Patent Publication Number: US-6701726-B1

Title: Method and apparatus for capacity valve calibration for snapp absorption chiller

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of absorption chiller systems, and more particularly to a method of controlling the leaving chilled water temperature of the system. 
     BACKGROUND OF THE INVENTION 
     One of the inherent control problems of an absorption chiller is that its thermodynamic properties create a slow moving cycle. This slow response to dynamic building loads is further amplified by a unique valve position vs. fuel consumption curve. That is, because of variations between installations, a curve relating the position of the capacity valve to the heat input for all installations is impossible to obtain. To overcome this problem, when the unit is first installed in the building, the service technician adjusts the capacity valve to the minimum and maximum heat input values. Between these two points, the combustion characteristics are adjusted for a “clean burn” for a gas installation, i.e., adjusted to meet various pollution control requirements. This adjustment is dependent on several variables unique to the specific installation site; thus, the adjustment is done on-site at the time of installation. Only a few data points relating the position of capacity valve to the heat input are known at the time of installation, so that the relationship between the valve position and the heat input is known only as either an assumed linear curve or as a step function. The data points have to be determined empirically during installation. 
     The controls that regulate the movement of the capacity valve have no feedback other than the leaving chilled water temperature to determine the valve position, which controls the heat input to the system. The combination of the unique non-linear combustion curve and a slow moving cycle is one component of a problem termed “capacity valve hunting”, which is an undesirable effect that causes oscillations in the leaving chilled water temperature. The system adjustments are either too much or too little, so that the actual leaving chilled water temperature oscillates around the setpoint. 
     SUMMARY OF THE INVENTION 
     Briefly stated, data points are determined for an absorption chiller system which relate a position of the capacity valve to the heat input into the system. A continuous curve is determined which estimates the relationship between the position of the capacity valve and the heat input for all of the data points and all the points in between. The slope of this curve is the valve gain. The error for the system is defined as the difference between the setpoint and the leaving chilled water temperature. The leaving chilled water temperature of the system is measured to determine the actual error for the system, after which a linearizing gain derived as a function of the inverse of the valve gain is used in the system control algorithm to linearize the overall valve gain, thereby eliminating capacity valve hunting and producing an improved transient response. 
     According to an embodiment of the invention, a method for calibrating a capacity valve for an absorption chiller system includes the steps of (a) empirically determining a plurality of data points for said system that relate a position of said capacity valve to heat input into said system; (b) determining a continuous curve which estimates a relationship between said position of said capacity valve and said heat input for all of said plurality of data points and all points therebetween; (c) measuring a leaving chilled water temperature of said system; (d) defining an error for said system as a difference between a setpoint and said leaving chilled water temperature; (e) determining said error for said system; and (f) using a function of said relationship in a control algorithm for said system to reduce said error. 
     According to an embodiment of the invention, an absorption control system for an absorption chiller includes means for empirically determining a plurality of data points for said chiller that relate a position of a capacity valve to heat input into said chiller; means for determining a continuous curve which estimates a relationship between said position of said capacity valve and said heat input for all of said plurality of data points and all points therebetween; means for measuring a leaving chilled water temperature of said chiller; means for defining an error for said chiller as a difference between a setpoint and said leaving chilled water temperature; means for determining said error for said chiller; and means for using a function of said relationship in a control algorithm in said control system of said chiller to reduce said error. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic representation of an absorption chiller system; 
     FIG. 2 shows a relationship between the position of the capacity valve and the raw heat input for the FTU- 1  Weishaupt Burner; and 
     FIG. 3 shows the steps of the method of the present 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. 
     As mentioned in the Background section, one of the inherent control problems of an absorption chiller is that its thermodynamic properties create a slow moving cycle. This slow response to dynamic building loads is further amplified by a unique valve position vs. fuel consumption curve. That is, because of variations between installations, a curve relating the position of capacity valve  52  to the heat input for all installations is impossible to obtain. To overcome this problem, when the unit is first installed in the building, the service technician adjusts the capacity valve  52  to the minimum and maximum heat input values. Between these two points, the combustion characteristics are adjusted for a “clean burn” for a gas installation, i.e., adjusted to meet various pollution control requirements. This adjustment is dependent on several variables unique to the specific installation site; thus, the adjustment is done on-site at the time of installation. Only a few data points relating the position of capacity valve  52  to the heat input are known at the time of installation, so that the relationship between the valve position and the heat input is known only as either an assumed linear curve or as a step function. The data points have to be determined empirically during installation. 
     The controls that regulate the movement of capacity valve  52  have no feedback other than the leaving chilled water temperature from sensor  82  to determine the valve position, which controls the heat input to system  10 . The combination of the unique non-linear combustion curve and a slow moving cycle is one component of a problem termed “capacity valve hunting”, which is an undesirable effect that causes oscillations in the leaving chilled water temperature. The system  10  adjustments are either too much or too little, so that the actual leaving chilled water temperature oscillates around the setpoint. 
     Referring to FIG. 2, a relationship between the position of capacity valve  52  and the raw heat input for the FTU- I Weishaupt Burner is shown. According to the method of the invention, five data points are preferably taken from the initial burner setup during field installation, and a curve fitting program is applied to the data points. Data points for valve positions of 0%, 25%, 50%, 75%, and 100% can be used, or other data points can be used as long as the range goes from 0% to max%, where max% is the most open position for the capacity valve that would be used in a particular installation. Based on the measured data points, the relationship between capacity valve  52  position and raw heat input is determined by the curve fitting program, with the output shown as curve  90  in FIG.  2 . 
     The continuous curve  90  so obtained is then used in the transfer function of the control algorithm that controls system  10 . Curve  90  is of the form y=f(x), where y is the heat input and x is the gas (or steam) valve position. Because the leaving chilled water temperature is a function of the heat input, and the heat input is a function of the valve position, curve  90  relates the valve position to the leaving chilled water temperature. By using curve  90 , the desired effect, i.e., the leaving chilled water temperature, is reached quicker than was the case with the prior art method. The gain of the value function  90  is found by taking the partial derivative of heat flow with respect to the valve position. From the relationship y=f(x), with y≡heat flow and x≡valve position, the valve gain is ∂y/∂x. This non-linear gain varies as the valve position varies. For example, in curve  90  the valve gain remains relatively constant around 0.026 for valve positions &lt;30 and approaches  0  for valve positions &gt; 30. If the capacity valve PI control gains had been tuned for normal operation at a valve position around  40 , they would become much too large when the capacity valve moved to  20  due to the increased valve gain going from 40 to 20. The net result would be capacity valve hunting. By multiplying the capacity valve PI control output by a function of the inverse of the valve gain, the non-linear effects of the valve gain are negated resulting in an overall linear, i.e., constant, valve gain characteristic. 
     Referring to FIG. 3, the steps of the method of the present invention are shown. In step  91 , the data points are determined for the system which relate a position of the capacity valve to the heat input into the system. In step  92 , the continuous curve is determined which estimates the relationship between the position of the capacity valve and the heat input for all of the data points and all the points in between. In step  93 , the leaving chilled water temperature of the system is measured. In step  94 , the error for the system is defined as the difference between the setpoint and the leaving chilled water temperature. In step  95 , the actual error is determined for the system, after which the relationship defined by the continuous curve is used in the system control algorithm to reduce the error. 
     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.