Abstract:
The present invention overcomes many of the disadvantages of prior art mobile oil field heat exchange systems by providing a portable heat exchange system. The present invention is a self-contained unit which is easily transported to remote locations. The present invention includes a single-pass tubular coil heat exchanger contained within a closed-bottom firebox having a forced-air combustion and cooling system. The rig also includes integral fuel tanks, hydraulic and pneumatic systems for operating the rig at remote operations in all weather environments. In a preferred embodiment, the portable heat exchanger system is used to heat water on-the-fly (i.e., directly from the supply source to the well head) to complete hydraulic fracturing operations. The present invention also includes systems for regulating and adjusting the fuel/air mixture within the firebox to maximize the combustion efficiency. The system includes a novel hood opening mechanism attached to the exhaust stack of the firebox.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of U.S. application Ser. No. 13/897,883 filed May 20, 2013, which is a divisional application of U.S. application Ser. No. 12/352,505 (now U.S. Pat. No. 8,534,235) filed Jan. 12, 2009, which claims the benefit of and priority to a U.S. Provisional Patent Application No. 61/078,734 filed Jul. 7, 2008, the technical disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to an apparatus and method for heating a water or petroleum based fluid for injection into an oil or gas well or into a pipeline system. 
         [0004]    2. Description of the Related Art 
         [0005]    It is common in the oil and gas industry to treat oil and gas wells and pipelines with heated fluids such as water and oil. For example, one such application commonly known as a hydraulic fracturing job or “frac” job, involves injecting large quantities of a heated aqueous solution into a subterranean formation to hydraulically fracture it. Such frac jobs are typically used to initiate production in low-permeability reservoirs and/or re-stimulate production in older producing wells. Water is typically heated to a specific temperature range to prevent expansion or contraction of the downhole well casing. The heated water is typically combined with a mixture of chemical additives (e.g., friction reducer polymers which reduce the viscosity of the water and improve its flowability so that it&#39;s easier to pump down the well), proppants (e.g., a special grade of light sand), and a cross-linked guar gel that helps to carry the sand down into the well. This fraccing fluid is then injected into a well hole at a high flow rate and pressure to break up the formation, increasing the permeability of the rock and helping the gas or oil flow toward the surface. As the fraccing solution cracks the rock formation, it deposits the sand. As the fractures try to close, the sand keeps them propped open. Frac jobs are typically performed once when a well is newly drilled, and again after a couple of years when the production flow rate begins to decline 
         [0006]    Another application, commonly referred to as a “hot oil treatment”, involves treating tubulars of an oil and gas well or pipeline by flushing them with a heated solution to remove build up of paraffin along the tubulars that precipitate from the oil stream that is normally pumped therethrough. 
         [0007]    Frac jobs and hot oil treatments are typically performed at the remote well sites and usually require less than a week to complete. Consequently, the construction of a permanent heating facility at the well site is not cost effective. Instead, portable heat exchangers, which are capable of transport to remote well sites via improved and unimproved roads, are commonly used. 
         [0008]    In the past, such portable heat exchangers have typically employed gas-fired heat sources using a liquefied petroleum gas (LPG) such as propane to heat treatment fluids at remote well sites. Such gas-fired heater units typically include a tubular coil heat exchanger configured above one or more open flame gas burners in an open-ended firebox housing. The tubular coil heat exchanger typically comprises a fluid inlet in communication with a plurality of interconnected tubes, which in turn communicate with a fluid outlet. The plurality of tubes are typically arranged in a stacked configuration of planar rows, wherein each tube in a row is aligned in parallel with the other tubes. The outlet of each tube is connected in series to the inlet of an adjacent tube in the row by means of a curved tube or return bend. Similarly, each planar row is connected to the adjacent rows above and below by connecting the outlet of the outermost tube in one row with the inlet of the outermost tube in another row by means of a curved tube or return bend. 
         [0009]    The one or more gas burners are typically positioned below the tubular coil heat exchanger so as to project a vertical flame up and through the heat exchanger. The gas burners are supplied with gas fuel from a nearby gas storage tank (e.g., a propane tank). Ambient air is also supplied to the burners via the opened-ended bottom of the firebox housing. The hot flue gasses generated from the burning of the LPG rise up and through the tubular coil heat exchanger within the firebox housing and exhaust via a vent at the top of the firebox housing. 
         [0010]    While gas-fired heat sources are adequate for performing many oil field servicing tasks, they exhibit a number of inherent drawbacks. These inherent limitations significantly impact their effectiveness in performing certain heating operations at remote oil field work sites. For example, frac jobs typically require the production of massive volumes of heated water. While gas-fired heat sources are certainly capable of heating fluids such as water, they are poorly suited to heating in a timely manner large volumes of continuously flowing water in many commonly occurring climactic and atmospheric conditions. Moreover, the logistics involved in conducting such heating operations at remote work sites negatively impacts the cost efficiencies of such a system. 
         [0011]    For example, LPG (e.g., propane gas) has a relatively low energy content and density when compared to other fuel options. For example, diesel fuel when properly combusted typically releases about 138,700 British thermal units (BTU) per US gallon, while propane typically releases only about 91,600 BTU per liquid gallon, or over 33% less. Thus, gas-fired heating units often lack sufficient heating capacity to produce sufficient quantities of heated water rapidly enough for the required operation to be completed. Consequently, in order to provide sufficient quantities of heated water on a timely basis for a typical frac job, the treatment water must often be preheated and stockpiled in numerous frac water holding tanks. These holding tanks range in size up to 500 bbl (i.e., approximately 21,000 gallons). It is not unusual for a typical frac job to require 10 or even 20 frac water holding tanks at the remote work site. The preheated water is typically overheated so as to allow for cooling while waiting to be injected into the well. Oftentimes, the preheated treatment water must be reheated just prior to injection into the well head. Needless to say, the logistics involved with providing additional holding tanks at the remote work site and the additional costs incurred in overheating or reheating the supply water negatively impacts the efficiency of the overall operation. 
         [0012]    While the technique of overheating and stockpiling supply water can ameliorate some the shortcomings in the heating capacity of gas-fired heat sources, in certain circumstances (e.g., severely cold weather or high altitude) it is inadequate. This is due to a number of reasons. First, the temperature change requirement for the system is simply greater in colder weather. That is, in colder weather the intake water supplied to the gas-fired heating unit is colder while the required injection temperature remains essentially the same. Thus, it takes longer for the gas-fired heating unit to preheat the supply water. The problem is further compounded by the fact that the stockpiled preheated water cools more rapidly in colder weather. Moreover, at higher altitudes there is less oxygen in the ambient atmosphere for combustion in the gas burner. Thus, at higher altitudes the heating capacity of gas-fired heat sources is further reduced. 
         [0013]    In addition, propane gas requires large and heavy high-pressure fuel tanks for its transport to remote sites. The size of such high-pressure fuel tanks is, of course, limited by the size of existing roads. Thus, a typical frac job may require the transport of multiple large high-pressure fuel tanks to a remote site to ensure an adequate supply of fuel to complete the operation. 
         [0014]    Furthermore, there are several safety concerns which must be taken into consideration when using gas-fired heat sources. As mentioned previously, current gas-fired heat exchangers typically use an open flame burner, i.e., a burner which is open to the ambient atmosphere. The fire boxes of such heat exchanger are typically elevated above the ground and opened on the bottom. The gas-fired burners are typically positioned near the open bottom of the firebox and directly below the heat exchange tubing. The gas-fired burners draw ambient air as necessary to assist in the combustion of the propane gas. While simple and efficient in providing air for combustion, open flame burners present a number of safety concerns. An open flame at the well site poses a substantial risk of explosion and uncontrolled fire, which can destroy the investment in the rig and injure or even cost the lives of the well operators. Moreover, open flame burners are particularly susceptible to erratic burning or complete blow-out in gusty wind conditions. Current U.S. government safety regulations provide that the open flame heating of the treatment fluids cannot take place within the immediate vicinity of the well. 
         [0015]    While safety concerns are of overriding importance, compliance with the no open-flame regulations requires additional time and expense to conduct heated fluid well treatments. Thus, there has been a long felt need for a safer and more efficient apparatus and method of heating a treatment fluid for injecting into the tubulars of oil and gas wells and pipelines without using an open flame heat source in the vicinity of the treatment location. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention overcomes many of the disadvantages of prior art mobile oil field heat exchange systems by providing an oil-fired heat exchange system. The present invention is a self-contained unit which is easily transported to remote locations. In one embodiment, the present invention is disposed on a trailer rig and includes a closed-bottom firebox having a forced-air combustion and cooling system. The rig also includes integral fuel tanks, hydraulic and pneumatic systems for operating the rig at remote operations in all weather environments. In a preferred embodiment, the oil-fired heat exchanger system is used to heat water on-the-fly (i.e., directly from the supply source to the well head) to complete a hydraulic fracturing operation. 
         [0017]    The present invention comprises a closed firebox that includes a novel heat exchanger comprised of a single-pass tubular coil configured in a highly oscillating or serpentine manner and oriented along multiple axes so as to maximize its exposure to the heat generated by the oil-fired burner assemblies. The design of the heat exchanger includes a horizontal tunnel configured within a bottom portion. The oil-fired burner assemblies are configured and oriented in relation to the tunnel so that their flames are initially generated in a horizontal fashion into the tunnel within the heat exchanger. 
         [0018]    The present invention further includes a novel forced-air combustion and cooling system. The forced-air system is comprised of a primary air system and a secondary air system. The primary air system provides pressurized air directly to the oil-fired burner assemblies to maximize atomization and combustion of the fuel oil. The secondary air system provides pressurized air to strategic positions within the firebox to assist in controlling the cooling of the firebox and to maximize the combustion of the fuel/air mixture. The primary and secondary air systems are powered by hydraulic pumps integral to the overall system. The present invention also includes systems for regulating and adjusting the fuel/air mixture within the firebox to maximize the combustion efficiency. 
         [0019]    The improved system of the present invention also includes several subsystems for maximizing the safety and efficiency of the heat exchanger system. The system includes a novel hood mechanism attached to the exhaust stack of the firebox. In addition, the system includes a novel intake air muffler/silencer system, which significantly reduces the noise generated by the intake of such large quantities of ambient air. 
         [0020]    The system also includes novel methods for heating large volumes of treatment fluids, such as water, in a continuously flowing fashion so that heating operations can be performed “on-the-fly”, i.e., without the use of preheated stockpiles of treatment fluid. For example, water at ambient conditions can be drawn into the device of the present invention and heated so that sufficient volumes of continuously flowing heated treatment fluid may be supplied directly to the well head for conducting hydraulic fracturing operations on the well. The system also includes novel methods for controlling the heating of the treatment fluid as it passes through the system. The system further includes novel methods for controlling the temperature change and volume flow of treatment fluid as it passes through the system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    A more complete understanding of the method and apparatus of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein: 
           [0022]      FIG. 1  is a perspective view of an embodiment of the Oil-Fired Heat Exchanger of the present invention; 
           [0023]      FIG. 2A  is a left side elevation view of the embodiment of the Oil-Fired Heat Exchanger of the present invention shown in  FIG. 1 ; 
           [0024]      FIG. 2B  is a right side elevation view of the embodiment of the Oil-Fired Heat Exchanger of the present invention shown in  FIG. 1 ; 
           [0025]      FIG. 2C  is a close-up view of the mechanism for opening and closing the opposing hood doors of the embodiment of the Oil-Fired Heat Exchanger of the present invention shown in  FIG. 2B ; 
           [0026]      FIG. 3  is a overhead plan view of the embodiment of the Oil-Fired Heat Exchanger of the present invention shown in  FIG. 1 ; 
           [0027]      FIG. 4A  is a front perspective view of an embodiment of the heat exchanger of the Oil-Fired Heat Exchanger of the present invention; 
           [0028]      FIG. 4B  is a back perspective view of the embodiment of the heat exchanger shown in  FIG. 4A ; 
           [0029]      FIG. 4C  is a cross-sectional view of the embodiment of the heat exchanger shown in  FIGS. 4A and 4B  installed in the embodiment of the Oil-Fired Heat Exchanger of the present invention shown in  FIG. 1 ; 
           [0030]      FIG. 5  is perspective view of a portion of the primary and secondary air systems of the Oil-Fired Heat Exchanger of the present invention; 
           [0031]      FIG. 6  is cut-away cross-sectional view of a portion of the secondary blower section of the secondary air system of the Oil-Fired Heat Exchanger of the present invention; 
           [0032]      FIG. 7  is a schematic depiction of the hydraulic, fuel, and air supply systems of the embodiment of the Oil-Fired Heat Exchanger of the present invention shown in  FIG. 1 ; and 
           [0033]      FIG. 8  is an overhead view of the schematic depiction of the hydraulic, fuel, and air supply systems of the embodiment of the Oil-Fired Heat Exchanger of the present invention shown in  FIG. 7 . 
       
    
    
       [0034]    Where used in the various figures of the drawing, the same numerals designate the same or similar parts. Furthermore, when the terms “top,” “bottom,” “first,” “second,” “upper,” “lower,” “height,” “width,” “length,” “end,” “side,” “horizontal,” “vertical,” and similar terms are used herein, it should be understood that these terms have reference only to the structure shown in the drawing and are utilized only to facilitate describing the invention. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    With reference to the Figures, and in particular to FIGS.  1  and  2 A-C, an embodiment of the improved oil-fired heat exchanger system  100  of the present invention is shown. The embodiment  100  shown in the Figures is configured to be an oil-fired frac water heater system. As depicted, the embodiment of the frac water heater system  100  is configured on a drop deck trailer  14  and suitable for transport to remote oil field sites. The system  100  includes a fuel storage and supply system, a firebox  40  containing a single heat exchanger  50 , primary  70  and secondary  80  air supply systems connected to the firebox  40 , and an auxiliary power plant  30  for driving an accessory gearbox  32 . The accessory gearbox  32 , in turn, drives multiple hydraulic pumps, which power a main fluid pump  94  and the air supply systems. The main fluid pump  94  is used to draw fluid, such as water, from a fluid source and supply it to the intake  51  of the heat exchanger  50 . The hydraulic pressure generated by the main fluid pump  94  effectively pumps the fluid through the heat exchanger  50  where it is heated. As the treatment fluid proceeds through a single pass of the heat exchanger  50  it increases in temperature until it reaches an outlet  52  of the heat exchanger  50  where it is directed via tubular conduits or hose to the well head for injection into the formation. The system  100  also includes a control quadrant  10  and control levers  12  for operating and monitoring the system  100 . 
         [0036]    As shown in the embodiment depicted in the Figures, the entire frac water heater system  100  is configured on a single drop deck trailer  14  having multiple wheels  16  and connected to a separate towing vehicle  2 . It is understood that alternate embodiments of the system of the present invention may be skid mounted or configured integral to a single vehicle. In addition, the subject invention may also be configured so that one or more of the various components of the system (e.g., fuel tank  20 , firebox  40 , auxiliary power plant  30 ) are configured on separate trailers, vehicles or skids for transport to the remote work site. 
         [0037]    With reference again to the Figures, and in particular to  FIGS. 2A-2C  and  3 , the components of the embodiment of the improved oil-fired heat exchanger system  100  of the present invention will be described in greater detail. As depicted in the Figures, the embodiment the present invention  100  is disposed on a single trailer rig  14  and includes a firebox  40  containing a single heat exchanger  50 , primary  70  and secondary  80  air supply systems connected to the firebox  40 , a fuel system for storing and supplying fuel to multiple burner assemblies  60  configured in the firebox  40 , and an auxiliary power plant  30 , which powers multiple hydraulic systems and assorted auxiliary systems. 
       Auxiliary Power Plant &amp; Hydraulic System 
       [0038]    As depicted in the Figures, the auxiliary power plant  30  is configured near the front end of the trailer  14 . The auxiliary power plant  30  provides power for driving an accessory gearbox  32  and assorted auxiliary systems (e.g., electric, pneumatic). In one embodiment, the auxiliary power plant  30  comprises a diesel engine, which includes an electric alternator and air compressor. Alternatively, the electric alternator and air compressor may be powered by the accessory gearbox  32 . The electric alternator provides electrical power to the system  100  and the pneumatic compressor provides pneumatic pressure for controlling the system  100 . 
         [0039]    The auxiliary power plant  30  provides the primary motive force for driving the accessory gearbox  32 . The accessory gearbox  32 , in turn, drives multiple hydraulic pumps that power the hydraulic systems of the present invention. Each hydraulic pump is used to power an independent hydraulic circuit. For example, in the depicted embodiment, the accessory gearbox  32  powers three hydraulic circuit systems. The first hydraulic circuit includes a first hydraulic pump  33  that supplies pressurized hydraulic fluid via supply/return line  33   a  to a first hydraulic motor  36 , which powers the first air blower system. The second hydraulic circuit includes a second hydraulic pump  34  that supplies pressurized hydraulic fluid via supply/return line  34   a  to the second hydraulic motor  37 , which powers the second air blower system. The third hydraulic circuit includes a third hydraulic motor  35  that supplies pressurized hydraulic fluid via supply/return line  35   a  to a third hydraulic motor  38 , which powers the main fluid pump  94 . The three hydraulic systems are supplied by a hydraulic reservoir  31  positioned near the accessory gearbox  32 . In a preferred embodiment, the three hydraulic pumps  33 ,  34 , each comprise a mechanically-driven, variable-displacement, hydraulic pump; while the three hydraulic motors  36 ,  37 ,  38  each comprise fixed displacement hydraulic motors. The hydraulic pumps  33 ,  34 ,  35  are rated at 5000 psi, but typically operated at approximately 2500-3000 psi. 
       Treatment Fluid Supply System 
       [0040]    The main fluid pump  94  is used to draw a treatment fluid, such as water, from a fluid source and supply it to the inlet  51  of the heat exchanger  50 . The main fluid pump  94  is typically integral to the system  100  and has sufficient power to both draw the treatment fluid from a source and to pump the treatment fluid through the heat exchanger  50  and on to the well head for subsequent injection into the formation. In one embodiment, the main fluid pump  94  comprises a hydraulically-powered centrifugal fluid pump that is capable of supplying treatment fluid to the heat exchanger  50  at a pressure of about 150 psi. The volume of treatment fluid pumped through the heat exchanger  50  will vary with the pump speed. In a preferred embodiment, the main fluid pump  94  is capable of pumping a maximum of 252 gpm of treatment fluid through the heat exchanger  50 . 
         [0041]    As shown in the Figures, the fluid supply system may include an intake  90  manifold for connecting one or more supply hose (not shown) to the system&#39;s respective intake. The intake manifold  90  may include one or more spigots  91  for receiving supply hose in fluid communication with the fluid source. Each inlet spigot  91  may further include a valve mechanism  92 , which selectively controls the fluid flow through its respective inlet spigot  91 . Tubular intake conduits  93   a ,  93   b  fluidly connect the inlet of the main fluid pump  94  with the intake manifold  90 . Conduit  93   c  fluidly connects the outlet of the main fluid pump  94  with the inlet  51  of the heat exchanger  50 . The hydraulic pressure generated by the main fluid pump  94  effectively pumps the fluid through the heat exchanger  50  where it is heated. As the treatment fluid proceeds through a single pass of the heat exchanger  50  it increases in temperature until it reaches an outlet  52  of the heat exchanger  50  where it is directed via tubular outlet conduit  95  and supply hose (not shown) to the well head for injection into the formation. As shown in the Figures, the fluid supply system may further include an outlet manifold  96  having one or more spigots  97  for connecting with supply hose. Each outlet spigot  97  may further include a valve mechanism  98 , which selectively controls the fluid flow through its respective outlet spigot  97 . 
       Fuel Supply &amp; Control System 
       [0042]    As shown in the Figures and schematically depicted in  FIGS. 7 and 8 , the fuel system includes a fuel tank  20 , which is configured near the rear or back end of the trailer  14 . The fuel tank  20  is typically unpressurized and used to store the liquid fuel used by the multiple burner assemblies  60  configured in the firebox  40 . In the depicted embodiment  100 , the fuel tank  20  is unpressurized and can hold up to 60 bbl of diesel fuel. The fuel system also includes an unpressurized fuel line  21 , which supplies fuel from the fuel tank  20  to the intake of a fuel pump  22 . The fuel pump  22  boosts the fuel pressure and directs it to the multiple burner assemblies  60  by means of a pressurized fuel line  26 . In one embodiment, the fuel pump  22  boosts the fuel pressure to approximately 50-100 psi, preferably 60 psi. 
         [0043]    The fuel system also includes a pressure relief valve  24  in fluid communication with the pressurized fuel line  26 . The pressure relief valve  24  permits fuel to vent back into the fuel tank by means of fuel line  25  when the fuel pressure in the pressurized fuel line  26  exceeds a certain pressure. 
         [0044]    The fuel system further includes a fuel pressure control motor valve  27 , which regulates the flow of fuel from the pressurized fuel line  26 . The pressurized fuel line  26  fluidly connects the outlet of the fuel pump  22  with the inlet of a fuel pressure control motor valve  27 . The fuel pressure control motor valve  27  controls the amount of fuel supplied to the multiple burner assemblies  60  via pressurized metered fuel lines  28 . As depicted in the drawings, the metered fuel lines  28  may be configured so as to supply pressurized fuel to sets of burner assemblies, which are comprised of more than one burner assembly  60 . The fuel pressure control motor valve  27  may be electrically, pneumatically or hydraulically actuated. In a preferred embodiment, the fuel pressure control motor valve  27  comprises a pneumatically-actuated flow control valve. 
         [0045]    The temperature of the treatment fluid exiting the heat exchanger outlet  52  is a function of three variables: the volumetric flow rate of the treatment fluid through the heat exchanger  50 ; the flow rate of the pressurized secondary air; and the heat generated by the multiple burner assemblies  60  configured in the heat exchanger  50 . The flow rate of the secondary air is typically held constant during all operations while the volumetric flow rate of the treatment fluid is typically constant for a given operation. Thus, the temperature of the treatment fluid exiting the heat exchanger outlet  52  is controlled by regulating the volume of fuel supplied to the multiple burner assemblies  60 . 
         [0046]    An adjustable temperature controller mechanism  68  is used to send a control signal, which causes the fuel pressure control motor valve  27  to open or close, thereby increasing or decreasing the volume of fuel supplied to the multiple burner assemblies  60  via pressurized metered fuel lines  28 . The control signal may comprise an electrical, wireless, pneumatic, or hydraulic signal. For example, in one embodiment, the adjustable temperature controller mechanism  68  comprises a simple manual rotary or slider rheostat device, which controls an electric signal that controls the actuation of the fuel pressure control motor valve  27 . In another embodiment, the adjustable temperature controller mechanism  68  comprises a simple manual rotary valve, which controls a pneumatic pressure signal that controls the actuation of the fuel pressure control motor valve  27 . 
         [0047]    The temperature controller mechanism  68  may further includes a thermostat mechanism, which continually monitors the temperature of the treatment fluid exiting the heat exchanger outlet  52  and automatically adjusts the control signal to the fuel pressure control motor valve  27  to open or close as necessary to maintain a set point temperature. 
         [0048]    Thus, the fuel pressure supplied to the multiple burner assemblies  60  is initially generated by the fuel pump  22  and regulated by the fuel pressure control motor valve  27 . For example, in the previously noted embodiment, the fuel pump  22  boosts the fuel pressure to approximately 50-100 psi, preferably 60 psi. The fuel pressure is limited to a maximum pressure of 100 psi by the pressure relief valve  24 , which permits fuel to vent back into the fuel tank by means of fuel line  25  when the fuel pressure in the pressurized fuel line  26  exceeds 100 psi. The fuel pressure control motor valve  27  regulates the maximum fuel pressure supplied to the multiple burner assemblies  60  via pressurized metered fuel lines  28  to approximately 60 psi. 
       Firebox 
       [0049]    As depicted in the Figures, the firebox  40  is configured near the center of the trailer  14 . The firebox  40  is a closed-bottomed box having one or more exhaust stacks  42  configured near the top. In a preferred embodiment, the outer shell of the firebox  40  is constructed substantially of 3/16″ carbon steel. The firebox  40  houses a single heat exchanger  50  and a plurality of burner assemblies  60  for heating a treatment fluid during a single pass through the heat exchanger  50 . The closed-bottom design of the firebox  40  ensures the plurality of burner assemblies  60  are less susceptible to changes in ambient conditions, such as wind direction or gustiness. The interior walls and bottom of the firebox  40  are lined with an insulating refractory material. The refractive lining  48  is configured between the interior walls and bottom of the firebox  40  and the heat exchanger  50 . In one embodiment, the refractive lining  48  comprises one or more layers of fiber-type insulation coated with a cementious refractive compound. 
       Exhaust Stacks 
       [0050]    As previously noted, one or more exhaust stacks  42  are configured near the top the firebox  40  providing an exhaust for flue gases to exit the firebox  40 . In the depicted embodiment, the firebox  40  further includes a tapered hood assembly  41 , which incorporates the one or more exhaust stacks  42 . The tapered hood assembly  41  is removable so as to allow access to the heat exchanger  50  for servicing. Each exhaust stack  42  also includes a hood door assembly  44 , which is opened when the system  100  is operating. As depicted in  FIG. 2A , each hood door assembly  44  includes two doors  44   a ,  44   b  which are pivotally mounted to opposing sides of a respective exhaust stack  42 . 
       Hood Door Opening Mechanism 
       [0051]    With reference to  FIG. 2B , each hood door assembly  44  may further include a novel mechanism  46  for opening and closing the opposing hood doors. As shown in greater detail in  FIG. 2C , the mechanism  46  comprises a series of bell crank mechanisms, which cause the hood doors to open or close when actuated. The embodiment in  FIG. 2C  depicts the hood door assembly  44  on the left side in an opened position and the hood door assembly  44  on the right side in a closed position. Each mechanism  46  comprises a piston  46   a  having one end attached to the firebox  40  and a second end attached to a first bell crank  46   b . The first bell crank is pivotally attached to the side of the firebox  40 . When actuated, the piston  46   a  causes the first bell crank  46   b  to rotate about its pivot point p 1 . The first bell crank  46   b  also includes a pivotally attached push rod linkage  46   c  that connects the first bell crank  46   b  to a second bell crank  46   d , which is fixably attached to the side edge of one of the hood doors  44   a . The second bell crank  46   d  is configured so that its pivot point p 2  is co-aligned with that of its respective hood door. The second bell crank  46   d  also includes a pivotally attached push rod linkage  46   e  that connects the second bell crank  46   d  to a third bell crank  46   f , which is also fixably attached to the side edge of the other of the hood doors  44   b . The third bell crank  46   f  is also configured so that its pivot point p 3  is co-aligned with that of its respective hood door. Actuating the piston  46   a  causes the extension or retraction of a piston rod r p , which causes each of the three bell cranks to rotate simultaneously about their respective pivot points. This, in turn, causes the hood doors  44   a ,  44   b  to pivot open or closed as desired. In a preferred embodiment, the piston  46   a  is a pneumatically actuated piston. 
       Burner Assemblies 
       [0052]    The firebox  40  also includes a plurality of burner assemblies  60 , which are configured in the lower side of the firebox  40 . As will be subsequently described in greater detail, each of the burner assemblies  60  are connected to the fuel system and a pressurized air supply. For example, as schematically depicted in  FIGS. 7 and 8 , liquid fuel is supplied to each burner assembly  60  via the metered pressurized fuel line  28 . Similarly, pressurized air for combustion is supplied to each burner assembly  60  via a primary air conduit  78   c . The pressurized air and fuel are combined in the burner assembly  60  and directed through an atomizer nozzle  64 , which projects an atomized fuel spay into the firebox  40  where it is combusted. Each burner assembly  60  is configured in the lower side of firebox  40  so as to initially generate a substantially horizontal combustion flow within the firebox  40 . Each burner assembly  60  includes self-contained controls for adjusting the fuel-air mixture and an ignition mechanism for initially igniting the fuel-air mixture. In a preferred embodiment, the burner assembly  60  comprises a 780-Series self-proportioning, oil-fired burner manufactured by the Hauck Manufacturing Company of Lebanon, Pa. 
       Heat Exchanger 
       [0053]    The heat exchanger  50  contained within firebox  40  is comprised of a tubular coil which is configured in a highly oscillating or serpentine manner and oriented along multiple axes so as to maximize its exposure to the heat generated by the oil-fired burner assemblies  60 . The heat exchanger coil  50  includes a single inlet  51  configured at or near the top of the heat exchanger coil  50  and a single outlet  52  configured at or near the bottom of the heat exchanger coil  50 . Such a configuration greatly improves the efficiency of the system  100  by minimizing the back pressure exerted on the main fluid pump  94  by the treatment fluid and providing a gravity assist to the flow of treatment fluid through the heat exchanger  50 . As the treatment fluid proceeds through a single pass through of the heat exchanger coil  50  it increases in temperature until it reaches the outlet  52  where it is directed, via an outlet conduit  95  and supply hose (not shown), to the well head for injection into the formation. 
         [0054]    With reference now to  FIGS. 4A-4B , an embodiment of the heat exchanger  50  of the present invention is depicted. The heat exchanger  50  is comprised of a tubular coil which is configured in a highly oscillating and serpentine manner and oriented along two axes so as to maximize its exposure to the heat generated by the oil-fired burner assemblies  60 . For example, the depicted embodiment of heat exchanger  50  includes an upper portion  53  configured in stacked horizontal rows of tubing faked down in a series of reversing loops oriented about a vertical axis; and a lower portion  56  configured in a helical coil oriented about a horizontal axis. The upper portion  53  is fluidly connected to the lower portion  56  forming the single heat exchanger  50 . In one embodiment, the upper  53  and lower  56  portions of the tubular coil of the heat exchanger  50  comprise approximately 1,300 ft. of 3″ seamless steel pipe with weld fittings. 
         [0055]    Each row of the upper portion  53  of the heat exchanger  50  is constructed of a plurality of tubes  54  aligned in parallel with each other. The outlet of each tube  54  is connected in series with the inlet of an adjacent tube  54  by means of an approximate 180° curved tube or return bend  55 . Similarly, each planar row is connected in series to the adjacent rows above and below by connecting the outlet of the outermost tube in one row with the inlet of the outermost tube in another row by means of a return bend  55   a . In a preferred embodiment, each planar row is laterally offset from the planar row above and below it so that the tubes  54  in one row are centered on the space between two adjacent tubes  54  in the rows above and below it. 
         [0056]    Each return bend  55  may further include an alignment bolt  47  extending from the approximate exterior inflection point of the return bend  56 . The multiple alignment bolts  47  correspond to holes formed in an alignment plate  98 , which is fixably attached to the upper portion  53  of the heat exchanger  50  by means of mechanical fasteners  45 , such as threaded nut fasteners. The alignment plate  98  maintains the alignment of the stacked planar rows of the upper portion  53  of the heat exchanger  50  so that the adjacent rows do not touch and space is maintained between all adjacent tubes  54 , thereby enabling the flow of heated air through the upper portion  53  of the heat exchanger  50  during operation. 
         [0057]    The upper portion  53  is fluidly connected in series to the lower portion  56  of the heat exchanger  50 . As shown in  FIGS. 4A-4B , the lower portion  56  transitions to an angled rectangular helical coil configuration, which is oriented about a horizontal plane and defines a five-sided cavity/chamber or tunnel  65 . As will be described infra, the tunnel  65  serves as an effective combustion chamber for the multiple oil-fired burner assemblies  60 . The lower portion  53  of the heat exchanger  50  comprises a tubular coil constructed a plurality of adjacently aligned upper  57   a  and lower  57   b  lateral tubes, which are vertically spaced and connected in series by means of quarter-bend (i.e., approximately 90° bend) tubes  58  and riser tubes  59 . The outlet of each lateral tube  57  is fluidly connected in series with the inlet of the next vertically spaced lateral tube  57  by means of a quarter-bend tube  58  followed by a riser tube  59  followed by another quarter-bend tube  58 . As shown in  FIG. 4A , the outlet of the last lateral tube  57  in the tubular coil forming the lower portion  53  is fluidly connected to the outlet  52  of the heat exchanger  50 . 
         [0058]    With reference now to  FIG. 4C , a cross-sectional view of the heat exchanger  50  shown in  FIGS. 4A-4B  installed in the firebox  40  of the present invention is shown. The firebox  40  includes a refractive lining  48  configured between the interior walls and bottom of the firebox  40  and the tubular coil of the heat exchanger  50 . As previously described, the heat exchanger  50  is comprised of a tubular coil which is configured in a highly oscillating and serpentine manner and oriented along two axes so as to maximize its exposure to the heat generated by the oil-fired burner assemblies  60 . The upper portion  53  configured in tightly stacked horizontal rows of tubing faked down in a series of reversing loops oriented about a vertical axis; and a lower portion  56  configured in a helical coil oriented about a horizontal axis. The upper portion  53  is fluidly connected to the lower portion  56  forming the single heat exchanger  50 . The attached alignment plate  98  maintains the alignment of the stacked planar rows of the upper portion  53  of the heat exchanger  50  so that the adjacent rows do not touch and space is maintained between all adjacent tubes  54 , thereby enabling the flow of heated exhaust or flue gases  88  through the upper portion  53  of the heat exchanger  50  during operation. The lower portion  56  of the heat exchanger  50  transitions to an angled rectangular helical coil configuration, which is oriented about a horizontal plane and defines a five-sided cavity/chamber or tunnel  65 . 
         [0059]    The tunnel  65  serves as an effective combustion chamber for the multiple oil-fired burner assemblies  60  configured in the lower side of the firebox  40 . Each burner assembly  60  is connected to the fuel system and a pressurized air supply. For example, as schematically depicted in  FIGS. 7 and 8 , liquid fuel is supplied from the fuel tank  20  to each burner assembly  60  via fuel pump  22 , pressurized fuel line  26 , fuel pressure control motor valve  27  and the metered pressurized fuel line  28 . Similarly, pressurized air for combustion is supplied to a primary air inlet  62  configured on each burner assembly  60  via a primary air conduit  78   c . With reference again to  FIG. 4C , the primary air and fuel are combined in the burner assembly  60  and directed through an atomizer nozzle  64 , which projects an atomized fuel spay F A  into the firebox  40  where it is combusted in the previously described cavity/chamber or tunnel  65  formed in the heat exchanger  50 . It is further noted that each burner assembly  60  is oriented so as to initially generate a substantially horizontal combustion flow  69  within the firebox  40 . Each burner assembly  60  includes self-contained controls  66  for adjusting the fuel-air mixture and an ignition mechanism for initially igniting the fuel-air mixture. 
         [0060]    The firebox  40  depicted in  FIGS. 4C ,  7  and  8  further includes ductwork  85   a ,  85   b , which supply pressurized secondary air to the interior of firebox  40 . The pressurized secondary air assists in directing and regulating the flow of heated flue gases  88  through the heat exchanger  50  during operation. The ductwork  85   a ,  85   b  supplies pressurized secondary air to vents  86 ,  87  configured on opposing sides of the firebox  40 . The vents  86 ,  87  are typically configured so that their respective airflows F B , F C  are generally directed into the cavity/chamber or tunnel  65  formed in the heat exchanger  50 . The secondary airflows F B , F C , which are projected from their respective vents  86 ,  87 , assist in regulating and directing the flow of heated flue gases  88  through the heat exchanger  50  during operation. 
         [0061]    For example, a first or front vent  86  is configured under the burner assemblies  60  and projects a first flow of secondary pressurized air F B  into the open front portion of the cavity/chamber or tunnel  65  formed in the heat exchanger  50 . In one embodiment, the first vent  86  comprises an individual nozzle vent configured under each burner assembly  60 . The first flow of secondary pressurized air F B  provides a thermal air barrier that partially insulates the lateral tubes  57   b  on the bottom of the heat exchanger  50  from the substantially horizontal combustion flame  69  generated by the burner assembly  60 . In addition, the first flow of secondary pressurized air F B  absorbs the heat produced by the substantially horizontal combustion flow  69  generating a flow of heated flue gases  88 , which exhausts up through the heat exchanger  50  during operation. In a preferred embodiment, the first vent  86  is angled at a slightly upward angle, so that the first flow of secondary pressurized air F B  combines with the atomized fuel spay F A  to effectively supercharge the resulting combustion flow  69  with additional air. 
         [0062]    The second or rear vent  87  is configured on the opposing wall or side from the first vent  86  and burner assemblies  60 , and projects a second flow of secondary pressurized air F C  into the rear portion of the cavity/chamber or tunnel  65  formed in the heat exchanger  50 . As depicted in Figures, the rear portion of the cavity/chamber or tunnel  65  formed in the heat exchanger  50  is partially obscured by the lateral tubes  57   c  traversing the tunnel  65 . Thus, the second or rear vent  87  is configured so as to project the second flow of secondary pressurized air F C  through gaps existing between adjacent lateral tubes  57 . The injection of the second flow of secondary pressurized air F C  provides a thermal air barrier that partially insulates the lateral tubes  57   c  traversing the back of the heat exchanger  50 . In addition, the second flow of secondary pressurized air F C  also absorbs the heat produced by the substantially horizontal combustion flow  69  generating a flow of heated flue gases  88 , which exhausts up through the heat exchanger  50  during operation. In one embodiment, the second vent  87  may also be angled at a slightly upward angle. 
       Air Supply System 
       [0063]    With reference again to the Figures, and in particular to  FIGS. 5 and 6  the air supply system of the present invention will be described in greater detail. The air supply system of the present invention a forced-air or pressurized system which is not susceptible to changes in ambient conditions, such as wind direction or gustiness. The air supply system of the present invention is comprised of primary and secondary air systems. The primary air system supplies large volumes of pressurized air to the multiple burner assemblies  60  configured in the side of the firebox  40 . The primary air system includes a high-pressure pump which compresses ambient air and directs it to the primary air inlet  62  of each oil-fired burner assembly  60  where it is used to atomize fuel. The secondary air system supplies large volumes of pressurized air to strategic locations within the firebox  40  to control and regulate the heating of the heat exchanger  50  and firebox  40 . The secondary air system includes a secondary air blower mechanism, which draws in large volumes of ambient air. The secondary air is then directed via ductwork to the previously described vents  86 ,  87  configured on opposing sides of the firebox  40 . The secondary air assists in maximizing the combustion of the fuel/air mixture while directing and regulating the flow of heated flue gases  88  through the heat exchanger  50  during operation. By controlling and regulating the heating of the heat exchanger  50  and firebox  40  during operation, the oil-fired heat exchanger system  100  of the present invention can continuously heat large volumes of treatment fluid safely. 
         [0064]    In the embodiment of the present invention  100  depicted in the Figures, the air supply system is comprised of matched sets of primary and secondary blower systems disposed on opposing sides (i.e., the front and rear) of the firebox  40  in a mirror-image configuration. Each set includes a primary blower system  70  and a secondary blower system  80 , which are powered by a single motor mechanism. For example, the first or front of blower system set is powered by motor  36  while the second or rear blower system set is powered by motor  37 . The single motor mechanism  36 ,  37  are preferably hydraulically powered. For example, in the depicted embodiment, the motors  36 ,  37  are powered by hydraulic pumps  33 ,  34 , respectively, which are driven by the accessory pump drive gear box  32 . As noted previously, in a preferred embodiment, the hydraulic pumps  33 ,  34  comprise mechanically-driven hydraulic pumps which are rated at 5000 psi, but typically operate at approximately 2500-3000 psi. 
         [0065]    As shown in  FIG. 5 , which depicts in greater detail the second or rear blower system of the present invention  100 , each primary air blower system  70  includes a high-pressure blower pump  74  having an intake which draws ambient air through an intake filter  72  and intake conduit  73 . In a preferred embodiment, each high-pressure blower pump  74  is a positive displacement rotary blower. Each high-pressure blower pump  74  is powered by its respective motor mechanism  36 ,  37  through a rotary driveshaft  84 . The high-pressure blower pump  74  compresses the air and directs it via primary air conduits  78   a ,  78   b ,  78   c  to the primary air inlet  62  of each oil-fired burner assembly  60 . The primary air conduits  78   a ,  78   b ,  78   c  may further include a primary air silencer  76 , which muffles the noise generated by the suction of ambient air into the primary air system  70 . In one embodiment, the primary air conduits  78   a ,  78   b ,  78   c  also include a pressure relief “pop-off” valve, which limits the primary air pressure to approximately 5 psi. 
         [0066]    Each secondary air system  80  includes one or more secondary air blowers  81 , which are also powered by the respective motor mechanism (e.g.,  37 ) through a common rotary driveshaft  84 . As shown in the  FIG. 6 , in one embodiment the one or more secondary air blowers  81  each comprise a conventional centrifugal or squirrel-cage fan mechanism  82  contained in a protective housing  83 . As depicted, the one or more fan mechanisms  82  are aligned in a parallel configuration along and coupled to a common rotary driveshaft  84  so that when the driveshaft  84  rotates, each fan mechanism  82  also rotates within its housing  83 . It is further noted that the co-alignment of the rotary shaft  84  with the fan mechanisms  82  of the secondary air system  80  and the high-pressure blower pump  74  of the primary air blower system  70  enables both air supply systems to be simultaneously powered by the same motor  37 . 
         [0067]    The protective housing  83  of each secondary air blower  81  includes an opening, which allows the fan mechanism  82  to draw ambient air into its housing  83  where it is directed to the ductwork of the secondary air system. The output of pressurized air from the secondary air blowers  81  is combined in a first ductwork  85 , which then divides into secondary ductwork  85   a ,  85   b , which supply pressurized secondary air to vents  86 ,  87  configured on opposing sides of the firebox  40 . In the depicted embodiment, secondary air is pressurized to approximately 2.5-3 psi. As previously noted, the vents  86 ,  87  are typically configured so that their respective airflows F B , F C  are generally directed into the cavity/chamber or tunnel  65  formed in the heat exchanger  50 . The secondary airflows F B , F C , which are projected from their respective vents  86 ,  87 , assist in regulating, directing, and enhancing the convective flow of heated flue gases  88  through the heat exchanger  50  during operation. 
         [0068]    As shown in the embodiment depicted in  FIG. 5 , the first or front vents  86  preferably comprise oblong circular vents positioned below the nozzles  64  of the burner assemblies  60 . The depicted oblong circular vents  86  extend away from the firebox  40  wall and project one secondary air stream F B  up towards the fuel/air mixture spray F A  generated by the burner fuel nozzle  64 . The second or rear vent  87  is configured on the opposing wall of the firebox  40 . As noted previously, the configuration of the second oblong circular vents  87  provides a layer of cooling air F C  between the main burner fire and the bottom of the firebox. Moreover, the angular set of the secondary vents  86 ,  87  causes their respective opposing secondary air flows F B , F C  to collide in the tunnel  65  formed in the heat exchanger  50 , thereby affecting the flow of heated exhaust or flue gases  88  up and through the upper portion  53  of the heat exchanger  50  during operation. 
         [0069]    The integrated temperature controller mechanism  68  in conjunction with forced-air supply system and refractive insulation lining  48  in the firebox  40  enable the oil-fired heat exchanger system  100  of the present invention to safely heat water continuously. Operation time is limited only by fuel supply. For example, the depicted embodiment of the present invention  100 , which is configured with six (6) burner assemblies  60 , typically consumes 150-165 gallons of fuel per hour. The burner fuel tank  20  on the unit holds about 2500 gallons and is therefore sized for 15-16.5 hours of continuous operation. The auxiliary powerplant  30  has its own fuel tank that holds approximately 150 gallons of fuel that allow it to operate up to 18 hours depending on operating conditions. In the field, operators may have additional fuel delivered every 12 hours or so to allow the system  100  to continue operations on large heating jobs. 
       Method of Operation 
       [0070]    The system  100  of the present invention includes novel methods for heating large volumes of treatment fluid in a continuously flowing fashion so that on-site heating operations can be performed “on-the-fly”, i.e., without the use of preheated stockpiles of treatment fluid. For example, the embodiment of the system  100  of the present invention depicted in the Figures, is capable of heating sufficient quantities of continuously flowing water to conduct “on-the-fly” hydraulic fracturing operations at remote well sites. The system  100  of the present invention also includes novel methods for controlling the heating of the treatment fluid as it passes through the system  100 . The system  100  of the present invention further includes novel methods for controlling the temperature change and volume flow of treatment fluid as it passes through the system  100 . 
         [0071]    With reference again to the Figures and in particular  FIGS. 7 and 8 , the method of the present invention is depicted. A treatment fluid, such as water, is drawn from an ambient fluid source into the system  100 . The treatment fluid is then pumped through a single pass of a tubular coil heat exchanger  50  contained within firebox  40  where it is heated. As the treatment fluid proceeds through the heat exchanger  50  it increases in temperature until it reaches the outlet  52  of the heat exchanger  50  where it is directed via tubular conduits or hose to the well head for injection into the formation. 
         [0072]    The main fluid pump  94  is used to control the flow rate of the treatment fluid through the system  100 . For example, a supply hose (not shown) extending to the fluid source is connected to the intake manifold  90  so as to put the system  100  in fluid communication with the fluid source. The main fluid pump  94  draws the treatment fluid via conduits  93   a ,  93   b  from the fluid source and supplies it to the inlet  51  of the heat exchanger  50 . The main fluid pump  94  has sufficient power to both draw the treatment fluid from the fluid source and pump the treatment fluid through the heat exchanger  50  and on to the well head for injection into the formation. 
         [0073]    For example, in one embodiment, the main fluid pump  94  is capable of supplying treatment fluid to the heat exchanger  50  at a pressure of about 150 psi. In a preferred embodiment, the main fluid pump  94  is also capable of drawing and pumping a maximum of 252 gpm of treatment fluid through the system  100 . The requisite volumetric flow rate of treatment fluid is typically dictated by the particular operational requirements desired at the well head. By adjusting the speed of the main fluid pump  94 , the volumetric flow rate of treatment fluid is controlled. The main fluid pump  94  is driven by a hydraulic motor  38  powered via supply line  35   a  by a hydraulic pump  35  attached to the accessory pump drive gear box  32 . Consequently, the speed of the main fluid pump  94  is controlled by the operator using a control lever  12  to increase or decrease the amount of pressurized hydraulic fluid supplied to hydraulic motor  38 . In a preferred embodiment, control lever  12  comprises an electronic joystick actuator, which regulates the displacement of the hydraulic pump to change the speed of its respective hydraulic motor. The hydraulic pressure depends on the loads placed on the hydraulic motors. 
         [0074]    As the treatment fluid is pumped through the heat exchanger  50  contained within the firebox  40 , the fluid is heated by the transfer of thermal energy generated by the combustion of a liquid-fuel/air mixture in the firebox  40 . As previously detailed, pressurized primary air and liquid fuel are combined in the multiple burner assemblies  60 , which each project an atomized fuel spay F A  into the firebox  40  where it is combusted. The burner assemblies  60  are configured near the bottom of the firebox  40  and oriented so as to initially generate a substantially horizontal combustion flow  69  within the firebox  40 . Pressurized secondary air assists in directing and controlling the thermal energy generated by the substantially horizontal combustion flow  69  to exhaust in a convective flow up and through the upper portion  53  of the heat exchanger  50 . 
         [0075]    The tubular coil heat exchanger  50  is designed to maximize the heat transfer of the thermal energy within the confines of the firebox  40 . The heat exchanger  50  is, therefore, comprised of a tubular coil which is configured in a two interconnected portions, which are oriented along two distinct axes so as to maximize exposure to the heat generated by the oil-fired burner assemblies  60 . The ambient or cool treatment fluid enters the heat exchanger  50  through the inlet  51  configured at or near the top of the heat exchanger coil  50 . As the fluid flows through the upper portion  53  of the heat exchanger  50  thermal energy is transferred by the convective flow of the hot flue gases  88  over and between the stacked horizontal rows of interconnected adjacent tubes faked down in a series of reversing loops oriented about a vertical axis. As the fluid continues through the lower portion  56  of the heat exchanger  50  it flows through a helical coil oriented about a horizontal axis, thermal energy is transferred by the both the convective flow of the hot flue gases  88  and the radiant heat emanating from the substantially horizontal combustion flow  69  within the cavity/chamber or tunnel  65 . 
         [0076]    The convective flow of flue gases  88  through heat exchanger  50  is substantially enhanced by the secondary air system, which continually supplies large volumes of pressurized air to strategically configured vents  86 ,  87  on opposing sides of the firebox  40 . The secondary air flow is essentially a forced air system which uses air as its heat transfer medium to extract thermal energy from the substantially horizontal combustion flow  69 . The vents  86 ,  87  are positioned near the bottom of the closed-bottom firebox  40  and configured so that their respective airflows F B , F C  are generally directed into the cavity/chamber or tunnel  65  formed in the heat exchanger  50 . 
         [0077]    The treatment fluid continues to absorb thermal energy as it flows through the lower portion  56  of the heat exchanger  50  until it reaches the outlet  52  of the heat exchanger  50  where it is directed via tubular  95  and supply hose (not shown) to the well head for injection into the formation. 
         [0078]    As the heated treatment fluid exits the outlet  52  of the heat exchanger  50  its temperature is monitored. The temperature of the treatment fluid exiting the heat exchanger outlet  52  is a function of three variables: the volumetric flow rate of the treatment fluid through the heat exchanger  50 ; the flow rate of the pressurized secondary air; and the heat generated by the multiple burner assemblies  60  configured in the heat exchanger  50 . The flow rate of the secondary air is typically held constant during all operations while the volumetric flow rate of the treatment fluid is typically constant for a given operation. Thus, the temperature of the treatment fluid exiting the heat exchanger outlet  52  is controlled by regulating the volume of fuel supplied to the multiple burner assemblies  60 . 
         [0079]    In one embodiment, the operator monitors the temperature of the heated treatment fluid as it exits the outlet  52  of the heat exchanger  50 . The operator then adjusts the temperature controller mechanism  68  sending a control signal to the fuel pressure control motor valve  27  to increase or decrease the volume of fuel supplied to the multiple burner assemblies  60  via pressurized metered fuel lines  28 . The control signal may comprise an electrical, wireless, pneumatic, or hydraulic signal. For example, in the depicted embodiment, the adjustable temperature controller mechanism  68  comprises a simple manual rotary valve, which controls the pneumatic pressure supplied to the fuel pressure control motor valve  27 . 
         [0080]    In another embodiment, the temperature controller mechanism  68  is an automated thermostat mechanism that continually monitors the temperature of the treatment fluid exiting the heat exchanger outlet  52 . An operator inputs a desired temperature reading (i.e., set point temperature). The temperature controller mechanism  68  compares the actual temperature of the treatment fluid exiting the heat exchanger outlet  52  with the set point temperature and automatically adjusts the control signal supplied to the fuel pressure control motor valve  27 . For example, if the temperature of the treatment fluid exiting the heat exchanger outlet  52  is less than the set point temperature, the temperature controller mechanism  68  adjusts the control signal supplied to the fuel pressure control motor valve  27  to increase the volume of fuel supplied to the multiple burner assemblies  60  via pressurized metered fuel lines  28  in order to maintain a set point temperature. Conversely, if the temperature of the treatment fluid exiting the heat exchanger outlet  52  is higher than the set point temperature, the temperature controller mechanism  68  adjusts the control signal supplied to the fuel pressure control motor valve  27  to decrease the volume of fuel supplied to the multiple burner assemblies  60  via pressurized metered fuel lines  28  in order to maintain a set point temperature. 
         [0081]    The temperature of the treatment fluid is also typically monitored at the inlet  51  of the heat exchanger  50 . The temperature spread between the inlet  51  and outlet  52  of the heat exchanger  50 , when combined with the volumetric flow rate of treatment fluid, is indicative of the heating capacity of the system. Field testing has determined that the depicted embodiment of the oil-fired heat exchanger system  100  of the present invention is capable of heating ambient water from 70° F. to 210° F. at a maximum volumetric flow rate of 252 gpm. Moreover, field reports further indicate that the system  100  is capable of heating water from 40° F. to 210° F. in ambient atmospheric temperatures below 25° F. at a slightly reduced volumetric flow rate (e.g., 200-250 gpm). 
         [0082]    It will now be evident to those skilled in the art that there has been described herein an improved heat exchanger system for heating large, continuously flowing volumes of treatment fluids at remote locations. Although the invention hereof has been described by way of a preferred embodiment, it will be evident that other adaptations and modifications can be employed without departing from the spirit and scope thereof. For example, instead of the treatment fluid being water, it could be a petroleum based liquid such as oil for hot oil well treatments. The terms and expressions employed herein have been used as terms of description and not of limitation; and thus, there is no intent of excluding equivalents, but on the contrary it is intended to cover any and all equivalents that may be employed without departing from the spirit and scope of the invention.