DOUBL OUBL E-PIPE HEAT EXCHANGER E XCHANGER by J effre ffrey B. Williams
Project No. 1H L aboratory boratory Ma M anua nual
Assign ssigne ed: Augu A ugust st 26, 26, 2002 Due: Septe September 18, 2002 Submitted: Submitted: Septe September 18, 2002 Proje roject Te Team Members for for Group Group B: Tho Thomas Walte lters, Gr Gro oup Lea Leader Dong-Hoon Han J effre ffrey B. Williams
_______________ _______________________ ________ J effre ffrey Williams
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TA BLE BL E OF OF CONTE CONTENTS NTS SUMMARY I. INTRODUCTION II. II . THEORY THEORY III. II I. EQUPMENT EQUPMENT SPECIFI SPECIFICAT CATIONS IONS IV. IV . SYSTEM OVERVIEW OVERVI EW V. PROCEDURE A. OPTO-22 B. USING DATA IN EXCEL EXCE L C. SYSTEM SY STEM STARTUP STARTU P D. TURNING TURNING ON THE STEAM E. CALIBRA CAL IBRATI TION ON OF THE FLOW FL OW METERS F. MAK MA K ING MEAS MEA SUREMENTS G. SAFETY TABLE OF OF NOMENCLATURE NOMENCLATURE REFERENCES
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iv 1 3 6 11 13 13 15 17 17
18 18 19 21 23
L IST OF FI GURES No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Title Pump Gate Valve Disk Globe Valve Ball Valve Control Valve Flow Meter Process Flow Diagramof Double-Pipe Heat Exchanger Photo of Double-Pipe Heat Exchanger Loading the Heat Exchanger Software Selecting the Double-Pipe Heat Exchanger Opto-22 Interface Opening the Double-Pipe Data Text Wizard Step One Text Wizard Step Two Text Wizard Step Three Pump Power Switch
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SUMMARY Double-Pipe Heat Exchanger, Project No. 1H Group B J effrey B. Williams (report author), Thomas Walters, Dong-Hoon Han Report Date: September 18, 2002 A Procedures Manual was created for the double-pipe heat exchanger. The theories of transient heat transfer in double-pipe heat exchangers were explained and followed by literature correlations. All of the instrument specifications were defined. A procedure for use of the equipment and the software was outlined. Safety and other concerns during operation were discussed. This manual will serve to direct anyone in how to start up and run the double-pipe heat exchanger. It is recommended that this Procedures Manual be filed with Robert Cox in MEB 3520. It is recommended that students be given access to the following manual, to aid themin their understanding of the use of the equipment. It is also recommended that students begin with acalibration of the flow meters and possibly thethermocouples before beginning use of the equipment. All calibration data, where possible, should be coordinated with the computer-generated data.
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I. INTRODUCTI ON Temperature can be defined as the amount of energy that a substance has.
Heat
exchangers are used to transfer that energy from one substance to another. In process units it is necessary to control the temperature of incoming and outgoing streams. These streams can either be gases or liquids. Heat exchangers raise or lower the temperature of these streams by transferring heat to or from the stream. Heat exchangers are a device that exchange the heat between two fluids of different temperatures that are separated by a solid wall. The temperature gradient, or the differences in temperature facilitate this transfer of heat. Transfer of heat happens by three principle means: radiation, conduction and convection. In the use of heat exchangers radiation does take place. However, in comparison to conduction and convection, radiation does not play a major role. Conduction occurs as the heat from the higher temperature fluid passes through the solid wall. To maximize the heat transfer, the wall should be thin and made of a very conductive material. The biggest contribution to heat transfer in a heat exchanger is made through convection. In a heat exchanger forced convection allows for the transfer of heat of one moving stream to another moving stream. With convection as heat is transferred through the pipe wall it is mixed into the stream and the flow of the streamremoves the transferred heat. This maintains a temperature gradient between the two fluids. The double-pipe heat exchanger is one of the simplest types of heat exchangers. It is called a double-pipe exchanger because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first. This is a concentric tube construction. Flow in a double-pipe heat exchanger can be co-current or counter-current. There are two flow configurations: co-current is when the flow of the two streams is in the same direction, counter current is when the flow of the streams is in opposite directions. As conditions in the pipes change: inlet temperatures, flow rates, fluid properties, fluid composition, etc., the amount of heat transferred also changes. This transient behavior leads to
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change in process temperatures, which will lead to a point where the temperature distribution becomes steady. When heat is beginning to be transferred, this changes the temperature of the fluids. Until these temperatures reach a steady state their behavior is dependent on time. In this double-pipe heat exchanger a hot process fluid flowing through the inner pipe transfers its heat to cooling water flowing in the outer pipe. The system is in steady state until conditions change, such as flow rate or inlet temperature. These changes in conditions cause the temperature distribution to change with time until a new steady state is reached. The new steady state will be observed once the inlet and outlet temperatures for the process and coolant fluid become stable. In reality, the temperatures will never be completely stable, but with large enough changes in inlet temperatures or flow rates a relative steady state can be experimentally observed.
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I I . THEORY The theory behind the operation of a double-pipe heat exchanger is covered in Incropera and Dewitt (1996). Also in this same textbook is the derivation of how transient behavior is treated with respect to heat transfer. As with any process the analysis of a heat exchanger begins with an energy and material balance. Before doing a complete energy balance a few assumptions can be made. The first assumption is that the energy lost to the surroundings from the cooling water or from the Ubends in the inner pipe to the surroundings is negligible. We also assume negligible potential or kinetic energy changes and constant physical properties such as specific heats and density. These assumptions also simplify the basic heat-exchanger equations. The determination of the overall heat-transfer coefficient is necessary in order to determine the heat transferred from the inner pipe to the outer pipe. This coefficient takes into account all of the conductive and convective resistances (k and h, respectively) between fluids separated by the inner pipe, and also takes into account thermal resistances caused by fouling (rust, scaling, i.e.) on both sides of the inner pipe. For a double-pipe heat exchanger the overall heat transfer coefficient, U, can be expressed as R fo di ,o R fi 1 1 1 1 + ln . = + + + U•A Aoho Ao 2• k • π • l di , i Ai hi • Ai
(1)
In a heat exchanger the log-mean temperature difference is the appropriate average temperature difference to use in heat transfer calculations. The equation for the log-mean temperature difference is
∆ TLM =
( i ,o − To,i ) − T ( i ,i − To,o ) T Ti ,o − To,i ln T T − o,o i ,i
3
.
(2)
Fluid properties such as density, viscosity and heat capacity are evaluated at the average temperatures. The averageis found by using the inlet and outlet values Ti , a = To, a =
Ti ,o + Ti ,i
2 To,o
+ To,i
2
.
(3) .
(4)
Thermal conductivity, k, can be evaluated at the average of the average temperatures. In a double-pipe heat exchanger the inner pipe is made of a conductive metal and is thin. The problem can be further simplified if the equipment is assumed to be clean, which means that no scaling exists. This is a poor simplification with the double-pipe heat exchanger in the laboratory, because it is many years old. The fouling factors Rfo and Rfi can be looked up from various sources, including Standards of the Tubular Exchange Manufacturers Association, or lumped together and determined experimentally. The only part of the overall heat-transfer coefficient that needs to be determined is the convective heat-transfer coefficients. Correlations are used to relate the Reynolds number to the heat-transfer coefficient. The Reynolds number is a dimensionless ratio of the inertial and viscous forces in flow. Rei
=
i di ,i m µ i ai
.
(5)
In the inner pipe if the Reynolds is less than 2000 this is considered to be laminar flow and the Nusselt number is equal to 4.36. If the Reynolds number is greater than 10,000, the Nusselt number is given by for turbulent, fullydeveloped flowwhere Nui = 0.023Rei4 5 Pri
n
l ≥ 10, 0.6 ≤ Pri ≤ 160, Rei ≥ 10,000, d i ,i n = 0.4 for Ts > Tm or n = 0.3 for Tm > Ts .
Pri =
Cpi µ i . ki 4
(6)
(7)
This gives a Nusselt number that can then be use to find hi Nui =
hi di ,i ki
.
(8)
The convective heat transfer coefficient in the annulus is more difficult to determine. The hydraulic diameter is used to find the Reynolds number. The hydraulic diameter is defined as the cross-sectional area perpendicular to flow divided by the wetted perimeter. With the Reynolds number calculated the same correlations apply and with these ho can be determined. Once all the separate heat-transfer coefficients are calculated an overall heat transfer coefficient is calculated. Now everything that was necessary for an energy balance is available. With the previous assumptions made earlier the dynamic equations would be
mi • Cpi •
dTi , a ( i ,i − Ti ,o ) − U • A • ∆ TLM . = qi • ρ i • Cpi T dt
(9)
mo • Cpo •
dT 0, a ( o,i − To,o ) + U • A • ∆ TLM . = qo • ρ o • Cpo T dt
(10)
With the transient data taken experimentally an overall heat-transfer coefficient can be determined at each time step. This can besolved numerically.
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I II . EQUIPMENT SPECIFI CATI ONS
The following is a list of all pieces of equipment and their specifications for the double-pipe heat exchanger. 1) Pump Manufactured by:
Dayton Electric Manufacturing
Model:
Teel Industrial Series (seeFigure 1)
Horsepower
2
RPM:
3485
Efficiency
80
Incoming pipe diameter:
2 in, Schedule 40 stainless steel
Outlet pipe diameter:
1 1/2 in, Schedule 40 stainless steel
Figure 1 – This Pump is used to pump the fluid from the tank to double-pipe exchanger. 2) Double-Pipe Heat Exchanger Material:
Schedule 40 stainless steel
Length:
14 ft
InsidePipeDiameter:
1 1/4 in
Outside Pipe Diameter:
2 in
Steam Pass
1
Cooling Water Pass
4
6
3) Valves
•
Gate Valves Manufactured by:
Stockham
Location:
Steam Valves (see Figure 2)
Figure 2 – This valve allows steam to enter the steam pipe in the annulus of the double-pipe heat exchanger.
•
Disk Globe Valve Manufactured by:
Nibco
Location:
Cold Water Valves (see Figure 3)
Figure 3 – When this valve is open the cold water can enter the double-pipe heat exchanger.
•
Ball Valves Manufactured by:
Watts Regulator or Apollo 7
Location:
Process Valves, Tank Valve, Drain Valve (see Figure 4)
Figure 4 – When the valve (Apollo) on the left is open it allows the cooling water to travel to thedrain. When the valve (Watts Regulator) on the right is open, the process fluid can travel to the drain.
•
Computer Controlled Valves Manufactured by:
Scott J ohnson
Model:
Valtek (seeFigure 5)
Operating Temperature:
0 to 55°C
MaximumAir Pressure:
30 psig
Figure 5- Control valve used to control amount of coolant flow to the heat exchanger.
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4) Flow meters Manufactured by:
Brooks Instruments
Model:
MT 3810 (see figure 6)
Accuracy:
±5% full scale from 100% to 10% of scale reading
Repeatability:
0.25% full scale
Operating Temperature:
-39 to 215°C
Flow Range:
4.1 to 41.6 gpm for inner-pipe flow meter 2.6 to 26.4 gpm for outer-pipe flow meter
Figure 6 – Meter measures the flow of process fluid coming from the pump. 5) Thermocouples Manufactured by:
Omega
Model:
Type T
Sheath Material:
304 Stainless Steel
Sheath Length:
12 in -60 to 100°C
Temperature Range:
1.0°C or 0.75% above 0°C (whichever is
Accuracy:
greater) 1.0°C or 1.5% below 0°C (whichever is greater)
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6) Low Pressure Steam Pressure:
27 psia 118°C
Temperature: 7) Computer Manufactured by:
Dell Systems
Operating System:
Windows NT
Software:
Opto-22 electronics and computer based software, Version R3.16. Copyright 19962000 Opto-22.
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I V. SY STEM OVERVI EW
The double-pipe heat exchanger used in experimentation is located in MEB 3520. Figure 7 describes the setup of double-pipe heat exchanger. Fluid from the tank is first heated in the by steam that is condensing in the annulus and is then cooled by the four cooling-water passes. In all instances low-pressure steamis used to heat the fluid and water is used to cool the fluid. Once cooled the fluid is then returned to the tank. There are six thermocouples that record temperature at six different points that can be seen in the following figure. The first records the temperature of the inlet process fluid, the second records the process fluid temperature after heating with steam, the third records the temperature after cooling with the water, the fourth records the cooling-water temperature at the inlet, the fifth records at the outlet and the sixth records the steam temperature at the inlet. There is a control valve that controls the steam inlet, the process fluid inlet and the cooling-water outlet. There are manual valves that also need to be opened before the process could begin, even if the control valves were open to 100%. Once the proper valves are opened the pump can be manually activated.
Figure 7- Process flow diagram for the double-pipe heat exchanger
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Figure 8 is a full overview of the double-pipe heat exchanger taken from the south side. The disk included with this manual includes this picture, as well as other pictures.
Figure 8- Picture of the double-pipe heat exchanger from the southwest corner
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V. PROCEDURE A. OPTO-22 Software The valves on the double-pipe heat exchanger are electronically controlled, and the data from the thermocouples and the flow meters are taken via computer. The following steps will explain how to start the software, and what thevarious sections of the software mean. 1. 2.
Turn on the computer. Click on the icon reading “Shortcut to Heat Exchanger MMI.” This will load the Opto-22 software.
3.
The first screen that appears will look like this:
Figure 9- Loading theheat-exchanger software. 4.
Click on the Heat-Exchanger Menu button. At this time, another menu will come up.
Figure 10- Selecting the double-pipe heat exchanger 5.
Select the “Double Pipe”.
6.
Figure 11 opens up next. This is the Opto-22 interface screen. All the work that is done while the heat exchanger is operating will be done here. For both the cooling and the water (process) flow the manipulated variable (MV) is the 13
percent opening of the respective control valves. 100% means that the valve is fully opened and 0% means that the valve is completely closed. The valve setting is changed by opening up the green MV and inputting avalue of 0-100.
Figure 11- Opto-22 interface and control screen 7.
The lower portion of Figure 11 shows values for the six different thermocouple readings, for the coolant and process flow meters and also for the control valves themselves. The colors in these boxes correspond with the colors of the lines in the graphs. This Opto screen provides numerical values and plots the numerical values to the graph. A new reading is taken and recorded at least every 5 seconds. Old data are saved to a file and are accessible in this screen.
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B. USING DATA IN EXCEL 1.
Open thefile in Excel. It is found in theC drive in “double-pipedata”.
Figure 12 – Opening the double-pipe data 2.
The data are saved in comma-delimited form. So Excel has to convert this to rows and columns.
Figure 13 – Text wizard Step one indicates that the data are in delimited form
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Figure 14 – The data are delimited using commas, this is apparent from thepreview
Figure 15 – The last step in the import wizard is to specify the data format of the columns and create the spreadsheet 3.
Once in Excel the data can then be studied and used in any necessary calculations. All the data (thermocouple readings, flow measurements and % control valve opening) given on theOpto-22 interface are recorded in this style for later use. Data such as this are very useful in the study of transient behavior.
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C. SY STEM STARTUP Between runs and at startup, the heat exchanger has to be cleaned in order to remove any rust or scale that has built up in the pipes. 1.
Make sure the computer is on, and the Opto-22 software is running on the double-pipe 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 time for thecooling water to flush the coolant pipe.
3.
Close the ball valve to the recycle, the tank and to the drain. Open the ball valve that allows mixing of cooling water and process fluid. This will allow the cooling water to run through the inner pipe and clean out any debris or deposits.
4.
Once the tank is about half way full open the tank valve and the drain valve and allow tank and inner pipe to drain, repeat this same process until the process fluid looks clean.
5.
Once the fluid is clean close the tank valve, close the drain valve and proceed to fill the tank.
6.
When finished, close the valve that allows the cooling water to mix with the process fluid. This will close the loop to the process. Open the drain valve to allow cooling water to cycle.
D. TURNING ON THE STEAM 1.
Get aladder, put on thermal resistant gloves and open the steamvalve above the heat exchanger.
2.
Open the inlet steam valve to the exchanger.
3.
Open the outlet steam(gate) valve.
4.
The steam trap will capture the steam and condense it. This will control the flow of steam to theexchanger. The steam will condenseas it transfers it’s heat to the process fluid. The steam trap will allow only liquid to the steam drain.
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E. CALIBRATION OF THE FLOW METERS Simple calibrations can be made for the flow meters. Calibrated data should be compared not only with the instrument itself, but when and where possible also with Opto-22.
The thermocouples should not be removed from the heat
exchanger, however the thermocouples still need to be calibrated regularly, if and when, necessary calibration of the thermocouples should take place 1.
Use a bucket or other container that can hold water. The container should have avolume of at least 3 gallons, but not exceed 5 gallons. Obtain ascale that can measure mass up to 10 kg. Obtain a stopwatch.
2.
Determine what range of flow rates that are necessary to give the required conditions. At least three separate flow rates should be used.
3.
Open or close the control valve to give the needed flow rates.
4.
Record the flow rateboth on the computer and on the flow meter; these should be the same.
5.
Have one person record time, one person hold the bucket and one person watch the flow meter to look for variation in the flow.
6.
Weigh the empty bucket.
7.
Fill the bucket from the tank inlet, record the time it takes to fill the bucket.
8.
Record the temperature of the outgoing stream.
9.
Weigh the bucket with water. Find the weight of the water.
10.
Determine the flow rate using density of the fluid at the recorded temperature
11.
Repeat the calibration two to four times.
F. MAKING MEASUREMENTS Once the flow meters have been calibrated and the system is flushed of any loose scaling, measurements can be made. Depending on the nature of the experiments to be performed, whether they be steady- or unsteady-state, the following procedure might vary. It is recommended however to first open the control valves to a setting that allows the type of flow that is needed, whether it be laminar or 18
turbulent. Second is to activate the pump to give the needed flow in the process fluid. Figure 16 shows the pump control on the west wall that needs to be used. The pump on the operators left is the one that is used for the double-pipe heat exchanger. Once the pump is activated, wait and see when the system reaches steady state and then make any necessary changes to the system.
Figure 16 – The pump power switch is the switch on the left. These power switches are located on the wall behind the heat exchanger. G. SAFETY Safety precaution is of utmost importance with any process. At all times operators of the equipment should wear safety glasses and hardhats, especially when the steam is turned on, or people are working on ladders (with the steam valves). When opening or closing the steam valves, always wear heat-resistant gloves. After the steam is turned on, care must be exercised as the water and pipes will become warm. Avoid touching the warm metal. Most of the critical areas are insulated; however, there are several exposed pipes that can become quite warm. The level of water in the tank should be kept at least one-third full to decrease the amount of splashing, especially when the water is hot. As water spills may occur, it is also important to have a mop and bucket on hand whenever the heat exchanger is used. Clean up any spills immediately to avoid damaging any electronics, especially on the computer
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controlling the equipment or the pump. It is important to keep the area surrounding the pump clear. While it is enclosed and mounted, the pump does require ventilation. Also, never run the pump dry or run it with the process and with the bypass valve shut. Not only does it wear out the valves and seals, but the pump can also overheat.
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TABLE OF NOMENCL ATURE Symbols A
=Area for heat transfer, ft2
Ai,i
=Surface area of the inside of the inner pipe, ft2
Ai,o
=Surface area of the outside of the inner pipe, ft2
ai
=Cross sectional area of inner pipe, ft2
Cpi
=Heat capacity of the fluid in the inner pipe, Btu/lbm. oF
Cpo
=Heat capacity of the fluid in the outer pipe, Btu/lbm.oF
Di,,i
=Inner diameter of inner pipe, ft
do,,i
=Inner diameter of outer pipe, ft
di,,o
=Outer diameter of inner pipe, ft
do,,o
=Outer diameter of Outer pipe, ft
hi
=Convective heat-transfer coefficient of fluid in inner pipe, Btu/hr .ft2.oF
ho
=Convective heat-transfer coefficient of fluid in outer pipe, Btu/hr .ft2.oF
k
=Thermal conductivity of inner pipe material, Btu/hr.ft.oF
l
=Total pipe length, ft
mi
=Mass of fluid in inner pipe, lbm
mo
=Mass of fluid in outer pipe, lbm
Nui
=Nusselt number of fluid in inner pipe
Nui
=Nusselt number of fluid in outer pipe
Pri
=Prandtl number of fluid in inner pipe
Pro
=Prandtl number of fluid in outer pipe
Rei
=Reynolds number of fluid in inner pipe
Reo
=Reynolds number of fluid in outer pipe
Ti,i
=Inlet temperature of fluid in inner pipe, oF
To,i
=Inlet temperature of fluid in outer pipe, oF
Ti,o
=Outlet temperature of fluid in inner pipe, oF
To,o
=Outlet temperature of fluid in outer pipe, oF
DTLM =Log-mean temperature difference Ti,a
=Average temperature of fluid in inner pipe, oF
To,a
=Average temperature of fluid in inner pipe, oF
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Rf i
=Fouling factor inside theinner pipe, Btu/hr.ft2.oF
Rf o
=Fouling factor outside theinner pipe, Btu/hr.ft2.oF
ui
=Velocity of the fluid in the inner pipe, ft/s
uo
=Velocity of the fluid in the outer pipe, ft/s
qi
=Volumetric flow rate of the fluid in the inner pipe, ft3/s
qo
=Volumetric flow rate of the fluid in the inner pipe, ft3/s
Q
=Heat transfer rate, Btu/hr
U
=Overall heat-transfer coefficient, Btu/hr.ft2.oF
Greek Symbols ρ i
=Density of the fluid in the inner pipe, lbm/ft3
ρ o
=Density of the fluid in the outer pipe, lbm/ft3
µ i
=Viscosity of the fluid in the inner pipe, lbm/ft.s
µ o
=Viscosity of the fluid in the inner pipe, lbm/ft.s
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REFERENCES
1. deNevers, N., Fluid Mechanics, McGraw Hill, (1991). 2. Emerson Process Management product guide.http://www.emersonprocess.com/ brooks/ products/products3d.html (accessed Sept 2002). 3. Incropera, F.P., D.P. DeWitt, Fundamentals of Heat and Mass Transfer, J ohn Wiley & Sons, Inc., pp. 460, 582-612. (1996). 4. Redding, Alyssa M., Shell-and-Tube Heat Exchanger, Project 1, Laboratory Manual. Sept. 21, 2001. 5.
Standards of the Tubular Exchange Manufacturers Association, 6th ed., Tubular
Exchanger Manufacturers Association, New Y ork, 1978.
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