Patent Publication Number: US-6658870-B1

Title: Absorption chiller control logic

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of absorption chillers, and more particularly to a non-linear controller for an absorption chiller. 
     BACKGROUND OF THE INVENTION 
     In an absorption chiller, the chilled water temperature in the leaving chilled water line is directly affected by disturbances such as the entering chilled water temperature and the entering cooling water temperature. Because the only control point for the system is a capacity valve which controls the heat to the system, whether from steam or gas flame, and because the system is chemical-based, the machine dynamics of the system are relatively slow. Changes created by the disturbances mentioned above are removed slowly by the existing capacity control. 
     SUMMARY OF THE INVENTION 
     Briefly stated, in an absorption chiller system, a control input for the chiller is a heat source controlled by a capacity valve, which is in turn controlled by a PI controller. The controller is controlled by a non-linear control function. During operation, a disturbance in the system is measured. A signal error is defined as a setpoint for the leaving chilled water minus the disturbance. The non-linear control function is represented as C(s)=K P0 (1+b|E|)+K I /s, where where K P0  is the gain when said signal error is zero, |E| is the absolute value of the signal error, b is an adjustable constant, and K I  is an integral gain. 
     According to an embodiment of the invention, a method for controlling an absorption chiller system, wherein a control input for said chiller is a heat source controlled by a capacity valve, and wherein said capacity valve is controlled by a PI controller, includes the steps of (a) measuring a disturbance in said system; (b) defining a signal error as a setpoint minus said disturbance; and (c) controlling said capacity valve based on a control function in said PI controller, wherein said control function is represented by C(s)=K P0 (1+b|E|)+K I /s, where where K P0  is the gain when said signal error is zero, |E| is the absolute value of the signal error, b is an adjustable constant, and K I  is an integral gain. 
     According to an embodiment of the invention, a control system for an absorption chiller, wherein a control input for said chiller is a heat source controlled by a capacity valve, and wherein said capacity valve is controlled by a PI controller, includes means for measuring a disturbance in said chiller; means for defining a signal error as a setpoint minus said disturbance; and means for controlling said capacity valve based on a control function in said PI controller, wherein said control function is represented by C(s)=K P0 (1+b|E|)+K I /s, where where K P0  is the gain when said signal error is zero, |E| is the absolute value of the signal error, b is an adjustable constant, and K I  is an integral gain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic representation of an absorption chiller system; 
     FIG. 2 shows a control schematic is shown for the absorption chiller system of FIG. 1; and 
     FIG. 3 shows the steps in a control method according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a schematic representation of an absorption chiller system  10  is shown. Other types of absorption systems may use more or fewer stages, and may use a parallel rather than a series cycle. It will therefore be understood that the absorption system of FIG. 1 is only representative one of the many types of absorption systems that might have been selected to provide a descriptive background for the description of the invention. The control method and apparatus of the invention may be applied to any of these types of heating and cooling systems. 
     The absorption chiller system  10  is a closed fluidic system that operates in either a cooling mode or in a heating mode, depending upon the concentration of the absorbent in the refrigerant-absorbent solution and on the total quantity of liquid within the system. When system  10  operates in its cooling mode, the solution preferably has a first, relatively high concentration of the absorbent, i.e., is relatively strong or refrigerant poor, while the total quantity of liquid within the system is relatively small. When system  10  operates in its heating mode, the solution preferably has a second, relatively low concentration of the absorbent, i.e., is weak or refrigerant-rich, while the total quantity of liquid within the system is relatively large. In the following brief description of the operation of system  10  in these modes, it is assumed that system  10  employs water as a refrigerant and lithium bromide, which has a high affinity for water, as the absorbent. 
     System  10  includes an evaporator  19  and an absorber  20  mounted in a side-by-side relationship within a common shell  21 . When system  10  is operating in its cooling mode, liquid refrigerant used in the process is vaporized in evaporator  19  where it absorbs heat from a fluid, usually water, that is being chilled. The water being chilled is brought through evaporator  19  by an entering chilled water line  23   a  and a leaving chilled water line  23   b . Vaporized refrigerant developed in evaporator  19  passes to absorber  20  where it is combined with an absorbent to form a weak solution. Heat developed in the absorption process is taken out of absorber  20  by means of a cooling water line  24 . 
     The weak solution formed in absorber  20  is drawn therefrom by a solution pump  25 . This solution is passed in series through a first low temperature solution heat exchanger  27  and a second high temperature solution heat exchanger  28  via a delivery line  29 . The solution is brought into heat transfer relationship with relatively strong solution being returned to absorber  20  from the two generators, high temperature generator  16  and low temperature generator  36 , employed in the system, thereby raising the temperature of the weak solution as it moves into generators  16 ,  36 . 
     Upon leaving low temperature solution heat exchanger  27 , a portion of the solution is sent to low temperature generator  36  via a low temperature solution line  31 . The remaining solution is sent through a high temperature solution heat exchanger  28  and then to high temperature generator  16  via a solution line  30 . The solution in high temperature generator  16  is heated by a burner  50  to vaporize the refrigerant, thereby removing it from the solution. Burner  50  is fed from a gas line  54  and an air line  56  via a capacity valve  52 . Controlling valve  52  controls the amount of heat delivered to the system. Alternately, the heat delivered to the system comes from a steam line controlled by a steam valve (not shown). The refrigerant vapor produced by high temperature generator  16  passes through a vapor line  35 , low temperature generator  36 , and a suitable expansion valve  35 A to a condenser  38 . Additional refrigerant vapor is added to condenser  38  by low temperature generator  36 , which is housed in a shell  37  along with condenser  38 . In low temperature generator  36 , the weak solution entering from line  31  is heated by the vaporized refrigerant passing through vapor line  35  and added to the refrigerant vapor produced by high temperature generator  16 . In condenser  38 , refrigerant vapor from both generators  16 ,  36  are placed in heat transfer relationship with the cooling water passing through line  24  and condensed into liquid refrigerant. 
     Refrigerant condensing in condenser  38  is gravity fed to evaporator  19  via a suitable J-tube  52 . The refrigerant collects within an evaporator sump  44 . A refrigerant pump  43  is connected to sump  44  of evaporator  19  by a suction line  46  and is arranged to return liquid refrigerant collected in sump  44  back to a spray head  39  via a supply line  47 . A portion of the refrigerant vaporizes to cool the water flowing through chilled water line  23 . All of the refrigerant sprayed over chilled water line  23  is supplied by refrigerant pump  43  via supply line  47 . 
     Strong absorbent solution flows from the two generators  16 ,  36  back to absorber  20  to be reused in the absorption cycle. On its return, the strong solution from high temperature generator  16  is passed through high temperature solution heat exchanger  28  and through low temperature solution heat exchanger  27  via solution return line  40 . Strong solution leaving low temperature generator  36  is connected into the solution return line by means of a feeder line  42  which enters the return line at the entrance of low temperature solution heat exchanger  27 . 
     Sensors are emplaced in various parts of system  10 , including temperature sensors  72 ,  74 ,  76 , and  78  in cooling water line  24 , temperature sensor  82  in the leaving chilled water line  23   b , and temperature sensor  84  in the entering chilled water line  23   a . The outputs of these sensors are connected to a controller such as PI controller  70 . Controller  70  also includes a connection to capacity valve  52 , in addition to receiving input from a thermostat, shown here as a set point  86 . 
     The chilled water temperature in the leaving chilled water line  23   b  is directly affected by disturbances such as the entering chilled water temperature (sensor  84 ) in water line  23   a  and the entering cooling water temperature (sensor  74 ) in cooling water line  24 . Because the only control point for the system is capacity valve  52 , and because the system is chemical-based, the machine dynamics of the system are relatively slow. Changes created by the disturbances mentioned above are removed slowly by the existing capacity control. 
     Currently, the capacity valve  52  control is based on proportional-integral (PI) control logic based in PI controller  70 . The output signal to capacity valve  52 , which controls burner  50 , is a function of the setpoint error, that is, the chilled water leaving setpoint value from setpoint  86  minus the measured chilled water leaving temperature from sensor  82 . As is known in the art, the proportional part of the PI control multiplies the error by a constant, the proportional gain K P , while the integral part consists of the error integrated over time and multiplied by an integral gain K I . The transfer function of a basic PID controller is Gc(s)=K P +K D s+K I /s, but when the controller is used only as a PI controller, the derivative gain is not used and the K D s term drops out. Thus, the basic transfer function of the PI controller is represented as Gc(s)=K P +K I /s. 
     Referring to FIG. 2, a control schematic is shown for absorption chiller system  10 . The existing capacity control law is shown as C(s), while G(s) is the transfer function for absorption system  10 . The idea behind the nonlinear adaptive gain of the present invention is that a nonlinear process is best controlled by nonlinear controllers. Essentially, the proportional gain K P  in the controller transfer function is made variable by expressing it as a function of the signal error, that is, the setpoint minus the measurement, as 
     
       
           K   P   =K   P0 (1 +b|E |) 
       
     
     where K P0  is the gain when the error is zero, |E| is the absolute value of the error, and b is an adjustable constant. Since the proportional gain K P  is already multiplied by the error, this expression results in the output signal being proportional to the error squared. Thus, C(s)=K P +K I /s=K P0 (1+b|E|)+K I /s. 
     An advantage of using this expression is that a low value for K P0  can be used so that the system is stable around the setpoint, resulting in greatly reduced overshoot and undershoot of the chilled water setpoint. 
     When a large disturbance enters the system, the magnitude of the error results in a large gain which serves to move the burner control rapidly to deal with the transient disturbance. Using this expression also has the advantage of reducing the effect of signal noise around the setpoint, thereby preventing continuous oscillation of the leaving chilled water temperature. This control algorithm requires minimal modification to the existing control routine, but it offers drastic improvement to the current proportional-integral control of the burner. 
     Referring to FIG. 3, the steps of the method of the present invention are shown. In step  90 , the disturbance entering the system is measured. The disturbance is preferably the chilled water temperature, and either the entering chilled water temperature or the leaving chilled water temperature may be used. In step  92 , the signal error is defined as the setpoint for the leaving chilled water temperature minus the disturbance. Then in step  94 , the capacity control valve for absorption chiller  10  is controlled by PI controller  70  using the non-linear control function described above. 
     While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.