Abstract:
A controller automatically determines drive signals by testing an exhaust system, either immediately after installation or at selected times thereafter, to determine the drive signal values that correspond to each of one or more selected flow rates. The drive signals are stored. Thereafter, the controller uses the stored values of drive signals to control the exhaust system. This avoids problems with real time control such as drift or failure of sensors and such which are very common in commercial exhaust installations.

Description:
BACKGROUND  
       [0001]     One of the problems with installing exhaust hoods in industrial, commercial, and large residential systems is adjusting the flow rate of each hood so that a minimum volume of air is exhausted to ensure capture, containment, and removal of effluent. The performance of a hood, however, is very variable depending upon how it is installed. Often, unforeseen adjustments made in the size and length of ducting and other variables established during installation make it impossible to select an exhaust blower configuration which will deliver a desired exhaust flow once a hood is installed. Because of the cost of unnecessarily high exhaust capacity, it is important to establish a desired exhaust flow upon installation.  
         [0002]     Currently, one way of dealing with this problem is for an installer to perform a flow measurement and make adjustments to a fan system to establish a desired flow. However, such field measurements and procedures are time consuming and subject to error and common sloppiness.  
       SUMMARY  
       [0003]     Briefly, A controller automatically determines drive signals by testing an exhaust system, either immediately after installation or at selected times thereafter, to determine the drive signal values that correspond to each of one or more selected flow rates. The drive signals are stored. Thereafter, the controller uses the stored values of drive signals to control the exhaust system. This avoids problems with real time control such as drift or failure of sensors and such which are very common in commercial exhaust installations. A variable frequency motor drive can be used, for example. The system may be used in combination with real time control. If a failure of the real time control system is detected such as by detecting out-of-range sensor or drive signal (for feed-forward control) values, the controller can default to the stored drive signal values. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is an illustration of an exhaust hood with a flow control system.  
         [0005]      FIG. 2  is a more detailed illustration of a control system shown in  FIG. 1 .  
         [0006]      FIG. 3  is a flow chart illustrating a control method.  
         [0007]      FIGS. 4A and 4B  illustrate alternative details of a simple feedback or feed-forward control loop with the escape.  
         [0008]      FIG. 5  illustrates a control method which is an alternative to the one of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0009]      FIG. 1  illustrates an exhaust hood  145  with a flow controller/drive unit  105 . A fan  310  draws air through a duct  180  that leads away from recess  135  of the exhaust hood  145 . A filter  115  separates the recess  135  from the duct  180  and causes a pressure drop due to the known effect of grease filters in such applications. A pressure sensor  140  measures a static pressure which can be converted to a flow rate based on known techniques due to the flow resistance caused by the filter  115 . A differential pressure reading may also be generated using an additional pressure sensor  142  or a differential sensor (not shown separately) with taps upstream and downstream of the filter.  
         [0010]     Instead of a filter, reference numeral  115  may represent an orifice plate or other calibrated flow resistance device and may include a smooth inlet transition (not shown separately) to maximize precision of flow measurement by means of pressure loss. Instead of pressure sensors  140  may represent a flow measurement device such as one based on a pitot tube, hot wire anemometer, or other flow sensor. The sensor  140  may be replaceable since, as discussed below, it is used only once or intermittently so that replacement would not impose an undue burden.  
         [0011]      FIG. 2  illustrates details of the controller/drive unit  105  according to an embodiment of the invention. A fan  31   1 , which may correspond to the fan  310  of  FIG. 1 , is driven at a selected speed by a variable speed drive  300 . The latter may be an electronic drive unit or a mechanical drive with a variable transmission or any other suitable device which may receive and respond to a control signal from a controller  320 . The latter is preferably an electronic controller such as one based on a microprocessor. The controller  320  accesses stored data in a memory  330 . The memory may contain calibration data such as required to determine flow rate from pressure readings or anemometer signals (illustrated generally as a transducer  340  and flow sensor  350 ). In addition, the memory  330  may also store a predetermined flow rate value at which the associated exhaust hood  145  (See  FIG. 1 ) is desired to operate. Thus, the controller  320  can determine a current flow rate and compare it to a stored value and make corresponding adjustments in fan speed (or otherwise control flow, such as by means of a damper).  
         [0012]     The memory  330  also stores fan speed value so that once a particular fan speed is determined to achieve a desired flow rate (e.g., one predetermined value stored in memory  330 ), the associated fan speed can be stored in memory  330  and used to control the fan after that. In this way, the required fan speed need not be determined, as in common feedback control, each time the system operates. This is desirable because the accuracy of flow measurement devices is notorious for its tendency, particularly in dirty environments such as exhaust hoods, to degrade over time.  
         [0013]      FIG. 3  illustrates a control procedure for use during set-up when a hood is installed. First a command is issued at step S 90  to start the exhaust hood. In step S 95 , it is determined whether a fan speed has been determined by a configuration procedure. If not, control proceeds to step S 20 . In step S 20  the fan is started and a flow rate measurement is made in step S 30 . The flow rate is compared with a value stored in the memory  330  at step S 40  and if it is equal (assumed within a tolerance) to the predetermined value, control proceeds to step S 80 . If the flow rate is unequal it is determined if the flow rate is higher at step S 50  and if so, the fan speed is increased at step S 70  and if not, the fan speed is decreased at step S 60 . After step S 60  or S 70 , the comparison is repeated at step S 40  until the predetermined and measured flow rates are substantially equal.  
         [0014]     In step S 80 , the value of the fan speed (or corollary such as a drive signal) is stored in the memory  330 . In addition, step S 80  may include the step of setting a flag to indicate that the procedure has been run and a desired fan speed value stored. The stored value is retrieved at step S 100  and applied to operate the fan at step S 105 . If the configuration process S 20  to S 80  had been run already, the flow would have gone from step S 95  to step S 100  directly resulting the exhaust hood operating at the fan speed previously determined to coincide with the desired flow.  
         [0015]     In another embodiment, the memorized driver signal is used as a default driver signal. Input control signals are permitted to supersede the default driver control when the difference between the desired level exceeds the default by a specified margin. The iterative control process is encapsulated in step S 115 . Iterative control may be according to any suitable real-time (feed-forward or feedback) control method, for example ones discussed in U.S. Pat. No. 6,170,480, hereby incorporated by reference as if set forth in its entirety, herein. In step S 115 , if the inputs of a feedback control signal lie outside a specified range, the default drive signal stored in the memory is used. Detection of an input range outside the specified range causes control to escape E 10  and return to the default drive signal. If the feedback control signal(s) lie within the specified range, feedback control is used to determine the drive signal.  
         [0016]      FIGS. 4A and 4B  illustrate the possible details of a simple feedback or feed-forward control loop with the escape. Step S 105  is the same as the similarly numbered step of  FIG. 3 .  FIG. 4A  corresponds to a feedback control method. A stored drive signal is applied by default to drive the fan. Then at step S 135  the real time conditions are detected and converted to values or levels that can be compared with stored values or signal levels defining a safe operating window. At step S 140 , it is determined if the detected real time conditions are within the safe window. If they are, control proceeds to step S 150  and if not, the escape path E 10  is taken and stored default drive signals are applied. In step S 150 , a feedback setpoint is compared to the detected real time values of the feedback control signal and adjusted accordingly as indicated by steps S 155  and S 145 , respectively whereupon control proceeds back to step S 135 .  
         [0017]      FIG. 4B  corresponds to a feed-forward control method. Step S 105  is the same as the similarly numbered step of  FIG. 3 ; a stored drive signal is applied by default to drive the fan. Then at step S 136  the real time conditions are detected and converted to values or levels that can be compared with stored values or signal levels defining a safe operating window or used to generate a drive signal, at step S 170 , using a feed-forward control method.  
         [0018]     Feed-forward control is not described here, but feed-forward control, in general, is conventional. An example of feed-forward control applied to a complex ventilation problem (among other things) is described in U.S. patent Ser. No. 10/638,754, entitled “Zone control of space conditioning system with varied uses” which is hereby incorporated by reference as if fully set forth in its entirety herein.  
         [0019]     At step S 180 , the detected signals or the predicted drive signal are compared with values defining an allowed window and determined to acceptable or not. In other words, S 180  may compare a drive signal value to an allowed range stored in a memory of the controller or it may compare the real time condition signal to specified values stored in a controller memory, similar to step S 140  of  FIG. 4A . Detection of a value outside the specified range causes control to escape E 10  and return to the default drive signal. Otherwise, the predicted drive signal is used to drive the exhaust system and control returns to step S 135 .  
         [0020]      FIG. 5  illustrates another control procedure for use during set-up when a hood is installed. First, as in the embodiment of  FIG. 3 , a command is issued at step S 90  to start the exhaust hood. In step S 95 , it is determined whether a fan speed has been determined by a configuration procedure. If not, control proceeds to step S 200 . In step S 200 , an index (counter value) n is initialized whose value will span the number of different control conditions to be covered by the instant procedure.  
         [0021]     In step S 20  the fan is started and a first stored value of a desired flow rate is read. Each of N flow rate values F n  corresponds to a respective desired flow rate associated with particular one of N operating conditions. Each F n  is stored in a controller memory. A flow rate measurement is made in step S 30  and compared with the current F n  (the value of F n  corresponding to the index value n initialized in step S 200 . If it is equal (assumed within a tolerance) to the predetermined value, control proceeds to step S 215 . If the flow rate is unequal it is determined if the flow rate is higher at step S 250  and if so, the fan speed is increased at step S 70  and if not, the fan speed is decreased at step S 60 . After step S 60  or S 70 , the comparison is repeated at step S 240  until the current flow value F n  and measured flow rates are substantially equal.  
         [0022]     In step S 215 , the value of the fan speed (or corollary such as a drive signal) drive signal is stored in the n th  one of N memory locations  330 . In addition, step S 215  may include the step of setting a flag to indicate that the procedure has been run and the desired fan speed values stored when n reach N. The value of the index n is incremented in step S 220  and if all values of F n  have not yet been set, control returns to step S 225 . Otherwise control goes to step S 240 . Conditions are detected in step S 240  and the associated stored value of the driver signal determined in step S 245 . The determined drive signal is then applied in step S 105  and control loops back to step S 240 .  
         [0023]     In another embodiment, the memorized driver signal is used as a default driver signal. Input control signals are permitted to supersede the default driver control when the difference between the desired level exceeds the default by a specified margin. The iterative control process is encapsulated in step S 115 . Iterative control may be according to any suitable real-time (feed-forward or feedback) control method, for example ones discussed in U.S. Pat. No. 6,170,480, hereby incorporated by reference as if set forth in its entirety, herein. In step S 115 , if the inputs of a feedback control signal lie outside a specified range, the default drive signal stored in the memory is used. Detection of an input range outside the specified range causes control to escape E 10  and return to the default drive signal. If the feedback control signal(s) lie within the specified range, feedback control is used to determine the drive signal.  
         [0024]     In step S 240 , the conditions detected may be, for example, the fume load predicted from one or more inputs. For example, the time of day (a restaurant that cooks according to a particular schedule) can be used to determine the fume load. Another input may be an indication of whether a protected fume source, such as a kitchen appliance, has been turned on and for how long. The fuel consumption rate may also be used. Other kinds of detection mechanisms may also be employed, such as described in U.S. Pat. No. 6,899,095 entitled “Device and method for controlling/balancing flow fluid flow-volume rate in flow channels,” hereby incorporated by reference as if fully set forth in its entirety herein. Expected flow values for the following exhaust conditions are listed here for an example: (1) full load; (2) intermediate load; (3) idle; (4) initialization (e.g., burners turned on, but no cooking yet) in winter; (5) initialization in summer. The reason summer and winter (or it could be based on temperature) may be different is that the heat liberated by a heat source may be undesirable in summer but more acceptable during winter time.  
         [0025]     The sensors used for feedback or feedforward control may include any of a variety of types which may be used to prevent escape of pollutants from an exhaust hood. The flow sensors used for determining drive signals associated with desired flow rates may be any type of flow sensor. Preferably, the flow sensor is one which is robust and which is not overly susceptible to fouling. One of the fields of application is kitchen range hoods, which tend to have grease in the effluent stream. For example, static pressure taps with pressure transducers in the exhaust duct may provide a suitable signal.