Title: Heat Exchanger Performance (Rev.

November 2016, NJL)

Purpose: The purpose of this experiment is to calculate the convective heat transfer
coefficients (h) and overall conductance (UA) for a counter flow heat exchanger using
both design calculations and experimental results.

Apparatus: The experimental apparatus consists of a Hilton H102 G single tube, water-
to-water turbulent flow heat exchanger test rig. Which is connected to the base unit
Hilton 102 Heat Exchanger Service Unit. The apparatus is designed so that temperatures
can be read at the locations illustrated in the figures shown in this lab. Flow meters are
provided to read the hot water and cold water flow rates.

The hot water is heated by an electric immersion heater and is re-circulated by a pump.
The cold water is from the building’s piping and is dumped after cooling the hot water.
The hot water inlet temperature can be controlled by the heater power control. The
control valves can regulate both water flow rates.

The heat exchanger is a concentric (tube within a tube) type with the hot water flowing
through the inner (core) tube and the cold water flowing through the annulus (outer tube).
By rearranging the water connections, the heat exchanger can be set up as a counter-flow
or parallel-flow configuration.

Temperatures at fourteen positions are read by thermocouples giving both water

temperatures and tube wall temperatures at inlets and outlets. The tube material is copper
and the heat exchanger dimensions are as follows:

Tube length, L = 1.05 m

Inner tube (inner diam.), di = 0.0079m
Inner tube (outer diam.), do = 0.0095m
Inner tube flow area, Ai = [( di2 ) / 4] = 49x10-6m2
Inner heat transfer area, Ah i = ( di L ) = 0.0261m2
Outer tube (inner diam.), Di = 0.0111m
Outer tube (outer diam.) Do = 0.0127m
Outer tube flow area, Aa = [((Di2 - do2 )) / 4] = 25.9x10-6m2
Outer heat transfer area, Ah o = ( do L) = 0.0310m2
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Schematic Key
1. Thermocouple sockets 5 to 16
2. Main Switch
3. Heater Switch
4. RCCB
5. Water Temperature Control
6. Temperature Indicator and Selector Switch
7. Coooling Water Flow Meter
8. Hot Water Flow Meter
9. Hot Water Out & In
10. Cold Water Out & In
11. Fill & Sight Tube
12. Vent(Top of Panel)
13. Tank Drain Valve

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Theory: The following symbols will be used in the calculations and theory explanations:

Symbol Designation Units

A Area m2
CP Specific Heat J/kgK
De,de Equivalent Diameter m
Q Heat Transfer Rate W
U Overall Heat Transfer Coefficient W/m2K
Nu Nusselt Number unitless
Pr Prandtl Number unitless
Re Reynolds Number unitless

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 h Convective Heat Transfer Coefficient W/m2K
k Thermal Conductivity of Fluid W/mK
kT Thermal Conductivity of Tube W/mK

Mass Flow Rate kg/sec

T Fluid Bulk Temperature C
v Fluid Velocity m/sec
x Wall Thickness m
Temperature Difference K
LMTD Log Mean Temperature Difference K
Dynamic Viscosity kg/(m-s)
Density kg/m3

Four dimensionless numbers are listed in the above table with no units; the
following expressions define those numbers:

Equation 5-1: Nu =

Equation 5-2: Pr =

Equation 5-3: Re =

Where:

NOTE: D = de = d i for the inner tube flow area (hot water)

D = De = Di – do for the annular flow area (cold water)

The following numbers designate the temperatures at the indicated locations:

7 Hot steam entering at the top of the heat exchanger

10 Hot steam leaving at the bottom of the heat exchanger
5 Metal wall at top of heat exchanger
6 Metal wall at bottom of heat exchanger
11 Cold stream leaving at the top of heat exchanger
14 Cold stream entering at the bottom of heat exchanger

Theory (Design):

Conduction is heat transfer through solids and through fluids in situations where there is
no movement of the fluid in the direction of heat flow. For one dimensional conduction
through a flat wall, the rate of heat transfer is given by :

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Equation 5-4:

Convection is heat transfer through a fluid system by the motion of the fluid. Forced
convection occurs when a mechanical device such as a pump causes the motion of the
fluid. Natural convection occurs when the heating process (through the density change of
the fluid) causes the motion of the fluid.

Within a heat exchanger, heat transfer occurs through a combination of conduction and
convection. According to the flow rate, and the resulting Reynold’s number, the flow in a
bulk of fluid may be laminar or turbulent. At higher Reynold’s numbers, ordered laminar
flow breaks down and is replaced by random and turbulent flow. The movement within
the fluid would then rapidly distribute the heat that has been transferred from the walls.
However, even when the bulk of the fluid has vigorous turbulent flow, the boundary layer
against the wall has the turbulent flow greatly suppressed. Due to this suppression, heat
transfer within the boundary layer is mainly due to conduction. In both laminar and
turbulent flow, the rate of heat transfer to a surface is given by the following equation:

Equation 5-7:

= Temperature of the fluid.

= Temperature of the surface

Radiation is the mode of heat transfer by electromagnetic waves, which requires neither
the contact between the hot and cold bodies or the use of an intermediate carrier.
Radiation is important at high temperatures, but at the moderate temperatures used in this
experiment, radiation effects are very small and will not be considered.

Overall Heat Transfer Coefficient

In a typical heat exchanger, heat is transferred from a hot fluid, through a separating wall,
to a cold fluid. The following diagram represents the temperature distribution:

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 Figure 5-2

The expression for the heat transfer in a simple heat exchanger from a hot fluid in
the inside tube to a cold fluid in the surrounding (outside) tube can be written as the
following expression:

Equation 5-8:

Forced Convection in Tubes

The large number of factors affecting convective heat transfer makes the theoretical
solution of heat exchanger problems almost impossible. To ease the problem,
dimensional analysis combined with experimental investigations has yielded a number of
relationships which can be easily handled. For turbulent flow in channels of uniform
cross-section, a well-known equation for this relationship is:

Equation 5-9: Nu = 0.023 (Re 0.8) (Pr 0.4)

For convenience, the properties used in the above calculations are those at the mean
temperature of the fluid. Thus, if the mean temperature values are known, or are
assumed, as well as the flow rates and dimensions of the heat exchanger, then all of the
values for Nu, Pr, and Re can be calculated.

The convective heat transfer coefficient, h can also be calculated by rearranging Equation
5-1 so that:

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 and
Finally, after those calculations are performed, UA may be calculated by noting from

equation 5-8 that

Theory (Experimental):

Temperature Distribution in Simple Concentric Tube Heat Exchangers

The temperature distribution in a concentric tube heat exchanger through which two
single-phase fluids flow in counter-current fashion is shown below. The temperature
difference between the two streams varies according to their position within the heat
exchanger.

Figure 5-3

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 Heat transfer calculations are eased if a mean value of the local temperature
differences can be found. This value is called the log mean temperature difference
(LMTD) and its value can be found using the following:

[ ( T 7−T 11 )−( T 10−T 14 ) ]

LMTD=
Equation 5-10:
ln
[ ( T 7−T 11)
( T 10−T 14 ) ]
The rate of heat transfer can be given by the following:

Equation 5-11:

Evaluation of Heat Transfer Coefficients

By observing the temperatures and mass flow rates of both streams and the temperatures
of the walls, the following may be calculated:

NOTE:The subscript i is used to denote the hot fluid flowing in the inner stream.
The subscript o is used to denote the cold fluid flowing in the outer stream.

1. The rate of heat transfer from the hot stream: Qi= ṁi C P ( T 7 −T 10 )
2. The rate of heat transfer to the cold stream: Q O =ṁ O C P ( T 11−T 14 )

Note: For Qi and Qo the appropriate mass flow rate should be used. Any
discrepancies between Qi and Qo should be explained in the conclusion
section of the report.
3. Overall heat transfer coefficient:

Qi
U=

[ ]
( T 7 −T 11 ) −( T 10 −T 14 )
A hi
( T 7−T 11 )
ln
( T 10 −T 14 )

4. The convection heat transfer coefficient between inner surface of tube and the
hot stream:

Qi
hi =

[ ]
( T 7−T 5 )−( T 10−T 6 )
A hi
( T 7−T 5 )
ln
( T 10−T 6 )

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5. The convection heat transfer coefficient between the outer surface of the tube
and the cold stream:

Qo
h o=

[ ]
( T 5−T 11) −( T 6−T 14 )
A ho
( T 5−T 11 )
ln
( T 6−T 14 )

Procedure:

1. Ensure that the water connections are aligned for a counter-flow

configuration. The heat exchanger should already be set up for this
configuration.

2. Turn of the cold water supply. Adjust the cooling water flow control valve to
50g/sec. If 50g/sec cannot be obtained pressure regulator may require
adjustment.

3. Full open hot water flow control valve on side of unit.

4. Fill clear sight tube to minimum water level line. Open the tank drain valve
so a small quantity of water is released to ensure the system is free of air.
Water level will fluctuate during experiment, it is important to maintain water
level at all times. WATER MUST BE VISIBLE AT ALL TIMES.

5. Close the blue master switch and main switch.

6. Let system run to make sure system is air free. Close heater switch and set
water temperature to 70oC.

7. Allow temperatures to stabilize, this process could take some time.

8. Adjust hot water flow rate to a convenient value close to100 % open. This
value will be the new value for fully open.

9. Record the six temperatures and mass flow rates of the hot and cold fluids.

10. Reduce the cold water flow in two 25% steps and record data. Allow the
temperatures to stabilize before taking readings.

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 11. Leaving the cold water valves in their current position, reduce the hot water
flow in two 25% steps and record data. Allow the temperatures to stabilize
before taking readings.

Shut Down Procedure

1. Turn off (open) heater switch.

2. Fully open cooling and hot water flow control valves
3. Turn off main switch when the system has cooled to approximately 40o C
4. Turn off (open) blue master switch
5. Turn off cold water supply

Presentation of results:

STEP A. CALCULATING THEORETICAL VALUES

1. Calculate the mean hot and cold temperatures.

T IN −HOT +T OUT− HOT

T HOT − AVG=
a. 2
T IN −COLD +T OUT −COLD
T COLD− AVG =
b. 2

2. Determine the thermal conductivity, density, dynamic viscosity, and specific

heat at those temperatures using the provided chart.

3. Calculate Re for each of those temperatures.

4. Calculate Pr for each of those temperatures.

5. Calculate Nu for each of those temperatures from Equation 5-9:

Nu = [0.023 (Re 0.8) (Pr 0.4)]

6. Calculate hi and ho

a.

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 b.

7. Calculate using the following equation:

8. Calculate the theoretical heat transfer, Q using equation 5-8 where

9. Repeat these steps for each trial and record values in data table.

STEP B. CALCULATING EXPERIMENTAL VALUES

1. Calculate Qi and Qo.

Qi= ṁi C P ( T 7 −T 10 )
Q O =ṁO C P ( T 11 −T 14 )

2. Calculate the OVERALL log mean temperature difference (LMTD).

LMTD=¿ ¿

3. Calculate and

a.

b.

4. Calculated the INSIDE and OUTSIDE LMTD.

LMTD inside =
[ ( T 7 −T 5 ) −( T 10−T 6 ) ]
ln
[ ( T 7 −T 5 )
( T 10−T 6 ) ]
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 LMTD outside =
[ ( T 5−T 11 )−( T 6−T 14 ) ]
ln
[ ( T 5 −T 11 )
( T 6 −T 14 ) ]
5. Calculate ho and hi

a.

b.

Report Requirements:

1. Compare and discuss theoretical and experimental values of hi, ho, and Q
2. Discuss any inconsistencies in heat transfer from the hot to cold stream
3. Discuss how each varies with flow rate.
4. Generally discuss experimental results.

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Raw Data Sheet:

Date:_____________
Ambient Temperature:__________
Barometric Pressure:___________

1 2 3
Hot 100% Hot 50%
Hot and Cold and Cold and Cold
Recorded Data 100% 50% 50%
T7 ºC
T10 ºC
T5 ºC
T6 ºC
T 11ºC
T14 ºC
Hot Flow Rate (gram/s)
Cold Flow Rate (gram/s)

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You should present your
calculated results in the
following format

Calculated Theoretical Data

Hot Water

Flow setting 1 2 3
Average Temperature (ºC)
Specific Heat (J/ kg ºC)
Density (kg/ m3)
Thermal Conductivity (W/ m ºC)
Viscosity (kg/ m s)
Prandtl Number
Reynolds Number
Nusselt Number
hi (W/m2 ºC)
UA (W/ºC)

Cold Water

Flow setting 1 2 3
Average Temperature (ºC)
Specific Heat (J/ kg K)
Density (kg/ m3)
Thermal Conductivity (W/ m ºC)
Viscosity (kg/ m s)
Prandtl Number
Reynolds Number
Nusselt Number
ho (W/m2 ºC)
UA (W/ºC)

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Calculated Experimental Data

Hot Water

Flow setting 1 2 3
Qi (W)
Overall LMTD
UAhi (W/ºC)
LMTD inside to the wall
hi (W/m2 ºC)

Cold Water

Flow setting 1 2 3
Qo (W)
Overall LMTD
UAho (W/ºC)
LMTD outside to the wall
ho (W/m2 ºC)

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