Abstract:
The present invention is a drainpipe heat exchanger to heat cold water using drainwater heat. A slit in a stiff exterior plastic sleeve with band clamps combine with internal hydraulic pressure to create a very high thermal contact force. Vertical and horizontal embodiments with and without protected heat storage are disclosed including a half jacket design for installation around operating drainpipes. A horizontal embodiment discloses a two-piece plastic-copper design. Double-wall construction and venting for visible leak detection satisfies plumbing code requirements. Use on vehicular or other combustion engine exhaust pipes is also contemplated.

Description:
[0001]    This application claims benefit from Provisional Application No. 60/998,670 dated Oct. 12, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is a drainpipe heat exchanger for drainwater heat recovery (DHR) from a building&#39;s regular drainpipe plumbing system. It includes a cold-drainwater-protected heat storage reservoir. Also disclosed is a one- and two-piece heat exchanger design that can be installed over existing drainpipes while they remain in full operation. The heat storage reservoir uses the thermosiphon principle and makes DHR available from both continuous plumbing fixture/appliance drain flows, such as a shower or running sink, and batch drain flows, such as from a dishwasher or filled sink. Thermosiphon is a well-known method of passive heat exchange based on natural convection. (See, for example, thermosiphon at Wikipedia.com.) 
       BACKGROUND OF THE INVENTION 
       [0003]    The traditional drainwater heat recovery (DHR) heat exchanger comprises a large diameter central copper tube (as used for drainpipes) wrapped with a small diameter cold water tube also of copper. It is based on the long-known Falling Film principle of heat transfer. In Falling Film heat exchangers, a liquid is ideally made to overflow into the top of a straight, large bore, vertical tube. The flow is meant to be circumferential, flowing down in an even, falling film clinging to the entire inner vertical tube wall, from top to bottom. (More information on falling film heat exchangers can be found at: The Chemical Educator, Vol. 6, No. 1, published on Web Dec. 15, 2000, 10.1007/s00897000445a, © 2001 Springer-Verlag New York, Inc., and, U.S. Pat. No. 4,619,311 to Vasile which discloses a equal flow Falling Film DHR heat exchanger.) The falling film DHR is, in many ways, ideal because it is not blocked by large solids and other matter contained in a building&#39;s drainwater. In operation, cold, ground water feeding a water heater first passes through the outer coil of tubing on its way to the heater while drainwater is ‘falling’ down the inside tube and transferring its heat to the cold water in the outer coil. Thus showering and sink rinsing are the principal appliances/fixtures where such heat exchangers can work because only then is cold water flowing into the hot water heater exactly while the drain is flowing with the now-dirty used hot water. 
         [0004]    However the traditional DHR design is not very cost effective because their payback time or return on investment (ROI) is too long in comparison to other energy saving strategies. 
         [0005]    This can be attributed to: 
         [0006]    1. Too little use of the expensive heat transfer material, which is usually copper, is actually used for heat transfer. For example thermal contact is limited to a narrow spiral contact strip between the outer coil&#39;s (conduit) contact surface with the inner tube&#39;s wall. Because heat transfer is a direct function of surface area, this limitation reduces performance which negatively affects ROI. This limitation is so greatly increased when it is laid horizontally which is often necessary (i.e., buildings without basements), that horizontal use is not recommended. Also, in regards the outer coil, the greatest part of the of its total surface area is not used for heat transfer. Only that small inner portion of the circumference actually contacts the drainpipe wall, the remaining, larger, outer portion of the circumference does not do heat transfer at all. 
         [0007]    In the instant invention, instead of a coiled tube conduit, sheet copper is used and is formed into a hollow jacket that serves as the cold water ‘tube’ or conduit. This dramatically lowers cost, while increasing thermal contact area to nearly 100%. For example, a 5 foot long, 4 inch diameter drainpipe, requires only ⅔ the weight of copper for the cold water exchanger; plus sheet-form copper is less expensive by weight than tube-form copper, and, a much higher percentage of that copper is used for heat transfer. Further, the instant invention allows for very compact, small diameter DHR (i.e., for a 1¼ inch diameter sink drainpipe) for individual fixtures and appliances which is not practical with wrapped tube designs due to the bend radius limitation of suitably sized outer tubing. Thus with the instant invention, DHR has offers a shorter ROI allowing for wider use in all size buildings. 
         [0008]    2. Lack of heat storage. The traditional DHR only works when both the drainwater and the cold water are flowing simultaneously, such as in showering or running sinks. This referred to as ‘continuous’ hot water use. It cannot recover heat from ‘batch’ hot water use such as from appliances/fixtures including wash machines and filled sinks and tubs, since there is nowhere for any meaningful amount of recovered heat to be stored. As a result, only about 40% of the total used hot drainwater (continuous use) is available for DHR with traditional non-storage DHR. And what little heat is stored in the outer coil is lost immediately to any cold drainwater which may flow at any time. 
         [0009]    The instant design uses a separate reservoir to receive and store heat from 100% of a building&#39;s drainwater no matter if it is from a continuous- or batch use source. This remote heat storage reservoir is mounted above the DHR so as to thermosiphon with the cold water jacket or conduit when hotter drainwater is flowing creating a thermal differential with the reservoir water. No moving parts or controls are required. Further, thermosiphoning provides automatic protection from heat loss to colder drainwater because thermosiphoning stops when the temperature differential is reversed. This further reduces the ROI. 
         [0010]    3. The long length of the coil tube (up to 100 feet long) and the fact that it flattens somewhat as it is wound creates internal resistance to flow and an unwanted drop in water pressure for the heater. This then requires either larger, more expensive tubing and/or a manifold arrangement of two or more coils to have multiple, parallel flow, tube coils which again adds cost and negatively affects the ROI. 
         [0011]    In the instant invention, the jacket offers a direct flow path from inlet to outlet and the passage can be as small or as large as needed. This eliminates pressure drop and reduces manufacturing cost. 
       SUMMARY OF THE INVENTION 
       [0012]    In a building, a first heat transfer fluid, referred to herein as drainwater, flows through a drainpipe. In the instant drainpipe heat exchanger invention, sheet copper is formed into a chamber or conduit. In one embodiment his chamber or conduit is in the form of a jacket with a longitudinal gap, to encircle a round, vertical drainpipe in the shape of a letter “C” in outline. In a second embodiment it is in the shape of a ‘bar’ or beam or trough that fits below the flattened, ‘D’ shaped, bottom portion of a horizontal drainpipe. In both, the spaced inner and outer walls are sealed at the ends and there are inlet and outlet fittings for connection to a second heat transfer fluid which may be under pressure such as the cold water supply for a water heater. The inner wall contacts the drainpipe and matches its shape so as to maximize the area of thermal contact. In the jacket, a longitudinal gap or slit is provided where the inner and outer walls U-bend back on themselves to create the chamber. This gap allows contraction of the heat exchanger&#39;s inner wall to clamp tightly onto a circular drain tube. The exterior wall has a stiff outer sleeve around which are several band clamps. The outer still sleeve provides clamping force distribution and heat insulation. The gap allows for intimate contact and easy sliding assembly onto the drainpipe. When connected to the pressurized water supply, the pressure adds to the thermal contact force much like a blood pressure measuring cuff, to further increase the all important rate-of-heat-transfer. 
         [0013]    In one application the jacket is slid over and clamped onto the exterior of an existing drainpipe. In another, it is pre-assembled with a drainpipe forming a complete DHR heat exchanger which then replaces a section of existing drainpipe. 
         [0014]    In a third embodiment, the instant invention is fabricated in two long half-cylindrical jackets (clam-shell like) which are assembled onto a operating drainpipe without disrupting drainwater flow. 
         [0015]    A the second flat embodiment, the instant invention is clamped between the flattened drainpipe and a shaped shoe or filler piece to spread the clamping force along the entire length. Again, the clamping plus the internal water pressure provide high performance thermal contact with the drainpipe. 
         [0016]    In a fourth embodiment, for flattened, D-shaped drainpipes, the cold water heat exchanger may be in two parts: an upper hemi-cylindrical plastic sealing portion bonded to a lower flat sheet metal heat transfer portion. This would further lower costs to improve the ROI. 
         [0017]    In use, a sink or shower may have the heat exchanger lying horizontally beneath it such that cold water is pre-heated before reaching the cold water faucet. In this way, less too-hot water is needed to mix with the now-warm-cold-water to achieve the desired final comfortable temperature. Less hot water use saves energy and money and pollution, and, if electrically heated, lowers peak power demand. 
         [0018]    During fabrication, the sheet copper should be slightly creased diagonally on the inner wall to serve as a vent for visible leak detection (a drip or air-drop onto the floor). The sheet is then formed into a hollow structure either a tubular ‘C’ shape or a flat bar shape. The outer wall of the jacket is pierced to receive soldered-on pipe fittings and the ends are sealed with appropriately shaped copper (tubing, rod or twisted wire), soldered into place. Alternatively, the jacket ends may be squeezed-closed and soldered shut. 
         [0019]    The unique, high-force hydraulic clamping action maximizes heat transfer by increasing thermal contact force. For example, if the drainpipe is 3 inches in diameter and the jacket 48 inches long and the cold water is at 50 pounds per square inch pressure, the contact force will be approximately: 3.14 (π)×3×48×50=22,000 pounds, or 11 tons of contact force! 
         [0020]    Not only does such an enormous force provide fast heat transfer over the entire length, but it forces intimate, conforming contact between the form-able sheet metal inner wall and drainpipe wall surfaces that may be imperfectly fitted. This would be extremely difficult or impossible to achieve by any mechanical clamping method. 
         [0021]    Where the instant invention is to be installed on an existing drainpipe already permanently in place, the jacket may be made in two halves (or hinged) with duplicate inlet and outlet fittings to connect to the cold water supply. The outer plastic sleeve would also be in two halves (or hinged). In some cases only a lower, half-jacket may be appropriate to reduce cost when using it on a large diameter, round, horizontal drainpipe, for example. 
         [0022]    In a sixth embodiment a remote reservoir is part of the pressurized cold water system and is located above the instant vertical or horizontal drainpipe heat exchanger. The reservoir is connected with inlet and outlet tubes to the cold water heat exchanger jacket or conduit. The reservoir preferably has a high, horizontal orientation to provide maximum thermosiphon effect. One tube between the reservoir and heat exchanger terminates low in the reservoir and the other tube terminates above the first. Natural temperature gradients (layering or stratification) in the reservoir means that lower layers are always colder and heavier that upper layers. Thus whenever warm drainwater (first heat transfer fluid) heats the cold water (second heat transfer fluid) in the cold water heat exchanger, it will also be made lighter and will therefore automatically be displaced upward into the reservoir by the heavier colder reservoir water sinking downward. This circulation of reservoir water will continue for as long as a temperature difference exists. In that way the reservoir become heated and the cold water heat exchanger is cooled for best heat transfer. 
         [0023]    When cold water is required by the water heater (hot water is being used) the cold water under pressure flows first into the center of the cold water heat exchanger, then through the connecting tubes at each end and into the reservoir, and then out of the reservoir into the water heater. The outlet tube therefore can have two way flow depending on whether thermosiphon or pressure flow is occurring. By having these two flow paths any heat received from the flowing hot drainwater by the cold water conduit will either be picked up directly under forced flow (hot water being used) or by thermosiphonic action (no hot water being used). If cold water is flowing as, for example, in the case of replacing the hot water being used in a shower, it will directly be heated by the hot shower drainwater. If no cold water is flowing but hot drainwater is, the heat will automatically transfer by thermosiphonic action into the reservoir. Here, the heat is stored until some future hot water use causes the now-pre-heated cold water from the reservoir to flow into the water heater to reduce energy use. 
         [0024]    In a seventh embodiment the same remote reservoir concept is applied to a horizontal drainpipe heat exchanger. here the reservoir may be vertical or horizontal. In the event that the water heater is properly positioned with appropriate upper and lower water connections, (one somewhat above the other) this embodiment may be plumbed directly to the heater using, for example, T-fittings at the heater&#39;s inlet and outlet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  shows a partial section end view a middle portion of one embodiment of the drainpipe heat exchanger having an upper conduit for drainwater and a lower conduit for cold water with forced thermal contact all along their flat surfaces; 
           [0026]      FIGS. 2 ,  3 ,  4  show the same embodiment in a sequence of forming steps to squeeze-close and solder-seal the two end portions of the lower exchanger; 
           [0027]      FIG. 5  shows the same embodiment in side view showing the sealed ends of the cold water heat exchanger, its lower fittings, and, the adapted ends of the upper conduit that connect to regular round drainpipes, and where the right end is shown to have an added adaptor while the left end is shown to have been formed into a short cylindrical shape, in both cases the flow path is flush such that there is no ‘step-up’ to impede drainwater flow in or out; 
           [0028]      FIG. 6  shows an adaptor for the drainwater heat exchanger formed, for example, from a suitable plastic material; 
           [0029]      FIG. 7  shows an end view of another embodiment where the drainwater heat exchanger&#39;s end&#39;s are formed to rectangular sockets to receive rectangular solder-type plumbing fittings and a plug, and where the excess material is closed off to be sealed by soldering at the same time that the fitting is inserted, and showing an internal fluid distribution tube enclosed therein; 
           [0030]      FIG. 8  shows a copper solder-type fitting having one end formed to a rectangular shape for insertion into the end socket; 
           [0031]      FIG. 9  shows a copper plug to be soldered in unused socket openings; 
           [0032]      FIG. 10  shows a side view of the same embodiment as  FIG. 7  showing the end location of the drainwater heat exchanger fittings; 
           [0033]      FIG. 11  shows a top view in section of a cylindrical, jacket-style heat exchanger having a longitudinal gap to allow clamping motion, which would be slid over a drainpipe/exhaust pipe; 
           [0034]      FIG. 12  shows a top section view of a two-piece design for clamping about an in-use drainpipe/exhaust pipe; 
           [0035]      FIG. 13  shows a side view of the embodiment in  FIG. 11  showing the outer sleeve and band clamps and showing the fluid fittings and the location of the end sealing members; 
           [0036]      FIG. 14  shows a top view of the sealing ring member made from tube or rod although a stamped sheet design may be more economical in production; 
           [0037]      FIG. 15  show a side view of the sealing member; 
           [0038]      FIG. 16  shows a possible use of the joint flange where it has various notches to distribute the fluid flow evenly over the jacket&#39;s inner wall so as to maximize heat transfer by maintaining the best temperature differential; 
           [0039]      FIG. 17  shows a thin, flat cold water (or other fluid) conduit clamped against the flat lower surface of the drainwater conduit; 
           [0040]      FIG. 18  is a cross section of the same embodiment and showing one internal stiffener in the cold water exchanger to prevent bulging; 
           [0041]      FIG. 19  is a cross section showing how the drainwater heat exchanger may be a two piece design with the upper, non-heat transfer portion in plastic and the lower heat transfer portion in sheet copper, bonded together along the length, and, with tension walls of sheet copper to transmit the internal pressure in the cold water exchanger to the external clamping member; 
           [0042]      FIG. 20  is a side view of the same embodiment showing how the drainwater flow may be made to enter from the top at the inlet end and to collect in a cross tube outlet arrangement at the exit end; 
           [0043]      FIG. 21  shows a perspective view of the outlet fitting of the embodiment; 
           [0044]      FIG. 22  is a top view looking into the vertical heat exchanger where the cold water is made to flow past a distribution gap formed adjacent an annular ring and the jacket&#39;s inner wall so as to sweep the entire surface along its vertical length; 
           [0045]      FIG. 23  is a cross section side view of the same embodiment showing how the cold water inlet is located between the sealing end cap and the annular ring with the single-sided arrows representing the resulting sheet-like flow; 
           [0046]      FIG. 24  is an end view of an embodiment of a upper conduit having a lower surface with a gully-shape along flow path, to resist upward bulging from the force of contact generated by the internal pressure in the shaped cold water jacket below; 
           [0047]      FIG. 25  shows the same embodiment but with an oval shaped lower flow surface; 
           [0048]      FIG. 26  shows another vertical embodiment with a remote heat storage reservoir connected with tubes for thermosiphoning with the cold water heat exchanger and central feed into the cold water heat exchanger; 
           [0049]      FIG. 27  shows a horizontal embodiment of a drainpipe heat exchanger with a remote heat storage reservoir; 
           [0050]      FIG. 28  shows the same embodiment in partial section; 
           [0051]      FIG. 29  shows the embodiment of  FIG. 12  but as would be used on a horizontal drainpipe where only a lower half is used (no exterior clamping shown); 
           [0052]      FIG. 30  shows the same embodiment having a plastic outer wall for fluid containment joined to a metal inner wall for fluid containment and for heat transfer; 
           [0053]      FIG. 31  shows another embodiment where the metallic cold water and drainwater conduits are fabricated to fit snugly within an outer plastic reinforcing tube; 
           [0054]      FIG. 32  shows the same embodiment where plastic is used in the conduits where no heat transfer takes place so as to lower costs; 
           [0055]      FIG. 33  shows an end seal for the embodiment shown in  FIG. 31  with the cold water fitting attached to the outside; 
           [0056]      FIG. 34  shows the same end seal but the inner surface with flow distribution holes. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0057]    Vertical drainpipe heat exchangers and horizontal drainpipe heat exchangers are disclosed each with unique embodiments. Each has two conduits in thermal contact. One conduit is a straight pipe or tube that typically carries a waste fluid from which heat is to be recovered, and the second conduit is for the second fluid to which heat is to be transferred (although the heat transfer could be reversed for cooling). Generally the conduits are metal and preferably copper for fast heat transfer. The instant drainpipe heat exchangers may comprise both conduits as a single assembly or just the second conduit which can be fitted to and existing first conduit. 
         [0058]    The two conduits are co-operatively shaped and tightly clamped together so as to provide maximum thermal contact area and high thermal contact force again for rapid heat transfer. In the horizontal embodiment the waste conduit is normally on top of the second conduit (waste fluid has heat to be recovered), while in the vertical embodiment the waste conduit is encircled by the second conduit. 
         [0059]    One novel feature of the instant invention is the use of the internal water pressure in the cold water conduit to add to the thermal contact force to provide even faster heat transfer. Faster heat transfer makes DHR more cost effective. 
         [0060]    In  FIG. 1  horizontal heat exchanger  200  has an upper drainwater conduit  60  and a lower cold water conduit  50  held tightly together with clamping bands  12  ( FIGS. 5 and 10 ) around a suitable force distribution sleeve (not shown). Drainwater conduit  60  comprises wall  1  with drainwater A flowing along flattened bottom surface  1 ′ (of wall  1 ) to thereby form a hemicylinder that transfers heat to fluid B which enters and exists cold water conduit  50  via underside fittings  10 ,  11  or alternately, via end fittings  80 . 
         [0061]    In  FIG. 1-5 ,  7 ,  10 , cold water conduit  50  is shown being in the shape of a trough made from sheet copper and formed with longitudinal hems  4  that are solder joined to create a generally “C shaped” hemicylindrical conduit with flat surface  5 . Hem  4  also serves as a heat conductive fin and, as a result of the bend curvature  6 , provides a longitudinal vent to the ambient for leak detection. 
         [0062]    In one embodiment, wall  2  of conduit  50  has wings  3  which contact the side of the drainwater conduit  60  to create additional surface for heat transfer. In  FIGS. 2 ,  3 ,  4  cold water conduit  50  is shown having a short end portion of hem  4  folded flat in preparation for sealing the ends. The wings  3  are pinched closed and excess metal is pulled into additional seams  3 ′. In  FIG. 4  is shown a dotted line  2  that represents the original cold water conduit  50  shape. 
         [0063]    In  FIG. 7  is shown an alternate way of sealing the ends of cold water conduit  50  so as to provide in-line connection sockets  33 ′,  34 ′. The two sockets at each end (4 in total) are formed on each side of hem  4  using an appropriate mandrel about which the remaining wall  3  and wing  2  are squeezed to bring them together as a seam to be soldered. Appropriate surfaces can be ‘tinned’ with solder prior to the forming in preparation for final soldering. 
         [0064]    In  FIG. 8 , fluid fitting  80  has rectangular end  33  inserted and soldered into socket  33 ′ or  34 ′ (at each end of cold water conduit  50 ), and has a round end  30  for connecting to standard plumbing. Fitting  80  may also be an end of a longer tube where installation conditions warrant. Alternatively one of the two rectangular shapes  33 ′ and  34 ′ may be blocked with a simple plug  34  as indicated in  FIG. 9 . Interior to cold water conduit  50  and inline with the socket  33 ′ and/or  34 ′ is a fluid distribution tube  35 ′ which extends full length and is closed at the far end and has cross apertures at intervals. The purpose of tube  35 ′ is to distribute fluid B (i.e., cold water) to cause a crossflow creating turbulence and evening out flow velocity across the width of cold water conduit  50 . 
         [0065]    In  FIG. 5  horizontal heat exchanger  200  is shown having the upper drainwater conduit  60  made from a flattened tube, and lower cold water conduit  50  (for, say, cold water) formed of sheet material bound together by exterior clamping bands  12 . In some uses the upper drainwater conduit  60  may also be formed from sheet to reduce cost. In either case the ends of drainwater conduit  60  can be adapted to connect with existing round drain pipes the right end of the drainwater conduit being shown having a separate, bonded-on adaptor  70 , while the left end  70 ′ is shown as having an integrally formed round end  20 ′. It is important that the drainwater conduit provides a flush flow path especially at the exit end so that solids in the drainwater will not hook and collect at the region of transition from flat to round. This can be achieved by forming a recess in the “D’ shaped end of the bonded on adaptor equal to the thickness of the drainwater conduit material. The bonding region is shown at overlap  20 ′. 
         [0066]      FIG. 5  shows fluid B, such as cold water for a water heater, entering fitting  10  at the left to counterflow horizontally under the drainwater water conduit  60  and exit via fitting  11  on the right having absorbed (or given up) heat from warmer (or colder) drainwater’. Drainwater A flows horizontally with a first temperature A′ at inlet on right side and a different temperature A″ at outlet on left side. 
         [0067]      FIG. 6  shows adaptor  70  having a “D” shaped first end  20 ′ for bonding to drainwater conduit  60  and a round end  20  for connecting to existing drainpipe. Adaptor  70  may also be made of molded rubber with a shaped shoe  22  (shown in dotted outline) under the flat portion  20 ′ to provide even clamping pressure for sealing. 
         [0068]    In use, by connecting cold water conduit  50  to a pressurized fluid supply, an enormous thermal transfer contact force is created between the flat surfaces of conduits  50  and  60 , restrained by bands  12  (over a stiff sleeve, not shown), to provide exceptional heat transfer therebetween. For example, with a 4 inch wide flat that is 50 inches long and with a pressure of 40 pounds per square inch, the contact force is some 8,000 pounds. This force custom forms typically imperfect flat surfaces  1 ′ and  5  into intimate contact. 
         [0069]    With the instant invention, horizontally flowing drainwater, whose valuable heat energy is normally wasted, can be cooled by heat transfer to the cold water supply of the water heater to thereby shorten the time it takes to fully heat hot water which, in turn, saves energy and money and provides more hot water due to faster recovery. It may also be used to cool a flow of warmer water feeding, for example, an ice cube maker, using colder drainwater from a ice-filled sink. 
         [0070]    In all figures the drainwater flow or exhaust gas inlet flow is indicated as A′ and A″ and the fluid whose temperature is to be changed is B and B′. Heat exchanger  200  may be used to heat or cool fluid B. Although gaps between surfaces are shown in the figures (for clarity) it is understood that there is intimate contact between heat transfer and clamping surfaces. 
         [0071]    In  FIGS. 11-13  heat exchanger  100  is a jacket(s) comprising an inner heat transfer wall  5  and outer retaining wall  2  spaced apart for fluid flow therebetween with minimal resistance. This space may be, say, ¼ inch. The walls are contiguous and formed from a single piece of thin sheet metal (copper) using reversing bends  112  and lap joint  5 ′. This leaves a longitudinal opening or gap  111  between bends  112  to accommodate movement from external mechanical clamping forces and internal hydraulic clamping forces. The jacket may also be formed by extrusion in which case finning  115  (representative fins only, shown in  FIG. 11 ) and fluid control elements  114  may be easily included on the inner wall  5  and/or outer wall  2 . Outer clamping sleeve  116  with gap  113  closes tightly around and distributes clamping forces from band or hose clamps  12  to prevent expansion or bulging of outer wall  2  from the internal pressure of fluid B such as that from a building&#39;s cold water supply. Inner wall  1  is however free to expand every so slightly to provide a tight, intimate thermal contact with drainpipe  1  using that same internal pressure. 
         [0072]    In  FIG. 11 ,  12  lap joint  5 ′ is a soldered and may include longitudinal joint flange  110  which can act as a fluid flow distribution ring and a stabilizer/spacer for aligning the sheet metal during soldering. Inlets(s)  10  and outlet(s)  11  are connections for fluid B (such as cold water) whose temperature is to be changed. Representative fluid control element  114  may be several in number and take various shapes such as mesh, rods, screen, angles, etc., that direct, for example, flow of fluid B over element  114  as indicated by dashed flow arrow  114 ′, to help effect best heat transfer from inner wall  5  by the fluid ‘sweeping’ the surface of the inner thermal contact wall as fully as possible. Element(s)  114  may also be used to create turbulent flow which is known to improve heat transfer. Element  114  may also be shaped and located to deflect fluid B inflow at inlet  10  to avoid erosion corrosion of the small area of the inner wall by the fluid impinging on it perpendicularly at full velocity over long years of daily use. 
         [0073]      FIG. 12  shows the hollow, tubular nature of the heat exchanger  100  as fitted onto a vertical drainpipe  1 . Sealing rings  34  are shown in dotted line and are soldered into the annular space between the inner and outer wall ends at top and bottom. Although a tubular shape is shown, other shapes such as oval are contemplated where, for example, fitting clearance is a concern. 
         [0074]      FIGS. 14 and 15  show the sealing member  34  which can be made from rolled rod, tube or twisted wire bundle to fit snugly into the annular space and have a gap  111 ′ to coordinate with gap  111 . They may be made by winding a long tube onto a mandrel of the correct diameter into the form of a coil spring and then sawing through the coil to free individual rings which are then made planar as in  FIG. 15 . Dip soldering is a fast method of construction. 
         [0075]      FIG. 16  shows a method of using the longitudinal joint flange  110  as a flow distributor by providing restriction to flow directly from fitting  10  such that fluid B is forced through spaced vias  120  to travel across inner wall  5  to reach outlet  11  thereby improving heat removal from drainpipe  1 . Flange  110  may also simply be more simply double-tapered (not shown) from full width at the center tapering to nil at each end to even out flow along its length, especially if the fittings  10  and  11  are positioned centrally and opposite one another. 
         [0076]      FIG. 12  shows the cold water conduit in two halves with inlets  10  and outlets  11  on each half. The outer sleeve  116  and clamps  12  of  FIG. 11  are not shown. The outer sleeve  112  would of course be in two pieces either separate or hinged for ease of assembly onto the drainpipe in a building while it remains in operation. The sealing rings  34  (not shown in  FIG. 12 ) would of course be four in number each being a half ring, one at each of the four ends. 
         [0077]      FIG. 17  shows another embodiment of horizontal heat exchanger  200  where the cold water conduit  2  comprises a sheet copper duct or tube in the form of a flat, rectangular hollow strip. It is sealed at each end and preferably has flow-formers to ensure that the cold water flows as a flat sheet of water across the entire width of the heat transfer surface so as to keep the surface as cool as possible, thereby maximizing delta T for faster heat transfer. 
         [0078]      FIG. 18  shows a cross section of the same embodiment where the drainwater conduit is shown to be a flattened, hemi-cylindrical tube  1  forced into intimate, conforming thermal contact with cold conduit  2  using shaped pressure distribution shoes  130 ,  131  and clamp bands  12 . 
         [0079]    In the embodiments shown in  FIGS. 18 and 19 , and all embodiments of the horizontal drainwater heat exchanger, the cold water conduit may have internal baffles  2 ″ comprising one or more flattened tubes soldered between the top and bottom surfaces that will prevent excessive bulging of the conduit in reaction to the water pressure inside. This will help maintain flat drainwater heat exchange surfaces. 
         [0080]    In  FIG. 19  drainwater conduit  1  is comprised of a trough-like lower portion in sheet copper through which heat transfer takes place and a U-shaped plastic upper portion bonded  1   b  thereto, the two creating a hybrid drainpipe of rounded rectangular or hemicylindrical form. This embodiment is for the lowest cost device. Interior longitudinal supports  1   c  act to transmit bulging force from cold water conduit  2  to shoe  130  and bands  12  thereby maintaining a flat profile for the trough. Supports  1   c  may be wavy to create a desirable turbulent flow. Supports  1   c  also act as fins to extend heat transfer surface area. Supports  1   c  may be eliminated and baffles  2 ″ in the cold water exchanger may be used to prevent pressure bulging of the flat surfaces. 
         [0081]      FIG. 20  shows the same embodiment with different drainpipe connection fittings. Inlet  200 ″ is a vertical right angle inlet centered on plastic top  1   a  and outlet  200 ′ is a horizontal right angle fitting shown in more detail in  FIG. 21 , having an end cap and a slot  201  which matches the shape of the end of heat exchanger  1 ,  1   a,    1   b  ( FIG. 19 ) and is bonded and sealed thereto. A slight slope to outlet  200 ′ carries away the final drainwater drips to leave drainwater conduit  1  dry. 
         [0082]    In  FIG. 22  vertical heat exchanger  100  has an inner wall  5  (heat transfer surface) and ring-shaped flow distribution ring  110 ′ which provides an even annular gap  120 ′ adjacent wall  5 . End seals  34  ( FIG. 23 ) and flow distribution ring  110 ′ are spaced apart vertically creating a circular chamber into which flows fluid B, which then must leave the chamber in a full curvilinear sheet flow B′ (half arrows) against inner wall  5  so as to sweep heated (or cooled) fluid towards the outlet, which is similarly configured. This ensures that a maximum temperature differential, or delta T, can be maintained to optimize heat transfer. This annular flow control arrangement may be used to advantage in all the aforementioned heat exchangers including the two-piece embodiment of  FIG. 12 . In the case of horizontal heat exchangers  200  the distribution ring would take the form of a rectangular bridge held a small distance below the heat transfer surface by stand-off elements. 
         [0083]      FIGS. 24 and 25  show variations on the profile of the flow surface  1 ′ of the drainwater conduit  1  with the purpose of stiffening the flow surface  1 ′ to resist upward bulging from the expansive potential of the pressurized cold conduit below. The cold water conduit  2  is shown to be conforming in shape so as to maintain maximum thermal contact. 
         [0084]      FIG. 26  shows a vertical drainpipe heat exchanger  500  having a remote heat storage reservoir  400  which is always pressurized with the cold water supply B and lies in series with the cold water flow into, say, a water heater. Outlet  11  connects to external plumbing to provide, pre-heated water C to a water heater or other fixture/appliance. Cold water B enters via fitting  10  into jacket or conduit  2 . Two central flow distribution rings  110 ′ ensure that the up and down vertical flow through jacket  2  is adjacent inner heat transfer wall where it then passes under two additional upper and lower flow distribution rings  110 ′ into the collection area (between end seal  34  and ring  110 ′) and out through fittings  213  and  214 . Now cold water B (preheated by drainpipe  1 , or not) passes through connecting tubes  401  and  402  into reservoir  400  via fittings  410  and  411  respectively. Tube  402  terminates higher in reservoir  400  than tube  401 . Thus tube  402  terminates in the warmer, lighter layers of water filling reservoir  400 . 
         [0085]    In operation four scenarios are possible:
   1. Hot water, is being used and used hot drainwater A′ is flowing, such as in showering. Here the cold water B will be pre-heated in jacket  2  and flow upwards through tubes  401  and  402  (arrows  403 ,  404 ) into reservoir  400  and out outlet  11 .   2. Hot water is being used but no drainwater is flowing such as when filling a wash machine. Here the cold water B simply passes through jacket  2  and through tubes  401  and  402  into reservoir  400  and outlet  11  (arrows  403 ,  404 ). With fitting  11  on top, any previously recovered heated water will be the first to flow out because it is lighter and rises.   3. Hot drainwater A′ is flowing but no hot water is being used, such as when an appliance drains. Then, if the in water B in jacket  2  is being heated by drainwater A′ and is thereby made lighter, thermosiphoning will automatically take place, whereby any water in reservoir  400  which is colder than that in jacket  2 , will cause the heavier cold water to sink down tube  401  (arrow  403 ) into jacket  2  via fitting  214 , then travel up through jacket  2  picking up heat and out outlet  213  to return to the upper region of reservoir  400  via tube  402 . This continues as long as there is a temperature differential (weight difference) between the water in the reservoir and the water in the jacket, that is as long as heated drainwater continues to flow. The net result is that the water in the reservoir is heated ready to flow into a water heater or other appliance/fixture   4. Cold drainwater is flowing. The water in the jacket  2  is the first to become cold and therefore also becomes heavier. Thermosiphoning cannot occur with the reservoir  400  since cold water cannot rise into it and therefore whatever heat is present in the reservoir will not be lost to the cold drainwater. This automatic cessation of thermosiphoning provides protected heat storage for the recovered heat in the reservoir.   
 
         [0090]    Cold water reservoir  400  may be the reservoir may be a rectangular shape or a square tube shape or a cylindrical shape and mounted or hung some distance from the drainpipe heat exchanger and as high as practical, such as being hung from a ceiling. This will increase thermosiphon action (speed the flow) to improve performance provided tubes  401 ,  402  are of sufficient diameter. In such cases tubes  401  and  402  should be well insulated to maintain the best temperature differential and to prevent heat loss to the ambient. 
         [0091]    In the event that it is desired to discard heat, as in, for example, a cold water drinking fountain, the arrangements may be reversed so that the coldest water remains in the reservoir ready to move to the drinking outlet. Then, the reservoir would be below the heat exchanger and the tubes  401  and  402  arranged such that hotter water in the reservoir rises to be cooled by colder drainwater from the fountain and returns cooler, thus keeping the reservoir cool and the drinking water cold as desired. 
         [0092]      FIG. 29  shows how drainpipe heat exchanger  600  is a half jacket with a continuous inner and outer wall  2 ,  5  that may be used on the bottom portion of a horizontal drainpipe  1  such as one of copper or steel (cast iron) carrying drainwater A. Cold water B passes through exchanger  600  on its way to the water heater. This minimizes material and so improves the ROI. In  FIGS. 29 and 30  none of the required external clamps or reinforcing sleeves are shown for added clarity. 
         [0093]      FIG. 30  shows horizontal drainpipe heat exchanger  700  of composite construction where inner wall  2  is metal joined to a thick outer wall  222  made of plastic to further reduce cost and improve ROI. 
         [0094]      FIG. 31  shows a preferred embodiment where the upper and lower conduits are formed each with outward radiuses (for example 2″) that match that of a plastic reinforcing and insulating sleeve  70 . Sleeve  70  has a longitudinal gap  70 ′ that allows external band clamps (not shown) to tighten the sleeve and compress the two conduits together under high force to improve heat transfer. The inward radiuses of both conduits are also matched but are much larger (for example 10″) to create a flatter, broader heat transfer flow path in the upper drainwater conduit for better heat transfer therebetween. The ends of the lower cold water conduit are closed with form-fitting end pieces  71  soldered in to make it a pressure tight conduit. The end pieces  71  may be made from tubing appropriately formed to match the radiuses and having a cold water fitting  10  towards the exterior. Its interior may have a row of holes  10 ′ to direct the cold water flow evenly across the heat transfer surface. The conduits  1  and  2  can be made from sheet material and solder joined. Filler piece  75  indicates how a join may be made to leave the exterior wall butted for a smooth circumference for seal-clamping with rubber couplers (not shown) to the building&#39;s round drainpipe stubs. 
         [0095]    In operation, the internal cold water pressure will urge the two conduit&#39;s thermal transfer walls  1 ′,  5  together under considerable hydraulic force as previously explained. In an unreinforced flat contact surface between the conduits, the conduits would bulge balloon-like under the hydraulic pressure into the interior of the upper conduit diverting flow from the surface. It would be difficult and expensive to contain such bulging. The solution shown in  FIG. 31  is to use a slightly curved contact surface and restrain bulging by using the opposite force generated by tightly clamping exterior sleeve  70 . Sleeve  70  will try to make surface  1 ′ more concave (smaller radius, downward push) and therefore apply force onto the top surface of wall  1 ′. Hydraulic force from within conduit  2  will do the opposite pushing wall  1 ′ up. The result is both forces increase the net thermal contact force and bulging is restrained. 
         [0096]      FIGS. 33 and 34  show each side of a boat-shaped or arcuate end piece  71 , made from tubing, with the cold water B inlet  10  and the distribution outlet holes  10 ′. The arcuate shape has two radii to match R 1  and R 2  of conduit  2 . Such a design may also be used on the opposite outlet end of conduit  2 . Fresh cold water B exits end piece  71  via holes  10 ′ (or other shapes of paths) across the interior wall  5  of the cold water conduit thereby providing more even heat removal from the whole heat transfer surface. This, in turn, lowers its temperature which improves the ΔT and thus the rate of heat transfer, raising performance. 
         [0097]      FIG. 32  shows the same embodiment with non-heat transfer surfaces replaced with plastic upper  223  for upper conduit  1  and plastic bottom  222  on lower conduit  2 , both to reduce cost. The plastic portions may be bonded and/or mechanically interlocked with the metallic portions. For example the thicker plastic may have a longitudinal slit formed along the edges to receive the thinner metal portion. An adhesive can be first applied in the slit.