Compound Heat Transfer Enhancement Methods …intheat.dcs.uni-pannon.hu/wp-content/uploads/2012/05...Compound Heat Transfer Enhancement Methods To Increase Heat Exchanger Efficiency

Compound Heat Transfer

Enhancement Methods To

Increase Heat Exchanger

Efficiency David J. Kukulka State University of New York College at Buffalo USA

[email protected]

Rick Smith

Vipertex Division of Rigidized Metals, Inc

Buffalo, New York USA

mailto:[email protected]

Problem

This study details the process of evaluating a

compound enhanced heat transfer tube.

Compound enhanced heat transfer tubes utilizes more

than one enhancement method (tubes and twisted

tapes) to change heat transfer

In order to design the compound enhanced heat

transfer tube, a fluid flow model of the tube and

twisted tape was created and studied.

Experimentation on the system validated the design

CAPE 2012 Introduction Procedure Results Conclusion 2

Outline Introduction

Waste Heat

Enhanced heat transfer surface background

Benefits

Design Considerations

Enhancement Techniques

Twisted Tapes

Compound Heat Transfer

Previous Work

Procedure

Results

Conclusions

3 CAPE 2012 Introduction Procedure Results Conclusion

4

Potential of Waste Heat

5

Essential Components Required

for Waste Heat Recovery

6

Important Concepts that Determine Waste

Heat Recovery Feasibility

The quantity of waste heat contained in a waste stream is

a function of both the temperature and the mass flow rate

of the stream.

Waste Heat Temperature is a key factor determining

waste heat recovery feasibility and can vary significantly.

Typical Examples of Waste Temperatures:

Cooling Water - Low temperatures around 100 - 200°F [40 - 90°C]

Glass Melting Furnaces - Flue temperatures above 2400°F [1320°C].

7

Sources of Waste Heat Users of Waste Heat

8

Important Parameters that Determine

Materials for Use in Waste Heat Recovery

Temperature of the waste heat source is important for

material selection in heat exchangers or a heat recovery

system

Composition of the stream affects the recovery process

and material selection.

The composition and phase of waste heat streams will impact

heat exchanger effectiveness.

9

Waste Heat Temperature Groups

High : 1200ºF [650°C]

Medium: 450ºF [230°C] to 1200ºF [650°C]

Low: 450ºF [230°C] and lower

10

Waste Heat Potential from Selected Processes

11

Simple Example Using Waste Heat to

Increase Process Efficiencies

12

Qualities of Waste Heat Available

13

Waste Heat Content for Selected

Processes

14

Key Opportunity Areas

Low temperature waste heat sources

Systems already including waste heat recovery that

can be further optimized to reduce heat losses

High temperature systems where heat recovery is less

common

Alternate waste heat sources typically not considered

for waste heat recovery

15

16

Waste Heat Potential as a Function of Furnace Size

17

Waste Heat Loss in the Glass Industry

18

Enhanced Heat Transfer Surfaces

Enhanced Surfaces have developed a great

deal of interest in the design of heat

exchange devices .

Various areas in industry are currently

working aggressively to incorporate enhanced

heat transfer surface technology into their

designs.

Virtually every heat exchange device is a potential

candidate for enhanced heat transfer.


Enhanced Surfaces

Increases process surface area

Interrupts the development of the boundary layer

Increases the degree of turbulence

Generates rotating secondary flows


Enhanced Surface Ratio

Ratio of h A of the enhanced surface to that

of a plain surface is the enhancement ratio.

PLAIN

ENHANCED

hA

hAtRatioEnhancemen

)(

)(

CAPE 2012 Introduction Procedure Results Conclusion

Heat

Transfer

Coefficient

Surface

Area

21

Overall Thermal Resistance

222111 Ah

L

Ak

Lt

Ah

L

UA

L

mw

w

Surface efficiency, if extended surfaces are used

•The performance of the heat exchanger will be enhanced if UA/L is increased.

• Enhanced Heat Transfer Surfaces Reduces the thermal resistance per unit length.

• Fouling Resistance may also be added in this calculation.

Subscripts 1 and 2

refer to inside fluid

and outside fluid


Parameters to Characterize Surface

Enhancements for this Study

Enhance Heat Transfer • Geometry of the roughness should break the laminar sub layer.

• Minimize disruption to the core in order to keep pressure drop within

range.

Several Enhancement Geometries to Consider: • Rib, Rectangular, Circular, Dimples, Grooves, Wedges, etc.

Surface Enhancement Arrangements to Consider: •Continuous, Staggered, Cavity, Groove, etc


Previous Geometry Enhancement

Investigations

• Several investigators have attempted to develop

accurate predictions of heat transfer coefficients and

friction factors for compound heat transfer tubes

• Little has been done in the design of twisted tape

surfaces in combination with enhanced tube

surfaces that are designed to enhance heat transfer

near a enhanced tube wall.


Enhanced Heat Transfer Surfaces Can be Used to

Provide Performance Improvements

Size Reduction If the rate of heat exchange (Q) is held constant, the

required length (L) of heat exchange may be reduced.

Increased Heat Transfer Rate If Q and the tube length (L) are held constant, ΔTm may be

reduced and the operating costs are reduced.

For fixed fluid inlet temperatures, L constant, and increase in UA/L will result in an increased heat exchange rate (Q).

Reduced power requirements Result of lower required flow rates.


Heat Exchanger Size as a Function of

Enhanced Heat Transfer Coefficient

26

Enhancement Techniques

Surface Enhancement Techniques can be segregated into passive and active categories.

Passive techniques employ techniques such as

special surface geometries, coatings or fluid additives.

Active techniques require external devices to enhance

heat transfer.

The concentration here will be on Passive Enhancement.


Passive Techniques

Coated surfaces Non Wetting Coating (Teflon) Applied to Promote

Dropwise Condensation

Fine Scale Porous Coating Applied to the Surface to Nucleate Boiling.

Rough Surfaces May be Integral to the Base Surface

machining

restructuring the surface

Enhancement can also be created by placing a roughness adjacent to the surface.


Two Fluid Tubular Enhanced

Heat Transfer Tubes are used in typical Heat Exchange Devices.

Enhancement may be desired on both the inner and outer surfaces.

Surface Enhancement Depends on the Application i.e. two phase flow may occur on one side and convection

cooling on the other side.

A variety of techniques are available to enhance heat transfer.


Surface Enhancement

Some Techniques Utilize the removal of

material to create an enhanced surface.

That technique may weaken the wall of the tube.

Material dependent.

The technique used in this study redistributed

the material


Restructured Process Surfaces

For single phase flow, a configuration is

chosen to promote mixing in the boundary layer

rather than only increase heat transfer area.


Tube Side Enhancements

Conventionally Produced Internally Finned

Tubes having high fins are:

quite expensive to produce

difficult to work with

Only select materials

Less Expensive Micro Fin

tubes are only available for

limited materials.


Design Variables to Consider

If the process is heating it may have a

different exchange process than if it is a

cooling process.

Laminar, transitional and turbulent flows have

different enhancement requirements.

Single Phase or Two Phase Flow also has

different enhancement requirements.


Corrugated Tube Flow

Ravigururajan and Bergles (1986)

summarized work done on various

types of corrugated tubes.

Blumenkrantz and Taborek (1971)

evaluated spirally indented tubes.

Ravigururaian, T. S., and Bergles, A. E., 1986. “An Experimental Verification of General

Correlations for Single-Phase Turbulent Flow in Ribbed Tubes,” in Advances in Heat

Exchanger Design, HTD-Vol. 66, ASME., New York, pp. 1-11.

Blumenkrantz, A., and Taborek, J., 1971. “Heat Transfer and Pressure Drop

Characteristics of Turbotec Spirally Deep Grooved Tubes in the Laminar and Transition

Regime,” Report 2439-300-8, April 1971, Heat Transfer Research, Inc.


Enhancement of Laminar Flow in

Roughened Circular Tubes Vicente et al. (2002 a, b) presents laminar flow data

for dimpled tubes.

Enhancement is influenced by several factors

Thermal boundary conditions

Entrance region effects

Natural Convection

Vicente, P. G., García, A., and Viedma, A., 2002. “Experimental Study of Mixed

Convection and Pressure Drop in Helically Dimpled Tubes for Laminar and Transition

Flow,” Int. J. Heat Mass Trans., Vol. 45, pp. 5091-5105.

Vicente, P. G., García, A., and Viedma, A., 2002. “Heat Transfer and Pressure Drop for

Low Reynolds Turbulent Flow in Helically Dimpled Tubes,” Int. J. Heat Mass Trans.,

Vol. 45, pp. 543-553.


Dimpled Tubes

Chen et al. (2001) studied inward-facing, raised dimples on the inner tube.

Values of the heat transfer coefficient increased when compared to smooth tubes. Heat transfer enhancement ranged from 25% to 137% at constant

Reynolds number.

Study in the turbulent range

Enhancement increased from 15% to 84% with constant pumping power

Chen, J., Müller-Steinhagen, H., and Duffy, G. G., 2001. “Heat

Transfer Enhancement in Dimpled Tubes,” Applied Thermal Eng. , Vol.

21, pp. 535-547.


Enhanced Tubes Evaluated Brahim et al. Numerical Simulation of Fouling, 2003 ECI Conference on Heat Exchanger Fouling


CFD

Analysis

of

Various

Surfaces


Vipertex EHT Development Included

a Primary Texture Evaluation

In Line Staggered


Primary Texture Variation


Inside Tube Outside Tube

Secondary Wall Enhancements

41

Enhancements Considered


Combination of Primary and

Secondary Enhancements

43

Models of Smooth Tubes current study


Max 0.132

44

Model of a Linear Rib Enhancements

in an Enhanced Tube


Max 0.198

45

Model of Enhanced Structures in Tubes


Max 0.184

46

Typical Enhanced Surface Pattern

Produced for Vipertex 2EHT Tube


Outside Inside

Typical Enhanced Tube Produced


1EHT Tube Produced


Experimental Process Fluids in the sample tubes were heated/cooled with

water

Flow and temperature regulated.


Tube in Tube, Counterflow Test Apparatus


Details of Tubes Evaluated

Description Surface Cross Sectional View

(a) Vipertube 2EHT: Type

304 stainless steel

(b) Vipertube 1EHT: Type

304 stainless steel

(c) Stainless steel

smooth tube: Type 304,

type D finish,


Tube Enhancement Test Conditions

54

Vipertex 1EHT Enhanced Tubes

55

Vipertex 2EHT Enhanced Tubes

56

Vipertex 1EHT Cooling Enhancement Ratio

57

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05

Nu

meas / N

up

red

,PT

Re

1EHT - Cooling

Predicted - VDI

Predicted - Ghajar

Vipertex 2EHT Cooling Enhancement Ratio

58

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05

Nu

meas / N

up

red

,PT

Re

2EHT - Cooling

Predicted - VDI

Predicted - Ghajar

Inside Heat Transfer Coefficient Equation for

Enhanced Tubes (1EHT) compared to Smooth Tubes

59

Smooth tube


Friction Factor of Enhanced Tubes (1EHT) Compared to Smooth Tubes

60

Smooth tube


Twisted Tape Inserts

Inserts influence the fluid flow.

Fluid moves with a higher velocity and produces

secondary flows.

Tangential velocity component moves the inner

core fluid closer to the wall for a better exchange

of energy.

Twisted tape inserts force the fluid to follow a

helical path rather than a straight path.

61

Flow Near the Wall

Swirl flow from the twisted tape:

Induces the turbulence near the tube wall

Increases the residence time of the fluid flow in the

tube.

Higher fluid turbulence caused by twisted

tapes close to the tube wall increases

convective heat transfer.

Results in an effective energy enhancing device.

Increases pressure drop (moderate increases)

62

Twisted Tape Advantages

Insertion of twisted tape is one of the most

popular passive heat transfer enhancement

techniques:

Low cost

Ease of installation

Low maintenance

Can be installed on new or existing exchangers

63

Twisted Tapes

Flow enhancements are possible in the laminar

and transitional flow regimes.

The heat transfer and pressure drop

characteristics can be varied somewhat by

changing the twist pitch of the device.

65

Laminar Flow

Laminar Flow is a relatively complex subject

Flow is influenced by :

Thermal boundary conditions

Entrance region effects

Natural convection at low Re

Fluid property variation across the boundary layer

Cross sectional shape

66

Twisted Tape in Smooth Tube with Laminar Flow

for a Twist Ratio of 5 and Various Spacing

67

Shift Region of Maximum Heat Transfer

Through the use of twisted tapes the Reynolds

Number of maximum enhancement can be shifted.

The type of twisted tape has an influence both on the

peak enhancement as well as the magnitude of the

enhancement.

70

Parameters that influence heat transfer

in twisted tapes

Heat Transfer is influence by:

entrance effect

fluid viscosity

ratio (bulk to wall conditions)

Prandtl number

tape twist ratio

swirl flow Reynolds number.

71

Fit of the Twisted Tape also

Influences Enhancement Lopina and Bergles (1969) developed a

practical correlation that includes the influence

of tape width on heat transfer in turbulent

flows.

They reported an increase in the Nusselt number

as much as 20% for the tight-fit tape over that of

the loose-fit tape.

72

Lopina, R. F. and Bergles, A. E. 1969. Heat transfer and

pressure drop in tape generated swirl flow of single-phase

water. J. Heat Transfer, 91, 434-442

Optimum Tape Width

Ayub and AI-Fahed (1993) have conducted

extensive experimental research on the

influence of twisted tape width on the

pressure drop.

Existence of an optimum tape width

Function of both the twist ratio and the Reynolds

numbers.

73

Ayub, Z. H. and AI-Fahed, S. F. 1993. The effect of gap width

between horizontal tube and twisted tape on the pressure drop in

turbulent water flow. Int. J. Heat Fluid Flow, 14, 64-67

Tape Clearance Between Tape and Wall

Results have demonstrated that:

As tape clearance decreases, the heat transfer

enhancement increases.

For practical designs of thermal systems,

operating under turbulent flow conditions

Small twist ratio and a tight-fit tape are desirable

in order to obtain the largest enhancement.

74

Sami AI-Fahed and Walid Chakroun, 1996,

Effect of tube-tape clearance on heat transfer for fully

developed turbulent flow in a horizontal isothermal tube, Int.

J. Heat and Fluid Flow 17: 173-178.

Dimple Tube with Twisted Tape

For the range of Re between 12000 and 44000,

experimental results reveal that both heat

transfer coefficient and friction factor in the

dimpled tube fitted with the twisted tape, are

higher than those in the dimple tube acting alone

or in a plain tube.

Heat transfer coefficient and friction factor in the

combined devices increase as the pitch ratio (PR)

and twist ratio (y/w) decrease.

75

Chinaruk Thianpong, Petpices Eiamsa-ard, Khwanchit

Wongcharee, Smith Eiamsa-ard, 2009, International

Communications in Heat and Mass Transfer, 36, 698–704

Compound Enhancement Test

Arrangement

76

Compound Enhancements at Low Flows

77

Enhanced Twisted Tapes Cause Earlier Transition and Higher HTC

78 1

10

100

1000

1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05

Nu

Re

Vipertex 1EHT - Cooling

Measured

Predicted-VDI

Predicted-Ghajar

1

10

100

1000

1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05

Nu

Re

Vipertex 1EHT – Cooling with twisted tape insert

Measured

Predicted-VDI

Predicted-Ghajar

Need and Justification

There exists a need in industry for high quality, enhanced compound heat transfer tubing that can be used in a variety of industries to meet the needs of heat recovery.

The enhanced products discussed here have been developed for a variety of industries and applications. Waste Heat Recovery

Power plants

Process applications

Energy Conversion


Economics is One of the Primary Considerations in

the Development and Evaluation of Process Surfaces

Total Cost Includes:

Initial development costs

Capital costs

Operating costs

Maintenance cost


Economic Analysis Parameters

81

Operating Cost Information

82

Economic Analysis

83

1% enhancement

$40045.77

Enhanced - Smooth

Economic Analysis

84

10% enhancement

$594,352

Enhanced - Smooth

Energy Savings with Enhancements

85

10% enhancement

1% enhancement

Summary and Conclusions

An evaluation of the need to enhance heat transfer was performed.

Available Techniques were evaluated.

Criteria Considered for Development

Cost

Material

Performance

Economics


Development of Enhanced Heat

Transfer Tubes These compound heat transfer tubes provide heat

transfer rates that are higher than the rates found in

smooth tubes under similar conditions.

This is an important development for the energy

conversion and process industries.

It was demonstrated that more heat transfer and an

earlier transition to high heat transfer can be

accomplished through the use of enhanced twisted

tapes. This was accomplished by: Modeling the surface to evaluate the. enhancement

Lab evaluation of EHT tubes further enhanced with twisted tapes

produced.


87

Summary

Compound Heat Transfer Tubes can be

designed to enhance heat transfer under various

conditions.

Requires different enhanced surfaces and twisted

tapes for different applications.

Tubes have been evaluated and can be designed to

produce more heat transfer than smooth tubes under

fouling conditions. Kukulka et al. (2012)

Kukulka D.J. , Smith R., J. Zaepfel (2012), Development and Evaluation of

Vipertex Enhanced Heat Transfer Tubes for use in Fouling Conditions,

Theoretical Foundations of Chemical Engineering.

.


88


These enhanced compound heat transfer tubes were tested under some limited operating conditions. Heat Transfer

Enhanced Surface Ratio Values Measured were near 600%

When compared to the results of Thianpong et al. (2009): Magnitude of enhancement is :

Maximum at lower flows;

Thianpong et al. (2009- ) did not evaluate at Re< 12000.

Turbulent Flows Similar magnitude

Surface enhancement configuration Similar

Present study also evaluated enhanced twisted tapes



Future

Additional Testing on Current Designs

Develop Additional Designs

Revised Designs for the Same Applications

New Designs for additional Applications/ Types of Heat

Transfer

Look at two phase flows.

Include Fouling and enhanced twisted tapes

Evaluate a wider range of conditions for the New

Designs.


Summary Future studies will examine surface texture

variations in greater detail, coated surfaces and

enhanced surfaces produced from engineered

alloys.

Improvements created through surface texture

enhancements of the heat transfer surface are

clear.

Penalty is the cost to produce the surfaces or twisted tapes.

Total costs over the life of the product will be less than

conventional designs.

Savings produced more than offset the costs.


91

Acknowledgements

Thank you to Rigidized Metals and Vipertex

for their Support in the Development and

Study of Compound Enhanced Heat Transfer

Tubes.


Thank You

Questions?


94

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