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Patent Searching and Data

Document Type and Number:
WIPO Patent Application WO/2009/008826
Kind Code:
A Heat Exchanger for domestic drain water heat recovery is described which reduces energy consumption in a domestic or light-duty commercial application, such as a shower, shower-in-bath, or other appliance where hot and cold water are to be mixed just before use. The drain water heat energy is reclaimed in order to preheat cold water before being mixed with hot water. The hot and cold water are supplied in separate hot and cold supply lines. The cold water supply is first directed through the heat exchanger where drain water heat is absorbed. The heat exchanger embodiment facilitates installation in several possible locations such that the end user has several options in order to reduce installation costs by opting for a simple hook up, or install the unit in a more hidden location to improve aesthetics. The embodiment utilizes the counter flow method, in a concentric tube type heat exchanger, employing plastic tubes, baffles, fins (in addition to other methods), in attempts to achieve the highest possible efficiency per cost ratio. This directly affects the payback period of the product. The embodiment is self draining to reduce fouling and anti-clogging by means of allowance for internal overflow.

Application Number:
Publication Date:
January 15, 2009
Filing Date:
July 08, 2008
Export Citation:
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International Classes:
Foreign References:
Attorney, Agent or Firm:
HYNELL PATENTTJÄNST AB (Patron Carls väg 2, Uddeholm, S-68340, SE)
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1. A drain water heat recovery device comprising a counterflow, tube heat exchanger (6) enclosed by a platform (7) and a base section (12), characteri sed in that said heat exchange (6) is formed by a single, continuous, overlapping tube, preferably made by plastic, having the length in the range of 10 - 100 m and a diameter in the range of 10 - 40 mm.

2. A heat recovery device as defined in claim 1, characteri s ed by platform channels (18) in the platform (7) to reduce cooling effects on water before it reaches the drain intake (1) by channeling it, thereby reducing the surface area of the shedding water.

3. A heat recovery device as defined in claim 1 or 2, arranged to have an overall dimension and profile such that it can easily be mounted underneath a bathtub, inside an existing shower tray, or under the floor, etc.

4. A heat recovery device as defined in claim 3, characteri s ed by when installed below the shower floor lends itself to the easy application of flooring tiles or linoleum whereby the desired sloping of the shower floor is accomplished by the shape of the platform.

5. A heat recovery device as defined in any preceding claim, characteri s e d by being arranged to temporarily retain water inside the device by means of a weir- sluice (14) upstream of all drain outlets (8,9,10,11) and preferably also to self- drain said water to prevent fouling, reduce corrosion and avoid damage due to freezing.

6. A heat recovery device as defined in any preceding claim, characteri se d in that said tubes (6) have ribs, fins, knurls, multiple tubes, coiling in a spiral, or a combination of these features on their external surface for improved heat transfer coefficient.

7. A heat recovery device as described in any preceding claim, characteri s e d by channel walls (13) and/or baffles arranged to direct the drain water in a direction opposite but parallel to the direction of cold water flow in order to

increase overall efficiency.

8. A heat recovery device as defined in any preceding claim, characteri sed by channel walls (13) which may support the upper platform but do not necessarily join with it in all locations such that clogs which might occur inside the heat exchanger can be bypassed by the water flowing overtop the channel walls.

9. A heat recpvery device as defined in claim 1, with a screen and trap insert (5) which can also be removed and plugged in order to utilize other drain intakes if installed in other installations.

10. A method for producing a device according to claim ^ characteri s e d by having multiple drain inlets (1, 2, 3) and outlets (8, 9, 10, 11) whereby the necessary outlets would be punched out of the plastic mold at the time of installation.

11. Method according to claim 10, characterized by when re-installing the heat exchanger, the inlets and outlets which are no longer used can be blocked with a screw cap or other sort of plug (either removable or fixed), whereby these inlets and outlets, along with the removable outer wall (20) enable the heat exchanger to be adaptable to any situation/location where it is used, and can be easily removed and re-used in another location including but not limited to: above the shower floor, under the bathtub, under the bathroom floor, under the shower floor or in the ceiling or wall of a room below the bathroom.



This invention relates to a drain water heat recovery device comprising a counterflow, tube heat exchanger enclosed by a platform and a base section.


The goal of drain water heat recovery is to reduce the costs of supplying hot or warm water to the home either via heating of the cold water just before entering the hot water tank (centralized), or just before being used by an appliance, shower, etc. (localized). This is done by recovering heat energy from the drain using a heat exchanger.

In the localized case, hot and cold water are typically mixed to obtain a desired temperature and flow rate in some type of blending or mixing valve. Many modern blending valves used in showers and bath stalls are temperature-regulated such that a constant output temperature is achieved despite fluctuations in the cold or hot water supply lines. Therefore preheating the cold water supply will equate to less hot water needed to maintain a given outlet temperature and flow rate.

Various other patents exist which have specified a typical installation, and have laid claims to various heat exchanger designs. The various designs have shortcomings in the following areas:

1) Preferred Location (Range of use / Adaptability) 2) Efficiency

3) Clogging

4) Build up of fouling agents, rust, scum, lime, etc and ability to clean these agents.

5) Aesthetics

6) Cost - Including manufacturing and installation costs, payback period, etc. 7) Possible damage from freezing

The term "waste water recovery heat exchangers" will simply be called heat exchangers herein.

The plumbing layout is essentially the same for all types of heat exchangers discussed in the background section, in that they are intended for showers which have a centralized hot water heating system with hot and cold supply lines to the shower.

There is one patent, however (GB 2835785) which specifically states that its use is intended only for "heat on demand" types of hot water heaters, typically located in or near the bathroom.

This patent will make no such limiting statements. Instead the embodiment can be used in virtually all domestic/commercial/industrial situations including but not limited to showers, bathtubs, kitchen and bathroom sinks, centralized installations, heat-on- demand installations, and even commercial use such as hair salons, sporting facilities, hotels, animal care salons, etc. It is important to note that, although the dimensions are provided for a domestic embodiment, an enlarged version of the same embodiment is also possible for more industrial applications, e.g. higher pressures or flow rates.

Assessment of current patents.

There are, in general, 5 categories for existing patented heat exchangers.

a) Heat Exchanging Mats b) Platform Surface Heat Exchangers c) Platform With Basin Heat Exchangers d) Under Bathtub Heat Exchangers e) localized or centralized Drain Tube Coil Heat Exchangers

Each will be discussed briefly, in the following.

A Heat Exchanging Mat has been patented (US 4821793) which lies over the existing bathtub (or shower) floor. Drain water is trapped between the mat and the bathtub (or shower stall) surface and is guided through channels before it reaches the drain. No retaining wall keeps the drain water from spilling over the edge, since the existing bathtub prevents water from escaping already. The solution has very little plumbing or installation work required. This known prior art is poor in the aspect that not all wasted water will make contact with entire heat exchange surface and also the mat gets in the way for those who sit or lie in the bath.

A Platform Surface Heat exchanger has been patented which lies over the existing shower floor. Drain water flows towards the center of horizontal metal plate (forming the platform). A coil/spiral of cold water pipe is bonded to the lower surface. A retaining wall keeps the drain water from spilling over the edge, and an outer housing encapsulates the platform. The platform can be removed for cleaning if necessary

though it is not likely required since water does not enter the inside of the device. The solution has very little plumbing or installation work required. This known prior art is poor in the aspect that drain water does not stay in contact with coil/spiral for a suitable length of time. Flow is likely laminar, not enough heat exchange area. Water will likely not come in contact with entire surface and concerns about getting cold feet while showering.

Several Basin Heat Exchangers have been patented which lie over the existing shower floor. Examples are GB 2420973 and FR 2868796. Drain water collects on the platform which has retaining walls to prevent overspill. In GB 2420973, drain water is channeled horizontally and then downwards into a chamber called the basin where water collects and must pass through a series of channels while exchanging heat with the submerged tube and then overflows past a dam into the second drain, hi FR 2868796 drain water collects in a bowl where the submerged tube is wrapped in a coil. In both examples, water then flows towards the final drain. The solutions have some plumbing and installation work required. The patent GB 2420973 also claims the feature of being able to lift off the platform with a special waterproof peripheral seal which easily snaps into place. This known prior art is poor in the aspect that it will limit the user to the manufacturers design only, i.e. they must remove their present shower and replace with their own heat exchanger model. Tubes are specified to be copper or metal. Similar patents exist, e.g. SE 52844, SE526061, NL1015561C, DE 440697I 5 and US 4821793

GB 2835785 describes an Under Bath Tub Heat Exchanger, which exchanger is located under an existing bathtub. The inventor limits his claims only for application where the heat exchanger is used to preheat cold water entering an electric "heat-on-demand" or "combi-boiler" type of system, not for use in installations which have a centralized hot water heater. Regardless, this is still a viable alternative for preheating cold water in a typical installation as well. Drain water leaves the bathtub and enters a canister or shell inside of which a coil of cold water picks up the heat. Some plumbing work is required, probably best handled by a professional. This known prior art is poor in the aspect that the design is complex and therefore rather expensive.

AU2004212549 describes a Drain Pipe Coil Heat Exchanger which fits into or below the floor. Drain water passes through a metal drain section which is wrapped in a coil of cold water pipe. The two pipes are bonded together to increase the heat transfer rate. If installed locally, (near the shower or bathtub) the solution requires extensive plumbing work and possibly removal of a section of floor or wall in order to install. A

professional would be needed. If installed centrally, a considerable length of piping would likely be needed depending on how far away the main drain is from the cold water pipes entering the home. This known prior art is poor in the aspect that the unit itself is entirely copper and the installation costs can be large.

Many of the existing patents seem to focus a solution where a shower tray is provided along with an integrated heat exchanger, thereby making the costs higher, not only for the removal of the old tray and installation of the new system but also for the unit itself. There is also the risk that the new system is not the correct size for many customers' preferences.

In addition to this, the tray that comes with the heat exchanger, for the patents discussed above, cannot be discarded altogether in favour of a hidden installation, since the heat exchanger and tray are made in one integral unit (with the exception of e) - a drain pipe heat exchanger)

Of the above mentioned designs, the patents tend not focus on efficiency or heat transfer improvement of their design, in other words make no preference between whether the flow is arranged in parallel or counter flow, make no distinction whether the system shall be single wall or double wall, whether the pipes will be thin walled or thick walled, finned or smooth, baffles or not, etc. They also make no mention of any special features in the platform itself, in terms of efficiency, such as surface channels.

A rather lengthy heat transfer analysis shows that, given the slow speed of the drain water, the convective heat transfer rate is so miniscule, that a regular smooth-walled shell-in-tube or concentric tube heat exchange would achieve less than 500 W/m 2 -K. This means that a product would require a heat transfer surface area of at least 1 m 2 to even begin reach a reasonable amount of performance. Anything less I would conjecture would not yield a commercially viable product. If the length of the cold water delivery tube can be expressed as:

A = πD x L

where D is 0.02 m diameter on the inside cold water tube, A is the 1 m 2 from above.

The tube length would need to be L = A / πD = 21.5 m. If the tube were to be made of metal, such as copper, 21.5 m would be a prohibitively expensive length of pipe, meaning that the payback period for the product would be very long.

No existing patents seem to adequately address this problem, although no dimensions are normally provided, it is hard to comprehend how 21 m of smooth-wall copper tubes could possibly fit under the shower trays shown. It is in this area that the current patent will also focus on a very practical alternative to metal. Namely, PEX plastic. This causes another problem, however, since PEX tubes are not easily bent, so fitting 21 m of tubes into a small box is difficult. There are two possible solutions to this presented.

Of the above mentioned designs, most tend to leave the heat exchanging pipes submerged during the operational life of the product, and occasionally discuss how these pipes are to be cleaned, etc. But they are not self draining. The ability to automatically drain the water from the exchanger can reduce fouling since the tubes would dry off somewhat.

Another issue facing most patents is the drain water will tend to back up into the tray if the drain length is too long. This is because friction losses in the drain channel will need to be overcome by a certain pressure. The longer the drain channel, the higher this pressure will need to be in order to meet the flow requirements. Since the drain water is open to the atmosphere, the only source of pressure to force the drain water along is head. The head is the difference in water level between the upstream and downstream water. If the head is insufficient, the water will back up causing the water level to rise until the head is sufficient to drive the flow along the channel. This problem is also addressed in the preferred embodiment by offering an overflow path to the drain water.

One final area of improvement is in the platform itself, which is typically flat. The water draining across the platform is subjected to cooling from convection and evaporation for the brief period of time it takes to make its way towards the drain.

Another type of existing patent describes hidden installations where the heat exchanger is installed in the drain under the floor or in a centralized location. These patents lack a platform to stand on, and are in somewhat of a different category than the preferred embodiment. Therefore, even if some of these patents may have claimed similar features, such as employing a counter flow design, etc. It is suggested that the preferred embodiment is substantially different in that it also has a platform.


In the following the invention will be described with reference to the enclosed drawings, wherein: Figs. 1 -4. schematically show some different possible installations according to the invention,

Figs. 5A, 5B, 6 show different views of a preferred embodiment of the invention,

Fig. 7 shows a further embodiment,

Fig. 8 shows a number of different embodiments of tubes according to the invention,

Figs. 9 A, 9B show a further embodiment according to the invention,

Fig. 10 shows different embodiments of sluices for self drainage, and

Figs. 11, 12 show some possible modifications.


According to the invention a Multi-Purpose Heat Exchanger has been envisioned, in accordance with claim 1, which can be installed in one of many places. It can be installed in a shower either above the floor or below it. It can also be installed under a bathtub. It can also be installed in the ceiling of a room below the bathroom if this is a more suitable location. It may even be installed in a centralized location in the cellar for example.

The aim of the preferred embodiment is to be commercially viable to as many customers as possible. The key factors have been identified as

• Preferred Location (Range of use / Adaptability)

• Efficiency

• Clogging

• Build up of fouling agents, rust, scum, lime, etc and ability to clean these agents. • Aesthetics

• Cost - Including manufacturing and installation costs, payback period, etc.

• Possible damage from freezing

Adaptability: The adaptability of the embodiment should attract a wider range of buyers, since the solution can be adapted to the specific needs of each customer, and can be re-installed as the customer's preferences or economic situation changes. It can be installed in any of the following locations:

• On the shower floor, or on top of an existing shower tray (Figure 1)

• Under the bathtub (Figure 2) - therefore it preferably should be thin, preferably around 15 cm or less. The current prototype is 12 cm tall and relatively small. A bathtub in Sweden is 67 cm wide, so the preferred embodiment preferably should be less than this.

• Under the shower floor (Figure 3)

• in the ceiling of a room below the bathroom (Figure 4)

• under the sink - due to space limitations under the sink, it preferably shall be possible to install the embodiment on its side. • in a centralized location, near the hot water heating tank

The heat exchanger will maximize efficiency:

• Firstly, the platform will have surface channels or grooves (Figs. 5 A, 11) to minimize the free surface area where evaporation losses remove energy before the water even makes it to the first drain intake.

• Secondly, and perhaps most importantly, the flow direction is counter flow (the drain water moved opposite direction to the incoming cold water supply). This unique arrangement allows for a theoretical limit of 100% efficiency, although this would take an infinite tube length, in practice, it still outperforms parallel or multi- stage systems.

• The heat exchanger base (Fig.5B) may be insulated with a polystyrene (or equivalent) foam

• In order to achieve ample heat transfer, a suitable heat transfer area must be available. With respect to the heat transfer discussion in the previous section, a cross linked polyethylene PEX tube length would need to be approximately 21 m in length to achieve a 1.25 square meter heat transfer area. Depending on a cost- benefit analysis, the actual tube length may in fact fall anywhere between 15 m and 100 m with the specified diameter of between 12 and 50 mm.

• There will preferably be a single wall through which heat exchange will occur. The wall thickness is determined by the tube diameter and the rated operating pressure.

To achieve 10 bars of rated pressure, for example, a 20 mm tube has a 2 mm wall thickness on standard domestic PEX tubes (even thinner for copper). However, if possible, the wall thickness would be preferably even thinner to increase the heat transfer rate. o It should be noted that using the PEX plastic instead of copper only marginally decreases the heat transfer rate, while significantly reducing

costs. Because of the large cost reduction, a greater amount of tubing can be used, thereby surpassing the original efficiency, with a reduced cost, o It should also be noted that this length of tube is difficult to fit into a shower tray with approximate dimensions of (65 cm x 65 cm x 15 cm), mainly because the 20 mm diam. tubes do not have a tight bend radius

(about 200 mm) and are therefore easy to kink. A concentric, overlapping coil/spiral arrangement, (Figs. 5B, 6) or alternately a flattened helix arrangement (Figs. 9 A, 9B) are considered ideal to fit as much tube as possible into the shower tray. Industrial-scale applications would require larger tubes, and a larger box, of course.

Clogging: The heat exchanger designs against clogging by allowing an internal overflow path, whereby drain water can simply move over the baffles (Fig. 6) and bypass the clog. The internal drainage path will be large to help prevent clogging from occurring in the first place. The reason for this is to prevent backup into the platform so that the end user is not standing in a puddle.

Fouling/Cleaning: The heat exchanger can be easily cleaned by lifting off the drain trap/screen and inserting a 2 foot long coil brush into the drain and simply scrub away any deposits. However, prevention is best. This is why the Drain intake is protected with a fine screen (Figure 5A, 6, 9B item 5)

To achieve the goal of being commercially viable, the actual costs must be minimized. This includes installation costs as well as manufacturing costs. The energy savings earned from using the heat exchanger should then save money for the users and the heat exchanger would eventually "pay for itself in a short number of years. The key factor to achieving this aim is not efficiency but rather the ratio of efficiency with respect to the overall product cost. The efficiency to cost ratio must be maximized. Manufacturing costs of the heat exchanger will be minimized.

• The heat exchanging surface will likely be a plastic tube, certified for about 10 bar of pressure. This would preferably be a cross-linked Polyethylene (PEX) plastic commonly used for domestic water lines. The diameter could be between 10 and 40 mm, but preferably either 20 or 22 mm because these are common standards and because this size is large enough so as to not introduce too much pressure drop, but can still be bent easily enough to fit a 21 -m length inside the embodiment.

• Alternately, the tube dimensions can be approximately 10 to 12 mm in diameter (approximately half the diameter) but having double the length (40 - 45 m). This has the effect that the tubes contain approximately half the volume of water as the preferred embodiment, which means the water can heat up more quickly, and that the tubes are easier to bend, which may mean that even more tube can be fit into the embodiment (perhaps up to 100 m).

• The base, platform and outer walls will preferably be molded from plastic as well, but may also be formed from a fiberglass-epoxy composite, or acrylic.

• The Installation costs of the heat exchanger will be minimized. • The product will preferably be packaged with some tools/accessories to aid in installation and cleaning such as simple punch out drain intake and outlets, bulkhead caps to plug punched out holes, adjustable feet to level the heat exchanger, a specialized brush for cleaning the tubes, and possibly fittings to adapt to different standards of plumbing in various countries. • The preferred embodiment shall preferably be made from recycled materials, or from new materials which are recyclable.

The preferred embodiment should be self-draining to minimize build up of fouling agents, and also to prevent possible damage due to freezing. The excess drain water will be automatically drained from the exchanger after every use.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT With reference to Figures 5 A, 5B and 6 features of a preferred embodiment will be described, by means of primarily describing/explaining each reference sign used in said figures.

1, 2, 3; Multiple Drain Intake Options allow the embodiment to be mounted in various locations. The unused intakes are either not punched out by the installer to begin with, or plugged with caps provided. The caps would be screwed into place and can be removed if needed, either for cleaning, or for mounting in a different location.

4; Adjustable feet are used to stabilize the embodiment on an uneven shower floor. These could also take the form of compressible foam or rubber pads (supplied in a variety of thicknesses).

5; Intake Drain Screen this is an integrated screen and trap together. This part would sit over the mouth of the drain intake and can be lifted out when it needs to be cleaned out. This helps to prevent fouling.

6; The Plastic PEX Tube carries the cold water to be warmed up. The tubes are supported by brackets (22). The direction of water flow in the pipe would ideally be in the opposite direction to the drain water flow (counter flow). This greatly improves the efficiency of the embodiment compared to if the flow were parallel flow. The tubes in the preferred embodiment would be arranged in a multilayer spiral/coil pattern as shown in Figures 5B and 6, or alternatively in a flattened helix as shown in Figures 9A and 9B. The number of layers in the spiral/coil or the number of turns in the helix may be greater than that shown in the figures.

The tubes could alternately have some kind of heat transfer augmentation, such as longitudinal fins, etc. See Figure 8.

The Tubes could alternately be made of copper or other metal. The required length of tube would therefore be shorter, but the manufacturing cost would increase dramatically.

7; Platform is a non-slip surface for standing on. It is slightly sloped to bring water towards the primary drain intake. The platform features surface channels or grooves (18).

8, 9, 10, 11 ; Drain Outlet Options allow the embodiment to be mounted in various locations. The outlet desired can possibly be punched out of the plastic mold and fitted with a bulkhead type fitting. The fitting could be screwed into place at the time of installation. Caps could be provided which would allow a punched out hole to be plugged if a different drain option is required. The unused outlets could thereby also be removed if needed, for cleaning, etc.

12; Base is the bottom of the unit. It is slightly sloped to bring water towards the primary drain outlets 8, 9, 10, or 11.

13; Baffles are used to guide the drain water through a spiral (Fig. 6) or helical/serpentine (Fig. 9A) course through the embodiment to roughly simulate a concentric tube heat exchanger. The flow is guided downstream towards one of the

drain outlets. The channels thus formed help to raise the heat transfer rate, which in turn helps to improve the efficiency. The baffles do not always join with the platform, but instead there is clearance so that if there is a clog somewhere in the embodiment, the drain water will overflow a baffle upstream and bypass the clog. Some baffles may however join with the platform in order to support it. Channel walls/baffles may also allow insertion of a cleaning brush over them in order to access the tubes for cleaning.

14; The Weir (see Figure 10a) is essentially a wall, slightly lower than the baffles, used to hold back the flow of water to cause the water level to rise inside the embodiment thereby submerging the tubes to allow heat transfer. The Weir has a small sluice (or opening) at the bottom to allow a small flow to bypass the weir. The purpose of the sluice is to allow all the drain water to exit the Embodiment when not in use. The reason for this is threefold. Firstly it keeps the tubes relatively dry to reduce corrosion, secondly, to reduce fouling, and thirdly, to reduce damage that may be caused by freezing in homes located in colder climates. Alternately, the weir may lack a draining opening and keep the tubes permanently submerged. The heat exchanger itself would then act like a trap.

16; Cold water input. The cold water supply first enters the embodiment at this point.

17; Warm water output. The cold water supply exits the embodiment at this point, and continues to the mixing/blending valve.

18; Platform Channels are essentially deep grooves in the platform (7) The depth of the channels would gradually increase towards the drain inlet (1). The purpose of the channels is to reduce the amount of time drain water is flowing over the platform, where it will rapidly lose heat due to convection and evaporation to the room. The platform's channel water will have a smaller surface area exposed to room temperature, and therefore retain more heat energy. The channels also help to reduce slipping while standing on the platform.

19; Peripheral Flanges are used to seal the platform and base together. This can be accomplished with a gasket and a series of C-section fasteners (23). The flange would preferably have a small groove or a lip around the perimeter to keep the fasteners from slipping outwards and falling off.

20; Wall (see figure 7). The wall would only be used in the case where the embodiment is installed in a shower, in a visible installation. It helps to reduce splashing outside of the shower. It is possible that the wall and platform are molded from the same piece of plastic, in this case, some other means of sealing the platform/wall to the base would be needed.

21; Bypass channel. This channel brings drain water from intakes 2 or 3 towards the centre of the unit. To do so, water must go over or under the PEX tubes (6) and channels (13).

22; Tube supporting brackets. These brackets hold the tubes in place.

23; C-section fasteners are essentially tubes with a slit down the side forming a "C". The fasteners are slid over all 4 of the peripheral flanges (19) to hold the base to the platform so that the C shape clamps the flanges together. An alternate method is to use several nuts and bolts. The C-section is preferred since they can be easily removed for cleaning purposes.

ALTERNATIVE EMBODIMENTS Alternate means to increase the external surface area of heat transfer tubes.

In order to increase the heat transfer on the outside of the heat exchange tube (6), a combination of increased area and more turbulent flow conditions is desired, increased area can be achieved using ribs, fins, multiple tubes in a bundle, knurling (not shown), or other protuberances.

Alternate means to increase heat transfer

The flow can be coerced into more turbulent behaviour through the use of baffles, weirs, trip wires, increased tube outer wall roughness, etc. Baffles (Fig. 8e) could also be used to move the flow in an 'S' pattern.

Alternate method to layout tubes in heat exchanger.

The tube could also be wound in an overlapping (flattened) helix tight spiral, coaxial with the channel, similar to a telephone cord (see figure 9A and 9B).

Alternate means to allow self-draining of the Embodiment.

The first and second weir in the preferred embodiment shall be weirs with fixed sluices of the type shown in figure 10 a) but can alternatively be of type b) or c)

The channeling platform

The platform channels could be arranged in a grid shape instead of radially. The grid- shaped design is shown in Figure 11.

Another possible method is to have the platform smooth (without channels) if it is too expensive to manufacture with the channels.

The platform channels may be either arranged in a grid (Figure 11), or radially (Figure 5A).