Abstract:
A method of controlling operation of a liquid-fuel combustion appliance. The method is characterized by including an “antiseize operating mode” in which an electric motor and a fuel pump, powered by the electric motor, are started periodically to prevent oxidation and/or polymerization of the liquid fuel in the fuel pump when the fuel pump is idle. More specifically, the fuel pump is a gear pump.

Description:
The present invention relates to a method of controlling operation of a liquid-fuel combustion appliance. 
   BACKGROUND OF THE INVENTION 
   Combustion appliances are known comprising at least one pump for feeding liquid fuel to at least one liquid-fuel atomizing head. 
   More specifically, burners or heating units have at least one pump for drawing fuel from a tank and feeding it under pressure to at least one nozzle where the liquid is atomized and made suitable for combustion (mechanical atomization). 
   Given the high pressures (5-20 bars) and low flow rates involved, the pumps are normally volumetric types. 
   Gear-type volumetric pumps are the most commonly used. 
   In pumps of this sort, one gear is driven by an electric motor, and in turn drives an idle second gear; and the liquid flows through an intake port into one of the cavities formed by the teeth of the two gears, and is discharged to the delivery port while gradually increasing in pressure. 
   The two gears and the seat in which they operate must be machined accurately to ensure optimum performance of the pump. 
   Moreover, the mechanical assembly defined by the two gears is lubricated by the pumped fuel itself. 
   When the pump is idle, however, fuel remains trapped inside the cavities formed by the geometry of the gears. 
   In certain conditions, and if the pump remains idle for any length of time, the fluid film deposited inside the cavities undergoes oxidation and/or polymerization, thus resulting in alteration of its properties, in particular viscosity. 
   When this happens, the fluid deposit may assume such a consistency as to prevent rotation of the gears when the pump is started up again. 
   That is, the starting torque of the electric motor is no longer enough to overcome the breakaway friction torque produced by the high-viscosity fluid deposit. 
   One of the causes of the problem is the addition of vegetable fuel to the mineral fuel. 
   Vegetable fuel may comprise unsaturated polymer chains, which, in the presence of oxygen or during prolonged stoppages, may combine to form macromolecules, the relative flow properties of which are greatly inferior to those of non-oxidized fuel. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention, therefore, to provide a method of controlling operation of a liquid-fuel burner, whereby the fuel pump is operated periodically to prevent seizing of the pump caused by polymerization of the fuel. 
   The liquid fuel, normally gas oil for heating, is pumped by the gear pump to two conduits formed inside the pump. 
   A first conduit is normally closed by a safety valve controlled by a central control unit; while a second conduit is fitted inside with a pressure regulator for maintaining a user-defined fuel pressure. 
   When heat demand (closure of the thermostat) starts the burner, this performs a predetermined operating sequence. 
   That is, the electric motor starts the fuel pump, which sets the circuit to the pressure set by the pressure regulator; and, when the pressure regulator opens, all the fuel flows back to the tank via the pressure regulator. 
   After a given length of time, a central control unit excites the delivery valve, and part of the fuel flows to the nozzle. 
   Actual operation commences when the flame is lit, as detected by a sensor. 
   As stated, start-up is determined by heat demand (closure of the thermostat) by the outside environment. 
   It is an object of the present invention to provide a combustion appliance operating mode distinct from the operating mode described above. 
   For the sake of clarity, in the following description, the term “standard operating mode” refers to the mode described above, and the term “antiseize operating mode” to a new mode on which the method which is the main object of the present invention is based. 
   When the combustion appliance operates in “antiseize operating mode”, the electric motor and the fuel pump are operated periodically so that fuel flows from the tank to the pump and from the pump back to the tank, but is never fed to or atomized by the nozzle. 
   The purpose of this mode, in fact, is to prevent an increase in the viscosity of the liquid fuel in the pump when the pump is idle. 
   “Antiseize operating mode” may be either “periodical” (possibly performed at intervals determined by an electronic central control unit as a function of typical fuel parameters and the particular type of combustion appliance), or “random”, in which case, the electronic central control unit governing the combustion appliance generates random checks. 
   The following is a more detailed description of the “antiseize operating mode” and the differences between this and the “standard operating mode”. 
   “Standard operating mode” is always and only activated by closure of the thermostat, whereas “antiseize operating mode” can only be activated when the burner is left idle for a prolonged period of time. 
   A device associated with the burner determines the time lapse since the last heat demand, and accordingly sets the burner to “antiseize operating mode”. 
   “Standard operating mode” comprises a sequence of operations, including exciting an appropriate valve and feeding fuel to the nozzle where it is atomized. In “antiseize operating mode”, on the other hand, no valve is excited, and the fuel is never supplied to the nozzle, but is circulated from the tank to the pump and from the pump back into the tank. 
   Moreover, in “antiseize operating mode”, the electric motor and pump are started and stopped with a frequency as determined by the electronics of the burner, whereas, in “standard operating mode”, operating frequency coincides with the frequency with which the thermostat closes, as determined by environmental conditions. 
   In “standard operating mode”, the electric motor always rotates in the same direction, whereas, in “antiseize operating mode”, the electronic devices controlling the combustion appliance can choose either rotation direction, depending on the antiseize function required. 
   It should be noted that “antiseize operating mode” is subordinate to “standard operating mode”, in the sense that, if heat demand (closure of the thermostat) occurs while “antiseize operating mode” is activated, the system switches to “standard operating mode”. 
   “Antiseize operating mode” may be activated at the user&#39;s discretion, and may be user-deactivated by means of an appropriate selector if not required by the system, e.g. on account of fuel containing particular antiseize compositions being used. 
   According to the present invention, therefore, there are provided a method and relative appliance as claimed in the attached Claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  shows a first embodiment of a combustion appliance for implementing the innovative method which is the main object of the present invention; 
       FIG. 2  shows a second embodiment of a combustion appliance for implementing the innovative method which is the main object of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Number  10  in  FIG. 1  indicates as a whole a combustion appliance for implementing the innovative method which is the main object of the present invention (see below). 
   In “standard operating mode”, heat demand by the outside environment closes a thermostat TR. 
   An electric signal is therefore sent to an electronic central control unit  100  for controlling all the operations performed by appliance  10 . 
   Central control unit  100  controls operation of an electric motor  11  powering a gear pump  12 . 
   Fuel is drawn by gear pump  12  from a tank  13  along an intake conduit  14  and via a filter  15 . 
   The fuel then flows along a conduit  16  closed by a normally-closed valve  18 , and along a conduit  17   a.    
   When the fuel pressure reaches such a value as to activate a spring  19  of a regulating device  20  along conduit  17   a,  device  20  opens to drain the fuel into tank  13  along a conduit  17   b.    
   For a few seconds, therefore, all the fuel pumped by pump  12  flows into tank  13  via regulating device  20  and along conduits  17   a  and  17   b.    
   This is what is known as the “pre-ventilation stage”, in that, during this time interval, electric motor  11 , in addition to operating gear pump  12 , also starts a fan  21  to expel any unburnt gases from a combustion chamber CC. 
   At the end of the “pre-ventilation stage”, electronic central control unit  100  opens valve  18 , so that part of the fuel flows along a conduit  23  to a nozzle  22 , and the pressurized liquid fuel is atomized by nozzle  22  and mixed with the air supplied by fan  21 . 
   Fan  21  and nozzle  22  together form a combustion head H fitted to a wall W of combustion chamber CC. 
   A heat source (not shown in  FIG. 1 ) inside combustion chamber CC ignites the fuel/air mixture issuing from combustion head H. 
   Lighting of the flame is detected by a sensor (not shown in  FIG. 1 ), which informs electronic central control unit  100  that the fuel/air mixture has been ignited and the flame lit. 
   In the event the fuel/air mixture is not ignited, central control unit  100  closes valve  18  to cut off fuel flow to combustion head H. 
   What has been described so far is the “standard operating mode” common to numerous currently marketed combustion appliances. 
   As stated, one of the objects of the present invention is to provide, in addition to the above “standard operating mode”, a second so-called “antiseize operating mode”. 
   Electronic central control unit  100  is therefore designed to start electric motor  11 , and therefore gear pump  12 , regardless of the actual heat demand of the environment. 
   For this purpose, a timer  24 —preferably, though not necessarily, an internal device of central control unit  100 —is provided. 
   The function of timer  24  is to measure the time lapse since the last heat demand. 
   When the time lapse exceeds a given value set by the user in central control unit  100 , central control unit  100  starts the electric motor  11  and, therefore, pump  12 . 
   The liquid fuel therefore flows through the gear assembly of pump  12  in tank  13 , through regulating device  20 , and along conduits  17   a,    17   b.  This stage is only followed by excitation of valve  18  if, in the meantime, a heat demand is received from thermostat TR to set electronic central control unit  100  to the “standard operating mode” described previously. 
   If no heat demand by the environment occurs, electronic central control unit  100  stops electric motor  11  and, therefore, combustion appliance  10 . 
   After a given time lapse, providing no heat demand is made by the outside environment, the above procedure is repeated in exactly the same way. 
   In other words, “antiseize operating mode” is subordinate to “standard operating mode” when heat demand is made by the environment. 
   Electronic central control unit  100  starts electric motor  11 , which in turn starts pump  12 , through which fuel flows for a given time, and the system is then stopped. 
   Obviously, the purpose of starting and stopping electric motor  11  is to prevent the liquid fuel from settling for a prolonged period inside the gaps between the gears of pump  12 . 
   If the gear assembly of pump  12  should seize despite repeated on-off cycles, a temperature sensor (not shown in  FIG. 1 ) provides for thermal protection of electric motor  11 , and informs electronic central control unit  100  of seizure of pump  12 . 
   By means of an appropriate selector (not shown in  FIG. 1 ), “antiseize operating mode” may be deactivated by the user when not required by the heating system or the liquid fuel used. In which case, the heating system operates solely in “standard operating mode”. 
   Activation of electric motor  11  and pump  12  is so programmed as to rotate electric motor  11  and pump  12  first clockwise then immediately anticlockwise, or vice versa. 
     FIG. 2  shows a further embodiment of the present invention. 
   More specifically, in this case, regulating device  20  is fitted along conduit  16 , and valve  18  along conduits  17   a ,  17   b.    
   Also, valve  18  is normally open. 
   When heat is demanded by the environment (closure of thermostat TR), electroni central control unit  100  starts electric motor  11 , and gear pump  12 , powered by electric motor  11 , pumps liquid fuel along conduits  17   a ,  17   b  into tank  13 . At this stage, liquid-fuel flow along conduit  16  is prevented by regulating device  20 , which the fuel pressure is not high enough to activate. 
   After the above pre-ventilation stage, electronic central control unit  100  closes valve  18 , the pressure along conduit  16  rises, regulating device  20  opens, and fuel flows to head H. 
   As in the  FIG. 1  embodiment, in this case too ( FIG. 2 ), the present invention provides for an additional so-called “antiseize operating mode”. 
   When “antiseize operating mode” is activated, electronic central control unit  100 , in the absence of heat demand, starts electric motor  11  and pump  12  with a given frequency to pump fuel through valve  18 , and, if no heat demand is made in the meantime, stops electric motor  11  and pump  12 . 
   In this case too, seizure of pump  12  is indicated by means of electronic central control unit  100 . 
     FIGS. 1 and 2  show so-called “two-pipe” combustion appliances. 
   The same, however, obviously also applies to “single-pipe” appliances, in which one pipe connects the tank to the burner circuit. 
   In these applications, the outlet conduit from the regulating device and the outlet conduit from the valve are formed inside the pump. 
   In real applications, valve  18  and regulating device  20  may be formed inside the body of pump  12 .