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This invention teaches how to reduce the copper content of a commercially available, all-copper, double-wall-vented, coil & tube Gravity Film Heat Exchangers (“GFX”) developed for the GFX™ system of U.S. Pat. No. 4,619,311. For example, a GFX+™ equivalent of a residential Model G3-60 GFX can be made using 71% less copper. Such significant copper-savings, with comparable heat transfer coefficient (“U”), effectiveness and coil pressure drop, is achieved by improvements in innovations disclosed in Provisional Patent Application 60/709,889 (Filing date: Aug. 22, 2005). Unlike conventional, single and multi-coil GFX's currently produced by several manufacturers, the present invention enables the use of a variety of metal and/or plastic components to achieve either single or vented-double-wall construction and lower manufacturing costs. In fact, higher U-values can be achieved by increasing the potable-water pressure drop of a GFX+ equivalent of any coil & tube GFX. The present invention also teaches how to simultaneously optimize U-values & pressure drop[s] and means of introducing controlled turbulence & mixing. Such trade-offs cannot be achieved in a conventional GFX design having one or more coils wound singly or in parallel. It also teaches how to manufacture an improved version of the Double-Helix Heat Exchanger (“DHX”), which is also disclosed in said Provisional Patent Application 60/709,889. Unlike a GFX, which must be installed vertically for best effectiveness, a DHX can be operated in any orientation. For applications permitting higher pressure drop, a DHX can be designed with a much higher U-value than a comparable GFX or GFX+.

Vasile, Carmine F. (Patchogue, NY, US)
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165/115, 165/71
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Primary Examiner:
Attorney, Agent or Firm:
Dr. Carmine F. Vasile (Patchogue, NY, US)
What is claimed is:

1. The apparatus shown FIG. 1(a) and illustrated in FIGS. 1(b)(c)(d)(e), comprised of a single-wall or double-wall tube having a tightly wrapped helical-fin to increase its “collapse pressure”.

2. The apparatus of claim 1 having a tightly wrapped helical-fin with a non-uniform or staggered pitch to promote turbulence, mixing, and control the pressure drop encountered by a fluid or gas constrained to flow in the helical channel formed by said tube and helical-fin by an outer shell made of metal or plastic as shown in FIG. 1(a)(b).

3. The apparatus of claim 2 having a tight-fitting outer jacket having inlet/outlet connectors and pressure seals so as to cause to a first fluid or gas entering said inlet connector to flow in a helical path between the fins of said helical-fin and against the outer wall of said tube to promote efficient heat transfer to a second fluid or gas flowing in thermal contact with the inner wall of said tube before exiting said outlet connector.

4. The apparatus of claim 3 mounted in a vertical position to cause a falling-film to flow down the inner wall of said tube, or in the alternative, the apparatus of claim 1 formed to fit inside said tube to form a “DHX” having two helical flow paths.

5. The apparatus of claim 4 having said first and second fluids (or gas) flowing in opposite (contra-flow) directions for best heat transfer effectiveness.

6. The apparatus of claim 5 having more than one helical fin to form parallel flow paths.

7. The apparatus of claim 6 having a double-wall tube formed by wrapping the outer surface of the larger single-wall tube with a metal foil thin enough to be held in intimate thermal-contact with said single-wall tube whenever said helical flow path is pressurized.

8. The apparatus of claim 7 wherein a vent is formed by overlapping said metal foil to form a leak-proof lap joint by soldering, seam welding or other fastening process.

9. The apparatus of claim 8 having more than one vent either machined into said inner tube or created by multiple lap joints.



1. Field of the Invention

The instant invention relates to a method for increasing heat transfer coefficient and lowering the manufacturing cost of a Gravity Film heat exchanger (“GFX”) developed for use with apparatus disclosed in U.S. Pat. No. 4,619,311, hereinafter called the GFX™ Patent.

Many all-copper coil & tube GFX's have been manufactured by various licensees and sub-licensees of the GFX™ Patent. All licensed GFX-products were manufactured using a trade-secret coiling technique. An unknown number of GFX-knock-offs have been sold.

For examples, see these Web sites: www.gfxtechnology.com, www.renewability.com/powerpipe.htm, www.apricus.com/html/waste_heat_recycler.htm & www.retherm.com, which show non-infringing and infringing GFX designs.

2. Description of the Prior Art

Prior art is exemplified by many models of commercially available non-infringing GFX's such as those listed in the Table @ www.gfxtechnology.com/contents.html#selection. These are all-copper, self-vented, double-wall GFX models having various coil combinations. The need for a multi-coil GFX arose because flow rates in single-coil models are limited by coil-pressure drop and/or erosion corrosion, and some residential plumbing codes prohibit installation of standard GFX models having ½″ inlet/outlet connections such as the G3-60, G4-60, G3-40, G4-40, G3-30, and G4-30, for example.

GFX-models such as the S3-60, S4-60, S3-40, S4-40, S3-30, and S4-30 have two coils wound on a single Type-DWV drainpipe having respective lengths of 60″, 40″ or 30″, with ¾″ inlet/outlet connectors to meet most residential plumbing codes. Unlike the aforementioned “G” models, an “S” model cannot be operated as true counter-flow heat exchanger. Consequently “S” models yield lower effectiveness for identical flow rates (as do three-coil & four-coil “G” models such as the G3-60-3 or G4-80-4) than a “G” model made with the same type and length of drainpipe. In general, for identical flow rates, any conventional GFX will exhibit lower U and effectiveness values when its coil-side pressure drop is reduced by any prior art.

The present invention overcomes these limitations and addresses a new problem, skyrocketing prices of copper, its alloys, and stainless steel. GFX+ designs such as those of FIG. 1 are therefore needed to reduce the amount of expensive, corrosion-resistant metals used in conventional GFX designs, without degrading performance.


The present invention enables replacement of a GFX's heaviest component[s], its copper coil[s], with components using less copper for applications requiring all-copper construction. Additional copper-savings can be achieved in potable water or food processing applications that permit the use of plastic pipe such as PVC or CPCV, for example.

To save copper without degrading effectiveness, a GFX's copper coil is substituted by a helical copper fin tightly wrapped onto a single- or double-wall copper drainpipe and an outer shell made of copper, PVC or other suitable material. Any suitable material can be used to make the helical-fin or shell, but copper is preferred to eliminate differential expansion effects, while increasing the effective contact area by acting like a heat fin. A standard PVC or CPCV shell can be used to maximize copper savings in many applications. One or more parallel helical flow paths, with or without a variable helix-pitch to induce turbulence and mixing can be used to also adjust the pressure drop.

For the first time, it will be possible for a designer to optimize both pressure drop and effectiveness of a GFX. By contrast, the only cost-effective way to increase the effectiveness of a conventional coil & tube GFX is to flatten its coil and deal with a rise in coil-pressure drop. This is discussed Provisional Patent Application 60/709,889 (Filing date: Aug. 22, 2005) in the section entitled “Improved Coil & Tube Gravity Film Heat Exchanger (GFX)”.

By tightly coiling said helical fin about a thin-wall copper drainpipe, its “collapse pressure” is significantly increased thereby enabling either single or double-wall construction with minimal use of copper. For example, according to FIG. 3 of the Copper Tube Handbook (CDA, 1993) the collapse pressure of a 3″ Type-DWV copper drainpipe is about 250 psi compared to about 1010 psi for a 3″ Type-L drainpipe having twice the wall thickness (0.09″ compared to 0.045″). Therefore, 3″ Type-L weighs 3.33 lb/ft compared to 1.69 lb/ft for 3″ Type-DWV. (See CDA Handbook Tables 2b & 2c)

For applications requiring vented-double-wall construction, the present invention permits significant weight savings because said helical-fin can be wound onto a thin-wall drainpipe covered by copper foil; typically between 0.003″ and 0.10″ thick, for example. The foil layer can be made to form a double-wall-vented structure having the lowest possible weight compared to convention double wall tubing made by forming 3″×56″ Type DWV copper tube onto a 3″×60″ Type DWV tube, for example.

    • Early GFX-prototypes (using heavy double-wall tubes having machined vent-groves and were much more expensive to manufacture than a coil & tube GFX) are described in UL Certification File SA8583, Project 89ME50865, Dec. 4, 1989.

A long strip of foil approximately 10.5″ wide by 56″ can be used to cover a section of a 60″ long, 3″ Type-DWV drainpipe while allowing an adequate lap joint to be formed with a vent as illustrated in FIG. 1(d). This will leave a vent space of about 0.5″ at each end between a pair of 1.5″ long 3″ CI to Copper no-hub connectors typically used to insert a G3-60 into a 3″ cast iron or PVC drainpipe. (See Proflex by Fernco, Part #3001-33, for example)

Multi-coil GFX's are presently made by interconnecting shorter GFX's; some having more than one coil wound on a single drainpipe. The present invention enables both conventional and multi-coil GFX's to be replaced by one simple design that is less expensive to manufacture because not only will much less copper be used, expensive coil-manifolds are eliminated.

For example, a single-coil GFX Model G3-60 can be made by coiling 100-feet of ½″ Type-L copper water tube weighing 28.5 pounds onto a 5-foot length of 3″ Type-DWV weighing 8.45 pounds for a total weight of about 37 pounds. (Some manufacturers have elected use a manufacturing process that winds tighter coils that require a slightly more than 100-feet of ½″ Type-L.)

By contrast to match the heat transfer coefficient and coil-pressure drop of a G3-60, a single-wall GFX+ (Model GP3-60-SW) unit made in accordance with the present invention will still require 8.45 pounds of 3″ Type DWV, but as illustrated in FIG. 1, a G3-60's 28.5 pound coil can be replaced with a 4″ Schedule 80 PVC “shell” having ¾″ inlet and outlet connectors, and a helical copper fin weighing about a pound; after being formed by from a copper strip approximately 61-feet×0.35″×0.01″. The net weight of copper used to make a “GP3-60-SW” will be close to 9.5 pounds; representing a substantial copper-savings of about 27.5 pounds (74%). If a 4″ Schedule 40 PVC shell is used (instead of Schedule 80), slightly more copper will be required because the helical-fin's height will have to be increased to 0.515″ and its pitch decreased to match the pressure drop of a G3-60.

Similarly, a double-wall GFX+ (Model GP3-60-DW) could use a 56″ length of copper foil measuring 0.007″×10.5″ and weighing about 1.3 pounds for a total copper-weight of about 10.8 pounds; representing a copper-savings of about 26.2 pounds (71%).

For applications requiring an all-copper construction, a single-wall GFX+ (Model GP3-60-SW-C) can use a shell made from a 56″ long length of 3.5″ Type-M or 4″ Type-DWV, respectively weighing 16.7 & 13.4 pounds. The corresponding fin height will have to be changed from 0.35″ to 0.167″ or 0.442″, respectively. The helix pitch will have to be adjusted to match the pressure drop of a G3-60 (8 psi @ 2.25 gpm) and/or its heat transfer coefficient “U”.

The aforementioned comparisons are summarized in the following Table A:

Copper Savings Provided By Various GFX+ Versions of a
Model G3-60 GFX
Standard All-Weight ofGFX+ Equivalent ofWeightCopper-
Copper GFXCopperModel G3-60 GFX(lbs)Savings
Model G3-6037 lbsSingle-Wall/PVC Jacket9.574%
Double-Wall/PVC Jacket10.871%
Single-Wall/All Copper23.337%
4″ Type-DWV Shell
Double-Wall/All Copper24.035%
4″ Type-DWV Shell
Single-Wall/All Copper25.631%
3.5″ Type-M Shell
Double-Wall/All Copper27.326%
3.5″ Type-M Shell

A GFX+ equivalent of any multi-coil GFX can be manufactured by using a single helical fin having a variable pitch as illustrated in FIGS. 1(c) & (d), more than one fin to form parallel helical channels, or both.

Using the present invention, some of the novel devices disclosed in Provisional Patent Application 60/709,889 (Filing Date Aug. 22, 2005) can be substituted with a more cost effective GFX+ design. For example, single-wall or double-wall DHX's can be made by inserting a smaller GFX+ into a larger GFX+ as illustrated in FIG. 1. Because the partial heat transfer coefficient of the inner helix can be made much higher than that of a falling film, a DHX can exhibit much higher U-value than a GFX or GFX+. Unlike the latter designs, both sides of a DHX can be flooded and operated in any orientation. Very high heat transfer effectiveness values can be attained with a DHX because very long helical fluid (or gas) flow-paths can easily be manufactured; especially in low flow applications that will allow short helix pitch.


FIGS. 1(a) to 1(e) show preferred embodiments of the invention. FIG. 1(a) is a photograph of one end of a typical GFX+ (Model GP3-30-SW) 10 showing one of two ¾″ threaded inlet/outlet connectors 11.

FIG. 1(b) shows the same GFX+ unit having a section of its PVC shell 12 cut away to reveal a copper helical-fin 13 wrapped tightly around a 3″ Type-DWV copper drainpipe 14. (Details of the PVC adaptor 15 or and pressure seal are not shown in the interest of brevity since there are no claims related to them.)

FIG. 1(c) illustrates another inventive feature; a helical fin 16 having a variable pitch denoted “S” &“B”. By interleaving a short helix-pitch “B” with a longer pitch “S” to reduce pressure drop, turbulence and mixing are introduced to help offset the drop in U associated with wide channels having thicker boundary layers caused by a lower fluid velocity. This optional innovation enables the use of the same, single-helix GFX+ design to match the performance of a multi-coil GFX without the use of a manifold.

A variable pitch is more cost-effective to manufacture than the fin pattern described in said Provisional Patent Application 60/709,889.

    • It's obvious to anyone skilled in the art that the same variable or zigzag pitch techniques can be used if more than one fin is wound to form parallel helical paths.

FIG. 1(d) illustrates a cost effective method to convert a GFX+ (Model GP3-30-SW) into a vented-double-wall GFX+ (Model GP3-30-DW) by laminating the 3″ drainpipe 14 with a sheet of copper foil 17 wide enough to form a lap joint 18 by a simple soldering operation. Using a foil thickness of about 0.007 inches, or less, will guarantee it is flexible enough to make thermal contact when the helical path is pressurized if the foil length is made shorter than the drainpipe so that the ends of vent path 19 are open to the atmosphere.

Unlike a conventional GFX having a constant coil pitch of about 0.5″, which dictates the pressure drop per unit length of every similar GFX, the helix pitch and pressure drop of a GFX+ can be varied to optimize performance for various applications. In general, a higher coil-pressure-drop causes an increase in heat transfer coefficient (“U”). For example, by reducing its helix-pitch, the pressure drop of a G3-30 equivalent can be increased to that of a G3-60 equivalent. Or the pressure drop of a G3-60 equivalent can be reduced to that of an S3-60 or a G3-60-3 or a G4-80-4 by increasing its helix pitch. (A G4-80-4 equivalent will require substituting the ¾″ connector 11 with a 1″ connector.) The latter models require expensive manifolds and additional labor to plumb 2 to 4 coils in parallel. Therefore, the present invention allows additional savings by eliminating the cost and weight of manifolds required to make every multi-coil GFX.

Inherent loss in heat transfer effectiveness & efficiency associated with existing multi-coil GFX designs, which cannot be operated as a true counter-flow heat exchanger, can be offset by using a single helical path having a variable pitch to introduce turbulence and mixing.

FIG. 1(e) illustrates how to convert a GFX or GFX+ unit into a double helix heat exchanger (“DHX”) by simply making a finned tube 20 having a helical fin 21 with an outer diameter (OD) slightly less that the inner diameter (ID) of the drainpipe into which it will be inserted 14; forcibly to ensure metal-to-metal contact between the fin 21 and outer tube 14. A pair of inlet/outlet holes and connectors 22, 23 can be fastened to the inner wall of the smaller finned-tube 20 to introduce fluid or gas into the inner helical path 21 thereby causing it to exchange heat with a fluid or gas flowing within the outer helix 16 in either a counter- or co-flow direction.

Because the partial heat transfer coefficient (U) of the inner helix can be made much higher than that of a falling film, a DHX can exhibit a much higher overall U-value than a GFX or GFX+. Much higher effectiveness values will be attained when a DHX is operated as a counter-flow heat exchanger. Unlike a GFX or GFX+, both sides of a DHX can be flooded and operated in any orientation. A DHX can offer effectiveness values well beyond the current state-of-the-art for heat exchangers having comparable flow passages because very long helical paths can be manufactured cost effectively; especially in low flow applications that will allow very small values of “B” and/or “S”.

While the form of apparatus illustrated in FIG. 1 constitutes preferred embodiments and obvious extensions of the invention, it is understood that the invention is not limited to this precise form of apparatus and that design and material changes may be made therein without departing from the scope of this invention.

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