US20010004009A1 - Drainwater heat recovery system - Google Patents

Drainwater heat recovery system Download PDF

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Publication number
US20010004009A1
US20010004009A1 US09/738,112 US73811200A US2001004009A1 US 20010004009 A1 US20010004009 A1 US 20010004009A1 US 73811200 A US73811200 A US 73811200A US 2001004009 A1 US2001004009 A1 US 2001004009A1
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Prior art keywords
heat exchanger
heat
water
drainwater
convection
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Abandoned
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US09/738,112
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Winston MacKelvie
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Mackelvie Winston
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Application filed by Mackelvie Winston filed Critical Mackelvie Winston
Priority to US09/738,112 priority patent/US20010004009A1/en
Publication of US20010004009A1 publication Critical patent/US20010004009A1/en
Priority claimed from US10/156,281 external-priority patent/US6722421B2/en
Application status is Abandoned legal-status Critical

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    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28HEAT EXCHANGE IN GENERAL
    • F28DHEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D7/00Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall
    • F28D7/0066Multi-circuit heat-exchangers, e.g. integrating different heat exchange sections in the same unit or heat-exchangers for more than two fluids
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24DDOMESTIC- OR SPACE-HEATING SYSTEMS, e.g. CENTRAL HEATING SYSTEMS; DOMESTIC HOT-WATER SUPPLY SYSTEMS; ELEMENTS OR COMPONENTS THEREFOR
    • F24D17/00Domestic hot-water supply systems
    • F24D17/0005Domestic hot-water supply systems using recuperation of waste heat
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28HEAT EXCHANGE IN GENERAL
    • F28DHEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D20/00Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00
    • F28D20/0034Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00 using liquid heat storage material
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28HEAT EXCHANGE IN GENERAL
    • F28DHEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D21/00Heat-exchange apparatus not covered by any of the groups F28D1/00 - F28D20/00
    • F28D21/0001Recuperative heat exchangers
    • F28D21/0012Recuperative heat exchangers the heat being recuperated from waste water or from condensates
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B30/00Energy efficient heating, ventilation or air conditioning [HVAC]
    • Y02B30/08Energy efficient heating, ventilation or air conditioning [HVAC] relating to domestic heating, space heating, domestic hot water heating [DHW] or supply systems
    • Y02B30/18Domestic hot-water supply systems using recuperated or waste heat
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B30/00Energy efficient heating, ventilation or air conditioning [HVAC]
    • Y02B30/50Systems profiting of external or internal conditions
    • Y02B30/56Heat recovery units
    • Y02B30/566Water to water
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00Enabling technologies or technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10Energy storage
    • Y02E60/14Thermal storage
    • Y02E60/142Sensible heat storage

Abstract

A fluid-to-fluid heat exchanger for use where two fluid streams may be of indeterminate composition, temperature, and flow rate, and, where the two fluid streams may flow at different rates and at different times. Such conditions are found, for example, in the flow patterns of a building's hotwater, cold feed water for a water heater, and drainwater. As defined for heat recovery from drainwater, the present invention comprises a first, central, straight-through heat exchanger tube for cooling flowing drainwater, a second heat exchanger to heat cold water encircling and spaced from the first, and a non-pressurized reservoir between first and second heat exchangers permanently filled with water in thermal contact with first and second heat exchangers. Submerged in the reservoir water there is at least one insulated convection chamber, of small volume, enclosing first heat exchanger with a convection opening uppermost. Opening allows upward convection and therefore heat transfer when drainwater heats convection chamber water making it lighter. Convection and heat transfer ceases when drainwater cools convection chamber water making it heavier, blanketing drainwater heat exchanger in a small volume of cold water. This one-way heat transfer prevents heat loss from remaining reservoir water to cold drainwater. With convection chambers inverted to have convection opening bottommost, the device serves to make coldwater, for drinking, as cold as possible by transferring unwanted heat to colder drainwater. Used in series, heat recovery and fresh water cooling can both be accomplished. Units for horizontal and vertical installation are disclosed.

Description

  • The present application is a continuation-in-part of application Ser. No. 09/237,611, filed Jan. 25, 1999. [0001]
  • BACKGROUND
  • Water heaters are well known to consume vast amounts of energy to heat cold water to make it hot for human use in washing and cleaning, and for industrial processes. The resulting hot drainwater (also referred to as wastewater) flows freely to the sewer taking with it all of that heat energy. Generation of energy to heat water releases pollutants including those that cause global warming. [0002]
  • Although it would seem obvious to use heat in drainwater to heat new cold water, thereby reducing energy use and saving money, this seemingly simple heat transfer idea has resisted successful solution in spite of many inventors having tried over a very long time. [0003]
  • It is, therefore, the objective of the present invention to provide a heat exchanger apparatus to remove heat from flowing drainwater, to store that heat within that apparatus, and to limit heat loss of that stored heat to cold drainwater that may flow thereafter. [0004]
  • Another objective is to cool drinking water using cold drainwater. [0005]
  • DESCRIPTION
  • By way of review, the water heater in a building has a continuous cold water pressure feed to it. When a hot water faucet or valve (hereinafter referred to as ‘valve’) is opened, the flow of hot water at the valve reduces pressure and allows cold water to instantaneously flow into the water heater, displacing the hot water out of the valve. Thus when hot water is used, cold water flows at exactly the same time and at exactly the same rate of flow. [0006]
  • Drainwater heat recovery involves removing heat from hotter drainwater (cooling it) and transferring the heat to the fresh cold water (warming it). This saves energy and money since no new energy is required. [0007]
  • U.S. Pat. No. 4,619,311, to Vasile, describes a drainwater heat recovery system comprising a copper drainpipe heat exchanger whose exterior is wrapped with a copper coil heat exchanger through which passes cold water to be pre-heated. This type of tube-on-tube heat exchanger has been long-available, such as that sold by the Solar Research in Brighton, Mich. 48116, as part number 5832. These devices use conductive heat transfer which is necessarily a two-way process because the two heat exchangers are in direct physical contact. U.S. Pat. No. 4,619,311, to Vasile is a simple, low-cost, easy-to-install heat exchanger. However, it transfers heat only when both drainwater and cold water are flowing simultaneously therethrough. This special flow condition, referred to as ‘continuous use’, occurs when showering. [0008]
  • However the other major use of hot water in a building, referred to as ‘batch use’, occurs when appliances or fixtures, such as a washing machines or sinks, fill with hot water, operate, and then, later, drain. In more detail, in batch use, when filling with hot water, cold water is flowing through the heat exchanger but there is no drainwater flow, so no heat is transferred with tube-on-tube heat exchanger and the cold water is not warmed before it enters the water heater. Then, when the wash machine drains, there is hot drainwater flow but no cold water flow (there being no hot water being used at that time) and so, again, no heat is transferred and the hot drainwater leaves the building uncooled. [0009]
  • The reason tube-on-tube heat exchangers do not work under batch hot water use is that their only heat storage is the exterior cold water coil which will transfer it's heat back to a cold drainwater flow. Lacking heat storage means that tube-on-tube heat exchanger can only recover heat from approximately half of the total drainwater available for heat recovery. This limits cost effectiveness of this important energy conservation device. A seemingly obvious solution to this is to enclose the entire heat exchanger in a reservoir of water to store heat. However, cold drainwater would again simple cool the reservoir by conductive heat transfer with no net gain. [0010]
  • Further, U.S. Pat. No. 4,619,311, to Vasile is not recommended for horizontal drainpipe applications, as found in a great many buildings with no basement, because the design requires a generally circular drainpipe upon which to wind the outer coil. Further the design cannot have exterior wall finning on the drainwater heat exchanger also due to the outer coil being wrapped against the exterior wall. This severely limits heat transfer and so cost effectiveness. Moreover this design cannot use twisted tube for the drainwater heat exchanger which may add useful heat transfer. [0011]
  • U.S. Pat. No. 5,736,059 to the present applicant, does teach of a drainwater heat recovery system with no-loss heat storage. However, for low volume hot water users, such as in homes, the system tends to be too large and, with its numerous components, too expensive. Further, its installation is essentially limited to vertical drainpipes unless mechanical pumping is added. [0012]
  • The object of the present invention is to provide a drainwater heat exchanger which solves all of the aforementioned problems by providing no-loss heat storage and low cost construction/installation. [0013]
  • A review of the physical principles involved in the present invention follows. [0014]
  • Firstly, heat is transferred by conduction, convection, and radiation. When a fluid such as water is adjacent a surface which is heated or cooled, heat is conducted to or from the water. [0015]
  • Secondly, when a fluid such as a body of water is heated or cooled, its density changes. When heat is added or removed at a particular region in a body of water the water adjacent the hotter or colder surface becomes more more less heavy, dense, or buoyant, compared to adjacent water. This added buoyancy causes the water to move vertically by convection whereupon the temperature affected water will convect to a vertical position within the body of water where it becomes a horizontal layer or stratum parallel to all other strata and parallel to the earth's surface. The hotter water will occupy the highest stratum and the coldest water will occupy the lowest stratum with all other strata in between being determined by relative temperature. This stratification is a natural phenomena and cannot be avoided save by agitating, mixing, or stirring the body of stratified water (or fluid). Thus heat is first transferred by conduction which thereafter causes convection. [0016]
  • Thirdly, fluid flow in a pipe tube or duct (all referred to as conduit) has components of flow that are called boundary layer, laminar layer, and central or main flow. The boundary layer is that thin, immobile layer of fluid that clings to the wall of the conduit and through which conductive heat transfer occurs first. The laminar layer is a slow moving thin layer between the boundary and central flow and is where conduction also takes place and where convective flow begins. [0017]
  • Fourthly, in a vertical conduit, liquid flow is principally adjacent the conduit wall with no flow down the hollow center. Capillary action and air motion diverts the liquid to the wall where it clings, spreads, and flows downwards as a relatively thin falling film. [0018]
  • Fifthly, by adding protrusions to a conduit wall, heat transfer can be further improved due to the turbulent mixing of the three flow regions. Such turbulence inducing protrusions may take the form of dimples or ridges on the conduit wall. [0019]
  • The present invention prevents unwanted conductive heat transfer of recovered heat to a cold drainwater flow by adding an intermediate convection section between the drainwater and freshwater heat exchangers. The convection section is a tubular reservoir which encloses the drainwater heat exchanger, is filled once with water, and is encircled by the freshwater heat exchanger. Within the water-filled reservoir there is also at least one convection chamber made of an insulative material that fills with reservoir water. It/they holds a small but sufficient volume of reservoir water to completely submerge the drainwater heat exchanger. The convection chamber's opening is in its upper portion while its lower portion is leak-proof. The small volume of water contained in the convection chamber exchanges heat with the drainwater heat exchanger by direct thermal conduction. However since the convection chamber is open at the top and insulated all around, it can only exchange heat with the reservoir water by convection. Since the convection chamber is leak proof and has only an upper opening, heat from warmer drainwater drives an upward convection into the reservoir, thereby effecting the desired drainwater heat recovery. Cooled convection chamber water, from a cold drainwater flow, is made heavier and so remains immobile within the convection chamber isolating the drainwater heat exchanger and thereby preventing the surrounding warmer reservoir water (and freshwater coil) from losing its heat to the cold drainwater. Thus the objective of no-loss heat storage is achieved at low cost in both horizontal and vertical units. [0020]
  • The drainwater heat exchanger of the present invention generally comprises at least one straight section of drainpipe that uses thin film heat transfer and has a central heat transfer portion located within the water-filled reservoir. End portions extend out of the reservoir for connecting inline to a building's drainpipe. The diameter of the drainpipe heat exchanger depends on drainwater composition and flow. For use with toilet flows, the drainpipe is generally a minimum of 3 inches in diameter and in this application the drainwater heat exchanger would be that diameter, or larger, and generally a straight through tube. For applications where there is contained in the drainwater smaller solids than the toilet, the diameter may be reduced, there being no lower limit. In an application, for example, where the present invention is to be installed within the cabinet of a dishwasher, the drainpipe heat exchanger may be 1 or 2 inches in diameter or even less. It is also within the scope of the present invention to have other than a straight through drainwater heat exchanger where fouling is not a problem such as in a laundry washing application. Here the drainwater heat exchanger may be coiled or have several parallel straight tubes or be made of a twisted tube. As well, oval or rounded rectangle shapes provide larger surface area for horizontally flowing drainwater. Any shape may be used consistent with fabricating a suitable convection chamber to work with that shape. For lowest cost, the may be made from a thin plastic film extruded tube or tube welded from sheet film, and backed by a thin seamed-metal tube on its exterior. A double drainwater heat exchanger tube of thin plastic film could also be used for added security. [0021]
  • In the present invention the convection chamber is/are located within the reservoir and may be one or more in number. The convective opening is arranged such that a horizontal plane through the lowest point of the opening, lies at least marginally above the highest point of the that portion of the exposed drainwater heat exchanger wall served by the respective convection chamber. In this way, the entire drainwater heat exchanger is fully submerged in the water contained in the convection chamber(s), and, the convection chamber cannot overfill with cold water. A convection chamber may be made entirely of plastic or if made of metal (to act as a heat transfer fin) it must have an insulated exterior surface. A convection chamber should hold as small a volume of reservoir water as possible, much smaller than the combined volumes of the reservoir water plus that water filling the cold water heat exchanger tubing. This volume, however, must not be so small as to restrict convection speed. Fins attached to the drainwater heat exchanger outer wall may advantageously take up volume in the convection chamber, reducing water volume, and add heat transfer performance. [0022]
  • The convection chamber takes on two distinct shapes depending on whether the heat exchanger is to be used horizontally or vertically. [0023]
  • For the horizontal embodiment, the convection chamber may be a long, channel-shaped trough which may advantageously be made of metal such as copper and attached directly (i.e., soldered) to the bottom of a copper drainwater heat exchanger to enhance heat transfer. Several such metal channels may be nested to further enhance heat transfer. The outside of the outermost convection chamber channel is covered with an insulating skin. It is closed and sealed at the ends to ensure that when cooled (heavier) water is contained therein it does not leak into the warmer reservoir cooling same. Because drainpipes are necessarily sloped downwards for flow, the entire drainwater heat recovery device may also be sloped. However cold water is created up to the level of the highest point of thermal conduction from the drainwater heat exchanger. This level, the cold water line, must be entirely within the convection chamber. If designed for dead horizontal use but then sloped to match the building's drainpipe, the cold water would flow down to the low end of the convection chamber and continually overflow the walls of the convection chamber defeating the no heat-loss objective. Therefore the sloped drainwater heat exchanger may have spaced fins along its horizontal length that act as bulkheads or sealing partitions, dividing the convection chamber into short separate sections where the cold water level in each segment will remain within a minimum overall convection chamber volume. For this arrangement, the outer insulation layer will need to wrap around the top of the convection chamber and down the inside wall so that no conductive heat transfer can take place anywhere above the cold water line. Alternatively the convection chamber walls may merely be made higher (taller) to enclose horizontal pooling of any cold water in the convection chamber(s). This alternative however, adds unwanted volume to the convection chamber. Also for this arrangement, the outer insulation layer will need to wrap around the top of the convection chamber and down the inside wall, at least at the high end, so that conductive heat transfer cannot take place anywhere above the cold water line. [0024]
  • The drainwater heat exchanger may be of any shape suited to the task including round, oval, twisted tube, multiple parallel, and labyrinth, as long as a suitable convection chamber can be constructed to enclose same and not hold too much volume. The opening in the convection chamber may be designed to enhance fluid flow having, for example, it may have a nozzle-shaped slit with a gentle upward and outward flare to ensure rising convection currents are not slowed by edges. [0025]
  • For the vertical embodiment the convection chambers are generally several in number and take the form of tapered cups with holes in their bottoms for them to slip and seal to the drainpipe heat exchanger. These cups, being open at their tops, are arranged to slightly nest one into the next such that, no horizontal strata of reservoir water can contact any exposed wall of the drainwater heat exchanger. Sufficient numbers of nested cups are used such that they extend the length of the drainwater heat exchanger and are submerged in the reservoir water. A short, bottommost portion of the drainpipe heat exchanger may be left plain (no convection chamber) as the coldest water will naturally collect there and so heat transfer to a cold drainpipe will be minimal. This would be done mainly in the interests of ease of assembly. If a metal convection chamber is used to enhance heat transfer, then the outside of the convection chamber must be covered with an insulating skin to prevent conductive heat transfer through it's wall. [0026]
  • The reservoir of the present invention is a water-filled, generally tubular container of any cross-sectional shape suitable for enclosing the drainwater heat exchanger and having a volume for the required heat capacity, where, size and weight aside, the larger the volume the better. The reservoir need not be pressurized. Depending on whether the cold water heat exchanger is installed inside or outside of this reservoir, the reservoir may be made from a several different materials. If the cold water heat exchanger is wrapped about the exterior of the reservoir and high heat transfer rates are desirable with cost being a secondary consideration, then the reservoir may be made from a highly thermally conductive metal, such as copper. For similar performance at a lower cost, the reservoir may be made of thin plastic membrane such as vinyl or polyethylene film welded into a ‘bag’ shape. This bag would be supported against the weight of contained water by the exterior cold water heat exchanger coil. The reservoir is sealed by clamp means to the tube ends of the drainwater heat exchanger. If the cold water heat exchanger is to be installed within the reservoir then a thick walled plastic tube may be used for the reservoir. Various combinations of these materials may be used for the reservoir. For example a membrane bag with an exterior thin metal foil for enhanced heat transfer at moderate cost increase. A metal insert (i.e., a sheet of metal) within the reservoir will allow temperature to even out within the reservoir. [0027]
  • The cold water heat exchanger of the present invention may be submerged inside the reservoir or wrapped in conductive contact around the exterior of the reservoir. The tubing used can advantageously be as large a diameter as practical in order to hold more volume of water for more heat storage. Heat is better stored in the cold water coil as it is then instantly available for delivery to the water heater. Cold water tubing may also be of a plastic material (for lower cost) since there are often long periods of time (i.e., overnight) that pass between a drainwater heat recovery event and a hot water use event. Such plastic tubing may be readily extruded to have a square cross section to increase heat transfer surface area in contact with the reservoir wall. Long time periods overcome the poor heat transfer coefficient of plastic. In addition, in a preferred embodiment, both a metal and a plastic coil may be co-jointly used being plumbed together in series or parallel and wrapped about the exterior of the reservoir wall. This dual material cold water heat exchanger arrangement will allow both fast and slow heat transfer to be utilized at the best possible cost-performance ratio. The plastic tube (low cost) would be wrapped outside of the copper coil (expensive) so that the water in the copper coil would heat first and fastest and conduct heat to the plastic. [0028]
  • Moving now to a description of operation. Any hotter drainwater flowing at any time (from either continuous and batch hot water use) heats the water in the convection chamber(s) by conductive heat transfer. This makes that water more buoyant which causes it to naturally convect upwards out of the convection chamber and become heated reservoir water. The main reservoir water being less buoyant (heavier), naturally convects downwards into the convection chamber for heating. This convection continues for as long as there is a temperature differential between the drainpipe heat exchanger and the reservoir water. The main reservoir water therefore becomes warmer as it stores more and more recovered drainwater heat energy. The cold water heat exchanger and the water it contains, are, of course, heated at this same time by conductive heat transfer from the warm reservoir water. If and when colder drainwater flows, convection chamber water is cooled first, becoming less buoyant (heavier) than the surrounding warmer reservoir water. The cooled convection chamber water therefore remains within the convection chamber and convective heat transfer with the reservoir water ceases. This prevents stored heat in the reservoir from being transferred to cold drainwater thereby achieving the objective of one-way-only heat transfer. The entire drainwater heat recovery device may be enclosed in insulation and protective jacket. [0029]
  • The present invention finally solves the problem of drainwater heat recovery from a building's entire drainwater supply by providing no-loss heat storage in a simple, low cost design, and with widespread installation potential. [0030]
  • The present invention in another embodiment may also be used for water cooling (i.e., cooling drinking water) by inverting the device such that convection chamber(s) have downwards facing opening. In such an orientation, the hot drainwater would just fill the insulated convection chamber(s) with more buoyant heated water preventing convection and thereby prevent heating the reservoir water. When drainwater is colder, convection would be downwards cooling the reservoir water, as intended. This of course, would cool water flowing through the fresh water heat exchanger. [0031]
  • With one of each embodiment of the present invention installed in-series, drainwater heat recovery and drainwater fresh water cooling may both be accomplished. [0032]
  • In the present invention three walls of separation exist between drainwater and fresh water which ensures absolute safety from contamination of fresh water, (drainwater heat exchanger wall, reservoir wall, fresh water heat exchanger wall). In addition the no-pressurized reservoir adds even more safety. [0033]
  • Additional details. To reduce fouling of the drainwater heat exchanger and to increase rate of heat transfer, dimpling of the exterior of the drainpipe heat exchanger may be used as disclosed in this applicant's U.S. Pat. No. 5,736,059, mentioned above, where high velocity punches or projectiles (such as fired shot) are applied to at least a portion of the drainpipe heat exchanger. Masks prevent dimpling in the portions where the convection chamber seals to the drainpipe. Other enhancements can be devised to create turbulence, including: rolled ridges, twists, fins, bubbled air, and vibration, and ultrasonics. [0034]
  • The present invention may be used in various combinations such as: more than one unit plumbed in series, or in parallel, in series-parallel, and where vertical and horizontal embodiments are combined. In addition, one or more miniature systems may be integrated into the cabinetry of sinks or dish- and laundry washing machines and used with tankless or instantaneous water heaters. [0035]
  • Further, it should be remembered that all indoor plumbing fixtures are heated by ambient air and so even cold water used at the fixture is warmed as it flows over warm fixture and drainpipe surfaces. When the drainwater heat is recycled by the present heat exchanger invention, this warm drainwater can provide fresh warm water with no need for a traditional hot water supply. The pre-heated fresh water provided may be plumbed directly to the fixture's faucet, providing warm water at no energy cost, the heat being repeatedly recycled from the drainwater to the fresh water. [0036]
  • Moreover the reservoir of the present invention could be pressurized with fresh water, thus avoiding the cost for the second cold water heat exchanger. This heated water could then be used as feed water to a fixture, or, used for toilet flush where the heated water would reduce condensation on the exterior of the tank and resultant dripping onto the floor. This dripping is known to cause structural damage, and to support fungus/mould growth which results in dangerous airborne spores in the building. [0037]
  • In yet another embodiment, a thermostatically controlled low wattage heater may be provided within the reservoir to maintain a minimum temperature for use at the site. [0038]
  • BRIEF EXPLANATION OF THE DRAWINGS
  • FIG. 1 shows a cross section of one horizontal embodiment where the coldwater heat exchanger is a series of straight tubes, the convection chamber is tubular, and lower water inlet ports are flap controlled; [0039]
  • FIG. 2 is a perspective detail view of one end of the same embodiment; [0040]
  • FIG. 3, 4, [0041] 5, 6 are end views of the horizontal embodiment showing different shapes and positions of components, so to best use minimal vertical space in the installation;
  • FIG. 7 is a partial section view of a vertical embodiment; [0042]
  • FIG. 8 is a cross section top view taken at line [0043] 8-8 of same embodiment and including heat conducting element next to reservoir interior wall;
  • FIG. 9 is a transparent view of one embodiment of a convection chamber for the vertical embodiment showing the hole in the convection chamber bottom which allows it to slide on and seal to the heat exchanger and, a single fin for heat transfer therein; [0044]
  • FIG. 9[0045] b shows a fin ring on drainwater heat exchanger within a convection chamber;
  • FIG. 10 shows a double drainwater heat exchanger vertical embodiment; [0046]
  • FIG. 11 shows a double vertical embodiment in series arrangement where the top unit shows the volume of convection chamber water only for clarity and with internal coldwater heat exchanger while the bottom unit shows the full reservoir with coldwater heat exchanger removed for clarity; [0047]
  • FIG. 12 shows an end cross section of a horizontal embodiment with slit tune convection chamber spread and sealed to drainwater heat exchange which leaves the lower portion exposed to the reservoir fluid for heat transfer; [0048]
  • FIG. 12[0049] a shows the same embodiment with end caps, reservoir body and coiled cold water heat exchanger therein, but with drainwater heat exchanger and convection chamber removed for clarity;
  • FIG. 13 shows a perspective of the embodiment of FIG. 12 with convection chamber horizontal on a sloped drainwater heat exchanger; [0050]
  • FIG. 14 shows a horizontal embodiment where the drainwater heat exchanger comprises several smaller drainwater heat exchanger tubes for use where no large solids are present in the drainwater; [0051]
  • FIG. 15 shows a partial phantom view of a vertical embodiment with internal coldwater heat exchanger; [0052]
  • FIG. 16 shows a partial phantom view of a vertical embodiment where the coldwater heat exchanger is coiled around the exterior of the reservoir wall and deflectors against the reservoir wall direct cooled reservoir water to the convection chambers; [0053]
  • FIG. 17 shows a partial phantom view of an embodiment where the reservoir wall is grove-threaded to improve heat transfer with the exterior coldwater heat exchanger; [0054]
  • FIG. 18 shows a conical element that deflects cooler, convecting reservoir interior wall water, as it descends, into convection chamber elements; the embodiment of FIG. 16; [0055]
  • FIG. 20 shows a cross section view of a preferred horizontal embodiment with the square tube coldwater heat exchanger coiled around the exterior of the metal reservoir wall, and showing a drainwater heat exchanger having partitioning fin-separator within a metal convection chamber trough and with exterior insulating sleeve, having upper convection openings, enclosing the entire assembly, convection chamber water and reservoir water are not shown for clarity; [0056]
  • FIG. 20[0057] a shows the same embodiment (less insulation) with rectangular drainwater heat exchanger and associated fin-divider to segment the convection chamber;
  • FIG. 21 shows a partial phantom perspective of a horizontal embodiment where the convection chamber is an open trough insulated on its exterior wall; [0058]
  • FIG. 22 shows a sloped drainwater heat exchanger and convection chamber only, and, a horizontal reference to show how the walls of the convection chamber need to taller to fully contain all water when cooled by cold drainwater; [0059]
  • FIG. 22[0060] b shows the effect of using multiple fin-dividers that keep the total volume of the convection chamber at the desired minimum in a sloped installation;
  • FIG. 23 shows a cross section end view of a preferred horizontal embodiment with nested convection chambers, the outer one of which has exterior insulation and and where the external coldwater heat exchanger is a large diameter tube and where the entire apparatus is enclosed in an insulating jacket; [0061]
  • FIG. 24 is a cross section of a preferred horizontal embodiment with doubled cold water heat exchanger; [0062]
  • FIG. 25 is a cross section of a water cooling embodiment for use with horizontal drainpipes. [0063]
  • DESCRIPTION OF DRAWINGS
  • There are two principal embodiments of the present drainwater heat recovery invention, a generally horizontal embodiment [0064] 45 (FIGS. 1 to 6, 12 to 14, 20-25) and generally vertical embodiment 50 (all other Figs). Each principle embodiment has reservoir 1 filled with a water 3 (or other suitable fluid) to serve as both a heat transfer medium and a heat storage medium. Drainwater heat exchanger 4 is located within reservoir 1 and has end portions extending therefrom for connection in-line to a drainpipe from which an appropriate section has been removed. Drainwater heat exchanger 4 may be made from seamless copper tube as is typically available from plumbing supply shops. It may be dimpled or grooved to enhance internal turbulent flow. Arranged on this drainwater heat exchanger 4 is a convection chamber(s) 5 made of an insulative material such as plastic to greatly minimize conductive heat transfer therethrough. A cold water heat exchanger 2 transfers recovered heat to cold water. The entire device is preferably enclosed in an insulating jacket 55 to maintain heat as long as possible.
  • In vertical embodiment [0065] 50 there are typically multiple convection chambers 5 each an open-topped tapered cup with a hole in the bottom, arranged in a slightly nested relationship such that the bottom of one sits just within the top opening of the next lower. For example for a 3 inch drainwater heat exchanger 4, they may be made from common polyethylene plastic tubs such as are commonly used for food stuffs such as yogurt. A hole 25 (FIG. 9) is punched through the bottom to be a tight fit onto the drainwater heat exchanger 4. They may be trimmed to a height such that the larger end will fit within the reservoir 1. A preferred material is a foamed polyethylene convection chamber cup to provide the best insulation. Fins 30 and 30 b may be added to the drainwater heat exchanger 4 as a strip or band of copper folded alternately to create a ‘vee’ corrugated band (FIG. 9b) that fits tightly to the drainwater heat exchanger 4 for heat transfer. Fin bands and convection chambers may then be alternately slid onto the drainwater heat exchanger 4 leaving plain ends for extension out of reservoir at each end.
  • For the horizontal embodiment [0066] 45 the convection chamber 5 is trough-shaped and may be made from a copper strip rolled to an open cylinder. Its side walls must be at lease marginally higher than the highest exposed point of the drainwater heat exchanger 4 (as represented by horizontal dotted line 6 c in FIG. 22-23) so as to prevent conductive heat transfer with reservoir water 3. Troughs may be nested for added heat transfer as shown in FIG. 23 where two are shown. They may be sweat soldered to the bottom of drainwater heat exchanger 4 to add maximum fin effect for heat transfer. Alternatively the two may be forced into thermal contact using spacers 26 b as shown in FIGS. 5, 6 and 12. One design is shown in FIG. 23 where insulative convection chamber 5 has a metal heat conducting core 5 b. The convection chamber 5 may be bound to conductive fin 5 b by wrapping with a string-like material, or, it may be adhesively attached or clipped in place. The volume of convection chamber water 24 should be as small as possible to minimize heat loss to cold drainwater yet allow unimpeded convection. The ends of convection chamber 5 may be butted against reservoir ends 74 (FIG. 21) with a gasket washer therebetween (not shown) to prevent leaking.
  • For the reservoir [0067] 1 it may be a simple tube of circular cross section or a more optimized shape depending on application such as a preferred shape shown in FIG. 20a. Where installation clearances are tight in the building, oval and flattened shapes may be more appropriate (see FIGS. 3 to 6). Doubled, side-by-side units may also suit certain conditions as shown in FIG. 3. The reservoir may be made of metal or plastic tube, or plastic film, depending on price and payback requirements. If the cold water heat exchanger is contained within the reservoir, the reservoir will best be of plastic with sufficient wall thickness (say ⅛ to ¼ inch thick) to withstand the weight and static pressure of the water contained therein. If the cold water heat exchanger is to be external then it can support a much thinner reservoir including a plastic film pieces made from, say, 0.005-0.010 inch thick PVC or polyethylene which are heat or high frequency welded into a suitable tubular reservoir shape. The assembled film reservoir is clamped or otherwise sealed to the drainwater heat exchanger 4, at each end for the horizontal embodiment 45 and at least to the bottom for the vertical embodiment 50.
  • A reservoir for a 3 inch drainwater heat exchanger may be metal such as a 4-5 inch copper tube. The sum of the masses of the materials of the reservoir and coldwater heat exchanger, and the waters contained therein, represents the heat storage capacity of the reservoir. The higher temperature they become the greater the amount of energy that has been recovered from the drainwater and the greater the energy savings for hot water. [0068]
  • The coldwater heat exchanger [0069] 2, 2 d may be a coil of tubing of large diameter to hold a maximum amount of water. It may be installed within reservoir 1 as shown in FIGS. 1, 3-8, 12, 12 a or exterior of reservoir 1 as shown in FIGS. 16, 17-25. Installed on the exterior adds a large measure of safety from contamination by the additional reservoir wall of separation. Cold water heat exchanger 2 a, 2 d (FIG. 24, 25) may also be a double coil where the outer cold water heat exchanger 2 a may be made of plastic tube while inner cold water heat exchanger 2 d may be made of copper tube where their respective ends are show as 2 b and 2 c in FIG. 24-25. This dual arrangement will allow fast heat transfer through the more expensive copper and a slower heat transfer into the lower cost plastic, this where long time periods for such slow heat transfer are the norm as is the case in many homes. Coldwater heat exchanger may also be of straight lengths of tubing as shown in FIGS. 3-6 with u-connectors at their ends to effect a single path or manifolded for parallel flow. More than one coldwater heat exchanger 2 may be manifolded together in parallel for higher water flow rates.
  • The following paragraphs describe some aspects of the present invention in greater detail. [0070]
  • Another design of convection chamber [0071] 5 shown in FIGS. 1-7, 12-14 for horizontal embodiment 45 may have outlet convection opening(s) 7 a in a split tube exposing bottommost portion of drainwater heat exchanger 4 to reservoir fluid 3. The outlet convection openings 7 a in convection chamber 5 are located only at the top of convection chamber 5. Convection chamber 5 also seals at edge 41 to exchanger 4.
  • FIG. 12[0072] a shows cold water heat exchanger 2 as a tubular coil that fits next to the interior wall of reservoir. This arrangement maintains an even temperature throughout the reservoir since the coil material will conduct heat readily from any warmer fluid 3 to any colder fluid 3 until temperature equilibrium is reached. Coil ends 31 and 32 extend out of reservoir for connection to water supply. Fitting 75 serves to fill and drain the reservoir. End caps 72 and 74 seal to reservoir 1 and extension 73 serves to seal to end of exchanger 4.
  • Referring to FIGS. [0073] 3-6 of horizontal embodiment 45, FIG. 3 shows how exchangers and volume of fluid 3 may be doubled-up to add volume for heat storage fluid 3 while maintaining a low profile for installation. FIG. 4 shows an off centered embodiment to add volume for heat storage fluid 3. FIG. 5 shows the use of an ovalized drainwater heat exchanger 4 and fin 51 which offer greater surface area for heat transfer from drainwater flowing therethrough and greater surface area for heat transfer into fluid 24, respectively. FIG. 6 shows an embodiment with large volume cold water heat exchangers 2. Although FIGS. 3-6 shown the horizontal embodiment, it is understood that this side-by-side arrangement can be used for the vertical embodiments also. In some applications where long periods lapse between hot water use, these large volume cold water exchangers 2 may be made of plastic to reduce overall system cost. The low thermal conductivity of the plastic is overcome by the longer time available for heat transfer and the larger volume of cold water thereby heated.
  • Reservoir [0074] 1 may be roll-threaded as shown in FIG. 17 such that the external coil fits into the thread to increase surface-to-surface contact. The entire unit may be dipped in zinc, solder, or tin, to further increase the rate of heat transfer. In the embodiments where exchanger 2 is outside reservoir 1 an advantage is gained from eliminating the liquid contact with exchanger 2. This embodiment more readily meets plumbing code and health code safety requirements that generally demand having potable water separated, from reservoir water which will become stagnant water, by at least two barrier walls. In this embodiment, the two heat exchanger tubing walls plus the reservoir wall, total three such safety barriers.
  • In FIGS. 1 and 2 the convection chamber [0075] 5 is shown as having separate inlet openings 6 a and outlet openings 7 a to allow convective flow 6 and 7 respectively. Inlet convective openings 6 a and outlet convective opening 7 a may be of any suitable shape, but their lowermost extremity must be at least marginally above the upper most surface of heat exchanger 4 represented by line 6 c in FIG. 1, 14, 22, 22 b, 23, 24, so that heavy, cold convection chamber fluid 24 will not flow out of any of these openings and cool reservoir fluid 3. Also in FIGS. 1 and 2 is shown a more sophisticated convection inlet opening 9 where a flexible flap valve 10 is attached with hinge 14 such that heavier cold convection chamber fluid 3 will force flap valve 10 against inlet opening 9, preventing leakage. However if reservoir fluid 3 is colder and heavier, then fluid 3 will force flap valve 10 inwardly from lower opening 9, and thereby flow 6 into convection chamber 5. In FIG. 1 closed flap valve 10 is shown in open position 11, as a dotted line. In FIG. 1 is shown dimple 26 a used to create a flow space between convection chamber 5 and drainwater heat exchanger 4. Alternatively a rod 26 may be used for that same spacing purpose.
  • In all embodiments of horizontal embodiment [0076] 45 the tube walls of heat exchangers 2 and 4 may be processed by dimpling or grooving the exterior to create interior ‘bumps’ that induce turbulent flow which reduces fouling and increases heat transfer.
  • In FIG. 10 of vertical embodiment [0077] 50, upper convection chambers 5 may be shorter to benefit overall heat transfer and may be flared 5 a at their tops so as to collect cold fluid 3 descending from cold water heat exchanger 2 (not shown). Funnel deflectors 60 shown in FIGS. 16 and 18 likewise serve to direct the cold fluid 3, descending by convection, towards the center such that fluid 24 in convection chambers 5 is as cold as possible to improve heat transfer.
  • Although not shown, heat exchanger [0078] 4 may be positioned off-center in reservoir 1 so as to allow vertical installation closer to a wall where existing drainpipe plumbing is close to the wall. FIG. 19 shows the components of vertical embodiment 50 with fresh water heat exchanger 2 a coiled around outside of reservoir 1.
  • In FIGS. 7, 8 and [0079] 9, and 9 b fins 30 are shown with broad thermal contact onto outer wall of heat exchanger 4. Fining may be a deeply corrugated clamp-on ring 30 b in each convection chamber (FIG. 9b), fitting tightly on exchanger 4. Internal cold feed water heat exchanger 2 is shown as an encircling coil but many other arrangements are possible, such as a vertical picket fence-like arrangement with u-loops at each end to interconnect the individual tubes. In vertical embodiments 50, reservoir 1 need only be sealed at the bottom while the top may have a removable cap. FIG. 9 also shows hole 25 of convection chamber 5 that seals against heat exchanger 4. FIG. 8 shows the inclusion of a thermal conductive liner 1 b that serves to even out fluid 3 temperature in reservoir 1 wherein fluid 3 would normally stratify in temperature layers. This is particularly useful at the upper end of the reservoir where, above the top convection chamber 5, there is only a small volume of fluid 3 to store heat from upward convecting fluid 7 (FIG. 16). Thermal conductive liner 1 b will conduct top-layer heat downward enabling more overall heat storage.
  • In FIG. 10 a dual vertical embodiment [0080] 50 is depicted with the entering ‘Y’ 31 made in plastic and outside of the reservoir 1, while the lower ‘Y’ 32 is preferably metallic and submerged, adding heat transfer surface area. The cold water heat exchanger is not shown but may be internal or external. This dual embodiment can be a triple, quadruple, or any number of exchangers 4. This embodiment is particularly suitable when vertical length is not sufficient to accomplish requires rate of heat transfer. Such multiple units provide large heat transfer surface in a short overall height. Convection chambers 5 of different heights are shown to compensate for low volume of reservoir water 3 above uppermost convection chamber 5.
  • FIG. 11 shows a vertical embodiment [0081] 50 of two identical units installed in tandem. Cold feed water heat exchangers 2 (shown only in upper unit) may be plumbed in series or parallel. This embodiment enables a single module to be manufactured and then multiples of them connected into the building's plumbing system so as to increase overall performance.
  • Heat exchanger [0082] 4 in all embodiments preferably has external dimples 40 completely covering the tube wall, save where a seal to convection chamber 5 is required. This will induce turbulence in the drainwater 8 (FIG. 1) to improve heat transfer and reduce fouling. In FIG. 12 the cold feed water heat exchanger 2 is shown to have dimples 40 to improve heat transfer. Such turbulence may also be achieved with grooves rolled into the exterior.
  • In FIG. 14 there is shown a horizontal embodiment comprised of several smaller pipes all enclosed in convection chamber [0083] 5, and manifolded at the entrance and exit (not shown) to single pipes. These may be dimpled to improve heat transfer (not shown). The convection chamber fluid 24 submerges all the tubes. Convection opening 6 a, 7 a is shown at the top of convection chamber 5 fully above the upper surface 6 c of the drainwater heat exchanger. The convection chamber 5 forms a seal 41 which, in FIG. 14 is shown sealing against one drainwater heat exchanger pipe 4 a. This embodiment is highly suited to washing machines, including commercial dishwashers, which use relatively small amounts of very hot water with no large solids. In FIG. 14 the reservoir and the cold feed water heat exchanger are not shown.
  • In another embodiment, where laws permit, reservoir [0084] 1 of both vertical and horizontal embodiments may be pressurized with the fresh cold feed water directly which, therefore, temporarily becomes fluid 3. Such an embodiment would avoid the expense of a cold water heat exchanger 2. Such an arrangement may also be used as a cold water pre-heater for non-potable water such as for toilet flushing. Such a warm water supply to a toilet would greatly reduce condensation and dripping, and the resultant water damage to the floor beneath toilets. Such wet areas also contribute significantly to mold and fungus growth in a building with attendant health hazards.
  • Drainwater heat exchanger [0085] 4 may be double walled (telescopic tubing) for potable water safety in such an embodiment.
  • Since heat transfer is well known to be a direct function of surface area, heat exchanger [0086] 4 may be made larger in diameter within the reservoir to increase internal surface. Inlet and outlet plumbing reducer fittings would funnel drainwater appropriately.
  • FIG. 15 is a partial phantom view that shows the relative placement of components in vertical embodiment [0087] 50.
  • FIG. 25 shows an water cooling horizontal embodiment [0088] 60 where the convection chamber 5 is inverted so that the convection opening is at the bottom at a position at least marginally above drainwater heat exchanger's lowermost surface represented by line 6 d in FIG. 25. This arrangement will trap drainwater heat as it floats upwards from the drainwater heat exchanger 4 preventing the heating of the reservoir water 3 and the cold water heat exchanger 2 j and 2 r. Used in this way the reservoir will receive heat from the cold water coils 2 j and 2 r (whose ends are shown respectively as 2 m and 2 p) cooling same, and give that heat to colder drainwater for the purpose of supplying cold drinking water in hot climates. An inverted vertical embodiment (not shown) will accomplish this same cooling function.
  • Both heat recovery (first) and heat rejection (second) can be used together in tandem to accomplish both objectives. [0089]

Claims (19)

I claim:
1. In a plumbing system for a building, the plumbing system including a cold water supply and a water disposal conduit, the improvement comprising an apparatus to transfer heat from waste water in said water disposal conduit to said cold water, said apparatus comprising a reservoir;
a heat transfer fluid substantially filling said reservoir;
a first heat exchanger for drainwater in thermal contact with said fluid;
a second heat exchanger having cold water from said cold water supply in thermal contact with said fluid;
convection chamber means partially surrounding said first heat exchanger, said convection chamber means having an opening therein allowing convection movement of said heat transfer fluid into and out of said convection chamber, said opening being located only at an upper portion of said convection chamber means,
the arrangement being such to heat said cold feed water by permitting convection of said fluid into and out of said convection chamber when said drainwater is hotter than said fluid, and, minimizing flow of said heat transfer fluid out of said convection chamber when said drainwater is colder than said heat transfer fluid.
2. The improvement of
claim 1
wherein said plumbing system includes means for heating said cold water, said apparatus being connected such that cold water passes through said first heat exchange means prior to entering said means for heating said cold water.
3. The improvement of
claim 1
wherein said second heat exchange means is connected to a water disposal conduit from a heat generating appliance.
4. The improvement of
claim 1
wherein said first heat exchange means is connected to a toilet.
5. The improvement of
claim 1
wherein said first heat exchange means is connected to a washing machine.
6. The improvement of
claim 1
wherein said building is a residential house.
7. The improvement of
claim 1
wherein said heat transfer fluid is water.
8. The improvement of
claim 1
further including means to disturb the boundary layer of liquid about at least one of said first and second heat exchange means.
9. The improvement of
claim 1
wherein at least one of said first and second heat exchange means comprise at least one tube having a plurality of inwardly directed dimples to thereby create turbulence in liquid flowing therethrough, said dimples being spaced sufficiently close together to provide continuous turbulence in said liquid passing therethrough.
10. The improvement of
claim 1
further including a plurality of said apparatuses connected in a plumbing system, said apparatuses being connected in series or in parallel or in series-parallel.
11. A heat transfer system suitable for recovering heat from waste water comprising:
a reservoir having a heat transfer fluid therein;
a first heat exchanger for receiving said waste water within said reservoir; said first heat exchanger having a first heat exchanger inlet and a first heat exchanger outlet;
a second heat exchanger within said reservoir in thermal contact with said heat transfer fluid, said second heat exchanger having a first heat exchanger inlet and a second heat exchanger outlet;
convection chamber means at least partially surrounding said first heat exchanger, said convection chamber means permitting convective movement of said heat transfer fluid from adjacent said first heat exchanger when said waste water flowing therethrough is warmer than said heat transfer fluid, said convection chamber means being arranged such that convective movement of said heat transfer fluid from adjacent said first heat exchanger is impeded when said waste water flowing therethrough is colder than said heat transfer fluid.
12. The heat transfer system of
claim 11
wherein said first heat exchanger includes a first heat exchanger conduit extending in a generally horizontal direction, said second heat exchanger including a second heat exchanger conduit also extending in a generally horizontal direction, said first heat exchanger conduit being located below said second heat exchanger conduit in said reservoir.
13. The heat transfer system of
claim 12
wherein said convection chamber means comprises a horizontally extending convection chamber encircling at least an upper portion of said horizontally extending first heat exchanger conduit.
14. The heat transfer system of
claim 13
further including spacers spacing said convection chamber from said first heat exchanger conduit.
15. The heat transfer system of
claim 13
wherein said convection chamber has a plurality of longitudinally extending slots formed therein.
16. The heat transfer system of
claim 11
wherein said first heat exchanger comprises a vertically extending conduit within said reservoir, said second heat exchanger comprising a coiled conduit coiled about said first heat exchanger conduit and spaced therefrom.
17. The heat transfer system of
claim 16
wherein said convection chamber means comprises a plurality of upwardly and outwardly extending flanges secured to said first heat exchanger conduit.
18. In a plumbing system for a building, the plumbing system including a cold water supply and a water disposal conduit, the improvement comprising an apparatus to transfer heat from said cold water to said waste water in said water disposal conduit, said apparatus comprising a reservoir;
a heat transfer fluid substantially filling said reservoir;
a first heat exchanger for drainwater in thermal contact with said fluid;
a second heat exchanger having cold water from said cold water supply in thermal contact with said fluid;
convection chamber means partially surrounding said first heat exchanger, said convection chamber means having an opening therein allowing convection movement of said heat transfer fluid into and out of said convection chamber, said opening being located only at a lower portion of said convection chamber means,
the arrangement being such to cool said cold feed water by permitting convection of said fluid into and out of said convection chamber when said drainwater is colder than said fluid, and, minimizing flow of said heat transfer fluid out of said convection chamber when said drainwater is hotter than said heat transfer fluid.
19. An apparatus for exchanging heat between drainwater and freshwater comprising,
a first heat exchanger comprising,
tube means having central portion for heat transfer and having end portions for connection to a supply of drainwater,
a second heat exchanger comprising tube means having central portion for heat transfer and having end portions for connection to a supply of freshwater,
reservoir means separating said first and second heat exchangers,
said reservoir means substantially filled with a fluid in thermal contact with said first and second heat exchangers,
at least one convection chamber submerged in said fluid comprising,
wall means,
said wall means being thermally insulative,
said wall means enclosing at least a portion said central portion said first heat exchanger,
said wall means including at least one convection opening,
the whole arranged to control the direction of heat transfer between drainwater and freshwater by orienting said convection opening upwardly or downwardly.
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