EP0035994B1

EP0035994B1 - Heat recovery system

Info

Publication number: EP0035994B1
Application number: EP19800901047
Authority: EP
Inventors: Arthur R. Kramert
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-09-10
Filing date: 1981-03-23
Publication date: 1984-04-11
Also published as: EP0035994A4; DE3067413D1; WO1981000760A1; EP0035994A1

Abstract

Apparatus for heat recovery from stacks and chimneys, using a heat exchanger scroll (2) mounted at the apex of such stacks, which consists of paired plates, spaced by deformed portions (21, 22) to define channels for flow of a liquid to be heated. A jacket (3) surrounds the scroll (2) is closed by a dome (5) having a filler cap (6). The scroll (2) is connected to the dome (5) by piping and a dome outlet (8) connected to a pump which circulates the heated water to a heating system by supply lines (11) or to other areas returning the liquid to the scroll (2) by return line (12). Sensible heat as well as latent heat of moisture in the fuels together with vapor produced as a combustion product can be scrubbed out. The use is to effect economies and to reduce environmental contamination wherever fuels are fired.

Description

Technical field

My invention is a system of mechanical apparatus for the recovery of waste heat from smokepipes, chimneys or stacks using natural draft.

Background art

Much effort has gone to placing heat exchangers of sorts somewhere in flues, chimneys or stacks with the common limitation of the need to keep stack temperatures hot enough to maintain adequate natural draft-and thus wasteful. Placement at the apex of a stack has been almost entirely overlooked.
The closest prior art is that of Kadan et al, US Patent number 4 222 349 which depicts a heat exchanger of coiled tubing at the top of a chimney to transfer the waste heat to water. Two basic deficiencies exist in that patent: (1) Kadan does not perceive, nor teach, nor claim, that locating at the apex has any virtue. Actually this location leaves the gases below still hot, the draft unimpaired, and as a consequence invites us to recover as much heat as we can use if only we contrive a truly efficient exchanger; (2) Kadan's exchanger is woefully inadequate because the gases go straight through an inner shell to the atmosphere while the exchanger tubing lies in an annulus of stagnant air surrounding that inner shell; the waste gases do not scrub the exchange surface; the gases cannot condense on the exchange surface to surrender latent heats; nor can the wetting by condensates be exploited to further enhance the heat transfer rates.

Disclosure of invention

My invention uses a special heat exchanger mounted on a stack or chimney and baffled so that the gases go up through the core and down through the exchanger before release to the atmosphere.
Condensibles are trapped out and their latent heats recovered. The column of hot gases still exists in the stack below and its normal draft is not impaired. The shrink in volume of the cooled gases and the condensing moisture within the unit actually augments the draft.
The reclaimed heat is transferred to liquid which is circulated to just those areas where it is desired. The heat obtained from fireplaces and stoves is greatly increased and without the aesthetic violence of blowers, heat tubes, or the sheer physical bulk of surface.
The fuel energy saved results in lesser environmental pollution; and of course, oil fuels conserved are available for other uses.
A number of problems arise when we install heat exchangers in various gas streams; the products of combustion vary with the nature of the fuels burned. The salient corrosives are pyro- ligneous acids with wood fuels and sulphur compounds with coal and oil.
We should avoid the use of dissimilar metals with their inherent galvanic vulnerability due to their separation in the electrochemical series. This also argues against any use of brazed joints.
Some metals in the stainless steel series, while they are acid resistance, have poor thermal conductivity and thus do not lend themselves to fabrication as finned tubing where the heat travel is a relatively long path.
Any exchanger in such service will eventually foul with soot and must have a configuration which permits of access to its surfaces for cleaning.
Ideally, it should be possible to enable the exchanger to be varied in capacity and function without the need for a large series of dies or die changes for different production runs.
Thus to make the basic system invention practical and perform well, it has been necessary to design a heat exchanger for it which specifically considers these criteria.
When the driving force, the spread between source and sink, the delta T, is high, the requisite area for the transfer of a given quantity of heat is low. As the delta T is reduced the area required increases directly; but if we are crafty and use all the methods at our disposal-a phenomenon can be exploited. When the gases are cooled enough to initiate condensation they not only surrender their sensible superheat and latent heats, but the condensables form droplets that trickle down a vertical surface and overall coefficient jumps up. Thus a further reduction in delta T does not require the brutal use of still more area because overall transfer is so greatly augmented. The condensables are there in the gases. Air dry wood has 25% moisture by weight; oil and natural gas burn to produce 1.1 pounds and over 2 pounds of water vapor respectively, per pound of fuel fired.
The exchanger surfaces must be directly in the gas stream; the benefits are more than a trade-off for the possible fouling and corrosion problems which may need to be faced.
The invention relates to a method of recovering heat from a gas stream exhausting from a fuel burner or other heat source through a chimney, flue or stack using a heat exchanger which is arranged to route the gas stream therethrough and which has associated means for circulating a heat transfer liquid to and from points of usage, which is characterised in that the heat exchanger is located at the apex of the chimney, flue or stack with all its heat exchange surfaces located in the exhausting gas stream, whereby the entire amount of exhausting gas below the exchanger remains hot and, without loss of natural draught, a desired amount of heat is capable of being recovered by the heat transfer liquid.

Brief description of drawings

Sheet 1 of the drawings shows a vertical section through the equipment and a schematic of the other system elements. The exchanger unit either sits on top of a leveled masonry chimney or is supported to embrace a flue. The basic elements are: a torus shaped condensate pan (1); a heat exchanger scroll (2); a jacket around the scroll (3); a hinged cap (4) which is tensioned to open by a spring; the cap incorporates a dome (5) and a filler cap (6); short flexible hoses lead from the exchanger outlet (7) to the dome and from the dome (8) down to piping which leads to a pump; hose (9) runs to tubing which leads to bottle (10).
The pump circulates the heated water through any conventional heating system or device, such as baseboard heaters, convectors, radiators, unit heaters, etc. or even to a hot water storage tank. This, via supply line (11) and return (12).
Bottle (10) is open and sized to hold the volume of liquid displaced by expansion plus the small volume in dome (5). The liquid in the dome is expelled automatically upon the generation of any steam-as would follow upon any power failure. The cap, now unloaded, is opened by the aforementioned spring. Thus the exchanger is by-passed and no liquid is boiled off or wasted out of the system.
Sheet 2 of the drawings shows the construction of the heat exchanger itself. This begins as a two ply sandwich of metal strip (Figure 2); these are embossed, fitted with liquid connections and rolled into a spiral scroll (Figure 4); the scroll is inserted into the jacket (Figure 5) which protects the scroll from abuse and resits the "hoop stress" or the spring-back of the scroll.

Best mode for carrying out the invention

Fabrication of the exchanger (see dwg. sheet 2) starts with slitting suitable metal to width. For example-for use with wood fires-a typical choice would be type 304 stainless steel, fully annealed, in .007 inch (.2 mm) thickness. Two metal coils are each run through an embossing roll and laid together in a sandwich. The embossing is in modular rows to permit varying the width of the strips without change to the rolls. Note the skew pattern across the strips (Figure 2).
One set of rolls impresses shallow tits to the interior of the sandwich (21); the other set of rolls impresses larger and wider spaced dimples to the outside of the sandwich (22). The small tits establish a liquid channel and will be on the concave face of the scroll; the larger dimples maintain a gap for the gases. Both patterns serve to rigidize and strengthen a metal gauge chosen for economy and light weight.
The edges of the lapped sheets are run through the wheels of a resistance seam welder or a high frequency resistance welder. The skew cut (Figure 6) is parallel with the dimples and the liquid and connection cuff (24) is inserted here. This piece is bench welded by Tungsten Inert Gas will filler metal, or alternately by Metal Inert Gas wire feed to embrace the half nipple (23). This cuff is a heavier gauge, say, .0375 inch (1 mm plus). The cuff wraps around the nipple but gradually blends off in a tapering run until it is pinched flat at the other end. These two liquid connection welds are the only manual welds required in the exchangerfabrica- tion; all others arae machine made and require lesser skills.
A machine seam is run across the skew to join the thin skins to the heavier cuff, with a temporary copper chill sheet inserted (25) through the end of the sandwich and then withdrawn and the end welded shut. The top nipple, the outlet nipple, should not be inserted deeper than 1 cm. into the sandwich. The whole is now rolled into a spiral scroll and leak tested. If quality control has been good, the scroll should be inserted into the jacket and then tested. The test is made by admitting only low pressure air to one of the ports with other capped, and submerged in lukewarm water with a wetting agent added.
Referring to sheet 1 of the drawings: the construction of the condensate pan (1), the cap (4), and the dome (5) are obvious and are made by a conventional spinning or stamping as is determined by volume and labor cost intersects in the country of manufacture.
In the interests of clarity and simplicity, minor elements of construction are not shown on the drawings when they can be conveyed by description, which follows:

The interior of the jacket should have no sharp edges or protrusions to gouge the exchanger scroll which may be withdrawn for ease of cleaning.

Connection (13) is a stub tube inserted low so that condensate may be drained off if desired.
A weatherproof, snap-acting thermostat, set for 140°F (60°C) is surface mounted on the exterior of the jacket anywhere close under the cap-preferably near the hinge. This is to start and stop the pump when a manual-automatic switch is set to automatic position.
The dome (5) has three functions: it is an air- separation chamber where air swept out from anywhere in the whole heating system will be trapped out; it has a small superheating effect; it is an unloader to prevent boiling off liquid if the power supply were interrupted or the heating system could not absorb as much heat as was produced. If steaming occurs, the liquid in the dome is forced out via hose (9) to small diameter tubing which runs to bottle (10) through check valve (14) which has a controlled leak in the reverse flow direction. When vapor condenses in the dome, it allows the return of liquid over a period of say half an hour to overcome the spring and again test for excessive heat. Incidentally, the cap in an open position without any firing going on, is a signal of air in the system to be bled out.
The tubes which project from the dome and to which the hoses connect, extend out over the hinge area and are given support at the outboard end to withstand shipping abuse.
The liquid in the heating system can well be clean, filtered rain water with only enough ethylene glycol type anti-freeze added to protect against minimum temperatures. This does not preclude other precautions against freezing which can be used.
The pump of choice selected for prototype models may be of interest. It is not universally known that small centrifugals are available with magnetically coupled drives which eliminates weeps from stuffing boxes or mechanical seals which may develop over years of service. Such pumps also protect themselves in that the impeller declutches should grit from a dirty piping system be entrained.

Industrial applicability.

The great amount of energy expended for heating looms foremost as an area of utility. It is an add-on; it does not require the discard of existing equipment; it improves the efficiency.
It has been demonstrated that the burning of airdry wood with a normal 25% moisture content, results in the loss of 29% of the heating value in expelling that moisture. Reducing any stack temperature to 200°F (83°C), and even lower at reduced firing rates, recovers that heat. There is also a loss of latent heat in burning oil, gas and coal, not only from moisture but largely as a product of combustion of their hydrogen content.
To assess the potential impact of fuel usage let us use the following premises and terms:

PE Present Efficiency-with existing equipment
NE New Efficiency-using this invention-95%
PFC Projected Fuel Cost-best estimate of annual fuel cost at some future time desired

Then: PE/NE=f_utu_re fuel costs as % of PFC; and when worked out for typical usages we have:
To restate: the final column in the above table is the multiplier to be used against the Projected Fuel Costs to determine costs enabled by this invention.
To take a specific example, say we have a poor oil furnace and we project our future oil bill to be $2500 per year, then with this invention it would be lowered to (2500x.558)=$1395 for a saving of $1105 annually, or $22,100 in 20 years. Payout is less than a year.
This is not a distortion; there are many homes which have, or will have, such fuel bills.
The inference is plainly that the invention has utility.

Claims

1. A method of recovering heat from a gas stream exhausting from a fuel burner or other heat source through a chimney, flue or stack using a heat exchanger (2, 3) which is arranged to route the gas stream therethrough and which has associated means (11, 12) for circulating a heat transfer liquid to and from points of usage, characterised in that the heat exchanger (2, 3) is located at the apex of the chimney, flue or stack with all its heat exchange surfaces located in the exhausting gas stream, whereby the entire amount of exhausting gas below the exchanger (2, 3) remains hot and, without loss of natural draught, a desired amount of heat is capable of being recovered by the heat transfer liquid.

2. A method according to claim 1, wherein the exchanger (2, 3) is arranged to divide the exhausting gas stream into fine streams in intimate contact with the heat exchange surfaces, thereby reducing the by-pass factor to an economic minimum and causing condensibles in the gas stream to condense on to the heat exchange surfaces such that the heat exchange rate between the gas and heat transfer liquid is increased.

3. A heat exchanger for use in a method according to claim 1 or 2, comprising two metal sheets coiled into a spiral scroll (2), to provide spiral heat transfer liquid flow between the two sheets and free axial gas flow between the turns of the scroll (2), and an open-ended cylindrical jacket (3) containing the scroll (2), characterised in that the open-ended, scroll-containing jacket (3) is arranged to facilitate ready withdrawal of the scroll (2) therefrom to enable the scroll (2) to spring apart for cleaning access and in that the free axial gas flow between the turns of the scroll (2) provides no lodgement of ash or solids therein.

4. A heat exchanger according to claim 5 fabricated completely from seven pieces of metal.

5. A heat exchanger according to claim 4 of all welded construction for withstanding high operating temperatures and corrosion in acidic environments.

6. A heat exchanger according to any of claims 3 to 5, including means (5-10) for protecting against the boiling-off of heat transfer liquid.