Abstract:
An exhaust control system ( 72 ) for a commercial or institutional kitchen exhaust system ( 32 ) is disclosed in which the exhaust fan speed is optimized for the amount of cooking heat and cooking by-product generated by the cooking units, as well as for comfort in the kitchen ( 12 ). Kitchen comfort is determined by sensing temperature, humidity, noxious gases, smoke, odor, or some combination thereof. In particular, exhaust air temperature can be used by the control system ( 72 ) to modulate fan speed from a minimum value to a maximum value based on the minimum and maximum temperatures that define a particular temperature span. During operation, the control system ( 72 ) continues to monitor environmental parameters of the kitchen ( 12 ) to determine if the current temperature span provides optimal performance. Upon determining that the current temperature span is no longer the optimal one, the control system ( 72 ) operates the exhaust system ( 32 ) according to a different temperature span.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     I. Field of the Invention  
         [0002]     The present invention relates to commercial and institutional kitchen exhaust systems, and more particularly, to an exhaust rate control method and apparatus for such exhaust systems.  
         [0003]     II. Discussion of Prior Art  
         [0004]     Commercial and institutional kitchens are equipped to prepare food for large numbers of people and may form part of or adjoin larger facilities such as restaurants, hospitals and the like. Such kitchens are typically equipped with one or more commercial duty cooking units capable of cooking large amounts of food. On such a scale, the cooking process may generate substantial amounts of cooking heat and airborne cooking by-products such as water vapor, grease particulates, smoke and aerosols, all of which must be exhausted from the kitchen so as not to foul the environment of the facility. To this end, large exhaust hoods are usually provided over the cooking units, with duct work connecting the hood to a motor driven exhaust fan located outside the facility such as on the roof or on the outside of an external wall. As the fan is rotated by the motor, air within the kitchen environment is drawn into the hood and exhausted to the outside atmosphere. In this way, cooking heat and cooking by-products generated by the cooking units follow an air flow path defined between the cooking units and outside through the hood to be exhausted from the kitchen before they escape into the main kitchen environment and perhaps into the rest of the facility.  
         [0005]     In many conventional installations, the motor driving the exhaust fan rotates at a fixed speed. The exhaust fan thus rotates at a fixed speed as well and, therefore, tends to draw air through the hood at a constant or fixed volume rate without regard to the amount of heat or cooking by-product actually being generated. As a result, there are often times throughout a working shift where the system may be under or over-exhausting. Under-exhausting allows heat and/or cooking by-products to build up in the kitchen or other parts of the facility, which can create discomfort and also overload the building heating and ventilation or air conditioning systems (“HVAC”). Similarly, over-exhausting wastes air that has been conditioned by the building HVAC, thus requiring further burden on the HVAC systems to make up the loss.  
         [0006]     To reduce the likelihood of over or under-exhausting, systems have been developed which vary the motor speed between a minimum and a maximum speed in fixed relationship to the exhaust air temperature, as shown in U.S. Pat. Nos. 4,903,685 and 6,170,480, both assigned to the assignee hereof and both of which are incorporated herein by reference in their entireties. While those systems offer substantial improvements to commercial kitchen exhaust systems, further improvements are desired.  
       SUMMARY OF THE INVENTION  
       [0007]     In those systems where the fan speed was varied in relation to exhaust temperature, for example, the relationship between that temperature and the fan speed could be seen as a fixed mathematical formula or as a single curve on a graph. I have discovered, however, that reliance on a fixed, single formula or single curve does not always provide optimal exhausting conditions. To this end, and in accordance with principles of the present invention, I have discovered that varying the formula or the curve (or by selecting from various formulae or curves) which defines the relationship by which fan speed is varied relative to exhaust temperature, can produce more optimal exhausting conditions within the facility. Advantageously, the relationship is varied in response to environmental parameters of the kitchen and/or ambient environment.  
         [0008]     More specifically, past efforts involving variation in the fan speed were based on a fixed linear relationship between temperature and fan speed, for example. Thus, in prior systems, the fan speed would vary over a temperature span defined by a fixed minimum and a fixed maximum temperature In such systems, the fan speed is, thus, operated at a minimum rate if the exhaust air temperature is below a predetermined minimum temperature, is operated at a maximum rate when the exhaust temperature exceeds a predetermined maximum, and is otherwise operated at a speed correlated to the temperature. I have discovered that there are various conditions in which the typical temperature span is not sufficient to provide the most desirable results. Rather, by providing different temperature spans, i.e., different curves which define different relationships between fan speed and temperature, for example, the fan speed may be different for the same temperature depending on the applicable temperature span, to thereby reduce the incidence of over or under exhausting. The different temperature spans also allow the exhaust system to use the exhaust heat to warm the kitchen or facility during cooler weather or to assist the HVAC system with cooling the kitchen during warmer temperatures.  
         [0009]     Other aspects of the present invention relate to further enhancements of a commercial kitchen exhaust system that determines which temperature span to use based on a number of different parameters. These parameters include such examples as whether the current temperature span results in the fan routinely operating at a speed above 90% (or some other threshold); whether the exhaust temperature routinely exceeds a predetermined temperature; whether operation at the current temperature span results in frequent, rapid rises in exhaust temperature through an operating day; and whether outside conditions can be used to effectively cool or heat the kitchen. With a number of different parameters available to effect the operating temperature span, different analyses based on these parameters may result in conflicting determinations about how to best change the operating temperature span. Accordingly, embodiments of the present invention advantageously include a “voting” system in which the different analyses are tallied to determine, by majority, how to change the operating temperature span temperature.  
         [0010]     Other aspects of the present invention relate to monitoring the exhaust temperature to determine when to automatically turn the exhaust hood on or off. Further aspects relate to determining an optimal minimum speed at which to operate the exhaust fan. With the data available from the monitored parameters, other aspects of the present invention relate to automatically determining if the exhaust system is out of balance or whether the system&#39;s maximum fan speed is set to operate within its designed capacity. Yet a further aspect of the present invention relates to monitoring both the intensity and the duration of smoke within an exhaust hood to effect a change in fan speed regardless of the fan speed specified by the current temperature span and exhaust temperature.  
         [0011]     By virtue of the foregoing, there is thus provided an exhaust system and method which provides for more optimal exhausting of a facility. These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1A  is a perspective view diagrammatically illustrating a restaurant or institutional facility, primarily the kitchen area and cooking units thereof, including a kitchen exhaust system according to principles of the present invention.  
         [0014]      FIG. 1B  is an exemplary depiction of multiple, different temperature spans over which exhaust fan speed may be modulated.  
         [0015]      FIG. 2  is a block diagram of an exemplary exhaust system according to principles of the present invention.  
         [0016]      FIG. 3  is a flowchart of an exemplary control algorithm implemented by embodiments of the exhaust system of  FIG. 2 .  
         [0017]      FIG. 4  is a flowchart of an exemplary algorithm for selecting an operating temperature span based on conditions within the environment of  FIG. 1A .  
         [0018]      FIG. 5  is a flowchart of an exemplary algorithm for selecting an operating temperature span based on other conditions within the environment of  FIG. 1A .  
         [0019]      FIG. 6  is a flowchart of an exemplary algorithm for selecting an operating temperature span based on still further conditions within the environment of  FIG. 1A .  
         [0020]      FIG. 7  is a flowchart of an exemplary algorithm for selecting an operating temperature span during cool weather.  
         [0021]      FIG. 8  is a flowchart of an exemplary algorithm for selecting an operating temperature span to convectively cool portions of the environment of  FIG. 1A .  
         [0022]      FIG. 9  is a flowchart of an exemplary algorithm for automatically turning on or off an exhaust hood according to principles of the present invention.  
         [0023]      FIG. 10 . is a flowchart of an exemplary algorithm for selecting a minimum fan speed according to embodiments of the present invention.  
         [0024]      FIG. 11  is a flowchart of an exemplary algorithm for modulating exhaust fan speed based on smoke density within the exhaust hood.  
         [0025]      FIG. 12  is a flowchart of an exemplary algorithm for determining if an exhaust system is out of balance.  
         [0026]      FIG. 13  is a flowchart of an exemplary algorithm for setting a maximum fan speed for an exhaust system based on a pressure differential between the kitchen and the outside.  
         [0027]      FIG. 14  is a flowchart of an exemplary algorithm for setting a maximum fan speed for an exhaust system based on a pressure differential between the inside of an exhaust hood and the environment outside of the exhaust hood.  
     
    
     DETAILED DESCRIPTION  
       [0028]     Exemplary Exhaust System and Environment  
         [0029]     Referring to  FIG. 1A , a facility  10  such as a restaurant or institutional facility includes a kitchen  12  and at least one adjacent room such as a dining room  14  with an interior wall  16  separating the two rooms  12 ,  14 . Kitchen  12  includes a plurality of commercial cooking units  18  such as one or more stoves, ovens, griddles and the like. The facility  10  is typically surrounded by an enclosure  20  (defined by a roof  22  and exterior walls  24  only one of which is shown in  FIG. 1A ) which separates the outside environment  26  from the inside ambient air environment  28  of facility  10  including kitchen  12 . Facility  10  is also equipped with a heating, ventilating and air conditioning system (“HVAC”) as at  30  which maintains the inside environment  28  at a suitable condition for the use of the occupants of facility  10 .  
         [0030]     Associated with kitchen  12  is kitchen exhaust system  32  including an exhaust hood  34  situated over the cooking units  18  and communicating with an exhaust assembly  36  through a duct  38 . Hood  34  generally defines a volume  46  that communicates through a downwardly facing opening to cooking units  18  and also communicates with exhaust assembly  36  via exhaust duct  38 . Exhaust duct  38  extends upwardly through the roof  22  of enclosure  20  and terminates in exhaust assembly  36  by which to exhaust air from volume  46  to the outside environment  26 . Exhaust assembly  36  may include a fan motor  50  and associated fan  51  as is well understood by which to expel air from assembly  36  at a volume rate. Thus, when motor  50  is running, an air flow path is defined between cooking units  18  and outside environment  26 . As air follows the air flow path, cooking heat and cooking by-products generated by the cooking units  18  are drawn along to be exhausted to the outside environment  26  rather than into the rest of the facility  10 .  
         [0031]     As will be explained in more detail below, a control system  72  controls the speed of the exhaust fan motor  50  according to a current operating temperature span and according to current environmental conditions. The control system  72  communicates with sensors  73 ,  76  over communication channels  75 ,  78  respectively and identifies operating conditions for the fan motor  50  and the fan  51  based on these environmental parameters. For example, sensor  73  is a temperature sensor for the ambient environment of the kitchen  12  and sensor  76  is a temperature sensor for the exhaust gasses exhausted from the hood  34 . The illustrated sensors  73 ,  76  are exemplary in nature and other types of sensors as well as their respective locations are contemplated.  
         [0032]     Facility  10  may advantageously include a make-up air system represented diagrammatically at  79  to bring air from the outside environment  26  to the ambient air environment  28  within kitchen  12  to compensate for the volume of air exhausted by the exhaust system  32 . Make-up air system  79  may be adapted to provide air in the vicinity just outside of the hood  34  to reduce the amount of air exhausted that has been conditioned by the HVAC system  30 . Alternatively, make-up air  79  may be introduced into other locations within kitchen  12  specifically, or facility  10  generally, as will be readily understood. The make-up air system  79  includes its own motor control  80  that communicates with the control system  72 .  
         [0033]     In accordance with principles of the present invention, the rate at which air is exhausted by the exhaust hood  34  is not simply limited to a fixed relationship with the exhaust temperature but, instead, is determined based on identifying one of a plurality of temperature spans (i.e., formulae, modifications to a formula, or curves) and then controlling the fan speed according to that temperature span.  FIG. 1B  depicts different temperature spans as curves on a graph. According to one embodiment of the present invention, the control system  72  selects, from stored memory, one of these temperature spans as the current operating span and then controls the exhaust fan speed accordingly.  
         [0034]     One exemplary temperature span  61 , over which embodiments of the present invention may control operation of the exhaust system  34 , varies fan speed in a linear manner between 75° and 95°. More particularly, according to this temperature span  61 , fan speed is at a minimum speed (e.g., 40%) when the exhaust temperature is at or below 75° and is at 100% when the temperature is at or above 95°. In between these two extreme temperatures, the fan speed varies linearly according to the temperature.  
         [0035]     Referring to  FIG. 1B , the temperature span  62  has the same minimum temperature (e.g., 75°) as span  61  but has a higher maximum temperature (e.g., 105°). Also, for example, the temperature span  63  includes a different minimum temperature and a different maximum temperature (e.g., 85° and 115°, respectively). As explained in more detail later, depending on different environmental parameters, the control system  72  may determine that one temperature span is a more optimal selection than another of the temperature spans and, therefore, change the current operating temperature span to the more optimal span. These are exemplary temperature spans and other, including, non-linear, temperature spans may be employed as will be appreciated by those skilled in the art.  
         [0036]     With respect to temperature spans  61  and  62 , a relationship can be defined wherein one temperature span is an “increase” or a decrease” from the other. A such, later descriptions herein describe the control system  72  “increasing” or “decreasing” the current temperature span. An increase in temperature span is one which increases the maximum temperature and, conversely, a decrease is one which decreases the maximum temperature. Although minimum temperatures for the temperature spans can be adjusted as well, the minimum temperature is typically set at a point considered to be comfortable, such as 75° F. Accordingly, to move from temperature span  61  to span  62  is an “increase”. The sloping portion of each span  61  and  62  can be characterized by a respective equation of the form y=Ax+B. Thus, to implement the different temperature spans, the control system  72  may store and retrieve the coefficients A and B for each temperature span or store the maximum temperature for a temperature span and calculate A and B accordingly.  
         [0037]     As one of ordinary skill would recognize, the curves of  FIG. 1B  are exemplary representations of a temperature span. More generally, a temperature span is a relationship between exhaust temperature and exhaust fan speed wherein the exhaust fan speed varies according to that relationship. Thus, a temperature span as used herein may encompass any of a variety of formulae, functions or curves that are linear, non-linear, continuous, or non-continuous in nature.  
         [0038]      FIG. 2  depicts a more detailed schematic view of the control system  72  and its interconnectivity with other aspects of the facility  10  and the exhaust system  32 . To this end, a motor speed controller  70  is provided by which to vary the speed of motor  50  and thus its associated fan so as to vary the volume rate of air exhausted through exhaust assembly  36 . Accordingly, the control system  72  is provided to couple volume rate signals over cable  74  to controller  70  by which to effect both the operating temperature span and the volume rate variations within that temperature span.  FIG. 1A  depicts exemplary sensors  73  and  76  that monitor one or more environmental parameters in the kitchen  12 , the ambient environment (e.g.,  26  or  28 ), or the exhaust hood  34  and provide such data to the control system  72 . Based, at least in part, on this data, the control system  72  determines the operating temperature span for the exhaust system  32 . However, the sensors  73  and  76  are exemplary in nature, only, and  FIG. 2  illustrates the variety of different sensors contemplated within the scope of the present invention. For example, a gas level sensor  96  and energy shut-off  112  may be present to provide safety features which are of benefit, for example, during an earthquake or other emergency. Also, a fire-suppression system  120  may be used to quickly squelch a fire under the hood  34 . Furthermore, various pressure gauges  101 ,  103  and temperature sensors  102  may be located inside or outside of the kitchen  12  to provide the control system  72  with data about other environmental parameters. A more complete description of these sensors and their operation are provided in the previously incorporated patents.  
         [0039]     With further reference to  FIG. 2 , it may be seen that control system  72  may include a microprocessor-based component or controller  130 , such as a model 807C52 microprocessor manufactured by Intel, with associated memory  132  which receives the signals from the various sensors  73 ,  76 ,  96 ,  82 , and  102  over cables  75 ,  78 ,  100 ,  90  and  104  and generates signals to the motor controller  70  (and  80 ) over cables  74  to achieve the above-described functions. By providing microprocessor capability in control module  72 , the various functions of the exhaust system  32  may be adjusted and more reliably controlled. The control system  72 , via a link  136 , may also include visual (or audible) indicators (not shown) and a user interface  134  for use by kitchen personnel.  
         [0040]     Control Algorithms  
         [0041]     The above description of an exemplary exhaust hood control system is provided to lay the foundation for discussing the aspects and features of embodiments of the present invention. A more complete description of the operation and systems within the exhaust system of  FIGS. 1A and 2  can be found in the aforementioned and incorporated U.S. patents. In general, however, in such a system, when the exhaust system  32  is first turned on, the exhaust fan  50  and make-up air fan may advantageously be set to start operating at a preselected minimum speed between 10% and 50% of a maximum rate. Alternatively, the fans can be briefly operated at 100%, to provide aural feedback to an operator that the fans are properly operating, and thereafter operated at the preselected minimum speed. As cooking takes place, the exhaust temperature within the hood  34  will likely increase. These increases are detected by the various sensors and, once some preset minimum exhaust temperature is reached, the speed of the fans is automatically increased, usually in proportion to the exhaust temperature according to the current operating temperature span. The speed continues to be increased until a maximum exhaust air temperature is reached, at which point, the speed is at 100% its maximum rate. Thus, there is a temperature span having a minimum temperature and a maximum temperature wherein the speed of the fans varies from a minimum rate (i.e., 10-50%) at the minimum temperature to a maximum rate (i.e., 100%) at the maximum temperature.  
         [0042]     Variations of cooking schedules, outside temperatures and inside temperatures that occur throughout a particular day, throughout a given week, and over the different seasons of the year result in different temperature spans being optimal at different times. Accordingly, the computer control system  72  of the exhaust system  32  includes provisions for determining which of a plurality of different temperature spans is an optimal span over which to control the fan speed and, thus, the air exhaust rate of the hood  34 . According to one embodiment, the optimal span is advantageously the one span which minimizes the energy consumed by operation of the hood  34  while maintaining comfortable conditions within the kitchen  12  or other space serviced by the hood  34 .  
         [0043]     The control system  72  monitors a number of parameters that provide information regarding which temperature span to use to control operation of the exhaust fan  51  (and, possibly, the makeup air fan  79 ). These parameters include, for example, the exhaust air temperature, the ambient kitchen temperature, and the outside temperature. Based on these parameters, the control system  72  adjusts the operating temperature span to effect different exhaust rates at a particular exhaust temperature within the exhaust system  32 . This change can occur almost immediately after testing certain conditions related to the parameters or postponed until the next operating day. Also, the change could be temporary or remain in place until conditions dictate otherwise. The control system  72  can also determine, based on the parameters, that the current temperature span remains the optimal span and, therefore, effects no changes.  
         [0044]     When the fans  51 ,  79  are first turned on, for an operating day, the control system  72  controls the fan speed and, thereby, the exhaust rate of the hood  34  according to a particular temperature span. For a newly installed system, a default temperature span is used for the first operating day. However, for a system that has been in operation, the selected temperature span may be a default span or, advantageously, be that which the control system  72  identified as the optimal temperature span during the previous operating day. During operation, the control system  72  monitors the conditions of the surrounding environment and modulates the fan speed according to the currently selected temperature span.  
         [0045]     In addition to using the values of the monitored parameters to control the fan speed within the selected temperature span, the control system  72  also uses these values to evaluate the appropriateness of the currently selected temperature span and change it if necessary.  
         [0046]     Temperature Span Selection  
         [0047]     The flowchart of  FIG. 3  illustrates a control method implemented by control system  72  to analyze the environment of facility  10  to determine whether the current temperature span over which the fans are operated is the optimal one. Thus, in step  170 , the control system operates the exhaust, and possibly the make-up fan, at an appropriate speed as explained above. While, the fans are operating, the control system monitors one or more of the environmental parameters of the facility  10 , in step  172 . These parameters may include the outside temperature, the kitchen temperature, the exhaust air temperature, and the presence of cooking effluents. Furthermore, for these parameters, in addition to instantaneous monitoring, historical data can be gathered, stored, and analyzed by the control system  72 .  
         [0048]     In step  174 , the control system  72  evaluates data about the monitored parameters according to one or more criteria. Although shown as a single block in the diagram of  FIG. 3 , the step  174  of evaluating the data can include a number of different tests conducted concurrently or sequentially. For each such test, the control system  72  determines in step  176 , based on the test results whether the temperature span should be increased or decreased. Also, the outcomes of the different tests of step  174  may conflict with one another such that one test indicates the temperature span should be increased, while another test indicates that the temperature span should be decreased. Thus, when making the decision to change the temperature span, the control system  72  could handle such conflicting results by imposing a simple majority rule or could weight the different test results differently so that some test results have more effect on the decision of step  176  than others.  
         [0049]     The control system  72  then changes the temperature span, in step  178 , in accordance with the determination made in step  176 . As explained in more detail below, the changing of temperature spans may occur almost instantaneously or can be delayed until the exhaust system  32  is turned-off and then back on.  
         [0050]     The monitoring of environmental conditions of the surroundings (step  172 ) and subsequent analysis (step  174 ) performed by the control system  72  of the exhaust system  32  involves a wide variety of inputs and decisions. For example, the control system  72  may determine if, throughout the day, the exhaust temperature exceeded the temperature span&#39;s maximum temperature for a predetermined time period, or whether the average fan speed exceeded 90% during the operating day.  
         [0051]     The flowchart of  FIG. 4  depicts one exemplary method that the control system  72  could implement to perform the above-described analyses and evaluations. In step  180 , the fans are turned on and begin operation within the currently set temperature span. During operation, the control system  72  modulates, in step  182 , the fan speed according to the conditions of the kitchen  12  such as exhaust temperature and effluent levels.  
         [0052]     In step  184 , the control system  72  analyzes the exhaust temperature to determine if it has exceeded the maximum temperature for the current temperature span for a particular time period. For example, the control system  72  determines if the maximum temperature of the current temperature span has been exceeded by more than 5° F. for at least ten minutes. If so, then the control system  72  decides, in step  186 , to raise the temperature span by one level.  
         [0053]     If not, then the control system  72  might still decide that the temperature span should be changed if other criteria are met. In step  188 , the control system determines if the average fan speed at the end of an operating day was more than a particular threshold such as, for example, 90%. If so, then the control system  72  returns to step  186  to decide to raise the temperature span. Advantageously, an operating day is one in which the exhaust hood has been turned on for at least 6 hours or some other predetermined time-period.  
         [0054]     If the average fan speed did not exceed the threshold, then the control system  72  performs additional analysis such as determining, in step  190 , if the minimum fan speed for the selected temperature span was exceeded during the operating day. If not, then the control system  72  decides that the temperature span should not change. If, however, the preset minimum speed was exceeded, then the control system  72  determines, in step  192 , if the fan speed exceeded 90% at any time during the day. If so, then the full temperature span is being utilized and the control system decides the current temperature span is optimal. If not, then the control system decides, in step  194 , that the temperature span should be changed to one having a lower maximum temperature in order to more fully utilize the entire temperature span.  
         [0055]     In the method depicted in  FIG. 4 , the decision to change the temperature span may be made during, or at the end of, one operating day but be implemented during the next operating day. However, embodiments of the present invention contemplate implementing temperature span changes during the same operating day as well.  
         [0056]     In addition to the criteria and determinations described above, a number of other operating parameters can be utilized by the control system  72  to decide whether the temperature span should be changed. The flowchart of  FIG. 5 , depicts an algorithm that investigates the change in exhaust temperature over time to determine if this data indicates a temperature span change is needed.  
         [0057]     As described earlier, the fan speed is controlled, in step  200 , according to exhaust temperature to vary between a minimum and maximum rate. During this operation, the control system  72  monitors the variation of exhaust temperature over time. If the control system determines, in step  202 , that the exhaust temperature has increased by more than 5° F., for example, in the last minute, then the fan speed can be increased, in step  204 . More particularly, if the fan speed is above 80%, then the fan speed is increased to 100%; otherwise, the fan speed is increased by 20%, in step  204 . The duration of this increase is for a predetermined time period, such as one minute. Other times and temperature changes may be used as well.  
         [0058]     The control system  72  monitors these temporary fan speed increases because each one indicates that the exhaust rate is insufficient to prevent relatively rapid increases in exhaust temperature. If the control system  72  determines that the fan speed is frequently increased, then a new temperature span should be selected. For example, if the control system  72  determines, in step  206 , that in a one hour period the fan speed was temporarily increased at least 10 times, then the control system  72  decides, in step  208 , that a different temperature span is needed having a lower maximum temperature. Alternatively, the control system  72  may average the data over the course of an entire operating day and make its decision, in step  206 , based on whether the temporary fan speed increases averaged more than 10 per hour over the entire operating day.  
         [0059]     Increases in temperature within the kitchen  12 , or other space, serviced by the hood  34  can be caused by spilling or dumping. If all the heat caused by cooking is not sufficiently exhausted by the hood  34 , then spilling occurs and the kitchen temperature increases. If the outside temperature is warmer than the desired temperature of the kitchen, then dumping can occur. Dumping conditions are caused by a exhaust fan speed that is too high. During dumping, even though all the heat from cooking is successfully removed by the hood  34 , more makeup air is introduced into the kitchen  12  than can be handled by the air conditioning equipment  30  of the facility  10  housing the kitchen  12 . Thus, when adjusting the temperature span to address increases in kitchen temperatures, the underlying cause of these increases is a factor to consider.  
         [0060]     According to the flowchart of  FIG. 6 , an exemplary algorithm is provided for adjusting the operating temperature span of an exhaust hood  34 . Similar to the previous techniques described, the control system  72  operates, in step  210 , the fan speed according to a current temperature span and varies the speed according to the continuously monitored exhaust temperature. Next, in step  212 , the control system  72  determines if the exhaust temperature is at least 20% more than the minimum temperature of the current temperature span. If not, then monitoring continues. If, however, the exhaust temperature is 20% into the temperature span, then the control system  72 , in step  214 , determines if either a) the fan has been operating for at least an hour or b) it has been an hour since this exemplary algorithm has increased the fan speed.  
         [0061]     If either of the conditions in step  214  are satisfied, then the control system  72  analyzes the exhaust temperature, in step  216 , to determine if it is stable. One exemplary test for stability is to determine if the temperature has not varied by ∓3° F. over a one-minute time period. If the exhaust temperature is stable, then the control system  72 , in step  218 , increases the fan speed by 20% (or to 100% if already operating above 80%) for a predetermined time period such as, for example, one minute. During and after the time period, the control system  72  monitors, in step  220 , the kitchen temperature to determine if the temperature increases or decreases.  
         [0062]     If the kitchen temperature increases, or stays the same, then increasing the fan speed caused dumping and the control system  72  decides, in step  224 , to maintain the current temperature span. If, however, in response to the increased fan speed, the kitchen temperature decreases, then spilling was occurring prior to the speed increase and the control system  72  decides, in step  622 , that a different temperature span is needed having a lower maximum temperature.  
         [0063]     Many of the previous algorithms address the effects that different cooking activities can have on the appropriate temperature span to select. However, even if cooking schedules remain the same over the course of a year, the optimal temperature span might change because of the outside temperature fluctuations due to seasonal differences. For example, during the winter for a given exhaust temperature, the fan speed can be decreased (by increasing the temperature span) so that some convective heat spillage can occur and provide free heat to the kitchen  12 . The control system  72 , according to  FIG. 7 , operates the variable fan speed, in step  230 , according to a current temperature span and monitors the environment  28 . In particular, through this monitoring, the control system  72  can determine, in step  232 , whether the kitchen temperature is less than 65° F. and, if so, changes, in step  234 , the temperature span to one having a higher maximum temperature thereby slowing the fans and warming the kitchen  12 .  
         [0064]     If the kitchen temperature is 65° F. or more, then the air temperature from the make-up air unit  79  is tested, in step  236 , to see if it is below 60° F. If so, then the control system  72  determines, in step  238 , whether the kitchen temperature is less than 68° F. If so, the control system  72  changes the temperature span, in step  234 , to one having a higher maximum temperature. However, if either the make-up air temperature is greater than or equal to 60° F., or the kitchen temperature is greater than or equal to 68° F., then the temperature span can remain the same, in step  240 . As this algorithm occurs throughout the operating day, a subsequent execution may determine that the temperature span no longer needs to be elevated. Under these circumstances, the decision made at step  240  can result in the temperature span reverting to the initial temperature span for that operating day. The above threshold temperatures are exemplary in nature and other embodiments of the present invention contemplate use of alternative threshold temperatures.  
         [0065]     According to one embodiment of the present invention, within the exemplary algorithm depicted in the flowchart of  FIG. 7 , step  234  is arranged so as to prevent increasing the temperature span by more than one level in a single operating day.  
         [0066]     Conversely to the algorithm of  FIG. 7 , the fan speed can be increased for a particular exhaust temperature (by decreasing the temperature span) to effect convective cooling within the kitchen  12 .  FIG. 8  illustrates an exemplary algorithm the control system  72  may implement to achieve this result. During operation according to a current temperature span, in step  244 , the control system  72  monitors, in step  246 , the kitchen temperature to determine if it is too warm. For example, a kitchen temperature of 80° F. would be considered uncomfortable by many kitchen personnel and some type of cooling would be desired. Thus, if the kitchen temperature indicates, in step  248 , cooling is needed, then the control system  72  decreases, in step  250 , the temperature span by selecting a different temperature span having a lower maximum temperature. According to the exemplary algorithm of  FIG. 8 , the control system  72  is prevented from decreasing the temperature span by more than one level during a single operating day. Once the kitchen temperature has decreased to a more comfortable temperature, such as 75°, the control system can revert back, in step  252 , to the original temperature span.  
         [0067]     According to the previously described algorithms, the control system  72  may make a number of independent decisions about whether the temperature span should be changed or remain the same. In certain circumstances, the control system  72  may make conflicting determinations about how to change a temperature span. For example, one algorithm may indicate that the temperature span should be increased while another, different algorithm indicates that the temperature span should be decreased. To handle such possibilities, one exemplary method of organizing the control system  72  is to consider each different criterium-test performed by the control system  72  for changing temperature spans as being a vote. At the end of an operating day, the control system  72  determines, for each algorithm, whether it indicates no change to the temperature span, an increase to the temperature span, or a decrease to the temperature span. A simple majority of the votes dictates how the control system will change the temperature span for the next operating day. Alternatively, it is also contemplated that different algorithms could be given different weights so that one algorithm can have more effect on the outcome than some other algorithm or that one algorithm will over-ride another.  
         [0068]     In addition to changing the temperature span for the next operating day, a change to the temperature span can be made almost immediately after detection of certain criteria by the control system. For example, the convective cooling algorithm described with respect to  FIG. 8 , or the winter setback algorithm described with respect to  FIG. 7 , may be implemented such that the control system  72  overrides the current temperature span by immediately increasing or decreasing the temperature span as indicated by the kitchen conditions. This override could be temporary until kitchen conditions indicate otherwise, could be temporary for a predetermined time period, could be for the remainder of the operating day, or could change the preset temperature span stored in the control system so that it is used to begin operation the next day.  
         [0069]     Automatic On/Off  
         [0070]     Typically, an exhaust hood  34  as described herein relies on kitchen personnel to manually power it on or off. However, the control system  72  and sensors can automatically control the on/off operation of such an exhaust hood  34  by following the exemplary flowchart depicted in  FIG. 9 .  
         [0071]     According to the exemplary flowchart, the control system  72  determines in step  256 , if the fans  51 ,  79  are running above the preset minimum speed. If they are, then their speed is controlled, in step  258 , according to the exhaust temperature or other monitored conditions as previously described. Furthermore, the control system  72  checks, in step  260 , whether the exhaust temperature has been below a particular minimum value for a predetermined time period likely indicating that cooking is not taking place. For example, the control system  72  could determine if the exhaust temperature has been below 75° F. for at least the last 15 minutes.  
         [0072]     Once the exhaust temperature satisfies the test of step  260 , any other connected exhaust hoods are tested in step  262 , if there are multiple hoods present. Once all the hoods satisfy the test of step  260 , the fans are operated, in step  264 , at their programmed speed at night. This night-time speed can be between 0% (i.e., “off”) or a value such as 20% that consumes little energy. Additionally, any displays or indicators associated with the hood  34  are deactivated as well, in step  264 .  
         [0073]     An alternative outcome to step  256  would be that the control system  72  determines the fans are not running at the minimum speed. In this instance, the control system checks, in step  266 , if a “turn-on” temperature threshold, such as 90° F., has been exceeded. If it has, then the control system  72  turns the fans on at the minimum speed, in step  268 , and afterwards modulates the fan speed according to the sensed conditions (step  258 ).  
         [0074]     Even if the turn-on temperature has not been exceeded, the control system  72  may still check, in step  270 , whether the exhaust temperature is rapidly increasing. If so, then the fans can be turned-on in step  268 . One exemplary indication of rapidly increasing temperature is whether the increase in exhaust temperature exceeds 5° F. over the last minute. However, other temperature deltas and time periods are contemplated as well. If a rapid temperature increase has not been detected, the control system  72  continues to monitor, in step  272 , the exhaust temperature in order to determine when to turn on the fans.  
         [0075]     Minimum Speed Selection  
         [0076]     In addition to identifying an optimal temperature span from among a plurality of temperature spans, the control system  72  can also adjust the minimum speed associated with the temperature span&#39;s minimum temperature. As described earlier, the control system  72  varies the fan speed, during operation, from a minimum speed to a maximum speed based on monitored conditions, typically, although not necessarily, in a linear manner. Thus, by increasing the minimum speed, the fan effectively operates at a higher speed along the entire temperature span.  
         [0077]     The flowchart of  FIG. 10  depicts an exemplary algorithm that can be implemented by the control system  72  to automatically determine if the minimum fan speed should be increased. The fans are operated, in step  276 , at the programmed minimum speed upon being powered-up. For example, this speed could be 20%. Next, in step  278 , the exhaust temperature is monitored so that the control system  72  can determines, in step  280 , whether there has been a rapid increase in exhaust temperature. The control system  72  may make this determination as the conditions are being monitored or merely collect the data over the course of an operating day and then perform the analysis of this data at the end of the day.  
         [0078]     If there has been a rapid increase in exhaust temperature during the operating day, the control system increases, in step  282 , the programmed minimum speed by some amount such as, for example, 10% for the next operating day. This increase can be limited by some maximum value so that the minimum speed is never allowed to be greater than, for example, 50% regardless of the outcome of step  280 .  
         [0079]     If, in step  280 , the increase in exhaust temperature did not exceed the threshold for “rapid”, then the historical data for the operating day is analyzed in step  284 . In particular, the control system  72  determines if, during the operating day, the exhaust temperature increased at a rate, for example, greater than 2° F./minute. If not, then the temperature variations during the operating day appear to be normal and no changes to the temperature span are warranted. If however, the temperature increases experienced during the day were above the 2° F./minute criteria of step  284 , then the control system  72  decides, in step  286 , to decrease the minimum speed by a predetermined amount such as 10% at the end of the operating day. The step  286  may be limited so that it never results in the minimum speed being decreased below a minimum value such as 10%.  
         [0080]     Smoke Density  
         [0081]     In addition to the different temperature data monitored, smoke density within the exhaust hood  34  may possibly indicate to the control system  72  when fan speed should be modulated. Referring back to  FIGS. 1A and 2 , an optical sensor  82  is typically connected with the control system  72  to detect the presence of smoke within the exhaust hood  34 . As explained in more detail within the aforementioned and incorporated patents, the sensor  82  is calibrated for an amount of light transmitted from a light source along a clear air path and detects the amount of light reduction caused by the presence of smoke within the exhaust hood  34 .  
         [0082]      FIG. 11  illustrates an exemplary algorithm by which the control system  72  can adjust the fan speed based on the presence of smoke even if the temperature data does not indicate that the fan speed should be adjusted. In accordance with this algorithm, the fans are operated, in step  290 , based on the exhaust temperature and the current operating temperature span, as described earlier. In step  292 , the control system  72  monitors the optic sensor  82  within the exhaust hood  34  to determine if it indicates the presence of smoke. The control system  72  determines, in step  294 , whether the sensor  82  indicates that there has been a noticeable reduction in the transmitted light. For example, a 5% reduction is one possible threshold at which to decide that corrective action is necessary. Otherwise, the control system  72  continues to monitor the sensor  82 , in step  292 .  
         [0083]     In step  296 , the control system  72  tests the smoke density a second time approximately one second after step  294  indicates smoke is present. By performing step  296  in this manner, the control system  72  can determine if the presence of smoke still exists and, further, it will recognize how long the smoke has been present. If smoke is still present, then the control system  72  adjusts the fan speed in step  298 . If no smoke is present, then the smoke has dissipated and control of the fan speed by the control system  72  can once again be based on exhaust air temperature. However, as a precaution, the fans are operated at their current speed, in step  300 , for a preset time period to ensure all the smoke has been successfully dissipated. An exemplary time period is one minute but the fans could be operated in this manner for a period ranging from a few seconds to over a minute.  
         [0084]     The adjusting of the fan speed, in step  298 , is performed in accordance with the following table:  
                                                                                 Elapsed Time   ≧5%   ≧7%   ≧9%                                        1s   60%   80%   100%           2s   80%   100%   100%           3s   100%   100%   100%                      
 
         [0085]     If the desired fan speed, determined according to one of the exhaust temperature control algorithms, is greater than an entry within the above table, this smoke density control algorithm will not decrease the fan speed as doing so would worsen the conditions in the exhaust hood.  
         [0086]     If, upon reaching step  296  the first time, the control system  72  detects that the light reduction remains greater than 5%, then a fan speed is selected from the first row of the table based on the detected percentage of light reduction. After adjusting the fan speed in step  298 , the control system  72  returns to step  296  and by now two seconds have elapsed. If the light reduction remains greater than 5%, then a fan speed is selected from the second row of the table. If the smoke has dissipated, however, the control system  72  operates the fans according to step  300  as explained above.  
         [0087]     If the control system  72  returns once again to step  296 , three seconds have elapsed and if the smoke persists, then a fan speed is selected from the third row of the table. Once step  296  is performed three times, the fan speed is at 100% regardless of the exact amount of light reduction detected. Thus, step  296  can be repeated over and over again until the smoke dissipates and control passes to step  300  but on these subsequent iterations, no new speed is selected from the table as the fans are already being operated at their maximum speed. Thus, both the duration and intensity of the smoke within the hood  34  is used to select a fan speed to help dissipate the smoke.  
         [0088]     Exhaust Hood Out of Balance  
         [0089]     There may be instances in which the exhaust hood  34  operates at the lowest temperature span (e.g., 75° F.-90° F.) and is still unable to effectively cool the kitchen  12 . Under these circumstances, the hood  34  is considered to be out of balance and the operator of the kitchen will need to use higher capacity motors and/or fans. The control system  72  can alert an operator to this condition by implementing the exemplary algorithm depicted in the flowchart of  FIG. 12 .  
         [0090]     According to this flowchart, the control system  72  operates and controls, in step  310 , the fans as described previously. The control system  72 , in step  312 , monitors the kitchen temperature as well as the exhaust temperature so that a determination may be made about the effectiveness of the exhaust system  32 . In step  314 , the control system  72  determines if the current temperature span is at the lowest level (i.e., has the lowest maximum temperature). If not, then no conclusions are made regarding the adequacy of the hood  34 . If, however, the temperature span is at the lowest level, then the control system  72  will analyze the data from step  314  to see if temperature fluctuations in these parameters occur concurrently. For example, the control system  72  determines, in step  316 , if temperature fluctuations in the kitchen  12  occur frequently, such as an increase of 5° F. in less than a minute occurring at least five times within one hour. The control system  72  also determines, if during these fluctuations, whether the exhaust temperature was increasing as well. If there are no exhaust temperature fluctuations or there is no correlation between the data, then the control system  72  continues to monitor, in step  312 , the temperature of the kitchen  12  and exhaust hood  34 .  
         [0091]     If, however, there are temperature fluctuations in both the kitchen temperature and the exhaust temperature and there is a correlation between the two data, then the cooking heat is likely spilling into the kitchen  12  because the hood is inadequate to fully capture it. In response, the control system  72  provides, in step  318 , an alert, or indicator, to an operator of this condition. Thus, when the exhaust system  32  is operating at the lowest temperature span and the control system  72  determines that exhaust heating results in kitchen heating, then the kitchen operator is notified that the hood, or hoods, are out of balance.  
         [0092]     Air Balance  
         [0093]     Before installing an exhaust system  32  such as that depicted in  FIG. 1A , a number of parameters are analyzed in order to design the system so that it is capable of performing in its intended environment. For example, the size of the exhaust hood  34  and the capacity of the fans  51 ,  79  are both design characteristics that are decided beforehand. In particular, analysis of the intended environment includes determining what pressure difference between the inside of the exhaust hood  34  and the outside of the exhaust hood will result in adequate performance that keeps the facility comfortable. Therefore, when installed, the exhaust fan  51  needs to be capable of operating at a maximum speed which results in that pressure differential and, in addition, the exhaust fan  50  should not unnecessarily expend power by operating above that maximum speed.  
         [0094]     As described herein, instead of specifying the fan speed as an absolute value such as 200 revolutions per minute (RPMs), fan speed has been referred to as a ratio such as 50% or 90%. This ratio is more precisely the ratio of a particular fan speed as compared to the maximum fan speed for a particular exhaust hood application. Thus, when the control system  72  is modulating the fan speed to a value such as 20% or 80%, it is using a pre-stored value of the maximum fan speed to do so. The maximum fan speed usually occurs when the control signal modulates the fan speed between 50 Hz and 60 Hz. By increasing the frequency sent to the motor control  70  (or  80 ), for example to 60 Hz, the control system  72  will cause an increase in the fan speed and by decreasing the frequency towards 50 Hz, the fan speed will decrease.  
         [0095]     Once an exhaust hood  34  is installed at a particular location  10 , some of the pre-programmed control parameters may need to be modified because the “real-world” exhaust system does not behave exactly as expected. Also, as equipment ages and wears, the original control parameters may no longer be at optimal values. One such parameter is the maximum speed for the exhaust fan and another such parameter is the maximum speed for the makeup air unit  79 .  
         [0096]      FIG. 13  depicts an algorithm for an exemplary method of using pressure differential measurements to set an maximum fan speed. This algorithm can be performed upon initial installation of an exhaust system  32  and periodically over its life. In step  320 , the control system  72  operates the fans before initiation of an auto-balance routine as described below. As described with respect to  FIG. 2 , a differential pressure gauge  101  can be positioned so as to sense the difference in pressure between the outside  26  of the kitchen  12  and the inside  28 . Furthermore, this gauge  101  includes a relay that changes state when a threshold difference is exceeded. Although one of ordinary skill would readily recognize alternative techniques for detecting when a differential pressure exceeds a threshold, the Photohelic® gauge and relay manufactured by Tierra Universal, Inc. is an exemplary simple technique for doing so. Thus, in step  322 , the control system  72  monitors the gauge and relay  101  to detect its condition. Also, in step  324 , a previously determined maximum speed signal is sent to the fans so that they operate at 100% speed. The control system  72  retains this information from previous operating days; however, if no maximum speed data is present, then the control system can start at a minimum threshold such as 50 Hz. In step  326 , the control system  72  determines whether the gauge and relay  101  indicate that the relay is open or closed. If the relay is open, then the pressure within the kitchen  12  is too low and the make up unit  79  fan speed should be increased. However, instead of rapidly increasing the fan speed, the exemplary algorithm, in step  330 , ramps the control signal slowly by, for example, +0.5 Hz/sec. As the fan speed is being increased, the control system  72  continues to monitor the gauge and relay  101  in step  326  to determine when it closes. The value of the control signal in step  330  that results in the gauge and relay  101  closing becomes, in step  332 , the “maximum speed’ used by the control system  72  when setting fan speeds. The control system  72 , thus, stores this value in its memory  132 .  
         [0097]     If, however, the gauge and relay  101  originally indicated that the relay was closed, then the pressure within the kitchen  12  is too high and the fan speed should be decreased. The control system  72  slowly decreases, in step  328 , the modulating control signal to the motor controller  80  by ramping down at −0.5 Hz/sec. When the control system  72  detects that the speed has been decreased enough to open the gauge and relay  101 , then the fan speed is increased slowly (as in step  330 ) until the gauge and relay  101  once again closes. The value of the control signal in step  328  that results in the gauge and relay  101  closing becomes, in step  332 , the “maximum speed” used by the control system  72  when setting fan speeds. The control system  72 , thus, stores this value in its memory  132 .  
         [0098]     In exemplary embodiments of the present invention, the modulating control signal is not decreased lower than 50 Hz in step  328 , nor increased past 60 Hz in step  330 . The specific actions described above with respect to the gauge and relay  101  being opened or closed can be alternatively performed. For example, the gauge and relay acts as a binary logic device, whether it is specifically the “open” or “closed” state that is associated with too little or too much pressure is immaterial to the scope of the present invention. The gauge and relay  101  provide a input to the control system  72  by which it can determine whether a maximum fan speed should be increased or should be decreased based on differential pressure data between the inside and outside of the building.  
         [0099]     A similar algorithm can be implemented with the exhaust fan speed, as shown in  FIG. 14 . However, in this instance, the pressure differential of interest would be that between inside the exhaust hood  34  and the outside of the exhaust hood  34  and, thus, a gauge and relay  103  would be located to detect and indicate a pressure differential between these two areas. The steps  334 - 346  of this algorithm are similar to those of  FIG. 13  and will not be discussed in as much detail. In summary, the control system  72  determines, in step  340 , whether the fan speed of the exhaust fan should be increased, in step  344 , or decreased, in step  342 , based on the state of the gauge and relay  103 . Once, the modulating control signal to the controller  70  of the exhaust fan  51  is changed to result in the pressure differential matching the design parameters of the hood  34 , then this value of the control signal is stored, in step  346 , in the memory  132  of the control system  72 . The control system  72 , then uses this value as the 100% reference when setting the fan speed according to the other control algorithms described herein.  
         [0100]     In use, embodiments of the present invention in its broader aspects are not limited to the specific details, representative apparatus and methods, and illustrative examples shown an described. Accordingly, departure may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept. For example, in its broadest sense, changing a temperature span results in changing the fan speed at a particular temperature between the two different spans. Thus, different temperature spans do not necessarily require different minimum and/or maximum temperatures. This is merely one way of accomplishing fan speed changes by linearly changing the fan speed according to temperature. Alternatively, operating the fan speed according to one of many different non-linear variations of fan speed across a temperature span will also result in changing the fan speed for a given temperature, even if the minimum and maximum temperatures remain constant. Additionally, in its broadest sense, a temperature span is a relationship between an exhaust temperature and a fan speed; thus, different temperature spans may be effected by a function that maps temperature to fan speed wherein each temperature span has different coefficients or different mapping functions. Thus, implementing different temperature spans does not necessarily require changing a minimum or maximum temperature.  
         [0101]     Additionally, the control algorithms described above included exemplary temperatures, rate of temperature changes, and predetermined time periods. These specific values are exemplary only and other, alternative values are contemplated within the scope of the present invention.