Compound Heat Transfer Enhancement Methods To Increase Heat Exchanger Efficiency

Transcription

1 Compound Heat Transfer Enhancement Methods To Increase Heat Exchanger Efficiency David J. Kukulka State University of New York College at Buffalo USA Rick Smith Vipertex Division of Rigidized Metals, Inc Buffalo, New York USA

2 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

3 Outline Introduction Waste Heat Enhanced heat transfer surface background Benefits Design Considerations Enhancement Techniques Twisted Tapes Compound Heat Transfer Previous Work Procedure Results Conclusions CAPE 2012 Introduction Procedure Results Conclusion 3

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5 Potential of Waste Heat 5

6 Essential Components Required for Waste Heat Recovery 6

7 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 F [40-90 C] Glass Melting Furnaces - Flue temperatures above 2400 F [1320 C]. 7

8 Sources of Waste Heat Users of Waste Heat 8

9 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

10 Waste Heat Temperature Groups High : 1200ºF [650 C] Medium: 450ºF [230 C] to 1200ºF [650 C] Low: 450ºF [230 C] and lower 10

11 Waste Heat Potential from Selected Processes 11

12 Simple Example Using Waste Heat to Increase Process Efficiencies 12

13 Qualities of Waste Heat Available 13

14 Waste Heat Content for Selected Processes 14

15 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

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17 Waste Heat Potential as a Function of Furnace Size 17

18 Waste Heat Loss in the Glass Industry 18

19 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. CAPE 2012 Introduction Procedure Results Conclusion 19

20 Enhanced Surfaces Increases process surface area Interrupts the development of the boundary layer Increases the degree of turbulence Generates rotating secondary flows CAPE 2012 Introduction Procedure Results Conclusion 20

21 Enhanced Surface Ratio Ratio of h A of the enhanced surface to that of a plain surface is the enhancement ratio. EnhancementRatio ( ha) ( ha) ENHANCED PLAIN Heat Transfer Coefficient Surface Area CAPE 2012 Introduction Procedure Results Conclusion 21

22 Overall Thermal Resistance L UA L Lt w L 1h1A1 kwam 2h2A2 Subscripts 1 and 2 refer to inside fluid and outside fluid 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. CAPE 2012 Introduction Procedure Results Conclusion 22

23 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 CAPE 2012 Introduction Procedure Results Conclusion 23

24 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. CAPE 2012 Introduction Procedure Results Conclusion 24

25 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, ΔT m 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. CAPE 2012 Introduction Procedure Results Conclusion 25

26 Heat Exchanger Size as a Function of Enhanced Heat Transfer Coefficient 26

27 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. CAPE 2012 Introduction Procedure Results Conclusion 27

28 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. CAPE 2012 Introduction Procedure Results Conclusion 28

29 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. CAPE 2012 Introduction Procedure Results Conclusion 29

30 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 CAPE 2012 Introduction Procedure Results Conclusion 30

31 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. CAPE 2012 Introduction Procedure Results Conclusion 31

32 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. CAPE 2012 Introduction Procedure Results Conclusion 32

33 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. CAPE 2012 Introduction Procedure Results Conclusion 33

34 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., 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 Blumenkrantz, A., and Taborek, J., Heat Transfer and Pressure Drop Characteristics of Turbotec Spirally Deep Grooved Tubes in the Laminar and Transition Regime, Report , April 1971, Heat Transfer Research, Inc. CAPE 2012 Introduction Procedure Results Conclusion 34

35 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., 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 Vicente, P. G., García, A., and Viedma, A., Heat Transfer and Pressure Drop for Low Reynolds Turbulent Flow in Helically Dimpled Tubes, Int. J. Heat Mass Trans., Vol. 45, pp CAPE 2012 Introduction Procedure Results Conclusion 35

36 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., Heat Transfer Enhancement in Dimpled Tubes, Applied Thermal Eng., Vol. 21, pp CAPE 2012 Introduction Procedure Results Conclusion 36

37 Enhanced Tubes Evaluated Brahim et al. Numerical Simulation of Fouling, 2003 ECI Conference on Heat Exchanger Fouling CAPE 2012 Introduction Procedure Results Conclusion 37

38 CFD Analysis of Various Surfaces CAPE 2012 Introduction Procedure Results Conclusion 38

39 Vipertex EHT Development Included a Primary Texture Evaluation In Line Staggered CAPE 2012 Introduction Procedure Results Conclusion 39

40 Primary Texture Variation Inside Tube Outside Tube CAPE 2012 Introduction Procedure Results Conclusion 40

41 Secondary Wall Enhancements 41

42 Enhancements Considered CAPE 2012 Introduction Procedure Results Conclusion 42

43 Combination of Primary and Secondary Enhancements 43

44 Models of Smooth Tubes current study Max CAPE 2012 Introduction Procedure Results Conclusion 44

45 Model of a Linear Rib Enhancements in an Enhanced Tube Max CAPE 2012 Introduction Procedure Results Conclusion 45

46 Model of Enhanced Structures in Tubes Max CAPE 2012 Introduction Procedure Results Conclusion 46

47 Typical Enhanced Surface Pattern Produced for Vipertex 2EHT Tube Outside Inside CAPE 2012 Introduction Procedure Results Conclusion 48

48 Typical Enhanced Tube Produced CAPE 2012 Introduction Procedure Results Conclusion 49

49 1EHT Tube Produced CAPE 2012 Introduction Procedure Results Conclusion 50

50 Experimental Process Fluids in the sample tubes were heated/cooled with water Flow and temperature regulated. CAPE 2012 Introduction Procedure Results Conclusion 51

51 Tube in Tube, Counterflow Test Apparatus CAPE 2012 Introduction Procedure Results Conclusion 52

52 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, CAPE 2012 Introduction Procedure Results Conclusion 53

53 Tube Enhancement Test Conditions 54

54 Vipertex 1EHT Enhanced Tubes 55

55 Vipertex 2EHT Enhanced Tubes 56

56 Vipertex 1EHT Cooling Enhancement Ratio 7.0 1EHT - Cooling Nu meas / Nu pred,pt Predicted - VDI Predicted - Ghajar E E E E E+05 Re 57

57 Vipertex 2EHT Cooling Enhancement Ratio 6.0 2EHT - Cooling Nu meas / Nu pred,pt Predicted - VDI Predicted - Ghajar E E E E E+05 Re 58

58 Inside Heat Transfer Coefficient Equation for Enhanced Tubes (1EHT) compared to Smooth Tubes Smooth tube CAPE 2012 Introduction Procedure Results Conclusion 59

59 Friction Factor of Enhanced Tubes (1EHT) Compared to Smooth Tubes Smooth tube CAPE 2012 Introduction Procedure Results Conclusion 60

60 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

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

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

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

64 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

65 Twisted Tape in Smooth Tube with Laminar Flow for a Twist Ratio of 5 and Various Spacing 67

66 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

67 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

68 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. Lopina, R. F. and Bergles, A. E Heat transfer and pressure drop in tape generated swirl flow of single-phase water. J. Heat Transfer, 91,

69 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. Ayub, Z. H. and AI-Fahed, S. F 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,

70 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. 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:

71 Dimple Tube with Twisted Tape For the range of Re between 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. Chinaruk Thianpong, Petpices Eiamsa-ard, Khwanchit Wongcharee, Smith Eiamsa-ard, 2009, International Communications in Heat and Mass Transfer, 36,

72 Compound Enhancement Test Arrangement 76

73 Compound Enhancements at Low Flows 77

74 Nu Nu Enhanced Twisted Tapes Cause Earlier Transition and Higher HTC Vipertex 1EHT Cooling with twisted tape insert Measured Predicted-VDI Predicted-Ghajar E E E E E Re Vipertex 1EHT - Cooling Measured Predicted-VDI Predicted-Ghajar E E E E E+05 Re 78

75 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 CAPE 2012 Introduction Procedure Results Conclusion 79

76 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 CAPE 2012 Introduction Procedure Results Conclusion 80

77 Economic Analysis Parameters 81

78 Operating Cost Information 82

79 Economic Analysis Enhanced - Smooth 1% enhancement $

80 Economic Analysis Enhanced - Smooth 10% enhancement $594,352 84

81 Energy Savings with Enhancements 1% enhancement 10% enhancement 85

82 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 CAPE 2012 Introduction Procedure Results Conclusion 86

83 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. CAPE 2012 Introduction Procedure Results Conclusion 87

84 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.. CAPE 2012 Introduction Procedure Results Conclusion 88

85 Summary and Conclusions 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< Turbulent Flows Similar magnitude Surface enhancement configuration Similar Present study also evaluated enhanced twisted tapes CAPE 2012 Introduction Procedure Results Conclusion 89

86 Summary and Conclusions 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. CAPE 2012 Introduction Procedure Results Conclusion 90

87 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. CAPE 2012 Introduction Procedure Results Conclusion 91

88 Acknowledgements Thank you to Rigidized Metals and Vipertex for their Support in the Development and Study of Compound Enhanced Heat Transfer Tubes. CAPE 2012 Introduction Procedure Results Conclusion 92

89 Thank You Questions? CAPE 2012 Introduction Procedure Results Conclusion 93

90 94