Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
  • F01K25/00—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for
  • F01K25/08—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for using special vapours
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F22—STEAM GENERATION
  • F22B—METHODS OF STEAM GENERATION; STEAM BOILERS
  • F22B15/00—Water-tube boilers of horizontal type, i.e. the water-tube sets being arranged horizontally
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F22—STEAM GENERATION
  • F22B—METHODS OF STEAM GENERATION; STEAM BOILERS
  • F22B21/00—Water-tube boilers of vertical or steeply-inclined type, i.e. the water-tube sets being arranged vertically or substantially vertically
  • F22B21/02—Water-tube boilers of vertical or steeply-inclined type, i.e. the water-tube sets being arranged vertically or substantially vertically built-up from substantially straight water tubes
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F22—STEAM GENERATION
  • F22B—METHODS OF STEAM GENERATION; STEAM BOILERS
  • F22B21/00—Water-tube boilers of vertical or steeply-inclined type, i.e. the water-tube sets being arranged vertically or substantially vertically
  • F22B21/40—Water-tube boilers of vertical or steeply-inclined type, i.e. the water-tube sets being arranged vertically or substantially vertically built-up from water tubes arranged in a comparatively long vertical shaft, i.e. tower boilers

Definitions

• the present invention relates to a direct heat exchanger for Organic Rankine Cycle (ORC) .
• ORC Organic Rankine Cycle
• the subject system is functional in applications using biomass or in the presence of combustion fumes or recovery or very hot gaseous effluents.
• the working fluid of the ORC turbo-generators is not directly exposed to the hot source, but it typically exchanges heat with a thermal oil circuit.
• the thermal oil due to its good thermo-chemical stability, is capable of withstand high temperatures without deteriorating, and therefore without compromising its hear carrier function. This is extremely important, for example, in applications with a biomass, where the fumes reach temperatures near 900°C.
• thermo oil film does not exceed, for example, 310-320°C, and then mineral or better synthetic oils of normal production can be used.
• An intermediate thermal oil circuit however, has some disadvantages, such as a greater cost of the circuit, a higher electrical absorption by the relative auxiliary equipment (for example, the oil circulation pump) , a significant environmental impact in case of spills or leakages, the introduction of an intermediate heat exchange phase which inevitably reduces the thermal power absorbed by the ORC (at the same minimum temperature minimum of the working fluid, the minimum fume temperature is still higher due to the presence of two different temperatures, the one between the ORC fluid and the thermal oil, the other between thermal oil and fumes), risk of oil flammability .
• a solution of a system with a kind of direct exchange, i.e. with the working ORC fluid which exchanges heat directly with the hot source, would allow a reduction of investment costs and an increase in efficiency.
• the working fluids are organic substances which have a limit of thermal stability comparable to that of the thermal oil. Above a certain temperature (dependent on the substance, but approximately equal to 300-400°C) they tend to react to form new organic compounds which degrade the system performances, up to require a replacement of the entire working fluid in the worst cases.
• Direct exchange ORC systems from recovery fumes are rare and limited to cases in which the fumes are clean (for example the recovery of gases from MCI or gas turbines) and have relatively low temperatures.
• the direct exchanger must provide some measures to limit the temperature of the fluid film, such as, for example, the injection of air from the environment or of any other colder gas, and the recirculation of already cooled and collected fumes downstream of the direct exchanger or according to the size of the direct exchanger with relatively low heat exchange coefficients of the fumes.
• measures cause an inevitable increase in the size of the direct exchanger compared to an optimal sizing of the exchanger, in the absence of the limits dictated by the maximum fluid temperature.
• the injection and recirculation of air reduce the fume temperature (with a decrease of AT_log) and therefore involve an increase of S.
• the fume flow rate must increase: equal to U, i.e. substantially at the same speed, the gas passage sections increase.
• U the choice of the size the exchanger with low fume coefficients (reduction of U) causes an increase in the exchange surface S.
• the exchange coefficients of fumes are naturally already relatively low compared to the inner side, where the organic fluid is present.
• the use of vanes is unadvisable because it facilitates the deposit of the powders on the exchange surfaces .
• FIG. 1 shows by a countercurrent heat exchanger of known type.
• the organic liquid enters a manifold at the base of the exchanger and is distributed into the different rows of tubes; the vapor is then collected in a manifold located at the opposite end.
• the heat exchanger could have greater dimensions (in height) so as being incompatible with the system layout or make it impossible to transport the same as a single body already assembled and welded at the factory.
• Subject of the present invention is therefore a novel direct heat exchanger for organic Rankine cycle systems, with a layout having reduced height size, as specified in the attached independent claim.
• FIG. 2 shows a direct heat exchanger according to a first embodiment of the present invention
• FIG. 3 shows the direct heat exchanger in a second embodiment
• FIG. 4 shows the direct heat exchanger according to a third embodiment
• FIG. 5 shows the direct heat exchanger according to a fourth embodiment
• FIG. 6 shows the direct heat exchanger according to a fifth embodiment
• FIG. 7 shows the direct heat exchanger according to a sixth embodiment
• FIGS. 9a and 9b show the direct heat exchanger applied to an ORC system according to two alternative configurations ;
• FIG. 10 shows the direct heat exchanger applied to an ORC system provided with a three-way valve
• FIG. 11 shows the direct heat exchanger applied to an ORC system provided with a separator.
• the heat exchanger 100 is configured so as to have a more compact encumbrance in height compared to known embodiments.
• the heat exchanger has a vertical axis and works, as in other known embodiments, in countercurrent with the organic fluid at the inside of the tube bundles 200, 201 and with the fumes outside.
• the heat exchanger comprises on the opposite sides of the axis, two portions: a first portion 101 conventionally also called “right portion” and a second portion 102 conventionally also called “left and portion.
• the term "high” means the edge of the heat exchanger provided with inlet and outlet of fumes.
• fumes enter from above of the left portion, run through the heat exchanger 100 downward along the second portion 102, up to the right side and then raise back along the first portion 101, coming out again at the top.
• the organic fluid enters the inlet manifold 103 at the top on the right side, runs in countercurrent the heat exchanger 100 and particularly to the right side 101, where it is collected in an intermediate manifold 104, placed downwards, and it is again distributed to the next section (second portion 102) and finally it is collected in the outlet manifold 105 to the left side in form of vapor.
• the inlet manifold 103 of the fluid is placed at the outlet of the fumes and the outlet manifold 105 of the fluid is placed at the inlet of the fumes, a "once-through" countercurrent heat exchanger is produced.
• the evaporation will preferably occur only in the second portion 102 of the heat exchanger, so as to exploit the natural tendency of vapor to rise upwards of the heat exchanger, due to the lower density of the vapor with respect to the liquid.
• the heat exchanger shown in Figure 3 is configured in an equivalent inlet and outlet arrangement of fumes (with a 90° rotation with respect to a vertical axis) .
• the two portions 101, 102 of the heat exchanger and of the respective tube bundles 200, 201 can have a number, a pitch and a different geometry of the tubes 106.
• the tubes 106 could be made with greater through sections in order to limit the crossing speed of vapor, which has a significantly lower density than the liquid.
• the first portion 101 in contact with the hotter fumes has a first heat exchange section of a radiation instead of a convection type, made for example with a arrangement of tubes 106 parallel to the flow of fumes and with a wall structure 108 made by means of tubes 106 connected together by flat surfaces 107.
• Figures 2 to 4 show the heat exchanger configurations permitting the numerous benefits not only in terms of encumbrance .
• 201 can occur through two intermediate manifolds placed at the base of each section 101 and 102, or a single intermediate manifold 104.
• the exchange tubes 106 are distributed between the two portions of the heat exchanger so that the last row of tubes 110 of the inlet portion 101 of the organic fluid are placed at the same height of the first row of tubes 111 of the next portion.
• the heat exchanger 100 therefore has a single intermediate manifold 104. This single manifold can also be exploited for installation of a single drain line of the heat exchanger.
• the intermediate manifold 104 can also be entirely absent and in such case the heat exchanger would take the configuration of Figure 7 with the consequent tube distribution: each tube of the tube bundle 200 exits from the portion 101 and continues through the portion 102 without any intermediate manifold, up to the outlet manifold 105.
• the two portions 101, 102 are further placed apart in order to allow to perform installation and maintenance phases.
• the presence of two separate portions 101, 102 of the direct exchanger 100 facilitates the application of methods for controlling the temperature of the organic fluid films, which must remain below a certain limit in order to avoid phenomena of thermal chemical decay 20. Obviously, such control must be targeted in order to limit the film temperatures in the most critical point of the heat exchanger, occurring in the first rows of tubes (starting from upwards) of the left portion 102 in Figures 2-3 or in the highest portion of the tubes of the walls 108 in Figure 4.
• ORC fluid therefore, it present in vapor phase (as it is well known, heat is exchanged with lower exchange coefficients with respect to the liquid phase) and finds outside the fumes at the highest temperature.
• the preheated fluid then passes into the first portion 101 of the heat exchanger and the runs through the same as already described in the previous cases.
• Another possible configuration employing heat exchange banks is represented in Figure 6, where the organic fluid initially passed through the first portion 101 up to the intermediate manifold 104', from where it passes into the second portion 102 but enters at an intermediate level 125 (not necessarily coincident with the intermediate level 124) and running along the rows of tubes 106 upwards (and then to the inlet area of the fumes, or where the fumes are warmer) . Finally, the organic liquid is brought downwards to the manifold 104 and runs along the remaining part of the left portion until exiting from it (into the manifold 105) at a level positioned at an intermediate height. In this way the most critical area of the heat exchanger is not exclusively run by vapor, but by the organic fluid in biphasic conditions (the vaporization being just at the beginning) or by a fluid still in the liquid state.
• the direct exchanger 100 is connected to a ORC system, comprising a pump 2, a condenser 4, a turbine 5, an electric generator or other mechanical users, a possible regenerator 3 (provided schematically in Fig. 8) and is supplied by the entire flow of organic fluid worked by the pump 2.
• a possible regenerator 3 provided schematically in Fig. 8
• the first portion 101 of the heat exchanger is supplied only by a fraction of the flow 112, 113 worked by the pump 2, as another portion of the same is introduced in the intermediate manifold 104. Said portion can be taken downstream (Fig. 9a) or upstream (Fig. 9b) of the regenerator 3.
• the flow rate 113 worked by the pump 2 is divided in a first section 112 and a second section 114 which bypasses the first portion 101 of the heat exchanger. Whether the subdivision of the pump flow rate is made upstream of the regenerator 3, or downstream of the same regenerator, the flow rate percentages of the pump are controlled by means of respective first valve 112' and second valve 114'.
• Such configuration permits to reduce the maximum film temperature reached by the organic fluid.
• the portion of flow rate 113 bypassing the first portion 101 of the heat exchanger 100 will remain at the pump outlet temperature or the exit temperature of the regenerator.
• the remaining portion flowing into the first portion 101 having a lower speed, recovers less power from the fumes but slightly increases its outlet temperature.
• a flow rate of fluid will be reached entering the second portion 102 at a lower temperature with respect to the configuration shown in Figure 7.
• the outlet temperature of the organic vapor will be the lower, as higher the bypassed flow rate.
• the configuration shown in Figure 8 can be applied during normal operation or only in particular situations, for example if an excessive temperature of the organic vapor is registered.
• FIG. 10 A further functional configuration in order to counteract sudden increases of temperature is shown in Figure 10.
• the organic fluid through a three-way valve 115' or more valves, partially or totally bypasses the regenerator 3, and then enters at a lower temperature in the direct heat exchanger 100 and consequently reaches a lower maximum temperature.
• Such solution can be adopted singly or in combination with the previous ones.
• a separator 116 shown in Figure 11 in a simplified way: vapor is directed to the turbine or to the bypass duct, whereas the liquid fraction is deposited at the base of the separator.
• the latter is discharged to the condenser by acting on a valve 117, which is periodically opened or left always at least partially open, depending on the liquid flow rate reaching the separator, or is left open according to a control of level 118 in the separator 116.
• the separator 10 can be realized with a specially designed tank or more simply as a tube with increased diameter .
• a further advantage of the configuration with two portions regards the fouling of the exchange surfaces.
• the reversal of the fumes flow at the base of the heat exchanger 100 is useful for reducing the content of dust and other solid particles which may be contained in the gaseous flow: in fact, the fumes change their direction with an angle of substantially 180°.
• the heaviest particles have difficulties to follow the flow as they tend to impact against lower surfaces and deposit by gravity.
• the inversion zone can be realized as a hopper, to permit the extraction of powders using tools such as clapet valves or screw feeders and with an increased section, with respect to the passage section at the outlet of section 102, so as to reduce the gas speed and facilitate the deposit of the powders.
• powder deposits tend to collect, which are removed through the cleaning systems of the tubes, made for example of compressed air lances.
• the intermediate removal of bigger particles reduces fouling of the second portion of the heat exchanger (which could be realized with finned tubes, non usable in the first section in case of very dirty fumes), in addition to reducing the powder load to be treated downstream with sleeve filters, electro-filters or other removal means.
• the subject heat exchanger can also comprise a plurality of sacrificial tubes, or tubes or sheet metal made of high-temperature resistant material, which are not crossed by the organic fluid. These sacrificial tubes can be installed in front of the first row of tubes, which is exposed to the hottest fumes. These sacrificial tubes act as a shield with respect to the first rows of heat exchange tubes, so reducing the erosive effect due the impact of powders and contribute to a more uniform distribution of temperatures.
• the heat exchange tubes can be arranged in line or staggered rows, with outer diameters which may vary approximately between 38 mm and 60 mm.
• the arrangement and the distance among tubes is chosen in function of the temperature and the level of powders contained in the gaseous flow.
• the Applicant has verified the convenience of adopting a gap between a tube and another one approximately comprised between 10 and 50 mm; greater distances will be applied in case of fumes with a high powder content. In case that finned tubes are used, such gap must be considered between the ends of the fins of adjacent tubes.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Heat-Exchange Devices With Radiators And Conduit Assemblies (AREA)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

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Family Cites Families (4)

• 2016
  • 2016-02-25 IT ITUB2016A000984A patent/ITUB20160984A1/it unknown
• 2017
  • 2017-02-22 WO PCT/IB2017/050993 patent/WO2017145057A1/en active Application Filing
  • 2017-02-22 EP EP17711334.7A patent/EP3420204B1/de active Active

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