Title of Invention

AMMONIA ABSORPTION REFRIGERATION SYSTEM WITH IMPROVED COEFFICIENT OF PERFORMANCE

Abstract

An ammonia absorption refrigeration system with improved coefficient of performance comprising heat source; means for generating ammonia vapours at atleast two desired pressure levels from the refrigerant ammonia by generating cooling effect in which one temperature being upto -60 °C; means for absorbing the said ammonia vapours with aqueous ammonia solution(s); means for generation of ammonia vapours at higher pressure level containing moisture from the said aqueous ammonia solution; means for controlled enrichment of ammonia with step wise reduction of the moisture content of the vapour; and means for rectification of the enriched ammonia vapours and condensing the vapours for generation of liquid ammonia refrigerant at controlled moisture concentration, generating ultra pure ammonia containing moisture up to 5ppm characterized in that the said system is adapted to provide refrigeration up to -60°C with ultra pure ammonia.

Full Text

FORM 2 THE PATENTS ACT 1970 COMPLETE SPECIFICATION (See Section 10)Rule 13 TITLE Ammonia Absorption Refrigeration System with Improved Coefficient Of Performance APPLICANT (S) Department of Atomic Energy, Government of India, Anushakti Bhavan, Chhatrapati Shivaji Maharaj Marg., Mumbai - 400 001, Maharashtra, India. & Thermax Ltd., D-13, MEDC Industrial Area Chinchwad, Pune -- 411019 Maharashtra, India The following specification particularly describes the nature of the invention and the manner in which it is to be performed. Field of Invention: This invention relates to the ammonia absorption refrigeration (AAR) system and more particularly to the system where refrigeration is required at one or multiple temperature levels. This invention relates particularly to the generation of ultra pure refrigeration grade ammonia (upto 5ppm moisture) for use in very low temperature applications (in the range of -60°C).- With this invention, the Coefficient of Performance (COP) of the •^ammonia absorption system is improved. Background of Invention: Many processes in various industries like pharmaceuticals / fertilizers / food processing etc. require cooling at sub-zero temperatures, which use heat as primary energy source. Refrigeration implies maintenance of a temperature below that of the surroundings. This requires continuous absorption of heat at a low temperature level, usually accomplished by evaporation of liquid. The vapour generated may be returned to its original liquid state for re-evaporation in two ways: i) it is simply compressed and then condensed (Vapour-Compression Refrigeration) or ii) it is absorbed by a liquid of low volatility and then distilled at higher pressure (Absorption Refrigeration). In vapour-compression refrigeration the work of compression is usually supplied by an electric motor. The source of electric energy is a heat engine used to drive a generator. Thus, the work of refrigeration comes ultimately from heat at higher temperature level. This suggests that direct use of heat as the energy source of refrigeration. The absorption refrigeration uses direct heat energy as the energy source for refrigeration. Ammonia absorption technology is normally characterized by low COP's and is viable when sufficient heat is available for driving the absorption cycle. Thus, the viability entirely depends on cost of the primary energy in the form of low/ medium pressure steam or hot water. COP is heat units withdrawn from external coolant load cycle [that is units of refrigeration achieved] divided by heat units input to achieve that refrigeration OR percent COP is COP x 100. In the absorption refrigeration (AR) system two complete cycles are in operation: a refrigerant cycle and an absorption cycle. Refrigerant coolant cycle external to this system is supported by this system or AR system is developed for supporting external coolant cycle requirements. Ammonia absorption refrigeration (AAR) is a specific example of absorption refrigeration where the vaporizing refrigerant is ammonia and the solvent is either water or dilute aqueous ammonia. The greatest asset of the system is that it can be driven by any available process heat source, where temperature is high enough for the distillation step. A conventional, ammonia absorption refrigeration system is shown in Figure.l in the accompanying drawing. It has following main components: 1. A Distillation column with 2. A Re-boiler at the bottom and 3. A Condenser at the top 4. A refrigerant Tank 5. An Expansion Valve 6. An Evaporator 7. An Absorber 8. A Solution Pump 9. Solution Heat Exchanger The refrigerant vapour generated from evaporator (6) is absorbed in absorber (7) by a low volatile solvent - water at evaporator pressure. The refrigerant rich liquid solution from absorber (7) passes to a solution pump (8), which raises the pressure of the liquid to the condenser (3) pressure. Heat from the higher temperature source is transferred in re-boiler (2) to the compressed liquid solution in a distillation column (1) raising its temperature and evaporating the refrigerant from the solvent. The vapour is condensed in a condenser (3) and the solvent left in the distillation column (1) is returned as weak aqua to the absorber (7) after transferring the heat to the pressurised refrigerant rich strong solution from solution pump (8) in solution heat exchanger (9) for absorbing the refrigerant vapour generated from the evaporator (6). The condensed refrigerant is collected in refrigerant tank (4) and is expanded through an expansion valve (5) to the evaporator pressure and sent to the evaporator (6) for re-evaporation. The ammonia vapour generated in the re-boiler contains considerable amount of water vapour. This ammonia-water vapour mixture is rectified in the distillation column (1) to the required level of purity and then condensed in the condenser (3) to generate refrigerant liquid. The purification is accomplished by sending some of the condensed liquid as reflux to the distillation column (!)• Prior Art: Following patents describe prior art close to the present invention: 1. US 3817050 " Two-stage ammonia absorption refrigeration system with at least three evaporation stages", 1974, Alexander D; Muenger J, TEXACO INC The US patent 3817050 describes an AAR system with at least three evaporators for producing refrigeration at different temperature levels with the absorbers maintained at two pressure levels. The system is based on generation of ammonia vapours from the aqueous solution in a tower, as in a conventional system, and improving the purity of ammonia in a second tower. The ammonia at different purity levels is used as refrigerant to produce the specified refrigeration levels. The very high purity ammonia being produced from the second tower contains 0.01 % water. The cooling water used is mentioned as available at 60 ° F (equivalent to 15.6 ° C). The minimum refrigeration temperature claimed in the prior art is minus 57 ° F (equivalent to minus 49.5 ° C). The ammonia streams generated from the second tower are used in the evaporators for producing refrigeration. The ammonia vapours generated in the evaporators are absorbed in weak aqua and are pumped back to the first tower. In the US patent 3817050 a. Refrigerant ammonia is at purity levels of (i) not less than 0.01 % (equivalent to 100 ppm) moisture and (ii) more than 0.2 % (equivalent to 2000 ppm) moisture. This higher moisture level in the refrigerant ammonia will reduce the performance of the AAR system, as it will require high amount of bleed or purge stream to control the moisture levels in the evaporator. b. A very low temperature cooling water namely, at 60 F (equivalent to 15.6 °C) is required. This is not a generally prevailing cooling water temperature. c. The lowest temperature generated is minus 57 ° F (equivalent to minus 49.5 ° C) only. It cannot cool to temperature less than minus 49.5 ° C. 2. US 5231849 " Dual -Temperature vehicular absorption refrigeration system", 1993, Joel H. Rosenblatt. The US patent 5231849 describes an automotive ammonia absorption refrigeration system for providing comfort zone cooling and a lower temperature cargo zone cooling utilizing the waste heat from the automotive engine exhaust gases. The system described comprises components of a conventional AAR system. The ammonia is generated using the hot exhaust gas from the engine of a vehicle and air cooled condenser is used for condensing the ammonia vapour. The objective of this prior art is to facilitate development of a vehicle engine system with its associated AAR sub-system as an integrated design. The US patent 5231849 a. is AAR system for a specific application, i.e., for vehicular refrigeration using heat of the exhaust gas of the vehicle engine. b. is not a versatile AAR system c. is capable of providing refrigeration up to -17.7 °C depending on the application requirement. d. achieves this low temperature with high grade energy input e. has more than one absorber vessel, more than one refrigerant solution pump. f. has more than one evaporator vessel, more than one evaporator pressure-reducing valve Object: The principal object of the present invention is to provide an improved AAR system with high / improved COP by providing means for controlling residual moisture levels in refrigerant ammonia and means to generate refrigerant ammonia liquid streams with different moisture levels. Another object of the invention is to develop AAR system with higher COP, working with lower temperature heat source and higher temperature cooling system compared to prior art processes. Main object of the present invention is to overcome the limitations of the prior art and to develop an improved AAR system with high coefficient of performance (COP). One part of the principle object of the present invention is to design a rectification column and condenser system for providing different purity level refrigerant ammonia with at least one stream containing upto 5 ppm moisture, so as to provide refrigeration up to minus 60°C. Another part of the principle object of this invention is to explore and develop open loop system by injection of steam as a source for heat. An additional objective is to provide multiple temperature level refrigeration by withdrawing refrigerant ammonia, at one or multiple levels, from the rectification column together with generation of pure water from the bottom of the stripping section with very low ammonia content, typically less than 2 ppm, after efficient heat recovery with improved coefficient of performance (COP) therby operating the system in open loop with respect to absorbant water. A further objective is to provide one or multiple level ammonia absorption system with improved coefficient of performance. Summary of the Invention: Accordingly the present invention relates to an ammonia absorption refrigeration system with improved coefficient of performance comprising means for providing heat or heat source; means for generating ammonia vapours at atleast two desired pressure levels from the refrigerant ammonia by generating cooling effect in which one temperature being upto - 60 °C; means for absorbing the said ammonia vapours with aqueous ammonia solution(s); means for generation of ammonia vapours at higher pressure level containing moisture from the said aqueous ammonia solution; means for controlled enrichment of ammonia with step wise reduction of the moisture content of the vapour; and means for rectification of the enriched ammonia vapours and condensing the vapours for generation of liquid ammonia refrigerant at controlled moisture concentration, generating ultra pure ammonia containing moisture up to 5ppm characterized in that the said system is adapted to provide refrigeration up to -60°C with ultra pure ammonia. The said system comprises boiler, a stripping column, atleast one partial condenser, a rectification column, a total condenser, atleast two evaporators, atleast one absorber, atleast one solution pump, atleast one solution heat exchanger, atleast one sub-cooler and atleast two expansion valves and all these parts are connected such that the refrigerant vapour generated in the said evaporator is absorbed in a weak first aqueous solution of ammonia keeping pressure in said absorber same as that in said evaporator; the said refrigerant rich liquid solution from said absorber passing to the said solution pump thereby raising the pressure of the said liquid to that of pressure of the said evaporator; absorbing the refrigerant vapour generated in said evaporator in to the rich second aqueous solution of ammonia from said absorber at pressure of said evaporator in said absorber; passing the strong third ammonia solution from said absorber to said solution pump; raising the pressure of the liquid to that of the said total condenser pressure; heating a part of the strong aqueous third ammonia solution from said solution pump in said solution heat exchanger using the heat of said first weak aqueous solution from the bottom of said stripping column and then feeding to the top of stripping column; providing re-boiling at the bottom of the said stripping column by heat source like steam in said re-boiler; passing the ammonia-water vapour from the top of said stripping column into one or more said partial condensers in series ; recycling the partially condensed liquids being fourth and fifth solutions predominantly rich in water from the said partial condensers into the said stripping column; feeding ammonia vapour containing little moisture to the said rectification column, wherein it is rectified by using the reflux from the top ; condensing the ultra pure ammonia vapour ( means for generating ammonia vapours at atleast two desired pressure levels from the refrigerant ammonia by generating cooling effect in which one temperature being upto -60 °C; means for absorbing the said ammonia vapours with aqueous ammonia solution(s); means for generation of ammonia vapours at higher pressure level containing moisture from the said aqueous ammonia solution; means for controlled enrichment of ammonia with step wise reduction of the moisture content of the vapour; and means for rectification of the enriched ammonia vapours and condensing the vapours for generation of liquid ammonia refrigerant at controlled moisture concentration, generating ultra pure ammonia containing moisture upto 5 ppm. Detailed Description of the invention: First we shall see description of the preferred embodiment of the present invention and later see in what way the present invention differs from the prior art systems described above. The Ammonia Absorption refrigeration system of present invention is typically described in the enclosed Figure-2 in the accompanying drawing. The basic components shown in the Fig 2 are: (10): heat source (11): re-boiler, (12): stripping column, (13, 14): two partial condensers, (15): rectification column, (16): total condenser, (17, 18): evaporators, (19,20): absorbers, (21,22): solution pumps, (23): solution heat exchanger, (24, 26): sub-coolers, and (25, 27): expansion valves The refrigerant vapour generated in evaporator (17) is absorbed in weak aqueous solution of ammonia (solution 1) keeping pressure in absorber (19) same as that in evaporator (17). The refrigerant rich liquid solution (solution 2) from absorber (19) passes to a solution pump (21), which raises the pressure of the liquid to that of the second evaporator (18). The refrigerant vapour generated in evaporator (18) is absorbed in the rich aqueous solution (solution 2) of ammonia from absorber (19) at pressure of evaporator (18) in absorber (20). The strong ammonia solution (solution 3) from absorber (20) passes to a solution pump (22) which raises the pressure of the liquid to that of the total condenser (16) pressure. A part of the strong aqueous ammonia solution (solution 3) from solution pump (22) is heated in solution heat exchanger (23) using the heat of weak aqueous solution (solution 1) from the bottom of the stripping column (12) and then fed to the top of stripping column (12) and the other part is introduced directly to the top of stripping column (12). Re-boil is provided at the bottom of the stripping column (12) by heat source, like steam by indirect heating in a re-boiler (11) or by direct steam injection (in case of an open loop configuration). The ammonia-water vapour from the top of stripping column (12) enters the partial condensers (13, 14) in series. The partially condensed liquid predominantly rich in water (solution 4,5) from the partial condensers (13,14) is recycled to the stripping column (12). Ammonia vapour containing little moisture is fed to the rectification column (15), wherein it is rectified by using the reflux from the top. The ultra pure ammonia vapour ( The system aescribed in the present invention can reduce the ammonia content in the weak aqueous solution (solution 1) to very low, typically below 2 ppm and thus is capable of keeping the entire AAR system with water in open loop. Thus the present invention can also be used for providing refrigeration effect with water circuit in open loop. The heat input to the stripping column in case of steam can further be direct injection of steam with out any re-boiler (11). The use of partial condenser(s) as indicated in the above description improves the performance of the process by making more amount of refrigerant ammonia available for the refrigeration process. The use of one or several partial condensers will enrich the ammonia vapours by removing the moisture in a gradual manner and thereby providing a purer ammonia vapour with respect to moisture to the rectification column. The partial condensers cool the vapours coming out of the vapour generation section to temperature range between 45°C to 85°C and enable pre-purification with respect to water content before sent to the rectification column. The ammonia-water vapour from the top of stripping column, if fed directly to the rectification column, the amount of reflux liquid required to get high purity ammonia from the top of rectification column will be substantially high and thereby reducing the amount of refrigerant ammonia for refrigeration. The use of partial condensers in the process enables use of reduced quantity of reflux for the rectification column to enrich this pure vapour and also make it possible to use the refluxed liquid drawn from the bottom of the rectification column to be used as refrigerant. The two evaporators (17), (18) are at the decreasing temperatures of-10 to -15 °C and -25 to -32°C respectively. The strong ammonia solution (solution 3) concentration is in the range of 20 to 24% ammonia. The weak aqueous solution concentration is in the range of In one embodiment of the present invention, the heat source is steam circulated in the closed coil in the stripping column and the absorbent water is in closed loop mode. In another embodiment of the present invention, it is a direct open steam injection into stripping column and the absorbent water either in open loop or in partially open loop mode. In the open loop mode,the absorbent water is directly fed to either one or both the absorbers and the heat source steam is directly fed to the bottom of the stripping column and both are removed from the bottom of the rectification column. In the partial open loop mode of the system absorbent water is added to one or both the absorbers and is removed from bottom of the stripper and the heat source steam is not direct steam but through the re-boiler. In another embodiment of the present invention, the heat source is sometimes partly steam circulated in the closed coil and partly direct open steam injection into stripping column and the absorbent water either in open loop or in partially open loop mode. In closed loop mode when steam is circulated in the closed coil as a heat source, some times the heat source can be a direct firing arrangement, waste heat process streams, waste exhaust gases etc., instead of steam, circulated appropriately in a closed coil. When heat source is steam it can be high pressure / low pressure steam or even hot water. The means for generating ammonia vapours at atleast two desired pressure levels from the refrigerant ammonia by generating cooling effect in which one temperature being upto -60 °C comprises, atleast two evaporators (17,18), atleast one sub-cooler (24) and atleast two expansion valves (25,27) and connected such that the refrigerant vapour is generated in the said evaporators. The means for absorbing the said ammonia vapours with aqueous ammonia solution(s) comprise atleast one absorber (19) and it is connected with pumps. In one embodiment of the invention, the means for generation of ammonia vapours at higher pressure level containing moisture from the aqueous ammonia solution is by supplying external heat; passing the strong ammonia solution (solution 3) from said absorber (19) to said solution pump (22); raising the pressure of the liquid to that of the said total condenser (16) pressure; heating a part or full of the strong aqueous ammonia solution (solution 3) from said solution pump (22) in said solution heat exchanger (23) using the heat of weak aqueous solution (solution 1) from the bottom of said stripping column (12) and then feeding to the top of stripping column (12); providing re-boiling at the bottom of the said stripping column (12) by heat source like steam in said re-boiler (11). The means for controlled enrichment of ammonia with step wise reduction of the moisture content of the vapour is by passing the ammonia-water vapouf'from the top of said stripping column (12) into atleast one or more said partial condensers (13,14) in series and recycling the partially condensed liquid predominantly rich in water from the said partial condensers into the said stripping column (12). The means for rectification of the enriched ammonia vapours and condensing the vapours for generation of liquid ammonia refrigerant at controlled moisture concentration, generating ultra pure ammonia containing moisture upto 5 ppm comprise feeding ammonia vapour containing little moisture to the said rectification column (15), wherein it is rectified by using the reflux from the top; condensing the ultra pure ammonia vapour (upto 5 ppm moisture) from the said column top in the said total condenser ; refluxing a part of the said condensed liquid into the column (15) and sending the rest to evaporator (17); sending the said reflux provided for the top of said rectification column(15) with drawn from the column bottom as second source of refrigerant to said evaporator (18) via said sub-cooler (24) and said expansion valve (25). The System has heat source as direct steam injection to stripping column (12) and the absorbent water either in open loop or in partially open loop mode. The stripping column may be replaced with any other mode of vapour generator and partial condenser(s) provided in series or parallel between the said vapour generator and rectification column. Distinguishing features of the invention: The present invention has low energy input, low temperature heat input; higher cooling water temperature and yet lower temperature achieved in refrigerator- coolant cycle, by the AAR system; thus giving higher COP compared to the prior art processes. a. The intention of the present invention is to generate substantially purer refrigerant ammonia and thereby increasing the performance of the system. b. The present invention is based on cooling water temperature of not less than 30 ° C,. c. Whereas the present invention is for generation of temperatures up to minus 60 ° C (equivalent to minus 76 ° F) with improved COP. The present invention of improved AAR overcomes the limitations of the prior art (prior art 1) and produce refrigeration at very low temperature making use of purer refrigerant ammonia. The purer refrigerant ammonia is obtained by use of one or several partial condensers, which enrich the ammonia vapours by removing the moisture in a gradual manner. The present invention is characterized by the use of refrigerant ammonia having different moisture purity levels including that with moisture levels upto 5 ppm and there by yielding an improved process. The present invention includes specific features of use of partial condensers to control the moisture levels of ammonia and use of the reflux liquid used for the rectification column also for production of the refrigeration effect. It also covers production and utilization of very pure refrigerant ammonia for refrigeration leading to generation of very low temperature of minus 60 ° C enabling an improved process performance. EXAMPLES The invention will now be illustrated with the help of examples. The examples are for the purpose of illustration only and in no way to restrict the scope of the invention. Example I This describes one operation of refrigeration system of FIG. 2 in the accompanying drawing in the closed loop manner. In this operation heating was done by steam in the closed coils at 6 kg/cm2g and temperature of ammonia water solution in the reboiler was 160°C . The cooling water temperature was 30 ° C. Evaporators 1 & 2 were in decreasings temperatures of-32°C -15°C. The refrigerant ammonia concentration from the bottom of the rectification column is in the range 99.8% to 99.9% ammonia, while the refrigerant ammonia from the top of rectification column is very pure with moisture content of less than 5ppm in ammonia. The pressure level of the operation of the system was aboutl6 kg/cm for the ammonia regeneration section. Average COP of the system was calculated after running the system for a day and it was found to be over 0.5. Example II This describes one operation of refrigeration system of FIG. 2 in the accompanying drawing in the open loop manner. In this operation open steam was injected in the stripping column and temperature was 190 °C. The cooling water temperature was 30 C. Evaporators 1 & 2 were in decreasing temperatures of -32°C -15°C . The refrigerant ammonia concentration from the bottom of the rectification column is in the range 99.8% to 99.9% ammonia, while the refrigerant ammonia from the top of rectification column is very pure with moisture content of less than 5 ppm. The reject water from the stripping column bottom was The pressure level of the operation of the system was about 17 kg/cm2. Average COP of the system was calculated after running the system for a day and it was found to be over 0.55. Advantages of the Invention The present invention is advantageous due to improved performance of the system as a result of use of refrigerant ammonia with different moisture levels with atleast one of the streams with moisture levels up to 5ppm. It is also advantageous, as the' specific energy requirement of the process is lesser resulting in improved coefficient of performance. In accordance with this invention Ammonia absorption refrigeration system with one or multiple level refrigerant liquid ammonia withdrawal using one or more partial condensers has been found advantageously operable even with low grade energy source like waste heat of flue gases, low pressure steam etc. This system with improved COP especially for one or more temperature level refrigeration becomes quite attractive over conventional vapour compression refrigeration system. The foregoing description and accompanying drawing set forth the preferred embodiments of the invention at the present time. Various modifications, additions and alternate designs will, of course, become apparent to those skilled in the art in light of the foregoing teachings without departing from the spirit and scope of the disclosed invention. Therefore, it is to be understood that the invention is not limited to the disclosed embodiments but may be practices within the full scope of the appended claims. We Claim : 1. An ammonia absorption refrigeration system with improved coefficient of performance comprising heat source; means for generating ammonia vapours at atleast two desired pressure levels from the refrigerant ammonia by generating cooling effect in which one temperature being upto -60 °C; means for absorbing the said ammonia vapours with aqueous ammonia solution(s); means for generation of ammonia vapours at higher pressure level containing moisture from the said aqueous ammonia solution; means for controlled enrichment of ammonia with step wise reduction of the moisture content of the vapour; and means for rectification of the enriched ammonia vapours and condensing the vapours for generation of liquid ammonia refrigerant at controlled moisture concentration, generating ultra pure ammonia containing moisture up to 5ppm characterized in that the said system is adapted to provide refrigeration up to -60°C with ultra pure ammonia. 2. An improved ammonia absorption refrigeration system as claimed in claim 1 wherein, said heat source is steam circulated in the closed coil in stripping column and the absorbent water is in closed loop mode. 3. An improved ammonia absorption refrigeration system as claimed in claim 1 wherein, heat source is a direct steam injection into stripping column and the absorbent water either in open loop or in partially open loop mode. 4. An improved ammonia absorption refrigeration system as claimed in claim 1 wherein, heat source is partly steam circulated in the closed coil and partly direct steam injection into stripping column and the absorbent water either in open loop or in partially open loop mode. 5. An improved ammonia absorption refrigeration system as claimed in claim 1 wherein, heat source is a direct fired boiler, waste heat process streams, waste exhaust gases etc. 6. An improved ammonia absorption refrigeration system as claimed in any claim 1 - 4 wherein, heat source is high pressure/ low pressure steam, hot water. 7. An improved ammonia absorption refrigeration system as claimed in any claim 1 - 6 wherein, said means for generating ammonia vapours at atleast two desired pressure levels from the refrigerant ammonia by generating cooling effect in which one temperature being upto -60 °C comprises atleast two evaporators(17,18), atleast one sub-cooler (24) and atleast two expansion valves(25,27) and connected such that the refrigerant vapour is generated in the said evaporators. 8. An improved ammonia absorption refrigeration system as claimed in any claim 1 - 7 wherein, said means for absorbing the said ammonia vapours with aqueous ammonia solution(s)comprise atleast one absorber(19) and connected with pumps(21,22). 9. An improved ammonia absorption refrigeration system as claimed in any claim 1-8 wherein, said means for generation of ammonia vapours at higher pressure level containing moisture from the said aqueous ammonia solution comprises stripping column(12), heat exchanger(23), absorber(19), solution pump (21), heating source(ll), total condenser (16); such that the heating source supplies external heat; said solution pump (22) receives the strong ammonia solution (solution 3) from said absorber (19) so as to raise the pressure of the liquid to that of the said total condenser (16) pressure; the stripping column (12) being provided reboiling at the bottom by the said heat source, which heats the weak aqueous solution (solution 1) in the stripping column; the heat exchanger (23) heats a part or full of the strong aqueous ammonia solution (solution 3) from said solution pump using the heat of weak aqueous solution (solution 1) from the bottom of said stripping column (12) and then feeds to the top of stripping column 10. An improved ammonia absorption refrigeration system as claimed in any claim 1 - 9 wherein, said means for controlled enrichment of ammonia with step wise reduction of the moisture content of the vapour comprise stripping column (12), one or more partial condensers (13, 14) such that the one or more partial condensers are connected in series and are adapted to receive the ammonia-water vapour from the top of said stripping column and the stripping column further receives and recycles the partially condensed liquid predominantly rich in water from the said condensers. 11. An improved ammonia absorption refrigeration system as claimed in any claim 1 -10 wherein, said means for rectification of the enriched ammonia vapours and condensing the vapours for generation of liquid ammonia refrigerant at controlled moisture concentration, generating ultra pure ammonia containing moisture up to 5 ppm comprise rectification column (15), total condenser (16), at least two evaporators (17,18), sub-cooler (24) and expansion valve (25) such that the rectification column receives ammonia vapour containing little moisture wherein it is rectified by using the reflux from the top; the total condenser condenses the ultra pure ammonia vapour (upto 5 ppm moisture) from the said column top ; and refluxes a part of the said condensed liquid into the rectification column and sending the rest to evaporator (17) via sub-cooler (24) and expansion valve (25); the sub-cooler (26) and the expansion valve (27) receives liquid ammonia from the bottom of the reflux column and introduce the same to the evaporator (18) as second source of refrigerant. 15; An improved ammonia Absorption refrigeration system as claimed in any claim 1 -11 wherein, said stripping column is replaced with any other mode of vapour generator and partial condenser(s) provided in series or parallel between the said vapour generator and rectification column and/or total condenser. 13. An improved ammonia Absorption refrigeration system substantially as herein described in the text and accompanying drawings. Dated this 16th day of November 2002 S.MAJUMDAR Of S.MAJUMDAR & CO. Applicant's Agents

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