IMPROVED TRIPLE EFFECT ABSORPTION CYCLE APPARATUS
Background of the Invention Concerns about the environmental impact of fluorocarbon and hydrofluorocarbons on the environment call for expande uses of environmentally sound refrigerants, such as water o ammonia. For commercial chiller applications in the capacit range of 15-10,000 refrigeration tons with little heatin requirements, water is usually the preferred refrigerant du to its non-flammable and benign nature. Aqueous absorption fluid cycles taking advantage of suc refrigerants have been known and used for many decades Single effect and various two-stage designs are commerciall used in many countries. However, the increasing concern abou the overall C0 2 generation in the process of converting fossi fuels into energy used for air conditioning or refrigeratio calls for higher energy conversion efficiencies than currentl obtained with single stage (COP = 0.6 to 0.8) or two-stag (COP = 0.9 to 1.25) absorption equipment.
U.S. Patent No. 4,732,008 teaches the use of two singl stage cycles coupled to achieve three refrigeration effects The independent loops carry different absorption fluids wit the lower stage using fluids such as aqueous LiBr solutions However, the needed upper stage fluid crystallization an vapor pressure suppression properties require the use of different fluid. Performance estimates lead to COPs of 1.5 t 1.7 if suitable upper stage fluids are used.
Summary of the Invention The present invention is for an apparatus having thre refrigeration effects in which either a single aqueou absorption fluid or two or three different absorbents o different absorbent concentrations with a single refrigerant i.e. water, are used throughout the system. Thus, becaus such a single refrigerant, water, is used as the refrigeran or working fluid solvent throughout the system, regardless o the stage, the invention offers a substantial improvement ove
prior art triple effect systems which rely solely on hea transfer coupling of three heat exchangers with no common mas flows. The apparatus and systems of the invention result i COPs similar to those used in the aforesaid two single stag cycle triple effect apparatus. However, because pea operating temperatures are lower than in such a dual loo system, the temperature lift and fluid crystallizatio requirements of the fluid composition in the third stag generator are reduced. These as well as other advantages o the apparatus in the system will be evident in the followin description.
Brief Description of the Drawings Fig. 1 shows a conventional double effect absorptio cycle phase diagram of the type typically resulting rom a aqueous LiBr absorption working fluid;
Fig. 2 shows a triple effect absorption cycle phas diagram according to one embodiment of the present invention;
Fig. 3 is a phase diagram of a system of the invention incorporating three evaporators operating with three absorbers;
Fig. 4 shows another phase diagram according to an embodiment of the invention using a different routing for the working fluid through the three generators;
Fig. 5 is a schematic illustration of a three generator, three absorber system configuration according to the invention using three separate absorption fluid loops;
Fig. 6 schematically illustrates another system of the invention incorporating three generators and three absorbers with a single absorption fluid loop; Figs. 7 - 10 schematically illustrate different embodiments of three generator, one absorber system configurations of the invention;
Figs. 11 and 12 schematically illustrate examples of three generator, two absorber system embodiments according to the invention.
Detailed Description of the Invention As previously noted. Fig. 1 shows a conventional double
affect absorption cycle using a typical aqueous LiBr workin fluid solution. Condenser heat generated by condensing wate vapor from the high temperature generator G H is used to driv the lower stage generator G L , which in turn releases wate vapor which is condensed with conventional cooling means, suc as evaporative condensers, cooling towers or air coolers. Th firing temperature FT is typically between about 300 β F-380°F, the absorber temperature A is typically between about 80° 110 β F, evaporator temperature between about 40°F-45 β F, and lo temperature condenser C L is between about 80-110°F.
A phase diagram of a triple effect absorption cycle o the invention is illustrated in Fig. 2. The cycle show illustrates three generators, high temperature, mediu temperature, and low temperature, G 3 , G 2 , and G 1f respectively, and high, medium, and low temperature condensers, C j , C 2 , G,, respectively. Firing temperatures FT are typically betwee about 400 P F and 520°F, and minimally between about 390 β F an 420°F. A schematic illustration of an apparatus of th invention having such an absorption cycle is illustrated i Fig. 8. In such an apparatus, a high temperature, third stag generator G 3 is used to generate water vapor of sufficien pressure and temperature to condense water refrigerant suc that the heat of condensation can be used to drive the mediu stage generator G 2 , which in turn generates vapor to b condensed at sufficient temperature to drive the lower, firs stage generator G-, which in turn generates refrigerant vapo which is condensed with conventional cooling and hea rejection means. The heat exchange between condenser C-, an generator G ≥ can be achieved with phase change heat transfe using an appropriate phase change heat transfer fluid capabl of operating in the approximate temperature range of 300°F t 400 β F. Alternatively, a pumped loop of heat exchange flui for sensible heat transfer may be used. Similarly, heat exchange between condenser C 2 and generator G 1 may be achieve with either a phase change heat transfer in the approximate temperature range of about 150°F and 275°F or by a pumped loo for a heat exchange fluid. The aforesaid temperature ranges
are approximate, and will depend on the thermal load on th equipment, as well as ambient reject temperatures which chang during the day as well as the season. Typical reject temperature equipment designs are in the range of 90 β F to 95°F for water cooled equipment, and approximately 25 β F higher for air cooled systems.
Because a single refrigerant, water, is used in the aqueous absorption fluids of the invention, there is provided a substantial number of different apparatus configurations, using different absorption fluid loop options as are illustrated in the apparatus examples of Figs. 5 - 11. Fig. 5 illustrates schematically an apparatus embodiment of the invention incorporating three absorber-generator pairs and using three separate aqueous absorption loops between each pair. Thus, fluid loop 11 directs aqueous absorption fluid between first stage absorber 14 (A,) and generator 12 (G. | ) , loop 21 between second stage absorber 24 (A g ) and generator 22 (G 2 ) , and loop 31 between third stage absorber 34 (A 3 ) and generator 32 (G 3 ) . Heat exchangers 18, 28, and 38 in each of the aforesaid working fluid loops heat the relatively dilute salt solution pumped from the absorbers to the generators, and cool the relatively concentrated solution directed from the generator back to the absorber. The specific fluid loop couplings between the absorbers and generators are not limited to those as shown and any absorber could be coupled with any generator. Moreover, any of the three evaporators E lf E 2 and E- j could communicate independently with condenser G,, or could be coupled together, as shown.
The generator/condenser heat couplings are quite important in the systems of the invention since the efficiency of a cycle depends substantially on effective heat transfer to the lower stages. Accordingly, it may be advantageous to use a heat transfer fluid loop 41 in which a heat transfer fluid is routed for being successively heated by medium stage condenser C 2 followed by high temperature condenser C j , and successively cooled by generator G 2 and generator G Excess heat available from heating means for driving generator G 3 may
also be introduced into the fluid loop by heat exchang conduit 43.
The operating temperatures of third temperature stag generator G 3 are higher than temperatures currently used i double effect equipment. While direct fired high temperatur generator heating may be more economical, occurrence of ho spots on the generator surface in contact with the absorptio fluid are to be avoided to prevent corrosion acceleration an an increase in material incompatibility. Accordingly indirect heating whereby burner flames are not in contact wit the high temperature generator, such as using a phase chang or pumped fluid loop, may be preferred. Moreover, regardles of the method of heating generator G 3 , any remaining energy o sensible heat below G 3 temperature which is unavailable fo heating generator G 3 , is advantageously used for combustio air pre-heating, or, for being directed to one or both of th lower stage generators. Thus, if a pumped fluid loop is use to provide generator G 3 with energy, it may be combined wit a pumped loop linking the lower generators and condensers, a previously described, or combined to link one of th communicating condenser/generator components, leaving th other generator/condenser set with an individual heat transfe loop. Moreover, although it is thermodynamicall disadvantageous to operate a fluid loop at lower temperature than necessary with heat transfer fluid reheating requirement using high second law availability heat, for example, ga combustion heat or high pressure steam, the aforesaid loo routing may simplify hardware needs and pump requirements an therefore have a cost advantage. Excess heat may also be use to provide hot water heating as commonly provided in curren chiller-heater systems.
The absorber and evaporator portions of the systems o the invention may either be single or multiple units respectively. The use of multiple evaporators and multipl absorbers as illustrated in the apparatus of Fig. 5 and th cycle diagram of Fig. 3 is particularly advantageous if flui crystallization limits endanger safe operation of the cycle i
the liquid solution field. In the embodiments illustrated, the salt concentration increases as the temperature level increases. Thus, salt concentration of absorber A 3 is low compared to absorber A g , which is also relatively low compared to absorber ,. Likewise, the respective absorber operating temperatures are also higher in the higher concentration absorbers. Additionally, the evaporators are operated at different temperatures, with the highest temperature evaporator E, cooperatingwith absorber A 1 handling the highest absorbent concentration fluid, and similarly, the lower temperature evaporators accordingly communicating with the more dilute solution absorbers. The use of different evaporator temperatures, typically in the range of about 37 β F to 60°F, requires proper routing of the heat transfer fluid used to provide cooling to the load. For example, if a building is cooled with a cold water distribution system, often referred to as chilled water loop, the return flow heated by the building load first enters the highest temperature evaporator heat exchanger, and is then routed to successively lower temperature evaporators. Although the system of the invention incorporates up to three evaporators, other apparatus configurations using different absorber/evaporator pairs may be used without increasing the number of generators by using more than one evaporator to communicate with a generator via one or more absorbers. However, excessive equipment costs may dictate a practical limit to be reached with two to three evaporator temperature levels. The system shown may also be modified by providing direct flow between C 1 and either or both evaporators E 2 and E- j rather than only with E,. It may also be advantageous to operate multiple evaporators at substantially the same temperature if component location in the equipment is an important factor, which may be particularly advantageous in multi-zone buildings with substantially the same working temperature requirements. Moreover, the use of multiple absorbents is not limited to absorber operation with different absorbent concentrations or different operating pressures or
different operating temperatures.
Because a single refrigerant is used in the system of th present invention, a substantial variation in proportions o fluid flow between the generators and the one or mor absorbers is available, with different flow selection option available depending on the operating conditions, loads an specific temperatures encountered. For example, the hig temperature generator G 3 may generate sufficient refrigeran to condense at C j such that the latter can feed G 2 as well a portions of G, requirements. Moreover, if C^ energy i insufficient to drive generator G 2 alone, flue gas from th combustion system, or heat transfer medium passing throug generator G 3 exhaust may be used. The aforesai configurations are understood to be only examples of differen types of various design options and are not intended to limi the scope of the invention.
Fig. 6 illustrates a system using a single absorben fluid loop with three absorbers, communicating in series. Th advantage of this apparatus configuration over that shown i Fig. 5 is the requirement for only a single absorption flui loop pump. With the three absorbers coupled in series alon the single absorption fluid loop, as shown, the sal concentration is successively decreased between A,, A j and A j Another useful alternative is to connect the three absorber in parallel using a conduit from each absorber drawin absorption fluid concurrently from all three absorbers to single pump supplying, the fluid to G 3 . Again, although thre evaporators are shown, only one or two need to be used, a well as their coupling options, as previously explained. Fo a loop that has to operate directly between one or mor absorbers and a third stage generator, a two stage pump, o two single stage pumps operating in series, may be desirabl because of the higher than usual pressure different betwee those components. Fig. 7 illustrates an embodiment of the system of th invention in which the absorption fluid flow is route successively through generators G,, G 2 , and G 3 starting wit
the lowest absorbent concentration leading to the highes concentration using a solution loop 44 and requiring thre pumps 45, 46 and 47 in the loop between the single absorbe and all three generators. Alternatively, two absorbers may b used with a single solution loop feeding generator G 1 operating in parallel with similar refrigerant concentrations In such a system, one evaporator for the two or more absorber or two evaporators for two or three absorbers, or on evaporator for each of the absorbers, may be used. Thus regardless of the number of absorbers, two or three loops with two or three pumps, may be used, such that each generato is fed with its individual solution f ow, or two loops may b used for three generators, or the return flows combined.
Fig. 8 illustrates yet another useful apparatu configuration using a single absorber and a single pump 45 fo absorption fluid loop 44. Figs. 9 - 11 are simplifie illustrations of other fluid loop alternatives. In Figs. 9 and 10, a single absorber and single fluid loop is provided, while in Fig. 11, two absorbers are used with two fluid loops. The fluid loop solution heat exchangers have not been shown, for simplicity, nor have the condensers, which cooperate with each generator, nor the evaporator(s) . Splitting the aqueous solution circuits has the advantage that the refrigerant concentration change generated in each generator can be designed for being optimized for each stage and does not have to take into consideration other design criteria. Figs. 9-11 illustrate different embodiments of splitting the solution circuits. In Fig. 9, the fluid is routed such that it is directed from the absorber to generator G 2 prior to G 3 , which has the advantage that a heat and mass transfer additive present in the solution, which is possibly not stable at generator G 3 temperatures, can be easily flushed off or separated in generator G 2 . However, such an option requires an additional pump for the solution to generator G3, which operates at a higher pressure. Yet another alternative is illustrated in Fig. 10 in which the solution is pumped from the absorber first to generator G 2 , and then successively to
G 1 and then finally to G 3 . Yet another alternative i illustrated in Fig. 11, in which two absorbers A, and A j ar used, each having a separate circuit 33 and 35, respectively, for directing working fluid to and from three generators G 1# G 2 and G 3 . Again, the flows between the generators may b selected such that the solution may be routed through th three generators, and split between the two absorbers.
Fig. 12 illustrates another apparatus alternative usin two absorption fluid loops and two absorbers. A,, which feed intermediate and low stage generators G 2 and G, along a firs loop, and A j feeding high temperature generator G 3 along second loop. The temperature ranges of the absorbers ma overlap as illustrated by the dotted lines between th absorbers. One or two evaporators may be used in such configuration. By proper use of evaporator/absorber pairin the Δ P, i.e., the evaporator pressure minus the absorbe pressure, will be sufficient to result in a relatively rapi sorption at low salt concentration, with the advantage that the need for a heat and mass transfer additive may not be required in the high temperature loop. A second evaporator may also be used, as shown with high pressure evaporator E H feeding A 2 and low pressure evaporator E L feeding A,. It should also be appreciated that the specific absorber- generator absorption fluid loops shown illustrates only one example of such a loop, and a number of other combinations of absorber-generator, two loop configurations evident to those skilled in the art may be used. Fig. 5 and 12 show embodiments using separate solution loops. such configurations can take advantage of the common refrigerant, water, and the separate solution cycles by using different absorbents or different absorbent concentrations in different solution cycles. The cycle of Fig. 12, if operated with two evaporators and two absorbers can take advantage of using the higher temperature evaporator in conjunction with the absorber connected to the high temperature generator G 3 such that the absorbent concentration in its solution is relatively low, the
operating and differential pressure between this evaporator and absorber is relatively high, and resulting heat and mass transfer in the absorber is improved, and may eliminate the need for a heat and mass transfer additive in the high temperature solution cycle. It is known that such conditions yield higher absorber heat and mass transfer rates. In general, separate solution loops allow for use of different absorbents or different absorbent concentrations as long as the same refrigerant is used. Solution heat exchange may be accomplished with conventional tube or plate heat exchangers and maximum efficiency reached by optimizing the temperature ' approach between entering and exiting solutions. It will be understood that in any of the system configurations within the invention, such a solution heat exchange between the generators and absorber or absorbers is to be used. If multiple absorption fluid loops are used, solution heat exchange may not be limited to heat exchange within each loop, but may incorporate energy exchange between different fluid loops. For example, high temperature fluid heats lower loop fluid after the higher temperature fluid performs its main function to preheat the medium temperature fluid. Excess heat may be available from cost driven design trade-offs in the solution heat exchangers as well as from the difference in specific heat capacity and mass flow between the concentrated and dilute refrigerant solutions entering or exiting the generators, respectively, as well as the exit generator temperatures.
As previously described, a single refrigerant, water, is used in the aqueous absorption fluid throughout the apparatus of the invention, regardless of the absorber-generator groupings or absorber fluid loop or loops and is used in all stages of the triple effect cycle carried out in the system. However, different salts or combination of salts, or different concentrations of the same salts, can be used in the different fluid loops. The aqueous absorption fluids which may be used in the present invention comprise aqueous solutions of LiBr,
LiCl, Lil, LiN0 2 , LiCNS, and LiC10 3 and mixtures thereof Preferred fluids operating in the high stage generato comprise LiBr - LiCNS, LiBr - Lil, LiBr - LiC10 3 , LiBr - LiN0 2 LiCl - Lil, LiCl - LiN0 2 and LiCl - LiC10 3 . Other usefu fluids are aqueous mixtures of one of the group LiBr, LiCl an Lil, together with a second salt of the group Ni(N0 3 ) 2 , CaBr 2 FeCl 2 and Mnl 2 . Another useful salt group is ZnBr 2 combine with CaBr 2 . Suitable concentrations of LiBr, LiCl or thei mixtures are between about 58% and about 68% ±2%, by weight while in the third stage, a lower concentration of LiBr, abou 55% or less, is used. The remaining salts may be used in an stage in concentrations of between about 40% up to about 75% However, the high concentration will be limited by the sal crystallization limit. Yet another useful salt grou comprises NaOH, KOH, or mixtures thereof. Suitabl concentrations of about 40% up to the crystallization limi may be used, and where mixtures are used, relative proportion of between 40% and 60% NaOH and 60-40% KOH respectively, ar preferred. Where LiBr, LiCl or mixtures thereof are used i stage one or in stages one and two, in stage three, a lowe concentration of LiBr, or any of the other described salts o combinations are preferred.
Lithium corrosion inhibitors are especially useful wit the aforesaid lithium salt compositions. Suitable corrosio inhibitors, include for example, lithium molybdate, lithiu nitrate or lithium chromate. Ph adjustments may be made, fo example, using LiOH. Because of the high temperatures and sal concentrations of the absorption fluids in the third stag generator it may be desirable to use corrosion resistan components or materials. Thus, for example, nickel-chromiu alloys or nickel-copper or other non-ferrous alloys fo construction of the high stage generator are preferred.
It is also desirable to use heat and mass transfe additives in the aqueous absorption fluids. Particularl useful additives include alcohols having between about 6 an about 10 carbon atoms, for example, 2-ethylhexanol and n
octanol. Aliphatic and aromatic amines such as nonylamine o benzylamine or its derivatives may also be used. Effectiv concentration ranges are from about 10 parts per million up t about 2000 ppm. It is desirable to separate the heat and mas transfer additive from the aqueous absorption fluid prior t entering the third stage generator. The heat and mas transfer additives are only slightly soluble or are insolubl in the aqueous salt solutions, and thus form a second phase typically floating on the heavier aqueous solution highl desirable in the absorber(s) . However, because thes additives are not normally stable at high temperatur generator temperatures, the use of a mechanical separator o skimmer, or other means for separating these heat and mas transfer additives prior to entry into the high temperatur generator is desirable. For example, in Fig. 7, a mechanica skimmer 40 is illustrated which provides such separation wher the aqueous solution is directed from lower temperatur generators to high temperature generator G 3 . Alternatively such separation may be achieved by providing an accumulatio chamber which avoids solution pumping from the surface wher the heat and mass transfer additive collects. Another mean for separating the additive is by use of a flash chamber i the lower temperature generator G 2 , or in such a chambe located along the fluid loop prior to entering the hig temperature generator. For example, observing also Figs. 9 11, by routing the absorption fluid such that it passes through generator G 2 prior to G 3 , has the advantage that the heat and mass transfer additive can be easily flushed off fro generator G 2 . This option, however, does require an additional pump for pumping solution to generator G 3 which operates at higher pressure. Regardless of the type of separator used, it should preferably reduce the amount of additive present in the aqueous absorption fluid to about or substantially to the solubility limit of the additive. Means should also be provided for returning the separated additive into the fluid in or just prior to entry into the absorber. Thus, as shown in Fig. 12, the separator 50 may be provided
with a return conduit 51 for supplying the additive separate from the fluid prior to entry into the high stage generator back to the loop after it leaves G 3 .
It may also be desirable to incorporate a conventiona purger in the system of the invention, for removing air o other non-condensable gases in the aqueous absorption flui loop. Such equipment and its use in absorption systems i well known to those skilled in the art. The systems may als be designed using a heat and mass additive transfer reservoi and means for introducing the additive into the absorber(s) Because of gradual decomposition of the additive over time means for periodically injecting a metered replacement amoun of the additive to maintain suitable concentrations in th fluid in or prior to the absorber(s) will also be preferred This requires that the heat and mass transfer additiv decomposition products are purgable as is the case, fo example, with 2-ethylhexanol.
Suitable heat transfer fluids used for heat transfe between components of the apparatus include water, hea transfer oil, Dowtherm® fluids, water/glycol mixtures, etc If the high temperature generator is steam fired, th condensate may be used for heating purposes at lowe temperatures in the system.
The significant advantage of the cycle and system of th present invention is in the requirement of only a singl refrigerant allowing for heat transfer and mass exchang coupling. The result is that no working fluid is require having an evaporator/absorber temperature lift of 13D°F fro about 50°F evaporator temperature to about 180°F solutio equilibrium temperature. Moreover, the lowest usefu operating temperature will be lower than required in a dua loop cycle since the temperature spread required betwee condenser and generator is proportional to the spread betwee the evaporator and absorber. Moreover, the highes temperature stage portion of the present invention operate with conventional evaporator/absorber temperature lifts o between about 45°F and about 90°F, and the highest stag
condensing temperature needs only to be sufficient to drive the second stage generator, which could operate as low as 300°F. Such conditions lead to minimal generator temperature requirements for the third stage of between about 390°F and 420 β F, depending on operating conditions and heat exchange surfaces used, which is lower than expected for a dual loop triple effect system which is currently estimated to be between about 440°F and 460°F at the lowest. These as well as other advantages of the system will be evident to those skilled in the art.
