1
ABSTRACT.
Heat exchanger is a device that built for efficient heat transfer from one medium to another. There are two type of flow in double pipe heat exchanger that is counter-flow and co-current flow. Both hot and cold fluids enter the heat exchanger at the same end and move in the same direction in parallel flow (co-current). On the other hand, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions in counter flow. The heat exchanger also affected by hot water temperature inlet and the flow rate variation. Hot water taken from the pump are discharge while the cold water is taken from the pipe. Both hot and cold water passes through along the concentric tube and the experiment of counter and co-current was carried out. On the panel, the stabilized temperatures that appear were taken.
2 TABLE OF CONTENTS. Abstract 1 Table of content 2 1.0 Introduction 3 2.0 Objectives 5 3.0 Theory 6
4.0 Diagram and Description of Apparatus 9
5.0 Experimental Procedures 11
6.0 Result and Discussions 13
7.0 Sample Calculation 18
8.0 Conclusions and Recommendations 21
9.0 References 23
3
1.0 INTRODUCTION.
Heat exchanger is an equipment built for efficient heat transfer from one
medium to another. Different applications of heat exchanger require different
types of hardware and configurations of heat transfer equipment. There are
several types of heat exchanger such as double pipe heat exchanger,
compact heat exchanger, shell-and-tube heat exchanger and plate and frame
heat exchanger. In this experiment, double pipe heat exchanger was the only
apparatus are used.
Heat exchangers are a device that exchanges 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.
Double-pipe heat exchanger is the simplest type of heat exchanger
consists of two concentric pipes of different diameter. One fluid in a
double-pipe heat exchanger flows through the smaller double-pipe while the other fluid flows
through the annular space between the two pipes. Two types of flow
4
co-current ) or counter flow. In parallel flow (co-current), both hot and cold
fluids enter the heat exchanger at the same end and move in the same
direction. In counter flow, on the other hand, the hot and cold fluids enter the
heat exchanger at opposite ends and flow in opposite directions.
The performance of heat exchanger usually deteriorates with time as a
result of accumulation of deposits on heat exchanger surfaces. The layer of
deposits represents additional resistance to heat exchanger and cause the
rate of heat transfer in a heat exchanger to decrease. The net effect of these
accumulations on heat transfer is represent by a fouling factor Rf, which is a
measure of the thermal resistance introduced by fouling. The most common
type of fouling is the precipitation of solid deposits in a fluid on the heat
transfer surfaces. This type of fouling can be notice by a layer of
calcium-based deposits on the surfaces at which boiling occurs. This is especially the
case in areas where the water is hard. The scales of such deposits come off
by scratching, and the surfaces can be cleaned of such deposits by chemical
treatment. Another form of fouling, which is common in the chemical process
industry, is corrosion and other chemical fouling. This form of fouling can be
avoided by coating metal pipes with glass or using plastics pipes instead of
metal ones. Heat exchanger may also be fouled by the growth of algae in
warm fluids. This type of fouling is called biological fouling and can be
prevented by chemical treatment. The fouling factor depends on the
operating temperature and the velocity of the fluids, as well as the length of
service. Fouling increases with increasing temperature and decreasing
5
conditioning, power plants, chemical plants, petrochemical plants, petroleum
refineries, natural gas processing, and sewage treatment.
2.0 OBJECTIVES.
In the experiment A and experiment B, the objectives is to
demonstrate the working principles of a concentric tube heat exchanger
operating under co-current and counter current flow conditions. Then, the
objectives in the experiment C is demonstrate the effect of hot water
temperature variation on the performance characteristics of a concentric tube
heat exchanger and experiment D is to demonstrate the effect of flow rate
variation on the performance characteristics of a concentric tube heat
6
3.0 THEORY.
The theory behind the operation of a double-pipe heat exchanger is
covered in Incorporeal and Dewitt (1996). Also in this same textbook is the
derivation of how transient behaviour is treated with respect to heat transfer.
The simplest type of heat exchanger is a double-pipe heat exchanger
consists of two concentric pipes of different diameter. In a double-pipe heat
exchanger, one fluid flows through the smaller pipe while the other fluid flows
through the annular space between the two pipes. There was two types of
flow arrangement are possible in a double-pipe heat exchanger is in parallel
flow (co-current) or counter flow. Both hot and cold fluids enter the heat
exchanger at the same end and move in the same direction in parallel flow
(co-current). On the other hand, the hot and cold fluids enter the heat
exchanger at opposite ends and flow in opposite directions in counter flow.
The counter current design is the most efficient, because it can transfer
the most heat from the heat transfer medium due to the fact that the average
temperature difference along any unit length is greater. In a co-current flow
heat exchanger, the fluids travel roughly perpendicular to one another
through the exchanger.
Before calculating the overall heat-transfer coefficient U, power emitted
and power absorbed must be calculated first to determine the value of power
lost by using formula:
Power emitted (W) = QHPHCpH (T Hin – T Hout )
7
Power lost (W) = power emitted – power absorbed The value of efficiency also must be calculated,
Efficiency ŋ =
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. For a double-pipe heat exchanger the overall heat transfer coefficient,
U, can be expressed as:
Overall heat transfer coefficient U =
Where,
Area = surface area of contact
= pi x ODinner pipe x Length
= ( 3.142 x 0.015 x 1.36 ) m2
= 0.0641 m2
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:
Log mean temperature difference Δtm =
8
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.
Reynold number, Re =
If reynold number in range between 2300-4000, this is considered to be
laminar flow and if reynold number greater than 4000 it will be considered to
be turbulent flow. Then entry lengths must to calculate to determine whether
it fully develops or developing region, but in this experiment we assume the
flow in tubes is turbulent and fully developing region. So the formula to
calculate the nusselt number used are:
Nusselt number, Nu = 0.023 . (Reᴧ0.8 ) . ( Pr ᴧ0.33 ) ( )
Prandtl number, Pr = µ . Cp / K
This gives a Nusselt number that can then be use to find h value.
Surface heat transfer coefficient, h = Nu . k / d
Last but not least, percentage error must be calculating to found out
how much error in this experiment. Before we calculated the percentage
error, we must calculated first the theoretical heat coefficient because to
calculate the error, the theoretical heat coefficient must be subtract the
experimental heat coefficient and then divide by theoretical heat coefficient.
The formula used to calculate the theoretical heat coefficients is:
9 Where, Ρ = density µ = dynamic viscosity Cp = specific heat K = thermal conductivity D = diameter of pipe
4.0 DIAGRAM AND DESCRIPTION OF APPARATUS.
1. Flowrate indicator 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 3 10 00 00 00 9 11 4 8 7 6 5 12 13
10 2. Temperature indicator 3. Temperature controller 4. Main switch 5. Concentric tube 6. Selector valve 7. Flowmeter 8. Control valve 9. Control valve 10. Flowmeter
11. Cold water inlet
12. Cold water outlet
13. Hot water inlet
14. Pump inlet
14 15 16
11
15. Bypass valve
16. Storage tank
17. Loose cover
5.0 EXPERIMENTAL PROCEDURES.
Before started the experiment make sure all valve is closed. After that,
opened the water supplies and main switch and then opened the water
pump.
EXPERIMENT A: CO-CURRENT FLOW ARRANGEMENT.
1. Set cold water flow direction control valve for co-current flow. Opened the
valve V1 and V3 but keep valve V2 and V4 closed.
2. Controlled hot water temperature at 60oC.
3. Adjust the hot water flow rate, QH at 2.0L/min and cold water flow rate, Qc
at 1.5L/min.
4. Recorded the hot water and cold water temperature at inlet, midpoint and
outlet once conditions have stabilized.
EXPERIMENT B: COUNTER-CURRENT FLOW ARRANGEMENT.
1. Set cold water flow direction control valve for counter flow. Closed the
valve V1 and V3 but opened the valve V2 and V4.
12
3. Adjust the hot water flow rate, QH at 2.0L/min and cold water flow rate, Qc
at 1.5L/min.
4. Recorded the hot water and cold water temperature at inlet, midpoint and
outlet once conditions have stabilized.
EXPERIMENT C: WATER TEMPERATURE VARIATION.
1. Set cold water flow direction control valve for counter flow. Closed the valve V1 and V3 but opened the valve V2 and V4.
2. Adjust the hot water flow rate, QH at 2.0L/min and cold water flow rate, Qc
at 1.5L/min.
3. Set the temperature at 50oC, 55oC, 60oC, and 65oC.
4. Recorded all water temperature at inlet, midpoint and outlet once the conditions have stabilized for a range of hot water inlet temperature as set
on the controller.
EXPERIMENT D: FLOW RATE VARIATION.
1. Set cold water flow direction control valve for counter flow. Closed the
valve V1 and V3 but opened the valve V2 and V4.
2. Controlled hot water temperature at 60oC. 3. Adjust the cold water flow rate QC at 2.0L/min.
4. Adjust the hot water flow rate QH at 2.0L/min, 3.0 L/min, 4.0L/min and
13
5. Recorded all water temperature at inlet, midpoint and outlet once the
conditions have stabilized for a range of hot water flow rates whilst
maintained a constant cold water flow rate.
6.0 RESULT AND DISCUSSION.
RESULT.
EXPERIMENT A: CO-CURRENT FLOW ARRANEMENT.
Rea d ing s TT1 (Thin) o C TT2 (Thmid) o C TT3 (Thout) o C TT4 (Tcin) 0 C TT5 (Tcmid) o C TT6 (Tcout) o C 60.4 56.1 53.0 32.2 37.2 40.8 Calcu lat ion Power Emitted W Power Absorbed W Power Lost W Efficiency % Δtm o C U W/m2oC 1015.30 891.44 123.86 87.80 19.10 728.12
14 Temperature T (oC) Flow rate Q (L/min) Reynold Number Re Nusselt Number Nu Surface heat transfer coefficient h (W/m2K) Theoretical U (W/m2K) Experimental U (W/m2K) Percentage error (%) Type of flow Hot Water 56.7 2.0 5969.28 34.61 1509 634.17 728.12 12.90 Turb ulent Cold water 36.5 1.5 2755.29 22.32 921.07 Tran sient
EXPERIMENT B: COUNTER-CURRENT FLOW ARRANEMENT.
Rea d ing s TT1 (Thin) o C TT2 (Thmid) o C TT3 (Thout) o C TT4 (TcOut) 0 C TT5 (Tcmid) o C TT6 (Tcin) o C 60.1 55.9 52.4 40.8 41.4 30.7 ca lcula tio n s Power Emitted W Power Absorbed W Power Lost W Efficiency % Δtm o C U W/m2oC 1056.46 1047.20 9.26 99.12 20.48 797.70
15 Temperature T (oC) Flow rate Q (L/min) Reynold Number Re Nusselt Number Nu Surface heat transfer coefficient h (W/m2K) Theoretical U (W/m2K) Experimental U (W/m2K) Percentage error (%) Type of flow Hot Water 56.25 2.0 5969.28 34.61 1509 625.78 797.70 21.55 Turb ulent Cold water 35.75 1.5 2637.39 21.92 898.72 Tran sient
EXPERIMENT C: WATER TEMPERATURE VARIATION.
Rea d ing s Temp Set o C TT1 (tHin) o C TT2 (tHmid) o C TT3 (tHout) o C TT4 (tCout) 0 C TT5 (tCmid) o C TT6 (TcIn) o C 50 50.4 47.7 45.0 36.8 34.1 30.9 55 55.4 52.0 48.7 37.6 34.9 31.1 60 60.4 55.8 51.8 42.0 35.3 31.0 65 66.0 60.2 55.2 61.0 35.8 31.0 ca lcula tio n s Temp Set o C Power Emitted W Power Absorbed W Power Lost W Efficiency % Δtm o C U W/m2oC 50 743.37 816.15 -72.78 109.79 13.85 919.31 55 920.85 899.07 21.78 97.63 17.7 792.43 60 1177.95 1520.29 -340.34 128.84 19.58 1211.31 65 1476.32 4129.84 -2650.52 279.17 12.18 5289.66
16
EXPERIMENT D: FLOW RATE VARIATION.
Rea d ing s QH L/min TT1 (tHin) o C TT2 (tHmid) o C TT3 (tHout) o C TT4 (tCout) o C TT5 (tCmid) o C TT6 (TcIn) o C 2.0 60.9 56.5 52.3 40.7 35.0 30.6 3.0 60.2 57.0 53.7 40.1 35.7 30.6 4.0 60.4 57.7 54.6 42.4 37.6 30.6 5.0 60.8 58.5 55.9 41.8 36.7 30.5 ca lcula tio n s QH L/min Power Emitted W Power Absorbed W Power Lost W Efficiency % Δtm o C U W/m2oC 2.0 1179.95 1396.27 -216.32 118.33 20.94 1040.24 3.0 1337.42 1313.61 23.81 98.22 21.57 950.08 4.0 1591.07 1630.86 -39.79 102.50 20.86 1219.68 5.0 1680.22 1561.80 118.42 92.95 22.05 1104.99
17 Discussion.
In the experiment A, the test are conducted under co-current flow and in
the experiment B the test are conducted under counter current flow. The
value of power lost in experiment A is 123.86W is much greater than
experiment B that is 9.26W but value of efficiency of experiment B, 99.12% is
much higher than experiment A, 87.80%.
Based on the result, experiment B (counter-current) is much more
better than experiment A (co-current). The counter current design is the most
efficient, because it can transfer the most heat from the heat transfer medium
due to the fact that the average temperature difference along any unit length
is greater. In a co-current flow heat exchanger, the fluids travel roughly
perpendicular to one another through the exchanger.
The higher the value of reynold number, Re the higher the value of
surface heat transfer coefficient, h.
In the experiment A and B, the value of the experimental U is higher
than value of theoretical U. The water could affect the efficiency of water by
composition in the water. The water supplies could contain contaminant such
as sand, dust, microorganism and others that can be affect the result and
heat exchanger cannot work with efficiently. There are several common
problems that always happen in the heat exchanger such as fouling, scale
and corrosive.
But there are several problems when conducting this experiment such
as, the value of power absorbed is much greater than power emitted. This
18
heating more than it supplies, so the value of power lost is negative and we
cannot calculate the efficiency (%).
7.0 SAMPLE CALCULATION.
Experiment A = CO-CURRENT FLOW ARRANGEMENT
Power emitted (W) = QHPHCpH (T Hin – T Hout ) = x x x ( ) x x = 1015.30 W
Power absorbed (W) = QCPCCpC ( T Cout – T Cin )
= x x x x x (313.8–305.2)k
= 891.44 W
Power lost (W) = power emitted – power absorbed = 123.86 W Efficiency ( % ) = = x 100 = 87.80% ΔTM ( °C ) = ( ) = ( )
= 19.1 o C U ( ) = =
19
=
Hot water. Reynold number, Re == = 5969.28 (turbulent) Prandtl number, Pr = µ . Cp / K = = 2.984
Nusselt number, Nu = 0.023. (Reᴧ0.8 ) . ( Pr ᴧ0.33 ) ( ) = 0.023. (5969.28ᴧ0.8) . (2. 984 ᴧ0.33)
= 34.61
Surface heat transfer coefficient, h = Nu . k / d
=
= 1509
Cold water.
20
= = 2755.79 (transient) Prandtl number, Pr = µ . Cp / K = = 5.15
Nusselt number, Nu = 0.023. (Reᴧ0.8 ) . ( Pr ᴧ0.33 ) ( ) = 0.023. (2755ᴧ0.8) . (5.15 ᴧ0.33)
= 22.32
Surface heat transfer coefficient, h = Nu . k / d
=
= 921.07
Area of hot water = surface area of contact
= pi x ODinner pipe x Length
= ( 3.142 x 0.013 x 1.36 ) m2 = 0.0556 m2
Area of cold water = surface area of contact
= pi x ODinner pipe x Length
= ( 3.142 x 0.02 x 1.36 ) m2 = 0.0855 m2
21 ( ) = ( ) ( ) U = 634.17 Percentage error = (
x 100%
= 12.90%
8.0 CONCLUSION AND RECOMMENDATION.
CONCLUSION.
Between co-current and counter-current, the counter current
design is the most efficient, because it can transfer the most heat from the
heat transfer medium due to the fact that the average temperature difference
along any unit length is greater. In a co-current flow heat exchanger, the
fluids travel roughly perpendicular to one another through the exchanger.
This can be shown at experiment A and B and the efficiency for counter
current is 99.12% while the co-current is 87.80%. The counter-current flow
has three significant advantages over the co-current flow design. First, it has
more uniform temperature difference between hot fluid and cold fluid and it
minimize the thermal stress throughout the exchanger. Second, the outlet
22
fluid. Third, more uniform temperature difference produces a more uniform
rate of heat exchanger throughout heat exchanger.
RECOMMENDATION.
There are a few problems regarding the result while the experiments are
conducted. The result may vary as the surrounding temperature may affect
the heat transfer.
Here are recommendations for experiment betterment in the time to come.
Confusion.
When experiment is conducted, the value of power absorbed is much
greater than power emitted. So the value of power lost is negative and
we cannot calculate the efficiency (%). This happen because, something
in the heat exchanger make the cooling water heating more than it
supplies. When we conducted this experiment, group before us already
make this experiment. This heat exchanger is not fully cooled to the room
temperature, and the remaining heat in the apparatus is transfer to the
cooling water. That is the reason why the power absorbed is much
greater than power emitted.
23
As a replacement for of using water, it will be more proper if we use other
materials or chemicals such as hydrocarbon or refrigerant .It may expose
student to experiment fluid with different physical and chemical properties.
9.0 REFERENCES.
1. Perry, J.H.(Ed.): “Chemical Engineers’ Handbook, “ 4th ed., McGraw-Hill Book Company, New York, 1993.
2. Chemical engineering laboratory report book.
3. Jeffrey, B.W. “DOUBLE-PIPE HEAT EXCHANGER, Laboratory Manual
24
10.0 APPENDICES.
Physical Properties of Component
o C Cp kJ/kg.K Density, p Kg/m3 21.11 4.179 997.40 26.67 4.179 995.80 30.00 4.176 995.26 31.00 4.175 995.10 32.00 4.174 994.94 32.22 4.174 994.90 34.00 4.174 994.23 34.30 4.174 994.14 34.65 4.174 993.99 35.15 4.174 993.83 35.65 4.174 993.61 35.90 4.174 993.53 36.20 4.174 993.38 36.40 4.174 993.35 37.25 4.174 993.02 47.20 4.174 989.42 48.89 4.174 988.80 50.00 4.175 998.18 51.50 4.176 987.36 54.44 4.179 985.70 54.65 4.179 985.61 55.00 4.179 985.46 55.05 4.179 985.42 55.50 4.179 985.22 56.50 4.180 984.71 57.00 4.180 984.48 57.25 4.180 984.41 59.70 4.181 983.16 60.00 4.179 983.30 65.00 4.183 980.60 65.55 4.183 980.30