner ppe, ft 3 /s Q =Heat transfer rate, Btu/hr U =Overall heat-transfer coeffcent, Btu/hr. ft 2.o F Greek Symbols ρ =Densty of the flud n the nner ppe, lbm/ft 3 ρ o =Densty of the flud n the outer ppe, lbm/ft 3 µ =Vscosty of the flud n the nner ppe, lbm/ft. s µ o =Vscosty of the flud n the nner ppe, lbm/ft. s 22

27 REFERENCES 1. denevers, N., Flud Mechancs, McGraw Hll, (1991). 2. Emerson Process Management product gude. brooks/ products/products3d.html (accessed Sept 2002). 3. Incropera, F.P., D.P. DeWtt, Fundamentals of Heat and Mass Transfer, John Wley & Sons, Inc., pp. 460, (1996). 4. Reddng, Alyssa M., Shell-and-Tube Heat Exchanger, Project 1, Laboratory Manual. Sept. 21, Standards of the Tubular Exchange Manufacturers Assocaton, 6 th ed., Tubular Exchanger Manufacturers Assocaton, New York,