WO2010021939A1

WO2010021939A1 - High thermal transfer spiral flow heat exchanger

Info

Publication number: WO2010021939A1
Application number: PCT/US2009/053940
Authority: WO
Inventors: Gregg C. Johnson; Stephen D. Neale; Scott A. Holland
Original assignee: Skye International Inc.
Priority date: 2008-08-19
Filing date: 2009-08-14
Publication date: 2010-02-25
Also published as: US20100046934A1; CA2734856A1

Abstract

A fluid heating system with a heat exchanger includes an elongated cylindrical tube with an inlet end and an opposed end. An array of film resistive elements is positioned on the outer surface of the cylindrical tube and extends circumferentially at least partially around the outer surface. A spiral channel guide is coaxially positioned within the cylindrical tube so as to force fluid flowing in the tube into a spiral path at least partially against an inner surface of the cylindrical tube. The spiral channel guide directs fluid flowing in the cylindrical tube so as to cause turbulence to occur in the flow and increase thermal transfer from the inner surface of the cylindrical tube to the fluid flowing in the tube.

Description

High Thermal Transfer Spiral Flow Heat Exchanger

Field of the Invention

This invention relates to tankless fluid heaters and more specifically to fluid heaters employing resistive heating elements.

Background of the Invention

There has been a drive to replace the conventional tank style water heater with something smaller, more energy efficient and with faster recovery. This has lead to the development of what is commonly called tankless water heaters. Sizes for these devices range from small point-of-use heaters designed to mount under sinks to whole house heaters that can deliver 5 gallons of water per minute or more. Smaller under sink heaters typically operate from electricity while the larger whole house heaters may operate from electricity or gas. Electric units are generally rated at 29 KW or less with efficiencies greater than 95% and gas units can achieve 175,000 Btu (51.2 KW) or more at 85% efficiency.

The need for tankless water heaters is on the rise. Increasing energy costs and the need to conserve water usage has prompted homeowners to replace large tank heaters with new compact tankless heaters. With their small size they can be located at optimum locations to minimize the distance from the water heater to the point of use. Further, only heating water when it is needed reduces the energy losses associated maintaining high storage temperatures in tank heaters. Studies have concluded as much as 50% of the energy used by conventional tank heaters can be lost to the surroundings.

Improvements in tankless heater design aimed at lower cost, smaller size and improved operation will allow more homeowners to conserve energy and scarce water resources.

Most electric tankless water heaters use immersion type heating elements. These are the same type of heating elements used for years in conventional tank heaters. The immersion elements are mounted so as to protrude into a small chamber or tube that is intended to have a continuous flow of water past each element. Effective heating occurs when the liquid being heated can be in uniform contact with the heating surface. Immersion elements are typically round or oval tubes with a much smaller surface area than the chamber in which they are inserted. As the fluid flows through the heating chamber, only a limited amount of the fluid actually comes in contact with the heating element. It is possible to put deflectors on each element to stimulate turbulence in the chamber and maintain full contact with the element but the surface area is still limited and part of the element is exposed to low turbulence areas especially at higher flow rates. This situation causes the element to operate with a surface temperature much hotter than the water it is attempting to heat. A characteristic of conventional immersion elements is when flow stops the residual energy stored in the element is dissipated into the surrounding water. Large temperature swings can occur during this event. For example, a typical tankless water heater with a 29 KW heating capacity will realize upwards of a 25° F increase over the normal heating temperature when immediately going from a full flow of 3 gallons per minute to zero. For reference, the chamber in this example holds approximately 1.5 pints of water.

Depending on the type and concentration of the minerals in the water being heated, elements that operate hotter than the surrounding water will tend to build up mineral deposits on the element surface. This further reduces the thermal transfer and further increases deposits until element failure occurs. With proper design the deposits will break off the element due to thermal cycling but may collect in the heater then causing other problems.

Temperature control in tank type heaters is a simple matter. Most often a mechanical thermostat is attached to the outside of the vessel and turns the heating elements on and off to maintain the desired water temperature. Tankless heaters require a more sophisticated control mechanism that must react quickly to varying inlet temperatures and flow rates to provide a regulated output water temperature. In an effort to apply only the necessary energy to tankless water heaters the common approach has been to cycle the elements on and off quickly using triacs or other similar electrical control devices. Depending on the heater capacity one or more individual heating elements can be used. Generally, practical limitations exist as to the number of elements that can be used. Adding additional elements to the heater requires increasing the size of the heater chamber and increasing the manufacturing costs. By limiting the number of elements in the heater, each element produces a proportionately large amount of energy.

A triac is a common solid-state switch used for element control, which can be turned ON with a relatively small control current and which is capable of carrying the relatively large currents used in the heating elements. These devices can switch on any time during the power cycle and by their design will turn off when the applied voltage drops to zero. To reduce EMI (electromagnetic interference), triacs are generally turned on at the beginning of the power cycle. The resulting maximum control resolution is therefore ¹A cycle of applied power or 8.33 msec in locations of 60 Hz power. A 28.8 KW heater with 4 elements would have an effective resolution of 60 watts. This would not be a problem except for the fact that the power step for this type of heater is 7.2 KW. Stepping the power in such dramatic increments leads to flickering lights and voltage surges.

The development of thick-film resistive (TFR) materials with high wattage per unit area has made it ever more difficult to transfer heat rapidly from a heat source. Applications such as fluid heat exchangers benefit from high power density to keep the size small and reduce cost but suffer if the transfer fluid cannot absorb the heat fast enough to prevent boiling or overheating.

A relatively new approach is to place thick-film resistive (TFR) material on a thermally conductive glass or ceramic insulator that is, in turn, fused to a stainless steel tube. United States Patent No. 7,206,506, issued 17 April 2007, describes some details of thick film use in a fluid heater, and is incorporated herein in its entirety by reference. In the use of TFR materials, according to the '506 patent, heat is conducted through a thermally conductive insulator, through a stainless steel tube and into a heat exchange fluid flowing through the tube. Advantages include lower thermal mass of the heater, reduced mineral deposit buildup due to temperature excursions and flexible element design.

Power density of the tankless fluid heater is still a limitation. Attempting to reduce the heater size and reduce cost drives the power density up and causes many of the same problems of the immersion heaters. A common solution includes increasing the flow rate in an attempt to absorb heat faster. Another is to increase the pipe diameter to decrease the power density. Both have problems. An increasing flow rate generally requires a larger pumping system or is not possible in fixed flow situations. Increasing the flow rate also leads to an increasing portion of the fluid flowing concentrated towards the center of the pipe. This can actually lead to a reduction in thermal transfer. A larger pipe reduces the power density but also reduces the proportion of fluid in contact with the inner wall of the heater tube.

With the innovation of TFR elements, lower cost and smaller point-of-use and whole house heaters have become more feasible. Still two major problems exist for these electric heaters. It is necessary to improve the thermal transfer from the heating element to the water to reduce the size and cost. Also, there is a need to improve the power control to reduce large load swings on the supply to the heater and, in turn, eliminate flickering lights and voltage fluctuations.

Summary of the Invention Briefly, to achieve the desired objects of the present invention in accordance with a preferred embodiment thereof there is provided a tankless fluid heating system with a heat exchanger including an elongated cylindrical tube with an inlet end and an opposed end. An array of film resistive elements is positioned on an outer surface of the cylindrical tube and extends circumferentially, at least partially, around the outer surface. A spiral channel guide is coaxially positioned within the cylindrical tube so as to force fluid flowing in the tube into a spiral path at least partially against an inner surface of the cylindrical tube. The spiral channel guide directs fluid flowing in the cylindrical tube so as to cause turbulence to occur in the flow and increase thermal transfer from the inner surface of the cylindrical tube to the fluid flowing in the tube. To further achieve the desired objects of the present invention, in a specific embodiment of the present invention, a high thermal transfer cylindrical heat exchanger using film resistive heating elements, provides a system and method for heating fluids under a broad range of flow rates with no appreciable residual heat and no mineral deposit buildup. A characteristic of film resistive elements is the lack of residual energy upon the removal of power. They have no appreciable thermal mass, therefore nowhere to store energy. The present invention also takes advantage of film resistive materials on insulator technology with the ability to subdivide resistive elements into various sizes without significantly increasing the heater size.

In one embodiment of the present invention, a process controller receives signals from a flow sensor, placed in the inlet stream, and determines the fluid flow rate. Additionally, the process controller receives signals from a plurality of temperature sensors placed on the cylindrical heat exchanger. This information allows the process controller to calculate the amount of power necessary to raise the fluid temperature to the desired level.

In another embodiment, the process controller provides localized monitoring of the electrical supply source in such a way as to provide overload protection to the supply circuit. This monitoring is done on a cycle-by-cycle basis, thus providing a methodology of self- control that can prevent circuit, as well as, service overloads. This information is used to identify local overloading conditions that may result from the potentially high current demand associated with the calculated power referenced above. If the supply voltage goes below a predetermined or acceptable level, the process controller will limit the ability to heat the fluid rather than allow an overload condition due to the heaters demand.

Characteristics and flexibility of the spiral channel guide allows control of the fluid flow direction and turbulence within the heating chamber, maximizing thermal transfer from the heater tube to the fluid. In one embodiment of the spiral channel guide, the core is solid and the root dimension of the spiral is reduced, as well as having the pitch adjusted, to effectively provide any equivalent pipe size up to approximately two (2) inches from a three (3) inch diameter heat exchanger. This allows a single heat exchanger to economically be configured for heating application demands ranging from low flow (high delta T) to high flow (low delta T), and anything in between.

Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following detailed description, when read in conjunction with the accompanying drawings. It is intended that all such features and advantages be included herein within the scope of the present invention and protected by the claims.

Brief Description of the Drawings

The foregoing and further and more specific objects and advantages of the invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof, taken in conjunction with the drawings in which:

FIG. Ia is an exploded view of a spiral flow heat exchanger in a straight- thru configuration, in accordance with the present invention;

FIG. Ib is a perspective view of another embodiment of a spiral flow heat exchanger;

FIG. Ic is an exploded view of the spiral flow heat exchanger of FIG. Ib;

FIG. Id is an enlarged side view of the internal component of the spiral flow heat exchanger of FIG. Ic; FIG. 2 is an exploded view of a spiral flow heat exchanger in a loop-back configuration, in accordance with the present invention;

FIG. 3 is a top plan view of the spiral flow heat exchanger of FIG. 1 with electrical and temperature sensor connections;

FIG. 4 is a top plan view of a contact assembly formed to overlie the electrical and temperature sensor connections of FIG. 3;

FIG. 5 is an end view of the contact assembly illustrated in FIG. 4;

FIG. 6 is a perspective view of a film resistive heating element on a conductive outer tube;

FIG. 7 is a perspective view of a film resistive heating element on the conductive outer tube of FIG. 6 with a ground layer;

FIG. 8 is a perspective view of a film resistive heating element on a non-electrically conductive outer tube; and

FIG. 9 is a block diagram illustrating an embodiment of a process controller and communications system for the spiral flow heat exchanger.

Detailed Description of the Drawings

In general, the present invention relates to a system and method for heating fluids by using film resistive elements fused to the outside of a cylindrical tube. Film resistive heating elements include thin film and thick-film resistive (TFR) elements. Thin film resistive elements are typically formed electro-statically on a surface, or using a very fine spray, while TFR elements typically employ a printing technique. It will be understood that substantially any technique and method of forming a film of resistive material on the outer surface of a cylindrical tube, such as deposition techniques, plating, painting, silk screening, and the like, can be employed. A spiral channel guide is disposed within the tube and creates the equivalent of a coil of tubing within the walls of a compact cylinder. This 'coil effect' allows the fluid to remain in contact with the heat source for longer durations than would normally be possible with immersion type heating elements. This combination of a spiral channel and film heating elements allows for a controlled watt density per linear inch that is unachievable by any other current technology. While the embodiments disclosed are illustrated primarily for heating water, it will be understood that they could be used for heating virtually any fluid.

Turning now to FIG. Ia, an exploded view of a spiral flow heat exchanger, designated

10, in a straight-thru flow configuration is illustrated. Heat exchanger 10 includes an elongated cylindrical outer tube 12 having an inlet end 14 and an opposed end, which in this embodiment is an outlet end 16. An elongated spiral channel guide 20 is positioned coaxially within tube 12 and includes a cylindrically shaped core 22 with a spiral shaped, fluid diverter 24 positioned relative to the outer surface. While the preferred embodiment of diverter 24 is as a continuous spiral, it should be understood that it could be formed in separate segments if desired. It will be understood that fluid diverter 24 may be formed as an integral part of core 22 or formed separately and attached to the outer surface thereof by any convenient means or otherwise affixed in the desired orientation, for example by affixing it to the inner surface of tube 12. Also, in this straight-thru flow configuration, core 22 is solid for extra rigidity but could be formed hollow, partially hollow, etc. for lighter weight. Also, for reasons that will become apparent presently, tube 12 is can be formed of electrically insulative material or has an outer electrically insulative surface. While cylindrical outer tube 12 is stainless steel in this embodiment, it will be understood that other materials of both an electrically conductive nature such as other metals, and a non-electrically conductive nature such as plastic, glass, quartz, ceramics, PVC and the like, can also be used. The film resistive elements on the electrically conductive materials and on the non-electrically conductive materials will be described presently.

The outer diameter of diverter 24 is substantially the same as the inner diameter of tube 12 so that when spiral channel guide 20 is properly positioned within tube 12 fluid flowing in inlet end 14 (the fluid being heated) is restricted to the spiral path defined by diverter 24. An inlet piping connection 26, inlet manifold 28, and optional flow sensor 30 are attached to inlet end 14 of tube 12 to provide fluid to be heated. An outlet manifold 32 and outlet piping connection 34 are attached to outlet end 16 of tube 12 to receive heated fluid and deliver the heated fluid to a selected location.

Spiral channel guide 20 controls the effective size of the flow restriction placed upon the fluid being heated. A desired flow restriction is achieved by selecting a specific core 22 and/or a specific fluid diverter 24. Depending upon the desired fluid flow rate, the root diameter and pitch of spiral channel guide 20 can be easily changed by the shape of diverter 24. An example of a change in pitch of the spiral channel can be seen in a comparison of FIG. Ia and FIG. 2. Also, the outer diameter of core 22 and the inner diameter of tube 12 can be selected during fabrication to provide a desired fluid flow. This varied fluid flow rate is accommodated not only by appropriate selection of spiral channel guide 20, but also by sizing the inlet piping connection 26, inlet manifold 28, outlet manifold 32, outlet piping connection 34, and flow sensor 30 (if included). This embodiment provides a solution to high flow rate applications requiring an inline heater to minimize flow restriction. As shown in FIG. Ia and 3, heat exchanger 10 includes an array 40 of film resistive element groups, including at least one, and preferably, a plurality of groups, herein shown and represented as groups 42, 43, and 44, each containing a plurality of film resistive elements which in this preferred embodiment are TFR elements. As will be understood more fully from the following explanation, the number of groups in array 40 and the number of elements in each group can vary in accordance with the number of heating steps desired, the amount of total heat and the amount of heat per step. Also, in this embodiment, multiple arrays 46 and 48 of TFR element groups are included. While any number of arrays and array configurations might be included if desired and convenient, in this embodiment each array 40, 46, and 48 is similar with a similar number of groups and a similar number of elements in each similar group. Further, when multiple arrays are included similar groups might be energized simultaneously in all of the arrays or might be energized in some sequence to provide a large variety of heating steps. The specific configuration or embodiment illustrated in FIG. 1 shows a three (3) phase electrical supply to be balanced to within the value of the smallest TFR group 42. Heat exchanger 10, when connected to a residential electrical supply (for example) or other supply source, may be configured so that TFR element groups 46 and/or 48 are not present, thereby reducing the capacity of the heat exchanger.

One characteristic of the film resistive material, whether thick-film or thin film, is that it has no appreciable thermal mass and, therefore, nowhere to store energy. Conversely, because there is substantially no mass to heat-up or cool-down (no energy to store), heating and cooling is very fast. Thus, cycling from cool to a heating step is very fast (nearly instantaneous) and cycling from a heating step to not heating is very fast. Thus, the desired heat is present upon supplying power to the heating element or elements and the heat is removed almost instantaneously as power is removed. This feature greatly simplifies heating cycles and the amount of power required.

A spiral heat exchanger, designated 10', similar to the heat exchanger illustrated in FIG. Ia is illustrated in FIGS. Ib through Id. Components similar to components in FIG. Ia are designated with similar numbers and a prime (') is added to indicate the different embodiment. In this embodiment an array 40' of thick-film resistive (TFR) element groups, including at least one, and preferably, a plurality of groups, is distributed substantially equally along the length of elongated cylindrical outer tube 12'.

Referring now to FIG. 2, an exploded view of a spiral flow heat exchanger 110 in a loop-back configuration is illustrated. Generally, components similar to components in the embodiment illustrated in FIG. Ia are designated with similar numbers with a "1" added in front to indicate the different embodiment. Heat exchanger 110 includes an elongated cylindrical outer tube 112 having an inlet end 114 and an opposed end 116. An elongated spiral channel guide 120 is positioned coaxially within tube 112 and includes a cylindrically shaped core 122 with a spiral shaped, fluid di verier 124 positioned relative to the outer surface. It will be understood that fluid diverter 124 may be formed as an integral part of core 122 or formed separately and affixed relative to the outer surface thereof by some convenient means.

In this loop-back configuration, core 122 of spiral channel guide 120 includes a hollow core 123, to enable input feed flow therethrough. A reversing header 133 is positioned in opposed end 116. Thus, any incoming fluid is directed to travel through the core to opposed end 116 of spiral channel guide 120, reversed in direction by reversing header 133, and returned via the spiral channel. Fluid to be heated enters through an inlet piping connection 126, flowing through a flow sensor 130, and is directed to the hollow core of spiral channel guide 120 by an inlet/outlet manifold 128. The reversing header 133 is positioned in opposed end 116 and, fluid, upon encountering reversing header 133, is redirected down the outside of the spiral channel guide 120 and forced into contact with the inside diameter of tube 112 before exiting via an outlet piping connection 134 through inlet/outlet manifold 128. This embodiment is applicable to low flow rate applications requiring a large temperature differential between the inlet and the outlet. In this embodiment, for illustrative purposes, the pitch of spiral channel guide 120 is greatly decreased, i.e., more turns per foot of length. Thus, spiral channel guide 120, as depicted in FIG. 2, has an effective length equivalent to fifteen (15) feet of one half (¹A) inch copper, yet is only fourteen (14) inches long from inlet end 114 to opposed end 116.

It is important to note that in either embodiment presented in FIG. 1 a or FIG. 2, the heat exchanger 10/110 has not been altered, i.e. remains a standard unit. The control strategy for turning the TFR elements on and off will change depending upon the application. However it is preferable for manufacturing convenience that the physical heat exchanger, remain constant. It will be understood that the heat exchanger can change in accordance with various applications and differing flow rates. Turning now to FIGS. 3, 4, and 5, an exploded view of a spiral flow heat exchanger

10/110 with electrical and temperature sensor connections is illustrated. For convenience, heat exchanger 10 and the associated array 40 of film resistive elements will be used in this example. The various film resistive elements of array 40, in this alternate embodiment, are formed by deposition of a thick film conductor on the outer surface of tube 12. It should be understood that a plurality of arrays, such as arrays 46 and 48, may be present in addition to array 40 and, if present, they are formed similar to and generally simultaneous with the formation of array 40. The formation may be performed using any of a wide variety of techniques or processes including, for example, silk screening, printing, spraying, dipping, etc. This same process allows for electrical traces, similar to printed circuit board traces, to be formed onto the heat exchanger 10. Taking advantage of the ability to place electrical traces on the heat exchanger 10 at virtually any location allows for the utilization of a multi- conductor high pressure contact assembly 50, to provide a quick assembly method that minimizes the physical stresses on the electrical connections at the heat exchanger 10.

Here it should be noted that because the arrays (40, 46, 48) of film resistive elements are formed by depositing the thick-film resistive material using any of the above described processes, the parameters of each element (e.g. the thickness, width, etc.) can be easily adjusted to provide any desired heating characteristics. It will be understood, for example, that a thinner layer of TFR material over a greater surface area will increase the heating area. Also, operating current and voltage can be selected by adjusting thickness and width parameters.

Rather than utilizing an attachment method that involves pushing/pulling or bolting contacts together, preferably, a plurality of compressive contacts 52 are supported by and secured to the multi-conductor high pressure contact assembly 50. Assembly 50 is an arcuately shaped section of a tube formed to fit over the ends and associated electrical contacts of arrays 40, 46, and 48 and provides external contacts to the arrays. In a preferred embodiment, the compressive contacts 52 are bolted to the multi-conductor high pressure contact assembly 50 (see FIG. 5), providing a secure contact point for the high current conductors providing power to the TFR elements of array 40, 46, and 48 on the heat exchanger 10. Electrical connection to a plurality of temperature sensors 60-63 are also supported by the multi-conductor high pressure contact assembly 50 and allows a plurality of nuts 70-73 to clamp the contact assembly 50, via studs 80-83, to the heat exchanger 10, thus placing no mechanical strain on the thick film conductors deposited on the surface of tube 12.

Referring now to FIGS. 6-8, different embodiments of film resistive elements are illustrated. A film resistive heating element 200 on an outer tube 202 of an electrically conductive material, such as stainless steel, is illustrated in FIG. 6. Since film resistive heating element 200 is formed on an electrically conductive substrate, an electrically insulative layer, such as dielectric layer 205 is formed on the outer surface of outer tube 202. A resistive layer 208 is then formed on dielectric layer 205 as described previously. An additional electrically insulative layer, dielectric layer 210 is formed overlying resistive layer 208 to prevent any extraneous electrical contact with and to protect resistive layer 208. As should be understood, the various layers and the material of outer tube 202 should have closely matching thermal coefficients of expansion.

With reference to FIG. 7, an additional ground layer 212 can be formed as part of film resistive heating element 200. In this instance, a ground layer 212 of a conductive material such as silver is formed on dielectric layer 205 and covered with another electrically insulative layer, dielectric layer 215. Resistive layer 208 is then formed on dielectric layer 205 as described previously. The additional electrically insulative layer, dielectric layer 210, is formed overlying resistive layer 208 to prevent any extraneous electrical contact with and to protect resistive layer 208.

With reference specifically to FIG. 8, a film resistive heating element 220 on an outer tube 222 of a non-electrically conductive material, such as plastic, PVC, glass, ceramic and the like, is illustrated. Since film resistive heating element 220 is formed on a non- electrically conductive substrate, an electrically insulating layer need not be provided on the outer surface of outer tube 222. A resistive layer 228 can be formed directly on the outer surface of outer tube 222. An electrically insulating layer, dielectric layer 230 can be formed overlying resistive layer 228 to prevent any extraneous electrical contact with and to protect resistive layer 228. As should be understood, the various layers and the material of outer tube 222 should have closely matching thermal coefficients of expansion.

Turning now to FIG. 9, a block diagram of an embodiment of a process controller, designated at 400, including a communications system is illustrated. The process controller

400 includes at least a micro-controller 401, an interface 430 with a communication link 431, a data storage device 410, and a memory device 420. Data storage device 410 includes at least the operational parameters 412 and logs 411. Memory device 420 includes at least control logic 421 and default parameters 422.

A flow sensor 405 is coupled to micro-controller 401 to supply flow signals. Flow sensor 405 may be, for example, flow sensor 30 of FIG. 1, or any sensor positioned to sense a flow of fluid in the system. When micro-controller 401 receives a signal from flow sensor 405 indicating the presence of fluid flow, micro-controller 401 retrieves and executes control logic signals from control logic 421 in memory device 420, via a connection 406, to determine the flow rate. Micro controller 401 simultaneously monitors the supply voltage, designated 404, and temperature sensors 403 (e.g. sensors 60-63 in FIG. 3). Micro-controller

401 then retrieves and executes additional control logic signals from control logic 421, via connection 406, to determine the amount of power necessary to raise the fluid temperature, provided by the plurality of temperature sensors 403 (e.g. temperature sensors 60-63), to a predetermined value. This calculated amount of power is then compared to the amount of power required by the plurality of TFR element arrays of groups (42/43/44 and 46/48, if included, from FIG. 1) to determine how many of the TRF element groups to energize. It should be noted that the process controller 400, may enable the supply voltage to be connected to a three (3) phase source. Here, the calculated required power, is divided by three (3) to determine how much power is needed from each phase. The capacity of the plurality of TFR elements 42/43/44, from FIG. 1, needed to provide the calculated power is subtracted from the calculated power, with the remaining additional power to be supplied by a single TFR array of groups (40/46/48 from FIG. 1).

Alternatively, the process controller 400, may enable the supply voltage 404 to be connected to a standard residential source. Here, the calculated power is supplied by one or more of the largest TFR elements (44 or 43 from array 40 and/or 46/48, if present, in FIG. 1) needed to meet the power requirement, with the remaining power supplied from one or more of the smaller TFR elements (42 or 43 from array 40 and/or 46/48, if present, in FIG. 1).

Still further, the process controller 400 may only receive a signal from flow sensor 405 to indicate the presence of flow rather than a flow rate. Here, micro-controller 401 will select the smallest TFR element 42 of array 40 (from FIG. 1) to energize initially. Values received from temperature sensors 403 allow micro controller 401 to approximate a flow rate and determine subsequent TFR elements to energize, according to the electrical supply source.

The energizing of a TFR element is done by micro-controller 401 sending a signal to an electrical control device such as a triac(s) controlling the appropriate TFR elements of the appropriate TFR element group or groups. This signal is correlated to the zero crossing of the electrical supply source for the respective TFR element.

Still further, the process controller 400 may enable the supply voltage 404 to be compared to the nominal supply voltage for the electrical supply source. If the supply voltage 404 is below a defined range, a power offset is applied to the calculated power before determining the energization of TFR elements. This offset is adjusted on a cycle-by-cycle basis to minimize the chances of an electrical supply overload. This embodiment will cause the heat exchanger 10 or 110 to provide a fluid outlet temperature that is below the predetermined value and allow other loads to consume power as necessary.

Furthermore, process controller 400 may enable communication with a plurality of heat exchangers using a Power Management Controller (PMC), such as described in US Patent No. 6,806,446, incorporated herein by reference. Communications link 431 enables the communication. The PMC allows multiple heat exchangers, each having a unique identifier, to have an electrical supply source that would normally be overloaded if multiple loads were allowed to energize at the same time. Here, micro-controller 401 communicates over interface 430 and communications link 431, via connection 408, with the PMC prior to energizing any of the TFR elements. If sufficient power is available, the process controller 400 will energize the TFR elements necessary to meet the calculated demand. If sufficient power is not available, the process controller 400 will limit the element energization accordingly. If another heat exchanger were to require power, the PMC would allocate available resources, as configured. This verification of available power is done multiple times per second.

One skilled in the art will appreciate that the process controller 400 may be configured in an infinite number of operating modes to provide any desired degree of control and flexibility. Although the present invention has been described with respect to a tankless water heater, it is to be appreciated that the fluid heated by the spiral flow heat exchanger 10 or 110 can range from gases, such as air, to liquids, such as water. This same flexibility extends to the materials used to manufacture the spiral channel guide 24 and 124, inlet/outlet manifolds 28/32 and 128, and reversing header 133, as well as tube 12 and 112. Thus, a high thermal transfer spiral flow heat exchanger is disclosed. Further, the high thermal transfer spiral flow heat exchanger is disclosed in conjunction with a fluid heating system. It should be understood that the high thermal transfer spiral flow heat exchanger can be used as a booster in any operating system, such as in an already installed and operating hot water heating system. Also, the high thermal transfer spiral flow heat exchanger can be used as a simple ON/OFF fluid heating system or the various disclosed flow and heat sensors can be incorporated in conjunction therewith. Further, the high thermal transfer spiral flow heat exchanger with sensors, etc. can be used as a whole house water heating system or for only portions thereof. Additionally, the high thermal transfer spiral flow heat exchanger can be used as a point-of-use fluid heater, and can be coupled in series or in parallel as desired for any of the uses herein.

The high thermal transfer spiral flow heat exchanger disclosed herein includes heating elements that evidence no appreciable residual heat and no mineral deposit buildup. Also, the high thermal transfer spiral flow heat exchanger uses thick-film resistive heating (TFR) elements having no appreciable thermal mass and, therefore, nowhere to store energy so that there is substantially no mass to heat-up or cool-down and cycle time can be made very accurate. Further, the high thermal transfer spiral flow heat exchanger is configured to allow a single (standard) heat exchanger to economically be configured for a variety of heating application demands. Also, a fluid heating system including a high thermal transfer spiral flow heat exchanger and a process controller provides localized monitoring of the electrical supply source in such a way as to provide overload protection to the supply circuit.

Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof, which is assessed only by a fair interpretation of the following claims.

Having fully described the invention in such clear and concise terms as to enable those skilled in the art to understand and practice the same, the invention claimed is:

Claims

1. A fluid heating system with a heat exchanger comprising: an elongated cylindrical tube with an outer surface, an inner surface, an inlet end and an opposed end; an array of film resistive elements positioned on the outer surface of the cylindrical tube and extending circumferentially at least partially around the outer surface; and a spiral channel guide coaxially positioned within the cylindrical tube so as to force fluid flowing in the tube into a spiral path at least partially against the inner surface of the cylindrical tube.

2. A fluid heating system with a heat exchanger as claimed in claim 1 wherein the array of film resistive elements includes a plurality of separate groups of film resistive elements.

3. A fluid heating system with a heat exchanger as claimed in claim 1 wherein the array of film resistive elements includes a film conductor formed on the outer surface of the cylindrical tube with a predetermined depth and width.

4. A fluid heating system with a heat exchanger as claimed in claim 1 wherein the spiral channel guide includes a central elongated cylindrical core and a fluid diverter extending along an outer surface of the cylindrical core.

5. A fluid heating system with a heat exchanger as claimed in claim 4 wherein the fluid diverter is one of an integral part of the outer surface of the cylindrical core and an element affixed to the outer surface of the central core.

6. A fluid heating system with a heat exchanger as claimed in claim 4 wherein the fluid diverter has an outer diameter substantially the same as an inner diameter of the cylindrical tube.

7. A fluid heating system with a heat exchanger as claimed in claim 4 wherein the fluid diverter is in the form of a continuous spiral.

8. A fluid heating system with a heat exchanger as claimed in claim 1 further including one of an inlet manifold and an inlet/outlet manifold positioned in the inlet end of the cylindrical tube.

9. A fluid heating system with a heat exchanger as claimed in claim 8 further including one of an outlet manifold and a reversing header positioned in the outlet end of the cylindrical tube.

10. A fluid heating system with a heat exchanger as claimed in claim 1 further including a process controller unit connected to the array of film resistive elements and adapted to be connected to an electrical supply, the process controller unit for energizing the array of film resistive elements.

11. A fluid heating system with a heat exchanger comprising: an elongated cylindrical tube with an outer surface, an inner surface, an inlet end and an opposed end; an array of film resistive elements positioned on the outer surface of the cylindrical tube and extending circumferentially at least partially around the outer surface; a spiral channel guide including a central elongated cylindrical core and a fluid diverter extending along an outer surface of the cylindrical core, the spiral channel guide coaxially positioned within the cylindrical tube so as to force fluid flowing in the tube into a spiral path at least partially against the inner surface of the tube; and one of an inlet manifold and an inlet/outlet manifold positioned in the inlet end of the cylindrical tube to introduce fluid into the cylindrical tube and one of an outlet manifold and a reversing header positioned in the opposing end to one of conduct fluid from the cylindrical tube and reverse the flow of fluid within the cylindrical tube, respectively.

12. A fluid heating system with a heat exchanger as claimed in claim 11 wherein the fluid diverter is one of an integral part of the outer surface of the cylindrical core and an element affixed to the outer surface of the central core.

13. A fluid heating system with a heat exchanger as claimed in claim 11 wherein the fluid diverter has an outer diameter substantially the same as an inner diameter of the cylindrical tube.

14. A fluid heating system with a heat exchanger as claimed in claim 11 wherein the fluid diverter is in the form of a continuous spiral.

15. A fluid heating system with a heat exchanger as claimed in claim 11 further including a process controller unit connected to an electrical supply for energizing the array of film resistive elements.

17. A fluid heating system with a heat exchanger comprising: an elongated cylindrical tube with an outer surface, an inner surface, an inlet end and an opposed end; an array of film resistive elements positioned on the outer surface of the cylindrical tube and extending circumferentially at least partially around the outer surface; a spiral channel guide including a central elongated cylindrical core and a fluid diverter extending along an outer surface of the cylindrical core, the spiral channel guide coaxially positioned within the cylindrical tube so as to force fluid flowing in the tube into a spiral path at least partially against the inner surface of the tube; one of an inlet manifold and an inlet/outlet manifold positioned in the inlet end of the cylindrical tube to introduce fluid into the cylindrical tube and one of an outlet manifold and a reversing header positioned in the opposing end to one of conduct fluid from the cylindrical tube and reverse the flow of fluid within the cylindrical tube, respectively; a plurality of temperature sensors positioned in the fluid flow within the cylindrical tube; a flow sensor positioned adjacent the inlet end of the cylindrical tube; and a process controller unit connected to receive a flow signal from the flow sensor and programmed to calculate a flow rate, the process controller unit further connected to receive temperature signals from the plurality of temperature sensors and further programmed to determine from the temperature signals the amount of power necessary to heat fluid from a source temperature adjacent the inlet end, at the calculated flow rate, to a specified temperature adjacent the outlet manifold and control energization of the film resistive elements to achieve the specified temperature.

18. A fluid heating system with a heat exchanger as claimed in claim 17 wherein the spiral channel guide directs fluid flowing in the cylindrical tube to cause turbulence to occur in the flow and increase thermal transfer from the inner surface of the cylindrical tube to the fluid flowing in the tube.

19. A fluid heating system with a heat exchanger as claimed in claim 17 wherein the array of film resistive elements includes a plurality of separate groups of film resistive elements, at least some of the separate groups including different numbers of film resistive elements.

20. A fluid heating system as claimed in claim 19 wherein an electrical supply is coupled to the process controller unit from a three (3) phase source and phase-to-phase loading is balanced to within the value of a smallest number of the different numbers of film resistive elements.

21. A fluid heating system as claimed in claim 20 wherein the process controller unit monitor includes means for coupling an electrical supply to the process controller unit and means for adjusting energization of the array of film resistive elements in response to the monitored voltage.

22. A fluid heating system as claimed in claim 20 further including a communication interface configured to provide access to a Power Management Controller.