design and experimental analysis of shell and tube heat...

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DESIGN AND EXPERIMENTAL ANALYSIS OF SHELL AND

TUBE HEAT EXCHANGER (U-TUBE) Divyesh B. Patel

1, Jayesh R. Parekh

2

Assistant professor, Mechanical Department, SNPIT&RC, Umrakh, Gujarat, India1

Assistant professor, Mechanical Department, SNPIT&RC, Umrakh, Gujarat, India 2

Abstract: Heat exchangers are one of the most important heat transfer apparatus that find

its use in industries like oil refining, chemical engineering etc. Shell and tube (U-tube) type

of heat exchangers have been commonly and effectively used in industries over the year. In

this paper, shell and tube (U-tube) heat exchanger is designed which includes thermal

design, mechanical design and hydraulic design. Different types of methods are carried out

for optimum design. According to design parameters experimental analysis is carried out,

which reveal the clear idea about temperature difference and dimension of heat exchanger.

General Design consideration and design procedure is also illustrated.

Keywords: Heat exchanger, heat transfer coefficient, LMTD, Mass flow rate, NTU.

INTRODUCTION

Heat exchanger is a device which is used to transfer the heat from one working fluid

to another or we can say transfer of internal thermal energy between two or more fluids

available at different temperatures. It is used in wide variety of applications i.e., power

production, food industry, electronics, environmental engineering, waste heat recovery,

air conditioning, refrigeration etc. Shell & tube type heat exchanger are most versatile

type of heat exchangers. In most heat exchangers, the fluids are separated by a heat-

transfer surface, and ideally they do not mix.

The U tube is least expensive construction because only one tube sheet is needed.

The tube side cannot be cleaned by mechanical means because of the sharp U bend.

Numbers of shell side & tube side flow arrangement are used in shell & tube heat

exchanger depending on heat duty, pressure drop, fouling, cost, corrosion control and

cleaning problems. Baffles are used in shell & tube heat exchanger to promote better

heat transfer coefficient on shell side and to support the tubes.

In this present work design of optimum size U – type SHTE was carried out. For

optimum heat transfer rate different method were carried out [1][2]. Sadic Kakac

et.al[1], Ramesh k. shah et.al[2], had given the equation for heat transfer rate for

hot and cold fluid by LMTD method, Є-NTU method, P-NTU method and also for Ѱ-

NTU method. Sadic Kakac et.al [1] also showed that approximate overall heat transfer

co-efficient for preliminary analysis for water to water is between 1300-2500 W/m2K.

He suggested that Tube Pitch, PT, is usually chosen in such a way that PT/dӨ is between

1.25 to 1.5. He gave that tube layout constant CL=0.87 for 300 triangular layout and

tube count calculation constant CTP=0.9 for two tube passes. The number of tubes that

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can be placed within a shell depended on a tube lay out, tube outside diameter, pitch

size, number of passes and shell diameter. He had conclude that optimum baffle

spacing, Bs is between 0.4 and 0.6 of shell diameter and baffle cut Bc, is 25% to 35% of

shell diameter. Sadic Kakac et.al[1], T. Kupprn et.al[3] had given following four type of

tube layout, Triangular (30°), Rotated Triangular (60°), Square (90°), Rotated Square

(45°). T. Kupprn et.al [3] had given the basic equation to find out, the number of baffles,

Nb = (L/Bc)-1

Bundle to shell clearance, Lbb = (12.0+0.005D)/1000 in m for U tube exchanger.

Outer tube limit diameter, Dotl = (Ds-Lbb),

Centerline tube limit diameter, Dctl = (Dotl-dӨ),

Baffle diameter = Ds-Bc

He also given Centriangle of baffle cut θds = 2×cos-1

[1-2×Bc/100]

DESIGN MEHODOLOGY

Following are the operating parameters while designing the shell & tube heat exchanger:

1. Inlet temperature of hot water, Th1= 63 °C

2. Outlet temperature of hot water, Th2 = 53 °C

3. Inlet temperature of cold water, Tc1 = 33 °C

4. Mass flow rate of cold water, ṁc = 0.6 kg/s

5. Mass flow rate of hot water, ṁh = 0.58 kg/s

A. Thermal Design

There are basic three approaches for thermal and hydraulic design:

1) LMTD METHOD

2) є-NTU METHOD

1) LMTD Method

Outlet Temperature Of Cold Water : [1][2]

Heat Transfer Rate: Q=U *A* ∆Tm

= ṁh *cph* (Th1 – Th2)

= ṁc *cpc* (Tc2 – Tc1)

Here, ṁh and ṁc are mass flow rate of hot fluid and mass flow rate of cold fluid

respectively. From the above equation we can find the outlet temperature of cold water.

Heat Transfer Rate Of Shell And Tube Side: [1][2]

Actual heat load can be estimated by Q = ṁh *cph* (Th1 – Th2)

Log Mean Temperature Difference: [1][2]

∆Tm=

Heat Transfer Area: [1][2]

Qact = U * A * ∆Tm

Overall Heat Transfer Co-efficient [U] for the water to water fluid is between 1300 to 2500

w/m2 °C

2) є-NTU Method

Maximum possible heat transfer rate is determined by

Qmax = Cmin (Th1-Tc1)

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Here Cmin=minimum heat capacity which is given by CC= ṁc *Cpc or Ch= ṁh *Cph

whichever is minimum.

Effectiveness: [2]

Heat exchanger effectiveness is defined as the ratio of actual heat transfer rate to the

maximum possible heat transfer rate

є= Qact / Qmax

Heat Capacity Ratio: [2]

It is a heat exchanger operating parameter since it depends on the mass flow rate and

temperature of the fluid in the heat exchanger

C* = Cmin/ Cmax = (ṁ*Cp)min /( ṁ*Cp)max

Effectiveness and number of transfer unit relationship: [2]

Effectiveness is function of NTU, heat capacity ratio and flow arrangement.

=

Number Of Transfer Unit: [2]

Number of transfer unit is estimated as the ratio of overall thermal conductance to the

minimum heat capacity

NTU = (U* A) / Cmin

From this equation we find Area of heat exchanger which is coming higher than the area

found by LMTD method so We adopt LMTD METHOD for Thermal Design.

B. Number Of Tube: [1]

Heat transfer area for the tube of the diameter d0 and length L can be given by following

equation from which numbers of tube can be estimated

A = π*d0*Nt*L

C. Shell Diameter: [1]

Shell diameter can be obtained as

Ds = 0.637 * *[A*(PR)2*do]

(1/2)

CL=0.87 for 30° and 60°

CTP=0.9 for two tube pass

PR is usually between 1.25 to 1.5

D. Pitch: [1]

Pitch can be determined as

PT=PR*d

E. Baffle Spacing: [1]

The practical range of single segmental baffle spacing is 40% - 50% of shell diameter

Bs =( 0.4 to 0.5) * Ds

F. Number Of Baffle: [3]

Numbers of baffles for the tube length L and baffle spacing Bs can be given by

Nb = (L / Bs) – 1

G. Baffle Cut Geometry:

Baffle Cut: [3]

The baffle cut vary from 20-35% of shell diameter with the most common being 20-25% as

it affords the highest heat transfer for a given pressure drop.

Bc = (0.20 to 0.35)* Ds

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Figure 1: baffle geometry

Bundle To Shell Clearance: [3]

Bundle to Shell Clearance is calculated based upon the following equation

Lbb= [12+ (0.005*Ds)]/1000 (m)

Outer Tube Limit Diameter: [3]

It is difference between the shell diameter and Bundle to Shell Clearance

Dotl=Ds- Lbb

Centerline Tube Limit Diameter: [3]

It is difference between the Outer Tube Limit Diameter and tube diameter

Dctl= Dotl-do

Diametral clearance

The diametral clearance between shell diameter and baffle diameter is given by

Lsb= 3.1+(0.004* Ds))/1000 (m)

Baffle Diameter: [3]

It is difference between the shell diameter and diametral clearance

Db= Ds- Lsb

H. Shell Side Heat Transfer Coefficient:

Equivalent Diameter: [1]

Figure 2: triangular pitch layout

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Triangular pitch layout is shown in the figure2, for which the equivalent diameter is given by

following equation.

For triangular pitch De= PT2

/4)-2/8) / ( *

Here do is tube outer diameter.

Clearance between Tubes: [1]

As shown in figure 2 clearances between tubes is estimated as below.

C= PT-

Bundle Cross Flow Area: [1]

Bundle Cross Flow Area can determined by following equation

As= Ds*C*Bs/ PT

Shell Side Mass Velocity: [1]

The variable that affects the mass velocity are shell diameter Ds, clearances between the

adjacent tubes and the cross flow area As. shell mass velocity is found with,

Us= s/ A

Here s is mass flow rate of fluid in shell side.

Shell Side Reynolds Number: [1]

Reynolds number for the shell side flow can be estimated as,

Res= Us* De / ʋs

Here ʋs is kinematic viscosity of fluid at given fluid temperature.

Approximate Wall Temperature: [1]

Approximate Wall Temperature is determined as,

Tw= (Th1 + Th2 +Tc2 +Tc1)/4

Shell side heat transfer coefficient is given by

To find the shell side heat transfer coefficient it requires to find the properties like viscosity,

thermal conductivity of working fluid at approximate wall temperature and viscosity at shell

side fluid temperature.

ho=[k/De*0.36]*[De*Us/ʋs]0.55

*[Cp* μs/k] 0.33

*[ μs/ μw]0.14

I. Tube Side Heat Transfer Coefficient: [1]

ρ = 985 kg/m3

μt=0.000485 at average tube side temperature,

Tube side flow area is determined as

Atp= [π/4*di2]*[ Nt/2]

Here di is tube inner diameter and Nt is numbers of passes.

Average Velocity In Tube:

Average fluid velocity in the tube is given by

Vm= ṁt/ (ρ* Atp)

Here ṁt is mass flow rate of tube side fluid; ρ is density of fluid at tube side fluid

temperature.

Tube Side Reynolds Number:

Tube Side Reynolds Number given by,

Ret= ρ*Vm* di/ μt

Friction factor for the tube is,

ft=(1.58ln Ret -3.28)-2

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It is require to find the properties like Prandtl number and thermal conductivity of fluid at

tube side fluid temperature to find tube side heat transfer coefficient,

Nusselt number for the tube side fluid is;

Nub= [( ft/2)*( Ret-1000)*Pr]/[(1+12.7(ft/2)0.5

*(pr2/3

-1)]

Tube side heat transfer coefficient is estimated as;

hi= Nub*k/di

J. Overall Heat Transfer Coefficient: [1]

Overall heat transfer coefficient is determined by following equation,

U=

CALCULATED RESULTS

Based on the above methodology the calculated results are summarised below.

TABLE 1: CALCULATED RESULTS

DESIGN PARAMETER VALUE

Cold outlet temperature (Tc2) 42.67 oC

Heat duty (Q) 24.28 kW

LMTD (∆Tm) 20.16 oC

Heat Transfer Area by LMTD (A1) 0.86016 m2

Maximum Heat Transfer Rate (Qmax) 75.36 kW

Effectiveness (є) 0.3222

Heat Capacity Ratio (C*) 0.9667

Number Of Transfer Unit ( NTU) 0.4716

Heat Transfer Area by NTU (A2) 0.8664 m2

Numbers of tubes (Nt) 26

Shell diameter (Ds) 0.1524 m

Pitch (PT) 0.021425 m

Baffle spacing (Bs) 0.0762 m

Numbers of baffles (Nb) 10

Baffle Cut (Bc) 0.0381 m

Bundle To Shell Clearance (Lbb) 0.01201 m

Outer Tube Limit Diameter (Dotl) 0.140399m

Centerline Tube Limit Diameter(Dctl) 0.125429 m

Diametral clearance between shell diameter

and baffle diameter (Lsb)

0.003101 m

Baffle Diameter (Db) 0.149299 m

Equivalent Diameter (De) 0.016038 m

Clearance Between Tubes (C) 0.0055545 m

Bundle Cross Flow Area (As) 0.003010 m2

Shell Side Mass Velocity (Us) 199.28 kg/s.m2

Shell Side Reynolds Number (Res) 4679.7019

Approximate Wall Temperature (Tw) 47.91 0C

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Shell side heat transfer coefficient (ho) 2594.577 W/m²k

Average velocity in tube (vm) 0.27495 m/s

Tube Side Reynolds Number(Ret) 8306.45

Tube side Friction factor (ft) 0.008295

Nusselt number (Nub) 48.95

Tube side heat transfer coefficient (hi) 2134.48 W/m²k

Overall heat transfer coefficient (U) 1127.87 W/m²K

I. EXPERIMENTAL RESULTS

Experimental Setup:

Figure 3: Experimental setup of shell and tube heat exchanger (u-tube)

In Experimental analysis of STHE , first we fix the mass flow rate of hot water at 0.401 kg/s

and vary the mass flow rate of cold water then we get the diff. Temperature reading. Here the

cold water and hot water flowing through shell and tube side respectively.

TABLE 2: EXPERIMENTAL DATA

Sr

.

N

o.

Mass

Flow

Rate

(kg/sec)

Inlet

Temp.

(°C)

T1

(°

C)

T2

(°

C)

T3

(°C)

Out

let

Te

mp.

(°C)

1 Tub

e

0.401 61.2 52.5

Shel

l

0.4785 33 35

.4

36

.8

38.

3

39.8

2 Tub

e

0.401 61.2 53.1

Shel

l

0.5173 33 34

.9

36

.7

38.

1

38.7

3 Tub

e

0.401 61.2 53.8

Shel

l

0.5595 33 34

.6

35

.4

37.

1

37.9

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Figure 4: Shell side temperature variation

Figure 4 shows the temperature variation in the shell side along the length at different mass

flow rate for the shell side fluid.

Keeping hot water mass flow rate constant at 0.401kg/sec and varying cold water mass flow

rate at 0.4785, 0.5173 and 0.5595 kg/sec, we get increase in cold water temperature of 6.8°C,

5.7°C and 4.9°C respectively.

CONCLUSION

Based on the design methodology we design and fabricate the shell and U tube heat

exchanger and perform the experimental test.

Keeping hot water mass flow rate constant at 0.401kg/sec and varying cold water

mass flow rate at 0.4785, 0.5173 and 0.5595 kg/sec, we get increase in cold water

temperature of 6.8°C, 5.7°C and 4.9°C respectively.

REFERENCES

[01] Karnik, Kakac Sadik , Hongten Liu, “Heat Exchangers Selection,Rating And Thermal

Design”, Second Edition. Page No. [33-71,288-307]

[02] Shah R.K , Sekulic D.P , John Wiley And Sons, INC. , “Fundamental Of Heat Exchanger

Design”, ISBN 0-47-3217-0

[03] T.Kuppan, Heat Exchanger Design Hand Book

design and experimental analysis of shell and tube heat...

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