US2828723A - Continuous flow water heater - Google Patents

Description

April 1, 1958 A. L. MILLER 2,828,723

' CONTINUOUS FLOW WATER HEATER Filed July 29, 1954 INVENTOR. I I

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rates and 3 Claims. (Cl. 122-264) The present invention relates generally to water heaters, and more particularly to such devices of the continuous flow type.

Continuous flow Water heaters are those in which water is heated only during the supply thereof as opposed to storage type heaters in which the water contained in the storage tank is maintained at the desired supply temperature, and the Water is recirculated to bring the temperature up to the desired hot water temperature whenever a portion of the contents of the storage tank is withdrawn.

One of the principal disadvantages encountered in the use of continuous flow heaters, particularly those which are gas fueled, is the fact that one end of the water tube is, due to the incoming cold water, constantly at a temperature considerably below the dew point of the combustion products surrounding the outside of the tube, while the other end of the tube is usually somewhat above the temperature of the desired hot water output due to the fact that the flow rate is fairly slow and of laminar rather than turbulent flow, thus reducing the efiiciency of heat transfer between the tube and the water inside the same. Thus, in short, one end of the water tube is too cold and the other end too hot.

Both the low temperature at the incoming end of the boiler tube and the high temperature at the opposite end cause operational difiiculties in addition to a loss of heating efiiciency. At the cold end of the tube, condensate is formed on the outer surface along a substantial length of the tube due to the fact that the products of combustion are heavily saturated with water vapor, and upon being cooled to the temperature of the inflowing water, condense out most of their moisture. This moisture drips down on the burners into the lining of the fire box, and in general creates serious corrosion problems.

In order to reduce as much as possible the effects of exterior condensation as above described, it has been the practice in some conventional continuous flow heaters, to bypass a major portion of the total flow of water through the heater and heat only a relatively small amount of such total flow to a high enough temperature so that the heated water, when mixed with the bypassed water produces the desired output temperature. Heating the water to such relatively high temperatures reduces the length of the cold portion of the heater'tubes which are below the dew point temperature of the flue gases, and thus somewhat reduces the effects of condensation. Such expedient only substitutes another difficulty, however, that of the deposit of minerals on the interior of the heater tubes. As is well known, the deposit of minerals in so-called hard waters, increases as the temperature of the water increases. Thus, the above-described expedient of heating a small portion of the total supply to a relatively high temperature may initially be fairly effective insofar as condensation is concerned, but in a relatively short time the interior of the tubes, due to the very high temperatures of the water atet ice therein, become clogged with mineral deposits and the entire system breaks down. Furthermore, even a very slight deposit of minerals acts as-an insulator and greatly reduces the heat transfer efficiency.

Another practical disadvantage of continuous flow water heaters of thetype heretofore available, has been the low efficiency of heat transfer between the hot products of combustion and the water tubes. In an effort to increase this efficiency, water tubes have been provided, carrying an external helical fin, thus to increase the heat transfer surface on the exterior of the tube. Because it is impractical to concentrate the externally applied heat on asingle flow tube, it has been the usual practice to provide a number of parallel water tubes, finned as above described, through all of which the water flows in parallel paths, the tubes being arranged side-by-side in the fire box above the burners.

While the provision of external fins in the manner above described greatly increases the external area of the water tube, and thus improves the elficiency insofar as any particular tube is concerned, these fins interfere with the close spacing of the tubes and thus-permit a considerable amount of hot gas to pass between the tubes without coming in intimate contact with either the tubes themselves or the fins thereon. While it would be theoretically possible to arrange the finned tubes in sideby-side relationship with the fins on one interlaced between the fins on the adjacent tube, such mounting presents many practical difiiculties, and in any event creates too great a restriction to the hot gases at the point where tube fins are interlaced or meshed with the fins of an adjacent tube.

Bearing in mind the above-mentioned difficulties encountered in continuous flow water heaters of the class described, it is a major object of the present invention to materially increase the efiiciency of a continuous flow water heater.

It is another object of the invention to provide an arrangement in a continuous flow water heater for preventing wide temperature differentials in the heating tubes, thus to eliminate condensation of moisture on the exterior thereof and deposit of minerals on the interior thereof.

It is still another object of the invention to provide a continous flow water heater in which the rate of flow through the heater tubes is several times greater than that dictated by the rate of hot water delivered whereby to decrease mineral deposits and increase the thermal etficiency at the heat exchange surfaces of the heater tubes.

A still further object of the invention is to provide for turbulent flow through the heater tubes of a continuous flow water heater whereby to increase the thermal efficiency.

Yet a further object of the invention is to provide a water tube mounting-fire box assembly wherein optimum contact with the hot gases is achieved while presenting a minimum constriction to the flow of gases around the tubes.

A still further object of the invention is to provide a water tube mounting assembly in a continuous flow heater wherein it is possible with a minimum of disassembly to clean the interior of the tubes.

The foregoing and additional objects and advantages of the invention will be apparent from the following detailed description of a presently preferred embodiment thereof, consideration being given likewise to the accompanying drawings in which:

Figure 1 is a partially cut away perspective view of a continuous flow water heater embodying the present invention;

Figure 2 is an enlarged fragmentary elevational section taken on a longitudinal plane through the water tube assembly and illustrating the flow path thereto; and

Figure 3 is an elevational transverse section taken on the line 3-3 in Figure 2.

Referring to Figure 1 in the drawings, it will be seen that the heater embodying the present invention is enclosed in an outer, generally rectangular sheet metal housing 10, having gas fired burners 11 near the bottom of an internal fire box 15, and horizontal water tubes 12, 13, and 14, extending longitudinally across the fire box 15 near the top. The burners 11 and the water tubes 12, 13, and 14, are completely enclosed by the fire box 15 which is lined with refractory tile and insulated from the outer housing 10 by a layer of rock wool 16 or the like. An interior hood 17 of sheet metal serves to collect the flue gases after they have passed in contact with the water tubes 12, 13, and 14, and direct such gases to the flue 18 at the top of the housing 10. Fuel gas is supplied by a fuel line 21 and conventional gas control elements are provided in the form .of a manual and automatic pilot control valve connected to the gas supply 21, a pressure regulator 22, a solenoid control 23 for the main burner, and a conventional safety pilot 24. Since the just-destcribed elemtns do not form an important part of the present invention, no further detailed description thereof is deemed necessary herein.

The water tubes 12, 13, and 14, are arranged in three transverse banks, one above the other, as can be seen in Figure 3, the lowermost bank being designated by the reference character 12, the intermediate bank being designated at 13, and the top bank being designated at 14. It will be noted that bottom bank of tubes 12, and the top bank 14 are provided with heat transfer fins 25 which are coined from the parent metal of the tube by a rolling operation, while the intermediate tubes 13 have no external fins. The relative arrangement of the lower bank of tubes 12, the intermediate bank 13, and the top bank 14 is such that the peripheries of the fins on the upper and lower banks 12 and 14 are close to, but not quite in tangential contact with the intermediate tubes 13.

I have found that, contrary to what might be expected, a much greater heating efiiciency for a given rate of gas consumption can be achieved by the use of an intermediate bank of unfinned tubes staggered with respect to the tubes in the other banks as shown in Figure 3 than can be achieved with an equivalent total number of tubes, all of which have fins. This appears to be due to the fact that the intermediate unfinned tubes 13 act as baffles, diverting and causing the hot flue gases to flow in curvilinear paths as shown by the arrows in Figure 3 whereby to pass the gases in considerably more intimate contact with the total surface of the fins on the finned tubes than would be the case if no intermediate tubes 13 were present. Separate bafiles, not containing water to be heated could be employed, but the heat consumed in heating the baffles would be of course be wasted.

If it is attempted to arrange eleven finned tubes in the general pattern indicated in Figure 3 whereby an intermediate bank of finned tubes would serve to a certain extent as bafiies, it is found that the increase in efiiciency is not achieved due to the geometry of the finned tubes and the necessary increase in spacing therebetween. On the other hand, if it is attempted to use all finned tubes, and to intermesh the fins on the intermediate banks with those of the banks above and below, the constriction to the flow of flue gases which is produced by the intermeshing of the fins, is such as to greatly reduce the efficiency.

The tubes 12, 13, and 14, are mounted between headers 28 and 29 as by brazing indicated at .30, which headers form ends for the fire box 15 and are hollow to provide water manifold chambers 31 and 32 which respectively divide the fiow and recombine it as water passes through the heater.

The cold water input to the heater is at the left end of the device through an external input manifold 33, having an internally threaded pipe connection 34 to receive the input pipe (not shown). The input manifold 33 forms a separate chamber from the manifold chamber 31 in the header 28.

The input manifold 33 is bolted to the lefthand header 28 as by bolts 35, thus forming a closure for the upstream manifold chamber 31. A similar closure is provided for the chamber 32 in the form of a plate 39 bolted to the header 29. With both the input manifold 33 and the plate 39 removed, free access is given for cleaning the tubes 12, 13, and 14, as by reaming, or otherwise.

Communication from the interior of the input manifold 33 to the manifold chamber 31 in the header is provided through a plurality of jet nozzles 37 which are secured in threaded openings in the inner wall 38 of the input manifold. The arrangement of the jet nozzles 37 in the wall 38 is such that each one of them aligns axially with one of the tubes 12 and 13 in the bottom and intermediate banks of tubes. No nozzles are provided in alignment with the upper bank of tubes 14. The internal bore of each nozzle 37 is relatively small whereby to produce a relatively high velocity jet of cold water introduced into each of the tubes 12 and 13. The kinetic energy in such water jet acts in the manner of an injector pump to draw water from the manifold chamber 31 into the respective tubes 12 and 13 whereby the water flowing from left to right through the tubes 12 and 13 constitutes a mixture of incoming cold water from the jet'nozzles 37 and the contents of the manifolding chamber 31. Such mixture is, due to the action of the jet nozzles 37, highly turbulent thus promoting a much greater efficiency of heat transfer between the water and the interior walls of the tubes 12 and 13 than would be the case with smooth or laminar flow.

The water issuing from the tubes 13 and 14 into the downstream manifold chamber 32 passes upwardly where the flow divides, a portion of the heated water passing out through the output connection 40 and conduit 41, and the other portion of the flow passing back to the upstream manifold chamber 31 through the top bank of tubes 14.

The back pressure in the hot Water supply line (output) 41 is such as to cause a substantial portion of the how to recirculate from right to left through the top bank of the tubes 14. To correctly proportion the flow division just described, a restricting orifice member may be inserted in the output connection 40 if needed. The portion of the flow passing from right to left through the top bank of tubes 14 is further heated by the flue gases, and thus the temperature of the water in the upstream manifold chamber 31 at the left header 28 is the highest in the entire system. Such high temperature water mixed with the cold input water issuing from the jets 37 so raises the temperature of the water Within the tubes 12 and 13, that the portion thereof which is below the dew point temperature of the line gases is substantially eliminated. Thus, the condensation of moisture on the exterior of the tubes 12 and 13 is virtually nil. Since the temperature of the water passing through the upper tubes 14 is even greater than that at the lefthand end of the tubes 12 and 13, there is no condensation on the top bank of tubes 14.

By an appropriate adjustment of the back pressure in the hot water supply conduit 41, the proportion of the flow which passes in a counter direction through the upper tubes 14 can be adjusted. It has been found that with the arrangement of 11 tubes illustrated in Figure 3, an optimum overall efiiciency is achieved when the flow rate is such as to cause approximately 25% of the total heat transfer to occur in the top bank of tubes 14. This percentage can be determined by measurement of flue gas temperatures at various points in the tube banks.

By way of example, but not by way of limitation, an

adjustment as just described when used in connection with aheater such as shown in Figure 1 adapted to heat 4.8 gallons of water per minute at a temperature rise of 50 F. was found to have a water temperature rise as between the right and left ends respectively of the top bank of tubes 14 of only 2. This relatively small temperature rise in the counter flow tubes 14 indicates the high flow rate through the individual tubes as compared to the overall effective flow rate through the entire system. Putting it another way, the sum in the flow rates of the individual tubes is much greater than the flow rate at which heated water is delivered atthe output 41. If the water flowed from left to right in all tubes as in most conventional heaters, the sum of the flow rates would merely equal the delivery rate and the fiow rate through any tube would be very much smaller than in the present case, and the efiiciency correspondingly reduced. The above-mentioned high rate of flow in the heater described herein has the advantages already mentioned of greatly reducing the deposition of minerals on the interior walls of the tubes, and also producing and augmenting the turbulence within the tubes thus to increase the heat transfer efliciency.

While the heater construction shown and described herein is fully capable of achieving the objects and providing the advantages hereinbefore stated, it is capable of some modification without departure from the spirit of the invention. For this reason I do not mean to be limited to the forms shown and described, but rather to the scope of the appended claims.

I claim:

1. In a continuous flow water heater: an input chamber having means to receive water to be heated; an output chamber having means to deliver heated water from the heater; a plurality of substantially parallel water tubes substantially horizontally disposed and arranged in suecessively vertically displaced banks; each of said tubes having open ends communicating directly respectively with said input chamber and said output chamber; said means to receive water to be heated comprising a plurality of jet nozzles, each positioned opposite and directed respectively toward a corresponding one of the plurality of tubes in at least the lowermost of said banks; at least the uppermost of said banks of tubes having no obstruction in said input chamber opposite the open ends of said tubes; the open ends of the tubes in said output chamber being unobstructed; said jet nozzles being arranged to eifect high velocity circulation of water through at least said plurality of tubes in said lowermost bank to said output chamber; means to pass a hot fluid upwardly around said banks of tubes to heat the same; whereby the apparatus is adapted to induct heated water into tubes in at least the lowermost bank wherein the same is mixed with injected unheated water and the mixture is passed at a high velocity to the outlet chamber, a part of the water is recirculated through tubes in the uppermost bank to the input chamber, and at least a portion of the highly heated water entering the input chamber from the upper bank of tubes is drawn downwardly and said induction takes place.

2. The apparatus of claim 1 in which said plurality of substantially parallel and horizontal water tubes are of equal diameter; the tubes in alternate banks having external circumferential heat transfer fins thereon and the tubes in any intermediate bank being unfinned; the tubes being spaced apart, in a horizontal direction, a distance substantially equal to the diameter of said tubes; the finned tubes being in vertical alignment with one another and spaced apart in the vertical direction a distance substantially equal to the diameter of said tubes; said unfinned tubes being in vertical alignment with the spaces between said finned tubes; whereby the tubes in one bank act as bafiles for said hot fluid passing transversely of said tubes to divert the said hot fluid into intimate heat transfer contact with the tubes in the next succeeding bank.

3. The apparatus of claim 1 in which said jet nozzles are mounted in a removable plate defining at least a portion of one wall of said input chamber.

References Cited in the file of this patent UNITED STATES PATENTS 834,444 Bausher et a1. Oct. 30, 1906 1,027,327 English May 21, 1912 FOREIGN PATENTS 510 Switzerland Oct. 4, 1904 28,953 Switzerland Apr. 16, 1903 116,209 Great Britain June 6. 1918 282,717 Great Britain May 3, 1928 603,646 Great Britain June 21, 1948 675,924 Great Britain July 16, 1952

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