WO2008127236A1

WO2008127236A1 - A method and system for extending a turndown ratio of an absorption chiller

Info

Publication number: WO2008127236A1
Application number: PCT/US2007/009098
Authority: WO
Inventors: Sung-Han Jung; Timothy C. Wagner; Lynn M. Rog; Jin Sang Ryu
Original assignee: Utc Power Corporation
Priority date: 2007-04-13
Filing date: 2007-04-13
Publication date: 2008-10-23
Also published as: KR20100019422A; EP2156109A1; CN101688704A

Abstract

A method and system for improving operation of an absorption chiller (10) having an evaporator (12), an absorber (14), a generator (16), and a condenser (20) varies the circulation of a refrigerant and/or an absorbent solution in order to allow for continued operation of the absorption chiller (10) during a low cooling and/or low heating demand. An overflow circulation loop (46, 50, 54) is configured to vary the circulation of the absorbent solution and selectively recycle excess absorbent solution from the generator (16) to the absorber (14). A refrigerant circulation loop (34, 52, 58, 60) is configured to vary the circulation of the refrigerant in the evaporator (12) to avoid running a refrigerant pump (36) when the evaporator (12) contains a minimum amount of refrigerant. The absorption chiller (10) may be a single-effect, double-effect or triple-effect absorption chiller. In some embodiments, the absorption chiller (10) may be capable of simultaneous heating and cooling.

Description

A METHOD AND SYSTEM FOR EXTENDING A TURNDOWN RATIO OF AN

ABSORPTION CHILLER

BACKGROUND

[0001] The present disclosure relates to an absorption chiller system. More particularly, the present disclosure relates to features for extending a turndown ratio of an absorption chiller system. [0002] An absorption chiller, which includes an evaporator, an absorber, a generator, and a condenser, uses an absorbent solution and a refrigerant to provide cooling and/or heating. An absorption chiller may have a limited turndown ratio, which is the ratio of maximum capacity to minimum capacity. The limitations may be a function, in part, of an amount of heat source energy supplied to the generator or limited space in the absorber for excess absorbent solution. Additionally, low refrigerant levels in the evaporator may hinder the absorption chiller from achieving a higher turndown ratio.

[0003] There is a need for improving operation of an absorption chiller system such that the chiller has an extended turndown ratio and is able to continue operation under a wider range of operating conditions. SUMMARY

[0004] The present disclosure relates to a method and system for improving operation of an absorption chiller having an evaporator, an absorber, a generator, and a condenser. The disclosure relates to varying the circulation of the refrigerant and/or the absorbent solution in order to allow for continued operation of the absorption chiller during a low cooling and/or low heating demand. An overflow circulation loop is configured to vary the circulation of the absorbent solution from the generator and selectively recycle excess absorbent solution from the generator to the absorber. A refrigerant circulation loop is configured to vary the circulation of the refrigerant in the evaporator to prevent a refrigerant pump from running when there is less than a minimum amount of refrigerant in a sump of the evaporator. The absorption chiller may be a single-effect, double-effect or triple-effect absorption chiller. In some embodiments, the absorption chiller may be capable of simultaneous heating and cooling. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a schematic diagram of an exemplary embodiment of an absorption chiller system that enables an extended turndown ratio.

[0006] FIG. 2 is a schematic diagram of a portion of the system shown in FIG. 1, illustrating use of overflow piping and a steam trap located between the absorber and the high stage generator for recycling excess absorbent solution back to the absorber. [0007] FIG. 3 is a schematic of a portion of the system shown in FIG. 1, illustrating use of liquid level sensors for monitoring a refrigerant level inside the evaporator to control operation of a refrigerant pump. DETAILED DESCRIPTION

[0008] FIG. 1 is a schematic diagram of absorption chiller system 10, which includes evaporator 12, absorber 14, high stage generator 16, low stage generator 18, condenser 20, high temperature solution heat exchanger 22, low temperature solution heat exchanger 24, and auxiliary heat exchanger 26. In the exemplary embodiment of FIG. 1, chiller system 10 is a double-effect absorption chiller with simultaneous heating and cooling capabilities, and as such, system 10 may be used to supply heating and cooling to a building. It is recognized that the method and system described herein for extending a turndown ratio of chiller system 10 may also apply to any type of absorption chiller, including, but not limited to, a single- effect or triple-effect absorption chiller, an absorption chiller configured only for cooling, and/or an absorption chiller configured for heating and cooling separately.

[0009] Chiller system 10 is configured to provide cooling to a building by decreasing a temperature of chilled water source 28, which passes through evaporator 12. System 10 is able to simultaneously provide heating to the building by increasing a temperature of hot water source 30, which passes through auxiliary heat exchanger 26. As is commonly used with absorption chillers, system 10 also includes cooling water loop 32 for flowing water from a cooling tower through absorber 14 and condenser 20 such that the cooling water is used for heat removal.

[0010] As is known in the art, absorption chiller systems, like system 10, are configured to use an absorbent solution, such as lithium bromide, and a refrigerant, such as water, to provide a cooling and/or a heating effect. Although chiller system 10 is described using lithium bromide and water, it is recognized that other combinations (for example, water as the absorbent and ammonia as the refrigerant) may alternatively be used in system 10. [0011] Evaporator 12 is configured to receive refrigerant in liquid form (i.e. water) from condenser 20 and store the water in evaporator sump 34. With the use of refrigerant pump 36, evaporator 12 pumps water from sump 34 to sprayer 38, located at a top of evaporator 12, or to a dripper system in evaporator 12. As a result of chilled water 28 running through tubes inside evaporator 12, water from sprayer 38 is vaporized, and chilled water 28 decreases in temperature. As shown, system 10 is a closed loop system and maintained in a vacuum such that water from sprayer 38 boils at a lower temperature. The refrigerant (water), now in vaporized form, travels to absorber 14 through eliminator 40, at which point the water is absorbed by a concentrated lithium bromide solution being sprayed through sprayer 42 at a top of absorber 14. A diluted lithium bromide solution then is delivered to high stage generator 16 using solution pump 44. High and low temperature solution heat exchangers 22 and 24, which transport lithium bromide solution to and from low stage generator 18, increase a temperature of the diluted lithium bromide solution flowing to generator 16, and thereby increase an efficiency of generator 16. [0012] Exhaust gas is supplied to high stage generator 16 to boil water from the lithium bromide solution, thus generating steam. In the exemplary embodiment of FIG. 1, exhaust gas is supplied from a microturbine or another type of prime mover. A benefit of system 10 is that it utilizes waste heat from another component used in the building. It is recognized that other types of heat sources may be used for supplying heat energy to generator 16. For example, in alternative embodiments, generator 16 may be direct-fired, steam fired or hot-water driven. Steam generated by generator 16 may then be directed to low stage generator 18 and to auxiliary heat exchanger 26. Moreover, steam from generator 16 may also reside in overflow piping 46.

[0013] Steam from high stage generator 16 flows to a tube side of low stage generator

18. Lithium bromide solution from high stage generator 16 flows through heat exchanger 22 and then flows to a shell side of low stage generator 18. The lithium bromide solution in generator 18 then boils off additional steam due to transferred heat from the steam on the tube side of generator 18. The additional steam on the shell side of generator 18 then travels to condenser 20 through eliminator 48 located between generator 18 and condenser 20. In condenser 20, cooling water 32 flows through a tube side of condenser 20. As the steam from generator 18 enters the shell side of condenser 20, the steam condenses and the condensate is recycled back to evaporator 12.

[0014] Steam in the tube side of generator 18 condenses and the condensate is recycled back to evaporator 12, along with the condensate from condenser 20. Lithium bromide from generator 18, which is again at a high concentration, flows through heat exchanger 24 and is recycled back to absorber 14. The cycle is repeated as concentrated lithium bromide is sprayed in absorber 14, thereby absorbing water from evaporator 12. [0015] Because system 10, in the exemplary embodiment of FIG. 1, is a simultaneous heating and cooling absorption chiller, system 10 also includes auxiliary heat exchanger 26, which may be used for heating. Steam from high stage generator 16 travels to a shell side of auxiliary heat exchanger 26, where the steam condenses, thus transferring heat to hot water source 30 flowing through a tube side of heat exchanger 26. After the steam condenses, the liquid condensate is recycled back to generator 16, where it may be reabsorbed by the lithium bromide solution in generator 16. [0016] In the embodiment shown in FIG. 1, system 10 includes three main valves that are used to control operation of system 10 — diverter valve 70 (also referred to as CVl), heat exchanger control valve 72 (also referred to as CV2), and low stage generator control valve (also referred to as CV3). Valve 70 (CVl) is configured to regulate an amount of exhaust gas supplied to high stage generator 16 based on the heating and/or cooling demands on system 10. Valve 72 (C V2) is configured to regulate an amount of liquid condensate in heat exchanger 26 recycled back to generator 16, as a function of the heating demand. Valve 74 (CV3) is configured to regulate an amount of liquid condensate in low stage generator 18 recycled back to evaporator 12, based on the heating and/or cooling demands and the conditions inside high stage generator 16. System 10 also includes bypass loop 80, configured in parallel with heat exchanger 26, and valve 82. It is recognized that bypass loop 80 and valve 82 are not required in chiller system 10, but may be used for improving operation of system 10, particularly in an absence of a heating demand. It is recognized that an absorption chiller system may include more or less valves, as compared to the embodiment shown in FIG. 1, depending on the particular type of absorption chiller. [0017] When a cooling or a heating load of system 10 is low, for example during mild weather conditions in the spring or fall, the load may fall below a minimum cooling or heating capacity of system 10 and system 10 may be required to shut down or enter a recycle shutdown mode. In a scenario in which the outside ambient temperature is relatively low, yet the building still has a minimal cooling demand, a level of lithium bromide solution in high stage generator 16 may rise to an undesirable level and system 10 may shut down (or enter recycle mode). This is described in further detail below. Alternatively or in addition to this, the low cooling demand may result in a depleted level of refrigerant (water) in sump 34 of evaporator 12, as also described below. A low refrigerant level in evaporator sump 34 may cause refrigerant pump 36 of evaporator 12 to cavitate, and ultimately lead to destruction of pump 36. Absorption chiller system 10 of FIG. 1 is configured to vary circulation of the lithium bromide solution from high stage generator 16 and to vary circulation of the refrigerant in evaporator 12, in order to allow for continued operation of absorption chiller system 10 during a low cooling demand. As such, system 10 is capable of an increased turndown (i.e. ratio of maximum capacity to minimum capacity).

[0018] When the building has a low cooling demand, yet the outside ambient air temperature is low, cooling water from the cooling tower, which flows through cooling water loop 32, is consequently at a lower temperature. The lower temperature of the cooling water as it passes through absorber 14 increases an absorption effect in absorber 14 such that more water from evaporator 12 is absorbed by the lithium bromide solution in absorber 14. As a result, an increased volume of diluted lithium bromide solution flows to high stage generator 16. Moreover, under a low cooling load, heat input (exhaust gas) to generator 16 is minimized. Because it is receiving less energy, generator 16 boils off less steam from the lithium bromide solution in generator 16. [0019] Both an increased absorption effect in absorber 14 and a reduced steam production in generator 16 result in a greater volume of lithium bromide solution residing in generator 16. In a standard absorption system, at some point the system would be required to shut down, or a solution pump would be required to stop, once there is a greater volume of solution in the high stage generator than what a dam in the generator is able to withstand. Moreover, as a result of an increase in the absorption of water in absorber 14 and a decrease in the generation of steam in generator 16, less water is recycled back to evaporator 12 (through low stage generator 18 and condenser 20). Thus, the low cooling demand also results in a reduced refrigerant level in evaporator sump 34. If refrigerant pump 36 is continuously running during operation of system 10, there is a risk that there may not be enough refrigerant in sump 34 and pump 36 may have insufficient NPSH (net positive suction head), which may cause pump 36 to cavitate.

[0020] The design features described herein and shown in FIGS. 1-3 are intended to address the above limitations that may inhibit a low turndown of system 10. The design features include overflow piping 46 installed between high stage generator 16 and absorber 14, steam trap 50 installed in line with overflow piping 46, and liquid level sensors 52 for monitoring and controlling a refrigerant level in evaporator sump 34.

[0021] Overflow piping 46, steam trap 50 and liquid level sensors 52 are shown in

FIG. 1 as being provided in simultaneous heating and cooling absorption chiller system 10. It is recognized that piping 46, trap 50 and sensors 52 may be incorporated into any type of absorption chiller. For example, these extended turndown features may be included in an absorption chiller that switches between a heating mode and a cooling mode, and is not configured for simultaneous heating and cooling. As described herein, the features are focused on enabling continued operation of chiller system 10 during a low cooling demand. It is recognized that piping 46, trap 50 and sensors 52 also may be used to enhance turndown during a low heating demand.

{0022] As shown in FIG. 1, overflow piping 46 is connected to generator 16. During normal operation of system 10 (i.e. a moderate to high cooling and/or heating demand), as steam is boiling in generator 16 and being removed from the lithium bromide solution, the concentrated lithium bromide solution flows over a dam inside generator 16 and then to low stage generator 18 through heat exchanger 22. At this point, a portion of the steam in generator 16 may flow into overflow piping 46, to auxiliary heat exchanger 26, and to low stage generator 18. [0023] In contrast, if there is a low cooling demand, a greater volume of lithium bromide solution is contained inside generator 16 and less steam is being generated. Moreover, the low heat input to generator 16 and the lower temperature of cooling water in cooling water loop 32 may reduce pressure inside high stage generator 16. This reduction in internal pressure may hinder a flow of lithium bromide solution from high stage generator 16 to low stage generator 18. As system 10 continues to operate, the lithium bromide solution may continue rising to an undesirable level within generator 16 such that the solution is above a predetermined level in the dam. At some point, without overflow piping 46, the lithium bromide solution may overflow in generator 16, resulting in a shutdown of system 10. [0024] System 10 has an overflow absorbent circulation loop, which includes overflow piping 46, configured to vary circulation of the absorbent solution based on the conditions in system 10, and specifically in generator 16. During normal operation of system 10, lithium bromide solution flows from high stage generator 16 to low stage generator 18. However, as described above, operating conditions, such as a low cooling demand, exist when it may be necessary or beneficial to vary circulation of the lithium bromide solution if the absorbent solution rises above a predetermined level in the dam of high stage generator 16. When the lithium bromide solution reaches the predetermined level in generator 16, the excess lithium bromide solution in generator 16 is directed through overflow piping 46 back to absorber 14. This allows chiller system 10 to continue operating under conditions which may cause a greater volume of absorbent solution to reside in generator 16. Because steam from generator 16 also may reside in overflow piping 46, system 10 includes steam trap 50 between absorber 14 and generator 16, as described in more detail below in reference to FIG. 2.

[0025] FIG. 2 is a schematic diagram of a portion of overflow piping 46 and steam trap 50 from FIG. 1. As part of the absorbent circulation loop, overflow piping 46 may be used to remove excess lithium bromide solution from generator 16 and prevent an overflow of the absorbent solution inside generator 16. Overflow piping 46 is configured to recycle excess lithium bromide solution in generator 16 back to absorber 14.

[0026] During operation of generator 16, overflow piping 46 may contain steam only, lithium bromide only, or lithium bromide and steam. Because steam is useful energy, it is undesirable to allow any steam from generator 16 to flow back to absorber 14 with the absorbent solution. Steam trap 50 is configured to selectively allow lithium bromide solution to recycle back to absorber 14, while preventing steam from passing through to absorber 14. [0027] As shown in FIG. 2, overflow piping 46 from generator 16 may be attached to a bottom end 50a of steam trap 50, and piping 54 extending from absorber 14 may be attached to a top end 50b of steam trap 50. The excess lithium bromide that is recycled back to absorber 14 through piping 54 may then be contained within a sump of absorber 14. In an exemplary embodiment, steam trap 50 may be an inverted bucket trap. It is recognized that other types of steam traps may alternatively be used in system 10, including, for example, thermostatic, mechanical and thermodynamic steam traps. [0028] FIG. 3 is a schematic diagram of a portion of evaporator 12 and absorber 14 from FIG. 1 to illustrate a refrigerant circulation loop, which includes sensors 52, to vary circulation of refrigerant in evaporator 12. The refrigerant circulation loop is configured to prevent running pump 36 when there is not enough water in sump 34. During normal operation of system 10, refrigerant (water) is continuously being removed from evaporator 12, due to absorption by lithium bromide in absorber 14, and then recycled back to evaporator 12 from condenser 20. As explained above, when system 10 has a low cooling demand, more refrigerant is absorbed in absorber 14 and less refrigerant is returned to evaporator 12, resulting in a decrease in refrigerant levels in evaporator sump 34. Under these conditions, it may be necessary to abort operation of pump 36 when the water level in sump 34 falls below a minimum level.

[0029] As shown in FIG. 3, container 56 is connected to evaporator 12 and contains liquid level sensors 52. Liquid refrigerant from condenser 20 travels to evaporator 12 through piping 58 and then is contained within sump 34 and container 56. A level of refrigerant in container 56 correlates to a refrigerant level in sump 34. Liquid level sensors 52 are configured to sense a level of refrigerant in container 56. Sensors 52 include low level sensor 52a, high level sensor 52b and common sensor 52c. Container 56 is configured such that sensors 52 may easily be removed and replaced as necessary. In a preferred embodiment, sensors 52 are connected to a controller of system 10 that controls operation of pump 36. In alternative embodiments, pump 36 may be manually controlled based on signals from sensors 52.

[0030] When low level sensor 52a senses that a water level in sump 34 has fallen below a minimum level, a signal from sensor 52a causes the controller to abort operation of pump 36, which stops liquid refrigerant from being delivered from sump 34 to sprayer 38 through piping 60. Once high level sensor 52b senses that a water level in sump 34 has returned to a predetermined level, a signal from sensor 52b results in pump 36 being restarted. Common sensor 52c extends the furthest into container 56- such that a water level in sump 34 should always be above a sensing end of common sensor 52c. As such, common sensor 52c acts as a reference point for sensors 52a and 52b. It is recognized that more than three sensors may be included in container 56.

[0031] During a period when pump 36 is turned off, no liquid refrigerant is supplied to sprayer 38 of evaporator 12. As a result, chilled water source 28 may increase above its set point during the period when pump 36 is shut down. Because pump 36 may typically be restarted after a short period of time, there should be a minimal overall effect on the cooling capacity, particularly if there is a low cooling demand. A minimal effect on cooling is more desirable than damaging pump 36 or having to shut down system 10 completely. [0032] In the exemplary embodiment shown in FIG. 1, chiller system 10 includes overflow piping 46 (with steam trap 50) and liquid level sensors 52 in combination. Overflow piping 46 and steam trap 50 vary circulation of the absorbent solution from generator 16 to allow continuous operation of chiller system 10 during a low cooling or heating demand. Liquid level sensors 52 vary circulation of refrigerant in evaporator 12 to allow continuous operation of chiller system 10 during a low cooling or heating demand. Overflow piping 46 and liquid level sensors 52 may be used together to vary circulation of the refrigerant and the absorbent solution, in order to increase a turndown ratio of system 10. It is recognized that a chiller system may include overflow piping 46 and steam trap 50, and exclude liquid level sensors 52; alternatively, the chiller system may include liquid level sensors 52, without overflow piping 46 and steam trap 50. In preferred embodiments, and for optimal operation of an absorption chiller system, the overflow piping, steam trap and liquid level sensors are used in combination. [0033] Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. A system for providing heating and/or cooling, the system comprising: an absorption chiller comprising: an evaporator configured to receive a refrigerant in liquid form and vaporize a portion of the refrigerant; an absorber configured to contain an absorbent solution and receive the refrigerant in vaporized form from the evaporator so that the absorbent solution absorbs the refrigerant to form a diluted absorbent solution; a generator configured to receive the diluted absorbent solution and a heat source so that refrigerant is vaporized from the diluted absorbent solution; and a condenser configured to receive refrigerant in vaporized form from the generator so that the refrigerant condenses and the refrigerant in liquid form is recycled back to the evaporator; and means for varying circulation of at least one of the refrigerant and the absorbent solution to allow continued operation of the absorption chiller during at least one of a low cooling demand and a low heating demand.

2. The system of claim 1 wherein the means for varying circulation includes an overflow circulation loop configured to selectively recycle excess absorbent solution from the generator to the absorber.

3. The system of claim 2 wherein the overflow circulation loop comprises: a first set of piping between the absorber and the generator; a pump for delivering the diluted absorbent solution from the absorber to the generator through the first set of piping; a second set of piping between the generator and the absorber and configured to receive the absorbent solution and refrigerant in vaporized form; and a trap connected to the second set of piping and configured to selectively allow the absorbent solution to flow back to the absorber and prevent the vaporized refrigerant from passing through the trap.

4. The system of claim 1 wherein the means for varying circulation includes a refrigerant circulation loop for controlling circulation of the refrigerant in the evaporator.

5. The system of claim 4 wherein the refrigerant circulation loop comprises: a sump in the evaporator for receiving and storing the refrigerant in liquid form from the condenser; a pump configured to deliver the refrigerant to a sprayer in the evaporator; at least one sensor for monitoring a level of the refrigerant in the sump to control operation of the pump.

6. The system of claim 5 wherein the at least one sensor includes a low level sensor and the pump is turned off when the low level sensor senses that the level of refrigerant in the sump is below a minimum level.

7. The system of claim 5 wherein the at least one sensor includes a high level sensor and the pump is turned on when the high level sensor senses that the level of refrigerant in the sump is at or above a predetermined level.

8. A system for providing heating and/or cooling, the system comprising: an absorption chiller comprising: an evaporator configured to receive a refrigerant in liquid form and vaporize a portion of the refrigerant; an absorber configured to contain an absorbent solution and receive the refrigerant in vaporized form from the evaporator so that the absorbent solution absorbs the refrigerant to form a diluted absorbent solution; a generator configured to receive the diluted absorbent solution and a heat source so that refrigerant is vaporized from the diluted absorbent solution; and a condenser configured to receive refrigerant in vaporized form from the generator so that the refrigerant condenses and the refrigerant in liquid form is recycled back to the evaporator; and a circulation loop configured to vary at least one of the refrigerant and the absorbent solution and enable continued operation of the absorption chiller during at least one of a low cooling demand and a low heating demand.

9. The system of claim 8 wherein the circulation loop is an absorbent circulation loop comprising: a first set of piping between the absorber and the generator for delivering the diluted absorbent solution from the absorber to the generator; and a second set of piping between the generator and the absorber and configured to receive the absorbent solution and the refrigerant in vaporized form.

10. The system of claim 9 wherein the absorbent circulation loop further comprises a trap connected to the second set of piping and configured to selectively recycle the absorbent solution back to the absorber.

11. The system of claim 8 wherein the circulation loop is a refrigerant circulation loop comprising: a sump in the evaporator for receiving and storing the refrigerant in liquid form from the condenser; a pump configured to deliver the refrigerant to a sprayer in the evaporator; and at least one sensor for monitoring a level of refrigerant in the sump, wherein the pump is turned off when the level of refrigerant in the sump is below a minimum level and the pump is turned on when the level of refrigerant in the sump is at or above a predetermined level.

12. The system of claim 11 wherein the at least one sensor includes a low level sensor and the pump is turned off when the low level sensor senses that a level of refrigerant in the evaporator is below a minimum level.

13. The system of claim 11 wherein the at least one sensor includes a high level sensor and the pump is turned on when the high level sensor senses that the level of refrigerant in the evaporator is at or above a predetermined level.

14. The system of claim 8 wherein the absorption chiller further comprises at least one solution heat exchanger configured to receive the absorbent solution.

15. The system of claim 8 wherein the absorption chiller further comprises an auxiliary heat exchanger configured to increase a temperature of a hot water source passing through the auxiliary heat exchanger.

16. The system of claim 8 wherein the refrigerant is water and the absorbent solution is lithium bromide.

17. The system of claim 8 wherein the absorption chiller is a double-effect absorption chiller and further comprises a second generator.

18. The system of claim 8 wherein the absorption chiller is a triple-effect absorption chiller.

19. The system of claim 8 wherein the absorption chiller is configured for providing simultaneous heating and cooling.

20. A method of operating an absorption chiller having an evaporator, an absorber, a generator and a condenser, the method comprising: flowing a refrigerant and an absorbent solution through the absorption chiller to provide at least one of cooling and heating; varying circulation of at least one of the refrigerant and the absorbent solution to enable continued operation of the absorption chiller during at least one of a low cooling demand and a low heating demand.

21. The method of claim 20 wherein varying circulation includes monitoring a level of refrigerant in the evaporator to control operation of a pump configured to deliver refrigerant to at least one of a dripper system and a sprayer of the evaporator.

22. The method of claim 21 wherein monitoring a level of refrigerant in the evaporator is performed by at least one sensor.

23. The method of claim 22 wherein the at least one sensor comprises: a low level sensor configured to sense a level of refrigerant in a sump of the evaporator, wherein the pump is turned off when the low level sensor determines that the refrigerant level in the sump is below a minimum level; and a high level sensor configured to sense the level of refrigerant in the sump, wherein the pump is turned on when the high level sensor determines that the refrigerant level in the sump is at or above a predetermined level.

24. The method of claim 20 wherein varying circulation includes recycling excess absorbent solution in the generator back to the absorber.

25. The method of claim 24 wherein recycling excess absorbent solution back to the absorber is achieved by providing overflow piping between the generator and the absorber.

26. The method of claim 25 wherein recycling excess absorbent solution back to the absorber includes preventing refrigerant in vaporized form from flowing back to the absorber.

27. A system for improving operation of an absorption chiller having an evaporator, an absorber, a generator and a condenser, the system comprising: a pump configured to deliver a refrigerant in liquid form from a sump of the evaporator to a top of the evaporator; a low level sensor configured to sense a level of refrigerant in the sump, wherein the pump is turned off when the low level sensor determines that the refrigerant level in the sump is below a minimum level; and a high level sensor configured to sense the level of refrigerant in the sump, wherein the pump is turned on when the high level sensor determines that the refrigerant level in the sump is at or above a predetermined level.

28. The system of claim 27 further comprising: a container connected to the evaporator and configured to receive and store a portion of refrigerant from the condenser, wherein the low and high level sensors are located inside the container, and a level of refrigerant in the container correlates to a level of refrigerant in the sump of the evaporator.

29. The system of claim 27 further comprising: an overflow system configured for selectively allowing excess absorbent solution in the generator to flow back to the absorber.

30. The system of claim 29 wherein the overflow system comprises: overflow piping located between the generator and the absorber, and configured to receive the absorbent solution and the refrigerant in vaporized form; and a trap connected to the overflow piping and configured to selectively enable the absorbent solution to flow back to the absorber and prevent the vaporized refrigerant from passing through the trap.

31. A method of operating an absorption chiller having an evaporator, an absorber, a generator and a condenser, the method comprising: sensing a level of a refrigerant in a sump of the evaporator; aborting operation of a pump_vof the evaporator when the level of the refrigerant in the sump is below a minimum level; and resuming operation of the pump when the level of refrigerant in the sump is at or above a predetermined level.

32. The method of claim 31 further comprising: enabling excess absorbent solution in the generator to flow back to the absorber.

33. An overflow system for improving operation of an absorption chiller having an evaporator, an absorber, a generator and a condenser, the overflow system comprising: piping located between the generator and the absorber, and configured to receive steam and an absorbent solution; and a steam trap connected to the piping and configured to selectively allow the absorbent solution to pass through the steam trap and into the absorber, and prevent steam from passing through the stream trap.

34. The overflow system of claim 33 wherein the piping is configured to remove excess absorbent solution from the generator and recycle the excess absorbent solution back to the absorber.

35. The overflow system of claim 33 further comprising: at least one sensor configured for monitoring a level of refrigerant in the evaporator to control operation of a pump configured to deliver refrigerant to at least one of a dripper system and a sprayer of the evaporator.

36. A method of improving operation of an absorption chiller having an evaporator, an absorber, a generator and a condenser, the method comprising: directing absorbent solution in the generator to flow back to the absorber when there is an excessive amount of absorbent solution in the generator; and preventing steam from flowing back to the absorber.