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Giovanni Molinari - Ascanio Trojani
The Heat Pipe - a Valuable Tool for the Energetics of the Latin American NICs

ENERG2 International Congress, Roma 1992

 



LOW PRICES OF ENERGY AND CONSTRAINTS TO RESEARCH, DEVELOPMENT AND COMMERCIAL PROMOTION OF INNOVATIVE ENERGY SYSTEMS. A CASE STUDY: THE HEAT PIPE HEAT EXCHANGER. G. Molinari - A. J. Trojani Università degli Studi di Roma la Sapienza - Dipartimento di Meccanica ed Aeronautica Via Eudossiana 18 - 00184 - Roma, Italy In the present scenario of low-priced fossil fuels (Brent barrel is under 20 US$), that is supposed to remain reasonably stable in the short-medium period, resources for R & D and commercial promotion of innovative energy systems are undoubtedly reduced, also with government interventions. In this situation, innovative technologies although well proven understated respect to the classical technologies, well developed and optimized for low energy prices. New technologies are then confined in niche applications, where some of their characters, that often do not even take in account their better energy performance, result competitive with the old ones. We here discuss an exemplary case study, the Heat Pipe Heat Exchanger, that although well known from more than a century still has not the commercial success its characteristics and performances should expect for. PAPER PRESENTED TO I T EE C 9 7 - TH I R D IN T E R N A T I O N A L TH E R M A L EN E R G Y & EN V I R O N M E N T C O N G R E S S MARRAKESH - MOROCCO - 9-12 JUNE 1997 - UNIVERSITY CADI AYYAD - FACULTY OF SCIENCE SEMLALIA 2 LOW PRICES OF ENERGY AND CONSTRAINTS TO RESEARCH, DEVELOPMENT AND COMMERCIAL PROMOTION OF INNOVATIVE ENERGY SYSTEMS. A CASE STUDY: THE HEAT PIPE HEAT EXCHANGER. G. Molinari - A. J. Trojani Università degli Studi di Roma la Sapienza - Dipartimento di Meccanica ed Aeronautica Via Eudossiana 18 - 00184 - Roma, Italy In the present scenario of low-priced fossil fuels (Brent barrel is under 20 US$), that is supposed to remain reasonably stable in the short-medium period, resources for R & D and commercial promotion of innovative energy systems are undoubtedly reduced, also with government interventions. In this situation, innovative technologies although well proven understated respect to the classical technologies, well developed and optimized for low energy prices. New technologies are then confined in niche applications, where some of their characters, that often do not even take in account their better energy performance, result competitive with the old ones. We here discuss an exemplary case study, the Heat Pipe Heat Exchanger, that although well known from more than a century still has not the commercial success its characteristics and performances should expect for. 1 - Premises. We are today in a scenario of low prices of energy from fossil fuels. Brent barrel is priced under 20 US$ and is predicted to remain stable in the short-medium period. We don t think that in this situation an investor, in a free trade environment, will spend resources to change the current mix of energy sources or to modify a well established model of energy production. Although the power plants we all know were slightly and slowly improved in their performance through the years, we still work out almost the same basic schemes we had years ago. The state-of-the-art energy technology is still basically linked to a low energy prices scenario, and new technologies, although well-proven and commercially available, are out of the mainstream market following their higher installation costs and too long payback periods. In this circumstances, new energy technologies are today mostly seen in niche applications, but very often not for their best energy efficiency but for reasons independent from this aspect. Let us think, for example, about the NIMBY (Not In My Back Yard) and NIMTOO (Not In My Term Of Office) syndromes. To build a new power plant is today a very high political risk [Ref. 1] even with the best technical, social and ecological postulates. Even the innocent and uncorrupted windfarms, epitomes of all that s green and clean, have fierce and resolute oppositors, facing serious risks to birds, noise, landscape [Ref. 2] and more. Energy companies are still holding the line - literally - on aged power plants, to maintain the siting and licensing positions conquered in the past, not to secure power but for using in perspective these positions to build new plants or to retrofit or repower the old plants themselves. We will now discuss a case study, on a well-known and well-proven innovative energy technology, that in a scenario of low energy prices is closed in niche applications: the Heat Pipe Heat Exchanger (HPHX). PAPER PRESENTED TO I T EE C 9 7 - TH I R D IN T E R N A T I O N A L TH E R M A L EN E R G Y & EN V I R O N M E N T C O N G R E S S MARRAKESH - MOROCCO - 9-12 JUNE 1997 - UNIVERSITY CADI AYYAD - FACULTY OF SCIENCE SEMLALIA 3 2 -The Heat Pipe (HP) and the Heat Pipe Heat Exchanger (HPHX) Amongst the energy-effective and innovative technologies developed in these decades, we must give prominent place to the techniques of heat transfer enhancement, which can secure very high energy savings. Reducing, for example, the size of a heat exchanger (for a specified heat flux), upgrading its performance or reducing the operative temperature difference between external flows doesn t mean only energy savings but - especially in this low price energy scenario - economy in valuable materials, smaller and cheaper equipment, more added value for the contractor, less environmental impact, less shipping, handling and assembly costs. Many advanced heat transfer enhancement methods and technologies are available [Ref. 3], and among which HPs show a very interesting combination of simplicity, high innovative grade, performance, economicity, reliability and easy technological accessibility. The HP is a well-known and well-proven class of devices [Refs. 4,5], consisting of sealed, evacuated cavities filled with a small quantity of working fluid that evaporates from one end at the application of an external heat flux and condensates on the other end, giving back the heat received. To restore the process the condensed working fluid must be returned to the evaporator with the help of a force field. HPs can be classified in their simplest configurations following the genesis of the forces acting to return the condensate to the evaporator. Most interest is centered on gravity, capillary and centrifugally-driven HPs, i.e. respectively, the so-called two-phase thermosyphon, the capillary HP and the rotating HP. More complex schemes were also studied and technically demonstrated using, for example, osmotic and electromagnetic forces. This variety of possible and robustly functional configurations is the original sin that excludes - even for the basic configurations here quoted - the possibility of defining an universal, self-contained mathematical model for this class of devices. Bps are extremely simple, do not have moving parts inside, are noiseless, highly reliable and sturdy, do not require energy for their operation, allowing, with a little engineering common-sense, their use in extremely difficult environments. HP are characterized by very high (equivalent) thermal conductance in comparision with solid materials, nearly isothermal operation, the capability to Function as active thermal diodes or switches, allowing them to active thermal field control. On the other hand, the sharp building simplicity and economicity of these devices allows the engineer to test extensively many prototypes derived from a simplified calculation record, creating a broad performance charting for the chosen configuration, for a wide amplitude of external parameters. The temperature fields within the HP devices can operate range from about 4K (liquid He) to 2000K and more (liquid metals like Ag or Hg and ceramic-carbon envelopes), their length from some millimeters to some tens of meters . Configuration of HP is then strictly laced to the specific application. HPs are known in their basic configurations (two-phase thermosyphon) from the past century. Quoting the UK Patent 22,272 filed in 1892 from the boiler builder Ludlow Perkins, HPs may be used for a variety of purposes among which are mentioned the heating of greenhouses, rooms, vehicles, drying-closets, and the like, the warming o the currents of volumes of air or water or other liquids contained in pipes, tanks, columns, such as water cranes (to prevent freezing) and the warming or heating of inflammable substances. Nothing less than the applications we see today in every reference list; we filed more than four hundred separate applications of HPs in the most unthinkable able areas, from cryostabilization of soils to roast-beef cooking, from nuclear fusion to toys, from heat recovery to thermoaerodynamics. Today , the areas of commercial standout of HP devices are in the field of heat recovery from flue gases, specially in the HVAC field, and in electrical and electronic systems thermal field control, with expanding interest in cryostabilization (the most extensive single HP application ever was the cryostabilization of the pillars of the Alyeskapipe, in the Seventies) and in evacuated solar thermal collectors. PAPER PRESENTED TO I T EE C 9 7 - TH I R D IN T E R N A T I O N A L TH E R M A L EN E R G Y & EN V I R O N M E N T C O N G R E S S MARRAKESH - MOROCCO - 9-12 JUNE 1997 - UNIVERSITY CADI AYYAD - FACULTY OF SCIENCE SEMLALIA 4 In the gas-to-gas HPHX, HP evaporation sections are immersed in the exhaust hot-gas duct and condensers are located inside the cool-gas duct, in a gas-to-gas air preheater for a medium-sized steam generator we can see several thousand single 1-IPs. A sealed partition prevents drastically every cross-contamination between the flows, and are excellent solutions in circulating fluidized bed boilers, where near-zero leakage between the fluxes is compulsory, and in coal-fired boilers where elimination of cross-contamination from ashes is highly appreciated. Recent laws and standards on air pollution impose also a review in the use of regenerative air preheaters, creating more attention on HPHX. Gas-liquid or liquid-liquid HPHX and HP recovery boilers were built, too. This system is characterized by an extremely high- modularity, since each HP is completely independent from each other, consenting the widest range of sizes, from small subwindow sized HVAC units to industrial boiler or furnace air preheaters. In the last case, the BPHX is cheaper and more cost-effective than tubular or rotating exchangers, as demonstrated from the Eighties [Refs. 6,7] in many field experiences in plants located in the US, Japan and Europe, both as substitutes to rotary regenerative air preheaters and as supplementary units to existing rotary to get more outlet gas temperature lowering, and even from the late Sixties in hydrocarbon processing plants. Because of its modularity, can be shipped economically from the factory in a number of small modules, not requiring customized transportation units, and can be assembled with very few problems once at the plant, cutting dramatically the related costs. Consisting of single, scaled, independent units (a failure of one or more HPs don t stop the plant as can happen with a coupled coil), the HPHX is extremely reliable and can be utilized in plants operating in difficult environments with little servicing. [Refs. 8, 9,10] Retrofitting and repowering of aged power plants, with some attention to the ones of low power (i.e. on the 30 Mwe class) is a practice progressively taking step in many countries, to face improved electrical power request without creating new plant sites. Aged plants are often maintained only to hold the line on siting and licensing, avoiding with their transformation all the operations connected with a new plant erection, although their poor economical and technical performance. For these applications HPHXs, as air preheaters meet a niche for its utilization, even in a depressed oil market. quotation. HPHX are generally more compact than other heat exchangers (about a fourth of a same class tubular, but still bigger than regenerative); high global heat transfer coefficients allows HPHXs to work at lower gas velocities, reducing pressure drops, noise and power absorption from fans. Fouling and corrosion result more manageable, because the HPHX is a static exchanger with single HPs working nearly-isothermal, and spacing between rows is relatively higher than in other heat exchangers. Recent studies open interesting perspectives for the regulation capabilities of HPHX- adopting Variable Conductance BP or Thermal Switch configurations, an HPHX performance can be widely adapted to variable load, modifying its thermal characteristics or excluding at all a part of the HP rows. Although the HP do not influence directly on the most important cause of exergy destruction and loss, i.e. the boundary layer, the engineer can optimize almost independently the thermophysical characteristics of each side of the heat exchanger itself, with thermotechnical optimum giving benefits also on the exergetic global performance. Taking in account the nearly isothermal behavior of the HP devices, and assuming the exergy destruction proportional to the temperature difference between the external fluxes, referring to the comparison of contending types of heat exchangers, we can say that regenerative types have no contest against HPHXs, as do tubular heat exchangers while plate heat exchangers show a little temperature drop, but their fouling problems could make the grade to HPHXs. PAPER PRESENTED TO I T EE C 9 7 - TH I R D IN T E R N A T I O N A L TH E R M A L EN E R G Y & EN V I R O N M E N T C O N G R E S S MARRAKESH - MOROCCO - 9-12 JUNE 1997 - UNIVERSITY CADI AYYAD - FACULTY OF SCIENCE SEMLALIA 5 3 - Economics of Heat Recovery As known, the efficiency of a steam generator can be written as: hG = 1 - åq p GC H i =1 - q pc + q pt + q ps GC H i (1) where Gc , and Hi are the fuel hourly consumption and the lower heating value and Y-qp is the sum of the various energy losses, respectively the imperfect combustion, imperfect insulation and for sensible heat. Usually qps > qpt + qps , more evident for little steam generators. We have in effect : q pc + q pt q ps = 0 .25 ¸ 0 .50 = c (2) where lower value of c refers to small steam generators (which show higher gas outlet temperature) and the higher to the large ones. Thus the Eq. (1) becomes: hG = 1 ( 1 + c ) q ps GC H i (3) Being sensible heat loss expressed by the formula: I q ps = GC 1 - e Gat c *pg T gu - Te [ ]( ) (4) I in which Gat is the theoretical combustion air [kg/kg], e is the excess air figure, Tgu is the outlet gas temperature, Te is the outside temperature and c*pg the mean value of the specific heat in the Tgu - Te range, Eq. (3), putting c I =1 + c , becomes: hG = 1 - 1 I c 1 + e Gat c *pg ( T gu - Te ) Hi [ ] (5) and by differentiation with respect to the outlet temperature. dh G 1I I =c 1 + e Gat c *pg dT gu Hi [ ] (6) Eq. (6) gives the gain in steam generator overall efficiency as a function of the gas temperature reduction at the outlet. PAPER PRESENTED TO I T EE C 9 7 - TH I R D IN T E R N A T I O N A L TH E R M A L EN E R G Y & EN V I R O N M E N T C O N G R E S S MARRAKESH - MOROCCO - 9-12 JUNE 1997 - UNIVERSITY CADI AYYAD - FACULTY OF SCIENCE SEMLALIA 6 On the other hand, from the energy overall balance of the steam generator: h G GC H i = GV Dh (7) by differentiation with constant rating condition the following relation can be carried outd h G GC + h G dGC = 0 and then (8) dh G dG =- C hG GC (9) Combining Eq.s (6) and (9) we obtain the expression of the relative fuel saving due to the gain in efficiency as an effect of Tgu reduction dGC 1 I = c I 1 + e G at c *pg dT gu GC h G H i [ ] (10) Looking at the whole range of steam rating, the only appreciable difference between a large steam generator and a small one in case of a given fuel can be semi in the h G values; referring to fuel oil as example, with Hi » 40000 [kJ/kg], assuming h G = 0.81 ¸ 0.93 for the smallest and largest ratings, respectively, and neglecting the variation of cpg due to the (little) different temperature range, Eq. (10) by integration gives : DGC = ( 0,00067 ¸ 0 ,00070 ) DTG GC (11) This means a fuel saving of 0.67 ¸ 0.70 % for each 10°C of decrease of Tgu. For steam ratings of 1000 t/h (GC = 65 t/h) and 10 t/h (GC = 0.7 t/h) fuel savings of 0.455 t/h and 0.0047 t/h can be achieved, respectively. It means a saving in energy consumption of 5055 kW and 52 kW, respectively. This fuel saving corresponds to three oil barrels approximately in the first case and to 0,032 oil barrels in the second one, i.e. 54 US$/h and 0,57 US$/h respectively, at mid-April 1997 oil prices. Since the current cost of an EPHX can be evaluated in about 45 US$ per thermal kW recovered, the total investment cost amounts to 227,475 US$ in the first case and 2,340 US$ in the second one. It appears that the only capital cost could be paid in one year approximately, in both cases. PAPER PRESENTED TO I T EE C 9 7 - TH I R D IN T E R N A T I O N A L TH E R M A L EN E R G Y & EN V I R O N M E N T C O N G R E S S MARRAKESH - MOROCCO - 9-12 JUNE 1997 - UNIVERSITY CADI AYYAD - FACULTY OF SCIENCE SEMLALIA 7 4 - References (1) Inhaber, H. - How to Slay the Nimby Dragon - XVI Intersociety Energy Conversion Engineering Conference, Boston 1991 , IV vol. , pp. 48/53 (2) Webb, J. - Can We Learn to Love the Wind ? - New Scientist , nr. 1934, vol. 143, 16 July 1994, pp. 12-14 (3) Reay, D. A. - Heat Transfer Enhancement - A Review of Techniques and Their Possible Role on Energy Efficiency in the UK - Heat Recovery Systems & CHP - vol. XI, 199 1, pp. 1-40 (4) Molinari , G. and Trojani , A. J. - Indagine Termofluidodinamica sul Tubo di Calore Rotante - XL Congresso Nazionale della Associazione Termotecnica Italiana, Trieste, 1985 (5) Faghri , A. - Heat Pipe Science and Technology - Taylor & Francis , London 1995 (6) Franklin, H.N. and Talmud, F.M. Economics of Heat Pipe Air Preheater for Primary Air Systems Joint Power Generation Conference , St. Louis , 1981 (7) Misner, T.L. and Franklin, H.N. - Coal Fired Operating Experience With a Heat Pipe Air Preheater - - American Power Conference , Chicago, 1983 (8) Smock, R. - Heat Pipe Finds Air Preheat Applications in Power Plants - Power Engineering March 1988, pp.40-41 (9) Franklin, N.R. Building a Better Heat Pipe - Mechanical Engineering - August 1990 pp.52-54 (10) Collins, S. - What to Look for in Heat Pipes , Gas-to-Gas Heat Exchangers - Power, May 1992, pp 102-106 PAPER PRESENTED TO I T EE C 9 7 - TH I R D IN T E R N A T I O N A L TH E R M A L EN E R G Y & EN V I R O N M E N T C O N G R E S S MARRAKESH - MOROCCO - 9-12 JUNE 1997 - UNIVERSITY CADI AYYAD - FACULTY OF SCIENCE SEMLALIA

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