Abstract:
Burner control systems and methods of operating a burner are for a large boiler such as might be used in hospitals, hotels, offices or other large commercial or domestic premises.

Description:
[0001]    This application claims priority to United Kingdom application serial no. 1018178.2 filed on Oct. 28, 2010, and which application is incorporated herein by reference. To the extent appropriate, a claim of priority is made to the above disclosed application. 
       TECHNICAL FIELD 
       [0002]    This invention relates generally to burner control systems and to methods of operating a burner. The invention relates in particular, but not exclusively, to a burner for a large boiler such as might be used in hospitals, hotels, offices or other large commercial or domestic premises. 
       BACKGROUND OF THE INVENTION 
       [0003]    A known burner control system is described in GB 2169726A which also refers to GB 2138610A. The disclosure of both those specifications is incorporated herein by reference. In that control system the flows of air and fuel are controlled by air and fuel valves respectively, and pairs of values of air and fuel valve settings are stored for varying firing rates of the burner. Those air and fuel valve settings are stored during a commissioning procedure by a commissioning engineer who is able to adjust the valve positions to obtain ideal combustion conditions for a given firing rate. 
         [0004]    In accordance with the teaching in GB 2169726, an exhaust gas analysis system is provided. That makes it possible to analyse the exhaust gases from the burner during operation and trim the setting of the air valve from the commissioned setting to a slightly different setting to obtain improved combustion in the burner. Such a closed loop control or feedback system enables good combustion conditions to be maintained, even when conditions change from those that applied during commissioning, since it enables the air to fuel ratio to be finely tuned. 
         [0005]    As is recognised in GB 2169726A, there is a significant time delay between the air for combustion passing through the air valve and the analysis of the combustion products from combustion of that air. The total length of that delay depends upon the firing rate of the burner: at relatively low firing rates, which are common during normal use of a burner, the delay is lengthened and may often be more than 60 seconds or even three times that. It is possible to allow for that time delay in designing the control system so that the results of analysis are associated with the correct, earlier, settings of the air and fuel valves. Even so, there is inevitably a significant period of time before the feedback from the exhaust gas analyser can be used to correct the setting of the air valve and, until the correction is made, the incorrect setting is maintained. That is disadvantageous both in respect of relatively small deviations that may be due to relatively common minor variations in conditions or in respect of less common and much bigger deviations, for example, arising from a major drop in airflow caused by a plastic bag being sucked into the air inlet of the burner and obstructing it. 
         [0006]    It is an object of the invention to provide an improved burner control system and an improved method of operating a burner. 
       SUMMARY OF THE INVENTION 
       [0007]    According to a first aspect of the invention there is provided a burner control system for controlling operation of a burner, the system including a regulator system for controlling the proportion of air to fuel fed to the burner, wherein the control system is arranged to receive an input signal relating to measurement of the ambient air pressure and/or the ambient air temperature and the signal is used to trim the proportion of air to fuel fed to the burner. 
         [0008]    It should be understood from the reference to measurement of “the ambient air pressure and/or the ambient air temperature” that there are three possibilities that are being referred to: firstly the ambient air pressure and the ambient air temperature may be measured; secondly, the ambient air pressure, but not the ambient air temperature, may be measured; and thirdly, the ambient air temperature, but not the ambient air pressure, may be measured. It is however preferred that both the ambient pressure and the ambient temperature are measured(the first possibility referred to above). 
         [0009]    The invention according to the first aspect firstly recognises that variations in ambient temperature and/or pressure can significantly affect the airflow rate (in terms of mass per unit time) for a given volume airflow rate and consequently lead to a need to trim the air to fuel volumetric flow ratio. Secondly, the invention uses the measurement of ambient conditions to trim the proportion of air to fuel fed to the burner. That trimming can be done pre-emptively, without having to wait for any feedback from an exhaust gas analyser. In that case, if there is some feedback control, for example from an exhaust gas analyser, it can still advantageously be employed but the amount of correction likely to be required is reduced because variation in ambient conditions have already been taken into account. 
         [0010]    It is possible for the proportion of air to fuel that is fed to the burner to be adjusted by adjusting a fuel valve, but it is generally preferred that an air valve is adjusted to alter the airflow to the burner. Thus it is preferred that the control system is arranged to provide an output signal for adjusting an air valve to alter the airflow to the burner by trimming the setting of the air valve according to the ambient air pressure and/or the ambient air temperature. 
         [0011]    As already indicated, preferably the control system further includes a system for analysing exhaust gases emitted by the burner and trimming the proportion of air to fuel according to the analysis. It should be understood, however, that the invention is applicable also to control systems that do not include any exhaust gas analysis, since it can still be advantageous to trim the proportion of air to fuel according to ambient conditions. 
         [0012]    The flows of air and fuel may be controlled by air and fuel valves respectively. The burner control system may further include a store for storing pairs of values of air and fuel valve settings for varying firing rates of the burner. Such a system provides a simple way of accurately controlling air and fuel flows at different firing rates of the burner. For firing rates between the stored settings, values of air and fuel valve settings can be interpolated. 
         [0013]    In an especially preferred control system, for each pair of values of air and fuel valve settings, a respective value of air pressure of the airflow to the burner is arranged to be stored. The control system may further include an air pressure sensor for sensing the respective value of air pressure of the airflow. The control system may then be arranged to monitor the air pressure at a given firing rate and compare the monitored air pressure with the stored air pressure value corresponding to that firing rate, or to an interpolated value if the firing rate is between those to which stored air and fuel valve settings relate. Similarly, for each pair of values of air and fuel valve settings a respective value of fuel pressure of the fuel flow to the burner may be arranged to be stored. This may be as an alternative to storing a respective value of air pressure but is preferably in addition to that. The control system may further include a fuel pressure sensor for sensing the respective value of pressure of the fuel flow. The control system may then be arranged to monitor the fuel pressure at a given firing rate and compare the monitored fuel pressure with the stored fuel pressure value corresponding to that firing rate, or to an interpolated value if the firing rate is between those to which stored air and fuel valve settings relate. With either of the arrangements described in this paragraph, it becomes possible to provide a control system which is able to react almost instantly to a sudden change in air or fuel pressure, rather than waiting for a feedback system such as an exhaust gas analyser to detect such a problem. That can improve the safety of the control system considerably. For example, the control system may be designed to shut down the burner in the event that the air or fuel pressure departs by more than, say, 10 percent from the stored value, or an interpolated value of the stored value. 
         [0014]    The storage of a fuel pressure and/or an air pressure for each pair of air and fuel valve settings is itself a special feature of the invention and may be used without the feature of the first aspect of the invention of measuring ambient air temperature and/or pressure. Accordingly, in a second aspect of the invention there is provided a burner control system for controlling operation of a burner, in which pairs of values of air and fuel valve settings are stored for varying firing rates of the burner, and in which for each pair of values of air and fuel valve settings a respective value of air pressure of the airflow and/or a respective value of fuel pressure of the fuel flow is arranged to be stored. 
         [0015]    It should be understood from the reference to “a respective value of air pressure of the airflow and/or a respective value of fuel pressure of the fuel flow” being stored that there are three possibilities that are being referred to: firstly both a respective value of air pressure of the airflow and a respective value of fuel pressure of the fuel flow may be stored; secondly, a respective value of air pressure of the airflow, but not a respective value of fuel pressure of the fuel flow may be stored; and thirdly, a respective value of fuel pressure of the fuel flow, but not a respective value of air pressure of the airflow, may be stored. It is however preferred that both a respective value of air pressure of the airflow and a respective value of fuel pressure of the fuel flow are stored (the first possibility referred to above). 
         [0016]    Preferably, the control system is arranged to monitor the air pressure and/or the fuel pressure at a given firing rate and compare the monitored air pressure and/or fuel pressure with the stored air and/or fuel pressure value corresponding to that firing rate, or to an interpolated value if the firing rate is between those to which stored air and fuel valve settings relate, and to emit an output signal for shutting down the burner in the event that the compared values differ by more than a predetermined amount 
         [0017]    In a preferred embodiment of the second aspect of the invention, the system further includes a system for analysing exhaust gases emitted by the burner and trimming the proportion of air to fuel according to the analysis. 
         [0018]    In either the first or second aspects of the invention, once installed, the burner control system includes a burner. Also there may be a fuel valve for adjusting the flow of fuel to the burner and an air valve for adjusting the flow of air to the burner. There may be a sensor for sensing ambient air pressure and/or a sensor for sensing ambient air temperature. Similarly, there may be a sensor for sensing the pressure in the airflow to the burner and/or a sensor for sensing the pressure in the fuel flow to the burner. These components may therefore all be part of the burner control system according to the first or second aspects of the invention. 
         [0019]    Commonly, the burner control system is employed in a boiler installation. Accordingly, the invention further provides a boiler installation including a burner control system according to the first or second aspects of the invention. 
         [0020]    According to the first aspect of the invention, there is also provided a method of operating a fuel burner in which a regulator system controls the proportion of air to fuel fed to the burner, wherein the ambient air pressure and/or the ambient air temperature are measured and the measurement is used to trim the proportion of air to fuel fed to the burner. 
         [0021]    Preferably an air valve is adjusted to alter the airflow to the burner and the setting of the air valve is trimmed according to the ambient air pressure and/or the ambient air temperature. 
         [0022]    Exhaust gases emitted by the burner are preferably analysed and the proportion of air to fuel trimmed according to the analysis. 
         [0023]    Also according to the second aspect of the invention, there is provided a method of operating a burner, in which the flows of air and fuel are controlled by air and fuel valves respectively, and pairs of values of air and fuel valve settings are stored for varying firing rates of the burner, and in which for each pair of values of air and fuel valve settings a respective value of air pressure of the airflow and/or a respective value of fuel pressure of the fuel flow is stored. 
         [0024]    It will be appreciated that the burner control systems of the various aspects of the invention are closely related to each other and to the various aspects of the methods of the invention and that therefore essential or preferred features of one may, unless indicated otherwise or clearly inappropriate, be incorporated into the other. For example, features described above in respect of the control system of the invention may be incorporated into the method of the invention and vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    By way of example an embodiment of the invention will now be described with reference to the accompanying schematic drawing, of which: 
           [0026]      FIG. 1  is a block diagram of a burner control system. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0027]    Referring to  FIG. 1 , a burner control system comprises a fuel burner  1  which in this case is a gas burner and to which gas is fed along a duct  2  via a fan  3  and a butterfly valve  4  and to which air is fed along a duct  5  via a fan  6  and a butterfly valve  7 . In the burner  1 , the gas and air are mixed and combustion takes place. The products of combustion pass from the burner  1  along a duct  8  where an exhaust gas analysis system  9  is placed for analysing the products of combustion. A control unit  10  is provided to control the operation of the burner  1 , by controlling the valves  4  and  7  via servomotors  4 A and  7 A to adjust the gas and air flow rates. The operation of the fans  3  and  6  by motors  3 A and  6 A is also controlled by the control unit  10 . 
         [0028]    The arrangement described above may be essentially as shown and described in GB 2138610A with the exhaust gas analysis system  9  being essentially as shown and described in GB 2169726A. As already indicated, the disclosures of both GB 2138610A and GB 2169726A are incorporated herein by reference. Furthermore the arrangement described above is commercially available from Autoflame Engineering Ltd, for example as the Mk 7 Evolution MM control unit and the Mk 7 Exhaust Gas Analyser. Such a burner control system is suitable for use as part of a boiler installation which may for example be employed in the heating system of large premises, for example a factory, offices, a hotel or hospital. 
         [0029]    In the arrangement described above the control unit  10  includes a store in which pairs of air and fuel valve settings are stored for different firing rates of the burner. Those settings are generated by a commissioning engineer when the control system for the burner is first set up. In the embodiment of the present invention, there are two other values stored for each firing rate: a reading from a gas pressure sensor  11  mounted in the duct  2  downstream of the gas valve  4 ; and a reading from an air pressure sensor  12  in the duct  5  downstream of the air valve  7 . In the expanded part of  FIG. 1 , a store  13  is shown with the left hand column, A, showing the numbered rows for different sets of values. There are then four further columns: the first two of those, B and C, store the settings of the gas and air valves as in GB 2138610A; in addition there is a further column, D, that stores a respective gas pressure reading from the sensor  11  for each pair of gas and air valve settings; in further addition there is a still further column, E, that stores a respective air pressure reading from the sensor  12  for each pair of gas and air valve settings. 
         [0030]    The control unit  10  is connected to receive a feedback signal from the exhaust gas analysis system  9  and that signal is used to trim the air valve setting from the stored value to a slightly different value, as described in GB 2169726A. In the embodiment of the invention described herein, there is in addition a temperature sensor  14  that is arranged to sense ambient air temperature and a pressure sensor  15  that is arranged to sense ambient air pressure. The control unit  10  is connected to receive signals from the sensors  14  and  15 . 
         [0031]    When the control system of  FIG. 1  is commissioned, the ambient temperature and pressure are recorded. Consequently, during subsequent operation of the burner  1 , the control unit is able to trim the setting of the air valve according to a comparison of the ambient pressure and temperature during operation as compared to the ambient temperature and pressure during commissioning. For example, if the ambient temperature is higher during operation than during commissioning, then to obtain the same mass flow rate of air as was obtained during commissioning, it is necessary to have a slightly higher volume flow rate of air and the control unit therefore calculates a trimmed setting of the air valve  7  which is slightly more open. Thus, without having to wait for a feedback signal from the exhaust gas analysis system  9 , the setting of the air valve  7  can be trimmed to provide an air to fuel ratio that is closer to the commissioned value. As will be understood, the ambient air pressure measurement is processed in a similar manner to the temperature measurement and used to generate the appropriate trim of the setting of the air valve  7 . 
         [0032]    The storing of gas and air pressure values in the ducts  2  and  5  enables a further safety feature to be incorporated in the control system: during operation of the burner, the control unit  10  can compare the stored values for the gas and air pressures in the store  13 , with actual measured pressures. If the measured pressures are within a predefined range (say not more than 10% away) of the stored values, then no action is taken. If, on the other hand, one of the measured pressures is outside the predefined range, then that is taken as an indication of some fault, for example failure of a fan, jamming of a valve or blockage of a duct, and the burner is shut down immediately. Without the additional pressure values stored against the different firing rates of the burner, the control unit  10  would not detect any such failure until a reading from the exhaust gas analysis system was obtained. Especially at relatively low firing rates of the burner, the delay in waiting for a reading from the exhaust gas analysis equipment can be as much as a few minutes. It will be seen that the extra controls provided by the control unit  10  enable errors and failures in the burner to be detected sooner and appropriate action taken. 
         [0033]    Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.