EP0018824A1

EP0018824A1 - Fluid heater and process of heating a fluid

Info

Publication number: EP0018824A1
Application number: EP80301415A
Authority: EP
Inventors: Martval John Hartig
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1979-05-01
Filing date: 1980-04-30
Publication date: 1980-11-12
Also published as: DK187380A; NO801257L; CA1130677A; JPS55146348A

Abstract

A high efficiency heater for air or water is disclosed in which the combustion products from a flame are forced through water in a contacting device (10) in a manner to transfer aimost all of the useable heat of the combustion products to the water, and to heat the water to a temperature of about 50 to 85°C. The cooled waste gases are vented to the atmosphere, and the heated water is passed through a heat exchanger (14) to heat air or water such as for a home heating system. As the heated water passed from the contacting device (10) to the heat exchanger (14) is at a temperature below the normal boiling point of water, a plastic heat exchanger can be used, which is low enough in costto permit using a large unit for maximum heat utilization.

Description

BACKGROUND OF THE INVENTION

At present most heaters for forced air or water burn a fuel which generally is either gas or No. 2 fuel oil, the combustion products of which pass through one side of a heat exchanger and then are vented into the atmosphere. In the case of forced air, the air from the building or other space to be heated is passed from the cold air return through the second side of the heat exchanger and back through the building. Similarly, in the case of hot water heating, the cold water returned from the radiators of the building is passed through the second side of the heat exchanger and back to the _i radiators of the building. Due to the high temperature of the waste gas going up the vent to the atmosphere a considerable amount of heat is lost. Generally, conventional gas fired heaters are, under steady state conditions, about 70 to 75% efficient while conventional oil fired heaters are about 65 to 70% efficient. Due to additional heat loss during both on and off cycles, during cold starts and from the pilot light, the overall efficiency is typically only 40 to 60%, and thus 40 to 60% of the potential heat value of the fuel is lost. The heater of the present invention generally will provide efficiencies both steady state and 'overall, generally over 90%, ordinarily greater than 95% and up to about 98%, which enables a considerable reduction in the amount of fuel consumed in heating the building or other space to be heated.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to an improved heater for forced air or water having high efficiency. While any combustible fuel can be used by the heater, the conventional gas or No. 2 fuel oil are the preferred fuels. The combustion products from the burning fuel and some excess air are drawn or forced through a water contacting device to cool the combustion gases and transfer heat to water. The contacting device is of suca design and operation that almost all of the recoverable heat of the combustion products is transferred to the water, the waste gases from the contacting device are vented to the atmosphere, and the heated water is then passed through one side of a heat exchanger to heat a fluid to be heated in the second side of the heat exchanger. The fluid to be heated can be air to be heated for a home, building or other use; a typical example is air in a forced air heating system. The fluid to be heated can also be a liquid such as water for a hot water heating system for a house or other building, or water for laundry and other household uses. The fluid to be heated can also be the liquid used in a solar heating system to store solar energy, such as a glycol or glycol/ water mixture, for example, 1:1 ethylene glycol/water.
More specifically, according to the present invention, there is provided a heater comprising a burner for producing hot combustion products, contacting means adapted for countercurrently contacting said combustion products with water, said means being capable of operation at at least 1.2 theoretical stages, a blower adapted to urge said combustion products from said burner through said contacting means and to urge the waste gases of said combustion products to exhaust means, and a heat exchanger which is adapted to receive said water heated by said combusticn products and to exchange the heat from said water to afluid, the product of the heat exchange area in said heat exchanger and the heat transfer coefficient of said fluid for which the heater is adapted being in the range from 20 to 500 per 1000 Btu/hour of fuel burned.
Further in accordance with the present invention there is provided a process of heating a fluid comprising (a) burning a fuel to produce combustion produces, (b) countercurrently contacting in at least 1.2 theoretical stages said combustion products with water to raise the temperature of said water to a temperature of from about 120°F to about 185°F, and (c) passing said water through a heat exchanger to heat said fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an elevation, not to scale, of one embodiment of the heater of the present invention.
Fig. 2 is an elevation, partially in section, not to scale, of one embodiment of the contacting device in the heater of Fig. 1.
Fig. 3 is an elevation, not to scale, of a portion of another heater of the present invention.
Fig. 4 is a schematic diagram showing a typical flow pattern within the heat exchanger of the heater of Fig. 1.
_Fig. 5 is a perspective view of a portion of a heat exchanger fabricated of a thermoplastic resin, suitable for use in the heater of the invention.
Fig. 6 is a sectional view of one of the elements of the heat exchanger of Fig. 5.
Fig. 7. is an enlarged perspective view, partially in section, of a portion of an element of the heat exchanger of Fig. 5.

DETAILED DESCRIPTION

The illustrative example shown in Fig. 1 is an embodiment of the invention suitable for use as a heater for a house, apartment or other building. In a contacting device 10 fuel is burned to form hot combustion products which are contacted countercurrently with water to produce heated water and cooled waste gases. A conventional burner assembly (not shown) in the lower end of contacting device 10 burns fuel introduced through pipe 11. The fuel could be, for example, natural gas, water gas, propane, liquified natural gas (LNG), No. 2 fuel oil, kerosene, diesel fuel, etc.; the particular burner assembly is chosen for the specific fuel used. Cold water introduced through pipe 12 into contacting device 10 is heated therein by the hot combustion products in a manner more fully described below in reference to Fig. 2, and hot water is removed through pipe 21. The cooled waste gases are urged through exhaust ducts 24 and 26 by blower 25, and exhausted to the atmosphere by any suitable means not shown, such as a chimney, plumbing stack, or other duct; the force provided by blower 25 replaces the thermal draft in the chimney of a conventional furnace. The action of blower 25 also serves to draw the hot combustion products up through contacting device 10.
Housing 13 comprises several sections, one of which contains heat exchanger 14. Cold air chamber 15 receives cold air from the house or other building through return duct 16, and main blower 17 forces the cold air through plenum 18, into heat exchanger 14, from which heated air exits into hot air chamber 19, thence through hot air supply duct 20 to the house or other space to be heated.
Hot water from contacting device 10 is carried by pipe 21 into heat exchanger 14, and, after removal of the heat therefrom in heat exchanger 14, flows as cold water through pipe 22 to reservoir 23. Pump 28 recirculates the water through pipe 12 to contacting device 10. Pump 28 can be any type of pump suitable for use with water, for example, a submersible centrifugal pump. Ordinarily the hot water will flow through pipe 21 by gravity, but in other arrangements a pump could be used.
One of the combustion products of the fuel is water, and although a small amount of water vapor will be carried away with the waste gases, excess water will ordinarily accumulate in the circulatory water system because the cold water introduced through pipe 12 into contacting device 10 will condense it. Accordingly, pipe 27 is provided in the upper part of reservoir 23 to permit accumulated excess water to drain away through any convenient waste water system. Reservoir 23, pump 28 and pipe 27 are preferably situated at a level to assure that the side of heat exchanger 14 which receives hot water from contacting device 10 remains filled with water at all times- so that no gas or air bubbles form, which bubbles may become lodged in the heat exchanger and lower its efficiency. However, the reservoir could be situated higher or lower if desired.
Blower 25 could be, for example, a small fan capable of creating a pressure differential of as little as 2 inches of water.
In a similar, but slightly modified, system, contacting device 10 could be located below reservoir 23, toward the lower end of housing 13, in which case, pump 28 would be located instead in line in pipe 12 to pump hot water removed from contacting device 10 up to the entrance to heat exchanger 14. Cold water from heat exchanger 14 would flow by gravity through reservoir 23 and associated pipes to contacting device 10. Reservoir 23 would then have two overflow pipes, the primary one througt which water would flow to contacting device 10, and a secondary one, situated above the primary one, for disposal of accumulated excess water. It is preferred, however, to use a pump in the cold water return system, rather than in the hot water line, as a pump will oridinarily operate more free of trouble when pumping water which is cold.
Contacting device 10, shown in greater detail in sectional view in Fig. 2, comprises a cylindrical outer wall 40, top plate 41, bottom plate 42, and cylindrical inner wall 43. Members 40, 41, 42 and 43 are suitably constructed of stainless steel, Inner wall 43 comprises lower portion 43a of relatively larger diameter, sloping section 43b, and upper portion 43c of relatively smaller diameter.
In the specific embodiment shown, a combustible gaseous fuel introduced through pipe 11 and air drawn in through sleeve 44 are mixed and burned at burner 45. Burner 45 is a conventional perforated metal or screen type burner closed at the top by solid disc 46 to pre- vens loss of unburned fuel. Hot combustion products accumulate in combustion chamber ⁴7 and move upwardly through space 48 into contacting section 49 in which the hot combustion products are brought into intimate and direct contact with cold water. Section 49 is filled with packing 50, which typically can be in the form of saddles, helices, short cylinders, rings or other suitable form, fabricated of ceramic, glass or other suitabl material, e.g., Raschig rings or berl saddles. Packing 50 is supported in the device 10 on plate 51, typically a shower plate comprising a solid central portion 51a and a peripheral perforated portion 51b which contains perforations 52.
Cold water is introduced through pipe 12 into the upper portion of contacting device 10, is received by pan 53, overflows onto the packing, flows downwardly over packing 50, through perforations 52 and accumulates in cylindrical well 54 which lies between outer wall 40 and inner wall 43. The central portion 51a of plate 51 is of diameter somewhat greater than the diameter of upper portion 43c of inner wall 43, to prevent downwardl flowing water from falling into combustion chamber 47 and contacting burner elements 45 and 46. Blower 25 (shown in Fig. 1) urges the hot combustion products accumulated in combustion chamber 47 upwardly through space 48, perforations 52, section 49 wherein they intimately contact the water flowing downwardly, and out through exhaust duct 24; blower 25 further serves tc draw the fuel released from pipe 11 and air through sleeve 44 to burner 45.
Hot water accumulated in well 54 flows through pipe 55, vented overflow 56, and pipe 21 to heat exchanger 14. Pipe 55 terminates at its upper end within vented overflow 56 at a level to insure that well 54 remains filled with water to the same level. This is done to reduce heat loss by radiation or conduction thrc the walls of contacting device 10 in the region adjacent burner 45.
Alternatively, in place of pan 53, a shower head or any other device for distributing water over the top of packing 50 can be used. In place of packing 50, it is also possible to use trays, such as perforated shower trays, bubble cap trays, seive plates, or any other device suitable for ensuring intimate contact of the water and combustion-products. Also, blower 25 could alternatively be relocated and connected by a fitting to sleeve 44, so as to push air into the combustion chamber, and thus to push the combustion products through the contacting device 10, instead of pulling them through the exhaust.
It should be understood that indicated burner elements 45 and 46 can be replaced by other suitable conventional burner elements, as may be required by the choice of another fuel, such as one of those listed above.
The controls used for burner ignition, for detection of flame, for turning the fuel supply on and off, for turning the exhaust blower, house air blower and water pump on and off, for prevention of overheating, etc., are not shown because they are conventional.
For this liquid-gas heat transfer, the heat transfer area in heat exchanger 14 is in the range of 3 to 30 square feet per 1000 Btu's per hour. For a typical 60,000 Btu home heater, the total heat transfer area in heat exchanger 14 could be about 500 square feet.
Contacting section 49 should be filled with sufficient packing 50, or alternatively with sufficient trays as noted above, to provide at least 1.2 and preferably from 3 to 5, theoretical heat exchange stages. Ordinarily there is no need for more than 6 theoretical heat exchange stages, as virtually all the recoverable heat can be transferred to the liquid with 6 theoretical stages. In one theoretical heat exchange stage, both thermal equilibrium and thermodynamic equilibrium are attained between the gas and liquid brought in contact.
The number of theoretical heat exchange stages can be calculated from the initial temeprature and weight of water introduced into the contactor; the final temperature and weight of water removed from the contactor; the temperature, weight and composition of the combustion products introduced into the contactor; and the temperature, weight and composition of the waste gases exhausted from the contactor. While a close estimate of the number of theoretical stages can also be made if the composition of the combustion products is inferred from the composition of the fuel and air mixture used, it is more accurate to use the composition of the combustion products determined by analysis. The calculation can be done as an iterated calculation using standard psychometric charts and the material and heat balance at each stage. The estimate can also be made from psychometric graphs using the same data indicated above.
In regard to theoretical heat exchange stages and calculating numbers of stages by iteration or graphically, reference is made to the following; Unit Operations of Chemical Engineering, McCabe and Smith, Third Edition, McGraw-Hill Book Co., N.Y., 1976, especially Section 18, Equilibrium Stage Operations, and Section 24, Humidification Operations; and Mass Transfer Process Calculations, Sawitoski and Smith, Interscience Publishers, N. Y., 1363, especially Chapter 5, Humidification and Water Cooling. For the case of air and water, psychrometric charts can be found in the Chemical Engineers' Handbook, Textbook Edition, Perry, Third Edition, McGraw-Hill Book Co., N.Y., 1950, pages 763-765.
During operation, cold water at a temperature of between about 70°F and 115°F is introduced into contacting device 10 at a rate from 7 to 50 pounds of water per 1000 Btu's of heat produced at the burner. With a water flow rate within these limits, and when contacting device 10 is operated as indicated with countercurrent contact of water and combustion products and has at least 1.2 theoretical stages, the water can be heated in the contacting device to a temperature in the range of about 120°F to 185°F, and is preferably heated to 150°F to 170°F. At the same time, most of the useable heat is removed from the hot combustion products, and waste gases are exhausted at a temperature of about 75°F to 120°F, preferably 75°F to 100°F. When operated in this- way, at least 90%, and up to about 98%, of the available heat is recovered from the hot combustion products. Inasmuch as most of the water vapor in the hot combination products is condensed by the cold water introduced into the contacting device, the heat value associated with the latent heat of vaporization of water is recovered from that water which is condensed, and the waste gases differ from the hot combustion products in that the waste gases lack most of the water in the combustion products, as well as in their lower temperature.
Contacting device 10 can be very compact; for a heater rated at 60,000 Btu per hour, which is a rated capacity typical for a house of average size, suitable overall dimensions of contacting device 10 can be about 6 inches in diameter and about 26 inches high.
If more than 50 pounds of water per 1000 Btu's is used, the temperature of the water will be increased by no more than about'20°F, and as a result the required exchange area in the heat exchanger would become excessively large and the rate of air circulation high. If less than 7 pounds of water per 1000 Btu's is used, sufficient water is evaporated and lost as steam with the waste gases that there is significant loss of heat value associated with the latent heat of vaporization of water. As the flow rate of water is decreased within the stated range, the number of theoretical heat exchange stages which are required in the contacting device is increased, and the amount of gas/liquid contact area required in the heat exchanger is decreased. In the case of a heater for heating forced air, the amount of water circulated is preferably in the range of 10 to 15 pounds per 1000 Btu's. In the case of heating water for radiators or laundry and other household use, the amount of water circulated through contacting device 10 is preferably in the range of 8 to 12 pounds per 1000 Btu's.
By way of example, typical operating conditions for a heater rated at 60,000 Btu/hour are as follows: Contactor:
pressure drop in contactor 2.5 inches of water heat exchanger:
In a heater operating thusly, the contacting device operates in the range of 4 to 5 theoretical heat exchange stages.
The heater and process of the invention are adapted for heating fluids of all kinds, both gases and liquids. The specific embodiment described above in reference to Figs. 1 and 2 is an example of heating a gas, in this case air for space heating. Other gases can be heated in the same way whenever required.
The fluid to be heated can also be a liquid such as water. One such example is heating of water for heating a home, where heat is carried to radiators throughout the house by hot water. In this case, both sides of the heat exchanger will carry water, i.e., hot water from contacting device 10 will be cooled in heat exchanger 14 as it heats water which is in turn circulated to hot water radiators. In such a system, elements 13 through 20, inclusive, as shown in Fig. 1 would be eliminated, and in place thereof the elements shown in Fig. 3 would be used. Referring to Fig. 3, there is shown a housing 68 comprising cold water plenum 71, hot water plenum 72, and a central section 69. adapted to hold heat exchanger 14. Hot water from contacting device 10 flows by gravity through pipe 21 to a first side of heat exchanger 14, loses heat therein, and exits as cold water through pipe 22. Water to be heated for radiators, laundry or other use is received as cold water from pipe 70 into plenum 71, which serves to distribute the water to be heated into all the passages of the second side of heat exchanger 14; after being heated therein, the hot water from heat exchanger 14 is collected in plenum 72. Pump 74 serves to urge water upwardly through plenums 71 and 72, heat exchanger 14, and through pipes 73 and 75 to locations where the hot water is needed. Pump 74 is preferably located to pump the water in the pipe receiving water from heat exchanger 14 so as to avoid creating excessive pressure in the thermoplastic heat exchanger which could cause damage to it, and which might occur if the pump were located in inlet pipe 70.
In the case of this liquid-liquid heat transfer, the heat transfer area in heat exchanger 14 need only be in the range of 0.2 to 10 square feet per 1000 Btu's per hour. For a typical 60,000 Btu home heater the total heat transfer area in heat exchanger 14 could thus be about 100 square feet. It is also possible to heat water in the same way for other household use, for example for laundry, sink, showers, etc. The invention is also adapted to further raise the temperature of fluids warmed in solar heating units, to bring them up to higher temperatures, such fluids being, for example, an ethylene glycol/water mixture. Other liquids can be heated in the same way when required, as heating of dye baths, chemical treating baths, etc.
Also possible is a dual system for heating both a gas and a liquid. As an example, for a house with a forced air heating system, both the heat for the forced -' air and the heat for hot water for sink and laundry use could be supplied by a heater having one contacting device 10 of sufficient capacity to supply two heat exchangers, in one of which forced air is heated and in the other of which water for sink and laundry use is heated.
It should also be understood that housing 13 can be of small dimensions, for example, no larger in cross section than typical duct 16 and 20 used for cold air return and hot air supply. This would be done in the case of installation of a heater of the invention in a newly constructed house or building. However, in the case of an existing heater in an older house or building, it is possible to retrofit the existing heater by discarding only those elements no longer needed, e.g., the burner and existing metallic heat exchanger, while retaining the heater housing, forced air blower and ductwork; in such a case, thermoplastic heat exchanger 14 would be installed in the existing housing, and contacting device and all other associated elements as described above would also be installed and the required connections to the existing housing would be made.
As indicated above, the heat transfer area in heat exchanger 14 will vary, depending on whether the fluid to be heated therein is a gas or a liquid. If it is a gas, the area required is 3 to 30 square feet per 1000 Btu's per hour, and if it is a liquid, the area required is 0.2 to 10 square feet per 1000 Btu's per hour. The area used in any particular case will vary within the indicated ranges depending on the amount of water (in pounds per 1000 Btu's) introduced into contacting device 10 to remove the heat from the combustion products, and, to a lesser extent, on the particular gas or liquid to be heated in the heat exchanger. The difference in the ranges of area in the heat exchanger for heating a gas or a liquid results primarily because of the difference in the heat transfer coefficients of cases and liquids. For example, when the fruid to be heated is air, the heat transfer coefficient U is in the range of to 8 Btu/square foot/hour/°F, but when the fluid to be heated is water, U is in the range of 30 to 300 Btu/square foot/hour/°F. The heat transfer coefficient varies within the indicated ranges, depending on the specific fluid being heated, the geometry of the system, and the flow rate. Thus, because the rate of heat transfer to a liquid is faster than the rate to a gas, the heat transfer area in the heat exchanger can be less for liquid/liquid heat transfer than for liquid/gas heat transfer. However, for any heater of the invention, the product of U and the heat transfer area A in the heat exchanger is the same regardless of whether the _; fluid to be heated is a gas or a liquid; the product U x A must be at least 20 per 1000 Btu/hour of fuel burned, and, though the upper limit is not so critical, need not exceed 500 per 1000 Btu/hour of fuel burned. A preferred range for U x A is from 30 to 100 per 1000 Btu/hour of fuel burned.
In heat exchanger 14, there are provided a plurality of first passages adapted to conduct the water received from contacting device 10, and adjacent thereto, a plurality of second passages adapted to conduct the fluid to be heated. The first and second passages can be arranged in different types of geometric and spatial relationships to one another, so long as the flow patterns of the two streams are adequate to transfer most of the recoverable heat from the liquid comprising water to the fluid to be heated.
Preferably, the heat exchanger is designed with a plurality of stages with the flow of the water being substantially at right angles to the flow of the fluid to be heated. For example, Fig. 4 is a schematic diagram which shows a flow pattern for a heat exchanger having six stages. Heat exchanger element 80 is divided into a multiplicity of first passages 81, 82, 83, 84, 85 and 86 by septums 87 so as to define a serpentine or tortuous path, indicated by arrows, for the water introduced through pipe 21 and removed through pipe 22, which path is substantially at right angles to the flow of the fluid to be heated in contact with element 80. In this case the indicated passages define plural heat exchange stages. There can, of course, be a greater or lesser number of passages and stages. There will ordinarily be a plurality of heat exchange elements in a heat exchanger, and in such cases, pipes 21 and 22 for feeding water to and receiving water from the heat exchanger can terminate in headers to service all the elements 80.
A suitable and preferred embodiment of a heat exchanger for use in the heater of the invention is depicted in Figs. 5, 6 and 7.
Referring first to the partial heat exchanger element shown in Fig. 7, sheets 151 and 152 of thermoplastic film, which serve as the heat exchange surfaces, are spaced apart from one another. Proximate faces of the two sheets are joined to one another near the edges thereof, in this embodiment by edge septa, only one of which, edge septum 153, is shown in this partial view. Throughout the space defined by the two sheets and the edge septa, there is a plurality of protuberances 168 which project from sheet 152 and extend toward sheet 151. Protuberances 168 serve to maintain sheets 151 and 152 in a spaced-apart relationship to one another, thus preventing collapse of the two sheets against one another, which would restrict or stop the flow of fluid between the sheets. Channel septum 165 extends between and is joined to proximate faces of sheets 151 and 152, and serves as a barrier between channels to direct the flow of a first fluid, water in this case, in a defined path between the two sheets.
As depicted at junction 171 of edge septum 153 with sheet 152, at junction 172 of channel septum 165 with sheet 152', and at junction 173 of protuberance 168 with sheet 152, the edge septa, protuberances, and channel septa are preferably formed integrally with sheet 152. Sheet 152, with protuberances 168 and the edge and channel septa, can conveniently be made by extrusion of a thermoplastic resin from a suitable die onto a patterned drum with the technique shown in any of U.S. Patents No. 3,509,005; 3,515,778; or 3,635,631. Sheet 151 is then joined to sheet 152 at the edges thereof, for example, by heat sealing or with a suitable adhesive, preferably by heat sealing, either directly as noted above, or by sealing to the top 174 of edge septum 153 and the tops of other edge septa not shown, and to the top 175 of channel septum 165 and the tops of all other channel septa. A suitable technique for sealing sheet 152 to the tops of the edge and channel septa is disclosed in U.S. 3,821,051.
It is preferred that protuberances 168 extend to and are joined to sheet 151. When protuberances 168 are joined to sheet 1₅1, this can also suitably be done by joining the top 176 of each protuberance 168 to sheet 151 by heat sealing or with a suitable adhesive, preferably heat sealing. It is preferred that protuberances 168 are joined to both sheets 151 and 152, as this prevents ballooning of sheets 151 and 152 away from one another when the pressure of the first fluid within the heat transfer element exceeds the pressure of the second fluid in contact with the exterior faces of the element. If such ballooning were not prevented, the flow of the second fluid in contact with the exterior faces of the element would be restricted or stopped.
Fig. 6 depicts a sectional view of one heat exchanger element. Sheet 152 of thermoplastic film carries edge septa 153, 154, 155 and 156 joined to the sheet at its edges. The space within the element is divided into channels 157, 158, 159, 160, 161 and 162 by channel septa 163, 164, 165, 166 and 167 which are joined to sheet 152 as lesoribed above. Protuberances 168. of which only three groups are shown in Fig. 6, project from sheet 152 throughout all of the channels. Sheet 152 contains two openings, a first opening 169 through which the first fluid enters the interior of the element, and a second opening 170 through which the first fluid is removed from the element. The flow of the first fluid through the channels is in the direction of the arrows shown.
The number of channels 157 etc. can vary from as few as one channel up to any number as may be desired or needed for a particular heat exchange. The six-channel element depicted in Fig. 6 is merely typical, and is suitable for many uses where six heat exchange stages are desirable. The heat exchange elements can have either an even or an odd number of channels, and the location of opening 170, through which the first fluid is removed, will vary, and it will be placed at the downstream end of the last channel through which the first fluid flows.
In Fig. 5 a portion of a typical heat exchanger of the invention is shown in perspective. The arrows associated with numerals 6,6 refer to the direction of the sectional view shown in Fig. 6. Shown are five individual heat exchange elements 101, 102, 103, 104 and 105. In this view sheets 151 and 152 and edge septa 153 and 154 are seen. Each of sheets 151 and 152, and the corresponding sheets of all the other elements 102 etc. except the last element (not shown), contain both first and second openings which are not seen in this view, such as first opening 169 and second opening 170 seen in Fig. 6. All of the first openings are in line, and all of the second openings are in line. The final element (not shown) contains first and second openings in only the first sheet, i.e., the sheet which faces toward the adjacent element, there being no openings in the second sheet, i.e., the sheet of the last element which is farthest away from the penultimate elemert. A first series of coaxial rings 106, 107, 103, 109 and 110 is situated such that each ring lies between and is joined to adjacent elements and is disposed to surround the first openings in the sheets they contact. For example, ring 106 joins onto sheet 152 to surround first opening 169, and joins onto sheet 150 to surround the corresponding first opening in that sheet. Similarly, a second series of coaxial rings 111, 112, 113, 114 and 115 is situated such that each ring lies between and is joined to adjacent elements and is disposed to surround the second openings in the sheets they contact. For example, ring 111 joins onto sheet 152 to surround second opening 170, and joins onto sheet 150 to surround the corresponding second opening in that sheet. Spacer bars 116, 117, 118, 119 and 120 are positioned between adjacent pairs of heat transfer elements to aid in maintaining the elements in a spaced apart relationship. The spacer bars should be secured in place to prevent them from shifing out of place; this can be done, for example, by joining them to the heat exchanger elements by heat sealing or with a suitable adhesive. The spacer bars need not be sealed along their entire length to the elements; it is adequate to secure them merely with seals near each end of the bar. Passages 131, 132, 133, 134 etc., and similar passages (not numbered) adjacent the opposite side of the spacer bars, thus formed carry the second fluid, which is to exchange heat with the first fluid. The spacer bars are disposed in a direction substantially perpendicular to the direction in which the first fluid flows in the channels within the elements, thus serving to guide the flow of the second fluid in a direction substantially perpendicular to the direction of flow of the first fluid. Two hollow cylindrical fittings are joined to sheet 151, a first fitting 121 surrounding the first opening in sheet 151 and a second fitting 122 surrounding the second opening in sheet 151. The fittings serve as means for connecting pipes, tubes, hoses or other ducts to the heat exchanger, so as to permit introduction and removal of the first fluid into and from the heat exchanger elements.
Taken together, fitting 121 and rings 106, 107, 108, etc. constitute a discontinuous duct and serve as means to distribute the first fluid into the space inside of all the heat exchange elements 101, 102, 103 etc. Similarly, taken together, fitting 122 and rings 111, 112, 113 etc. constitute a discontinuous duct and serve as means to collect the first fluid from the space inside of all the heat exchange elements 101, 102, 103 etc.
It should be understood that a heat exchanger may comprise as few as one heat exchange element to as many such elements as may be desired for a particular use, which may number in the hundreds.
Rings 106-110 and 111-115 are suitably circular in shape. They can be joined to sheets 152, 150 and other like sheets by heat sealing or with a suitable adhesive, preferably by heat sealing. A preferred method of joining the members is by a technique variously termed as electromagnetic bonding or magnetic heat-sealing, wherein a composition comprising a suitable thermoplastic resin such as polyethylene and a magnetic material such as iron, steel, iron oxide or a ferrite in the form of micron or submicron size particles is applied at all places where a sealed joint is to be formed, and then the assembly is placed in a high frequency magnetic field of an electric induction generator, whereby said composition heats and forms a secure bond to the thermoplastic members which it contacts. Such sealing compositions are known in the art and are commercially available in numerous forms including molded and extruded shapes such as films and gaskets, liguid or paste compositions in aqueous or solvent binder systems, and hot melts. Information concerning the technique can be found in the Modern Plastics Enc^yclo- pedia, 1977-78 Edition, McGraw-Hill, N.Y., October 1977, pages 420-21, "Electromagnetic Bonding", and pages 424-25, "Magnetic Heat-Sealing". More information concerning the technique,, and typical compositions suitable for making such bonds is found, for example, in U.S. Patents No. 3,620,375; 3,520,876; 3,461,014, and 3,779,564.
One method, which is a preferred method, of forming the distribution and collection systems for the first fluid to and from the inside of the heat exchange elements is to seal the rings 106, 107 etc. and 111, 112 etc. to sheets 152, 150, etc. before the first and second openings typified by openings 169 and 170 have been formed. That is, each heat exchange element is first fabricated without any first or second openings in it; rings 106 etc and 111 etc. are then sealed at sites where openings 169 and 170 and corresponding openings in the remaining sheets are to be formed. Fittings 121 and 122 are also sealed to sheet 151 at sites where the openings in sheet 151 are to be formed. The openings are then cut by inserting a tubular cutter tnrough the assembly inside of each series of coaxial rings. The openings can be cut through all the sheets except the second sheet of the last heat exchange element; alternatively, the openings can be cut through both sheets of all the elements including the last element, following which the two openings in the second sheet of the last element are sealed shut with a thermoplastic film, disk, or cup-shaped member. If cup-shaped members are employed, they can be sealed in place before the openings are cut, i.e., at the same time that the rings and fittings are sealed in place.
Protuberances 168 suitably can be of circular cross-section, or they can be any other shape desired, the cross-section being, for example, triangular, oval, streamlined, rectangular, etc. The protuberances need not be of uniform cross-section, and can be tapered, provided with chamfer, etc., as desired.
The arrangement of protuberances can be staggered or in line, und can be ordered on triangular centers, on rectangular or square centers or in any pattern desired, or it can be randon.
The amount of heat exchange area in such a heat exchanger, and its overall dimensions, will vary greatly depending on the type and flow rate of the fluids between which heat is to be exchanged, the heat transfer coefficient of the particular system, and the amount of heat to be exchanged. The active heat exchange area may be only a few square feet, or as much as thousands or tens of thousands of square feet. The two long dimensions of individual heat exchange elements can range from a few inches to many feet; and one long dimension can be 100 feet or more. Spacer bars, when used, will ordinarily be placed at distances of 2 to 6 inches from one another. The heat exchange may have only a few heat exchange elements or as many as hundreds of elements.
Such a heat exchanger suitable for an air-water heat exchange wherein heat from water at 120 to 185°F is tranferred to air at a rate of up to 60,000 Btu per hour is made of high density polyethylene. The elements are 2 feet long by 1 foot high, and have a layout as shown in Fig. 6. Sheets 151 and 152 are 3 mils thick. The spacing between the sheets is 34 mils. The edge septa are 34 mils thick and 0.1 inch wide. The channel septa are 34 mils thick, 8 mils wide, and traverse 22 inches of the sheets starting alternately from opposite ends of the sheets, so that the channels are connected end-to-end to provide a serpentine flow path for the first fluid which in water. The protuberances 168 are circular in cross-section with a 35 mil diameter, are 34 mils long, are joined to both sheets 151 and 152, having been formed upon extrusion of one sheet and subsequently heat-sealed to the second sheet, and are arranged triangularly on 0.125 inch centers. The overall thickness of the element is 40 mils. The six channels are each approximately 2 inches wide.
The heat exchanger has 125 such heat exchange elements. The elements are joined together by two series of polyethylene rings 106, 107 etc. and 111, 112 etc, cath ring ceing 0.1 inch thick and having an outside liameter of 0.75 inch and an inside diameter of 0.5 inch, the two series being placed at adjacent corners of the elements, one coaxial series at the beginning of the first flow channel and the other coaxial series at the end of the last flow channel. Two fittings 121 and 122, each fitting being 1.5 inches long and having an outside diameter of 0.75 inch and an inside diameter of 0.5 inch, are joined to the first sheet of the first element, one fitting being placed coaxially with each series of rings. The rings and fittings are joined to the elements by sealing with the aid of an inductively heatable composition comprising polyethylene and magnetic iron oxide. The first openings typified by opening 169 and second openings typified by opening 170 are then cut by inserting a tubular cutter 0.5 inch in diameter through the whole assembly, once within the first series of coaxial rings and cne within the second series of coaxial rings. The first and second openings cut through the second sheet of the last (125th) heat exchange element are then closed by sealing over each opening a polyethylene disk 0.1 inch thick and 0.75 inch in diameter, using the same sealing technique described above. Spacer bars 12 inches long, 0.1 inch thick and 0.1 inch wide are inserted between adjacent elements, disposed parallel to edge septa 153 and 155, i.e., in a direction generally perpendicular to the channel septa and the direction of fluid flow in the channels, and are joined to both adjacent elements near each end of each bar by the sealing technique described above. Five bars are placed between each adjacent pair of sheets, on centers approximately 4.8 inctes apart, one bar being placed near the edges of the elements adjacent edge septum 155 and corresponding edge septa of the remaining elements. Thus passages cetween adjacent elements are formed for the second fiuid, which is air, to flow in said passages in a direction parallel to the spacer bars. As noted above, the planar dimensions of the elements ire 1 foct by 2 feet. The thickness of the heat exchanger through the 125 elements and 124 passages which separate the elements is approximately 17.₅ inches.
In use, the heat exchanger is mounted in housing 13 which fits closely around four sides of the heat exchanger and which provides chambers or plenums for distributing cool air into the passages between the elements and for collecting hot air as it exits from the passages. The four sides around which the housing fits closely are (1) the side adjacent edge septum 153 and corresponding edge septa of the remaining elements, (2) the side adjacent edge septum 155 and corresponding edge septa of the remaining elements, (3) the side adjacent the first sheet of the first element, i.e., sheet 151, and (4) the side adjacent the second sheet of the last element, the second sheet being the sheet facing away from the penultimate element.
The heat exchanger of the invention will operate efficiently when the direction of flow within the elements is such that the channel first fed with the firsi fluid,water in the above example, is adjacent the downstream end of the passages which carry the second fluid, air in this example.
Further information concerning such heat exchangers and related modifications of such heat exchangers can be found in copending application No. 20301414 1 filed on equal date in our name, the entire disclosure of which is incorporated by reference.
Another type of thermoplastic heat exchanger which can be used in the heater of the invention, and which has narrow channels in which the first fluid, water, flows is described in U.S. 4,069,807, the disclosure of which is incorporated by reference.
The thermoplastic resin used for fabrication of such heat exchangers should have a softening point about 220°F or above and be reasonably rigid. A modulus of elasticity in flex of greater than 10,000 psi provides adequate rigidity. The preferred thermoplastic resins are high density polyethylene, isotactic polypropylene, acrylonitrile-butadiene-styrene resins, styrene-acrylonitrile copolymers, and fluorocarbon resins such as fluorinated ethylene-propylene copolymers and p_olyt_et_ra- fluoroethylene. High density polyethylene and isotactic polypropylene are especially preferred because of their low cost. The invention, however, is not limited as to materials of construction. For example, heat exchangers of known construction fabricated of anodized aluminum foil or copper plated steel can be used if desired.
The heater and process of the present invention offer numerous advantages, foremost of which is operation at efficiencies up to about 98%. The invention substantially reduces heat loss up the chimney, because waste gases are exhausted at a temperature of about 75° to 100°F, usually about 90°F, as compared with an exhaust gas temperature of about 400°F to 600°F in a conventional heater, and because heat is recovered from the gaseous water of combustion by condensation of the water vapor to liquid water. As a result of this more efficient and almost complete recovery of heat from the combustion products, the fuel requirement is reduced to an amount of about 50 to 60% of that required with a conventional heater. Conversion of residential heaters to those of the invention would result in saving tremendous quantities of oil and natural gas. Furthermore, conventional gas heaters operate in such manner that some carbon monoxide and/or soot are formed, two disadvantages of which are that carbon monoxide is highly toxic, and incomplete combustion signifies some loss of the full potential fuel value; in contast thereto, the present invention is hignly officiant even when using excess air, for example from 10% excess to as much as 200% excess air over that which is stoichicmetrically required for complete combustion, under which conditions the formation of toxic carbon monoxide is substantially avoided and the full potential fuel value is realized. Additionally, in new house or building construction, the chimney and its associated cost can be eliminated; in place thereof a simple duct to an outside wall or the roof, or a connection to the plumbing stack, is fully sufficient. The ability to retrofit a conventional heater in existing houses and buildings, as described above, is also an advantage.

Claims

1. A heater comprising a burner for producing hot combustion products, contacting means adapted for countercurrently contacting said combustion products with water, said contacting means being capable of operation at at least 1.2 theoretical stages, a blower adapted to urge said combustion products from said burner through said contacting means and to urge the waste gases of said combustion products to exhaust means, and a heat exchanger which is adapted to receive said water heated by said combustion products and to exchange the heat from said water to a fluid, the product of the heat exchange area in said heat exchanger and the heat transfer coefficients of said fluid for which the heater is adapted being in the range from 20 to 500 per 1000 Btu/hour of fuel burned.

2. The heater of Claim 1 wherein there is a pump adapted to urge said water received from an exit from said heat exchanger to the upper end of said contacting means. or Claim 2

3. The heater of Claim 1/wherein there is a pump adapted to urge said water received from said contacting means to said heat exchanger, and means to return said water received from an exit from said heat exchanger to the upper end of said contacting means.

4. The heater of any one of Claims 1 to 3 wherein said contacting means is capable of operation at 3 to 5 theoretical stages. anyone of to 4

5. The heater of/Claim_{s 1/}wherein said heater is adapted for flow of said combustion products upward and said water downward through said contacting means.

6. The heater of any one of Claims 1 to 5 wherein said contacting means and said heat exchanger are adapted to convey said water in an amount of 7 to 50 pounds of water per 1000 _Bt_u per hour of burner capacity.

anyone of to 6 7. The heater of/Claim_s 1/wherein said heat exchanger is made from a thermoplastic resin and has from 0.2 to 30 square feet of heat exchanger area per 1000 Btu per hour of burner capacity, and there are provided in said heat exchanger first passages adapted to conduct said water, and adjacent thereto, second passages adapted to conduct said fluid in a direction substantially perpendicular to the direction in which said water is conducted.

8. The heater of Claim 7 wherein said first passages are arranged in a plurality of stages, the stage first fed with said water being adjacent the downstream end of said second passages. Claim ₇ or

9. The heater of/Claim 8 adapted for use when said fluid is a gas, wherein said heat exchanger has from 3 to 30 square feet of heat exchange area per 1000 Btu per hour of burner capacity.

10. The heater of Claim 9 wherein said heat exchanger has a heat exchange area of 400 to 1000 square feet, and said gas is air. Claim 7 or

11. The heater of/Claim 8 adapted for use when said fluid is a liquid, wherein said heat exchanger has from 0.2 to 10 square feet of heat exchange area per 1000 Btu of burner capacity.

12. The heater of Claim 11 wherein said heat exchanger has a heat exchange area of 50 to 200 square feet, and said second liquid is water or a water-glycol mixture.

13. A process of heating a fluid comprising (a) burning a fuel to produce combustion products, (b) counter currently contacting in at least 1.2 theoretical stages said combustion products with water to raise the temperature of said water to a temperature of from about 120°F to about 185°F, and (c) passing said water through a heat exchanger to heat said fluid.

14. The process of Claim 13 wherein said water is employed in step (b) in 3 to 5 theoretical stages at a rate of 7 to 50 pounds of water per 1000 Btu of heat to be removed from said combustion products, and is recirculated from step (c) back to step (b).

27 Claim 13 or 15. The process of/Claim 14 wherein said water is introduced in step (b) at a temperature of from about 70°F to about 115°F, and the waste gases of said combustion products are exhausted from step (b) at a temperature of from about 75°F to about 100°F. to 15

16. The process of any one of Claims 13 to 15 wherein said heat exchanger is made of a thermoplastic resin and is operated in a plurality of stages with the flow of said fluid substantially perpendicular to the flow of said water.

any one of to 16 17. The process of/Claims ₁3/wherein said water instep (b) is employed at a rate of 10 to 15 pounds per 1000 Btu, and said fluid is forced air.

anyone of to 16 18. The process of/Claims 13/wherein said water in step (b) is employed at a rate of 8 to 12 pounds per 1000 Btu, and said fluid is water or a water-glycol mixture.