Abstract:
A system and method of controlling air quality within a room. The method includes determining a first discharge air temperature setpoint based upon a room temperature setpoint and a first value indicative of an air temperature within the room, and determining a first air flow control signal based upon the first discharge air temperature setpoint and a second value indicative of an air temperature of discharge air being provided into the room. The method further includes controlling an air flow device based upon the first air flow control signal, and maintaining a heating device employed to influence the air temperature of the discharge air at a maximum heating level. The method additionally includes monitoring a level of carbon dioxide within the room to determine whether the level is below a predetermined threshold.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the implementation of heating, ventilation and air conditioning (HVAC) systems for controlling the air quality within one or more rooms of a building. In particular, the present invention relates to the control of HVAC systems to maintain air quality within desired temperature and carbon dioxide level parameters. 
     BACKGROUND OF THE INVENTION 
     Heating, ventilation and air-conditioning (HVAC) systems are widely used to control the air quality within rooms of buildings. A variety of different parameters concerning the air quality can be controlled. Two commonly controlled parameters include the air temperature within the room(s) and the carbon dioxide (CO2) levels within the room(s). 
     In typical HVAC systems, air quality within a room is controlled by mixing some fresh air (outside air) with some return air (existing room air), heating or cooling the mixture, and then providing that heated or cooled mixture back to the room as discharge air. Control of the ratio of the fresh air component to the return air component within the room is a key mechanism for controlling air quality. 
     With respect to controlling the CO2 levels within a room, the number of people in a room is the major source of CO2 generation. HVAC systems that employ Demand Control Ventilation (DCV) vary the amount of fresh air provided to the room in order to maintain the CO2 at or below desired levels, despite changes in the number of occupants within the room. 
     The proportion of fresh air within the discharge air can also be varied to influence the room temperature. 
     For example, when the fresh air temperature is lower than an economizer switch-over temperature setpoint, additional fresh air can be added to the return air to reduce the overall temperature of the discharge air and the room. 
     Although the relative mixture of the fresh air and return air components in the discharge air can be varied widely to control temperature and CO2 levels, the temperature of the discharge air cannot be reduced below a certain low limit without reducing the comfort of occupants within the room or causing excessive down draft. Consequently, the HVAC system typically must monitor the temperature of the discharge air and make sure that it does not fall too low. 
     Under many circumstances, an HVAC system can control (or at least influence) both the temperature of a room and the CO2 levels within the room simultaneously by varying the amount of fresh air being provided to the room. However, when CO2 levels are high but outdoor temperatures are low, control of both parameters simultaneously can become difficult. Because CO2 levels are high, presumably because of a high number of occupants within the room, a greater amount of fresh air is desirable to reduce the CO2 levels. At the same time, because the outdoor temperatures are low, large amounts of fresh air can overly reduce the discharge air temperature and create discomfort for the occupants. 
     More specifically, as long as the HVAC system is able to sufficiently heat the mixture of the fresh air and the return air to keep the discharge air temperature from falling below a desired discharge air temperature (DAT) setpoint, desired control of both the room temperature and the CO2 levels is possible. However, if the discharge air temperature falls below the DAT setpoint but the HVAC system is providing heat at or above its capacity, desired control of both the room temperature and the CO2 levels is limited. 
     It would therefore be advantageous if an HVAC system and method were developed that enabled optimal control of the air quality within a room when (i) high levels of fresh air are desirable in order to reduce excessive CO2 levels due to a large number of occupants in the room, and yet (ii) the fresh air temperature is sufficiently low that the HVAC system cannot provide sufficient heat to warm up the discharge air temperature to above a DAT setpoint. It would further be advantageous if such a HVAC system and method were still capable of providing optimal control of air quality under normal conditions, that is, under conditions where the HVAC system could provide sufficient heat to keep the discharge air temperature above the DAT setpoint. It would additionally be advantageous if, in order to implement such a system and method, major modifications to existing HVAC systems were not required. 
     SUMMARY OF THE INVENTION 
     The present inventors have discovered that a cascaded PI control loop control system and a CO2 alarm can be provided within an HVAC system which prioritizes the action of the HVAC system in situations where desired control of both CO2 levels and temperature levels within the room is limited as discussed above. Upon the occurrence of a situation in which the HVAC system is unable to provide any additional heating capacity, the HVAC system switches from a normal heating state to a low limit state. 
     In the low limit state, a low limit proportional integral (PI) control element provides a control signal to a fresh air damper to control the amount of fresh air being added to form the discharge air. The low limit PI control element bases its output upon the difference between the actual discharge air temperature and the discharge air temperature setpoint produced by a room PI control element, which in turn bases its output upon the difference between the actual room temperature and a room temperature setpoint. Also in the low limit state, the HVAC system continues to provide the maximum amount of heating possible. The HVAC system leaves the low limit state and returns to the normal state once the room CO2 level falls below the CO2 setpoint, such that a CO2 alarm shuts off. 
     The present invention relates to a method of controlling air quality within a room. The method includes determining a first discharge air temperature setpoint based upon a room temperature setpoint and a first value indicative of an air temperature within the room, and determining a first air flow control signal based upon the first discharge air temperature setpoint and a second value indicative of an air temperature of discharge air being provided into the room. The method further includes controlling an air flow device based upon the first air flow control signal, and maintaining a heating device employed to influence the air temperature of the discharge air at a maximum heating level. The method additionally includes monitoring a level of carbon dioxide within the room to determine whether the level is below a predetermined threshold. 
     The present invention further relates to a system for controlling air quality within a room. The system includes a damper for controlling an amount of a first type of air to be combined with a second type of air to form an air mixture, and an air pathway to which the damper is coupled, and within which the first and second types of air are combined to form the air mixture. The system additionally includes a coil assembly including a coil that is positioned within the air pathway and a valve, the coil affecting a temperature of the air mixture before the air mixture is output to the room as discharge air. The system further includes a controller coupled to the damper and to the valve, the controller providing first and second control signals respectively thereto. The controller operates in at least a first state and a second state, wherein in the first state the first and second control signals are varied to allow for the control of both an air temperature and a carbon dioxide level within the room, and in the second state the second control signal is maintained at a fixed level. 
     The present invention additionally relates to a system for controlling the air quality within a room. The system includes a first means for regulating an amount of air being added to the room, a second means for influencing the temperature of the air being added to the room, and a third means for controlling the operation of, and coupled to, the first and second means. The third means operates in at least a first control state and a second control state. In the first control state, the third means can control the first and second means so that both a temperature of the air within the room and a carbon dioxide level within the room are within desired ranges. In the second control state, the third means can control the first means so that at least the temperature of the air within the room is within a desired range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a heating, ventilation and air-conditioning (HVAC) system being used to control the air quality and climate within a room; 
     FIG. 2 is a state diagram showing two states of operation of the HVAC system of FIG. 1, namely a heating state and a low limit state, and the transition of the HVAC system between those two states; 
     FIG. 3 is a block diagram showing operation of the HVAC system of FIG. 1 in the heating state; and 
     FIG. 4 is a block diagram showing operation of the HVAC system of FIG. 1 in the low limit state, in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a HVAC unit  10  that is employed to control the air quality in a room  12  pumps discharge air  14  into the room by way of a fan  16 . The discharge air  14  is typically a mixture of fresh air  18  (outside air) and return air  20  (existing room air) that has been filtered by a filter  28  and then heated or cooled by way of a hot water/chilled water (HW/CW) coil  22 . The relative amounts of fresh air  18  and return air  20  that comprise the discharge air  14  are respectively controlled by the position of a fresh air damper  24  and a return air damper  26 . The dampers  24 , 26  are usually mechanically or otherwise coupled to one another so that the dampers are coordinated in their movement. Although in most circumstances, fresh air  18  is mixed with return air  20 , in certain circumstances, the discharge air  14  includes only one of fresh air  18  or return air  20 . 
     By combining fresh air  18  with return air  20  in order to produce the discharge air  14 , CO2 and other contaminants within the return air  20  are diluted. Additionally, depending upon the desired room temperature and the existing room temperature within the room, fresh air  18  is also in some cases mixed with the return air  20  in order to contribute to the heating or cooling of the air within the room  12 . For example, when the existing room temperature is higher than the desired room temperature, and the fresh air temperature is lower than an economizer switch-over temperature setpoint, the fresh air  18  can be added to the return air  20  to reduce the overall temperature of the room without additional cooling action by the HW/CW coil  22 . In these ways, control of the relative amounts of fresh air  18  and return air  20  allows for the maintaining of the indoor air quality (IAQ) of the room  12 . 
     Typically, the operation of the HVAC unit  10  is controlled by a controller  30  that is part of or coupled to the HVAC unit. The controller  30  receives indications of the temperature of the room  12  from a room sensor  32 , indications of the temperature of the discharge air  14  from a discharge air sensor  34 , and indications of the CO2 levels within the room  12  from a CO2 sensor  36 . Based upon these indications, as well as other information concerning a desired room temperature (“room temperature setpoint”) and desired CO2 levels (“CO2 setpoint”) within the room  12 , the controller  30  provides control signals which determine the positions of the fresh air damper  24  and the return air damper  26  and thereby determine the relative mixture of fresh air  18  and return air  20  within the discharge air  14 . The controller  30  further provides control signals to control the operation of a HW/CW valve  38  which determines the amount of hot or chilled (cold) water provided to the HW/CW coil  22  and thereby controls the heating and cooling action of the HW/CW coil. 
     Referring to FIG. 2, the system of FIG. 1 operates in at least two states, a normal heating state  100  and a low limit state  110 . The controller  30  operates differently depending on whether it is in the heating state  100  or the low limit state  110 , as shown in the block diagrams of FIGS. 3 and 4, which are discussed below. The two transition points between the two states are set so that there is smooth transitioning between the heating state  100  and the low limit state  110 . 
     As shown in FIG. 2, the controller  30  switches from the heating state  100  to the low limit state  110  when a heating proportional integral (PI) control element  56  (see FIG. 3) employed by the controller  30  in the heating state is saturated high, that is, the output of the heating PI control element remains at (or above) its maximum value for a saturation time period. The length of the saturation time period is adjustable, and in one embodiment is two minutes. The requirement that the heating PI control element  56  remain at its maximum value for the saturation time period before the controller  30  switches from the heating state  100  to the low limit state  110  prevents the system from leaving the heating state  110  merely as a result of instantaneous (or very short) periods of maximization of the heating output of the system. 
     The heating PI control element  56  typically becomes saturated high in situations where both the temperature of the fresh air  18  is low and the CO2 level within the room  12  is high (presumably because a large number of occupants are within the room). In such situations, the system attempts to bring in larger amounts of the cold fresh air  18  to reduce CO2 levels, but then must compensate for the coldness of the fresh air by providing additional heating of the mixture of the fresh air and the return air  20 . 
     When the output of the heating PI control element  56  is saturated high, this is indicative that the HW/CW coil  22  is providing the maximum amount of heat possible. That is, the HW/CW valve  38  is supplying the maximum amount of hot water to the HW/CW coil  22  that is possible to heat the mixture of fresh air  18  and return air  20  used to produce the discharge air  14 . Because the heating capacity of the system is at its maximum level when the heating PI control element is saturated high, the temperature of the discharge air  14  cannot be raised by further heating provided by the HW/CW coil  22 , and the room temperature cannot be controlled if the temperature of the discharge air  14  falls further. Consequently, the controller  30  switches to a new state of operation, namely, the low limit state  110 . 
     Once the controller  30  is in the low limit state  110 , it only returns to the heating state  100  when it is determined that a CO2 alarm turns off and a low limit timer has timed out, as discussed further with respect to FIG.  4 . When the CO2 alarm turns off, currently measured CO2 levels within the room  12  as measured by the CO2 sensor  36  are no longer higher than a CO2 setpoint. For the low limit timer to time out, the CO2 alarm must remain off (that is, the CO2 levels must remain at or below the CO2 setpoint) for a predetermined period of time, which in one embodiment is 15 minutes. Because the CO2 levels are less than (or equal to) the CO2 setpoint, less of the fresh air  18  is needed to be added to the discharge air  14  in order to dilute the existing CO2within the return air  20 . Consequently, the HW/CW coil  22  need not continue to provide maximum heat, and the system can operate in the heating state  100 . 
     The shutting off of the CO2 alarm and timing out of the low limit timer is indicative of a situation in which the system can return to the “normal” heating state  100  because either the temperature of the fresh air  18  has risen or the CO2 levels are no longer excessive (presumably because the number of occupants in the room  12  has decreased), or both. That is, CO2 levels could have fallen below the CO2 setpoint either because the number of occupants in the room  12  has decreased, or because the temperature of the fresh air  18  has increased and consequently the system has been able to bring in a greater amount of fresh air and dilute the CO2 levels in the room. 
     Referring to FIG. 3, a block diagram concerning the operation of the controller  30  in the heating state  100  includes two control loops, a heating control loop  40  and a CO2 control loop  42 . With respect to the heating control loop  40 , the controller  30  provides a control signal  55  to the HW/CW valve  38  based upon a room temperature signal  51  from the room sensor  32 , a discharge air temperature signal  53  from the discharge air sensor  34 , and the room temperature setpoint, which may be stored in memory in the controller  30  or provided from another source that is coupled to the controller. 
     The control signal  55  provided to the HW/CW valve  38  is determined as follows. The controller  30 , at a first comparator  50 , determines a difference between the room temperature setpoint and the room temperature signal  51 . This difference is provided to a room PI control element  52 , which in turn provides the discharge air temperature (DAT) setpoint as its output. The DAT setpoint is compared at a second comparator  54  with the discharge air temperature signal  53  from the discharge air sensor  34 . The difference between these two signals is provided from the second comparator  54  to the heating PI control element  56 , which in turn provides the control signal  55  to the HW/CW valve  38 . Therefore, the heating control loop  40  is actually a pair of cascaded control loops, the first generating the DAT setpoint based upon the room temperature signal  51  and the room temperature setpoint, and the second generating the control signal  55  based upon the discharge air temperature signal  53  and the DAT setpoint. 
     Further, with respect to the CO2 loop  42 , the CO2 setpoint that is provided from memory or from some other location is compared at a third comparator  59  with a CO2 level signal  57  from the CO2 sensor  36 . The difference between the CO2 setpoint and the CO2 level signal  57  is provided from the third comparator  59  to a CO2 PI control element  58 , which then provides an output signal to control the positioning of the fresh air damper  24 . Thus, in the heating state, the amount of fresh air  18  provided to the room  12  relative to the amount of return air  20  is determined based upon the CO2 levels in the room. In order to control the heat level in the room  12  in the heating state  100 , the controller  30  controls the heating provided by the HW/CW coil  22 . That is, the heating within the room is not controlled even in part by varying the relative proportions of the fresh air  18  and return air  20 . 
     As discussed, if the output of the heating PI control element  56  is saturated high (at its maximum level), the controller  30  switches to the low limit state  110 . In the low limit state  110 , the controller  30  operates in accordance with FIG.  4 . As shown, the controller  30  provides control of the fresh air damper  24  by way of an air control loop  46 . Also, the controller  30  operates to provide a CO2 alarm output at a CO2 alarm branch  49  and maximum heating capacity at a heating control branch  48 . The heating control branch  48  provides a 100% signal  68  to the HW/CW valve in order to maintain the output of the HW/CW valve  38  at its highest output. 
     Additionally, the fresh air damper  24  is controlled so that the amount of fresh air  18  being added to the discharge air  14  does not overwhelm the system. The fresh air damper  24  is controlled by a damper control signal  67  provided from the controller  30 , which is determined as follows. The room temperature signal  51  from room sensor  32  is compared with the room temperature setpoint at a comparator  60  (which may be the same as comparator  50 ), the output of which is provided to a room PI control element  62  (which may be the same as the room PI control element  52 ). The output of the room PI control element  62  is a discharge air temperature (DAT) setpoint that is then compared against the discharge air temperature signal  53  from discharge air sensor  14  at a second comparator  64  (which may be the same as comparator  54 ). The output of the second comparator  64  is in turn provided to a low limit PI control element  66 , which outputs the damper control signal  67  to the fresh air damper  24 . Typically, the fresh air damper  24  is controlled to be less than its maximum open position in the low limit state. As with respect to the heating control loop  40  of FIG. 3, the air control loop  46  is made up of two cascaded control loops, the first generating the DAT setpoint and the second generating the damper control signal  67 . 
     The CO2 level signal  57  provided by the CO2 sensor  36  is provided to a third comparator  69  (which may be the same as comparator  59 ) at which it is compared with the CO2 setpoint. The output of the third comparator  69  is provided to a CO2 alarm  70 , which provides either an on or an off signal depending upon whether the CO2 levels within the room are excessive or are within a range of acceptable levels (e.g., below the CO2 setpoint), respectively. Although the output of the CO2 alarm  70  does not directly control any device within the HVAC unit  10 , once the CO2 alarm continuously produces an off signal for the time out period of the low limit timer (shown as part of the CO2 alarm  70 ), the controller  30  returns to the heating state  100 . Thus, the system remains in the low limit state  110  only as long as the CO2 levels are excessively high. The requirement that the CO2 alarm  70  remain off for the entire length of the time out period prevents the system from leaving the low limit state  110  merely as a result of short-term dips in the CO2 levels below the CO2 setpoint. 
     In alternate embodiments, the proportional integral (PI) control elements can be replaced with other types of control elements (e.g., proportional integral differential or “PID” control elements). Although the comparators  50 ,  54  and  59  employed in the heating state  100  need not be the same as the comparators  60 ,  64  and  69  of the low limit state  110 , in certain embodiments these are the same elements. Likewise, the room PI control element  52  need not be the same as the room PI control element  62 , although in certain embodiments this is the case. In the preferred embodiment all of the comparators, control elements, alarms, timers and other control elements described as part of the heating state  100  and low limit state  110  are embodied in software within the controller  30 ; however in other embodiments, these elements may be hardwired elements. 
     While the foregoing specification illustrates and describes the preferred embodiments of this invention, it is to be understood that the invention is not limited to the precise construction herein disclosed. The invention can be embodied in other specific forms without departing from the spirit or essential attributes. Accordingly, reference should be made to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.