ABSTRACT
The objective of this experiment is to evaluate the performance of counter-current flow of heat exchanger and to study the routine of the heat exchanger under various flow rates by using concentric tube heat exchanger or simply double pipe heat exchanger. The experiment was divided into two parts. The first part is where the hot water volume flowrate was set to be constant at 10L/min while the cold water flowrates were varied at L/min! "L/min! #L/min! $L/min and 10L/min. %or the second part! the cold water volume flowrate was set to be constant while the hot water volume flowrates were varied. TT1! TT! TT& and TT" were recorded every 10 minutes. 's the temperature of the hot water increases! the overall heat transfer coefficient is also increasing. The increasing of the overall heat transfer coefficient will causes the rate of heat transfer also increases. The higher the flow rate of the fluids flowing in the heat exchanger! the higher the rate of heat transfer! thus the better the performance of the heat exchanger. This can be shown in the %igure 1.( and %igure 1.#. 's the flow rate rate was increase from L/min to 10L/min! 10L/min! the overall heat transfer coefficient also increase from ()(."*/m. ℃ to +#1.)*/m.℃ for constant hot flowrate! while "1(.#*/m.℃ to +#1.)*/m.℃ for constant cold water flowrate. The increasing in heat transfer coefficient shows increasing in rate of heat transfer which reflects in increasing of the performance of the heat exchanger.
1
1.1
INTRODUCTION
,eat exchangers are devices that facilitate the exchange of heat between two fluids that are at different temperatures while eeping them from mixing with each other unus '. engel and hajar! 0112. ,eat transfer in heat exchanger usually involves convection in each fluid and conduction through the wall separating the two fluids. The rate of heat transfer between the two fluids depends on the magnitude of the temperature difference and the flow rate of the fluids. ,eat exchangers are usually can be found in extensive range of applications! from heating and air-conditioning systems in household! to chemical processing and power production in large plants. 3n this experiment! concentric tube heat exchanger is used or simply called double pipe heat exchanger in used in order to get better understanding on how this device wors. 3t is called a double-pipe exchanger because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first! where one fluid in a double pipe heat exchanger flows through the smaller pipe while the other fluid flows through the annular space between the two pipes. oncentric tube heat exchanger is the simplest type of heat exchanger which consists of two coaxial tubes of different diameters! the outer diameter and the inner diameter.
%igure 1.1 4 ounter current flow in double pipe heat exchanger Typically! there are two types of flow arrangements are possible in this concentric tube heat exchanger! parallel flow and counter-current flow. ounter-current flow arrangement is used in the experiment! where the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite direction. 5ased on %igure 1.1! heat from the hot fluid diffuses through the tube wall and been transferred into cold fluid by convection! then pass through the wall by conduction! and then from the wall through 2
cold li6uid by convection. The hot water! cold water circuits and hot water tans all are well insulated to minimi7e heat loss or heat gains between the tan and the heat exchangers. are used for li6uid-li6uid! condensing and gas cooling applications. 8sually the counter current movement is used as the rate of heat transfer is more as compared to co-current movement. The 9:LT;< ,eat ;xchanger Training 'pparatus =odel4 ,; 1($2 has been designed to allow students to get familiari7ed with different inds of heat exchangers and to collect the necessary experimental data for the calculation of heat losses! heat transfer coefficient! log mean temperature difference and more. 5esides! studen ts will also be able to study the effect of flow rate on the heat transfer rate. The unit used in the experiment comes with four different types of heat exchangers and two stainless steel sump tans for hot and cold water source. The hot tan is fitted with an 11.( * immersion type heater that is protected against possible overheating. ;ach tan has a centrifugal pump capable of delivering the re6uired 10 L>= of water. 's stated by >.' ,ilton LT? 00"2! the inner tube of the double pipe heat exchanger is mostly made from stainless steel while the outer side is from clear acrylic. The primary advantage of concentric heat exchanger or double pipe heat exchanger compared to shell and tube! spiral and plate heat exchangers is the simplicity in the design of it. Thus! it is very easy to construct and to operate. ?ue to simple design! it is very easy to clean and maintain both of the surfaces of the heat exchanger which can decrease the rate of fouling.
1.2
OBJECTIVES The experiment is conducted to evaluate and study the performance of the concentric
tube heat exchanger at various operating conditions. 1.3
THEORY ,eat exchangers usually operate for a long time period with no change in their operating
conditions. Thus! they are labelled as steady-flow devices. ?esign of heat exchanger is to relate the inlet and outlet temperatures! the overall heat transfer coefficient! and the 3
geometry of the heat exchanger! to the rate of heat transfer between the two fluids. 's stated by unus '. engel! 'fshin @. hajar 00"2! the theory starts with the first law of thermodynamics which re6uired that the rate of heat transfer from the hot fluid be e6ual to the rate of heat transfer to the cold one. That is4
Q =mCpc ( Th , ∈−Th,out ) Tc , out −Tc ,∈¿ Q=mCph ¿ *here! c and h stand for cold and hot fluids! respectively! m A =ass flowrate g/s2 pc!ph A specific heat capacity B@/Bg.℃2 Tc!out! Th!out A outlet temperature ℃2 Tc!in! Th!in A inlet temperature ℃2 < A heat exchange rate between fluids B@/s or *) Cote that the heat transfer rate! < is taen to be a positive 6uantity! and its direction is understood to be from the hot water to the cold water! obeys the second law of thermodynamics. p at specific temperature of li6uid water was obtained by interpolation.
The overall heat transfer rate is expressed in terms of an overall heat transfer coefficient and a mean temperature. This is based on the analogous manner to CewtonDs law of cooling as4 Q =U As∆Tm
9ince the total heat transfer coefficient! h for the whole system is approximately e6ual to 8! h is expressed in terms of overall heat transfer coefficient. Power emitted , Qe Overall Heat Transfer Coefficient ,U = As ∆ Tm The overall 8 value is calculated by an e6uation specific to the geometric configuration of a heat exchanger. The overall 8 value is calculated over the total surface area ' of the heat exchanger! across which the fluids exchange heat. ' is determined from the formula4
4
Total surfacearea , As =π × tubeouter diameter× lenth of tube 3nner diameter of the tube is used for calculation! since the different between outer diameter and inner diameter was too small! the tube wall is thin enough to facilitate heat transfer process between hot fluid and cold fluid. =ean temperature is not taen simply as the difference between the average bul temperature of the hot fluid and the cold fluid but it is calculated according to the formula of log mean temperature difference! ETm as follow4 ∆ Tm ( ℃ )=
∆ T 1− ∆ T 2 ln ( ∆T 1 ! ∆ T 2 )
where for counter-current flow! ∆ T 1 ( ℃ )= Th, ∈−Tc , out ( Hot inlet temperature−Cold outlet temperature ) ∆ T 2 ( ℃ ) =Th,out −Tc , ∈ ( Hot outlet temperature−Cold inlet temperature )
The temperature profile in the heat exchanger can be revised by looing at the characteristics of the heat exchangers which are the flow arrangement either the hot and cold fluids move in the same or opposite directions and the type of construction. The temperature profile obtained from chart of temperature difference between the hot fluid and cold fluid at inlet and outlet. 3t may vary along the length of the heat exchanger. This is due to the fact that the hot fluid temperature decreases as it transfers heat to the cold fluid! while the cold fluid temperature increases. %or counter-current flow arrangement! the difference between the temperature of the hot and cold fluid almost uniform! means that the heat transfer rate at any location is usually maximum at any location throughout the tube. The temperature difference decease less dramatically compared to parallel flow arrangement as we move towards hot fluid exit. %or either flow arrangement! it can be observed that the ETm is not constant and changes along the length of a heat exchanger.
1.4
APPARATUS AND MATERIALS
5
?escription of diagram4 The 9:LT;< ,eat ;xchanger Training 'pparatus ,; 1($2 was used to study the heat exchanger process between hot and cold fluid by varying the flow rates and flow direction counter current2. The flowrate are being controlled by using %1 valve and % valve of %T1 for hot water and %T for cold water. F1# and F1+ are open to perform counter- current flow direction Temperature reading at each part is taen at TT1! TT! TT&! and TT". The operating limit for this unit is (0℃.
%igure 1. 4 9:LT;< ,eat ;xchanger Training 'pparatus ,; 1($2 1. 9piral heat exchanger &. 9hell G tube heat exchanger (. Falve 1( +. Falve 1$ ). old water centrifugal pump 11. %lowrate meter cold water2 1&. TT hot water outlet reading2 1(. TT" old water outlet reading2 1+. E>T1 reading 1). >ower pump switch 1. old water pump switch
. oncentric heat exchanger ". Falve 1+ #. Falve 1# $. ,ot water centrifugal pump 10. %lowrate meter 1 hot water2 1. TT1 ,ot water inlet reading2 1". TT& old water inlet reading2 1#. TT( 1$. E>T reading 0. ,ot water pump switch
6
1.5
PROCEDURE eneral start-up procedure 1. ' 6uic inspection was performed to mae sure that the e6uipment is in a proper
woring condition. . =ae sure all valves are closed! except F1 and F1. &. ,ot water tan is filled via a water supply hose connected to F+. The valve is closed when the tan is full. ". Falve F$ is opened to fill up the cold- water tan and leave the valve opened for continuous water supply. (. ?rain hose is connected to the cold water drain point. #. The main power is switched on and heater is opened for hot water tan to set the temperature controller to (0 ℃. +. The water temperature in the hot water tan is allowed to reach the set point. ;xperiment4 ounter urrent oncentric ,eat ;xchanger 1. eneral start-up procedure is performed. . >ump >1 and > are switched on. &. Falve F& and F1" are opened to obtain the desired flowrates for hot water and cold water streams! respectively. ". The system reached steady state for minutes. (. %T1! %T! TT1! TT! TT& and TT" are recorded #. >ressure drop measurements for shell-side and tube-side are recorded. +. 9teps & to # are repeated for different combinations of flowrate %T1 and %T. $. >umps >1 and > are switched off after the completion of experiment. ). eneral shut down is performed. eneral shut - down procedure 1. The heater is switch off and wait until the hot water temperature drops below "0 ℃. . >ump >1 and > are switched off. &. =ain power is switched off. ". 'll water in the process line is drained off (. 'll valves are closed. 1.6
RESULTS
Table 1.1 4 ounter-current flow for constant hot water flow rate FT 1 (LPM)
FT 1 (LPM)
1&.& 1&.& 1&.&
.0 ".0 #.0
TT 1 (TCo!) (oC) &#.+ &&.( &&.1
T"#$ (oC)
&".(( &.+( &.(
TT 2 (TC#%) (oC) &." &.0 &1."
TT 3 (THo!) (oC)) "+.$ "+.# "$.(
T"#$ (oC)
"$.10 "+.)0 "$.$(
TT 4 (TH#%) (oC) "$." "$. "). 7
$.0 10.0
1&.& 1&.&
&&. &".1
&.$0 &&.((
&." &&.0
"+.$ "$.1
"$.1( "$."(
"$.( "$.$
T"#$ (oC)
TT 4 (TH#%) (oC) "$.0 "$.+ ").$ "$." "$.$
Table 1. 4 ounter-current flow for constant cold water flow rate FT 1 (LPM)
FT 1 (LPM)
.0 ".0 #.0 $.0 10.0
1&.& 1&.& 1&.& 1&.& 1&.&
TT 1 (TCo!) (oC) &&.$ &&.) &".1 &".1 &".1
T"#$ (oC)
&&.(0 &&.#0 &&.+0 &&.+0 &&.((
TT 2 (TC#%) (oC) &&. &&.& &&.& &&.& &&.0
TT 3 (THo!) (oC)) "#.0 "+." "$.& "+.( "$.1
"+.00 "$.0( ").0( "+.)( "$."(
Table &4 alculation of power emitted! power absorbed and heat loss for the constant hot volume flow rate Co'$ !*+F'o,+!* (L-"#%) " # $ 10
Po,*+ *"#!!*$ /* ()
Po,*+ *"#!!*$ /* ()
H*! 'o00 ()
"1&." "1&." "$. "$.& "$.
()(." "1(.+ +0#.$ ""&." +#1.)
-1$ -.& -".# &$.) -+).+
Table "4 alculation of power emitted! power absorbed and heat loss for the constant cold volume flow rate Ho! !*+ F'o,+!* (L-"#%) " # $ 10
Po,*+ *"#!!*$ /* () +(.+ &($.& #1).) ")#.1 "$.&
Po,*+ *"#!!*$ /* () "1(.# "1(.( ((".0 ((".0 +#1.)
H*! 'o00 ()
-1&).) -(+. #(.$# -(+.)& -+).#( 8
Table (4 Log mean temperature and the overall heat transfer coefficient for the constant hot volume flow rate Co'$
T1 (oC)
T2 (oC)
T" (oC)
O*+'' *!
!*+F'o,+!*
!+%0*+
(L-"#%)
o*##*%! U
" # $ 10
(-"2.7C) #(0. (+$.1 #1(.$0 ##(.#$ #$(.#"
11/+ 1".+ 1#.1 1(.& 1".+
1(." 1(.# 1+.1 1(." 1(.1
1&."+ 1(.1( 1#.() 1(.&( 1".)0
Table #4 Log mean temperature and the overall heat transfer coefficient for the constant cold volume flow rate Ho! !*+
T1 (oC)
T2 (oC)
T" (oC)
F'o,+!* (L-"#%)
" # $ 10
O*+'' *! !+%0*+ o*##*%! U
1". 1".$ 1(.+ 1".& 1".+
1.$ 1".1 1(.0 1". 1(.1
1&.") 1"."( 1(.&( 1".( 1".)0
(-"2.7C) "&.)) (1(.&" $((.#0 +&+.#0 #$(.+
9
T*"8*+!+* P+o#'* o+ Co%!*+9C++*%! F'o, o+ Co%0!%! Ho! F'o,+!*
48.4
"$.1
"+.$ Hot Water
&#.+
Cold Water
&".((
&."
%igure 1.& 4 Temperature >rofile for ounter urrent %low for onstant ,ot *ater %lowrate T*"8*+!+* P+o#'* o+ Co%!*+9C++*%! F'o, o+ Co%0!%! Co'$ F'o,+!*
48
"+
"# Hot Water
&&.$
&&.(
Cold Water
&."
%igure 1." 4 Temperature >rofile for ounter urrent %low for onstant old *ater %lowrate
10
Power vs Cold Water Flowrate 800 700 600 500
Power (W)
400
Power Absorbed
300
Power Emitted
200 100 0 2
4
6
8
10
Cold Water Flowrate (L/min)
%igure 1.( 4 raph of >ower vs old *ater %lowrate
Power vs Hot Water Flowrate 800 700 600 500
Power (W)
400
Power Absorbed
300
Power Emitted
200 100 0 2
4
6
8
10
Hot Water Flowrate (L/min)
%igure 1.# 4 raph of >ower vs ,ot *ater %lowrate
11
Overall Heat Transfer Coecient vs Water Flowrate 900 800 700 600 500
U (W/m2.oC)
Cold Water lowrate
400
Hot Water lowrate
300 200 100 0 2
4
6
8
10
Water Flowrate (L/min)
%igure 1.+ 4 raph of :veral ,eat Transfer vs *ater %lowrate
1.:
CALCULATIONS 12
To find the properties of water for the constant hot volume flow rate Tc , mid ( ℃ ) =
32.4 + 36.7 2
=34.55 ℃
*ater properties for cold water from Table '-) in 'ppendix %rom interpolation! "= 0.99518 # / $ Cp= 4.1782
#% #&'
Th,mid ( ℃ )=
48.4 + 47.8 2
=48.10 ℃
*ater properties for hot water from Table '-) in 'ppendix %rom interpolation! "= 0.98886 # / $
Cp= 4.18062
#% #&'
To find the properties of water for the constant cold volume flow rate
Tc,mid ( ℃ ) =
33.8 + 33.2 =33.50 ℃ 2
*ater properties for cold water from Table '-) in 'ppendix %rom interpolation! "= 0.994 6 # / $ Cp= 4.1782
#% #&'
Th,imid ( ℃ )=
46.0 + 48.0 =47.00 ℃ 2
*ater properties for hot water from Table '-) in 'ppendix %rom interpolation! "= 0.9893 # / $
13
Cp= 4.1804
#% #&'
9ample calculation of power emitted! power absorbed and heat loss during flow rate of cold water at L/min
Poweremitted ,Qe =(h"hCph ( Th ,∈−Th,out )
Qe=
10 $
min
×
1 min 1 000 ) # #% × 0.98886 × 4.181 062 × ( 48.4 − 47.8 )= 0.4134 #) × =413.4 ) 60 s 1 #) $ # & ℃
Tc , out − Tc ,∈¿ Power absorbed , Qa =(c "c Cpc ¿
Qa=
2 $
min
×
1 min # #% × 0.994 18 × 4.1782 × ( 36.7 −32.4 ) ℃ 60 s $ # & ℃
¿ 0.5954 # ) ×
1000 ) =595.4 ) 1 #)
Heat $oss *ate =Qe−Qa = 413.4− 595.4 =−182 )
To find the log mean temperature difference for counter-current flow ∆ Tm ( ℃ )=
∆ T 1− ∆ T 2 ln ( ∆T 1 ! ∆ T 2 )
∆ T 1 ( ℃ )= Th, ∈−Tc,out
∆ T 2 ( ℃ ) =Th,out −Tc , ∈¿ %or cold water flow rate of L/minH ∆ T 1 ( ℃ )= Th, ∈−Tc , out = 48.4 −36.7 =11.7
14
∆ T 2 ( ℃ ) =Th,out −Tc , ∈¿ 47.8 −32.4 =15.4
∆ Tm ( ℃ )=
11.7 −15.4 ln ( 11.7 ! 15.4 )
=13.47 ℃
To find heat transfer coefficient d0 A 0.0&&" m di A 0.0##" m L A 0.(m dm A 0.0&&" I 0.0##"2 / A 0.0&00 m
Total surfacearea , As =π × diameter ×lenthof tube
¿ π × 0.03002 m × 0.5 m=0.0 472 m ² Power emitted , Qe Overall Heat Transfer Coefficient ,U = As ∆ Tm
¿
413,4 )
(0.0472 m ² )( 13.47 ℃ ) ¿ 650.22
1.;
) 2
m &℃
DISCUSSION
The experiment is aims to evaluate the performance of counter-current flow of heat exchanger and to study the routine of the heat exchanger under various flow rates by using concentric tube heat exchanger or simply double pipe heat exchanger. The experiment was divided into two parts. The first part is where the hot water volume flowrate was set to be constant at 10L/min while the cold water flowrates were varied at L/min! "L/min! #L/min! $L/min and 10L/min. %or the second part! the cold water volume flowrate was set to be constant while the hot water volume flowrates were varied. TT1! TT! TT& and TT" were recorded every 10 minutes.
15
%rom data obtained! the exit temperature of the hot fluid is higher than the exit temperature of the cold fluid. This shows that the heat transfer must occur from hot body to the cold body. 3t is impossible for heat transfer to tae place from a cold body to hot body. 's stated by unus '. engel! 'fshin @. hajar 01(2! the outlet temperature of the cold fluid can never exceed the inlet temperature of the hot fluid! since this would be a violation of the second law of thermodynamics. Thus! the results obtained are correct. 's the temperature of the hot water increases! the overall heat transfer coefficient is also increasing. The increasing of the overall heat transfer coefficient will causes the rate of heat transfer also increases. This is because the rate of heat transfer is affected by both temperature and overall heat transfer coefficient as the following e6uation. The increasing of the rate of heat transfer will causes the performance of heat exchanger to increase. The overall heat transfer coefficient is generally wealy dependent on temperature. 's the temperature of the fluids change! the degree to which the overall heat transfer coefficient will be affected depends on the sensitivity of the viscosity of the fluids to temperature.
The higher the flow rate of the fluids flowing in the heat exchanger! the higher the rate of heat transfer! thus the better the performance of the heat exchanger. This can be shown in the %igure 1.( and %igure 1.#. 's the flow rate was increase from L/min to 10L/min! the overall heat transfer coefficient also increase from ()(."*/m.℃ to +#1.)*/m.℃
!or
"o#sta#t
$ot
%owrate&
w$ile
415.6*/m.℃
to
761.9*/m.℃ !or "o#sta#t "old water %owrate . The increasing in heat transfer
coefficient shows increasing in rate of heat transfer which reflects in increasing of the performance of the heat exchanger. ,owever! the higher the flow rate! the greater the pressure loss then the bigger the pump needed to circulate the water. :ther than that! low flow rate means low in velocity which is helpful in avoiding erosion! tube vibration and noises as well as pressure dropH henceforth a standard velocity used in heat exchanger is established. ?espite of that! supposedly the efficiency of the heat exchanger increasing along with the increasing of 16
the heat transfer coefficient! however the efficiency calculated is decreasing as the flow rate is increasing. This decrement in efficiency might because of the temperature is taen before it is stabili7e. 5ased on the results calculated! the experiment reached the main objectivesH howerver! there might be some error occurred while conducting the experiment. The water flow rate may not not constant during the experiment and this led to inaccuracy of taing the results for every 10 minutes. Lastly! the inaccuracy of calculations for the heat exchangerDs heat loss may because of fouling occ ur in the pipe. The most common heat exchanger problem for many chemical engineers is fouling which can occur within the inside of a tube wall and decrease performance and even damage the heat exchanger in the long run. %ouling is a term used to describe material which builds up on the inside of a tube wall within heat exchangers and in turn affects the performance of the heat exchanger. =aterial or fluids which might collect on the side wall will build up over time and! if not taen care of! can reduce the heat transfer within your heat exchanger and increase the pressure drop which can cause more problems in the heat exchangers. The performance of the heat exchanger can be increased in many ways. :ne of them is by increasing the overall heat transfer coefficient! 8. 8 can be increase by increasing turbulence in the flowing fluids on tube! by reducing fouling rates through pretreatment of the fluids or increasing cleaning schedules and by eliminating stagnant areas inside the exchanger by judicious designs.
1.<
CONCLUSION
?ouble pipe heat exchanger has two types of flow which are co-current low or parallel ! and counter-current flow. 3n this experiment! the flow used is counter-current. 5oth the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions in counter flow. 5ased on the data taen from the experiments! the characteristics of the heat exchanger such as power lost! power absorbed! power emitted! 17
log mean temperature Etm! and overall heat transfer coefficient can be calculated. The results show that at higher temperature gives higher efficiency of the heat exchanger. The higher the flow rate of the fluids flowing in the heat exchanger! the higher the rate of heat transfer! thus the better the performance of the heat exchanger. This can be shown in the %igure 1.( and %igure 1.#. 's the flow rate was increase from L/min to 10L/min! the overall heat transfer coefficient also increase from ()(."*/m. ℃ to +#1.)*/m.℃ !or "o#sta#t $ot %owrate& w$ile 415.6 */m.℃ to 761.9*/m.℃ !or "o#sta#t "old water %owrate . The increasing in heat transfer coefficient shows increasing in
rate of heat transfer which reflects in increasing of the p erformance of the heat exchanger.
1.1&
RECOMMENDATION
3n order to recover the mistaes and perform a better experiment progress! a few suggestions that can be made are4 1. 3t might be wisely to mae a regular maintenance and cleaning of the pipes to avoid fouling and error to the performance of the heat exchangers and to increase the efficiency of heat transfer process. . 3t is much better to particularly chec the dual between the pipes to minimi7e heat loss to the surroundings. &. 3t more wisely to install a new high sensitivity thermometer as to increase the accuracy of the temperatures readings. ". 3t is more easily to have an electronic flowmeter device as to reduce the inaccuracy of changing the flow rates for the heat exchangers to wor.
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1.11
REFERENCES-APPENDICES
1.
http4//www.solution.com.my Experimental Manual Heat Exchanger Training
Apparatus
Model
HE158C.
Jetrieved
#th
'pril!
01#!
from
9olte64
http4//www.solution.com.my/pdf/TJ1"'"2.pdf .
unus '. engel! afshin @. hajar! 01(2. hapter 114 ,eat ;xchangers. ,eat G
=ass Transfer! fundamentals G 'pplications! (th ;dition! #"+-#$+. &.
Double Pipe Heat Exchanger Design. Jetrieved (th 'pril 01#! from
http4//www.brighthubengineering.com/hvac/#"("$-double-pipe-heat-exchanger-design/ ".
>. '. ,ilton LT?! =ay 00"2. ptional Concentric Tube Heat Exchanger
H1!1A. Concentric Manual" Experimental perating # Maintenance procedures.
19
