METHOD AND APPARATUS FOR ENHANCING HEAT TRANSFER IN A FLUID CONTAINER

Abstract

An apparatus and method for enhancing the transfer of heat in a fluid within a reservoir. The interposition of heat exchangers between a primary heating arrangement and an auxiliary heating arrangement results in greater utilization efficiency in heat transfer to the fluid within the container. A predetermined arrangement of heat exchangers within the reservoir containing the same is also disclosed.

Description

FIELD OF THE INVENTION

The present invention relates to a method of enhancing the efficiency of heat transfer into a liquid fluid container and more particularly, the present invention relates to a method, system and apparatus for efficiently transferring heat into a fluid requiring the maintenance of a certain temperature in order to have the fluid remain effective.

BACKGROUND OF THE INVENTION

Fluid heating arrangements have been known for many years and these permeate a wide variety of industries. One industry where the temperature maintenance of the fluid is important is in the fracturing fluid heating industry. As is known “frac” fluid requires heating in order to be effective in an oil field operation. The temperature of the frac fluid must be maintained at certain temperatures in order to prevent fallout of sand within the fracturing environment.

A panoply of methods and different apparatuses to effect the method have been proposed in the prior art. An example of one such arrangement is one that which is shown and used by McAda Fluids Heating Services. The arrangement provides a truck arrangement where hot oil is used. The arrangement requires the use of fairly significant equipment that is transported to the site by a tractor trailer vehicle. In some instances, the vehicle must remain at the job site in order to provide for the necessary equipment to be available for heating the frac water.

A further variation of the frac water heating systems is shown in the website of Rapid Hot Flow LLC. In the website, there is an indication that Rapid Hot Flow's heating trucks are equipped with 21 million BTU input updraft style burner that runs on propane. It is indicated that once the heater trucks arrive at the specified location, the trucks can be heating within minutes. There is a further indication that when the hoses are connected, and the trucks have started circulating water, an operator runs through a list of procedures to ensure that all the requirements have been met. In this system, it is obvious that the vehicle is an integral part of the heating system and further that there is input required by the operator.

A further variation on the concept of maintaining the temperature of the frac water in the reservoir is shown on the Powerblanket website where effectively a blanket or a tank wrap is positioned around the holding tank. In the website there is an indication that the tank wrap creates a barrier of insulation to keep fluids from freezing and viscous materials flowing.

Although this is generally useful, it appears to be quite cumbersome not only from a handling point of view, but also for positioning about the tank. It is well known that holding tanks or reservoirs are very large and it would be somewhat cumbersome to wrap the blanket about a large vessel.

Turning to the patent art, in U.S. Pat. No. 5,983,889, issued Nov. 16, 1999, to Thomas, there is disclosed a portable water tank heating system. The reference teaches a portable system which uses a hollow continuous tubular loop containing a fluid this is circulated through the loop by convection to prevent water in the tank from freezing. The fluid in the loop flows from a reservoir and travels past a gas burner in the hot chamber for heating the fluid. The fluid flows through a heat exchanger and releases heat to the cold water in the tank before being returned into the housing through the return line. The fluid then returns to repeat the cycle.

The system would appear to completely rely on the burner coil for transmitting the heat to the water body. It is well established that such arrangements have limited efficiency.

In United States Patent Publication No. US 201110211818, published Sep. 1, 2011, the Applicant, Grady, teaches a fracturing tank fluid heating tank.

This patent application is very broad and effectively provides a self contained tank into which is disposed in a fracturing tank fluid healing unit. In the document, the tank comprises a closed tank having an access point such as a manhole cover. The unit further includes a dimensional coil where a tube like heating unit is designed to heat the fluid once contained within the tank.

As with the previous reference, this reference is confined to a relatively inefficient heating means in terms of the coil type arrangement. The inefficiency in terms of the energy is pronounced; in arrangements with the heat exchanger in the tank only, efficiency is typically 50%.

Chandler, in United States Patent Publication No. US2010/0000508, published Jan. 7, 2010, teaches an oil fired frac water heater. The document teaches a portable system for heating treatment fluids at a remove worksite. The arrangement has a firebox, heat exchanger within the firebox, a fluid supply system including a fluid supply pump connected to an inlet of a tubular coil associated with the heat exchanger and a plurality of burner assemblies in the firebox. The arrangement further provides for a primary air system for supplying pressurized air flow to each of the burners and a secondary air system for supplying a second pressurized air flow to the firebox. The secondary pressurized air flow increases the convective heat transfer of thermal energy from the combustion flow to the treatment fluid. The arrangement is fairly complex and does not indicate any thermal energy augmenting system which would elevate the efficiency of the unit.

In U.S. Pat. No. 6,516,754, issued Feb. 11, 2003, Chadwick, teaches a convective heating system for liquid storage tanks. The arrangement incorporates a heating chamber with an inlet and an outlet for convective fluid flow past a flameless heater. Heated liquid is circulated through the upper outlet of the tank from the heating chamber and back into the tank. It is indicated that the heated liquid reenters the tank through a floating discharge flexibly connected to the upper section of the tank to remain dynamically in contact with the liquid at all times. This is said to avoid airlocks which interrupt the convective flow of liquid through the heating system.

Hefley, in United States Patent Publication No. US2010/0294494, published Nov. 5, 2010, teaches a water heating apparatus for continuous heated water flow and method for use in hydraulic fracturing. Essentially, the publication includes a discussion regarding mixing device for ensuring that the frac fluid is maintained at the proper temperature during operation.

Other references which are generally relevant to the area of technology of the instant application include U.S. Pat. Nos. 2,067,063; 3,933,205; 3,512,239; 3,937,275; 4,318,549; 5,115,491; 6,662,861; 7,793,707; and United States Patent Publication Nos. US 2006/0196958; US 2007/0000453; US 2008/0217420; US 2008/0236275; US 2008/0206699; US 2010/010156; and US 2010/0193155.

Despite the comprehensive prior art and numerous methods that have been set forth in the fracturing frac fluid heating methods, there still exists the need to improve on the existing systems and operation where heat transfer can be maximized using a portable system and further where there is not any requirement that the vehicle be present at all times for operation.

The present invention has successfully unified various technologies in a unique manner to result in an elegant solution for maximizing heat energy in a frac fluid environment. It will be appreciated by those skilled that the fluid referred as frac fluid could be any fluid, drilling water, acid, inter alia.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved method, system and apparatus for augmenting the heat enthalpy available for transfer into a frac fluid or other fluid in an efficient manner.

A further object embodiment of embodiment of the present invention is to provide a method of enhancing the rate of heat exchange in a fluid retained in a reservoir, comprising: providing a heater circuit having a primary heater for heating a reservoir heat exchanger disposed within the reservoir; providing an auxiliary heater; and interposing a secondary heat exchanger in fluid communication between the primary heater. The auxiliary heater and the reservoir heat exchanger is present to augment heat content of cool fluid returning from the reservoir heat exchanger with heat from the primary heater and the auxiliary heater.

Conveniently, the use of the secondary heat exchanger within the environment of the other components which are high efficiency, results in significant operational efficiency for heating the frac fluid. Any of the suitable heat exchangers may be used such as those manufactured by the Dry Air Corporation, the Alfa Laval Corporation. The Alfa Laval unit is referred to as a brazed plate heat exchanger (CB Series). Of particular convenience is the fact that the heat exchanger arrangement can be incorporated without significant increase in mass or footprint to the overall heating arrangement. In one of embodiment, the entire heating arrangement can be configured and loaded onto a single skid and easily transported to a worksite and left at the worksite to therefore free the use of the transporting vehicle for other purposes. Further, the unit can be remotely controlled to avoid significant operator input and repetitive visits to the worksite with replacement heating units which would result in heavy traffic flow at the worksite. As noted supra, the utilization efficiency of existing arrangements is typically 50%; by incorporating the instant technology, utilization efficiency can reach 100%. This represents a significant advance in the art.

It has been found that by incorporating an auxiliary heater together with the secondary heating exchanger, the overall utilization efficiency of the system can be significantly increased. The result is that the secondary auxiliary heater functions to warm, cool fluid within the heat exchanger system leaving the frac fluid reservoir. In this manner, there is effectively a dual heating process taking place, namely one from the auxiliary heater and the second from the heater circuit from the primary heater. By the introduction of the heat exchanger, the method may be optimized in terms of output utilization efficiency.

In accordance with a further object of one embodiment of the present invention, there is provided a system for enhancing the rate of heat exchange in a fluid retained in a reservoir comprising: a heater circuit having a primary heater for heating a reservoir heat exchanger disposed within the reservoir; an auxiliary heater and a secondary heat exchanger interposed and in fluid communication between the primary heater, the auxiliary heater and the reservoir heat exchanger to augment heat content of cool fluid returning from the reservoir heat exchanger with heat from the primary heater and the auxiliary heater.

The system incorporates a highly efficient reservoir heat exchanger. It has been found that by making use of a dimpled closed heat exchanger of planar configuration, in combination with the secondary heat exchanger and heater circuit, as well as auxiliary heater, results in a very effective compact modular system. The dimpling effect of the heat exchanger within the reservoir significantly increases the available surface area for heat transfer. The exchanger, as noted above, is closed and uses for example, propylene glycol as the heating fluid for transferring the heat enthalpy sensed by the fluid.

In accordance with a further object of one embodiment of the present invention, there is provided an apparatus for enhancing the rate of heat exchange in a fluid retained in a reservoir having a heater circuit having a primary heater for heating a reservoir heat exchanger disposed within the reservoir. The improvement comprises an auxiliary heater; and a secondary heat exchanger interposed and a fluid communication between the primary heater, the auxiliary heater and the reservoir heat exchanger to augment heat content of cool fluid returning from the reservoir heat exchanger with heat from the primary heater and the auxiliary heater.

Having thus generally described the invention, reference will now be made to the accompanying documents illustrating preferred embodiments.

Copious advantages flow from the practice of the methodology and use of the apparatus. These include:

• • i) the lack of moving parts;
  • ii) the portability of the arrangement due to the reduced size;
  • iii) lighter weight for certain embodiments;
  • iv) reduced capital cost;
  • v) no fluid mixing of fluid to be heated with the heat exchanger fluid;
  • vi) expedited assembly of the apparatus;
  • vii) the ability to use the main generator; and
  • viii) no extraction of the fluids in the tanks, therefore avoiding fluid leaks from the tank.

INDUSTRIAL APPLICABILITY

The invention has utility in the head transfer art,

Having thus generally described the invention, reference will now be made to the accompanying drawings, illustrating preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the apparatus employed to effect the method according to one embodiment;

FIG. 2 is a schematic illustration of the heat exchange system according to one embodiment;

FIG. 2A is a further variation of FIG. 2;

FIG. 3A is a schematic illustration of the reservoir heat exchanger according to one embodiment;

FIG. 3B is a side view of FIG. 3A;

FIG. 4 is a plan view of the heat exchanger in FIG. 3A;

FIG. 5 is a section along line 5-5 of FIG. 4;

FIG. 6 is a top view of the reservoir illustrating the positioning of the heat exchangers; and

FIG. 7 is a top view of an alternate embodiment.

Similar numerals used in the Figures denote similar elements,

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, schematically depicted is the overall apparatus employed as an example. The overall arrangement is denoted by numeral 10. A reservoir 12 retains a fluid, one example of which is fracturing fluid, commonly referred to as “frac” fluid. Such vessels typically include a liner 14 which secured to the reservoir by, for example, clamps or other suitable fasteners 16 (not shown). As is well known to those skilled in the art, frac fluid needs to be maintained at an elevated temperature in order to be useful. This requires the injection of heat into the fluid.

Heat injection in the example is achieved by making use of a modular heating station, generally denoted by numeral 18. With the instant technology, the station 18, is particularly effective since it is retained on a skid 20 which may be easily transported to a worksite and disengaged from a truck for use. This has the advantage of preventing the invasiveness of several trucks at the site for prolonged periods and avoiding the concomitant costs and environmental impact. The skid and ancillary equipment is thus independent of a vehicle in use and therefore is self-contained. The skid 20 includes a plurality of heaters 22, generator 24, storage tank 26 for a heat transfer liquid, such as propylene glycol, fuel tank 30, inter alia. The heaters 22 each include secondary heat exchangers 32 which will be discussed in greater detail herein after. A plurality of fluid lines 34 fluidly communicate with reservoir 12 and more particularly with reservoir heat exchangers 36 which will now be discussed with respect to FIG. 2.

FIG. 2 is a schematic representation of the overall layout in accordance with one example. Reservoir heat exchangers 36, shown as two such units in the example for purposes of explanation, are planar elements and will described in detail later in the specification. The units 36 fluidly communicate with the water heaters 22 by lines 34 supra and 38. Conventionally, the water heaters 22 would simply run heating fluid, such as glycol, through heating units positioned in the tank 12 to heat the fluid retained in the reservoir. In the present system, by making use of ancillary heaters and interposition of secondary heat exchangers, utilization efficiency has been greatly improved with a complementary small portable system.

The secondary heat exchangers 32 are interposed in fluid communication with the heaters 22 and reservoir heat exchangers 36 as well as auxiliary heater 40. In operation, cool water from the reservoir heat exchangers 36 denoted by A on line 34 enters a respective secondary heat exchanger 32.

From the respective exchanger 32 cool fluid exits each exchanger 32 as stream B and with passive enthalpy transfer at C. Stream C then enters auxiliary water heater 40 where it is heated exiting as stream D for introduction back into the individual exchangers 32 as separate streams E. Streams E exit the respective exchangers 32 as warmed streams F for introduction into the heaters 22 for upgrade heating to hotter streams exiting the respective heaters 22 as stream G. The latter are then introduced into the reservoir heat exchangers 36. In this manner of operation, the utilization efficiency of the system is significantly boosted while avoiding the use of vehicle based power, heavy equipment and costly human intervention.

Referring now to FIG. 2A, shown is a further embodiment of the method. In this embodiment, hot fluid exists primary heater 22 via stream G and travels through heat exchanger 36 exiting cold via stream A. Cold fluid enters heat exchanger 32 receiving heat via passive transfer from auxiliary heater 40 and particularly by stream D and exits heat exchanger 32 wam 1 via stream F returning to primary heater 22. This is generally the primary circuit. Subsequently, hot fluid exists auxiliary heater 40 via stream D and travels through heat exchanger 32 (or heat exchangers in applications where one auxiliary unit is used to boost more than one primary circuit) transferring heat passively to primary stream F, returning cool to auxiliary heater 40 via stream C. This is referred generally as the second circuit.

Of particular advantage is the fact that the method and apparatus related to FIG. 2A allows the second circuit component (auxiliary heater 40 and heat exchanger 32 and related connections) to be removed from the primary circuit, once the desired temperature has been reached in the tank. The secondary circuit could then be transported to another tank to elevate the utilization efficiency and heat the contents of the tank. In this manner, for the arrangements of FIG. 2A the secondary circuit functions as an optimization module.

Returning now to the reservoir heat exchangers 36, FIGS. 3A, 38, 4 and 5 illustrate the structure in more detail. The arrangement as noted above is planar and generally rectangular in the example. The exchanger 26 includes an inlet connection 40 and an outlet connection 42 in a sealed body. For enhanced efficiency, the entire surface area of the exchanger 26 is dimpled, the dimples being denoted by numeral 44. This is more clearly illustrated in FIGS. 4 and 5 where the exchanger is shown in plan view. As shown, the dimples 44 extend over the Whole area of exchanger 26. A cross section of the exchanger 26 is shown in FIG. 5. The heating fluid, using glycol as the example, flows through the channels 46 which are in alternation with the dimples 44. As will be appreciated by those skilled, this arrangement provides enormous surface area for heat transfer to the reservoir 12.

In order to further augment the efficiency of the arrangement, the exchangers 26 provide supports 48 for positioning the exchangers onto the bottom 50 of the reservoir 26 as shown in FIG. 6.

The supports 48 include a vertical component 52 and a horizontal component 54. The connection of the vertical component 52 is such that the body of the exchanger is angularly disposed relative to the horizontal. It has been found that this angular disposition has ramifications in terms of heat transfer to the fluid in the reservoir. The effective range for the disposition is from 0.1 degrees to 90 degrees relative to the horizontal. As a preferred range, the angle may be between 15 degrees and 20 degrees. The vertical component 52 spaces the exchanger from the bottom 50 (FIG. 6) of the reservoir 12 to facilitate effective heat transfer. A suitable elevation has been found to be, for example 6 centimeters. This will largely depend on the reservoir volume and other specific individual requirements. As an option, the elevation and angle may be changed using suitable linkages and motors (both not shown) in order to provide the highest de gee of flexibility for the operator.

Perhaps one of the most advantageous features of the arrangement set forth herein relates to the fact that there is no need for supplementary pumps or other forms of drivers to have effective heat transfer within the reservoir. This has posed a problem in the prior art; typically existing systems required pumping either extraneously or internally of the reservoir to enable uniform heat distribution. This was necessary to avoid thermoclines or temperature stratification within the reservoir which inherently would cause regular cycling of the heating elements to maintain a uniform temperature. The present invention has discovered a method to avoid the need for pumps or other distribution means for the heat.

It has been found that natural convection can be achieved in the arrangement by arrangement of the exchangers 26 on the bottom surface 50 of the reservoir 12. Returning to FIG. 6, the Figure illustrates D1 D2, D3 and D4 in a counter clockwise array. By varying the distance in an increasing amount from D1 through with progressive additions to D4, natural convection has been observed thus obviating the need for extraneous energy input to induce a homogeneous temperature. In terms of the useful range between D1 through D4 a ratio 1:1 to 1:7 has been found effective. The result is very pronounced in light of the previously described heat exchanger and the secondary heat exchangers. These elements work in concert to result in an environmentally effective, small footprint and easily deployable arrangement with the possibility for remote operation.

Although four heat exchangers are shown in FIG. 6, it will be understood that any number of such units may be included in the reservoir 12. This will, of course, depend on the size of the reservoir 12 and ambient temperature conditions, etc.

As illustrated in FIG. 6 by dashed line numeral 52, a cover layer partially covering the top surface of the reservoir 12 is depicted. This cover may consist of a suitable buoyant cover that prevents or significantly reduces evaporation of the fluid from the reservoir 12, as well as providing an insulating factor to prevent any temperature drop of the fluid. Suitable materials include, for example, Styrofoam®, polyethylene, metalized plastic, etc. It will be readily apparent to those skilled in the art which other suitable materials may be used.

In respect of numeral 54, this represents an evaporation prevention layer which, may comprise, a suitable oil material. An example of a suitable material would be canola oil as canola oil is useful for purposes of insulation and preventing evaporation. Further, the canola oil is useful in that it does not interfere with the composition of the frac fluid where frac fluid is the fluid stored in the reservoir 12. Other fluid possibilities have been mentioned. Provided the liquid cover material does not interfere with the composition of the frac fluid, any suitable material achieving the insulation and liquid surface coverage features could be used and is envisioned for a possibility. Another example could be a sealed air blanket, suitable foam, etc.

Turning to FIG. 7, shown is a further possible variation of the arrangement that could be used in the practicing of the instant methodology. In this embodiment, numeral 56 shown in dashed line indicates a further series of heat exchangers 26 which are vertically elevated from the existing heat exchangers 26 positioned on the bottom of the reservoir 12 as discussed herein previously.

In the arrangement shown, the additional heat exchangers 26 are not only vertically elevated from the existing heat exchangers 26 but also staggered radially therefrom such that a respective vertically disposed heat exchanger 26 is from a relative distance point of view adjacent to heat exchangers 26 on the bottom of the reservoir, in this manner, the entire volume of the reservoir 12 benefits from the heat exchangers in the vertically disposed position. Simple connectors 56 may be employed to connect the vertically disposed heat exchangers 26 into the reservoir 12 as shown by the dotted line represented by numeral 56.

Claims

1. A method of enhancing utilization efficiency of heat exchange in a fluid retained in a reservoir, comprising:

providing a heater circuit having a primary heater for heating a reservoir heat exchanger disposed within said reservoir;

providing an auxiliary heater; and

interposing a secondary heat exchanger in fluid communication between said primary heater, said auxiliary heater and said reservoir heat exchanger to augment heat content of cool fluid returning from said reservoir heat exchanger with heat from said primary heater and said auxiliary heater.

2. The method as set forth in claim 1, further including the step of disengaging said auxiliary water heater and said secondary heat exchanger upon reaching a predetermined fluid temperature in said reservoir.

3. The method as set forth in claim 2, further including engaging a disengaged auxiliary water heater and said secondary heat exchanger on a heater circuit having said primary heater and reservoir with said reservoir heat exchanger for enhancing said utilization efficiency of heat exchanger at least until said predetermined temperature is reached.

4. The method as set forth in claim 1, wherein at least two heater circuits are connected to said auxiliary heater and said secondary heat exchanger.

5. The method as set forth in claim 1, wherein said reservoir comprises a fracturing fluid reservoir for retaining fracturing fluid.

6. The method as set forth in claim 1, further comprising a water reservoir for retaining water.

7. The method as set forth in claim 1, further comprising a drilling fluid reservoir for retaining drilling fluid.

8. The method as set forth in claim 1, further comprising an acid reservoir for retaining acid.

9. The method as set forth in claim 1, including providing a plurality of reservoir heat exchangers in spaced relation disposed within said reservoir.

10. The method as set forth in claim 9, further including the step of positioning said reservoir heat exchangers in said reservoir to be spaced from the bottom of said reservoir.

11. The method as set forth in claim 9, wherein said reservoir heat exchangers are positioned in an angular relationship relative to the bottom of said reservoir.

12. The method as set forth in claim 11, wherein said angular relationship is between 0.1 degrees and 90 degrees relative to the horizontal bottom of said reservoir.

13. The method as set forth in claim 12, wherein said angular relationship is between 15 degrees and 20 degrees.

14. The method as set forth in claim 9, wherein said reservoir heat exchangers are spaced in a predetermined relationship relative to one another to facilitate natural convection.

15. The method as set forth in claim 14, wherein the relative distance between adjacent heat exchangers increases in a counter clockwise direction.

16. The method as set forth in claim 12, wherein the relative distance is between the ratio of 1:1 to 1:1.7.

17. The method as set forth in claim 16, wherein the relative distance is 1:1.5.

18. The method as set forth in claim 1, further including engaging a disengaged auxiliary water heater and secondary heat exchangers on a heater circuit having said primary heater and reservoir with said reservoir heat exchangers for enhancing said utilization efficiency of heat exchange at least until said predetermined temperature is reached.

19. The method as set forth in claim 1, characterized in that at least two heater circuits are connected to said auxiliary heater and said second heat exchanger.

20. The method as set forth in claim 1, characterized in that said reservoir comprises a fracturing fluid reservoir for retaining fracturing fluid.

21. A system for enhancing the rate of heart exchange in a fluid retained in a reservoir, comprising:

a heater circuit having a primary heater for heating a reservoir heat exchanger disposed within said reservoir;

an auxiliary heater; and

a secondary heat exchanger interposed and in fluid communication between said primary heater, said auxiliary heater and said reservoir heat exchanger to augment heat content of cool fluid returning from said reservoir heat exchanger with heat from said primary heater and said auxiliary heater.

22. The system as set forth in claim 21, further including a reservoir.

23. The system as set forth in claim 18, characterized in that said reservoir is a fracturing fluid reservoir.

24. The system as set forth in claim 21, characterized in that said reservoir heat exchanger comprises a planar configuration.

25. The system as set forth in claim 24, wherein said heat exchanger is self-contained and includes a plurality of impressions over the surface area thereof.

26. The system as set forth in claim 25, wherein said impressions comprise dimples.

27. The system as set forth in claim 25, wherein said system includes a plurality of reservoir heat exchangers.

28. The system as set forth in claim 27, wherein said reservoir heat exchanger of said plurality of reservoir heat exchangers includes a heater circuit and a secondary heat exchanger.

29. The system as set forth in claim 25, further including a temperature monitor at least provided in said reservoir.

30. The system as set forth in claim 21, characterized in that said system is a portable system.

31. The system as set forth in claim 21, characterized in that said system is self-contained and independent of a vehicle in use.

32. The system as set forth in claim 21, characterized in that said auxiliary heater and said secondary heater exchanger are connected to a plurality of heater circuits.

33. An apparatus for enhancing utilization efficiency of the rate of heat exchange in a fluid retained in a reservoir having a heater circuit having a primary heater for heating a reservoir heat exchanger disposed within said reservoir, the improvement comprising:

an auxiliary heater; and

a secondary heat exchanger interposed and in fluid communication between said primary heater, said auxiliary heater and said reservoir heat exchanger to augment heat content of cool fluid returning from said reservoir heat exchanger with heat from said primary heater and said auxiliary heater.

34. The apparatus as set forth in claim 32, wherein said auxiliary heater and said secondary heat exchanger are portable.

35. The apparatus as set forth in claim 33, wherein said auxiliary heater and said secondary heat exchanger are connected to a plurality of heater circuits.