GB2379006A

GB2379006A - A waste water heat recovery system

Info

Publication number: GB2379006A
Application number: GB0115642A
Authority: GB
Inventors: David Thomas; Peter Thomas
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-06-27
Filing date: 2001-06-27
Publication date: 2003-02-26
Anticipated expiration: 2021-06-27
Also published as: GB2379006B; GB0115642D0

Abstract

A waste water heat recovery system comprises a combined heat exchanger and accumulator (storage) having a waste pipe 83 for communicating waste water from a waste inlet 82 to a waste outlet 84, a cylindrical body 81 disposed around the pipe 83 for communicating cold water through an inlet 86 to an outlet 87 so that heat transfers from the waste water to the cold water. Cylindrical body 81 has baffle plates 88 which help locate waste pipe 83 via apertures 92 and stiffens and braces the system when filled with fluid. The baffle plates 88 have holes 93 angularly offset to direct and impart spiral flow of the cold water so that turbulent mixing takes place. A vent pipe 91 is connected to the waste pipe 83 to vent entrapped air and to prevent an air lock and cylindrical body 81 is provided with a thermal insulation layer 97. Waste pipe 83 and cylindrical body 81 may be made from stainless steel, a plurality of combined heat exchangers and accumulators may be interconnected in series into single rows or multiple interconnected rows.

Description

Waste [Water] Heat Extraction & Recovery This invention relates to'fluid'-eg a liquid, such as water-heat transfer, extraction and recovery.

As such, aspects of the invention are particularly, but not exclusively, concerned with (partial) recovery of heat energy from a fluid.

An example would be heat energy previously imparted to the fluid, for a specific end purpose or intent-at the user's expense-and not all of which has been exhausted in meeting that purpose.

The overall intention is to reduce energy input for ongoing (re-) heating-of another fluid.

More specifically, to this end, heat transfer, extraction and recovery can be undertaken once its primary (heated) role has been fulfilled, and upon discharge to what otherwise would represent a waste of both the fluid and its heat energy.

Heat Energy (Re-) Cycling Thus heat energy or thermal re-cycling, without necessarily fluid re-cycling, is contemplated.

In the case of fluid, in particular a liquid such as water, consumed for cleansing purposes, pre-heating is commonplace to promote solubility of soap or detergent, soften and dissolve dirt and grime, and promote overall cleansing or wash efficacy.

For personal (skin) contact, warm, if not hot, water is more comfortable, the average bath temperature being some 38 degrees Celsius to 40 degrees Celsius, yet even modest temperature elevations require significant heat energy input-which is simply wasted when the water is discarded after use.

An issue of heat energy recovery cost remains-dictating a special low cost, and low maintenance, heat exchange configuration.

Heat Exchange Generally, the heat exchanger art embraces complex constructions reflecting sophisticated thermodynamic considerations-but which would not necessarily be appropriate, or economically justified, for hot water plumbing systems.

Thus, one established approach for heat exchange between otherwise independent and separate fluid (eg liquid) media would involve routing fluid through a convoluted conduit flow path.

An internal combustion engine cooling system would be a case in point-although recovered heat is simply dissipated to ambient air stream, with fan assisted, forced flow.

Commonly, a finned matrix, called a radiator (albeit heat transfer is by conduction and convection), is fitted to the conduit for one fluid, in order to increase contact surface areas between fluids.

This is complex and costly to fabricate, install and maintain-for example being vulnerable to internal sedimentation, clogging and corrosion.

Domestic waste water heat recovery arrangements have been explored, but are considered by the Applicants as impractical, for example prohibitive in complexity and expense.

Thus, for example: Local Appliance Heat Exchanger-heat exchanger installation in shower floor : GB 2295666 (Jackson)-waste warm water from shower discharged via spiral outlet conduit with internal mains feed; heat from waste water discharge transferred to mains water before inlet to shower head.

US 5791401 (Nobile)-cold water feed to shower coils around shower waste pipe, configured as water retention trap, or passes coaxially within waste pipe, to extract heat and pre-warm water, before inlet to shower head. Conduit surface area increase through corrugation also envisaged.

Heat Extraction from Waste Water Discharge-no accumulator: US 4502529 (Varney)-incoming cold water passes through coiled feed surrounding waste, for heat recovery from continuous waste water flow.

Continuous & Stored Waste Water: US 4454911 & US 4550771 (Arbabian)-overall system with intermediate heat transfer medium (eg water) to store and transfer heat from waste water to cold water feed. Opposed feed and discharge flows, internal heat exchanger baffles and separate insulated waste water accumulator with bypass envisaged, vis: US 4454911-separate, helical, waste water and cold water supply conduits, located by helical baffle, immersed in heat transfer medium.

US 4550771-separate, non-helical, waste water and cold supply water conduits, located by baffles, surrounded by heat transfer medium.

Heat Extraction from Waste Water Storage: GB 1595319 (Rump)-waste water stored in insulated tank, to heat incoming cold water passing through a floating heat-exchanger tank. Re-use of waste after heat extraction, for cistern flushing also envisaged. Local tank surface area increase and turbulence promotion through corrugation utilised.

DE 3113784 (Heimann) & EP 0058636 (Kalberer)-heat extraction installations with cold water pre-heated by passage through coiled pipe within insulated waste water container storage.

US 5301745 & EP 0532910 (Seib)-household installation waste water collection from multiple consumption points and delivery to distribution tank. Waste water above pre-determined temperature diverted from tank to heat exchanger in which cold supply water preheated by passage though coiled pipe. Waste water discharge upon heat surrender.

Heat Exchanger Configuration : DE 433774 (Stiebal Eltron GMBH & CO KG)-heat exchanger with conical baffle wall, supporting conduit array, located within fluid collection tank FR 2579312 (Nibart)-with multiple tanks, and selective flow distribution by temperature, for waste water heat energy recovery.

US 5736059 (Mackelvie)-elaborate convector heat exchanger with thermal storage, for waste water heat recovery, with an In-line solids separator, turbulent flow creation, base vibrator to promote heat transfer and automatic flush.

EP 0114583 (Indesit) - heat pump condenser sunk in cistern, with evaporator to inhibit waste and calcareous deposits upon conduit heat transfer surfaces.

Waste Accumulator and Heat Exchanger Separation: GB 2304877 (Selley) separates hot waste water accumulation from heat exchange, by using a header tank and diverter pump.

EP 0088055 (Indesit) features hot or cold waste water storage, with an evaporator coupled to a waste pump in an overlying water heater cistern, allowing heat recovery even from a relatively cold mains feed Heat Recovery from Water Heater Exhaust Gases: US 4251028 (Nicolai), US 4397297 (Wie), & GB 2178513 (Original Fuel Co. Ltd)address water heater efficiency, by utilising hot exhaust gases from gas water heater, to pre-heat water inlet feed.

Aspects of the present invention are concerned with a simplification in heat exchanger construction-to admit of replication and repetition, to improve heat recovery, without prohibitive cost, consistent with low installation cost.

No supplementary or intervening heat transfer medium is necessarily envisaged, beyond the material of the heat exchanger itself.

The intention is also to provide a heat sink, in which heat exchange can be undertaken over long or short periods-such as to achieve, say, a nominal temperature rise of some 100 degrees Celsius over approximately 10 minutes, for a

given volume.

To that end, fluid from which heat is to be recovered is allowed to form a static reservoir, accumulator, or (heat energy) pool-and through which a separate fluid to be heated can be passed.

Heat transfer is by conduction through the respective fluids and the walls of intervening pipes or conduits.

Statement of Invention According to one aspect of the invention, a waste water heat energy recovery system comprises: an integrated heat exchanger and waste water accumulator module, with a (direct) through passage for waste water, from which heat is to be recovered, a chamber, disposed around the waste water through passage, for relatively cold (mains) water to be heated, and couplings for external supply and waste.

Such a combination of the roles of heat exchanger and waste water accumulator has been found advantageous in practice.

In particular, fabrication, installation and maintenance costs are relatively low.

For convenience, either the terms accumulator or heat exchanger are used herein interchangeably to reflect both accumulation and heat exchange roles.

The accumulator role is consistent with the system remaining dormant for long periods of low or no water consumption, such as between morning and evening peak consumption periods, during the day or overnight.

The system retains heat, ready for heat exchange with fresh cold water feed, and any dissipation is to the surroundings, raising the ambient temperature, and easing insulation requirements.

An individual accumulator is conveniently configured as inter-fitting elongate (thin walled) cylinders, of heat conducting material.

Thus such cylinders are desirably fabricated from proprietary (eg stainless steel 316) pipe work, of 1.5mm or 2. 0mm sheet, fitted with end plates, of some 8. 0mm sheet.

The particular length or diameter admits of considerable variation-to suit installation considerations-an example being a 100mm diameter internal waste tube located generally coaxially within a 150mm diameter outer tube housing, with an intervening 25mm channel or chamber, for mains water to be heated.

A 20-35mm insulation wrap layer around the outer tube is also envisaged, to promote heat retention.

An elongate overall form is desirable, for accommodation within floor or ceiling voids and between structural joists.

Capacity is readily increased by, say, serial, interconnection of multiple modules.

Overview In the aforementioned diverse body of waste water heat extraction and boiler preheating art, the modular approach of the present invention is not envisaged.

Specifically, the emphasis of this art IS upon centralisation, rather than distribution, of heat exchange-and, on occasion, the separation of heat accumulation from heat exchange itself.

In contrast, the Applicants have adopted multiple (appliance) localised, seriallyconnected, combined accumulators and heat exchangers, of uniform modular construction.

This in turn enables accumulator location at different floor levels and serial interconnection, to increase overall capacity.

The Applicants have devised a straightforward, robust, space-saving, configuration of proprietary interfitting elongate cylindrical conduit forms-consistent with between joist installation-arguably assists fabrication, installation and maintenance (ie modular replacement).

A long straight waste run per accumulator module is consistent with either dynamic direct waste through flow or heat soak stagnation.

Embodiments There now follows a description of some particular embodiments of waste water heat energy recovery according to the invention, by way of example only, with reference to the accompanying diagrammatic and schematic drawings, in which: Figure 1 shows a part-sectioned, part cut-away view of a two storey (domestic) dwelling house, with a (mains supply) pressurised water feed, typically some 1. 5-2. 0 bar, to an in-line so-called'combination'boiler installation, and waste water heat recovery for pre-heating a pressurised (relatively cold) water feed to the boiler ; Figure 2 shows a corresponding (domestic) hot water plumbing installation to Figure 1, but adapted for a gravity-fed (and thus relatively low pressure) cold water cistern header tank feed to a hot water tank, with internal electrical immersion heater and indirect hot water re-circulation heater coil provision; Figure 3 shows a part cut-away view of an individual heat exchange accumulator, for the systems of Figures 1 or 2; and Figure 4 shows a cross section X-X of the individual heat exchange accumulator of Figure 3.

Referring to the drawings, the intention is to convey general principles, not a detailed plumbing system.

In the mains pressure hot water system of Figure 1, a combination domestic boiler 11 IS depicted, with a relatively cold water feed 14, derived from an external mains pressure supply 21.

The external mains supply 21 is routed to an internal supply riser feed 22, through a one-way isolator valve 23-in a manner conforming with local water supply authority regulations, to inhibit back flow and attendant mains contamination.

The mains feed 22 is subsequently pre-heated, at successive ground and first floor levels, by passage through a series of modular heat exchangers 18,19, associated with various hot water-consuming appliances and plumbing installations.

The heat exchangers 18,19 are also configured as heat accumulators, with a significant capacity for waste water-as detailed in Figure 3.

The waste outlets from individual appliances or plumbing installations, on each level or floor, are coupled to serially interconnected heat exchangers 18,19 on that leveland discharged to a common waste outlet 15,17, at that same level.

The waste outlets 15,17 at different levels may be connected to a common downpipe 29, with a stack vent 27.

Ground Floor At ground floor level 30, merely for the purposes of illustrative example, are depicted a washing machine appliance 31, a wash basin or sink 32 and a dishwasher appliance 33, each supplied with a common (mains pressure) hot water feed 12 from the output of the boiler 11.

More specifically, the hot water feed 12 is connected to a hot tap 34 of a sink 32, the hot water inlet of washer 31 and a single (cold) inlet of the dishwasher 33.

The respective waste outlets of appliances or installations 31,32, 33 are connected to a common waste feed 16, itself coupled to one end of a series of ground floor heat exchangers 18.

The last heat exchanger 18 in the series is coupled to the waste outlet 17, itself connected to the common waste downpipe 29.

The downpipe 29 leads to an external underground soiled water drain (not shown).

The cold water external mains supply 21 is routed as a pressurised, initially cold, water feed line 22-routed through the serially interconnected heat exchangers 18, in the opposite direction to the waste flow from common waste outlet 16 through to discharge at 17.

Water in emergent feed line 24 from the ground floor heat exchangers 18 is thus pre-

heated somewhat-and then passed to first floor level 40.

First Floor At first floor level 40, serially interconnected heat exchangers 19 receive a common waste feed 26 from various plumbing installations at that level.

More specifically, a bath 36 and a hand basin 37 are sited at first floor level 40 and their respective hot taps 46,47 are supplied with a common riser feed 13 from the boiler output line 12.

The common waste 26 is run through the serially interconnected first floor level heat exchangers 19 in the opposite direction to the pre-heated cold water feed 24-and emerges at outlet 15 to downpipe 29.

Each plumbing installation retains a respective local (waste water) trap, such as a'U'bend (not shown)-for onward connection to the common waste 26.

The pre-heated mains water feed 24 emerges from the first floor heat exchangers 19 with a further temperature rise, and is connected as a pre-heated feed 14 to the inlet of the boiler 11.

Such pre-heating reduces the heat energy input demand upon the boiler-with attendant shorter run times and reduced heating fuel consumption.

Vent In practice, individual heat exchangers 18,19 are fitted with vent stacks 91 to their respective waste water chambers, as shown in Figure 3.

These stack heights would exceed the highest fill level of any of installation whose waste is discharged into the waste chambers.

This avoids an overflow discharge from the stacks through over-filling the installations.

Similarly, waste chamber vents may be fitted with respective traps (not shown), such as a'U'-bend, in order to contain waste water odour.

These vents avoid air entrapment and incomplete filling and may either supplement or substitute for downstream provision such as the stack vent 27, or upstream venting through the plumbing installations.

Figure 2 shows a variant installation of Figure 1, adapted for a gravity fed-and thus relatively low pressure-hot water feed from a header tank 51 at roof space or loft level 50.

{Corresponding features to Figure 1 are given the same references.} A low pressure (down) feed line 52 from the header tank 51 is connected to one end of a row of ground floor serially interconnected heat exchangers 18.

A pre-heated, low pressure, output riser feed 54 from the heat exchanger series 18 is applied to upper level heat exchanger series 19-from which it emerges as a further pre-heated low pressure input feed 64 to the lower level of a hot water tank 61.

The hot water tank 61 incorporates a heater provision (not shown)-such as a direct electric immersion heater element, and/or an indirect re-circulated hot water heater coil connected to a hot water boiler circuit.

A hot water tank output feed 62 is connected to the top of the hot water tank 61 and feeds a hot water tap 46 of a bath 36 and the hot water tap 47 of a hand basin 37-with common waste outlet 56 coupled to one end of the heat exchanger series 19, whose eventual outlet 15, is connected to a down pipe 29, at first floor level 40.

A hot water (down) feed 63 from the tank outlet line 62 supplies ground floor appliances 31,32 and 33-with a common waste outlet 16 coupled to one end of the ground floor heat exchanger series 18, whose eventual outlet 17 is connected to a down pipe 29, and onward to an external underground foul water drain (not shown).

Again, waster water flow through the heat exchanger series 18 is opposite to that of the low pressure supply feed 52 through 54, in order to promote internal heat exchange.

Although, for ease of illustration, Figures 1 and 2 show single rows of heat exchangers 18,19, multiple interconnected rows may be employed.

Thus for example, several parallel orientated rows each of serially interconnected heat exchangers 18,19, with rows themselves also serially interconnected, could be employed-with individual rows nesting between floor or ceiling joists.

As shown in Figure 3, the individual heat exchanger bodies are conveniently configured as elongate structures, in order to fit between standard floor or ceiling joists, and to distribute the load evenly.

For ease of construction, installation and maintenance, a cylindrical heat exchanger form is adopted, allowing fabrication from proprietary tubing.

In practice, an outer cylinder of some 2 metres in length and some 15cm nominal diameter is constructed of (thin-walled, 1.5mm to 2. 0mm) stainless steel tubing-for corrosion resistance and longevity.

The tube is fitted with a series of internal diametral baffle plates 88, to locate internal pipework, direct internal flow, sub-divide the internal volume-and help stiffen and brace the structure when filled.

The baffle plates 88 feature mutually aligned apertures 92, to locate and support a waste through pipe 83.

The through waste pipe 83 is also desirable of stainless steel, and is connected, at bespoke end fittings, to a proprietary (plastic) plumbing waste inlet 82 and waste outlet 84, featuring internal seals.

Desirably, large capacity (say, nominal 2 inch, rather than 1.5 inch, outside diameter) proprietary plumbing fittings are employed, for enhanced (flow and static storage) capacity.

The through waste pipe 83 is of still larger diameter, in order to increase the surface area for (conductive) heat exchange.

The cylindrical body 81 is closed by end plates 89, say edge seam welded in place, to form a water-tight joint.

A vent pipe 91 is connected to the upper side of the waste pipe 83, to allow egress of what might otherwise be entrapped air, forming an air-lock inhibiting waste pipe fill.

An automatic air pressure release valve (not shown) might be fitted in this vent pipe 91, operable once a predetermined pressure differential had been reached between internal chamber pressure and ambient atmospheric pressure.

A standard (say, 15mm or 22mm) cold water inlet feed 86, and outlet 87, are connected to respective (BSP) standard (threaded) coupling, fitted upon end walls 89, to communicate with the main cylinder internal volume, around the through waste conduit 83.

Transfer holes 93 in successive, axially-spaced, baffle plates are angularly offset, to impart a spiral (turbulent mixing) flow within and through successive internal chambers defined between baffles 88.

Moreover, in operation, water feed and waste water flows are opposed.

An outer insulation layer or wrap 97, of some 20-35mm depth, envelopes the entire cylinder body.

Operational Considerations In practice, the combined internal capacity of the through waste is sufficient to accommodate at least the combined (one-off) waste discharge of all associated installations or appliances.

Effective heat transfer is contingent upon a certain accumulation of waste water-still at an elevated temperature after its initial consumption for the prime purpose of heating it in the first instance-to allow transfer of residual heat from the waste into the heat exchanger body.

The heat exchanger insulation 97 retains this heat until required, but any residual leakage from this effective heat sink simply warms the immediate environment-a tolerable side effect for most circumstances.

Thereafter, heat exchange between the heat exchanger body and the relatively cold water feed is feasible.

Generally, once the heat exchanger body has been pre-warmed, a continuous,

(modest) steady flow of initially relatively cooler water can still be warmed significantly.

Thus, for example, some 20-25% of the temperature rise which would normally be imparted for water heating (say to 40 degrees Celsius for bath water) could be achieved by heat recovery from preceding water heating.

Repeated discharge or flushing of any individual installation or appliance would-at worst-simply displace an equivalent volume from the accumulator to be discarded.

A working assumption for the common discharge path is that the waste is relatively warm-otherwise a repeated cold discharge would risk undermining heat recovery, in which case it would be better discharged Independently.

A cold water rinse or flush cycle of, say, a washing machine, or cold water sink waste, might have this effect-so might not be included, or selectively bypassed, according to sensed actual temperature.

Trials It has been found in preliminary trials that, even with a fairly rudimentary version, over a period of some 10 minutes, a temperature rise of some 10 degrees Celsius-eg from nominal 10 degrees Celsius mains supply temperature to some 20 degrees Celsius pre-heated temperature-can be imparted to a mains pressure cold water flow, through a standard nominal 15mm pipe.

This is to an initially static-then flowing-relatively cold water feed, at an initial nominal temperature of some 10 to 12 degrees Celsius, by extracting residual heat energy from a static waste water reservoir of some 40 or 50 litres of water, initially heated to a nominal average bath temperature of some 40 degrees Celsius.

Thus, for example, in Figure 1, the boiler feed 14 is of mains water elevated in temperature by some 10-12 degrees Celsius, achieved over some 10 minutes in passage through the serial integrated heat exchangers and accumulators 18,19.

Overall, the straightforward heat exchanger construction allows ready repetition for enhanced capacity, without elaborate convoluted pipe work or fins.

It is recognised that a balance would need to be struck between installation and maintenance costs and heating savings-aside from wider environmental considerations.

Large scale hot water users, such as hotels, guest houses, or nursing homes could be expected to benefit from significant savings, without penal installation or maintenance costs.

Component List 11 (combination) boiler 12 hot water feed 13 riser feed 14 water feed 15 waste outlet 16 waste feed 17 waste outlet 18 heat exchanger 19 heat exchanger 21 external mains pressure supply 22 internal supply riser feed 23 isolator/non-return valve 24 feed line 26 waste feed 27 stack vent 29 waste downpipe 30 ground floor level 31 washing machine 32 sink 33 dishwasher 34 hot tap 36 bath 37 hand basin 40 first floor level 46 hot tap 47 hot tap 50 loft level 51 header tank 52 feed line 54 riser feed 56 waste outlet 61 hot water tank/cylinder 62 output feed 63 hot water feed 64 input feed 81 cylindrical body 82 waste inlet 83 waste pipe 84 waste outlet 86 waste inlet feed 87 mains outlet 88 baffle plates

89 end plate 91 vent stacks 92 aperture 93 transfer hole 97 insulation layer

Claims

Claims 1.

A waste water heat recovery system comprising : an integrated heat exchanger and accumulator (18,19) module, with a (direct) through passage (83) for waste water, from which heat is to be recovered, a chamber, disposed around the waste water through passage (83), for relatively cold water to be heated, and coupling for external supply and waste.
2.

An accumulator, as claimed in Claim 1, configured as inter-fitting elongate cylinders.
3.

An accumulator, substantially as hereinbefore described, with reference to, and as shown in, the accompanying drawings.
4.

A waste water heat recovery system, including a plurality of (say, serially) interconnected accumulators, as claimed in any of the preceding claims.
5.

A waste water heat recovery system, substantially as hereinbefore described, with reference to, and as shown in, the accompanying drawings.