- Email: [email protected]
Thermal-hydraulic performance of Vipertex 1EHT enhanced heat transfer tubes
Applied Thermal Engineering 61 (2013) 60e66
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Thermal-hydraulic performance of Vipertex 1EHT enhanced heat transfer tubes David J. Kukulka a, *, Rick Smith b a b
State University of New York College at Buffalo, Mechanical Engineering Technology, 1300 Elmwood Avenue, Buffalo, NY 14222, USA VipertexÔ, 658 Ohio Street, Buffalo, NY 14203, USA
h i g h l i g h t s Vipertex 1EHT tubes produce heat transfer increases of almost 550% for a friction factor penalty of only 33%. 1EHT tubes at a Re ¼ 750 provides the same amount of heat transfer that smooth tube does for twenty times that value. Vipertex 1EHT series of tubes provides a means to significantly advance many heat exchange processes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 November 2012 Accepted 31 December 2012 Available online 18 March 2013
Heat transfer enhancement plays an important role in improving energy efficiency. Transition from laminar to turbulent flow for smooth tubes is typically assumed to occur for a Reynolds Number of approximately 2,300. Vipertex 1EHT enhanced tubes produce an early transition at Reynolds Numbers near 750 and for the same conditions they can provide the same amount of heat transfer that smooth tubes produce for flows that are twenty times greater. Low Reynolds Number flow (sometimes due to the lack of process water) is a typical process requirement in many areas of the world and can cause major design challenges. Use of Vipertex 1EHT enhanced heat transfer tubes can decrease process water requirements and provide higher performance levels within the same equipment footprint. In a comparison of the heat transfer for some constant flow rates, the Vipertex 1EHT surface can produce heat transfer increases of more than 500% when compared to smooth tubes. Advantages of the Vipertex 1EHT design (when compared to smooth tubes) include the maximization of heat transfer; minimization of operating costs; and/or minimization of the rate of surface fouling. These enhanced tubes recover more energy and provide an opportunity to advance the design of many heat transfer products. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Enhanced heat transfer surfaces Enhanced tubes Heat exchanger design Boiler tube Enhanced alloy tubes Three dimensional heat transfer surfaces
1. Introduction Enhanced heat transfer techniques have been around for years, however the field is still relatively young and the results are not always predictable. Experimentation is difficult and modeling can be complicated; with meaningful results sometimes difficult to obtain. Single phase heat transfer enhancements for high Reynolds Number conditions in tubular products are established; however many enhanced tubes used in that flow regime are expensive to operate and produce. Flow enhancement in the laminar and transitional regimes is difficult and has largely been ignored. Increasing
* Corresponding author. Tel.: þ1 716 878 4418; fax: þ1 716 878 3033. E-mail addresses: [email protected], [email protected] (D.J. Kukulka). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.12.037
overall efficiency in a process plant is always a priority for engineers who are constantly looking for new ways to reduce energy requirements. Many industrial processes that involve the transfer of heat energy employ old technology; making them ideal candidates for a redesign utilizing enhanced surfaces that improve process performance. Gough [1] discusses the increased demand on the performance of heat exchangers and the need to enhance their performance. Enhanced heat transfer surfaces increase performance through a combination of: increased turbulence; boundary layer disruption; secondary flow generation and increased heat transfer surface area. These factors lead to an increase in the heat transfer coefficient; smaller unit footprint; more economic operation costs and a prolonged product life. There are a few different scenarios to be considered in heat exchanger optimization. In the case of a one-for-one replacement of smooth tubes with enhanced tubes of equal length; there is an increase in heat transfer for a
D.J. Kukulka, R. Smith / Applied Thermal Engineering 61 (2013) 60e66
61
Fig. 1. (a) Cross sectional view showing details of the inner surface of Vipertex 1EHT (Type 304 L stainless steel) enhanced tube (b) details of the outer surface of the Vipertex 1EHT (Type 304 L stainless steel) enhanced tube.
constant fluid flow rate. Typically this is accomplished with an increase to the pumping power of the enhanced tube exchanger since there is increased friction. Another scenario to consider is constant pumping power designs; in this case the required tube length could be reduced. Finally for the case of constant heat transfer in the same unit footprint; the use of enhanced tubes will reduce the pumping power requirements. Many enhanced heat transfer surfaces increase efficiency in the turbulent region. Associated with those heat transfer increases are substantial increases to the friction factor. For some enhanced designs, the increase in the heat transfer outweighs the penalty due to the friction factor increase; however for many designs in the turbulent region, the benefit of the increased heat transfer cannot be justified since the increase in the friction factor greatly outweighs the heat transfer enhancement. Vipertex optimized several enhanced heat transfer tube designs. This study details the heat transfer and fluid flow results of the Vipertex 1EHT enhanced heat transfer tube for a range of flows that were traditionally classified as being in the laminar, transitional and turbulent regimes. Optimized Vipertex 1EHT tubes increase heat transfer on both the inside and outside surfaces, while requiring only a modest increase in the friction factor. Heat transfer enhancement using the Vipertex 1EHT series of tubes provides a means to significantly advance many heat exchange processes, with the largest enhancements for the 1EHT tubes seen at low flows. Low flow heat transfer enhancement was one of the primary goals considered in the development of the Vipertex 1EHT series of enhanced tubes. Utilization of enhanced heat transfer tubes is necessary in the development of high performance thermal systems. Their use will allow a reduction in the cooling water mass flow rate required to obtain the designed heat transfer. Transition from laminar to turbulent flow for smooth tubes is typically assumed to occur for a Reynolds Number (Re) of approximately 2,300. In reality, a transition point is not as well defined and transition for some process designs actually occurs over Reynolds Numbers varying between 2300 and 10,000. In many geographical regions, cooling water design criteria for heat transfer devices require operations in the low flow region; in or near the traditional laminar flow regime. Vipertex 1EHT enhanced tubes can be employed to design more efficient heat exchangers since they create an earlier transition; producing more heat transfer at lower flow rates and allow designs in the low flow regime, saving both
energy and water. Vipertex 1EHT tubes have been designed and produced through material surface modifications, creating flow optimized heat transfer tubes. Increases to heat transfer occur through a combination of factors that include: increasing fluid turbulence, secondary flow development, disruption of the thermal boundary layer and increasing the heat transfer surface area. In considering 1EHT tubes, the benefit of heat transfer enhancement greatly outweighs the penalty produced by an increase in the friction factor. Transition in smooth tubes was observed by Lindgren [2] to occur in a gradual manner; with bursts of turbulence occurring down the length of the tube. Nunner [3] presents an early study on tube augmentation and the use of artificial roughness to effect transition. Tam and Ghajar [4] present transitional flow characteristics and a detailed discussion on smooth tube transition occurring for a range of Reynolds Numbers between 2300 and 10,000. Additionally, they point out that some inlet conditions cause transition to occur at higher Reynolds Numbers. A variety of enhanced surface studies have been previously performed and include: a performance study of dimpled tubes by Kalinin et al. [5]. Chen et al. [6] compares the performance of enhanced dimple tubes with other methods of heat transfer augmentation designs in terms of heat transfer and friction factor performance. Wang et al. [7] models various tube enhancements and presents heat transfer results for internally finned tubes. Gee and Webb [8] studied the effect of two-dimensional roughness geometry on single-phase forced convection in a circular tube. Liu
Table 1 Measurement uncertainty of data. Measurement
Uncertainty
Pump discharge and tank static pressure Steam pressure Cooling water and condensate tank static pressure Fluid differential pressure Condensate accumulator level Fluid flow rate Steam condensate mass Annulus water flow rate Fluid, steam and cooling temperature
þ/0.35 psi (þ/2413 Pa) þ/0.30 psi (þ/2069 Pa) þ/0.10 psi (þ/690 Pa) þ/0.5 in H2O (þ/124 Pa) þ/0.20 in H2O (þ/49.8 Pa) þ/0.09 lbm/min (þ/0.0408 kg/min) þ/0.02 lbm (þ/0.009 kg) þ/0.12 gal/min (þ/0.0454 L/min) þ/0.2 F (þ/0.36 C)
62
D.J. Kukulka, R. Smith / Applied Thermal Engineering 61 (2013) 60e66
Fig. 2. Comparison of the measured Nusselt number data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid cooling arrangement.
and Jensen [9] presented results for a numeric model of single phase, turbulent flow and heat transfer in helical finned tubes. Vincente et al. [10] presents heat transfer and pressure drop data for helically dimpled tubes; finding early transition for flows with Re greater than 1400. Christians et al. [11] studied film condensation of refrigerants on two types of three dimensional, enhanced tubes. Christians et al. [12] presents observations and descriptions of the bundle effect on the heat transfer performance of two types of three dimensional enhanced tubes. A new predictive method for falling film condensation on bundles was proposed. A condensation and boiling analysis of the 1EHT tube is currently ongoing and the subject of a future study. Kukulka et al. [13] evaluated the surface geometry of enhanced tubes and that study formed the groundwork for the present study. Wei et al. [14] presented a study of micro fin tubes for a wide range of Prandtl Numbers (Pr < 220); with Reynolds Numbers from 2500 to 90,000. For high Pr fluids (oil), they found the critical Reynolds Number to be approximately 6,000; while for a low Pr fluids (water) it was w10,000. Kukulka et al. [15] evaluated enhanced tubes under fouling conditions and
detailed transient results are presented. Meyer and Olivier [16] present heat transfer and friction factor results for helically finned tubes, in the “transition region”, for developing and fully developed flows. None of the conditions and/or geometries from the previous enhanced studies was the same as the present study; therefore no direct comparison could be made to previous enhanced works. Previously reported plain tube heat transfer results were used as a basis to compare relative thermal performance gain associated with the Vipertex 1EHT enhanced tubes. DittuseBoelter’s [17] classic correlation was used to calculate the inside film coefficient for turbulent conditions. Flow rates in the current study extended into the fully-developed turbulent flow regime, with conditions evaluated for Reynolds numbers to w20,000. Additionally, a modified form of the DittuseBoelter equation is used to evaluate the outside heat transfer coefficient for smooth tubes as a function of the annulus Reynolds number. For a comparison of the Nusselt Number (Nu) based upon the 1EHT enhanced tube performance, with the Nu based upon a smooth tube, in the region typically
Fig. 3. Comparison of the measured friction factor data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid cooling arrangement.
D.J. Kukulka, R. Smith / Applied Thermal Engineering 61 (2013) 60e66
63
Fig. 4. Comparison of the measured Nusselt number data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid heating arrangement.
classified as laminar, two previous studies known for accurate predictions were utilized for comparison; the pure forced laminar convection study (from Gnielinski as given in the VDI Heat Atlas [18]) and the mixed laminar convection study of Ghajar and Tam [19]. These comparisons allow a determination of the heat transfer enhancement due to surface modification. Measured friction factors of the 1EHT tubes are compared to smooth tube predictions from Wilson et al. [20]. Government legislation and specific energy conservation targets have been set for overall energy reduction on a national basis by many countries. Additionally, government incentives are available to reduce energy usage and environmental impact. Gough [1] points out that recent events in Japan have prompted the Japanese government to take a more active role in its serious drive to reduce energy use. Recently, additional countries (i.e. United States, Korea, Denmark, etc.)have been promoting energy efficiency,
making the development of enhanced heat transfer tubes and other enhanced heat transfer technologies even more important. 2. Experimental details Fig. 1 provides inner and outer surface views of the 0.75 inch (19.05 mm) outer diameter Vipertex 1EHT enhanced stainless steel tube that was evaluated in this study. Heat transfer and hydraulic characteristics of the enhanced Vipertex 1EHT heat transfer tubes were evaluated and then verified with an experimental study at the Heat Transfer Research, Inc. (HTRI) Research and Technology Center. The heat transfer experimental setup was a horizontal doublepipe heat exchanger with propylene glycol used for the inside test fluid; testing covered a wide range of Reynolds numbers (between 250 and 18,000 for heating; and between 50 and 20,000 for cooling). Saturated steam (creating a constant surface temperature) was
Fig. 5. Comparison of the measured friction factor data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid heating arrangement.
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used as the heating medium in the outer tube of the test apparatus when fluid heating was studied and water was used as the cooling medium in the outer tube of the test apparatus for the fluid cooling portion of the experiment. The temperature of the cooling water could be varied between ambient and 250 F (121.1 C). In this manner, tubeside heating and cooling with a constant wall temperature boundary could be studied. For all tests, the heated/cooled length of the tested tube was 15 ft (4.572 m). Data measurements were performed using multi-point calibrated instruments that when possible have been calibrated to NIST-traceable standards, while other instruments (flow meters, weight scale and pressure transmitters) were factory calibrated, with calibration certificates; multipoint calibration checks are used for verification. Uncertainty of the data measured in this study is given in Table 1. Constant bulk to wall viscosity ratios for each set of heating or cooling condition was targeted. The bulk to wall viscosity ratio targeted was approximately: 2.0 for heating and 0.2 for cooling. Average Prandtl number values of the propylene glycol ranged from 33 to 130, with the majority of the values in the range 33 < Pr < 53. From the measured tubeside duty a measured heat transfer coefficient on the tube inside can be determined. 3. Results Single phase, heat transfer evaluations were carried out in a horizontal tube for inner fluid heating and cooling over a range of Reynolds Numbers to w20,000. In all tests, the enhanced tubes outperformed smooth tubes under similar conditions. Performance was evaluated using the enhancement ratio which compares overall performance of the enhanced tube to that of a smooth tube. Fig. 2 shows a comparison of heat transfer for Vipertex 1EHT tubes compared to smooth tubes, for interior fluid cooling. A peak gradually forms for the Vipertex 1 EHT tubes in the range of Re from 100 to 2,000, with the maximum forming at approximately 750. In this range, the maximum heat transfer enhancement ratio is approximately 6 (see Fig. 2). For flows above a Re of 2000 the inside heat transfer enhancement of the Vipertex 1EHT tube is approximately
100% greater than that of a smooth tube. Fig. 3 shows the increase in friction factor that accompanies the increases of heat transfer for the interior fluid cooling case. At very low flows (in the region of Re ¼ 100) the maximum friction factor ratio (fENHANCED/fSMOOTH) is approximately 5. Near the region of maximum heat transfer (Re w 750) the friction factor ratio decreases to ratio values in the range of 1.25e1.35. This ratio then forms another local maximum of 2.8 for an approximate Re value of 1500. For Re values greater than 3000 the friction factor ratio is approximately 1.8 and almost constant. In the fluid heating case there is also a region of maximum heat transfer (not as large as the cooling case) forming near a Re of 1000 (see Fig. 4). Once again there is a local minimum for the friction factor ratio near a Re of 800, with the ratio then increasing slightly for Re values larger than 2000 (see Fig. 5). Vincente et al. [10] finds transitional values of Re to be 1,400, while this study finds much smaller transitional values of Re. Fig. 6 presents a comparison of performance for inside fluid cooling at constant heat transfer. At the intersection of lines A and B, the maximum heat transfer for the 1EHT enhanced tube is shown. In order to obtain the same amount of heat transfer in the smooth tube (this is shown by the intersection of lines A and C), roughly twenty times the flow would be required. Fig. 7 compares the heat transfer and friction factor for inside fluid cooling at a constant flow rate (shown by Line E). Vipertex 1EHT tubes provide 545% more heat transfer (shown at the intersection of Line A and Line E) when compared to the heat transfer of a smooth tube (shown at the intersection of Line B and Line E) for the same flow rate. The increase of heat transfer for the 1EHT tube is accompanied by an increase of approximately 33% in the friction factor (Line C and E) when compared to the friction factor of the smooth tube (Line D and Line E). Vipertex 1EHT enhanced heat transfer tubes provide an unusual combination of surface characteristics that produce heat transfer increases of almost 545% for a friction factor penalty of only 33%. Replacing smooth tubes with 1EHT tubes provides the opportunity to cut operating costs and the ability to obtain more heat transfer out of the same equipment footprint.
Fig. 6. Comparison of the measured Nusselt number data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid cooling arrangement. Line A/Line B intersection indicates the flow rate and maximum heat transfer for the Vipertex 1EHT enhanced tube. Line A/Line C intersection indicates the flow rate in a smooth tube for the heat transfer indicated by the intersection of Line A/Line B.
D.J. Kukulka, R. Smith / Applied Thermal Engineering 61 (2013) 60e66
65
Fig. 7. Comparison of the measured Nusselt number data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid cooling arrangement. Line A/Line E intersection indicates the maximum heat transfer and the Line C/Line E intersection indicates the friction factor, f, at that maximum heat transfer for the Vipertex 1EHT tube. The Line B/Line E intersection indicates the heat transfer and the Line D/Line E intersection indicates the friction factor, f, at that heat transfer for the smooth tube.
4. Conclusions The purpose of this study was to characterize the thermal- hydraulic performance of the 1EHT enhanced heat transfer tube that has been enhanced on both the inside and outside surfaces. Evaluation was carried out in a double-pipe heat transfer test apparatus that utilized water, steam and propylene glycol. The experiments evaluated both inside fluid heating and cooling conditions over a wide range of flow conditions. A number of conclusions can be drawn from the results of this study: 1. The Vipertex 1EHT enhanced tube showed outstanding thermal performance characteristics in the traditional laminar flow regime (Re 2200) under inside fluid cooling conditions. A local Nusselt number maximum was observed at a Reynolds numbers of w750 for the Vipertex 1EHT tube. The heat transfer at this point is more than five times greater than the heat transfer of a smooth tube at the same conditions. Although the underlying phenomena giving rise to it are not fully understood, this peculiar performance characteristic has been verified as being repeatable for the test fluid, tube geometry, and flow conditions considered. At higher flows there is an approximate two fold increase in heat transfer when compared to smooth tubes under inside fluid cooling conditions.
2. In the transition and turbulent flow regimes, the Vipertex 1EHT tube was found to produce an average 100% increase in heat transfer when compared to smooth tubes for heating conditions. The corresponding increase in the friction factor, in the same flow regimes, suggests that increases to heat transfer performance is also the result of enhanced turbulence caused by the surface modification. 3. For heating conditions, the 1EHT tube shows a small increase in heat transfer in the traditional laminar flow regime when compared to the mixed convection results. In this regime the 1EHT tubes appear to promote an early transition to turbulence at Reynolds numbers near 1000 with surface enhancement the likely reason for this enhancement. All this leads to an important and exciting advancement in process design. The patented Vipertex 1EHT surface enhances heat transfer, conserves energy and minimizes cost. Results from this study verify previous designs and direct the development of future designs. Additional surfaces are being developed by Vipertex to address specific applications. Further studies of different patterned, Vipertex surfaces are ongoing with an additional study in the near future detailing a condensation and boiling evaluation of the 1EHT tube. Future plans included a study that evaluates new enhancement characters; and through the use of computational fluid dynamic methods, a new optimized three dimensional, enhanced
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D.J. Kukulka, R. Smith / Applied Thermal Engineering 61 (2013) 60e66
heat transfer surface that will move the peak performance of the EHT tube from a Re of 800 to higher values is planned. Additional plans also include a detailed experimental study of these improved tubes. References [1] M.J. Gough, Process heat transfer enhancement to upgrade performance, throughput and reduced energy use, Chemical Engineering Transactions 29 (2012) 1e6. [2] E.R. Lindgren, Some aspects of the change between laminar and turbulent flow of liquids in cylindrical tubes, Arkiv för Fysik 7 (1953) 293e308. [3] W. Nunner, Heat transfer and pressure drop in rough tubes, VDI-forschungsheft 455-B (1956) 5e39. [4] L. Tam, A.J. Ghajar, Effect of inlet geometry and heating on the fully developed friction factor in the transition region of a horizontal tube, Experimental Thermal and Fluid Science 15 (1997) 52e64. [5] E.K. Kalinin, G.A. Dreitser, N.V. Paramonov, A.S. Myakochin, A.I. Tikhonov, S.G. Zakirov, E.S. Levin, L.S. Yanovsky, Comprehensive study of heat transfer enhancement in tubular heat exchangers, Thermal Fluids Engineering 4 (1991) 656e666. [6] J. Chen, H. Muller-Steinhagen, G. Duffy, Heat transfer enhancement in dimpled tubes, Applied Thermal Engineering 21 (2001) 535e547. [7] Q.-W. Wang, M. Lin, M. Zeng, Effect of lateral fin profiles on turbulent flow and heat transfer performance of internally finned tubes, Applied Thermal Engineering 29 (2009) 3006e3013. [8] D.L. Gee, R.L. .Webb, Forced convection heat transfer in helically ribroughened tubes, International Journal of Heat and Mass Transfer 23 (8) (1980) 1127e1136. [9] X. Liu, M. Jensen, Geometry effects on turbulent flow and heat transfer in internally finned tubes, Journal Heat Transfer 123 (6) (2001) 1035e1044.
[10] P.G. Vincente, A.G. Garcia, A. Viedma, Heat transfer and pressure drop for low Reynolds turbulent flow in helically dimpled tubes, International Journal of Heat and Mass Transfer 45 (2002) 543e553. [11] M. Christians, M. Habert, J.R. Thome, Film condensation of R-134a and R-236fa, part 1: experimental results and predictive correlation for single row condensation on enhanced tubes, Heat Transfer Engineering 31 (10) (2010a) 799e808. [12] M. Christians, M. Habert, J.R. Thome, Film condensation of R-134a and R236fa, part 2: experimental results and predictive correlation for single row condensation on enhanced tubes, Heat Transfer Engineering 31 (10) (2010b) 809e820. [13] D.J. Kukulka, R. Smith, K. Fuller, Development and evaluation of enhanced heat transfer tubes, Applied Thermal Engineering 31 (13) (September 2011) 2141e2145. [14] Li Xiao Wei, Ji An Meng, Zhi Xin Li, Experimental study of single-phase pressure drop and heat transfer in a micro-fin tube, Experimental Thermal and Fluid Science 32 (2007) 641e648. [15] D.J. Kukulka, R. Smith, J. Zaepfel, Development and evaluation of Vipertex enhanced heat transfer tubes used in fouling conditions, Theoretical Foundations of Chemical Engineering 46 (6) (2012) 627e633. [16] J.P. Meyer, J.A. Olivier, Transitional flow inside enhanced tubes for fully developed and developing flow with different types of inlet disturbances: part IIeheat transfer, International Journal of Heat and Mass Transfer 54 (2011) 1598e1607. [17] F.W. Dittus, L.M.K. Boelter, Heat transfer in automobile radiators of the tubular type, University of California Publications in Engineering 2 (13) (1930) 443e461. [18] V. Gnielinski, in: VDI Heat Atlas (Ed.), Heat Transfer in Forced Single-phase Flow, Chapter G, Verein Deutscher Ingenieure, VDI-Verlag GmbH, Dusseldorf, Germany, 1993, pp. 691e711. [19] A.J. Ghajar, L.M. Tam, Heat transfer measurements and correlations in the transition region for a circular tube with three different inlet configurations, Experimental Thermal and Fluid Science 8 (1994) 79e90. [20] R.E. Wilson, W.H. McAdams, M. Seltzer, The flow of fluids through commercial pipe lines, Industrial & Engineering Chemistry 14 (1922) 105e110.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Thermal-hydraulic performance of Vipertex 1EHT enhanced heat transfer tubes David J. Kukulka a, *, Rick Smith b a b
State University of New York College at Buffalo, Mechanical Engineering Technology, 1300 Elmwood Avenue, Buffalo, NY 14222, USA VipertexÔ, 658 Ohio Street, Buffalo, NY 14203, USA
h i g h l i g h t s Vipertex 1EHT tubes produce heat transfer increases of almost 550% for a friction factor penalty of only 33%. 1EHT tubes at a Re ¼ 750 provides the same amount of heat transfer that smooth tube does for twenty times that value. Vipertex 1EHT series of tubes provides a means to significantly advance many heat exchange processes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 November 2012 Accepted 31 December 2012 Available online 18 March 2013
Heat transfer enhancement plays an important role in improving energy efficiency. Transition from laminar to turbulent flow for smooth tubes is typically assumed to occur for a Reynolds Number of approximately 2,300. Vipertex 1EHT enhanced tubes produce an early transition at Reynolds Numbers near 750 and for the same conditions they can provide the same amount of heat transfer that smooth tubes produce for flows that are twenty times greater. Low Reynolds Number flow (sometimes due to the lack of process water) is a typical process requirement in many areas of the world and can cause major design challenges. Use of Vipertex 1EHT enhanced heat transfer tubes can decrease process water requirements and provide higher performance levels within the same equipment footprint. In a comparison of the heat transfer for some constant flow rates, the Vipertex 1EHT surface can produce heat transfer increases of more than 500% when compared to smooth tubes. Advantages of the Vipertex 1EHT design (when compared to smooth tubes) include the maximization of heat transfer; minimization of operating costs; and/or minimization of the rate of surface fouling. These enhanced tubes recover more energy and provide an opportunity to advance the design of many heat transfer products. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Enhanced heat transfer surfaces Enhanced tubes Heat exchanger design Boiler tube Enhanced alloy tubes Three dimensional heat transfer surfaces
1. Introduction Enhanced heat transfer techniques have been around for years, however the field is still relatively young and the results are not always predictable. Experimentation is difficult and modeling can be complicated; with meaningful results sometimes difficult to obtain. Single phase heat transfer enhancements for high Reynolds Number conditions in tubular products are established; however many enhanced tubes used in that flow regime are expensive to operate and produce. Flow enhancement in the laminar and transitional regimes is difficult and has largely been ignored. Increasing
* Corresponding author. Tel.: þ1 716 878 4418; fax: þ1 716 878 3033. E-mail addresses: [email protected], [email protected] (D.J. Kukulka). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.12.037
overall efficiency in a process plant is always a priority for engineers who are constantly looking for new ways to reduce energy requirements. Many industrial processes that involve the transfer of heat energy employ old technology; making them ideal candidates for a redesign utilizing enhanced surfaces that improve process performance. Gough [1] discusses the increased demand on the performance of heat exchangers and the need to enhance their performance. Enhanced heat transfer surfaces increase performance through a combination of: increased turbulence; boundary layer disruption; secondary flow generation and increased heat transfer surface area. These factors lead to an increase in the heat transfer coefficient; smaller unit footprint; more economic operation costs and a prolonged product life. There are a few different scenarios to be considered in heat exchanger optimization. In the case of a one-for-one replacement of smooth tubes with enhanced tubes of equal length; there is an increase in heat transfer for a
D.J. Kukulka, R. Smith / Applied Thermal Engineering 61 (2013) 60e66
61
Fig. 1. (a) Cross sectional view showing details of the inner surface of Vipertex 1EHT (Type 304 L stainless steel) enhanced tube (b) details of the outer surface of the Vipertex 1EHT (Type 304 L stainless steel) enhanced tube.
constant fluid flow rate. Typically this is accomplished with an increase to the pumping power of the enhanced tube exchanger since there is increased friction. Another scenario to consider is constant pumping power designs; in this case the required tube length could be reduced. Finally for the case of constant heat transfer in the same unit footprint; the use of enhanced tubes will reduce the pumping power requirements. Many enhanced heat transfer surfaces increase efficiency in the turbulent region. Associated with those heat transfer increases are substantial increases to the friction factor. For some enhanced designs, the increase in the heat transfer outweighs the penalty due to the friction factor increase; however for many designs in the turbulent region, the benefit of the increased heat transfer cannot be justified since the increase in the friction factor greatly outweighs the heat transfer enhancement. Vipertex optimized several enhanced heat transfer tube designs. This study details the heat transfer and fluid flow results of the Vipertex 1EHT enhanced heat transfer tube for a range of flows that were traditionally classified as being in the laminar, transitional and turbulent regimes. Optimized Vipertex 1EHT tubes increase heat transfer on both the inside and outside surfaces, while requiring only a modest increase in the friction factor. Heat transfer enhancement using the Vipertex 1EHT series of tubes provides a means to significantly advance many heat exchange processes, with the largest enhancements for the 1EHT tubes seen at low flows. Low flow heat transfer enhancement was one of the primary goals considered in the development of the Vipertex 1EHT series of enhanced tubes. Utilization of enhanced heat transfer tubes is necessary in the development of high performance thermal systems. Their use will allow a reduction in the cooling water mass flow rate required to obtain the designed heat transfer. Transition from laminar to turbulent flow for smooth tubes is typically assumed to occur for a Reynolds Number (Re) of approximately 2,300. In reality, a transition point is not as well defined and transition for some process designs actually occurs over Reynolds Numbers varying between 2300 and 10,000. In many geographical regions, cooling water design criteria for heat transfer devices require operations in the low flow region; in or near the traditional laminar flow regime. Vipertex 1EHT enhanced tubes can be employed to design more efficient heat exchangers since they create an earlier transition; producing more heat transfer at lower flow rates and allow designs in the low flow regime, saving both
energy and water. Vipertex 1EHT tubes have been designed and produced through material surface modifications, creating flow optimized heat transfer tubes. Increases to heat transfer occur through a combination of factors that include: increasing fluid turbulence, secondary flow development, disruption of the thermal boundary layer and increasing the heat transfer surface area. In considering 1EHT tubes, the benefit of heat transfer enhancement greatly outweighs the penalty produced by an increase in the friction factor. Transition in smooth tubes was observed by Lindgren [2] to occur in a gradual manner; with bursts of turbulence occurring down the length of the tube. Nunner [3] presents an early study on tube augmentation and the use of artificial roughness to effect transition. Tam and Ghajar [4] present transitional flow characteristics and a detailed discussion on smooth tube transition occurring for a range of Reynolds Numbers between 2300 and 10,000. Additionally, they point out that some inlet conditions cause transition to occur at higher Reynolds Numbers. A variety of enhanced surface studies have been previously performed and include: a performance study of dimpled tubes by Kalinin et al. [5]. Chen et al. [6] compares the performance of enhanced dimple tubes with other methods of heat transfer augmentation designs in terms of heat transfer and friction factor performance. Wang et al. [7] models various tube enhancements and presents heat transfer results for internally finned tubes. Gee and Webb [8] studied the effect of two-dimensional roughness geometry on single-phase forced convection in a circular tube. Liu
Table 1 Measurement uncertainty of data. Measurement
Uncertainty
Pump discharge and tank static pressure Steam pressure Cooling water and condensate tank static pressure Fluid differential pressure Condensate accumulator level Fluid flow rate Steam condensate mass Annulus water flow rate Fluid, steam and cooling temperature
þ/0.35 psi (þ/2413 Pa) þ/0.30 psi (þ/2069 Pa) þ/0.10 psi (þ/690 Pa) þ/0.5 in H2O (þ/124 Pa) þ/0.20 in H2O (þ/49.8 Pa) þ/0.09 lbm/min (þ/0.0408 kg/min) þ/0.02 lbm (þ/0.009 kg) þ/0.12 gal/min (þ/0.0454 L/min) þ/0.2 F (þ/0.36 C)
62
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Fig. 2. Comparison of the measured Nusselt number data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid cooling arrangement.
and Jensen [9] presented results for a numeric model of single phase, turbulent flow and heat transfer in helical finned tubes. Vincente et al. [10] presents heat transfer and pressure drop data for helically dimpled tubes; finding early transition for flows with Re greater than 1400. Christians et al. [11] studied film condensation of refrigerants on two types of three dimensional, enhanced tubes. Christians et al. [12] presents observations and descriptions of the bundle effect on the heat transfer performance of two types of three dimensional enhanced tubes. A new predictive method for falling film condensation on bundles was proposed. A condensation and boiling analysis of the 1EHT tube is currently ongoing and the subject of a future study. Kukulka et al. [13] evaluated the surface geometry of enhanced tubes and that study formed the groundwork for the present study. Wei et al. [14] presented a study of micro fin tubes for a wide range of Prandtl Numbers (Pr < 220); with Reynolds Numbers from 2500 to 90,000. For high Pr fluids (oil), they found the critical Reynolds Number to be approximately 6,000; while for a low Pr fluids (water) it was w10,000. Kukulka et al. [15] evaluated enhanced tubes under fouling conditions and
detailed transient results are presented. Meyer and Olivier [16] present heat transfer and friction factor results for helically finned tubes, in the “transition region”, for developing and fully developed flows. None of the conditions and/or geometries from the previous enhanced studies was the same as the present study; therefore no direct comparison could be made to previous enhanced works. Previously reported plain tube heat transfer results were used as a basis to compare relative thermal performance gain associated with the Vipertex 1EHT enhanced tubes. DittuseBoelter’s [17] classic correlation was used to calculate the inside film coefficient for turbulent conditions. Flow rates in the current study extended into the fully-developed turbulent flow regime, with conditions evaluated for Reynolds numbers to w20,000. Additionally, a modified form of the DittuseBoelter equation is used to evaluate the outside heat transfer coefficient for smooth tubes as a function of the annulus Reynolds number. For a comparison of the Nusselt Number (Nu) based upon the 1EHT enhanced tube performance, with the Nu based upon a smooth tube, in the region typically
Fig. 3. Comparison of the measured friction factor data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid cooling arrangement.
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Fig. 4. Comparison of the measured Nusselt number data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid heating arrangement.
classified as laminar, two previous studies known for accurate predictions were utilized for comparison; the pure forced laminar convection study (from Gnielinski as given in the VDI Heat Atlas [18]) and the mixed laminar convection study of Ghajar and Tam [19]. These comparisons allow a determination of the heat transfer enhancement due to surface modification. Measured friction factors of the 1EHT tubes are compared to smooth tube predictions from Wilson et al. [20]. Government legislation and specific energy conservation targets have been set for overall energy reduction on a national basis by many countries. Additionally, government incentives are available to reduce energy usage and environmental impact. Gough [1] points out that recent events in Japan have prompted the Japanese government to take a more active role in its serious drive to reduce energy use. Recently, additional countries (i.e. United States, Korea, Denmark, etc.)have been promoting energy efficiency,
making the development of enhanced heat transfer tubes and other enhanced heat transfer technologies even more important. 2. Experimental details Fig. 1 provides inner and outer surface views of the 0.75 inch (19.05 mm) outer diameter Vipertex 1EHT enhanced stainless steel tube that was evaluated in this study. Heat transfer and hydraulic characteristics of the enhanced Vipertex 1EHT heat transfer tubes were evaluated and then verified with an experimental study at the Heat Transfer Research, Inc. (HTRI) Research and Technology Center. The heat transfer experimental setup was a horizontal doublepipe heat exchanger with propylene glycol used for the inside test fluid; testing covered a wide range of Reynolds numbers (between 250 and 18,000 for heating; and between 50 and 20,000 for cooling). Saturated steam (creating a constant surface temperature) was
Fig. 5. Comparison of the measured friction factor data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid heating arrangement.
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used as the heating medium in the outer tube of the test apparatus when fluid heating was studied and water was used as the cooling medium in the outer tube of the test apparatus for the fluid cooling portion of the experiment. The temperature of the cooling water could be varied between ambient and 250 F (121.1 C). In this manner, tubeside heating and cooling with a constant wall temperature boundary could be studied. For all tests, the heated/cooled length of the tested tube was 15 ft (4.572 m). Data measurements were performed using multi-point calibrated instruments that when possible have been calibrated to NIST-traceable standards, while other instruments (flow meters, weight scale and pressure transmitters) were factory calibrated, with calibration certificates; multipoint calibration checks are used for verification. Uncertainty of the data measured in this study is given in Table 1. Constant bulk to wall viscosity ratios for each set of heating or cooling condition was targeted. The bulk to wall viscosity ratio targeted was approximately: 2.0 for heating and 0.2 for cooling. Average Prandtl number values of the propylene glycol ranged from 33 to 130, with the majority of the values in the range 33 < Pr < 53. From the measured tubeside duty a measured heat transfer coefficient on the tube inside can be determined. 3. Results Single phase, heat transfer evaluations were carried out in a horizontal tube for inner fluid heating and cooling over a range of Reynolds Numbers to w20,000. In all tests, the enhanced tubes outperformed smooth tubes under similar conditions. Performance was evaluated using the enhancement ratio which compares overall performance of the enhanced tube to that of a smooth tube. Fig. 2 shows a comparison of heat transfer for Vipertex 1EHT tubes compared to smooth tubes, for interior fluid cooling. A peak gradually forms for the Vipertex 1 EHT tubes in the range of Re from 100 to 2,000, with the maximum forming at approximately 750. In this range, the maximum heat transfer enhancement ratio is approximately 6 (see Fig. 2). For flows above a Re of 2000 the inside heat transfer enhancement of the Vipertex 1EHT tube is approximately
100% greater than that of a smooth tube. Fig. 3 shows the increase in friction factor that accompanies the increases of heat transfer for the interior fluid cooling case. At very low flows (in the region of Re ¼ 100) the maximum friction factor ratio (fENHANCED/fSMOOTH) is approximately 5. Near the region of maximum heat transfer (Re w 750) the friction factor ratio decreases to ratio values in the range of 1.25e1.35. This ratio then forms another local maximum of 2.8 for an approximate Re value of 1500. For Re values greater than 3000 the friction factor ratio is approximately 1.8 and almost constant. In the fluid heating case there is also a region of maximum heat transfer (not as large as the cooling case) forming near a Re of 1000 (see Fig. 4). Once again there is a local minimum for the friction factor ratio near a Re of 800, with the ratio then increasing slightly for Re values larger than 2000 (see Fig. 5). Vincente et al. [10] finds transitional values of Re to be 1,400, while this study finds much smaller transitional values of Re. Fig. 6 presents a comparison of performance for inside fluid cooling at constant heat transfer. At the intersection of lines A and B, the maximum heat transfer for the 1EHT enhanced tube is shown. In order to obtain the same amount of heat transfer in the smooth tube (this is shown by the intersection of lines A and C), roughly twenty times the flow would be required. Fig. 7 compares the heat transfer and friction factor for inside fluid cooling at a constant flow rate (shown by Line E). Vipertex 1EHT tubes provide 545% more heat transfer (shown at the intersection of Line A and Line E) when compared to the heat transfer of a smooth tube (shown at the intersection of Line B and Line E) for the same flow rate. The increase of heat transfer for the 1EHT tube is accompanied by an increase of approximately 33% in the friction factor (Line C and E) when compared to the friction factor of the smooth tube (Line D and Line E). Vipertex 1EHT enhanced heat transfer tubes provide an unusual combination of surface characteristics that produce heat transfer increases of almost 545% for a friction factor penalty of only 33%. Replacing smooth tubes with 1EHT tubes provides the opportunity to cut operating costs and the ability to obtain more heat transfer out of the same equipment footprint.
Fig. 6. Comparison of the measured Nusselt number data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid cooling arrangement. Line A/Line B intersection indicates the flow rate and maximum heat transfer for the Vipertex 1EHT enhanced tube. Line A/Line C intersection indicates the flow rate in a smooth tube for the heat transfer indicated by the intersection of Line A/Line B.
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Fig. 7. Comparison of the measured Nusselt number data versus Reynolds number for the Vipertex 1EHT enhanced tube and a smooth tube, for an inside fluid cooling arrangement. Line A/Line E intersection indicates the maximum heat transfer and the Line C/Line E intersection indicates the friction factor, f, at that maximum heat transfer for the Vipertex 1EHT tube. The Line B/Line E intersection indicates the heat transfer and the Line D/Line E intersection indicates the friction factor, f, at that heat transfer for the smooth tube.
4. Conclusions The purpose of this study was to characterize the thermal- hydraulic performance of the 1EHT enhanced heat transfer tube that has been enhanced on both the inside and outside surfaces. Evaluation was carried out in a double-pipe heat transfer test apparatus that utilized water, steam and propylene glycol. The experiments evaluated both inside fluid heating and cooling conditions over a wide range of flow conditions. A number of conclusions can be drawn from the results of this study: 1. The Vipertex 1EHT enhanced tube showed outstanding thermal performance characteristics in the traditional laminar flow regime (Re 2200) under inside fluid cooling conditions. A local Nusselt number maximum was observed at a Reynolds numbers of w750 for the Vipertex 1EHT tube. The heat transfer at this point is more than five times greater than the heat transfer of a smooth tube at the same conditions. Although the underlying phenomena giving rise to it are not fully understood, this peculiar performance characteristic has been verified as being repeatable for the test fluid, tube geometry, and flow conditions considered. At higher flows there is an approximate two fold increase in heat transfer when compared to smooth tubes under inside fluid cooling conditions.
2. In the transition and turbulent flow regimes, the Vipertex 1EHT tube was found to produce an average 100% increase in heat transfer when compared to smooth tubes for heating conditions. The corresponding increase in the friction factor, in the same flow regimes, suggests that increases to heat transfer performance is also the result of enhanced turbulence caused by the surface modification. 3. For heating conditions, the 1EHT tube shows a small increase in heat transfer in the traditional laminar flow regime when compared to the mixed convection results. In this regime the 1EHT tubes appear to promote an early transition to turbulence at Reynolds numbers near 1000 with surface enhancement the likely reason for this enhancement. All this leads to an important and exciting advancement in process design. The patented Vipertex 1EHT surface enhances heat transfer, conserves energy and minimizes cost. Results from this study verify previous designs and direct the development of future designs. Additional surfaces are being developed by Vipertex to address specific applications. Further studies of different patterned, Vipertex surfaces are ongoing with an additional study in the near future detailing a condensation and boiling evaluation of the 1EHT tube. Future plans included a study that evaluates new enhancement characters; and through the use of computational fluid dynamic methods, a new optimized three dimensional, enhanced
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heat transfer surface that will move the peak performance of the EHT tube from a Re of 800 to higher values is planned. Additional plans also include a detailed experimental study of these improved tubes. References [1] M.J. Gough, Process heat transfer enhancement to upgrade performance, throughput and reduced energy use, Chemical Engineering Transactions 29 (2012) 1e6. [2] E.R. Lindgren, Some aspects of the change between laminar and turbulent flow of liquids in cylindrical tubes, Arkiv för Fysik 7 (1953) 293e308. [3] W. Nunner, Heat transfer and pressure drop in rough tubes, VDI-forschungsheft 455-B (1956) 5e39. [4] L. Tam, A.J. Ghajar, Effect of inlet geometry and heating on the fully developed friction factor in the transition region of a horizontal tube, Experimental Thermal and Fluid Science 15 (1997) 52e64. [5] E.K. Kalinin, G.A. Dreitser, N.V. Paramonov, A.S. Myakochin, A.I. Tikhonov, S.G. Zakirov, E.S. Levin, L.S. Yanovsky, Comprehensive study of heat transfer enhancement in tubular heat exchangers, Thermal Fluids Engineering 4 (1991) 656e666. [6] J. Chen, H. Muller-Steinhagen, G. Duffy, Heat transfer enhancement in dimpled tubes, Applied Thermal Engineering 21 (2001) 535e547. [7] Q.-W. Wang, M. Lin, M. Zeng, Effect of lateral fin profiles on turbulent flow and heat transfer performance of internally finned tubes, Applied Thermal Engineering 29 (2009) 3006e3013. [8] D.L. Gee, R.L. .Webb, Forced convection heat transfer in helically ribroughened tubes, International Journal of Heat and Mass Transfer 23 (8) (1980) 1127e1136. [9] X. Liu, M. Jensen, Geometry effects on turbulent flow and heat transfer in internally finned tubes, Journal Heat Transfer 123 (6) (2001) 1035e1044.
[10] P.G. Vincente, A.G. Garcia, A. Viedma, Heat transfer and pressure drop for low Reynolds turbulent flow in helically dimpled tubes, International Journal of Heat and Mass Transfer 45 (2002) 543e553. [11] M. Christians, M. Habert, J.R. Thome, Film condensation of R-134a and R-236fa, part 1: experimental results and predictive correlation for single row condensation on enhanced tubes, Heat Transfer Engineering 31 (10) (2010a) 799e808. [12] M. Christians, M. Habert, J.R. Thome, Film condensation of R-134a and R236fa, part 2: experimental results and predictive correlation for single row condensation on enhanced tubes, Heat Transfer Engineering 31 (10) (2010b) 809e820. [13] D.J. Kukulka, R. Smith, K. Fuller, Development and evaluation of enhanced heat transfer tubes, Applied Thermal Engineering 31 (13) (September 2011) 2141e2145. [14] Li Xiao Wei, Ji An Meng, Zhi Xin Li, Experimental study of single-phase pressure drop and heat transfer in a micro-fin tube, Experimental Thermal and Fluid Science 32 (2007) 641e648. [15] D.J. Kukulka, R. Smith, J. Zaepfel, Development and evaluation of Vipertex enhanced heat transfer tubes used in fouling conditions, Theoretical Foundations of Chemical Engineering 46 (6) (2012) 627e633. [16] J.P. Meyer, J.A. Olivier, Transitional flow inside enhanced tubes for fully developed and developing flow with different types of inlet disturbances: part IIeheat transfer, International Journal of Heat and Mass Transfer 54 (2011) 1598e1607. [17] F.W. Dittus, L.M.K. Boelter, Heat transfer in automobile radiators of the tubular type, University of California Publications in Engineering 2 (13) (1930) 443e461. [18] V. Gnielinski, in: VDI Heat Atlas (Ed.), Heat Transfer in Forced Single-phase Flow, Chapter G, Verein Deutscher Ingenieure, VDI-Verlag GmbH, Dusseldorf, Germany, 1993, pp. 691e711. [19] A.J. Ghajar, L.M. Tam, Heat transfer measurements and correlations in the transition region for a circular tube with three different inlet configurations, Experimental Thermal and Fluid Science 8 (1994) 79e90. [20] R.E. Wilson, W.H. McAdams, M. Seltzer, The flow of fluids through commercial pipe lines, Industrial & Engineering Chemistry 14 (1922) 105e110.