Abstract:
An HVAC system for controlling the temperature and humidity of air supplied to a comfort zone includes a heater and a cooler operating concurrently. In response to a supply air temperature sensor, a PID control loop with substantially constant gain controls the cooler to maintain the zone air at a certain comfortable temperature. The heater, also under PID control, is controlled in response to a humidistat. To prevent the heater from overloading the cooler during periods of high cooling demand, a variable gain multiplier decreases the heater&#39;s gain when the cooler&#39;s output exceeds a predetermined limit; otherwise, the heater&#39;s gain remains substantially constant. When the cooler is operating above the predetermined limit, the multiplier varies linearly between positive-one and negative-one and does so inverse-proportionally with the cooling load to smoothly change the heater&#39;s PID control from direct acting to reverse acting as the cooler approaches its maximum capacity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The subject invention generally pertains to heating ventilating air conditioning systems (HVAC systems) and more specifically to control schemes for such systems. 
         [0003]    2. Description of Related Art 
         [0004]    Heat exchangers, referred to herein as coolers, are used for cooling air supplied to a comfort zone, such as a room or other area of a building. The cooler not only cools the air but also helps dehumidify it. To achieve greater dehumidification without over-cooling the comfort zone, some systems include a heater that is activated along with the cooler. An example of such a system is disclosed in U.S. Pat. No. 6,973,795. Although such a system is effective, it can be challenging to coordinate the control of both a heater and a cooler to provide an appropriate balance of heating and cooling. In some attempts to lower humidity, for example, the heater might overload the cooler to a point where the cooler is unable to maintain a desired target temperature of the comfort zone. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of some embodiments of the invention to provide a temperature conditioning system with a control scheme for a heater and a cooler, wherein a gain multiplier for the heater is varied as a function of the cooler&#39;s operation. 
         [0006]    Another object of some embodiments is to change the control of a heater from direct acting to reverse acting in response to the cooler approaching its maximum cooling capacity. 
         [0007]    Another object of some embodiments is to control a cooler in response to a temperature sensor while controlling a heater in response to a humidistat. 
         [0008]    Another object of some embodiments is to apply PID control to both a heater and a cooler, wherein the respective heater and cooler gains are substantially constant for the vast majority of their operating ranges; however, the heater gain only varies near the maximum operating point of the cooler while the cooler gain remains substantially constant over its entire range of operation. 
         [0009]    In some embodiments, the present invention provides a temperature conditioning system for simultaneously heating and cooling a current of air flowing to and passing through a comfort zone, wherein the current of air includes a coolable current of air and a heatable current of air. The temperature conditioning system comprises a heater connected in heat transfer relationship with the heatable current of air with a heat regulator connected to adjust a heat output of the heater. The system also includes a cooler connected in heat transfer relationship with the coolable current of air with a cooling regulator connected to adjust a cooling capacity of the cooler. The cooling regulator provides the cooler with a range of capacities including a maximum cooling capacity, a minimum cooling capacity, and a predetermined intermediate cooling capacity therebetween. The system further includes a sensor system exposed to the current of air. The sensor system provides a feedback signal representative of a thermodynamic condition of the current of air. Examples of the thermodynamic condition include, but are not limited to, comfort zone air temperature, supply air temperature, absolute humidity, relative humidity, etc. The system also includes a control system connected in signal communication with the sensor system, the heat regulator and the cooling regulator. The control system has a cooler control loop to control the cooling regulator. The control system has a heater control loop with a heater gain to control the heat regulator, wherein the heater gain varies more when the cooler is between the predetermined intermediate cooling capacity and the maximum cooling capacity than when the cooler is between the predetermined intermediate cooling capacity and the minimum cooling capacity. 
         [0010]    In some embodiments, the present invention provides a temperature conditioning system for simultaneously heating and cooling a current of air flowing to and passing through a comfort zone, wherein the current of air includes a coolable current of air and a heatable current of air. The temperature conditioning system comprises a heater connected to convey the warm fluid therethrough to place the warm fluid in heat transfer relationship with the heatable current of air. A warm fluid flow adjustor is connected in fluid communication with the heater to control the warm fluid flowing through the heater. A cooler is connected to convey the cool fluid therethrough to place the cool fluid in heat transfer relationship with the coolable current of air. A cool fluid flow adjustor is connected in fluid communication with the cooler to control the cool fluid flowing through the cooler. The cool fluid flow adjustor has a range of flow modes including a maximum flow mode, a minimum flow mode, and a predetermined intermediate flow mode therebetween. A blower is positioned to force the current of air sequentially through the heater and the cooler, so the heatable current of air passes through the heater, and the coolable current of air passes through the cooler. A desiccant wheel is exposed to the current of air. The desiccant wheel being rotatable to rotate generally opposite radial ends of the wheel between the heatable current of air and the coolable current of air. A sensor system is exposed to the current of air. The sensor system provides a feedback signal representative of a thermodynamic condition of the current of air. A control system is connected in signal communication with the sensor system, the warm fluid flow adjustor and the cool fluid flow adjustor. The control system has a heater control loop with a heater gain to control the warm fluid flow adjustor. The control system has a cooler control loop with a cooler gain to control the cooler fluid flow adjustor. The heater gain varies more when the cooler flow adjustor is between the predetermined intermediate mode and the maximum flow mode than when the cooler flow adjustor is between the predetermined intermediate flow mode and the minimum flow mode. 
         [0011]    In some embodiments, the present invention provides a temperature conditioning method for simultaneously heating and cooling a current of air. The current of air provides a coolable current of air and a heatable current of air. The current of air flows to and passes through a comfort zone. The temperature conditioning method comprises heating the heatable current of air; controlling the heating via a heating control loop that has a variable gain; cooling the coolable current of air; varying the cooling over a range of capacities that includes a minimum cooling capacity, a maximum cooling capacity and a predetermined intermediate cooling capacity therebetween; and adjusting the variable gain more noticeably when operating between the predetermined intermediate cooling capacity and the maximum cooling capacity than when operating between the predetermined intermediate cooling capacity and the minimum cooling capacity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic diagram of a temperature conditioning system according to one example of the present invention. 
           [0013]      FIG. 2  is a graph showing a heater gain multiplier changing as a function of a cooler&#39;s rate of cooling. 
           [0014]      FIG. 3  is a schematic diagram of another example temperature conditioning system. 
           [0015]      FIG. 4  is a schematic diagram of yet another example temperature conditioning system. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]      FIG. 1  schematically illustrates one example of a temperature conditioning system  10  for conditioning a current of air  12  supplied to one or more comfort zones  14 , such as rooms or other areas of a building. In this example, system  10  comprises a cooler  16  for cooling and/or dehumidifying air  12 , a heater  18  for reducing the relative humidity of air  12 , an active desiccant wheel  20  for transferring moisture from one air stream to another, and a blower  22 . Blower  22  forces force air  12  through heater  18 , wheel  20 , cooler  16  and comfort zones  14 . A controller  24  with a novel control scheme controls the heat transfer rates of cooler  16  and heater  18 . A return air duct  26  returns air to heater  18  and/or an exhaust air blower  28 , wherein blower  28  can be used to force some exchange of indoor air  12  with fresh outside air  12 ′. Variable air volume valves  30  can be independently controlled to apportion conditioned air  12  from cooler  16  to zones  14  as needed. 
         [0017]    Cooler  16  is schematically illustrated to represent any heat exchanger capable of cooling air  12 . A coolable air current  12   a  is the segment of air current  12  that is cooled by cooler  16 . Examples of cooler  16  include, but are not limited to, a finned tube heat exchanger, a stacked plate or microchannel heat exchanger, a coil, an evaporator of a refrigerant circuit, etc. Heater  18  is schematically illustrated to represent any heat exchanger capable of heating air  12 . A heatable air current  12   b  is the segment of air current  12  that is heated by heater  18 . Examples of heater  18  include, but are not limited to, a finned tube heat exchanger, a stacked plate or microchannel heat exchanger, a coil, a condenser of a refrigerant circuit, an electric resistance heating element, etc. 
         [0018]    For sake of example, system  10  will be described with reference to cooler  16  and heater  18  being coils or finned tube heat exchangers. To cool air current  12 , a pump  32  circulates a cool fluid  34  between cooler  16  and a chilled fluid source  36 . The term, “cool” means cooler than air current  12   a . Examples of fluid  34  include, but are not limited to, water, glycol, water/glycol mixtures, and refrigerant. The tubes of cooler  16  convey cool fluid  34  while air current  12   a  flows across the exterior of the tubes, thereby placing air current  12   a  in heat transfer relationship with cool fluid  34 . A supply air duct  38  conveys the cooled air current  12   a  to comfort zones  14 . 
         [0019]    To regulate the supply air temperature, one or more temperature sensors  40  sensing cool air current  12   a  and/or the room temperature of one or more comfort zones  14  provide a feedback signal  42  to controller  24 , which in response thereto provides a cooling output signal  44  that controls the degree of opening of a cooler valve  46 , which in turn controls the flow rate of cool fluid  34  through cooler  16 . Valve  46  comprises not only a closing element (e.g., valve plug) and its element seat but also a conventional actuator that moves the closing element. Through output signal  44 , controller  24  can vary the opening of valve  46  over a range of positions including, for example, completely closed or minimally open (minimum flow mode) for minimum or zero cooling capacity, fully or nearly fully open (maximum flow mode) for maximum cooling capacity, and various intermediate open positions therebetween. 
         [0020]    In this example, controller  24  is a microprocessor based controller applying a PID or proportional-integral-derivative control loop, wherein output signal  44  is based on the following equation: delta-output(n)=[K 1 ×delta-error(n)]+[K 2 ×error(n)]+[K 3 ×delta-squared-error(n)]. In this equation, delta-output(n) is the incremental change in output signal  44  at each step-n, K 1  is a proportional gain constant, K 2  is an integral gain constant, K 3  is a derivative gain constant, error(n) is the difference at step-n between the target air temperature and the actual air temperature as measured by temperature sensor  40 , delta-error(n) is the change in error at step-n, and delta-squared-error(n) is the change in delta-error at step-n, and “n” represents the step or number of executions. The constants K 1 , K 2  and K 3  can be various numbers (positive, negative or zero) chosen to achieve a desired response with the particular system to which it is applied. 
         [0021]    Condensate  48   a  draining from cooler  16  removes moisture from air current  12   a , thus cooler  16  helps dehumidify the air supplied to comfort zones  14 . For greater dehumidification of comfort zones  14  without overcooling them, heater  18  and desiccant wheel  20  can be activated along with cooler  16 . When a motor  50  rotates wheel  20  so that opposite radial ends of wheel  20  rotate between air currents  12   a  and  12   b  while cooler  16  and heater  18  are active, wheel  20  absorbs additional moisture  48   b  from air current  12   a . As wheel  20  rotates, wheel  20  later releases the absorbed moisture as indicated by the arrow representing moisture  48   c  being released to the relatively dry air current  12   b . The relative humidity of air current  12   b  has been reduced by heater  18  raising the temperature and thus increasing the moisture-holding capacity of air current  12   b . As air current  12  passes through cooler  16 , at least some of moisture  48   c  condenses on cooler  16  and drains therefrom as additional condensate  48   a , thus further dehumidifying the air. 
         [0022]    To control the humidity, one or more humidity sensors  52  sensing air  12  flowing to or through comfort zones  14  provide a feedback signal  54  to controller  24 , which in response thereto provides a heating output signal  56  that controls the degree of opening of a heater valve  58 . Valve  58 , in turn, controls the flow rate of a warm fluid  60  that a pump  59  circulates between a warm fluid source  61  and heater  18 . The term, “warm” means warmer than air current  12   b . One or more humidity sensors  52  and/or one or more temperature sensors  40  are considered herein as a “sensor system.” As for the warm fluid flowing through heater  18 , examples of fluid  60  include, but are not limited to, water, glycol, water/glycol mixtures, and refrigerant. The tubes of heater  18  convey warm fluid  60  while air current  12   b  flows across the exterior of the tubes, thereby placing air current  12   b  in heat transfer relationship with warm fluid  60 . Through output signal  56 , controller  24  varies the opening of valve  58  to adjust the heat output of heater  18  to achieve a desired humidity level or reading from feedback signal  54 . 
         [0023]    In some embodiments of the invention, controller  24  generates output signal  56  based on the following equation: delta-output(n)=M×{[C 1 ×delta-error(n)]+[C 2 ×error(n)]+[C 3 ×delta-squared-error(n)]}. In this equation, delta-output(n) is the incremental change in output signal  56  at each step-n, C 1  is a proportional gain constant, C 2  is an integral gain constant, C 3  is a derivative gain constant, error(n) is the difference at step-n between the target humidity and the actual humidity as measured by humidity sensor  52 , delta-error(n) is the change in error at step-n, and delta-squared-error(n) is the change in delta-error at step-n, and “n” represents the step or number of executions. The constants C 1 , C 2  and C 3  can be various numbers (positive, negative or zero) chosen to achieve a desired response with the particular system to which it is applied. In some examples, for instance, C 3  equals zero. The term, “M,” is a heater gain multiplier  62  ( FIG. 2 ) having a value that varies based on the cooling rate of cooler  16  or the extent to which the cooler&#39;s valve  46  is open. Strategically varying the value of heater gain multiplier  62  provides a means for preventing heater  18  from overheating air  12  to an extent that cooler  16  would be overloaded and unable to maintain air  12  at the desired target temperature. 
         [0024]    For the example illustrated in  FIG. 2 , heater gain multiplier  62  is substantially constant and equal to positive-one when the cooler&#39;s valve  46  is no more than 85% open to meet low to moderate cooling loads. The 85% point  64 , in this example, is a predetermined intermediate flow mode that provides cooler  16  with a predetermined intermediate cooling capacity. When valve  46  opens more than 85% to meet higher cooling loads, multiplier  62  decreases linearly from positive-one (valve  46  at 85% open) to negative-one (valve  46  100% open). In this example, multiplier  62  equals zero when valve  46  is open 92.5%. When the cooler&#39;s valve  46  shifts from being less than to greater than 92.5% open, the control of heater valve  58  shifts from direct acting to reverse acting. The decreasing gain or decreasing response of heater valve  58  as cooler valve  46  opens from 85 to 92.5% and the negative gain or reverse acting response of heater valve  58  when cooler valve  46  is open more than 92.5% prevents heater  18  from delivering more heat to air  12  than what cooler  16  can handle. 
         [0025]    It should be appreciated that the particular values of 85% and 92.5% are only examples and other reasonable values are certainly within the scope of the invention. However, since the idea is to prevent overloading cooler  16 , the predetermined intermediate flow mode to provide the predetermined intermediate cooling capacity should be closer to the maximum cooling capacity than to the minimum cooling capacity, as shown in the example of  FIG. 2 , wherein the predetermined intermediate flow mode of 85% (point  64 ) and the predetermined intermediate cooling capacity of 85% (point  64 ) are in fact closer to the maximum value of 100% (line  66 ) than to the minimum value of 0% (line  68 ). Moreover, multiplier  62  does not have to be perfectly constant and equal to one when valve  46  is open less than a predetermined degree, and multiplier  62  does not have to decrease necessarily linearly as valve  46  opens beyond the predetermined degree. 
         [0026]    In the example shown in  FIG. 3 , a temperature conditioning system  10 ′ includes a pump  70  driven at variable speed to replace valve  58  as the warm fluid adjustor or heat regulator of heater  18 . System  10 ′ also includes a pump  72  driven at variable speed to replace valve  46  as the cool fluid flow adjustor or cooler regulator. Through output signal  44 , in this example, controller  24  varies the speed of pump  72  over a range of speeds including, for example, zero or near zero speed (minimum flow mode) for minimum or zero cooling capacity, full speed (maximum flow mode) for maximum cooling capacity, and various intermediate speeds therebetween. 
         [0027]    Similar to system  10 , output signal  44  still is based on the following equation: delta-output(n)=[K 1 ×delta-error(n)]+[K 2 ×error(n)]+[K 3 ×delta-squared-error(n)]. Also, in this example, controller  24  generates output signal  56  based on the equation: delta-output(n)=M×{[C 1 ×delta-error(n)]+[C 2 ×error(n)]+[C 3 ×delta-squared-error(n)]}. 
         [0028]    Instead of driving valve actuators, however, output signals  44  and  56  command the operation of variable speed drives connected to the motors of pumps  72  and  70 . Similar to system  10 , heater gain multiplier  62  varies as shown in  FIG. 2 , wherein gain multiplier  62  is substantially constant and equal to positive-one when pump  72  is running at no more than 85% full speed to meet low to moderate cooling loads. When the speed of pump  72  is greater than 85% full speed to meet higher cooling loads, multiplier  62  decreases linearly from positive-one (85% full speed) to negative-one (pump  72  at 100% full speed). In this example, multiplier  62  equals zero when pump  72  is driven at 92.5% of full speed. 
         [0029]    In another example, shown in  FIG. 4 , a temperature conditioning system  10 ″ is similar to system  10  of  FIG. 1 ; however, system  10 ″ includes an electric heater  74  that replaces heater  18 , pump  59  and valve  58 . Otherwise, systems  10  and  10 ″ are the basically the same in structure and function. To vary the heat output of heater  74 , output signal  56  controls the electrical power from the heater&#39;s variable power supply  76 , thus variable power supply  76  serves as a heat regulator for heater  74 . The electrical power varies over a range of power levels from zero or near zero power for minimum heat capacity to full power for maximum heating capacity. 
         [0030]    Similar to system  10 , output signal  44  still is based on the equation: delta-output(n)=[K 1 ×delta-error(n)]+[K 2 ×error(n)]+[K 3 ×delta-squared-error(n)]. Also, in this example, controller  24  generates output signal  56  based on the equation: delta-output(n)=M×{[C 1 ×delta-error(n)]+[C 2 ×error(n)]+[C 3 ×delta-squared-error(n)]}, wherein the term, “M” or heater gain multiplier  62  varies as shown in  FIG. 2 . 
         [0031]    Referring to  FIGS. 1 and 2 , air  12   b  passing across heater  18  represents heating the heatable current of air  12 . Controller  24 , heater gain multiplier  62  of  FIG. 2 , and the equations mentioned herein represent controlling the heating via a heating control loop having a variable gain. Air  12   a  passing across cooler  16  represents cooling the coolable current of air. Controller  24 , output signal  44 , and  FIG. 2  represent varying the cooling over a range of capacities that includes a minimum cooling capacity, a maximum cooling capacity and a predetermined intermediate cooling capacity therebetween. The graph of  FIG. 2  represents adjusting the variable gain more noticeably when operating between the predetermined intermediate cooling capacity and the maximum cooling capacity than when operating between the predetermined intermediate cooling capacity and the minimum cooling capacity. The graph of  FIG. 2  represents maintaining the variable gain substantially constant when operating between the minimum cooling capacity and the intermediate cooling capacity. The graph of  FIG. 2  represents changing operation from the predetermined intermediate cooling capacity to the maximum cooling capacity and changing a polarity of the variable gain upon changing from the predetermined intermediate cooling capacity to the maximum cooling capacity. Arrows  48   b  and  48   c  of  FIG. 1  represents transferring moisture from the coolable current of air to the heatable current of air. 
         [0032]    Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those of ordinary skill in the art. Outputs  44  and  56 , for example, are described as incremental outputs; however, converting those outputs to absolute or cumulative rather than incremental is certainly within the scope of the invention. The scope of the invention, therefore, is to be determined by reference to the following claims: