Experiments in Chemical Engineering Double- Pipe Heat Exchanger PERFORMANCE OF A DOUBLE-PIPE HEAT EXCHANGER INTRODUCTION Modern manufacturing industries employ processes that require heating and cooling. From the preparation of the raw materials, to their processing, to the conditioning of the final products into sellable items and even down to the treatment of process effluents, heat transfer mechanisms are always applied. Most of the time, heating and cooling are done using heat exchangers and a double-pipe heat exchanger is one of the commonly used type. Being such a vital industrial tool, it is of great importance that chemical engineering students learn the basic concepts and theories especially the operation of a double-pipe heat exchanger. The fundamental concepts applied will enable the students to analyze and design other types of heat exchanger. OBJECTIVES 1. To familiarize the students with the characteristics, parameters and problems involved in the operation of a double-pipe heat exchanger when operated using countercurrent or co-current flow. 2. To determine and compare measured and calculated mean temperature difference between hot and cold water in both countercurrent and co-current flow. 3. To compare experimental overall heat transfer coefficient obtained using data from direct measurements with the theoretical Unit Operations Laboratory Page 1
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Double- Pipe Heat Exchanger
PERFORMANCE OF A DOUBLE-PIPE HEAT
EXCHANGER
INTRODUCTION
Modern manufacturing industries employ processes that require heating and
cooling. From the preparation of the raw materials, to their processing, to the
conditioning of the final products into sellable items and even down to the treatment
of process effluents, heat transfer mechanisms are always applied. Most of the
time, heating and cooling are done using heat exchangers and a double-pipe heat
exchanger is one of the commonly used type. Being such a vital industrial tool, it is
of great importance that chemical engineering students learn the basic concepts
and theories especially the operation of a double-pipe heat exchanger. The
fundamental concepts applied will enable the students to analyze and design other
types of heat exchanger.
OBJECTIVES
1. To familiarize the students with the characteristics, parameters and problems
involved in the operation of a double-pipe heat exchanger when operated
using countercurrent or co-current flow.
2. To determine and compare measured and calculated mean temperature
difference between hot and cold water in both countercurrent and co-current
flow.
3. To compare experimental overall heat transfer coefficient obtained using data
from direct measurements with the theoretical overall heat transfer
coefficients calculated using available empirical equations.
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Double- Pipe Heat Exchanger
THEORY
Although there are several ways of transferring heat between fluids, the most
common is the use of a heat-exchanger wherein the hot fluid and cold fluid are
separated by a solid boundary. Different types of heat exchangers have been
developed. The simplest type is a double-pipe heat exchanger. This consists
essentially of two concentric pipes with one fluid flowing through the inside of the
inner pipe while the other fluid moves co-currently in the annular space. This type of
heat exchanger, however, is not recommended for processes that require very large
heating surfaces.
The heat transfer analysis of a double-pipe heat exchanger deals with the
application of several equations that relate the different parameters involved.
Consider the heat exchanger,
Where:
mh = Mass flow rate of hot fluid, lbm/hr
mc = mass flow rate of cold fluid, lbm/hr
Tc = temperature of cold fluid, °F
Th = temperature of hot fluid, °F
**subscript 1 refers to entrance conditions, 2 refers to exit conditions
To determine the rate of heat loss by the hot fluid or the heat gained by the cold
fluid, we apply a steady overall energy balance between the two ends of the heat
exchanger. On the basis of 1 lbm/sec of fluid flowing, we have,
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Double- Pipe Heat Exchanger
W + JQ = ΔZ( ggc ) + v2
2αgc + JΔH (1)
Where: W = shaft work
ΔZ( ggc ) = mechanical potential energy
v2
2α gc = mechanical kinetic energy
α = kinetic energy velocity correction factor
(α = 1.0 for turbulent flow; 0.5 for laminar flow)
Since no shaft work W, is involved, ΔZ( ggc ) and v2
2α gc, are small compared with
the thermal energy transfer. Then for one fluid, the equation reduces to,
Q = ΔH = (H2 – H1) (2)
If no change in phase involved,
ΔH = CpΔT (3)
Therefore, the rates of heat transfer for the cold and hot fluids are respectively,
qc = mcCpc(Tc2 – Tc1) (4)
qh = mhCph(Th1 – Th2) (5)
If heat losses to the surroundings are neglected,
qc = qh or (6)
mcCpc(Tc2 – Tc1)= mhCph(Th1 – Th2) (7)
To relate the heat transfer rate with the size of the heat exchanger, we apply the
transfer around the differential element of length, dL. Thus,
dq = U1(Th – Tc)dA = Uo(Th – Tc)dAo (8)
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Double- Pipe Heat Exchanger
Where: U = Overall heat transfer coefficient, Btu/hr-ft2·°F
A = heat transfer area, ft2
ΔT = temperature driving force, °F = (Th – Tc)
**subscript 1 refers to the inside of the heating surface and
subscript o refers to the outside of the heating surface
For double-pipe heat exchangers, the overall heat transfer coefficient is almost
constant along the length of the heat exchanger and the driving potential ΔT may
be considered almost linear with q so that Equation (7) can be integrated to give,
q = UiAiΔTln = UoAoΔTln (9)
where: ΔTln = Logarithmic mean temperature difference
logarithmic mean temperature difference is defined by the
equation,
ΔTln = ΔT 1−ΔT 2
lnΔT 1ΔT 2
(10)
Where: ΔT1 = Temperature approach in one end
ΔT2 = Temperature approach in the other end
The ΔTln is fairly accurate if the ΔT is linear with q or L. however, in most situations,
this relationship is not always true. Let us compare therefore the log mean
temperature difference as defined by equation (9) and the arithmetic mean
temperature difference, ΔTo defined by,
ΔTo = ΔT 1+ΔT 2
2 (11)
With the true mean temperature difference, ΔTm which is obtained directly from
equation (7) by expressing ΔT in terms of L,
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q = 2πUD∫0
L
ΔT dL = 2πUDL(ΔT)tm (12)
therefore,
(ΔT)tm = ∫0
L
ΔT dL
L
(13)
Equation (11) is evaluated using graphical or numerical integration by plotting
values of ΔT against exchanger length and getting the area under the curve. These
are then divided by the total length of the exchanger.
It is given that the experimental overall heat transfer coefficient may be calculated
based on equation (8) by determining the rate of heat transfer by direct
measurements. To determine theoretical overall heat transfer coefficient, express U i
or Uo in terms of the individual transfer coefficients by considering resistances
involved when heat travels from the hot to the cold fluid. Such a relationship,
assuming relatively clean surface, is given by:
1UoAo
= 1UiAi
= 1hiAi
+ XmkmA
+ 1hoAo
(14)
Where: xm =Thickness of the tube wall
km = Thermal conductivity of the metal
A = Average heat transfer area
If Uo is desired, equation (12) simplifies to
1Uo
= 1ho
+ XmDokmD
+ DhiDi
(15)
If Ui is desired, we get
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Double- Pipe Heat Exchanger
1Uo
= 1hi
+ XmDikmD
+ DihoDo
(16)
Since the values of the xm, Do, and km can easily be obtained from available data, the
problem now boils down to the evaluation of the individual heat transfer coefficients.
This involves the choice of a particular empirical equation based on several factors
such as mechanism of heat transfer, character of flow, geometry of the system type
of fluid involved, etc.
Since most of the conditions in this experiment can be set, the equations for h may
be limited to only several choices. Based on mechanism, we can limit it to forced
convection by using flow rates that yield turbulent flow. This will eliminate the
effects of natural convection. Based on geometry, we are limited to horizontal tubes
with fluids flowing inside the conduits, circular and annular. Based on the type of
fluid, we are limited to usng hot and cold water.
In general, for forced convection in turbulent flow, (NRE > 10,000), k may be
calculated considering the effect of tube length by
( hCpG )(Cpμ
k )3
( μμ )0 .14
= 0 .023¿¿ (17)
Where the properties Cp, μ, k are evaluated based on the arithmetic mean bulk
temperature of the fluid defined by,
Tave = T 1+T 22
(18)
The viscosity, based on the wall temperature, μw will have to be determined by
estimating Tw by iterative calculation using individual resistances evaluated by first
neglecting the effect of μw.
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Double- Pipe Heat Exchanger
If the effect of the tube length can be ignored, (L/D > 60) and the (μw/μ)0.14 is
approximately equal to 1, the simpler Dittus-Boelter Equation (Foust 13-77), given
by
NNu = 0.023(NRe)0.8(NPr)n (19)
May be applied, where n= 0.4 where the fluid is heated and 0.3 when it is being
cooled. Here, the dimensionless numbers are defined as
NNu = hDk
Nusselts’s Number
NRe = DVρμ
=DGμ
Reynold’s Number
NPr = Cpμk
Prandtl Number
Another equation which is limited to water based temperature range of 40°F to 220°
F, turbulent flow, may be used. This is given by
h = 150 (1 + .011 T) V 0 .8
( D )0 .5¿
¿ (20)
where: T = Arithmetic temperature of fluid, °F
D’ = Tube diameter, inches
Equations (15), (16) and (17) are used to determine both h i and ho. to get hi, the
corresponding inside diameter of the tube is used for D. to get ho, the D is replaced
by the equivalent diameter, De, which is four times the hydraulic radius RH, defined
to be the ratio of the cross-sectional area of the annular space to the wetted
perimeter. For an annular space,
RH = 4 (Di j2−Do t2)
π ¿¿ = 14
¿ (21)
Where: Dij = inside diameter of jacket (outer tube)
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Double- Pipe Heat Exchanger
Dot = outside diameter of inner tube
It is possible that flow with Reynold’s number less than 10,000 will be encountered.
In this case, Equations (15),(16) and (17) are no longer valid. For NRe = 2100 and for
fluids of moderate velocity.
hiaDk
=1 .75NGr 1/3 = 1.75 (mCp
L )0 .33
(22)
For NRe between 2100 and 10,000, Figure 9-22 (MC) will have to be used. Also, if the
flow is laminar, the effect of natural convection should not be discounted. This effect
can be accounted for by multiplying h ia (computed from equation (19) or figure 9-22)
by the factor
Ǿn = 2.25(1+0 .010Ng r0 .33)
logNRe(23)
NGr = DeρtβgcΔT
μt 2(24)
Where: De = equivalent diameter
β = coefficient of thermal expansion, °F-1
f = subscript indicating that fluid properties should be based on
Tf = Tw+T2
EQUIPMENT
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Double- Pipe Heat Exchanger
Figure 1. Side view of Double-pipe Heat Exchanger
Equipment Description
The double-pipe heat exchanger set-up as shown in the previous figure consists
essentially of concentric pipes welded in series. The inner is made of brass with an
inside diameter of 0.625 inch and an outside diameter of 0.815 inch. The outer tube
made of standard 1 ¼ steel pipe. The unit is composed of 12 sections in series. Each
section is approximately 50 inches long. Hot water, which comes from the nearby
tubular heat exchanger, is passed through the inner pipe and the cold water, coming
from the supply main is passed through the annular space between the tubes.
Valves are provided for reversing the direction of the cold stream to obtain either
countercurrent or co-current flow. Valves on both lines are also provided to control
the flow rates of the streams. Each section is provided with thermometer wells,
which contain small amount of oil, to measure the temperature of the streams at
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Double- Pipe Heat Exchanger
appropriate points along the heat exchanger. At the exit ends of the pipes, weighing
tanks with calibrated levels are provided for measurement of flow rates.
PROCEDURE
It is important that this experiment should be performed with proper coordination
with Experiment B2, Performance of a Tubular Heat Exchanger, since the hot water
used in this experiment is the hot water discharged from the tubular exchanger. Any
valve movement in Experiment B2 will affect the temperature and flow rate of the
hot water. Therefore, each run for both experiments should start and end
simultaneously.
1. Familiarizing yourself with the parts and operation of the equipment,
especially the use of the valves provided in the lines. Place the thermometers
at the appropriate wells provided.
2. Open the supply valve for cold water, check whether water is flowing out the
measuring tanks, if not, checks exit valves. Pressure gauge provided should
indicate a constant reading. Adjust this valve to have a feel of the range of
flow rates to be used. Approximately determine the setting so as to get six
different flow rates later for each run. The exit valves in the measuring tanks
should be open to drain the liquid to avoid overflowing when flow is not being
measured.
3. Adjust the four valves in the cold water line to get either co-current or
countercurrent flow. This is done by fully opening or closing two opposite
valves. Trace the direction of flow from inlet to exit to determine this.
4. If hot water is already available, allow this to flow through the lines by fully
opening the exit valves.
Note: you should not move any valve along the hot water line without the
consent of the people operating the tubular exchanger nor they should move
anything without you knowing it. The flow rate of the hot water is usually at
their control, so regular consultation is advised.
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Double- Pipe Heat Exchanger
5. If flow rates have been established, prepare to continue the run by regularly
checking the temperature indicated by the thermometers at regular intervals
of time to determine whether steady conditions have already been
established and by measuring the flow rates of the two streams. The flow rate
is measured by closing the first exit valve for the water level to pass between
pre-selected points in the level gauge. The more time you spend in the
measurement, the better. The volumetric flow rate is obtained by dividing the
volume of water collected by the time interval.
6. If reasonable steady state conditions have been established, record all
temperature readings and flow rates i.e., no significant changes are observed,
and the run is completed.
7. Proceed with another run by adjusting the flow rate of the cold fluid and/or
the flow rate of the hot fluid. Each run should last approximately 20 minutes.
8. Perform a total of six runs; three countercurrent and three co-current flows.
9. Tabulate all data collected, measure the length of each section accurately,
check diameter of tubes, etc.
DATA SHEET
A. CO-Current Flow Operation
Trial 1
Well Number 1st Reading 2nd Reading 3rd ReadingTH (°C) TC (°C) TH (°C) TC (°C) TH