Abstract:
Blowdown of a pressurised steam boiler heated by a burner  1  and including a blowdown flow control valve  17  is controlled by opening the control valve in response to receiving a control signal from a control unit  20  to allow water to leave the boiler. The control unit receives one or more signals indicative of the steam production rate of the boiler and adjusts the amount of blowdown in dependence upon the aggregate steam production.

Description:
TECHNICAL FIELD  
       [0001]    This invention relates generally to blowdown of steam boilers heated by burners. More particularly, the invention relates to control systems for use in such installations and to methods of commissioning and operating such installations. The invention relates in particular, but not exclusively, to a steam boiler such as might be used in hospitals, hotels, offices or other industrial, commercial or domestic premises. 
       BACKGROUND OF THE INVENTION 
       [0002]    Steam boilers are well known. Examples of US patents referring to aspects of the design of pressurised steam boilers are U.S. Pat. No. 6,520,122 and U.S. Pat. No. 7,249,573, the contents of which are incorporated herein by reference. 
         [0003]    Standard steam boilers are fitted with a bottom blowdown valve which, when opened for a short period of time allows an amount of hot water to leave the boiler, carrying with it solid deposits that have accumulated at the bottom of the boiler as it has been operating. The removal of such solids is important to ensure that the heat transfer efficiency of the boiler is maintained. It is therefore conventional practice to effect a bottom blowdown operation at regular intervals, for example once every 8 hours. For a boiler producing about 20,000 lbs (about 10 tonnes) of steam an hour, a blow down of the order of 10 to 15 seconds would be typical. During the blow down a substantial amount of waste heat passes out from the boiler in the water that is blown down. It is generally accepted that 2 to 3 percent of the total heat input to a boiler is lost through such blow down operations. Given that a boiler of the size indicated might use of the order of £500,000 or $800,000 of fuel in a year, that 2 to 3 per cent of extra fuel to support the blow down operations is very significant. 
         [0004]    It is an object of the invention to provide an improved method of controlling blowdown of a boiler and an improved steam boiler. 
       SUMMARY OF THE INVENTION 
       [0005]    According to the invention there is provided a method of controlling blowdown of a steam boiler heated by a burner, the boiler comprising a blowdown flow control valve that can be opened in response to receiving a control signal from a control unit to allow water to leave the boiler, wherein the control unit receives one or more signals indicative of the steam production rate of the boiler and adjusts the amount of blowdown in dependence upon the aggregate steam production of the boiler. 
         [0006]    By adjusting the amount of blowdown in dependence upon the aggregate steam production of the boiler, it becomes possible to tailor the amount of blowdown to that which is needed. That can avoid in particular operating with excessive blowdown when the boiler is operating well below its maximum continuous rating and can enable significant fuel savings to be made, and also reduce wastage of water. 
         [0007]    Typically, the control unit opens and closes the control valve a multiplicity of times over a time period. For example it may open and close the control valve three times each day. Each opening and closing of the control valve results in a blowdown operation. Accordingly, the control unit may open and close the control valve a multiplicity of times over a time period, causing a multiplicity of blowdown operations. It is possible to adjust the amount of blowdown in a variety of ways, for example, by adjusting the frequency of the blowdown operations, but it is preferred that, in order to adjust the amount of blowdown, the control unit adjusts the amount of water blown down from the boiler in each blowdown operation. 
         [0008]    There are also various ways in which the amount of water blown down from the boiler in each blowdown operation can be controlled; for example, the degree of opening of the blowdown control valve may be controlled; it is preferred, however, that the control unit adjusts the amount of water blown down from the boiler in each blowdown operation by adjusting the time for which the control valve is open during each blowdown operation. 
         [0009]    Where reference is made to “signals indicative of the steam production rate of the boiler”, it should be understood that those signals may take various forms which may be directly or indirectly related to the rate of steam production. For example, the signals may be derived from measurement of many different variables, including but not limited to the following: steam flow rate from the boiler; amount of fuel fed to the burner; amount of air fed to the burner; amount of exhaust gases generated. Generally, since the amount of evaporation, which corresponds to the amount of steam produced, is that most closely related to the optimum amount of blowdown, it will be preferred, for greatest accuracy, that it is the aggregate steam production since the previous blowdown that is assessed. 
         [0010]    The method preferably further includes an initial step of setting a blowdown amount for a given steam production rate of the boiler, the control unit thereafter adjusting the amount of blowdown according to a comparison of an actual steam production amount of the boiler with that which would apply for the given steam production rate. The given steam production rate of the boiler is preferably the maximum continuous rate of steam production of the boiler. The initial setting step is preferably carried out during commissioning of the boiler. By setting the amount of blowdown at the time of commissioning, the opportunity is provided for a commissioning engineer to consider the appropriate setting at the maximum continuous rate of steam production of the boiler, taking all known factors into account. 
         [0011]    The steam production rate may be measured directly by a steam flow meter which may provide a signal representing that rate to the control unit, which can then integrate that rate to arrive at the aggregate steam production. Another option is to measure the rate of steam production indirectly, for example as described in U.S. Pat. No. 6,520,122. 
         [0012]    The time for which the control valve is open during each blowdown operation may be adjusted downwardly in dependence upon the reduction of the aggregate steam production from that which would apply for the maximum continuous rate of the boiler. Even a crude adjustment can be effective in reducing heat loss. Preferably, the time is adjusted downwardly approximately in proportion to the reduction in the aggregate steam production. 
         [0013]    According to another aspect of the invention, there is provided a pressurised steam boiler including: 
         [0014]    a boiler housing for containing water in the boiler, 
         [0015]    a burner for heating water in the boiler and converting the water into steam, 
         [0016]    a blowdown control valve that is openable to allow water to leave the boiler in a blowdown operation, and 
         [0017]    a control unit for controlling the opening of the blowdown control valve, the control unit being connected to receive one or more signals indicative of the steam production rate of the boiler and to adjust the amount of blowdown in dependence upon the aggregate steam production of the boiler. 
         [0018]    The control unit may be arranged to open and close the control valve a multiplicity of times over a time period, causing a multiplicity of blowdown operations, and, in order to adjust the amount of blowdown, the control unit may be arranged to adjust the amount of water blown down from the boiler in each blowdown operation. 
         [0019]    The control unit may be arranged to adjust the amount of water blown down from the boiler in each blowdown operation by adjusting the time for which the control valve is open during each blowdown operation. 
         [0020]    The control unit may further include a store arranged to store pairs of values of air and fuel valve settings at various firing rates of the burner and the control device may be arranged, when the firing rate of the burner is to be changed, to control the air and fuel valve settings in dependence upon the stored air and fuel valve settings. A burner control unit of this kind is described in GB2138610A, the contents of which is incorporated herein by reference. 
         [0021]    Where reference is made to a “control unit”, it should be understood that such a unit may comprise one physical entity or two or more entities. In an illustrated embodiment of the invention described below, the control unit is a single physical entity, but as explained in the description below, it is possible for there to be two or more separate control modules. 
         [0022]    It will be appreciated that the method of controlling blowdown of the boiler and the steam boiler are closely related to each other and that features described in respect of the method may be adopted in the steam boiler and vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    By way of example an embodiment of the invention will now be described with reference to the accompanying schematic drawing, of which: 
           [0024]      FIG. 1  is a block diagram of a pressurised steam boiler installation. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0025]    Referring to  FIG. 1 , a pressurised steam boiler installation generally comprises a fuel burner  1 , which in this case is a gas and oil burner, and a boiler housing  3 . Gas is fed along a pipe  2  via a butterfly valve  4 , and/or oil is fed along a pipe  5  via a butterfly valve  6 . Air is driven along a duct  7  by a fan  8  via a damper valve  9 . The burner also has a pilot fuel feed  10 . 
         [0026]    In the burner  1 , the fuel and air are mixed and combustion takes place. The products of combustion pass from the burner  1  through a heat exchanger  11  in the boiler housing  3  containing water  14  into the bottom of a stack  15 . The combustion products pass up the stack  15  and into the atmosphere. 
         [0027]    An outlet pipe  16  extends from the bottom of the boiler housing  3  via a blowdown flow control valve  17  to a blowdown receiver (not shown), providing a bottom blowdown facility that is in most respects conventional, but employs an electrically controlled blowdown valve and servomotor assembly. The use of an electrically controlled blowdown valve is unconventional. In order to ensure operation, even in the event of a power failure, the assembly is also connected to a battery supply and, in the event of a power failure during blowdown, the blowdown valve is closed under battery power. 
         [0028]    A control device  20  is provided to control the operation of the installation. The control device receives many inputs and controls the operation of many parts of the installation in a conventional manner; in the interests of clarity, it is only the control aspects of more significance to the present invention that will now be described and that are shown in  FIG. 1 . The butterfly valve  4  controlling the flow of gas to the burner  1  is set by a servo motor  21  connected to the control device  20  and able to receive control signals determining the position adopted by the servo motor  21 . The butterfly valve  6  controlling the flow of oil to the burner  1  is set by a servo motor  22  connected to the control device  20  and able to receive control signals determining the position adopted by the servo motor  22 . The damper valve  9  controlling the flow of air to the burner is set by a servo motor  23  connected to the control device  20  and able to receive control signals determining the position adopted by the servo motor  23 . The bottom blowdown valve  17  is controlled by a servo motor  24  connected to the control device  20  and able to receive control signals for opening and closing the valve  17 . 
         [0029]    The control unit  20  includes a store  30  which is also shown in  FIG. 1  in an expanded schematic form. In the expanded part of  FIG. 1 , the store  30  is shown with the left hand column, A, showing the numbered rows for different sets of values. There are then two further columns, B and C, which store the settings of the fuel and air valves, as described in more detail in GB 2138610A. 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. 
         [0030]    In operation, when the control system of  FIG. 1  is commissioned, the commissioning engineer sets the air and fuel valves to settings that provide optimum combustion conditions at a given firing rate of the burner. Once the engineer is satisfied that the best settings have been achieved for a given firing rate, they are stored in the store  30 . The commissioning engineer can then adjust the firing rate of the burner upwards or downwards and store a set of optimum values for that firing rate. By repeating that process, values can be entered across the full firing range of the burner. If the burner is to operate on only one fuel then the commissioning can be carried out just with that fuel, but if the burner is also to operate with a second fuel, the commissioning procedure described above can be repeated for the second fuel. 
         [0031]    The commissioning engineer also sets the time period for which the blowdown valve  17  is opened when the boiler is operating at its maximum continuous rating; that setting, referred to herein as the “MCR blowdown time” can take account of all relevant factors known to the commissioning engineer, including for example measurements of water quality that may have been provided by a water analyst. Usually the boiler will be set to perform a blowdown at set times which the commissioning engineer does not vary; for example, in this particular example a boiler operates for three 8 hour shifts each day and one blowdown is performed at the end of each shift. A typical time period for which the blowdown valve  17  is opened when the boiler is operating at its maximum continuous rating is 12 seconds, but as just explained, this time period is chosen by the commissioning engineer. 
         [0032]    Once the installation has been fully commissioned, it is ready for operation. When the burner is set to a given firing rate, the control device looks up in the store  30  the settings of the appropriate servo motors  21 ,  22  and  23  for that firing rate and adjusts them accordingly. When the time comes for the control device to open the blowdown valve  17  at the end of a shift, it calculates the aggregate steam production of the boiler during the shift; in a case where the boiler has been operating throughout the shift at its maximum continuous rating, then the blowdown valve  17  is opened by the control device  20  for the “MCR blowdown time”. In the common case, however, when the burner has been operating at a lower average firing rate and the aggregate steam production has therefore been lower, the blowdown valve  17  is opened by the control device  20  for only a part of the “MCR blowdown time”, in proportion to the reduction in the rate of steam production. For example when the boiler has been operating at an average of 25% of maximum continuous rating, the blowdown valve  17  is opened by the control device  20  for only 25% of the “MCR blowdown time”; thus, if the “MCR blowdown time” is 12 seconds, the blowdown valve would in this case be opened for 3 seconds only. As will be understood the aggregate steam production can be ascertained by the control device  20  in various ways. For example the amount of steam generated by the boiler can be measured by a steam flow meter or calculated, for example as described in particular in columns 11 and 12 of U.S. Pat. No. 6,520,122, the description of which is incorporated herein by reference. The control device can then apply a proportionate adjustment to the time for which the valve  17  is opened using conventional proportionate control techniques. 
         [0033]    Whilst one particular example of the invention has been described, many modifications may be made to it without departing from the invention. Some such modifications will now be explicitly mentioned, but it should be understood that many others may be made. 
         [0034]    In the particular example described above, the installation runs in shifts of 8 hours each with one blowdown at the end of each shift. It will be understood that the invention may be employed with different lengths of shift and/or with each blowdown in the middle of a shift and/or with more than one blowdown in a shift. It is also possible that when the boiler is operating at its maximum continuous rating, it is arranged to have two or more blowdowns which differ in length; if that were the case, then at a lower steam production rate the duration of one or more of those blowdowns would be reduced in order to reduce the total amount of blowdown at a lower steam production rate. 
         [0035]    In the embodiment described the blowdown valve is opened fully and the total amount of blowdown controlled by controlling the time for which the valve is fully opened; an alternative possibility would be to alter the degree of opening of the valve instead of or in addition to altering the time for which it is open. 
         [0036]    In the embodiment of the invention described above, it is the steam production rate that is directly or indirectly measured. That is advantageous in that it is the steam production rate that is most closely related to the optimum amount of blowdown, but it should be understood that another possibility is to measure the firing rate of the burner and use the average firing rate to determine the amount of blowdown. The firing rate may be measured by monitoring the fuel flow rate to the burner. 
         [0037]    In the illustrated embodiment, a single control unit is shown in which all the control functions are carried out. It should be understood, however, that it is within the scope of the invention for the “control unit” to be provided by two or more control modules which may be physically separate from one another. For example, there may be a physically separate control module for the blowdown valve  17 , and that module may adjust the time for which the valve  17  is open according to a signal it receives from another control module and/or some other device such as a steam flow meter. In that way the control of the amount of blowdown can be carried out independently of an overall control system for the boiler installation. 
         [0038]    It is possible for other values to be stored in the store  30  alongside the settings of the air and fuel valves. For example in WO2012/056228A2, the contents of which are incorporated herein by reference, a modified version of the control arrangement of GB2138610A is described in which respective values of fuel pressure and air pressure upstream of the burner are stored in the store  30  for each pair of fuel and air valve settings. 
         [0039]    In GB 2169726A, the contents of which are incorporated herein by reference, the control device  20  is connected to receive a feedback signal from an exhaust gas analysis system and that signal is used to trim the air valve setting from the stored value to a slightly different value. That arrangement, with or without the modifications and developments described in WO2012/056228A2, may be employed in embodiments of the present invention. 
         [0040]    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.