Abstract:
A heater and a method of its use are configured for use at cold operating temperatures. The heater has a supply line for transporting a volume of fuel between a fuel tank and burner. An inline heater is supplied in a supply line for the burner. The heater also has a return line that normally returns unused fuel from the burner to the heater, hence reducing the volume of fuel that needs to be heated by the heater and reducing system power requirements. The heater may be thermostatically controlled to maintain the temperature of the heated fuel to a value that is at or above a temperature required for good fuel atomization but below a flashpoint of the fuel. A valve is provided in the return line to permit diversion of the returned fuel to the fuel tank during a purge operation at initial startup.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to fuel burning heaters, more particularly, relates to a fuel burning heater having an inline heater for heating fuel that is bound for the burner. The invention additionally relates to a method of operating such a machine. 
     2. Discussion of the Related Art 
     Performing construction work in cold weather climates faces many challenges that are not confronted in warmer climates. In the context of excavation and earth-moving, frozen soil, as is typically confronted in arctic environments, requires substantially more energy, time and resources to move and manipulate. Also, the curing of concrete and other paving materials may be negatively impacted by such extreme cold temperature as required water evaporation and drying are particularly challenging when the liquid components freeze prior to evaporation. 
     These difficulties can be mitigated through the use of heaters to warm the work site area. One commonly used type of heater is a so-called indirect fired (IDF) heater that heats air and directs the hot-air to the area to be heated by blowing the air through large hoses. The air is heated by a burner that may be fueled by any of a variety of fuels including diesel fuel, kerosene, natural gas, or propane. Heaters that burn a liquid fuel, such as diesel fuel, typically use an atomizing burner supplied with the liquid fuel from a fuel tank via a pump. Atomizing burners operate by pressurizing a fuel oil and forcing it through a nozzle. The nozzle causes the fuel oil to atomize into fine droplets that are readily burned. The atomized fuel is exposed to an electric arc to begin the combustion reaction. When the reaction has stabilized, it is self-sustaining, and the electrode is no longer needed to maintain a flame. The fuel may be supplied in either a “one-pipe system”, in which a pump is sized to deliver only as much fuel to the burner as is needed at any given time, or a “two-pipe” system in which the pump delivers much more fuel than is typically required for combustion and the unused fuel is recycled back to the fuel tank. As much as 70-90% of the fuel pumped by a two-pipe system may be returned as unused fuel. Two-pipe systems typically are considered to be preferable to one-pipe systems because they are self-purging after an out-of-fuel condition. That is, air trapped in the fuel lines is automatically purged back to the tank as opposed to having to be manually bled from the fuel lines in a one-pipe system. 
     Most atomizing burners are designed for indoor use at near room temperature conditions. Several are designed to withstand temperatures now lower than 0° C., and no commercially available burner known to the inventors is capable of starting and operating at temperatures of −40° C. without some degree of modification to either the burner or the fuel supply. The limiting factor preventing operation below these temperatures is the fact that fuel viscosity increases as temperature decreases, resulting in the ejection of larger fuel droplets from the burner&#39;s nozzles at low temperatures. At low temperatures of on the order of −20° C. and lower, the larger atomized droplets are difficult to ignite and may not ignite at all. Even if ignition is established, the burner will run with excessive smoke because of ineffective precombustion mixing of the fuel and air. 
     After-market heaters are available for heating fuel as it is being ejected from the burner&#39;s nozzle, but such heaters are minimally effective, even for start up, at extremely low temperatures of on the order of −30° C. Even if these small heaters are effective at improving burner start-up, they are insufficient for maintaining a stable flame over prolonged use. Furthermore, installation of the after-market inline heater requires modification to the heater, and may compromise manufacturer warranties. 
     In addition, at extremely low temperatures, such fuel may form a solid wax precipitate which can clog both the fuel filter and the burner nozzle. Nozzle heaters are completely ineffective at preventing the formation of such a precipitate in a fuel filter. 
     Various tank-based or inline heaters have been proposed in an effort to alleviate these problems, but all such heaters have disadvantages. Some are supplied with energy with heat from the burner and, as such, are completely ineffective at start-up when the heater&#39;s components are at or near ambient temperature and heating is most critical. Other, electrically powered heaters, require so much energy to operate that they dramatically increase the electrical power draw of the heater. 
     Despite these prior attempts to design a heater for use in cold weather climates, there remains need for improvement. In light of the foregoing, a heater configured to recirculate and effectively pre-warm fuel is desired. 
     SUMMARY OF THE INVENTION 
     One or more of the above-identified needs are met by providing a fuel burning heater having an inline fuel heater and a plumbing system for recirculating warmed fuel. The heater is ideally suited for use with air heaters, but is usable with other devices that require burning fuel in cold weather climates. 
     In accordance with a first aspect of the invention, a heater is provided, having a supply line for transporting a volume of fuel between a fuel tank and burner. An inline heater for heating the fuel and a fuel filter are located in the supply line between the fuel tank and the burner. The heater also has a return line in fluid communication with the burner and returning a volume of unused fuel from the burner to a valve provided in the return line. The valve is selectively movable between two positions, the first position directing fuel into the fuel tank, and the second position directing fuel into the supply line upstream of or into the inline heater. The recirculation of warmed unused fuel into the supply line at a position upstream of or into the inline heater allows the warmed recirculated fuel to mix with cold fuel drawn from the fuel tank. This results in a pre-heating of the fuel being drawn into the inline heater from the fuel tank, and thereby significantly decreases the electrical burden on the heater. 
     In one embodiment, the valve is manually operated so as to normally deliver fuel to the heater and to be switchable to deliver fuel back to the tank only, e.g., during a purge operation following an out-of-fuel condition. 
     The heater may be thermostatically controlled to deliver fuel to the burner at a set, possibly controllable temperature. That temperature preferably is above a temperature at which the fuel is effectively atomized by the burner but below the flash-point of the fuel. 
     In accordance with yet another aspect of the invention, a method of operating a heater is provided including the steps of directing a first volume of fuel from a fuel tank to an inlet of an inline heater, directing a second volume from a burner to the inline heater via a return line, combining the first and second volumes of fuel in or upstream of the inline heater to form a combined volume of fuel to preheat the first volume of fuel, and heating the combined volume of fuel with an electrical heating element. Additional steps include directing the combined volume of fuel through an outlet of the inline heater to an inlet of the fuel filter, filtering the combined volume of fuel using the fuel filter, directing the combined volume of fuel to the burner, burning a portion of the combined volume of fuel at the burner, directing an unused volume of the combined fuel to a valve in the return line. The valve is switchable to selectively deliver fuel to the inline heater or to the fuel tank, respectively. 
     These and other objects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
         FIG. 1  is a perspective view of an indirect fired air heater constructed in accordance with a preferred embodiment of the invention; 
         FIG. 2  is a partially cut away perspective view of the interior of the heater of  FIG. 1 ; 
         FIG. 3  is another partially cut away perspective view of the interior of the heater of  FIG. 1 ; and 
         FIG. 4  is a schematic illustration of the heater of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A wide variety of heaters could be constructed in accordance with the invention as defined by the claims. Hence, while the preferred embodiments of the invention will now be described with reference to an indirect-fired air heater, it should be understood that the invention is in no way so limited. 
       FIGS. 1-2  illustrate a perspective view of a heater assembly  10  constructed in accordance with one embodiment of the invention.  FIG. 1  shows that the heater assembly  10  can be mounted on a trailer  12  for transport. If a trailer  12  is provided, the heater assembly  10  can remain on the trailer  12  during operation. Alternatively, the heater assembly  10  can be moved to and from a worksite by some other mode of transport and supported directly on the ground during operation. 
     As can be seen in both  FIGS. 1 and 2 , the heater assembly  10  includes a casing  14  having air inlet and outlet vents  16 ,  18  that can be connected to hoses (not shown) to convey air from and to the worksite, respectively. Located within the casing  14  are a blower  20 , a fuel tank  22 , and an indirect fired heater, i.e. burner  24 . The blower  20  is a centrifugal blower powered by a motor  26 . The blower  20  has an axial inlet  28  connecting the air supply inlet  16  to a radial outlet  30 . A generator  32  is mounted on the trailer  12  in front of the heater assembly  10  for powering electrically-powered components of the heater assembly  10 , including the inline heater  34 , discussed below. Alternatively, electric power could be supplied to those components via a cable coupled to a main electrical source located at the worksite. It is also conceivable that the electrical components of the heater assembly  10  could be powered by an onboard battery or bank of batteries, but rapid power drains at low temperatures render batteries a less-preferred option, particularly in cold climates. 
     Referring particularly to  FIG. 2 , the heater assembly  10  includes a burner  24 , a fuel supply assembly  36  that supplies fuel to and from the burner  24 , a combustion chamber  31 , and a heat exchanger  33 . The burner  24  comprises an atomizing burner having an internal gear pump (not shown) and one or more nozzles (also not shown) that open into the combustion chamber  31 . The burner  24  heats air in the combustion chamber that indirectly heats air flowing through the heat exchanger  33  from the outlet  30  of the blower  20  to the air supply outlet  18  of the heater assembly  10 . Referring to  FIG. 4 , the burner  24  of this embodiment is part of a two-pipe system having an internal gear pump (not shown), having a fuel inlet  46  coupled to the fuel supply system and unused fuel outlet  50 . The burner  24  further comprises an electric ignition source which, when activated, triggers the combustion of the atomized fuel delivered to the nozzle by the gear pump. Once the combustion reaction has been initiated, the electric ignition source is not required to maintain the flame. 
     Still referring particularly to  FIG. 4 , the fuel supply system  36  includes a fuel tank  22 , a supply line  40 , an inline heater  34 , a fuel filter  42 , and a valve  44 . For the sake of visual clarification,  FIG. 3  further illustrates these elements without depicting the fuel supply system  36 . The supply line  40  connects the fuel tank  22  indirectly to the inlet  46  of the gear pump. The inline heater  34  is located within the supply line  40 , between the fuel tank  22  and the burner  24 . The fuel filter  42  is also located within the supply line  40  between the inline heater  34  and the burner  24 . A return line  48  connects the unused fuel outlet  50  of the gear pump to the valve  44 . The valve  44  has a housing  38  ( FIG. 3 ), one inlet  52  for receiving unspent fuel from the burner  24 , and first and second outlets  54 ,  56 . The first outlet  54  is coupled to the fuel tank  22  via a first downstream branch  58  of the return line  48  that serves as a purge line. The second outlet  56  is connected to the supply line  40  via a second downstream branch  60  of the return line  48 . The second downstream branch  60  of the return line  48  may open into the supply line  40  upstream of the inline heater  34  or into the inline heater  34  itself, preferably at or near an inlet  66  thereof. Since the valve  44  is intended to supply fuel to the second downstream branch  60  of the return line  48  at all times except during a purge operation following an out-of-fuel condition, the valve  44  can be a simple manual operated valve, such as a ball valve. 
     The inline heater  34  is an electrically powered, thermostatically controlled heater that heats the combined volume of fuel supplied thereby via the supply  40  and return lines  48 . Since the vast majority of the fuel being heated (typically on the order of 70% to 80%) is warm recirculated fuel being supplied from the return line  48 , the power requirements of the inline heater  34  are dramatically reduced when compared to a heater that heats the entire volume of fuel being withdrawn from the fuel tank in a two-pipe system. Referring again to  FIG. 3 , the inline heater  34  preferably is formed of an external housing  64  having an inlet  66  and an outlet  68 . The housing  64  may be an aluminum tube tapped at both the inlet  66  and outlet  68  ends of the tube. Around the exterior of the housing  64 , a layer or multiple layers of thermal insulation may be provided to prevent heat loss, and improve efficiency of the inline heater. Within the housing  64 , the inline heater  34  has an electric immersion heater (not shown) formed from electrical heating elements (also not shown) in contact with fuel flowing through the inline heater  34 . The heating elements may be of various sizes, as is required to adequately heat the volume of fuel flowing through the inline heater  34 . In one embodiment, the heating element may be a heating pad wrapped along the inner circumference of the inline heater housing  64 . A thermostat (not shown), such as a bimetallic thermostat, preferably is provided for controlling the inline heater  34  to heat the fuel to a desired, settable temperature. That temperature preferably is within a range above that required to achieve adequate fuel atomization and below the fuel&#39;s flashpoint. In the case of #2 diesel fuel oil (the fuel most commonly used in heaters of the disclosed type), that range preferably is between 0° C. and 65° C. An additional backup, such as a thermally actuated electrical fuse (not shown), may be integrated into the inline heater  34 , as to disrupt the flow of electricity to the inline heater  34  at a predetermined temperature beneath the fuel flashpoint. 
     Still referring to  FIGS. 3 and 4 , the fuel filter  42  is located downstream of the inline heater  34 , and is in fluid communication with the inline heater outlet  68  by means of the fuel supply line  40 . The fuel filter  42  is formed of an external housing  70  having an inlet  72  and an outlet  74 . The warmed fuel is received at the inlet  72 , and subsequently passes through an internal filter element (not shown), before exiting the outlet  74 . Filtration of the fuel is critical for removing undesirable contaminant, which may damage the gear pump or clog the burner  24 , unless removed. When using diesel fuel additional contaminants, such as water, may also be separated at the fuel filter. 
     In operation, as illustrated in  FIG. 4 , activation of the burner  24  and the gear pump assembly draws fuel from the fuel tank  22  into the supply line  40 . The fuel, which in cold weather climates may be at a temperature of approximately −40° C., is then mixed with fuel being returned from the gear pump assembly via the valve  44  and preheated by that fuel to form a combined volume of preheated fuel that may be of a temperature of 0° C. to 40° C. As mentioned above, returned fuel typically will comprise in excess of 50%, and up to 80% or more of the total volume exiting the inline heater  34 . The combined volume is warmed to a final temperature of 10° C. to 65° C., by way of passing over the heating element located within the inline heater  34 . The warm fuel subsequently travels through the fuel filter  42  where undesirable contaminants are removed. Since the filtered fuel is well-above the temperature above which wax may precipitate in the filter  42 , filter clogging is avoided. The filtered fuel then flows to the burner  24  and gear pump assembly. At the burner  24 , a fraction of the warm fuel is combusted to heat the surrounding air in the combustion chamber. Because the warm fuel is easily atomized by the burner  24 , efficient (i.e. relatively smokeless) combustion without the use of a nozzle heater can be easily achieved. The unspent or non-combusted fuel then travels into the return line  48 , where it is received at the valve inlet  52 . During normal operation in which the inlet  52  of the valve  44  is connected to the second outlet  56 , the returned fuel is delivered to the inline heater  34 , via the second downstream branch  60  of the return line  48 , where the process is repeated. 
     Prior to start up, it may be desirable to purge the fuel lines, i.e. fuel supply assembly  36 , of the heater assembly  10 . This is particularly important following a complete fuel burn off, during which the fuel lines  36  of heater assembly  10  may become filled with air, as opposed to fuel. The fuel lines  36  can be purged by switching the valve  44  to connect the inlet  52  to the first outlet  54 , and thereby the purge line  58  and operating the pump for a sufficient period of time to fully purge the air from the fuel supply assembly  36 . This purging may be performed either with or without operating the inline heater  34 . The valve  44  is then switched back to the second position, in which the valve inlet  52  is in communication with the second outlet  56 , and the burner  24  is ignited to heat air. 
     Tests of the heater assembly  10  according to the embodiment of the present invention have been performed by retrofitting of a Wacker Neuson Cub 700 Mobile heater assembly  10  with the inline heater  34  and recirculation fuel supply assembly  36 , as discussed above. The inline heater  34  was connected to an external generator  32  by way of a 115V 60 Hz male plug. At negative thirty degrees Celsius (−30° C.), with the inline heaters  34  not operating, the heater assembly  10  could not be started. However, at negative thirty degrees Celsius (−30° C.), with the inline heaters  34  operating, the heater assembly  10  could both be started and maintain a flame at the burner  24  throughout an overnight operating test. Subsequent testing has also indicated that, at negative forty degrees Celsius (−40° C.), the heater assembly  10  of the present invention was able to start and maintain a flame at the burner  24 , after the inline heater  34  was allowed to warm the fuel in the fuel supply assembly  36  for ten minutes. 
     Many changes and modifications could be made to the invention without departing from the spirit thereof. The scope of these changes and modifications will become apparent from the appended claims.