Shell&Tube Heat exchanger

1

1. INTRODUCTION

The aim of that experiment is to investigate the performance of shell and tube heat

exchanger both operating in counter-current and co-current modes and also to investigate the

effect of Reynolds number on individual heat transfer coefficients by using the experimental

data. To achieve this aim, firstly, the working principles of heat exchangers are researched.

Heat exchangers are devices that are used in wide variety of purposes in engineering

application such as electric resistance heaters, boilers, condensers, radiant heat dryers. Briefly,

they work as a heat transfer medium that is transferred from one matter to the desired one.

Heat exchangers are classified according to type of construction and flow arrangement. As

flow arrangement, heat exchangers classified under two main groups: parallel flow heat

exchangers and counter flow heat exchangers. In parallel flow, hot fluid and cold fluid enter

the exchanger at the same end, and travel in parallel to one another to the other side.

In counter-flow heat exchangers, on the other hand, the fluids enter the exchanger from

opposite ends. In this experiment, these two modes are examined by plotting the temperature

profile of each data. As construction arrangement, there are mainly three types: shell and tube

heat exchanger, concentric tube and compact heat exchanger. [1] In this experiment, a shell

and tube heat exchanger which has 1 pass is studied in both co-current and counter- current

modes. The heat exchanger is made of Borosilicated Glass in the shell side and AISI Stainless

Steel in the tube side and the properties are given in Table 1.1.

Table 1.1: Technical details of examined heat exchanger

2

The shell and tube heat exchangers used widely in industry since they have many

advantages such as having large heat exchange area, having good shape for pressure

operation, using well-established fabrication technique, ability to be constructed from wide

range of materials, ability to be cleaned easily and having well established design

procedures.[2]

Figure 1.1: Shell and tube heat exchanger with counter flow.[2]

In the calculations, the fouling factor effect is neglected since the pipes are said to be

clean. However, baffles are considered in calculation. Baffles are vanes and panels that give a

direction to the flow of fluids in heat exchangers to increase the fluid velocity and improve

rate transfer. The baffle cut term is used for the height of segment removed to form the baffle.

In this project, the designed heat exchanger has 13 baffles and baffles cut at 25% of diameter.

The two correlations for shell side and tube side and overall heat transfer coefficient

equation are given below.

Gnielinski Equation(for tube side)

3

Donohue Equation(for shell side):

Overall heat transfer coefficient calculation by using hi and ho:

4

2. EXPERIMENTAL METHODS

In this experiment, the purposes were to see the effect of Reynolds number on the

individual heat transfer coefficients, to calculate and compare the overall heat transfer

coefficient (U) and to observe the performance of shell and tube heat exchanger for both

cocurrent and counter-current modes.

In the first part of the experiment, the counter-current flow operation was observed

and for this purpose, the valves V1 and V3 are closed and V2 and V4 are opened. For sounter-

current observation, the cold water stream was adjusted to 400, 500 and 600 L/h and hot water

stream was adjusted to 400L/h for three different cold water flow rate values. After the

adjustments were done, the system was operated and each three minutes, the data were

recorded for the temperature values, that is for TI1, TI2, TI3, TI4 and TW1 till the system

reached the steady state. The steady state values of inlet and outlet temperatures of both

streams were also recorded. After the first part finished, in order to compare the performance

of co-current and counter-current operations, co-current operation at a studied value of first

part is chosen which is 500 L/h for cold water stream. The valves V2 and V4 are closed and

V1 and V3 are opened.

Figure 2.1: Experimental setup

5

3. RESULTS

In this experiment, we aimed to calculate and compare the overall heat transfer

coefficients (U) that obtained for both co-current and counter-current modes of shell and tube

heat exchanger. Also, we were able to see the effect of Reynold’s Number on the heat transfer

coefficients.

Temperature profiles of each run in the heat exchanger;

Table 3.1: Temperature and flow rate values for the counter-current flow operation

Flow rate (L/h) Temperature (°C)

Cold Fluid Hot Fluid Tc,i Tc,o Th,i Th,o

400 856 14.7 29.9 62.8 48.7

500 856 14.5 27.1 60.9 46.5

600 856 14.6 24.8 58.2 44.1

Table 3.2: Temperature and flow rate values for the co-current flow operation

Flow rate (L/h) Temperature (°C)

Cold Fluid Hot Fluid Tc,i Tc,o Th,i Th,o

500 856 14.6 26.3 58.8 45.7

Figure 3.1: Temperature profile for the counter-current flow operation for Run 1

6

Figure 3.2: Temperature profile for the counter-current flow operation for Run 2

Figure 3.3: Temperature profile for the counter-current flow operation for Run 3

7

Figure 3.4: Temperature profile for the co-current flow operation for Run 1

Some physical properties that assumed constant at average temperatures;

Table 3.1: Physical Properties of fluid at average temperatures

Counter-current Flow Co-current flow

Run no:1 Run no:2 Run no:3 Run no:1

Tube

Side

Shell

Side

Tube

Side

Shell

Side

Tube

Side

Shell

Side

Tube

Side

Shell

Side

Tav(K) 328.75 295.3 326.7 293.8 324.15 292.7 325.25 293.3

Pr 3.22 6.57 3.33 6.85 3.48 7.05 3.41 6.94

ϻ*10^(-

6)(N.s/m) 498.75 953 515 988 536 1014.6 526 1000

Cp(kj/kgK) 4.184 4.181 4.183 4.182 4.182 4.183 4.182 4.182

k*10^(-3) 648.75 606 646.7 604 644.2 602.3 645.25 603.3

ρ(kg/m3) 985 998 986 998.5 987 998.4 987 998.4

And this table also shows the difference between overall heat transfer coefficients, heat

transfer coefficient with respect to each side of the shell and tube heat exchanger, heat values

of hot and cold fluid, Reynolds and Nussle numbers.

8

Table 3.2: Overall heat transfer coefficients and Reynolds numbers

Counter-current flow Co-current flow

Run no:1 Run no:2 Run no:3 Run no:1

Tube

Side

Shell

Side

Tube

Side

Shell

Side

Tube

Side

Shell

Side

Tube

Side Shell Side

Q(L/h) 856 400 856 500 856 600 856 500

Tcin - 14.7 - 14.5 - 14.6 - 14.6

Tcout - 29.9 - 27.1 - 24.8 - 26

Thin 62.8 - 60.9 - 58.2 - 58.8 -

Thout 48.7 - 46.5 - 44.1 - 45.7 -

m(kg/s) 0.234 0.111 0.234 0.237 0.237 0.166 0.235 0.139

v(m/s) 0.946 0.093 0.946 0.116 0.946 0.139 0.946 0.116

Re 14950 9612 14490 11590 14100 13550 14200 11450

Flow

regime Turbulent Turbulent Turbulent Turbulent Turbulent Turbulent Turbulent Turbulent

Nu 90.087 91.307 88.964 103.601 88.481 114.828 88.305 103.291

hio(W/m2K) 5844 - 5753 - 5344 - 5698 -

ho(W/m2K) - 1197 - 1352 - 1508 - 1344

U(W/m2K) 993.5 1095 1176 1088

q(kW) -13.817 7.047 -14.122 7.308 -14 7.1 -12.857 6.611

These all datas are calculated by mathcad on computer. And sample calculations about all

procedure are given in appendix for shell side and tube side for run1 of counter current flow.

These are the some formulations that we use in results ;

Reynolds Number For Shell Side

9

If we substitute all equations into first one,

where,

fb = 0.1955 P = 20 mm B = 48 mm

D0=10 mm Ds=50mm Nt = 5

Res = ( Do . Ge ) / μ

Reynolds Number For Tube Side

If we substitute all equations into first one,

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Prandtl Nr. For Shell Side

Or you can find the values for water in cropera [1]

Prandtl Nr. For Tube Side

Or you can find the values for water in cropera [1]

Donohue Equation for Shell Side

Friction Factor Calculation for Tube Side

11

Gnielinski Equation for Tube Side

12

4. DISCUSSION

The aims of this experiment are to investigate the effects of Reynolds number on

individual heat transfer coefficients, while comparing the empirically and experimentally

calculated overall heat transfer coefficients. Both fluids are water; one is heated and the

cold one is simply softened tap water. Shell side water, which is cold one, is fed to the

exchanger at 14.6oC temperature, through piping and the cold fluid gets hotter by the heat

supplied from hot water. However, the gain of cold fluid is less than the amount of supplied

by hot fluid. Therefore, it can be said that there is a heat loss.

Other case is the overall heat transfer coefficients (U), which is both for empirical and

experimental. First of all, to calculate empirical overall heat transfer coefficient, the

individual heat transfer coefficients have to be calculated for each run. The result is in the

Table 3.2. The heat transfer coefficient changes when the flow rate changes. They are

proportional to each other. In theoretically, the heat transfer coefficient increases as

concluded experimentally because of the eddies, due to the turbulent flow regime.

The other assumptions were about the baffle spacing. Again our preliminary

calculations showed that choosing baffle spacing closer to 0.2 of Ds gives better results

which were given as 0.05. If we consider the cost, since baffle spacing affects area for cross

flow and the heat transfer coefficient directly. Moreover, heat transfer coefficient affects

clean overall coefficient and dirt factor.

When results which is listed in Table 3.2 are compared, theoretically, it is expected

that higher heat transfer coefficient for counter-current than for co-current. Similar behavior

is also observed experimentally. As expected that counter-current system is more efficient

with higher heat transfer coefficients.

Finally, while counter current and co-current flows have the same flow rates; their

heat transfer rates are different, because of the effect of log mean temperatures as the

same area. However, the theoretical and calculated values are nearly same in each other.

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5. CONCLUSION

In this experiment, the performance of heat exchanger for different operating modes

and heat transfer coefficient dependence are investigated. At different flow rates, overall heat

transfer rates, shell side and tube side heat transfer coefficients were calculated and compared.

According to our evaluation of experimental data, heat transfer coefficients are higher

in counter-current flow with respect to co-current flow. Accordingly, we can conclude that

counter-current operating heat exchangers are more efficient.

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6. REFERENCES

1. Dewitt, D., Incropera, F. & Bergman, T.L. “Fundementals of Heat and Mass Transfer”.

Sixth Edition

2. Shell and tube Heat Exchanger Design. Retrieved from

http://www.engr.iupui.edu/me/courses/shellandtube on May 23, 2012

3. Effectively Design Shell and Tube Heat Exchangers. Retrieved from http://www-

unix.ecs.umass.edu/~rlaurenc/Courses/che333/Reference/exchanger.pdf on May 23, 2012

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7. APPENDIX

TUBE SIDE (for Run1)

Di 0.008 m Cp 4.184 kj

kgK 985

kg

m3 Pr 3.22

Tcin 14.7 Tcout 29.9

Tincelcius( ) k 648.7510

3

W

mK

Thin 62.8 Thout 48.7

498.75106

Ns

m

Q 856 3600000

Q 2.378 104

m3

s

Tm

Thin Thout

2273

Tm 328.75 TminKelvin

at Di

2

4

Nt 5 nt 1

At

Nt at

nt

m Q

m 0.234 m in kg/s

q m Cp Thout Thin

q 13.817 kW

vm

At

v 0.946

Re v Di

Re 1.495 104

16

0

A Di

2

4

X 4 log

Di

3.7065

5.0452

Relog A( )

f1

X2

f 7.758 103

Nu

f

2Re 1000( ) Pr

1 12.7f

2

1

2

Pr( )

2

31

Nu 90.087

hiNu k

Di

hi 7.306 103

hio hi 0.8

hio 5.844 103

17

SHELL SIDE (for Run1)

Dt 0.01 m Ds 0.05 m Cp 4.181 kj

kgK tav 328.75 K

Tcin 14.7 Tcout 29.9 Tincelcius( ) hio 5844

Thin 62.8 Thout 48.7

Prs 6.57 998 kg

m3 953 10

6

Ns

m k 606 10

3

W

mK

P 0.02 m B 0.048 fb 0.1955

Q 400 3600000

Q 1.111 104

m3

s

Tav

Tcin Tcout

2273

Tav 295.3 Tav inKelvin

m Q

m 0.111 m in kg/s

q m Cp Tcout Tcin

q 7.047 kW

Sc P Dt BDs

P

Sc 1.2 103

m2

v Q Sc

v 0.093 m

s

Gcm

Sc

18

Nb 1

Sb fb

Ds2

4 Nb

Dt2

4

m2

Sb 3.053 104

Gbm

Sb

kg

m2s

Gb 363.185

Ge Gc Gb

Ge 183.197 kg

m2s

Res

Ds Ge

Res 9.612 103

A 0.2Res0.6

Prs0.33

k

Ds

A 1.107 103

where A=hs/ϕ s

tw tavA

hio ATav tav

tw 323.424 Kelvin

w 544 106

Gc 92.407 kg

m2s

19

s

w

0.14

s 1.082

ho A s

W

m2K

ho 1.197 10

3

NuA Ds

k

Nu 91.307

Uhio ho

hio ho

W

m2K

U 993.506