Abstract:
An absorption chiller system has an efficient start-up control that monitors system condition, and in particular the absorption solution temperature. The system limits the amount of heat delivered into the absorption chiller generator, to provide a gradual rise in the absorption solution temperature at start-up. In this manner, undesirable noise vibration and rapid thermal expansion, which may have occurred in the past is reduced or eliminated.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a control that utilizes the absorption solution temperature as feedback for controlling the amount of heat delivered into an absorption chiller at start-up. 
   Absorption solution/refrigerant systems are utilized in many applications to provide chilled or heated water for environmental control in buildings, etc. In a basic system, an absorber separately receives a flow of an absorption solution (concentrated LiBr), and a refrigerant. The refrigerant is often water. The refrigerant is directed onto a tube, which would carry water to be chilled for part of a building air conditioning system. The absorption solution is also directed into the absorber and is cooled. Within the absorber, the absorption solution and the liquid refrigerant combine. The absorption solution, at this point, is at the proper temperature and concentration that enables it to absorb the refrigerant vapor. It will then carry this refrigerant with it as it is returned to a generator. 
   In the generator, a heat source is applied to the mixed absorption solution and refrigerant, which boils off a portion of the refrigerant. This warmer refrigerant vapor is passed through a condenser where it is cooled and changed into liquid refrigerant. The liquid refrigerant is then sent to the evaporator where it cools the tubes carrying water for building air conditioning. The refrigerant evaporates and changes into vapor. This vapor is allowed to flow into the absorber unit. Here the water vapor is absorbed into the absorption solution being returned from the generator 
   Notably, an absorption chiller is utilized to provide chilled water for use in an air conditioning system. The same system can be utilized to provide heated water, with some rearrangement of flow, etc. For purposes of this application, the invention extends to both heating and cooling systems utilizing an absorption solution. While the application describes an “absorption chiller,” the invention extends to absorption solution/refrigerant systems that can be utilized to either chill or heat water. A worker of ordinary skill in the art would recognize how the disclosed system would be modified to function as an absorption heater. 
   At start-up, it would be desirable not to immediately subject the generator to the full heat of the system under some conditions. If the heat is directed into the chiller immediately, there is undesirable vibration, and noise, due to vapor being mixed with solution in the heat exchanger pipes, and rapid thermal expansion. These effects can reduce product life and also cause undesirable noise. The example above is for startup conditions but those skilled in the art would appreciate that a similar problem could occur under any conditions where a large change in supply heat is applied to the chiller. 
   In the prior art, some fixed ramp over time is utilized to control the amount of heat delivered into the chiller system. This is not as efficient as would be desirable, as a fixed ramp does not change with the current system conditions. 
   When driving the system with exhaust heat from a co-generation application, the rate of heat flow should be regulated by the response of the chiller. There is a maximum temperature rise rate for the chiller that is a function of its capacity and design. If too much heat is being applied to the chiller it will exceed this rate and will have the undesirable effects described above. Conversely, if the chiller is heated at a very slow rate, the chiller will not be able to get up to full operating capacity in a reasonable amount of time. 
   This invention is the control approach which is used to quickly bring the chiller up to full capacity while managing the temperature rises in the solutions to preventing the undesirable effects of uncontrolled rapid heat-up rates 
   SUMMARY OF THE INVENTION 
   In a disclosed embodiment of this invention, system feedback is utilized to control the amount of heat delivered into the generator for an absorption chiller at start-up. In a preferred embodiment, the feedback is taken from the temperature of the solution leaving the generator. Thus, the amount of heat delivered into the system is controlled to reach a steady state as soon as possible, without risking undesirable rapid thermal expansion, vibration, noise, or other undesirable results. 
   In one preferred embodiment, the solution temperature is compared at two closely spaced time periods. If the difference between the two temperatures exceeds a first threshold, then some limit on the heat entering the system is maintained. As an example, a damper valve controlling the flow of heat into the system may be kept partially closed if the change in solution temperature exceeds this first threshold. If the change in solution temperature does not exceed the threshold, but is still greater than some lesser second threshold, the start-up mode would continue. 
   At some point, the change in the two temperatures is less than both thresholds. At that point, all limits on the amount of opening of the damper valve are removed. In this manner, the control of the heat entering the system is maintained until the change in temperature of the solution is below a predetermined amount. In this manner, full heat being delivered into the generator will not occur until the solution has generally reached the desired state condition. 
   A second embodiment is particularly useful for a system that has the ability to provide “step” heating. As an example, in a preferred embodiment, a system for providing the heat includes several micro-turbines, and the outlet of the micro-turbines is utilized as a heating fluid to be delivered into the generator. The control of damper valve opening is changed dependent upon how many micro-turbines are operational or on how much heat and flow is being generated by them. As an example, if less than all of the micro-turbines are being utilized, perhaps no damper limit will be put in place. On the other hand, when the micro-turbines are all being utilized, some control on the amount of damper valve opening will be maintained until the solution temperature reaches a predetermined amount. 
   While the present invention is most directed to a start-up mode wherein the system is starting, many of the benefits and goals would have equal application to any point wherein the temperature of the system is to increase rapidly from a lower temperature to a higher temperature. Thus, the invention is not limited to simply a start-up mode, but would also extend to a temperature increase mode. 
   These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a system incorporating the present invention. 
       FIG. 2  shows plots of curves of heat into the system compared to the amount of valve openings. 
       FIG. 3  is a flow chart for the  FIG. 2  system. 
       FIG. 4  shows a plot of the amount of valve opening and corresponding solution temperature for two distinct operational conditions. 
       FIG. 5  is a flow chart of the  FIG. 4  system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An absorption chiller system  20  is illustrated in  FIG. 1 . As shown, an evaporator  24  receives flow from a refrigerant line  30 . Refrigerant in line  30  is delivered through an outlet to drip or fall on a water tube. In the event that the system  20  is a chiller, the tube will carry water that is to be cooled, and then utilized to cool environmental air in a building. Alternatively, as mentioned above, the water could be heated, with the refrigerant leaving the outlet being a heated refrigerant. Again, the details of the change to provide this function are within the skill of a worker in this art. 
   A second line  28  delivers an absorption solution into the absorber, positioned next to the evaporator  24 . Ultimately, a mixture of the refrigerant and absorption solution, or diluted LiBr solution, gathers at  30 , and is returned through a line  32  to a generator  22 . A source of heat is delivered through a line  44  into the generator  22 . This source of heat boils refrigerant out of the mixture, and into line  30 . A second line delivers the remaining concentrated absorption solution, with lower levels of refrigerant, through a line  28 , returning to the absorber or evaporator  24 . This concentrated absorption solution in the line  28  is cooled on the path to the absorber, increasing its ability to absorb the water vapor that is created as the refrigerant evaporates in the “Evaporator” 
   The present invention relates to control of the amount of heat delivered into the system at start-up. As shown, a sensor  34  may sense the temperature of the solution on line  28 . While a preferred location is shown, it should be understood that other locations that sense a temperature of the solution would come within the scope of this invention. Sensor  34  communicates temperature information to a control  36 . Control  36  controls a diverter valve  38  for diverting a heating fluid source, line  40 , either to an exhaust dump  42 , or into a line  44  leading into the generator at  22 . The diverter valve  38  may be generally as disclosed in co-pending provisional patent application Ser. No. 60/501,366, and entitled “Diverter Valve For Capacity Control Of A Refrigerant Absorption Cycle During Heat Service.” However, other valve systems for controlling the flow into the line  44  may also be utilized. 
   As shown, a source  46  of heating fluid provides the fluid to the line  40 . In a preferred embodiment, the source  46  is a micro-turbine arrangement. The micro-turbine arrangement has four turbines  48 ,  49 ,  50 ,  51 , shown schematically. A control  36  can separately control the source  46  such that one, two, three or all four of the turbines are operational. Alternatively, the number of operational turbines may be controlled by some other outside system and they can also be controlled to run at different output levels. That is, the purpose of the micro-turbines at source  46  may be completely removed from the system  20 , and the exhaust from the micro-turbines (source  46 ) simply utilized as the heating fluid source into line  40 . If the control of the turbines  48 ,  49 ,  50 , and  51  is not driven by the control  36 , then at least the number of turbines operational at any one point is preferably provided as feedback to the control  36 . 
   While the capacity is specifically illustrated as being provided by a varying number of micro-turbines, other methods of limiting capacity would also come within the scope of this invention. As an example, the valve could be controlled under this algorithm by sensing the heat flow from the turbines, the power generated by them, etc. 
   The present invention broadly utilized a system variable to provide feedback of how much heat should be delivered to the generator  22  at start-up of the absorption chiller while still minimizing the risk of undesirable noise, vibration, rapid thermal growth, or other undesirable results. In a more preferred embodiment, the system feedback is the temperature of the solution. 
   Two different algorithms are disclosed for providing this start-up control.  FIG. 2  shows a chart of the amount of heat delivered into the generator  22  as the percentage of opening of the valve  38  changes, and as the number of micro-turbines that are operational changes. As shown, a curve of some sort would result. 
   Also, the capacity of the turbines would also be lower simply if they were operating below their maximum output. 
     FIG. 3  is a flow chart of a method that does not directly monitor the output of the micro-turbines. Instead, in the start-up mode, the change in the solution temperature over a short period of time is utilized to determine whether a start-up mode is still proper. If the chiller is in a start-up mode, the temperature of the solution at  34  is measured (G 1   — SOL 1 ), and a second reading is taken a short period of time later (G 1   — SOL 2 ). At box X, these two temperature readings are compared. If the rise in the solution temperature over the short period of time exceeds a first threshold, then the system continues to limit the maximum amount of damper valve opening. It should be understood, that during steady state operation, the valve opening is controlled dependent upon capacity or other system needs by the control  36 . The present invention limits the maximum amount of valve opening to 50%. Of course, other valve opening amounts are within the scope of this invention, including both some other limit, and a series of incremental limits based upon, as an example, the amount of solution temperature rise. 
   In the event that the difference in solution temperature does not exceed the threshold, a box Y is considered. If the change is still above a certain amount, then the control stays in the same mode. This is to anticipate the fact that there might be variation in the amount of temperature rise, while the system is still in a start-up mode. However, if the change is also less than this second threshold at box Y, then all limits on the damper opening are removed. As is also clear, should the initial temperature reading be greater than a maximum (e.g., 212° F.), then the control moves immediately to remove any damper opening limit, recognizing that the start-up mode has passed. 
     FIG. 4  shows a graph of a second algorithm that is based upon the number of micro-turbines that are operational, or on the heat, power generated, etc. The control utilizes either a full micro-turbine operational mode that limits the valve opening, or perhaps have no limit on the valve opening during start-up mode if a fewer number of micro-turbines are operational. As shown in the flow chart of  FIG. 5 , the change in temperature is not necessary taken; instead the temperature is simply read. At box Z, this control asks whether all of the micro-turbines are on, and whether the solution temperature is below a predetermined amount. If the answer to either question is “no,” then all limits on the damper opening are removed. If, however, the answer to both questions is “yes,” then the damper limit is maintained. 
   Again, while the simple question of whether all of the micro-turbines are on is specifically disclosed in the application, similar controls come within the scope of the invention and may be provided by temperature sensors, mass flow sensors, power output levels, and other approaches. This capacity is then utilized to control the position of the valve. 
   This algorithm is perhaps sometimes more efficient than the  FIG. 3  algorithm in that the chiller start-up time would be longer with the  FIG. 3  flow chart in the event that fewer micro-turbines are operational. 
   While microturbines are disclosed as the source of heat, this invention would extend to many other sources. As examples, reciprocity piston engines, process heat, etc. could all be used. 
   Although preferred embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.