DOUBLE-PIPE HEAT EXCHANGER

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

TABLE OF CONTENTS
SUMMARY
iv
I. INTRODUCTION 1
II. THEORY 3
III. EQUPMENT SPECIFICATIONS 6
IV. SYSTEM OVERVIEW 11
V. PROCEDURE 13
A. OPTO-22 13
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
ii

LIST OF FIGURES
No. Title Page
1. Pump 6
2. Gate Valve 7
3. Disk Globe Valve 7
4. Ball Valve 8
5. Control Valve 8
6. Flow Meter 9
7. Process Flow Diagram of Double-Pipe Heat Exchanger 11
8. Photo of Double-Pipe Heat Exchanger 12
9. Loading the Heat Exchanger Software 13
10. Selecting the Double-Pipe Heat Exchanger 13
11. Opto-22 Interface 14
12. Opening the Double-Pipe Data 15
13. Text Wizard Step One 15
14. Text Wizard Step Two 16
15. Text Wizard Step Three 16
16. Pump Power Switch 19
iii

SUMMARY
Double-Pipe Heat Exchanger, Project No. 1H
Group B
Jeffrey 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 them in their understanding
of the use of the equipment. It is also recommended that students begin with a calibration of the flow
meters and possibly the thermocouples before beginning use of the equipment. All calibration data,
where possible, should be coordinated with the computer-generated data.
iv

I. INTRODUCTION
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 stream removes 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
1

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.
2

II. 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 U-
bends 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
1 1 R
fo 1 ⎛ d
i,
= + + ln
U A Ao
ho
Ao
2 k l
⎜
• • • π • ⎝ di,
o
i
⎞ R
fi
⎟ +
⎠ Ai
1
+ .
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
∆T
LM
=
( T − T ) − ( T − T )
i,
o
o,
i
⎛ Ti
ln⎜
⎝ Ti
, o
, i
− T
i,
i
− T
o,
i
o,
o
⎞
⎟
⎠
o,
o
.
( 2)
3

Fluid properties such as density, viscosity and heat capacity are evaluated at the average
temperatures. The average is found by using the inlet and outlet values
T i,
a
T o,
a
=
=
T + T
i, o i,
i
2
T + T
o, o o,
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 R fo and R fi 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.
d
m
i,
i i
Re
i
= . (5)
µ
iai
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,
fully developed
flow where
Nu
i
= 0.023Re
4 5
i
Pr
n
i
⎛ l ⎞
Rei
≥ 10,000, ⎜ ⎟
10, 0.6 Pri
160,
d
≥ ≤ ≤
⎝ i,
i ⎠
n = 0.4 for T > T or n = 0.3 for T > T .
s
m
m
s
( 6)
Cp
iµ
i
Pri
= .
k
i
( 7)
4

This gives a Nusselt number that can then be use to find h i
Nu =
i
h d
i
k
i,
i
i
.
( 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 h o 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
dTi,
a
mi
• Cpi
• = qi
• ρi
• Cp
dt
dT 0, a
mo
• Cpo
• = qo
• ρ
o
• Cp
dt
−
i
( Ti,
i
Ti,
o
−
o
( To,
i
To,
o
) −U
• A • ∆T
LM
) + U • A • ∆T
.
LM
.
( 9)
( 10)
With the transient data taken experimentally an overall heat-transfer coefficient can be
determined at each time step. This can be solved numerically.
5

III. EQUIPMENT SPECIFICATIONS
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 (see Figure 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
Inside Pipe Diameter: 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 the drain. When the valve (Watts Regulator)
on the right is open, the process fluid can travel to the drain.
• Computer Controlled Valves
Manufactured by: Scott Johnson
Model: Valtek (see Figure 5)
Operating Temperature: 0 to 55°C
Maximum Air Pressure: 30 psig
Figure 5- Control valve used to control amount
of coolant flow to the heat exchanger.
8

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
5) Thermocouples
Figure 6 – Meter measures the flow of process
fluid coming from the pump.
Manufactured by:
Model:
Sheath Material:
Sheath Length:
Omega
Type T
304 Stainless Steel
12 in
Temperature Range: -60 to 100°C
Accuracy:
1.0°C or 0.75% above 0°C (whichever is
greater)
1.0°C or 1.5% below 0°C (whichever is
greater)
9

6) Low Pressure Steam
Pressure:
27 psia
Temperature: 118°C
7) Computer
Manufactured by:
Operating System:
Software:
Dell Systems
Windows NT
Opto-22 electronics and computer based
software, Version R3.16. Copyright 1996-
2000 Opto-22.
10

IV. SYSTEM OVERVIEW
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 steam is 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
11

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
12

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 the various sections of the software mean.
1. Turn on the computer.
2. 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 the heat-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 a value 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.
14

B. USING DATA IN EXCEL
1. Open the file in Excel. It is found in the C drive in “double-pipe data”.
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
15

Figure 14 – The data are delimited using commas, this
is apparent from the preview
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 the Opto-22 interface are recorded in this
style for later use. Data such as this are very useful in the study of transient
behavior.
16

C. SYSTEM 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 the cooling 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 a ladder, put on thermal resistant gloves and open the steam valve 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 the exchanger. The steam will condense as it transfers it’s
heat to the process fluid. The steam trap will allow only liquid to the steam
drain.
17

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 a volume of at least 3 gallons, but not exceed 5 gallons. Obtain a scale
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 rate both 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
19

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.
20

TABLE OF NOMENCLATURE
Symbols
A =Area for heat transfer, ft 2
A i,i =Surface area of the inside of the inner pipe, ft 2
A i,o =Surface area of the outside of the inner pipe, ft 2
a i =Cross sectional area of inner pipe, ft 2
Cp i =Heat capacity of the fluid in the inner pipe, Btu/lbm . o F
Cp o =Heat capacity of the fluid in the outer pipe, Btu/lbm .o F
D i,,i
d o,,i
d i,,o
d o,,o
h i
h o
k
l
m i
m o
Nu i
Nu i
Pr i
Pr o
Re i
Re o
T i,i
T o,i
T i,o
T o,o
=Inner diameter of inner pipe, ft
=Inner diameter of outer pipe, ft
=Outer diameter of inner pipe, ft
=Outer diameter of Outer pipe, ft
=Convective heat-transfer coefficient of fluid in inner pipe, Btu/hr . ft 2.o F
=Convective heat-transfer coefficient of fluid in outer pipe, Btu/hr . ft 2.o F
=Thermal conductivity of inner pipe material, Btu/hr . ft .o F
=Total pipe length, ft
=Mass of fluid in inner pipe, lbm
=Mass of fluid in outer pipe, lbm
=Nusselt number of fluid in inner pipe
=Nusselt number of fluid in outer pipe
=Prandtl number of fluid in inner pipe
=Prandtl number of fluid in outer pipe
=Reynolds number of fluid in inner pipe
=Reynolds number of fluid in outer pipe
=Inlet temperature of fluid in inner pipe, o F
=Inlet temperature of fluid in outer pipe, o F
=Outlet temperature of fluid in inner pipe, o F
=Outlet temperature of fluid in outer pipe, o F
DT LM =Log-mean temperature difference
T i,a
T o,a
=Average temperature of fluid in inner pipe, o F
=Average temperature of fluid in inner pipe, o F
21

Rf i =Fouling factor inside the inner pipe, Btu/hr . ft 2.o F
Rf o =Fouling factor outside the inner pipe, Btu/hr . ft 2.o F
u i
u o
=Velocity of the fluid in the inner pipe, ft/s
=Velocity of the fluid in the outer pipe, ft/s
q i =Volumetric flow rate of the fluid in the inner pipe, ft 3 /s
q o =Volumetric flow rate of the fluid in the inner pipe, ft 3 /s
Q =Heat transfer rate, Btu/hr
U =Overall heat-transfer coefficient, Btu/hr . ft 2.o F
Greek Symbols
ρ i =Density of the fluid in the inner pipe, lbm/ft 3
ρ o =Density of the fluid in the outer pipe, lbm/ft 3
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
22

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, John 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, 6 th ed., Tubular
Exchanger Manufacturers Association, New York, 1978.
23