Patent Publication Number: US-9897309-B2

Title: Forced circulation steam generator

Description:
FIELD OF THE INVENTION 
     The present invention relates to oil recovery processes and more particularly to oil recovery processes that treat produced water and utilize a steam generator to produce steam from the treated produced water and inject the steam into an injection well. 
     BACKGROUND 
     Steam assisted gravity discharge (SAGD) refers to a widely used process where high pressure steam is injected into an injection well to melt bitumen or to generally reduce the viscosity of heavy oil to facilitate its removal. The bitumen or heavy oil and condensed steam flows by gravity to drain pipes buried below the oil deposit and the bitumen or oil is pumped out as an oil-water mixture. Once the oil-water mixture is pumped to the surface, a number of processes are utilized to treat the oil-water. First, oil is separated from the oil-water mixture to yield an oil product and produce water. The produced water is then treated to remove total dissolved solids and suspended solids. Various types of treatments can be employed such as filters for removing suspended solids and warm lime softeners or evaporators to remove dissolved solids. Cyclic Steam Simulation (CSS) process also works in the same principle as SAGD process with intermittent steam injection followed by oil-water mixture extraction. 
     There are several types of steam generators that can be utilized to generate steam for use in a SAGD process for example. One type of steam generator is referred to as the once through steam generator. Once through steam generators have a number of disadvantages or drawbacks. They tend to have high blowdown and hence this gives rise to thermal inefficiencies and water wastage. Once through steam generators typically utilize inline steam separators and this results in additional blow down and additional heat recovery equipment. Many once through steam generators are designed with refractory/insulated furnaces. These typically require substantial maintenance. In addition, once through steam generators have uncooled supports for supporting steam generation coils. This also leads to high maintenance. With once through steam generators the turn down is limited and they typically have very complex flow circuits to manage. Moreover, the steam capacity is limited to about 300,000 LB/HR. Typically once through steam generators require a relatively large footprint and the capital cost is high. When once through steam generators are used in heavy oil recovery processes such as commercial bitumen production, the resulting designs require numerous one through steam generation units and this results in high capital and operating costs. 
     A second type of steam generator is what is referred to as a drum boiler. Drum boilers have limited operating experience in heavy oil recovery processes and in particular, have not been widely used with feed water from an evaporator. Further, there is not a great deal of experience with drum boilers in handling upsets in water quality, a real concern for oil producers. Furthermore, with drum boilers it is expensive and time consuming to clean the tubes of the drum boiler. Finally, mechanical tube failures that result from water quality issues are expensive to repair. 
     Therefore, there is and continues to be a need for a steam generator design for use in heavy oil recovery processes that overcomes the shortcomings and disadvantages of once through steam generators and drum boilers. 
     SUMMARY 
     The present invention relates to a method of recovering oil and producing steam for injection into an injection well to assist in the recovery of oil. The method includes recovering an oil-water mixture from an oil bearing formation. The oil-water mixture is separated into an oil product and produced water which includes suspended solids and dissolved solids. The produced water is directed to a treatment system that removes suspended solids and dissolved solids from the produced water. This yields treated water. The treated water is then directed to a forced circulation steam generator that includes a furnace having a burner and at least one water cooled wall and an evaporator unit. The treated water is pumped through the water cooled wall and the evaporator unit. The water being pumped through the water cooled wall and the evaporator unit is heated and yields a water-steam mixture that comprises approximately 10% to 30% quality steam. The water-steam mixture is then directed to a steam drum that separates the steam from the water-steam mixture to form injection steam that comprises 95% or more quality steam. The injection steam is then injected into an injection well to facilitate recovery of the oil-water mixture from the oil bearing formation. 
     Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of such invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of the oil recovery process of the present invention. 
         FIG. 2  is a perspective view of the forced circulation steam generator of the present invention. 
         FIG. 3  is a cross-sectional view of the furnace of the steam generator as shown in  FIG. 2 . 
         FIG. 4A  is a perspective view of a tube element that forms a part of a heat exchanger module. 
         FIG. 4B  is a perspective view of the heat exchanger module comprised of a series of tube elements. 
         FIG. 5A  is a perspective view of a tube element that makes up an evaporator unit. 
         FIG. 5B  is a perspective view of the evaporator unit. 
         FIG. 6  is a fragmentary perspective view showing a water cooled wall assembly of the furnace that forms a part of the steam generator. 
         FIG. 7  is a perspective cut-away view illustrating portions of the furnace of the steam generator as well as the water cooled walls and evaporator unit in the furnace. 
         FIG. 8  is a graphical illustration showing the relationship between tube metal temperature and quality steam and particularly comparing tube metal temperature and quality steam of the forced circulation steam generator of the present invention with a conventional once through steam generator. 
         FIG. 9  is a schematic illustration showing the basic operation of the forced circulation steam generator of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings, particularly  FIG. 1 , there is shown therein an oil recovery process that employs a forced circulation steam generator  10 . As will be appreciated from subsequent portions of the disclosure, the forced circulation steam generator  10  functions to produce steam that is injected into an injection well  200  that is typically spaced from an oil well or oil bearing formation. More particularly, in one embodiment, the present invention is a heavy oil recovery process that employs steam assisted gravity discharge, commonly referred to as a SAGD process. 
     Viewing  FIG. 1  in more detail, the forced circulation steam generator  10  produces steam that is directed into the injection well  200 . Once in the injection well  200 , the steam functions to fluidize heavy oil, sometimes referred to as bitumen, in the oil bearing formation which is typically horizontally separated from the injection well  200 . The process of the present invention can be utilized in a wide range of heavy oil recovery processes where it is desired to utilize steam to facilitate the removal of heavy oil from an oil bearing formation. For example, one area in the world that is particularly suited for the process disclosed herein is the tar sands region in Alberta, Canada for example. 
     Steam entering the injection well  200  eventually condenses and an oil-water mixture  204  results and this oil-water mixture moves through the oil bearing formation  202 . Eventually the oil-water mixture  204  is consolidated in an oil-water gathering well and the oil-water mixture  204  is pumped to the surface. 
     Once the oil-water mixture  204  reaches the surface, it is directed to an oil-water separator  206 . Oil separator  206  separates oil from the mixture and produces an oil product  208 . The remaining water is referred to as produced water  209 . The produced water  209 , after separation from the oil, is further de-oiled by a de-oiling process  210 . De-oiling process  210  may be accomplished in various ways such as by utilizing a dissolved air flotation system with the assistance of the addition of a de-oiling polymer. 
     After the de-oiling process  210  and prior to the produced water reaching the forced circulation steam generator  10  it is necessary to treat the produced water to remove contaminants such as suspended solids and total dissolves solids (TDS) including contaminants such as hardness and silica. At various points downstream from the de-oiling process  210 , various types of filtration devices, such as nutshell filters, multi-media filters, membranes, etc. can be employed to remove suspended solids or particulates from the produced water. These processes are generally included in the section of the process denoted treatment system  212  in  FIG. 1 . There are various processes that may be utilized in the treatment section  212  to deal with hardness, silica, organics and other dissolved solids. For example, warm lime softeners in combination with downstream filtration devices and ion exchange units can be utilized to remove hardness and silica as well as other dissolved solids. In the alternative, evaporators can be utilized to remove hardness, silica and other dissolved solids and again further downstream polishing processes can be utilized to purify a distillate produced by the evaporator. In the end, it is the aim of the process of the present invention to remove sufficient contaminants from the produced water before entering the forced circulation steam generator so as to prevent scaling and fouling of metal surfaces found in the steam generator and any associated equipment. 
     Various softening chemicals such as lime, flocculating polymer and soda ash may be used in a warm lime softening process. Typically the warm lime softener produces waste sludge which can be further treated and disposed. As noted above, polishing downstream from the warm lime softener can include an ion exchange process which typically includes hardness removal by a weak acid cation ion exchange system that can be utilized to remove hardness and in some cases at least some alkalinity. 
     Various types of evaporators can be utilized to treat the produced water prior to reaching the steam generator  10 . For example, the produced water  209  can be treated and conditioned in a mechanical vapor recompression evaporator. Such an evaporator will concentrate the incoming produced water. Pretreatment prior to reaching the evaporator can be employed when necessary. For example sulfuric acid or hydrochloric acid can be used to lower the pH of the produced water prior to reaching the evaporator so that bound carbonates are converted to free gaseous carbon dioxide which can be removed along with other dissolved gases by an upstream deaerator. After pretreatment, if necessary, the produced water is directed to the evaporator which produces a concentrated brine and steam which condenses to form a distillate. Generally the concentrated brine in the evaporator is recirculated and a small portion of the recirculating concentrated brine is removed. In the evaporator, the dissolved solids in the produced water are concentrated since water is being removed from the produced water. 
     In some cases, the distillate produced by the evaporator may require further treating to remove organics and other residual dissolved solids. In some cases it may be necessary to remove ions from the distillate produced by the evaporator. In many cases the residual dissolved solids in the distillate include salts other than hardness. In one process, the removal of dissolved solids downstream from the evaporator can be accomplished by passing the distillate, after being subjected to a heat exchanger, through an ion exchange system. Such ion exchange systems may be of the mix bed type and aimed at removing selected solids. In other designs, the removal of residual dissolved solids can be accomplished by passing the distillate through a heat exchanger and then through an electrodeionization (EDI) system. The reject or waste stream from all of these polishing processes can be recycled upstream of the evaporator for further treatment by the evaporator. As noted above, various treatment systems  212  can be utilized upstream of the steam generator to remove various contaminants from the produced water stream. It is contemplated that utilizing evaporators to remove total dissolved solids from the produced water stream may be preferable. But it is understood and appreciated that other pretreatment processes may be employed to treat the produced water prior to its introduction into the downstream generator. 
     Downstream of the treatment system  212  is the forced circulation steam generator  10 . Details of the forced circulation steam generator  10  will be discussed later but it is beneficial to briefly review the forced circulation steam generator and discuss how it receives the treated produced water from the treatment system  212  and produces steam for injection into the injection well  200 . Generally the effluent from the treatment system  212  is directed to a steam drum  16  that forms part of the forced circulation steam generator  10 . Water from the steam drum  16  is pumped by one or more pumps through what can generally be described as two heat exchanger systems or circuits incorporated into the furnace of the steam generator  10 . First there is an evaporator unit contained in the furnace. In addition there is provided water cooled walls that form a part of the furnace unit. The one or more pumps pump water from the steam drum  16  through both the evaporator unit and the water cooled walls. In each case a water-steam mixture is produced and returned to the steam drum  16 . The forced circulation steam generator  10  includes flow controls for independently controlling the flow of water through the evaporator unit and the water cooled walls such that approximately 10% to approximately 30% quality steam is produced in each circuit. Steam drum  16  separates steam from the water in the steam drum  16  and produces a steam that exceeds 95% quality steam and in a preferred embodiment produces 99% or higher quality steam. Steam produced by the steam drum  16  is directed into the injection well  200 . Steam drum  16  also produces a blow down stream that is on the order of 1 to 2% compared to the feed to the steam drum. 
     Turning to  FIGS. 2-9 , the forced circulation steam generator  10  is shown therein in more detail. The forced circulation steam generator  10  comprises three basic components: a furnace indicated generally by the numeral  12 , a burner indicated generally by the numeral  14 , and a steam drum indicated generally by the numeral  16 . As discussed above, water from the steam drum  16  is forced and circulated through water cooled walls forming a part of the furnace  12  and through an evaporator unit indicated generally by the numeral  40 . Burner  14  supplies heat to the furnace  12  that heats the water passing through the water cooled walls and the evaporator unit  40  resulting in a water-steam mixture being produced in the water cooled walls and the evaporator unit. The water-steam mixtures are directed to the steam drum  16  where the steam is separated from the water. One of the features of the forced circulation steam generator  10  of the present invention is that the heat supplied by the burner  14  and the flow of water through the water cooled walls and the evaporator  40  are controlled so as to limit the quality of steam produced in the water cooled walls and the evaporator unit. As discussed below, controls are instituted such that the water cooled walls and the evaporator unit  40  produce steam that is 30% or less quality steam. Furthermore, the amount of water pumped and circulated through the water cooled walls and the evaporator unit  40  is substantially greater than the amount of steam produced by the water cooled walls and the evaporator unit  40 . In one design illustrated herein, the amount of water pumped from the steam drum  16  to and through the water cooled walls and evaporator unit  40  is greater than five times the amount of steam produced in the water cooled walls and the evaporator unit  40 . In a steam generator circulation circuit context, the flow of water and steam is expressed lbs\hr unit or as a ratio of water to steam flow in the circuits. In this particular case the flow of water into the two circuits is at least a 5:1 circulation ratio. That is, the flow of water from the steam drum  16  into the two circuits is at least 5 parts water to 1 part of steam produced in the circuits. That is, 5 parts of water directed into the two circuits exits the two circuits as 4 parts water and 1 part steam. This enables a relatively high wetted area in both water cooled walls and evaporator circuits and resultant lower tube wall temperatures. The quality steam produced at the steam drum exceeds 95% and in a preferred design is 99% greater. 
     Forced circulation steam generator  10  comprises a furnace indicated generally by the number  12 . See  FIG. 7 . Furnace  12  comprises water cooled walls. In the embodiment contemplated herein, the sides, bottom and top of the furnace  12  includes water cooled walls. 
     The water cooled walls are shown in  FIGS. 4A, 4B, 6 and 7 . The water cooled walls form a part of a wall assembly that is particularly illustrated in  FIG. 6 . Essentially each water cooled wall includes a heat exchanger module indicated generally by the numeral  18 . See  FIG. 4B . Each heat exchanger module  18  includes a series of parallel tubes or pipes through which water flows. In the embodiment illustrated herein, each side as well as the top and bottom of the furnace  12  will include a heat exchanger module  18 . That is, for example, one module  18  (shown in  FIG. 4B ) would extend along one side of the furnace  12 . Likewise, one module  18  would extend along the top of the furnace and another module  18  would extend along the bottom of the furnace. In the end, all of the exterior walls of the furnace  12  would include a module that would enable the exterior walls to be water cooled. Each module  18  includes a series of tube elements with each tube element being indicated generally by the numeral  20  and shown in  FIG. 4A . In the case of the module  18  shown in  FIG. 4B , the same includes multiple tube elements  20  that are stacked or nested together. Each tube element  20 , shown in  FIG. 4A , includes a water inlet  20 A, an outlet  208 , and a series of parallel tube segments  20 C. Each tube element  20  is designed such that a series of the tube elements can be integrated to form the module  18  in such a fashion that the tube segments  20 C lie in generally the same plane. 
     Module  18  includes a plurality of webs or fins  22 . These are elongated pieces of metal that are welded between the respective tube segments  20 C. The tube segments or sections  20 C along with the fins  22  form a generally impervious wall. 
     Continuing to refer to  FIG. 4B  and the module  18 , it is seen that the module includes a surrounding frame structure that imparts rigidity to the module and at the same time functions as a manifold for directing inlet water into the various tube elements  20  and for directing a water-steam mixture from the various tube elements. In the particular embodiments shown herein, the manifold structure being referred to includes an inlet manifold  24  and an outlet manifold  26 . Inlet manifold  24  for each module  18  is connected directly or at least indirectly to a source of water and to the inlet  20 A. Outlet manifold  26  is connected to the outlet  20 B of each module  18  and is also directly or indirectly connected to a fluid connection between the furnace and the steam drum  16 . 
     Module  18  comprises a part of an exterior wall that is partially shown in  FIG. 6 . Module  18  is disposed along the inside of the wall assembly. Disposed outside of the wall assembly is an outer skin  30 . Disposed between the module  18  and the skin  30  is an insulation layer  32 . In one embodiment of the present invention, the wall assembly shown in  FIG. 6  forms the side walls, top and bottom of the furnace  12 . 
     As viewed in  FIG. 7 , the left end of the furnace  12  includes an opening  34  that permits the flame to be projected from the burner  14  into the furnace  12 . Continuing to refer to  FIG. 7 , the right end of the furnace  12  also includes an opening indicated generally by the numeral  36  for permitting exhaust gases to be exhausted form the steam generator  10 . 
     Returning to the evaporator unit  40 , as shown in  FIG. 5B , the evaporator unit includes a series of stacked tube elements indicated generally by the numeral  42 .  FIG. 5A  shows one tube element  42 . Each tube element  42  includes an inlet  42 A and an outlet  42 B. In addition, each tube element  42  includes a series of parallel tube segments or sections  42 C. Evaporator unit  40  is formed by stacking a series of the tube elements  42  one over the other. Like the modules  18 , the evaporator unit  40  is communicatively connected to at least two manifolds that facilitate the flow of water into the evaporator unit  40  and which receive the water-steam mixture produced by the evaporator unit. As seen in  FIG. 7 , there is provided an inlet manifold  44  that is operatively connected to the inlets  42 A of the tube elements  42 . Further, there is provided an outlet manifold  46  that is operatively connected to the outlets  42 B of the tube elements  42 . Thus, it is appreciated that water entering the evaporator unit  40  passes into and through the inlet manifold  44  while the water-steam mixture produced by the evaporator unit is directed out the outlet manifold  46 . As seen in  FIG. 7 , the evaporator unit  40  is disposed in an end portion of the furnace  12  opposite the burner  14 . 
     As seen in  FIG. 1 , the forced circulation steam generator  10  includes a steam drum indicated generally by the numeral  16 . As is appreciated, the steam drum  16  functions to receive water-steam mixtures from the wall modules  18  and the evaporator unit  40 . Once the steam mixtures have been received in the steam drum  16 , the steam drum functions to separate the steam from the water. The system and process disclosed herein is designed to result in the steam drum  16  producing a very high quality steam, a quality steam of at least 95% and in a preferred system and process a quality steam of 99% or more. 
       FIG. 9  is a schematic illustration showing the steam drum  16 . The steam drum includes inlets  60 A and  60 B with inlet  60 A being operative to receive the water-steam mixture from the wall modules  18  while inlet  60 B is operative to receive the water-steam mixture from the evaporator unit  40 . Further, the steam drum  16  includes various ports for enabling access for sensors and other instruments. 
     The forced circulation steam generator  10  is powered with a conventional gas burner  14 . Details of the burner  14  are not dealt with herein because such is not per se material to the present invention and further, burners of the type employed in the forced circulation steam generator  10  are well known and conventional. One exemplary burner  14  that is suitable for the forced circulation steam generator  10  is the “NATCOM” Ultra Low NO x  burner supplied by Cleaver-Brooks of Lincoln, Nebr. Briefly, however, the burner  14  is at least partially housed in a housing  14 A. See  FIGS. 2 and 7 . Burner  14  is mounted in the housing  14 A at the left end of the furnace  12  as viewed in  FIG. 7 . In this position the burner  14  shoots a substantial flame into the left end of the furnace  12  and in the process is effective to heat water passing through the water cooled walls as well as the evaporator unit  40 . 
     Turning to  FIG. 9 , shown therein is a schematic illustration showing basic components of the forced circulation steam generator  10  and how steam is produced and injected into the injection weld  200 . As shown in  FIG. 9 , the forced circulation steam generator includes a pair of pumps  80  and  82 . Pumps  80  and  82  can be of various types but in one embodiment they are centrifugal pumps and their output or flow is generally a function of pressure. Pumps  80  and  82  are connected to an outlet of the steam drum  16  via line  100 . Furthermore, the pumps  80  and  82  are operatively interconnected between the evaporator unit  40  and the water cooled wall modules  18  and the steam drum  16 . Pumps  80  and  82  function to pump water from the steam drum  16  through the evaporator unit  40  and the water cooled wall modules  18 . 
     As shown in  FIG. 9 , the output of the pumps  80  and  82  are coupled by line  83 . Extending from line  83  are two lines  104  and  106  with line  104  functioning to feed the evaporator unit  40  and line  106  functioning to feed the water cooled wall modules  18 . Disposed between the pumps and the evaporator unit  40  and the water cooled wall modules  18  is a flow control system which functions to vary the flow of water through the evaporator unit  40  and water cooled wall modules  18 . The control mechanism utilized is a pair of flow sensors  88  and  90 . Flow sensors  88  and  90  are each operatively connected to a controller  92 . In the embodiment illustrated herein, two controllers are shown but it is appreciated that a single controller with the ability to produce a series of independent control signals could be utilized. In any event, each controller  92  is operatively connected with a flow control value  84  and  86 . As noted above, the function of the controller  92  is to control the flow of water through the evaporator unit  40  and the water cooled wall modules  18 . Controller  92  is programmed to exercise control based on one or more parameters or variables. The system and process is designed to produce approximately 10% to approximately 30% quality steam in each of the circuits, i.e., evaporator unit and the water cooled wall modules  18 . It is known that there is a relationship between the burner firing rate and flow. That is, to achieve a certain quality steam, the firing rate and flow are directly proportional. That is, as the firing rate is increased, the flow should also increase. Further, as the firing rate is decreased, the flow should be decreased in order to produce the same quality steam. Therefore, the controller  92  is programmed to control the flow control valves  84  and  86  in response to the firing rate of the burner  14 . Generally speaking, as the firing rate is increased, the flow control valves  84  and  86  are actuated so as to increase flow from the pumps  80  and  82  through the evaporator unit  40  and the water cooled wall modules  18 . Likewise, as the firing rate of the burner  14  is decreased, the controllers  92  generally control the flow control valves  84  and  86  so as to generally decrease the flow of water through the evaporator unit  40  and the water cooled wall modules  18 . As noted above, the controllers  92  can be programmed in various ways to achieve the desired quality steam produced. For example, in addition to firing rate, the controllers  92  could also be programmed to consider the water quality being fed into the evaporator unit  40  and the water cooled wall modules  18 . 
     The forced circulation steam generator  10  and the basic system and process disclosed herein is designed to produce a relatively low steam quality in the evaporator unit  40  and the water cooled wall modules  18  compared to conventional once through steam generator (OTSG) or drum boilers. In particular, the quality steam of the water-steam mixtures produced by the evaporator unit  40  and the water cooled wall modules  18  is generally 50% or less. In one particular embodiment, the system and process is designed such that the evaporator unit  40  produces approximately 10% to approximately 30% of quality steam. Likewise, the system and process is designed and programmed such that the water cooled wall modules  18  produce approximately 10% to approximately 30% of quality steam. These two circuits are controlled independently. These steam qualities are conveyed in lines  108  and  110  to the steam drum  16 . Once in the steam drum  16 , the steam drum separates the steam from the steam-water mixtures. Here the steam drum  16  accumulates steam and produced steam directed out the outlet  62  is at least 95% quality steam and in a preferred design is 99% or more quality steam. 
     To achieve 99% or more of quality steam while only producing 10% to 30% quality steam in the evaporator unit  40  of the water cooled wall modules  18  it is necessary to direct substantially more water to and through the evaporator unit  40  and the water cooled wall modules  18  than the amount of steam produced by the evaporator unit and the water cooled wall modules. In a preferred design the flow of water from the steam drum  16  to the pumps  80  and  82  should be at least five times greater that the amount of steam produced by the evaporator unit  40  and the water cooled wall modules  18 . Again, this means for every one part of steam produced in the evaporator unit  40  and the water cooled modules  18 , that the flow of water from the steam drum  16  to the pumps  80  and  82  should be at least  5  parts water. That means that the ratio of the water pumped to the steam produced in the two circuits is at least 5:1. 
     The forced circulation steam generator  10  is operated to assure that the temperatures of the heat exchange surfaces (i.e., the surface of the tubes or pipes that form the evaporator unit  40  and modules  18 ) remain relatively low and the variation of tube wall temperatures is generally small. This mode of operation is illustrated in  FIG. 8  where the temperatures are plotted versus steam quality. The lower curve indicates the temperature of the fluid, in this case, water, as a function of steam quality. Fluid temperature increases with the heat supplies till it reaches the saturation temperature at the operating pressure and remains constant at the saturation temperature from 0% to 100% steam quality. Supplying heat beyond 100% quality, of course, would result in producing superheated steam. 
     The curve immediately above the fluid temperature curve represents the tube wall temperature for a moderate heat flux or energy transfer rate while the curve above that is for a high heat transfer rate. It is seen that for steam quality above 30%, the tube wall temperature can increase significantly as a function of steam quality for the same heat flux or energy transfer rate. Likewise, for steam quality above 30% the wall temperature varies considerably as well. However, for 10% to 30% steam quality, tube wall temperature shows only a small increase with heat transfer rate. Likewise, the tube wall temperature for a given heat transfer rate when producing 10% to 30% quality steam remains generally constant over that interval of steam quality. 
     While operating in a regime that produces 10% to 30% quality steam, robust water boiling occurs, producing a turbulent condition that is favorable for efficient heat transfer. This is typically referred to as the bubbling regime and it is in this regime where the present invention is most effective and efficient in terms of the basic design objectives for the forced circulation steam generator  10  and its use in the SAGD process discussed above and shown in  FIG. 1 . Further, operating in this regime avoids the development of hot spots on the heat transfer surfaces thereby maintaining effective heat transfer and improving the reliability. 
     In a typical design, the forced circulation steam generator of the present invention is capable of a maximum heat input of approximately 400 mm BTU/hr and a maximum steam output of approximately 353,000 lb/hr (160 ton/hr). The maximum steam pressure for a typical design would be approximately 2,300 PSIG. As noted above, the forced circulation steam generator  10  of the present invention is capable of producing greater than 99.5% quality steam with 2% or less of blow down. The turndown for the forced circulation steam generator  10  of the present invention is typically about 10 to 1, but a turndown of 30 to 1 is possible. The entire forced circulation steam generator  10  of the present invention can be delivered on a skid to an oil recovery area or facility which simplifies installation and reduces overall cost. The water treatment capacity of the forced circulation steam generator  10  of the present invention is similar to drum-type boilers, however, the power consumption is similar to once through steam generators. 
     The forced circulation steam generator  10  of the present invention and the system and process for recovering heavy oil has many advantages. First, the forced circulation steam generator includes 100% piggable circuits with a tolerance to sub-ASME quality water. In addition, the forced circulation steam generator of the present invention includes membrane water cooled walls with a 1% to 2% blow down while producing in some cases 99.5% pure steam. The design of the forced circulation steam generator of the present invention reduces maintenance time and cost, lowers furnace temperatures which yields a longer life, and avoids expansion issues that are prevalent with refractory seals and un-cooled tube supports. The water cooled furnace walls and the ability to cleaning by conventional pigging serve as insurance against water quality upsets. In the case of the design described and shown herein, flow is managed in two independent circuits. This makes the total control scheme for the forced circulation steam generator  10  simple and easy to execute. The forced circulation steam generator  10  can be operated at lower capacities and higher flows during water quality upsets. This reduces expensive down time associated with shut downs for short duration upsets. 
     The two main circuits, that is the circuits comprised of the evaporator unit  40  and the water cooled wall modules  18 , are limited to producing a certain steam quality. In one design the steam quality in each circuit is limited to approximately 30% steam quality and operates in the robust bubbling regime which yields certainty in metal temperatures and improves reliability and turn down significantly. Finally, the forced circulation steam generator  10  reduces the footprint of the steam generating device for a given application and generally eliminates hot spot maintenance issues associated with refractory wall furnaces. 
     The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.