An Analysis of Residential Heat Exchangers for Heat Transfer from
Ground Water to Domestic Water
by
Carmen Kelly Chan
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING
Major Subject: MECHANICAL ENGINEERING
Approved:
_________________________________________
Professor Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
April 2012
(For Graduation August 2012)
i © Copyright 2012
by
Carmen Kelly Chan
All Rights Reserved
ii
CONTENTS
An Analysis of Residential Heat Exchangers for Heat Transfer from Ground Water to
Domestic Water ............................................................................................................ i
LIST OF SYMBOLS......................................................................................................... v
LIST OF TABLES............................................................................................................ vi
LIST OF FIGURES ......................................................................................................... vii
ACKNOWLEDGMENT ................................................................................................ viii
ABSTRACT ..................................................................................................................... ix
1. Introduction.................................................................................................................. 1
1.1
Problem Description .......................................................................................... 1
1.2
Heat Exchangers................................................................................................. 2
1.3
1.2.1
Shell and Tube Heat Exchanger............................................................. 2
1.2.2
Plate Heat Exchanger ............................................................................. 3
Related Past Research ........................................................................................ 5
2. Theory / Methodology ................................................................................................. 7
2.1
2.2
Theory ................................................................................................................ 7
2.1.1
Fluid Flow .............................................................................................. 7
2.1.2
Heat Balance .......................................................................................... 7
2.1.3
Overall Heat Transfer Coefficient.......................................................... 8
2.1.4
Log Mean Temperature Difference........................................................ 8
2.1.5
Effectiveness .......................................................................................... 9
Methodology .................................................................................................... 10
2.2.1
Shell and Tube Heat Exchanger........................................................... 10
2.2.2
Plate and Frame Heat Exchanger ......................................................... 11
3. Results and Discussion .............................................................................................. 12
3.1
Shell and Tube Heat Exchanger Design........................................................... 12
3.1.1
Design Parameters, Features and Performance.................................... 12
iii
3.2
3.1.2
Heat Transfer and Temperature Difference ......................................... 15
3.1.3
Fluid Flow ............................................................................................ 18
3.1.4
Heat Exchanger Geometry ................................................................... 18
Plate and Frame Heat Exchanger Design......................................................... 18
3.2.1
Design Parameters, Features and Performance.................................... 18
3.2.2
Heat transfer and Temperature Difference........................................... 19
3.2.3
Plate Geometry..................................................................................... 21
4. Cost Estimate ............................................................................................................. 22
5. Conclusions................................................................................................................ 23
6. References.................................................................................................................. 25
Appendix A – Shell and Tube Heat Exchanger............................................................... 27
A.1. HTRI Output Summary ..................................................................................... 28
A.2. HTRI Final Results ............................................................................................ 29
A.3. Rating Data Sheet .............................................................................................. 32
A.4. TEMA Specification Sheet ................................................................................ 33
A.5. 3D model............................................................................................................ 34
Appendix B – Plate and Frame Heat Exchanger ............................................................. 35
B. 1. Output Summary ............................................................................................... 36
B. 2. Final Results...................................................................................................... 37
B. 3. Specification Sheet............................................................................................ 39
B. 4. Drawing............................................................................................................. 40
Appendix C – Energy Savings Calculations.................................................................... 41
iv
LIST OF SYMBOLS
ρ – Fluid density [lb/gal]
Cp – Specific heat capacity [BTU/lb-°F]
Q – Heat duty or heat transfer rate [BTU, BTU/hr or kWh]
μ – Fluid viscosity [lb/ft3]
D – Diameter [in or ft]
Re – Reynolds number
v – Velocity [ft/sec]
m – Fluid mass or mass flow rate [lb or lb/hr]
T – Fluid temperature [°F]
A – Heat transfer area [in2 or ft2]
ε – Effectiveness
v
LIST OF TABLES
Table 1: Shell and tube heat exchanger design parameters ............................................. 12
Table 2: Shell and tube heat exchanger design features .................................................. 13
Table 3: Shell and tube heat exchanger performance summary ...................................... 15
Table 4: Plate and frame heat exchanger performance summary.................................... 19
Table 5: Plate geometry ................................................................................................... 21
Table 6: Shell and tube vs. plate and frame comparison. ................................................ 23
Table 7: Energy Savings Calculations............................................................................. 41
vi
LIST OF FIGURES
Figure 1: U-tube heat exchanger [2].................................................................................. 3
Figure 2: Straight-tube heat exchangers [3] [4]................................................................. 3
Figure 3: Plate heat exchanger [6] ..................................................................................... 4
Figure 4: Bleed port gasket design [6]............................................................................... 5
Figure 5: Lock-in gasket (left). Glue on gasket (right) [6] ................................................ 5
Figure 6: Fluid temperatures through a heat exchanger. ................................................. 10
Figure 7: Shell and tube heat exchanger tube layout....................................................... 13
Figure 8: Shell and tube heat exchanger baffle configurations and flow directions ....... 14
Figure 9: Shell and tube heat exchanger design heat transfer and temperature difference.
......................................................................................................................................... 16
Figure 10: Correction factor F (R, P) for 2 tube passes, 1 shell pass heat exchanger [14]
......................................................................................................................................... 17
Figure 11: Single-pass plate arrangement........................................................................ 19
Figure 12: Plate and frame heat exchanger design heat transfer and temperature
difference ......................................................................................................................... 20
vii
ACKNOWLEDGMENT
I am forever indebted to my parents Sunny and Mable Chan for their loves,
especially to my dad, for his understanding, devotion and endless patience. I am also
grateful to my significant other, Brett Tufano, for his support and encouragement.
Lastly, I would like to express my deepest love to my late grandmother who was a
giving, kind, hardworking and strong woman. She will always be part of me and will
always be remembered.
viii
ABSTRACT
This paper sizes and compares the feasibility of two different types of heat
exchangers, (shell and tube and plate and frame) for use in medium size residential
buildings. These heat exchangers result in energy savings by transferring heat from
ground water to cold domestic water in the winter. The heat exchangers are sized to
meet the water usage demands of approximately 160 people. The plate and frame (cost
~$500) removes 65,000 BTU/hr and saves up to $1927 per month, whereas, the shell and
tube heat exchanger (cost ~$8,000) removes 52,800 BTU/hr and saves up to $1565 per
month.
In this hypothetical application, a plate and frame heat exchanger is
recommended.
ix
1. Introduction
The most commonly used electric water heaters in the U.S. are only 33% efficiency
[15]. An average person in the United States uses approximately 70 gallon of water
indoor daily [16]. It is reasonable to assume that half of the usage comes from the water
heater at 140°F and the other half comes from the cold stream at 32°F. The mixture of
the cold and hot water is roughly 90°F, which is a comfortable indoor water usage
temperature. Therefore, the hot water usage daily per person is 69.3 gal / 2 = 35 gallons.
At 90°F, water density is 8.30 lb/gal. The energy necessary to heat the water is as
follow:
Q = m Cp ΔT = (290.5 lb)(1 BTU/lb °F)(140°F – 32°F) = 31,374 BTU = 9.19 kWh
Connecticut Light & Power charges 14.051 cents per kWh, which includes generation
and service charges but does not include a customer service charge of $16 per month. It
cost (9.19kWh * $0.14051/kWh) = $1.29 to heat 35 gallon of water daily (~$39 per
month) from 32°F to 140°F during winter months. However, if the water could be
heated from 32°F to ~55°F (ground water temperature) using a heat exchanger, the
energy bill could be reduced up to $8.26 per month per person.
Q = m Cp ΔT = (290.5 lb)(1 BTU/lb °F)(55°F – 32°F) = 6,682 BTU = 1.96 kWh
Electricity usage of 1.96 kWh per person daily costs $8.26 per person monthly. In this
calculation, 290.5 lb of water is only account for half the amount that an average person
uses daily, which comes from the hot faucet. However, the other half that comes from
the cold faucet can also be warmed up by ground water to ~55°F before one turns on the
cold faucet.
1.1 Problem Description
In this project, a shell-and-tube heat exchanger and plate and frame heat
exchanger will be sized to transfer heat from ground water to domestic water for a
medium size residential building with 160 people. The demand of domestic water is
roughly 8 gallons per minute. Two heat exchanger designs will be sized and compared.
1
1.2 Heat Exchangers
Heat exchangers are used to control the temperature of the contained fluid by adding
or removing heat. Two fluids of different temperatures are separated by a solid wall
where heat transfer takes place. Heat exchangers have been widely used in many
different areas from industrial practices to ships and vehicles as well as residential
buildings. Heat exchangers come in different forms and shapes depending on the system
and purpose of usage.
1.2.1
Shell and Tube Heat Exchanger
A shell and tube heat exchanger consists of a large vessel (shell) with a bundle of
tubes inside. One fluid runs through the shell side and the other fluid runs through the
tubes to prevent mixing of the two fluids. Temperature difference between the two
fluids results in heat transfer across the tube surface. There are two different types of
shell and tube heat exchangers: U-tube and straight tube. [1]
An U-tube heat exchanger is a two tube pass heat exchanger where the tubes are
bent in the shape of a U at the middle of the tube length. Tube side fluid enters and
leaves the heat exchanger on the same side. Fluid enters the tube bundles, runs through
the heat exchanger length to the other side, then turns at the U bend and runs back out to
the side in which the fluid enters (as shown in Figure 1). [1]
A straight tube heat exchanger consists of one tube pass construction where the tube
is straight. Tube side fluid enters on one side and leaves on the other. Fluid enters the
tube bundles, runs through the heat exchanger length and then exits the heat exchanger.
A two tube pass, straight tube heat exchanger is a hybrid between a U-tube heat
exchanger and straight tube heat exchanger. The tube side fluid enters and leaves on the
same side of the heat exchanger as shown in Figure 2. [1]
2
Figure 1: U-tube heat exchanger [2]
Figure 2: Straight-tube heat exchangers [3] [4]
Most shell-and-tube heat exchangers are one, two, or four tube pass designs. This
refers to how many times the tube side fluid travels along the length of the heat
exchanger before the fluid exits. Heat transfer takes place on the surface of the tube. To
increase heat transfer efficiency, baffle plates are often put in the shell side to direct the
fluid into the tube bundle instead of around the bundle.
1.2.2
Plate Heat Exchanger
A plate and frame heat exchanger provides a larger heat transfer area hence higher
efficiency.
Plate and frame heat exchangers usually operate with medium to low
pressure fluid. They are made of a series of metal plates bolted together. Each plate is
3
lined with rubber gaskets which are glued or locked around the edge of the plate and seal
the fluid from one plate to another. The metal plates are pressed during manufacturing
to form troughs, at an angle to the direction in which the fluid enters the heat exchanger.
The pattern of these troughs is arranged so that they interlink with the next plate, where
the other fluid flows, to form channels. The cold fluid flows on one side of the plate
while the hot fluid flows on the other side, maximizing the heat transfer area. The
troughs create and maintain a turbulent flow even at a low flow rate, thus maximizing
the heat transfer coefficient. Mixing of the fluids in turbulent flow eliminates stagnant
area in the fluid flow and minimizes fouling deposits. As shown in Figure 3, the red
plates represents where the hot fluid flows and blue plates represent cold fluid flow.
Each fluid flows through alternate plates and the entire plate works as the heat transfer
area. [5]
Figure 3: Plate heat exchanger [6]
Gaskets are used to keep the fluids from mixing. Double gaskets around the fluid
inlet are often used as a safety net to fully prevent intermixing of the fluids. As shown in
Figure 4, in case the outer gasket fails, the cold fluid will flow through the bleed ports.
The red arrows in Figure 4 represent the hot fluid flowing out of the plate, where the
gasket around the port hole is absent. The gasket surrounds the blue port hole prevent
the cold (blue) fluid from entering the plate.
4
The gaskets are either glued or locked in the plate as shown in Figure 5.
Sometimes, both glue and lock are used to secure the gasket in place. For lock-in
gaskets (left side of Figure 5), there are hooks that grip on the edge of the metal plate to
keep the gasket in place. For glue-on gaskets (right side of Figure 5), adhesive is applied
to the groove, where the gaskets sit on the metal plate.
Figure 4: Bleed port gasket design [6]
Figure 5: Lock-in gasket (left). Glue on gasket (right) [6]
1.3 Related Past Research
Since energy shortages are becoming a world crisis, many scientists and engineers
are looking for alternative energy sources, mainly environmental friendly energy
sources. Geothermal heat pump system installations have been increasing in residential
5
and commercial buildings for energy saving purposes. These systems use energy stored
in underground water for heating in the winter and cooling in the summer. Since
underground water temperatures are fairly constant year round (around 55°F) a heat
pump can obtain higher efficiency by using constant temperature ground water instead
of outside air. A geothermal heat pump requires conduction between the refrigerant and
ground water. Other researchers have combined a geothermal heat pump with heating
domestic hot water using the already-pumped-to-the-surface ground water for further
energy savings. Domestic hot water can be heated using the same ground water. This
hybrid system was studied and simulated for a small apartment located in Hong Kong,
China [7]. The energy savings for such a hybrid system is up to 70% compared with an
electric water heater.
In this project, it is assumed that ground water is available, either from combining
with a geothermal system or by using an additional pump. Pre-heating domestic hot
water using ground water will increase the efficiency of an electric or gas water heater.
6
2. Theory / Methodology
2.1 Theory
2.1.1
Fluid Flow
The properties of the fluid play an important role in the heat transfer process. The
viscosity (μ) and density (ρ) of the fluid affect greatly whether or not the fluid is laminar
or turbulent. This can be determined by the Reynolds number (Re) for flow in a tube.
Re 
D

(1)
Where ν is the velocity of the fluid and D is the diameter of the tube. The fluid is
laminar when the Re < 2000 and turbulent when Re > 6000. The transition region is
when the Re is between 2000 and 6000. In a laminar flow, the pressure drop is small
and is linearly correlated to the fluid velocity. As the Re number increases beyond the
laminar flow region, the pressure drop also increases and it is a function of velocity to a
power ranging from 1.6 – 2.0 [8].
Turbulent flow produces a high heat transfer
coefficient due to increased mixing of the fluid, however, the high pressure drop requires
higher pumping power. A heat exchanger designer shall strike a balance between the
thermal performance and the pumping power required.
2.1.2
Heat Balance
The specific heat or heat capacity (Cp) of a fluid is the measure of how readily the
fluid absorbs or releases heat. This characteristic of the fluid affects how fast the heat
transfer rate (Q) occurs. The heat removed from the hot fluid must be equal the heat
gained by the cold fluid in accordance with the following equation.
Q  mc C p ,c (Tc ,o  Tc ,i )  mh C p ,h (Th ,i  Th ,o )
(2)
Where the subscript ‘c’ stands for cold fluid, ‘h’ stands for hot fluid, ‘i’ stands for inlet
fluid and ‘o’ stands for outlet fluid. This equation is simplified by treating the heat
capacity constant within the temperature range Tin and Tout.
7 2.1.3
Overall Heat Transfer Coefficient
The overall heat transfer coefficient (U) is the most uncertain and also the most
essential in predicting the performance of a heat exchanger. This is a coefficient that
defines the total thermal resistance to heat transfer between two fluids. One way to
estimate the overall heat transfer coefficient is through the use of log mean temperature
difference in the equation below.
Q  UATlm
(3)
Where U is the overall heat transfer coefficient, A is the heat transfer area, and ΔTlm is
the log mean temperature difference (see Equation (4)). [9]
2.1.4
Log Mean Temperature Difference
The log mean temperature difference is the logarithmic average temperature
difference between two fluids at the front and rear end of the heat exchanger. A higher
log mean temperature difference means more heat is transferred. [12]
The log mean temperature difference method can be used with the following
assumptions:
1. No heat transfer to the surrounding of the heat exchanger.
2. Neglect potential and kinetic energy changes.
3. No conduction takes place along the axial direction of the tube.
4. Constant specific heat capacity.
5. Constant overall heat transfer coefficient.
The log mean temperature difference is defined as:
Tlm 
T1  T2
T2  T1

ln T2 / T1  ln T1 / T2 
(4)
For a parallel-flow exchanger, where the hot and cold fluids travel in the same direction
inside the exchanger:
T1  Th ,i  Tc ,i
T2  Th ,o  Tc ,o
8
(5)
For a counter-flow exchanger, where the hot and cold fluids travel in opposite direction
inside the exchanger:
T1  Th ,i  Tc ,o
T2  Th ,o  Tc ,i
(6)
Most heat exchangers are neither pure counter nor parallel flow, but a combination
of both, or cross flow [13]. For example, a heat exchanger with baffles creates a cross
flow pattern, where one fluid travels through the tube side horizontally and the other
fluid travels up and down over the baffles.
In this case, a correction factor F is
multiplied by ΔTlm. Factor F is a function of R and P, where:
R
Th ,i  Th ,o
Tc ,o  Tc ,i
P
Tc ,o  Tc ,i
Th ,i  Tc ,i
(7)
(8)
The chart for the correction factor F(R, P) varies with the design of the heat exchanger.
2.1.5
Effectiveness
A heat exchanger’s effectiveness (ε) compares the performance of a realistic
exchanger against an ideal heat exchanger. The outlet temperature of the cold fluid (Tc,o)
equals to the inlet temperature (Th,i) of the hot fluid in an ideal heat exchanger. An
infinitely large heat exchanger can achieve these ideal characteristics. The effectiveness
of an ideal heat exchanger is 1. In reality, the effectiveness is always less than 1, that is,
Tc,o < Th,i. Effectiveness is calculated
ε = Q / Qmax
Where Qmax is the maximum possible heat transfer, that an ideal heat exchanger can
achieve.
Figure 6 compares the fluids temperatures inside an actual and an ideal
counter-current heat exchanger. The red solid line represents the hot fluid entering an
actual heat exchanger at a higher temperature Thot,in and leaving the exchanger with a
lower temperature, Thot,out. On the other hand, the blue solid line represents the cold
fluid in an actual exchanger enters at a lower temperature, Tcold,in and leaves at a higher
temperature, Tcold,out.
In an actual heat exchanger, Tcold,out always less than Thot,in.
9
However, in an ideal heat exchanger, the cold fluid (dotted blue line) leaves the heat
exchanger at a temperature equals to the temperature of the entering hot fluid (dotted red
line). This ideal condition shows that the heat exchanger has 100% efficiency.
Figure 6: Fluid temperatures through a heat exchanger.
2.2 Methodology
2.2.1
Shell and Tube Heat Exchanger
Heat Transfer Research, Inc., (HTRI) is commercial computer software that
incrementally calculates and predicts the heat transfer, velocity profile and pressure drop
of a heat exchanger. HTRI validates the program code using research data obtained
from different manufacturers’ heat exchangers performance test data and all available
literature. HTRI Xist is used to size shell and tube heat exchangers. Xist accommodates
all TEMA standard1 shell and tube geometries with horizontal, vertical or inclined shell
position, multiple tube passes and various baffle cuts and configurations, such as single
segmental, double-segmental, segmental with no-tubes-in-windows, etc. Xist operates in
three different modes;
 The rating mode requires the user to specify the geometry and process
conditions. In return, Xist calculates the heat transfer coefficients and
pressure drop across the heat exchanger. The heat duty calculated by
1
Tubular Exchanger Manufacturers Association, Inc. (TEMA) is an association that standardizes shell and
tube heat exchanger. TEMA members continuously present the most up to date design and fabrication.
10
Xist is then compare to the specified heat load to report how under- or
over designed of the heat exchanger is.
 The simulation mode requires the user to specify the heat exchanger
geometry and partial process conditions.
Xist calculates the heat
exchanger performance and fills in the missing specified conditions for
the design to have zero heat transfer margin (e.i. the calculated heat
transfer area of the designed heat exchanger is equal to the area required
for the heat duty, no excess area is added for margin).
 The design mode contains the options for program and user-specified.
Program mode requires the user to specify partial heat exchanger
geometry and partial process conditions. Xist calculates the heat load
and reports the missing geometry and expected heat transfer coefficient
and pressure drops. In user-specified mode, the user chooses which
geometry is varied and which shall be calculated by Xist. Xist then
calculates all possible geometries and selects the best available design.
In this project, the design mode with user-specified option will be used to size the
shell and tube heat exchanger. When the most suitable heat exchanger geometry is
selected by Xist, the rating mode will be run to adjust and make minor changes to obtain
the desire heat transfer margin. [10]
2.2.2
Plate and Frame Heat Exchanger
HTRI Xphe is used to size a plate and frame heat exchanger.
It is a fully
incremental program that calculates each plate channel using local fluid properties and
process conditions.
Xphe allows either laminar or turbulent flow for a single fluid
phase. Xphe contains a data bank in which the user can choose the most common
commercially available plate-and-frame heat exchanger patterns and sizes. However,
the user is also allowed to specify different channel patterns and sizes. Similar to Xist,
Xphe also operates in rating, simulation, and design modes. [10]
11
3. Results and Discussion
3.1 Shell and Tube Heat Exchanger Design
The shell and tube heat exchanger was sized using HTRI Xist and the results are shown
in Appendix A. The performance and features of the exchanger are summarized below.
3.1.1
Design Parameters, Features and Performance
The input parameters used are shown in Table 1. The shell and tube heat exchanger
is designed with the ground water flowrate of 14 gpm or 7000 lb/hr and domestic water
flowrate of 8 gpm or 4000 lb/hr. The ground water enters through the tube side of the
heat exchanger at 55°F, which is the average ground water temperature. The domestic
water, which comes from outside of the house, enters through the shell side of the heat
exchanger at 32°F in the winter. The outlet temperatures of the hot and cold fluid along
with the heat load are calculated (see Appendix A.1).
Fluid
Temperature In
Temperature Out
Flowrate
Heat Load
Tube side (Hot)
Shell side (Cold)
Ground Water
Domestic Water
55°F
32°F
To be calculated
To be calculated
14 gpm
8 gpm
52,800 BTU/hr (program calculated)
Table 1: Shell and tube heat exchanger design parameters
The heat exchanger design features are summarized in Table 2. The heat exchanger
uses 3/8 inches normal pipe size, plain copper tube. It is a two tube pass (U-tube) and
one shell pass with a 4 inches diameter shell. There are 14 U-tubes (28 straight tubes)
with the straight tube length (measure from the tangent before the U bend to the end of
the tube) of 5 ft. Figure 7 shows the tube layout of the heat exchanger. The tubes are
arranged symmetrically. The baffles cut percentage are shown as the horizontal lines.
There are 4 tie rods and 2 pairs of seal strips, which force fluid in the shell side to flow
into the tube bundle, where the heat transfer takes place, rather than around the tube
bundle.
12 Feature
Tube
Parameters
Copper 3/8 inch outside diameter, plain tube,
U-tube
Number of U-tubes
14
Number of tube passes
2
Number of shell passes
1
Shell inside diameter
4.26 inches (nominal pipe size 4 inches)
Number of baffles
23 single segmented (see Figure 8)
Heat transfer area
13.845 ft2
Tube Length
5 ft
Table 2: Shell and tube heat exchanger design features
Figure 7: Shell and tube heat exchanger tube layout
The baffle configuration and the direction of flow of the heat exchanger is shown in
Figure 8. A TEMA type “BEU” was used. It is a combination of TEMA Standard
13
waterbox, shell and rear end of a heat exchanger [11]. The first letter stands for the type
of waterbox. In this case, ‘B’ is the bonnet integral cover. ‘E’ represents a one pass
shell where the inlet and outlet nozzles of the shell are located at opposite end. ‘U’ is the
U-tube bundle chosen for this design. There were 23 single segmented baffles, which
produced for 24 cross passes.
Figure 8: Shell and tube heat exchanger baffle configurations and flow directions
The performance summary is shown in Table 3. The calculated outlet temperatures
of the domestic water and ground water are 45.12°F and 47.48°F, respectively. The
domestic water pressure drop across the heat exchanger is 3.3 psi. The 8.8 psi pressure
drop of the ground water is not a significant matter since the ground water is discharged
back to the ground. The estimated overall heat transfer coefficient for this design is
356.93 BTU/hr-ft2-°F.
14
Shell (cold) side
Tube (hot) side
Domestic water
Ground water
8 / 4000
14 / 7000
32
55
45.12
47.48
Pressure Drop (psi)
3.3
8.8
Flow Velocity (ft/s)
0.80
4.39
Fluid
Flow (gpm) / (lb/hr)
Inlet Temperature (°F)
Outlet Temperature (°F)
52,800
Duty calculated by HTRI (BTU/hr)
Overall Heat Transfer Coefficient
356.93
(BTU/hr-ft2-°F)
Table 3: Shell and tube heat exchanger performance summary
3.1.2 Heat Transfer and Temperature Difference
The heat exchanger was sized to heat up domestic water using the year-round
constant temperature of ground water for a small residential building in the winter. As
shown in Table 3 and Figure 9, the domestic water coming from the street could be
heated up to 45.12°F from 32°F, whereas, the ground water in the tubeside entered at
55°F and exited at 47.5°F. The computed heat duty is 52,800 BTU/hr. This can be
estimated using the simplified
Q  mc C p ,c (Tc ,o  Tc ,i )  mh C p ,h (Th ,i  Th ,o ) (2)
For the cold fluid (domestic water):
Q  4000
lb
BTU
45.1F  32F   52,400 BTU
1
hr
lb F
hr
For the hot fluid (ground water):
Q  7000
lb
BTU
55F  47.5F   52,500 BTU
1
hr
lb F
hr
The simplified heat duty calculated above assumes constant heat capacity throughout the
heat exchanger. HTRI, however, use an integrated method to calculate the heat duty.
Figure 9 below shows the schematic of the fluids inlet and outlet temperatures.
15
Figure 9: Shell and tube heat exchanger design heat transfer and temperature
difference
Although the overall design was a counter-flow heat exchanger with the hot and
cold fluid entering from different ends of the exchanger, the exchanger is neither parallel
nor counter-flow. The baffle configurations inside the heat exchanger created a crossflow pattern. For purely counter-flow, the log mean temperature difference was:
T1  Th ,i  Tc ,o  55  45.1  9.9 F
T2  Th ,o  Tc ,i  47.5  32  15.5 F
Tlm 
from Equation 6
T2  T1
15.5  9.9

12.49 F from Equation 4
ln T2 / T1  ln 15.5 / 9.9 
R
P
T h , i  Th , o
Tc ,o  Tc ,i
Tc ,o  Tc ,i
Th ,i  Tc ,i

55  47.5
 0.573 from Equation 7
45.1  32

45.1  32
 0.570 from Equation 8
55  32
The correction factor F (R, P) chart for a one tube pass, two shell passes heat
exchanger is shown in Figure 10.
16
Figure 10: Correction factor F (R, P) for 2 tube passes, 1 shell pass heat exchanger
[14]
The correction factor F was about 0.9 for R = 0.573 and P = 0.570 according to
Figure 10. The overall heat transfer coefficient was calculated below.
Q  UAFTlm
U
U
Q
AFTlm
52,450 BTU
BTU
hr
 337 2
ft hr  F
13.845 ft  0.9  12.49 F
2
The overall heat transfer coefficient predicted by HTRI was 353.74 BTU/ft2-hr-°F,
which was within 5% of the calculated value. In an ideal heat exchanger (Figure 6) the
outlet temperature of the domestic water is equal to 55°F. The maximum heat duty was
Qmax  4000
BTU
lb
55F  32F   92,000 BTU
1
hr
lb F
hr
The effectiveness of this shell and tube design was

Q
52,400

 0.57
Qmax 92,000
17
3.1.3
Fluid Flow
The tubeside velocity (ground water) was 4.39 ft/sec. It is essential to maintain the
tubeside velocity above 3 ft/sec to minimize fouling deposits. With a tubeside flowrate
of 14 gallons per minute, the tubeside inlet nozzle was sized to a 1 inch inside diameter.
The ρV2 was 664.88 lb/ft-s2 and the velocity is 3.26 ft/sec (as shown in section A.3).
This was less than the maximum recommended value of 6,000 lb/ft-s2 as specified in
TEMA standard [11]. A tubeside ρV2 that is higher than 6,000 lb/ft-s2 would likely
cause erosion. The ρV2 of the shell and tube bundle entrance and exit shall not exceed
4,000 lb/ft-s2 as specified in TEMA standard [11]. In a case where such situations are
unavoidable, installation of a special device to protect the tube end or impingement
device to protect the tube bundle are necessary.
3.1.4
Heat Exchanger Geometry
The heat exchanger used U-tubes, which created 2 tube passes for a smaller unit and
higher tubeside velocity. The 3/8 inch tubes were plain, instead of finned, to reduce
cost. For smaller tube diameter sizes, finned tubes adds more cost then the equivalent
heat transfer area.
A single segmental baffle design was chosen for this design as shown in Figure 8.
There is flow over the U-bend. The U-bend section was responsible for 100% heat
transfer capability. A 20% baffle cut was used, that is, each baffle covers 80% of the
inside diameter of the heat exchanger.
3.2 Plate and Frame Heat Exchanger Design
3.2.1
Design Parameters, Features and Performance
The same fluid flow parameters were used to design the plate and frame heat
exchanger. The heat load of 65,000 BTU/hr was higher than that obtained for the shell
and tube exchanger. This is due to a higher outlet temperature of the domestic water.
The heat exchanger design contained 13 plates and 12 channels. Each fluid passed
through 6 channels. This was a single-pass plate arrangement with 1 pass for each fluid
as shown in Figure 11. In Figure 11, the blue represents the cold domestic water flows
18
through each plate once and leave the heat exchanger, and the red represents the hot
ground water with the same flow pattern.
Figure 11: Single-pass plate arrangement
The performance summary is shown in Table 4. Same fluids flow rate and inlet
temperatures as the shell and tube heat exchanger design are used to design the plate and
frame exchanger. The calculated outlet temperatures of the domestic water and ground
water are 48.18°F and 45.73°F, respectively.
Shell (cold) side
Tube (hot) side
Domestic water
Ground water
8 / 4000
14 / 7000
32
55
Outlet Temperature (°F)
48.18
45.73
Pressure drop (psi)
4.04
10.87
Velocity (ft/sec)
1.17
2.05
Fluid
Flow (gpm) / (lb/hr)
Inlet Temperature (°F)
Table 4: Plate and frame heat exchanger performance summary
3.2.2
Heat transfer and Temperature Difference
The heat duty for the plate and frame heat exchanger is calculated as follow:
For the cold fluid (domestic water):
Q  4000
lb
BTU
48.18F  32F   64,720 BTU
1
hr
lb F
hr
For the hot fluid (ground water):
Q  7000
lb
BTU
55F  45.73F   64,890 BTU
1
hr
lb F
hr
19
Figure 12 is a schematic representation of the situation.
Figure 12: Plate and frame heat exchanger design heat transfer and temperature
difference
Hot and cold fluid flow through each plate channel is in one direction, therefore
cross flow that occurs in the shell and tube heat exchanger is avoided. The plate and
frame heat exchanger design was counter-flow as shown in Figure 11.
T1  Th ,i  Tc ,o  55  48.2  6.8 F
T2  Th ,o  Tc ,i  45.7  32  13.7 F
Tlm 
from Equation (6)
T2  T1
13.7  6.8

 9.86 F from Equation (4)
ln T2 / T1  ln 13.7 / 6.8
64,805 BTU
Q
hr  797 BTU
from Equation (3)
U

2
ATlm 8.25 ft  9.86 F
ft 2 hr  F
The overall heat transfer coefficient predicted by HTRI was 838 BTU/ft2-hr-°F.
As in a shell and tube heat exchanger, an ideal plate and frame heat exchanger
would yield Qmax = 92,000 BTU/hr. The effectiveness of this design is:

Q
64,720

 0.70
Qmax 92,000
20
3.2.3
Plate Geometry
The plate geometry used is summarized in Table 5. The stainless steel plates have a
channel width of 3.88 inches with 0.094 inch spacing between each channel. The port
hole diameter where the fluids flow to the plate is 1 inch. Each plate thickness is 0.0236
inch. It is a relatively thin plate for higher heat conductivity.
Channel width (inch)
3.88
Channel spacing (inch)
0.094
Equivalent diameter (inch)
0.161
Average plate pitch (inch)
0.118
Port diameter (inch)
1.00
Tightened pack length (inch)
1.44
Horizontal port c-c (inch)
2.38
Vertical port c-c (inch)
25.19
Plate thickness (inch)
0.0236
Table 5: Plate geometry
21
4. Cost Estimate
The commercial price for the shell and tube heat exchanger in this design is
approximately $8,000 ± 20%. The commercial price for the plate and frame heat
exchanger in this design is approximately $500 ± 20%.
For alternative choice, Xylem, Cheektowaga, NY, also manufactures brazed plate
heat exchanger. Each plate is made from a copper sheet clad with a titanium sheet. The
plates are welded together instead of using gaskets and bolts, therefore, preventing
leakage and eliminating the need to replace the gaskets. Less maintenance is needed for
the brazed unit since the plates cannot be separated; hence, elimination of the guide bar
and a smaller unit. These units are 1/3 the price of a conventional plate and frame heat
exchanger since steel guide bars, frames, and bolts that are used in conventional plate
and frame units are eliminated. However, the brazed unit should be avoid when the fluid
is susceptible to heavy fouling, since the plates cannot be taken apart for cleaning.
22 5. Conclusions
As shown in Table 6, the performance and energy savings from the plate and frame
heat exchanger exceeds that of the shell and tube exchanger. The shell and tube heat
exchanger costs approximately 16 times more than the plate and frame. This is due to
highly expensive cost associated with fabricating tubes, tubesheet and drilling tube holes
into the tubesheet, waterbox, and unit assembling cost. It is much easier to manufacture
and assemble a plate and frame heat exchanger. However, there is more maintenance
associated with using a plate and frame heat exchanger.
approximately 3 – 5 years.
The gaskets life is
Leakage may be experienced when gaskets fail and
replacement of gaskets are then required. In this application, since both fluids are not
hazard, small potential leakage from the plate and frame heat exchanger is acceptable.
Shell and Tube
Plate and Frame
45.1°F
48.2°F
52,800 BTU/hr
65,000 BTU/hr
13.85 ft2
8.25 ft2
354 BTU/ft2-hr-°F
838 BTU/ft2-hr-°F
Effectiveness
0.57
0.70
Cost estimate
$8,000
$500
371 KWH/day
457 KWH/day
$52.17 per day
$64.24 per day
$1565 per month
$1927 per month2
Domestic water outlet temperature
Duty (HTRI)
Heat transfer area
Overall
heat
(HTRI)
transfer
coefficient
Energy Savings
Table 6: Shell and tube vs. plate and frame comparison.
The effectiveness of a plate and frame heat exchanger is 70% versus 57% of the
shell and tube.
The plate exchanger yields a much higher overall heat transfer
coefficient, hence; less heat transfer area is required as shown in Table 6. Therefore, the
2
Energy savings calculated here is only for the electric water heater. One must take into account the
energy spent to drive all necessary pumps to deliver 14 gallon per minute of ground water to the heat
exchanger. Cost savings calculation is shown in Appendix C.
23
unit is smaller but still capable of transferring more heat than the shell and tube
exchanger. With higher heat transfer, the domestic water outlet temperature of the plate
and frame heat exchanger is 3°F higher than the shell and tube heat exchanger. As a
result, the plate and frame heat exchanger produce higher energy saving.
Having a heat exchanger that uses ground water to warm up the domestic water
before entering the electric water heater could save up to $1,927 (for plate and frame) or
$1,565 (for shell and tube) monthly in the winter. In the application of a medium size
residential building, the plate and frame heat exchanger is recommended3.
3
This recommendation is based on a rough sizing of a shell and tube and plate heat exchanger for a
hypothetical residential building. Conditions such as ground water and domestic water quality, space
accessibility and constrain, etc. have not been taking in account. More optimization of the heat exchanger
design shall be done to meet each specific situation and condition.
24
6. References
[1]
“Shell and Tube Heat Exchanger.” 13 September 2011. Shell and Tube Heat
Exchanger – Wikipedia, the free encyclopedia.
23 September 2011.
<http://en.wikipedia.org/wiki/Shell_and_tube_heat_exchanger>.
[2]
U-tube Heat Exchanger. 5 June 2006. File:U-tube heat exchanger.PNG –
Wikipedia, free encyclopedia. 23 September 2011.
< http://en.wikipedia.org/wiki/File:U-tube_heat_exchanger.PNG>.
[3]
Straight-tube heat exchanger 1-pass. 28 May 2006. File:Straight-tube heat
exchanger 1-pass.PNG – Wikipedia, the free encyclopedia. 23 September
2011.
<http://en.wikipedia.org/wiki/File:Straight-tube_heat_exchanger_1-
pass.PNG>.
[4]
Straight-tube heat exchanger 2-pass. 28 May 2006. File:Straight-tube heat
exchanger 2-pass.PNG – Wikipedia, the free encyclopedia. 23 September
2011.
<http://en.wikipedia.org/wiki/File:Straight-tube_heat_exchanger_2-
pass.PNG>.
[5]
“Plate Heat Exchanger.” 11 September 2011.
Wikipedia,
the
free
encyclopedia.
Plate Heat Exchanger –
24
September
2011.
<http://en.wikipedia.org/wiki/Plate_heat_exchanger>.
[6]
Product Literature SUPERCHANGER® Plate & Frame HE. Tranter® The
heat transfer people. Tranter, Inc. 2009.
[7]
Cui, Ping, Yang, Hongxing, Spitler, Jeffrey D., and Fang, Zhaohong.
“Simulation of Hybrid Ground-coupled Heat Pump with Domestic Hot Water
Heating Systems using HVACSIM+.” Energy and Buildings, v 40, n 9, p
1731-1736, 2008.
[8]
Bartlett, Dean A. “The Fundamentals of Heat Exchangers.” The Industrial
Physicist. © 1996 American Institute of Physics.
[9]
Incropera, Frank P, Dewitt, David, Bergman, Theodore L., and Lavine,
Adrienne S. Fundamentals of Heat and Mass Transfer. New Jersey: John
Wiley & Sons, Inc. 2007.
25 [10]
HTRI – Heat Transfer Research, Inc. © 2011. HTRI. 18 September 2011.
<http://www.htri.net/>
[11]
“Standards of the Tubular Exchanger Manufacturers Association.” New
York: Tubular Exchanger Manufacturers Association, Inc. (TEMA) 1999.
[12]
“Log Mean Temperature Difference.” 15 March 2012. Log mean temperature
difference – Wikipedia, the free encyclopedia.
20 March 2012.
<http://en.wikipedia.org/wiki/Log_mean_temperature_difference>.
[13]
Bell, Kenneth J. “Fundamental Concepts.” Heat Exchanger Design
Handbook. Hemisphere Publishing Corporation. 1983.
[14]
“LMTD Correction Factor (F) Charts.” 2007-2010. LMTD Correction Factor
Charts. Online Chemical Engineering Softwares (ChemSOF). 25 March 25,
2012. <http://www.chemsof.com/lmtd/lmtd.htm>.
[15]
“Electric Hot Water Heater.” Household Energy Use. 25 September 2011.
<http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/houseenergy.html>.
[16]
“Water Use Statistics.”
1999.
American Water Works Association.
DrinkTap.org. 9 April 2012.
<http://www.drinktap.org/consumerdnn/Home/WaterInformation/Conservati
on/WaterUseStatistics/tabid/85/Default.aspx>.
26
Appendix A – Shell and Tube Heat Exchanger
27 A.1. HTRI Output Summary
28
A.2. HTRI Final Results
29
30
31
A.3. Rating Data Sheet
32 A.4. TEMA Specification Sheet
33 A.5. 3D model
Note:
The shell has been turned off to show the tube bundle and baffles.
Red: Tube bundle
Green: Baffles
Yellow (front end): waterbox, where the ground water enters into the tubeside of the heat exchanger, indicated by the red arrows.
Yellow (rear end): the cap for the U-bend end.
Blue arrows: inlet (rear end) and outlet (front end) of the domestic water into the shell side of the heat exchanger.
34 Appendix B – Plate and Frame Heat Exchanger
35 B. 1. Output Summary
36
B. 2. Final Results
37
38
B. 3. Specification Sheet
39 B. 4. Drawing
Side View
40 Appendix C – Energy Savings Calculations
Potential
energy
savings
per
person
Shell and
Tube heat
exchanger
energy
savings
per 75
people
Plate and
Frame
heat
exchanger
energy
savings
per 75
people
9.19
4.25
371.28
457.2
$0.08279
$0.01489
$0.76
$0.14
$0.35
$0.06
$30.74
$5.53
$37.85
$6.81
$0.02757
$0.25
$0.12
$10.24
$12.61
$0.00128
$0.01
$0.01
$0.48
$0.59
$0.00838
$0.08
$0.04
$3.11
$3.83
$0.00560
$0.14051
$0.05
$1.29
$38.74
$0.02
$0.60
$17.92
$2.08
$52.17
$1,565.06
$2.56
$64.24
$1,927.24
Total
hot
Connecticut water
Light &
usage
Power
per
4
charges
person
Electric usage
(KWH) per day
generation service
charge
Transmission charge
distribution charge
per KWH
CTA charge per
KWH
FMCC delivery
charge
Comb public benefit
charge
Total per day
Total per month
Table 7: Energy Savings Calculations
4
Connecticut Light & Power charges as of March 2012.
41