Heat Transfer Enhancement in Helix Exchanger Using Grooved Tubes At Different Baffle Angles

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International Journal of Emerging Technology and Advanced Engineering

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477

Heat Transfer Enhancement in Helix Exchanger Using Grooved

Tubes At Different Baffle Angles

Vineet Kumar

1

, Pardeep Kumar

2

, Sunil Nain

3

1,2

Research Scholar, 3Assistant Professor, UIET, Kurukshetra University, Haryana

Abstract Grooves on plain tubes in a helix exchanger improves the performance by increasing Reynold’s number, reduce flow induced vibration and fouling while maintain a higher heat transfer capability. In the present work, an experimental analysis of grooved tube helix exchanger with different helical baffle angles was done and the influence of Reynolds number, hot water inlet temperature on the heat transfer rate and on overall heat transfer coefficient was tabulated and shown by graphs. In the present study attempts were made at 5,15,25,35 and 45 degree baffle inclination at different mass flow rate of hot water, which gives insight of all parameters affect on pressure drop, friction factor and heat transfer rate. Through experimental analysis of helix exchanger it was found that angle 25 degree behaved the best in case of grooved tube helix exchanger.

KeywordsKern`s methods, Wolverine technology for grooved tubes, Grooved tubes, Helical baffle angles, Hot water mass flow rate, Reynolds number, Heat transfer coefficient and pressure drop.

I. INTRODUCTION

Helix exchanger with grooved tubes is a shell and tube type heat exchanger which eliminates principle shortcomings caused by shell side zigzag flow induced by conventional baffle arrangements with plain tubes. Helical baffle heat exchangers have shown very effective performance especially for the cases in which the heat transfer coefficient in shell side is controlled or less pressure drop and less fouling are expected. It can also be very effective, where heat exchangers are predicted to be faced with vibration condition. Quadrant shaped baffle segments are arranged at an angle to the tube axis in a sequential pattern that guide the shell side fluid to flow in a helical path over the tube bundle. Helical flow path of the shell-side fluid can also be achieved by helix shaped baffle running throughout the length of the shell and tube heat exchanger with a baffle pitch.

Helix exchanger are most widely used in various industries such as chemical process, power generation, oil refining, refrigeration and air - conditioning. Among different type of heat exchangers, a helix exchanger has best performance so it has been commonly used in industries.

It has been reported that more than35-40% of heat exchangers are of STHE type, due to their robust construction geometry as well as easy maintenance and possibility of upgrades [1]. Baffle is an important shell side component of STHEs. Besides supporting the tube bundles, the baffles form flow passages for the shell side fluid. The most commonly used baffles are segmental baffle, which forces the fluid in a zig-zag manner, thus improving the heat transfer but with a large pressure drop penalty. This type of exchanger has been developed [2-4] &probably is still the most commonly used types of STHE. The major draw backs of STHE with segmental baffles are (i) it causes large side pressure drop. (ii) it results in a dead zone in each component between adjacent segmental baffles, leading to increase of fouling resistance. (iii) zig-zag flow pattern also causes high risk vibration failure on tube bundle.

To overcome the above mentioned drawbacks of conventional STHE, segmental baffled, a no. of improved structure were proposed for the purpose of higher heat transfer coefficient, low possibility of tube vibration and reduced fouling factor [4-7]. Moreover, the principal shortcomings of the conventional segmental baffle still remain in the above mentioned studies, even though the pressure drop across the heat exchanger has been reduced to some extent. A new baffle called helical baffle, provides further improvement. This type of baffle was first proposed by D.Kral & Nemcansky [8], where they investigated the flow pattern produced by helical baffle geometry with different helix angles. They found that these flow patterns were much close to plug flow conditions, which was expected to reduce shell side pressure drop and to improve heat transfer performance.

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Desirable feature of Helix exchangers using grooved tubes-

The desirable features of a helix exchanger using grooved tubes would be to obtain maximum heat transfer to Pressure drop ratio at least possible operating costs without comprising the reliability.

1.1 Higher heat transfer co-efficient and larger heat transfer area-

A high heat transfer coefficient can be obtained by using heat transfer surfaces, which promote local turbulence for single phase flow or have some special features for two phase flow. Heat transfer area can be increased by using larger exchangers, but the more cost effective way is to use a heat exchanger having a large area density per unit exchanger volume, maintaining the Integrity of the Specifications.

1.2 Lower Pressure drop-

Use of segmental baffles in a Heat Exchanger result in high pressure drop which is undesirable as pumping costs are directly proportional to the pressure drop within a Heat Exchanger. Hence, lower pressure drop means lower operating and capital costs

II. HELIX EXCHANGER

The concept of helical baffle heat exchangers was developed for the first time Czechoslovakia. The helical baffle heat exchanger also known as Helix changer is a superior STHE over conventional baffle exchangers.

It provides the necessary characteristics to reduce flow dispersion and generate near plug flow condition. It also ensured a certain amounts of cross flow to the tubes to provide high heat transfer coefficient. The Shell side flow configuration offer a very high conversion of pressure drop to heat transfer.

The helix changer with grooved tubes provides: 1. Improvement of shell side heat transfer. 2. Less pressure drop for a given mass flow rates. 3. Decreasing of fouling in shell side.

4. Prevention of bundle vibration. 5. Reducing by pass effect in shell side.

Advantages of helix changer with grooved tubes: 1. High thermal & hydraulic performance.

2. High thermal effectiveness. 3. Lower fouling & cleaning ability. 4. Vibration elimination.

5. Cost saving on total life cycle basis. 6. Improving plant run length.

Concept of Grooved tubes

It is a passive heat enhancement technique generally uses surface or geometrical modification to the flow channel. These geometrical modifications promote higher heat coefficients by disturbing or altering the existing flow behavior. The flow pattern inside the grooved tubes has better fluid mixing particle which leads to better heat enhancement as compare to the plain tubes. Figure shows grooved copper tubes having sine surface of radius 2mm, pitch 100mm, outer dia. of tube 12.7 mm, length 740 mm.

Fig: Grooved copper tube

III. EXPERIMENTAL STUDY &PROCEDURE

In the current paper, comparative thermal performance analysis of helix exchanger with plain copper tubes or with grooved copper tubes by using parallel flow at different baffle angles5, 15, 25,35,45 deg. has been carried out. During this experimental investigation attempts were made for both exchangers at same operating conditions to determine the thermal performance. Their results clearly indicate the significant increase in thermal performance of heat exchanger with grooved tubes helix exchanger.

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This process fluid was supplied to the tube side part and then to the copper tube of outer diameter of 12.7mm with the help of a pump (0.5 HP) and through the hot flow meter with different rate of flow . After passing this process fluid through the test section this fluid is fed to the hot water reservoir by a by-pass valve. Similarly, the process cold fluid was supplied to the shell part over the copper tubes by a pump of 0.5 HP and through cold flow meter with different rate of flow. In the test section the temperature of the process fluid at different location is measured by the use of RTD PT-100 sensor. The total numbers of sensors are 05 used in the test section

In the present experimental work on grooved tube helix exchanger was done at 5,15,25,35 and 45 degree different baffle inclination to know the thermal performance.

Figure 3. Experimental set-up layout.

Fig: Photographic view of Experimental set up.

Geometrical Parameters of Heat Exchanger

Name of part

Material Size & specification

Shell

G.I Pipe

d-101mm

D-106mm

L-740 mm

Tube Copper

do-12.7mm

l-762mm,tb-0.65mm Qty –

15 no.

Baffle Aluminium 1 mm thick

Stationary tube sheet

M.S

5mm thick, 100mm dia

Qty-2 no

Shell cover (Front and Rear Head)

M.S

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480 Fluid properties

Property Symb ol

Unit water

Specific heat Cp Kj/kg-k 4.178

Thermal conductivity

k W/m-k 0.615

Viscosity µ Kg/m-s 0.001

Density ρ Kg/m3 996

The basic experimental procedure is as follows: (1) Filling the system with enough water;

(2) Keeping the water valves in full-open state, and running the cool water pump, the hot water pump and the Electric heater in turn;

(3) Regulating the water flow rate in the hot water loop to a certain value through adjusting the water valve;

(4) Regulating the hot water temperature in the tank to a certain stable value through adjusting the power regulator that controls the input power of the heater.

(5) Recording the temperatures of the water entering and leaving the heat exchanger (the time interval of the data acquisition system is set as 1s), and meantime, observing the water flow rate in the hot water loop for every minute; (6) After one experimental condition is completed, repeating steps (3)-(5) for the other conditions

IV. RESULTS

Governing Equations

The heat transfer equation in conduction and convection are used to find out the heat transfer rate, heat transfer coefficient, pressure drop, Reynolds number, Prandtls number at different baffle angles 5,15,25,35and 45 degree with different mass flow rate 0.5,0.75,1 kg/s at shell side and tube side respectively.

1. Tube Clearance (C)

C’ = Pt – Dot

Where C = tube clearance; Pt = tube pitch; Dot = outer

diameter of tube

2. Baffle spacing (Lb)

Lb = π Dis.tan (ϕ)

Where Lb = baffle spacing; Dis = shell inside diameter; ϕ =

baffle angle

3. Cross-flow Area (As)

As = (Dis C Lb) / Pt

Where As = cross-flow area; Dis = shell inside diameter; Lb

= baffle spacing; Pt = tube pitch

4. Equivalent Diameter (De)

De = 4 [ ( pt 2

– π ∙ Dot 2

/ 4 ) / (π ∙ Dot )

Where De= equivalent diameter; Pt = tube pitch; Dot = outer

diameter of tube

5. Maximum Velocity (Vmax)

Vmax =QS /A

Where Vmax = maximum velocity; QS = volume flow rate

shell side; A= area of shell

6. Reynolds number (Re),

Re = (ρ ∙ Vmax ∙ De) / μ

Where Re = Reynolds number; ρ = density; Vmax =

maximum velocity; De = equivalent diameter; μ = viscosity

Vmax = s / A

7. Friction factor can be calculated by using Petukhov’s factor correlation for smooth tube

valid for 3000 < Re < 5*106.

ƒ = (1.58lnRe-3.28)-2

= {1+ (29.1Re(0.67-0.06p/di-0.49β/90).( )(1.31-0.157p/di).( )

(-0.00000166Re-0.33β/90)

.( )(4.+0.00000411Re-0155p/di).(1+ )sinβg)15/16}16/15

Where fg = friction factor for grooved tubes; e = height

of grooves; p = axial pitch from one groove to the next groove; β = helix angle of groove relative to the tube axis; Sin βg= surface angle of grooves in tube; nc = no of sharp

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8. Heat Transfer Coefficient (αo)

αo = (0.36 ∙ K ∙ Re 0.55

∙ Pr0.33) / R ∙ De

where αo = heat transfer coefficient; K thermal

conductivity; Re = Reynolds number; Pr = Prandtl number; De = equivalent diameter

9. No. of Baffles (Nb)

Nb = 5

Where Nb = No. of Baffles

10. Pressure Drop (ΔPS)

ΔPS = [4 ∙ fg ∙ms2 ∙ Dis ∙ (Nb + 1)] / (2 ∙ ρ ∙ De)

Where ΔPS = Pressure Drop; fg = friction factor grooved

tubes; ms = shell side mass flow rate; Dis= = shell inside

diameter Nb= No. of baffles

Variation of Heat Transfer Properties at Different Baffle Angles

Baffle angle changed with the variation in the position with respect to the central axis of tube bundle.

At different baffle angle the variation in pressure takes place which result in variation of heat transfer coefficient i.e. heat transfer is directly related to helix angle. Now, the variation of heat transfer coefficient and pressure drop with baffle angles at different mass flow rates 0.5, 0.75 and 1 kg/s are represented graphically below.

Helix angle v/s Heat transfer coefficient

Graph: helix angle v/s heat transfer coefficient

From the graphical representation we analyzed that of with the increases of helix angle; the turbulence reaches to the highest peak values 3914.65, 4868.56 and 5774.61 at 0.5, 0.75 and 1 kg/s mass flow rate at 15 degree. After the increase of helix angle the fouling is increases in the region of contact of tube surface and helical baffle. The fouling region created decrease the heat transfer rate for different baffle angles with respect to mass flow rate 0.5, 0.75 and 1 kg/s respectively.

Helix angle v/s Pressure drop

Graph: helix angle v/s pressure drops

From the graphical representation we analyzed that the smoothness in the flow of water is achieved by introducing helical baffles with changing the angles of baffle. The correspondence pressure drop decreases at different mass flow rates 0.5, 0.75 and 1 kg/s. At 5 degree pressure drop is large, the change of pressure drop is large in the small inclination angle region. However, the effect of baffle inclination angles on pressure drop is small when helix angle > 25 degree.

Helix angle v/s Reynolds number

Reynolds number is the ratio of inertia force to the viscous force and i.e Re = fi/fv = ρVD/µ.

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International Journal of Emerging Technology and Advanced Engineering

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482 Graph: helix angle v/s Reynolds number

Helix angle v/s Friction factor

Graph: helix angles V/s friction factor

Comparison of heat transfer rate at different helix angles with different mass flow rates:

At different mass flow rate of shell inlet side 0.5, 0.75 and 1kg/sec respectively, the temperature of hot and cold water at inlet side is same as 50 and the inlet temperature on cold water taken as 30 , the outlet temperature of both hot and cold water calculated at different baffle angle using following formulas as:

Heat transfer rate given by

The amount of heat transfer (KW) between the hot water flow inside the tubes and cold water outside the tubes can be calculated as

Q = mh×cph×(th1-th2)

Where, mh mass flow rate of hot water in kg/sec; cph,

specific heat of water, J/kg ⁰C; th1, th2 inlet and outlet

temperature of hot water in ⁰C.

The heat transfer coefficients (U= w/m2⁰C) can be written as;

U= Q/ AӨm

Where A is total flow area of copper pipes, m2; Өm is

mean temperature difference between hot water and cold water,⁰C and calculated by Eq. (3)

Өm = (Ө1-Ө2)/ (InӨ1/Ө2)

Where, Ө1= th1-tc1; Ө2 = th2- tc2, where tc1, tc2,

temperature of cold water at inlet and outlet, respectively.

Table

Experimental reading for helical baffle heat exchanger at 0.5 kg/s

Ms

(kg/s)

θ

T

h1

T

h2

T

c1

T

c2

Q

0.5 5 49.4 46.2 28.2 31.1 6.68

0.5 15 49.4 46.4 28.2 31.5 6.85

0.5/ 25 49.4 45.8 28.2 31.8 7.52

0.5 35 49.4 45.9 28.2 31.6 7.35

0.5 45 49.4 46.1 28.2 31.9 7.20

Table

Experimental reading for helical baffle heat exchanger at 0.75 kg/s

Ms

(kg/s)

θ

T

h1

T

h2

T

c1

T

c2

Q

0.75 5 49.4 45.6 28.1 31.6 11.9

0.75 15 49.4 45.8 28.1 31.4 11.28

0.75 25 49.4 44.9 28.1 32.3 14.44

0.75 35 49.4 45.1 28.1 31.9 13.47

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International Journal of Emerging Technology and Advanced Engineering

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483 Table

Experimental reading for helical baffle heat exchanger at 1kg/s

Ms

(kg/s)

θ

T

h1

T

h2

T

c1

T

c2

Q

1 5 49.4 46.0 28.1 35.1 14.28

1 15 49.4 45.6 28.1 34.6 15.88

1 25 49.4 45.3 28.1 34.2 17.13

1 35 49.4 45.5 28.1 35.7 16.29

1 45 49.4 45.7 28.1 36.1 15.46

Graph helix angle v/s heat transfer rate (Q)

Comparison of Segmental Baffle with Helical Baffle

Calculation parameters

Segmental baffles

Helical baffle at 25 degree

Mass flow rate, kg/s 0.5 0.5

Maximum velocity, m/s

0.2 0.24

Reynolds number 1892.4 2270.88

Heat transfer

coefficient(αO)

2581.22 2851.50

Heat transfer rate (Q), (KW)

6.72 7.52

Overall heat transfer coefficient (U)

755 837

Pressure drop (ΔPs) 502.23 61.304

αO / ΔPs 5.13 46.51

V. DISCUSSION ON RESULT

1. The flow pattern in the shell side with helical baffle is near plug flow. Therefore dead region eliminatedand the heat transfer area is used effectively.

2. Helix angle 15 degree may provide higher heat transfer as compare to 25 degree but at this angle pressure drop is less that’s why 25 degree provides better results as compare to others.

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4.For all the helical baffle heat exchanger studied, the ratio of heat transfer coefficient to pressure drop are higher than those of a conventional segmental heat exchanger. This means that the heat exchanger with helical baffle will have a higher heat transfer coefficient, when consuming the same pumping power.

5.It can be concluded that proper helix angle will provide an optimal performance of heat exchangers, the detailed knowledge of heat transfer characteristics provided in this investigation may serve as a basis for further optimization of helical heat exchanger. 6.The ratio of heat transfer coefficient to pressure drop

at 25 degree helix angle has the most desired results for industrial heat exchanger as it creates a perfect balance between the heat transfer coefficient and shell side pressure drop in heat exchanger.

NOMENCLATURE :

D = shell outer diameter (mm) d = shell internal diameter (mm) do = tube outer diameter (mm)

L = shell length (mm) l = tube length (mm) tb = tube thickness (mm)

αo = heat transfer coefficient (W/m2k)

ΔPs = pressure drop (kpa) Ө = helix angle (degree)

REFERENCES

[1] B.I.Master,chungad,A.JBoxma,D.Kral,P.Stheik;―Most frequently used heat exchanger from pioneering research to world wide applications,‖ vol.no.6,2008 ppt-8.

[2] Young-Gang Lei, Ya-Ling He,Rui,Ya-Fu Crea,―Effects of inclination angle on flow and heat transfer of a heat exchanger with helical baffles‖. Science direct-Chemical engineering and processing pp.1-10(2006).

[3] B.Peng, Q.W.Wang, C.Zhang,―An experimental study of STHE with continuous helical baffles‖. ASME journal of heat transfer, vol.129,pp. 1425-1431(2007).

[4] M.R.Jafari and A.Shafeghat,― Fluid Flow analysis and extension of rapid design algorithm Science direct- applied thermal engineering,(2006).

[5] Bashir Master,K.chunghad,― Fouling mitigation using helix changer heat exchanger,‖. Conference on heat exchanger fouling and cleaning‖. Fundamentals and application. pp 312-317,(2003). [6] Kevin Lunsford,― Increasing heat exchanger performance,‖.

Hydrocarbon Engineering, pp1-13,(1998).

[7] Sadik Kakaq and Hongfun Lui,― Heat exchanger selection rating and thermal design,‖(2003).

Figure

Fig: Photographic view of Experimental set up.
Fig: Photographic view of Experimental set up. p.3
Figure 3. Experimental set-up layout.

Figure 3.

Experimental set-up layout. p.3
Table Experimental reading for helical baffle heat exchanger at 0.5 kg/s

Table Experimental

reading for helical baffle heat exchanger at 0.5 kg/s p.6
Table  Experimental reading for helical baffle heat exchanger at 0.75 kg/s

Table Experimental

reading for helical baffle heat exchanger at 0.75 kg/s p.6
Table Experimental reading for helical baffle heat exchanger at 0.5 kg/s

Table Experimental

reading for helical baffle heat exchanger at 0.5 kg/s p.6
Table  Experimental reading for helical baffle heat exchanger at 1kg/s

Table Experimental

reading for helical baffle heat exchanger at 1kg/s p.7

References

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