EASTERN MEDITERRANEAN UNIVERSITY
MECHANICAL ENGINEERING DEPARTMENT
ME 345 / HEAT TRANSFER
Experiment 5
Name of the Experiment: Concentric Tube Heat Exchanger
Objective: a) To demonstrate the working principles of a concentric flow heat exchanger
under parallel flow conditions.
b) To demonstrate the working principles of a concentric flow heat exchanger
under counter flow conditions.
c) To demonstrate the effect of hot water inlet temperature variation on the
performance characteristics of a concentric tube exchanger.
d) To find the effect of flow rate variation on the performance characteristics of a
concentric tube heat exchanger operating under counter flow and parallel
conditions.
Apparatus: Concentric tube exchanger with heater, water tank, hot and cold water flowmeters and thermometers, pump.
THEORY:
Consider the double-pipe heat exchanger shown in Fig 1. The heat transfer rate at any
distance x along the tubes between the hot and the cold fluids is given by
qx = UA(Th –Tc)
……………………..(1)
where
A : surface area for heat transfer consistent with definition of U
Th : temperature of hot fluid
Tc : temperature of cold fluid
and U is the overall heat transfer coefficient based on either the inside or outside area of the
tube.
Figure 1.
If U is based on the inside area Ai of the tube it is given by
Ui 
1
1 Ai ln(ro / ri ) Ai 1



hi
2kL
Ao ho
……………………(2)
and if it is based on the outside area Ao
Uo 
1
Ao 1 Ao ln(ro / ri ) 1
 

Ai hi
2kL
ho
………………………(3)
where
ri
ro
hi
ho
=
=
=
=
inner radius of the tube
outer radius of the tube
convection heat transfer coefficient at the inner surface of the tube.
convection heat transfer coefficient at the outer surface of the tube (Fig.2).
Figure 2.
The fluids may flow in either parallel flow or counter flow, and the temperature profiles for
these two cases are also shown in figure 1. As it is seen the temperature of the hot and cold
fluids change along the tube and so in order to calculate the heat transfer between the two
fluids equation (1) should be integrated between the inlet and outlet conditions. If this is done
the heat transfer rate can be expressed as
q = UATm
………………………..(4)
where Tm is the suitable mean temperature difference across the heat exchanger and it can
be found to be (Fig.3)
Tm 
T1  T2
……………………………………………………(5)
ln(T1 / T2 )
Figure 3
This temperature difference is called the log mean temperature difference (LMTD) and is
valid for both parallel and counter flow conditions. The derivation of equation (5) is based on
two important assumptions: (1) the fluid specific heats do not vary with temperature and (2)
the convection heat transfer coefficients are constant throughout the exchanger. The second
assumption is usually the most serious one because of entrance effects, fluids viscosity and
thermal conductivity changes etc.
The heat loss from the hot fluid flowing in the inner tube can be determined from
 C ( T - T
q
)
m
hin hout
h p
…………………………..(6)
h =
h
where

m
Cp
Th
in
Thout
: mass flow rate
: specific heat
:
temperature of the hot fluid at the entrance to the heat
:
exchanger.
temperature of the hot fluid at the exit of the heat exchanger
and subscript h refers to the hot fluid .
Similarly the heat gained by the cold fluid flowing in the space between the inner and outer
pipes can be determined from
q
c

(
)
= m c Cpc Tcin Tcout
………………………………..(7)
Although the outer surface of the concentric tubes is insulated some heat is lost through the
insulating material to the surrounding air so that qc is less than q . Accordingly the
h
efficiency of the heat exchanger may be defined as

qc
qh
……………………………………(8)
The effectiveness of a heat exchanger is defined as
Effectiveness   
actual heat transfer
MAXIMUM POSSIBLE HEAT TRANSFER
(9)
The actual heat transfer may be computed by calculating either the energy lost by the hot fluid
from equation (6) or the energy gained by the cold fluid from equation (7). Since some of the
energy gained by the cold fluid is lost through the insulating material to the surroundings we
shall use the energy lost by the hot fluid as the actual heat transfer in equation (9).
To determine the maximum possible heat transfer for the exchanger, we first recognize that
this maximum value could be attained if one of the fluids were to undergo a temperature
change equal to the maximum temperature difference present in the exchanger, which is the
difference in the entering temperatures for the hot and cold fluids. The fluid which might
 C
undergo this maximum temperature difference is the one having the minimum value of m
p.
So, maximum possible heat transfer is expressed as
q
max

= ( m Cp )min ( Thin - Tcin )
…………………………(10)
The minimum fluid may be either the hot or cold fluid, depending on the mass flow rates and
specific heats, and so the effectiveness ε is
q
  h
…………………………(11)
qmax
In the counter flows heat exchanger if the minimum fluid is the cold fluid then Th - Tc = T
will converge at the hot inlet end. Alternatively if the minimum fluid is the hot fluid T will
converge at the cold inlet end. In the parallel flow heat exchanger T will always converge at
the outlet end.
Experiment Set – up
The equipment consists of a concentric tube exchanger mounted on a support frame.
(Figures 4 and 5). The external surface of the exchanger is insulated. Three temperature
measuring devices are installed in both the inside and outside tubes, to measure the fluid
temperatures accurately. To minimize losses in the system, the hot water is fed through the
inner pipe, with the cooling water in the outer annulus.
Control valves are incorporated in each of the two streams to regulate the flow. The flow rates
are measured using independent flow meters installed in each line.
The hot water system is totally self – contained. A hot storage tank is equipped with an
immersion type heater and an adjustable temperature control, which can maintain a
temperature to within approximately ± 1ºC.
Circulation to the heat exchanger is provided by a pump, and water returns to the storage tank
via baffle arrangement to ensure adequate mixing. The cold water required for the exchanger
is taken from the laboratory mains supply.
A readily identifiable valve arrangement allows simple changeover between parallel and
counter flow configurations.
PART A
PROCEDURE
1- Set the hot and cold fluids in parallel flow across the heat exchanger
2- Set the hot water temperature control to 60C
3- Adjust the hot water flow rate to 2000 cc/min
4- Adjust the cold water flow rate to 1000 cc/min
5- Take readings of the hot and the cold water at inlet, mid point and outlet once
conditions have stabilized
Readings;
Temperatures
Tcin
Tcmid
Tcout
Th
Th
Th
in
mid
out
CALCULATIONS
1- Calculate q from equation (6)
h
2- Calculate qc from equation (7)
3- Find the efficiency of the heat exchanger using equation (8)
4- Find LMTD using equation (5)
5- Find the overall heat transfer coefficient U from equation (4) using q as the heat
h
transfer rate.
6- Find the effectiveness  of the heat exchanger using equations (11), (10) and (6)
7- Tabulate the results
q
q
h
c
(Watt)
(Watt)

Tm
U
(C)
(W/m2-C)

8- Draw the temperature profiles of the hot and cold fluids along the heat exchanger.
PART B
Repeat Part A for counter flow conditions and compare the results.
PART C
PROCEDURE
1- Set the hot and cold fluids in counter flow across the heat exchanger
2- Adjust the hot water flow rate to 2000 cc/min
3- Adjust the cold water flow rate to 2000 cc/min
4- Set the hot water temperature to 50 ºC and take reading of the water temperature
5- Increase the hot water temperature in increments of 10 ºC and repeat the above steps
6- Tabulate the readings as in part A
CALCULATIONS
1- Calculate q , qc ,  , Tm , U , and  as in part A and tabulate the results.
h
2- Draw the temperature profiles of the hot and cold fluids along the heat exchanger.
3- Comment on the results obtained.
PART D
PROCEDURE
1-Set the hot and cold fluids in counter flow across the heat exchanger
2-Set the hot water temperature control to 60C
3-Set the hot water flow rate at a constant value of 2000 cc/min
4-Adjust the cold water flow rate to 1000 cc/min and take reading of the water temperatures
5-Increase the flow rate of cold water in steps of 1000 cc/min and take reading of the water
temperatures.
6-Tabulate the readings
Qc
TH
in
(C)
(cc/min)
TH
mid
(C)
TH
out
(C)
Tcin
(C)
Tcmid
(C)
Tcout
(C)
CALCULATIONS
1-Calculate q , qc ,  , Tm , U , and  as in part A and tabulate the results.
h
2-Draw the temperature profile of the fluids along the heat exchanger
i)
When the cold fluid is the minimum fluid
ii)
When the hot fluid is the minimum fluid
3-Draw the effectiveness of the heat exchanger  versus ( Mmin / Mm ax ), where
4-Comment on the results obtained
PART F (optional)
Repeat Part D for parallel flow conditions
CONCLUSION
Comment on the results obtained
REFERENCES
1 . 'Heat Transfer, A Practical Approach' , Y. A. Çengel ,Mc Graw Hill

M
 m C