-2-
FIELD OF APPLICATION
The present invention relates to a method for producing finned and structured surface heat transfer tubes. In particular, the invention relates to manufacturing of finned and structured surface heat transfer tubes, which have applications in the areas of heat exchange equipments requiring enhanced heat transfer performance on tubeside. Typical applications include, single-phase convection, boiling and condensation, among others.
The invention also relates to finned and structured surface heat transfer tubes.
BACKGROUND OF THE INVENTION
Enhanced heat transfer performance is desired for compactness, improved efficiency and overall economics, among others. The approaches typically employ finned and structured surfaces for obtaining better heat transfer performance. While finned surfaces provide additional heat transfer area, the
-3-
structured surfaces provide stable nucleation sites for example in boiling, in addition to the improvements obtained through the surface modifications (Bergles, 1998; Manglik, 2003). There are many different ways to manufacture such tubes.
US 4, 476, 704 discloses a finned tube for a heat exchanger with fins that extend in the longitudinal direction of the tube and are continuous therealong before the two cold drawing operations in which plurality of fins are produced, which are separated from each other along the longitudinal direction. The longitudinal fins can, for example, be obtained by extruding the tube. The turbulence provided in the flow by the fins that extend in the longitudinal direction of the tube throughout as disclosed in this document is not very effective compared to three dimensional fins (raised portions). The disadvantage with raised portions however is that of higher pressure drop penalty. Bending of these three dimensional fins in longitudinal direction (one of the invention of this patent) therefore provides improved "efficiency index" because of reduced pressure drop.
A heat exchange tube made from a ductile metal strip with embossed enhancement is known from US 6, 067, 712. This document discloses fin
-4-
structure modification (bending and other changes) carried out before the strip is formed into tube, i.e. on the strip surface itself. The bending of fins is an additional operation after embossing, carried out by any suitable means such as passing through a rolling mill. Furthermore, such a process does not have the advantage of fin bending during tube drawing where while the tube is reduced in its size, improved structured surfaces can be obtained on the inside with the fins bending and getting modified (in their shapes and sizes) because of drawing of tube over the smooth (i.e. not patterned) plug having size(s) suitable for this operation.
Japanese patent number 54044256 discloses grooves processed by drawing on the inner wall of a copper tube, which is then processed using a drawing plug to perform drawing to obtain tunnel like voids. These two drawing operations are followed by a processing operation on the outside surface of the tube using processing discs to obtain small openings in the voids on the inside surface of the tube. Such multiple processing using drawing plugs and processing discs is not cost effective.
-5-
US 4, 050, 125 Al discloses enhanced heat transfer surface with tunnels and holes produced by sequentially knurling, cutting and wire brushing the pipe. The sizes of the holes, which represent openings in the tunnels can be adjusted by shallow grooves to be formed by knurling and the pressure with which the brushes are held in contact with the work during wire brushing. In Hitachi ThermoExcel-E™ tube (Thome, 1990 and Webb, 1994), which has an outer enhanced boiling surface with tunnels and pores, fins are rolled to bend them over to form reentrant channels or tunnels. The disadvantages of these approaches for applications for inside or tubeside enhancement are that the knurling and cutting operations may not be practical to be carried out cost-effectively.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a process for producing finned and structured surface by embossing such that the fins of any dimension do not extend in the longitudinal direction of the tube. Such fin patterns of the present invention provide better turbulence in the flow with their forms as distinct
-6-
extended protrusions on the inside surface of the tube which disturb the flow in a more effective way than the fins which extend in a longitudinal direction of the tube.
Another object of the present invention is to carry out the bending operation of fins on the inside surface of tubes after the tube is formed by drawing the finned tube using a die and plug. The advantage of fin bending during tube drawing where while the tube is reduced in its size, improved structured surfaces can be obtained on the inside with the fins bending and getting modified (in their shapes and sizes) because of drawing of tube over the smooth (i.e. not patterned) plug having size(s) suitable for this operation.
Yet another object of the present invention is to produce on the tube inside surface additional reentrant cavities and tunnels.
A further object of the present invention is to produce reentrant cavities and tunnels in a single tube drawing operation using suitable finned pattern on the strip. This will provide considerable advantages over multiple processing using drawing plugs and processing discs as disclosed in JP 54044256.
-7-
The present invention uses tube drawing operation employing smooth surface plug for a finned tube to produce the finned and structured heat transfer surfaces for tubeside (inside) enhancement. This invention differs from the known processes for the manufacturing of finned and structured surface heat transfer tube for tubeside (inside) enhancement with respect to the approaches for the production of finned surfaces (integral fins) on finned tube used for the tube drawing operation (US 4476704) and fin modifications and bending (US 6067712).
In the present invention finned tube has fin structures (profiles) that do not extend in the longitudinal direction of the tube. Such profiles can be obtained by many approaches such as embossing (US 6067712) the strip, which is then formed into a tube. This tube is then drawn, which reduces its size and modifies fin structures (US 4476704).
The embossed structures (profiles) of the present invention have no fins extending in the longitudinal direction of the tube. Moreover, the fin structure modification (bending and other changes) is carried out in the tube drawing operation (i.e., after the finned tube is formed) and not before (i.e., on the strip surface itself) as disclosed in US 6067712. The modified fins, which may be
-8-
bent at an angle can take any shape and size and can also form structured surfaces, which may represent reentrant cavities. Additionally, tunnel like voids as disclosed in JP 54044256 can also be obtained.
Thus the present invention provides a method for producing finned and structured surface heat transfer tubes comprising the steps of: obtaining fin patterns on a strip by embossing; forming said finned strip with integral finned surface into a tube, having the finned surface of the strip on the inside of the tube, using rolls; obtaining an ERW tube; and drawing the finned tube using smooth surface plugs, thereby modifying the fins and obtaining a structured surface like reentrant cavities and tunnel like voids.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be described in detail with the help of the figure of the accompanying drawing in which:
Figure 1 shows in schematic form the manufacturing process of the present invention.
-9-
Figure 2 shows in schematic form two-stage embossing of strip by rolls and in illustration the rolled out embossed pattern after second stage for heat transfer surface with tunnels and pores
Figure 3 shows in schematic form an enhanced pattern as an illustration for heat transfer surface with tunnels and pores (a) embossed pattern, (b) after cold drawing
Figure 4 shows in schematic form three-dimensional fin pattern as an illustration after embossing and finished bent fin enhanced heat transfer surface after cold drawing
DETAILED DESCRIPTION
According to the present invention fin structures or profile of patterns are obtained by embossing.
- 10-
Embossing on strip is carried out to obtain fin patterns or profiles. One way to do this is to use hydraulic press with desired die, which will emboss fin pattern on the strip. A number of such dies can be employed to obtain various fin patterns (profiles). The fins in the present invention do not extend in the longitudinal direction of the tube to be formed.
Another approach of the present invention and shown in Fig. 2 is two-stage embossing of strip by rolls. The first stage imparts the notch characteristic of the raised fins. These notches, which are of varying shapes such as dimples, later represent the holes or pores and provide openings to the tunnels. The opening ratio as defined in US 4, 060, 125 is obtained with these. In the second stage of embossing, the patterned roll surface embosses a raised projections or finned patterns, which are continuous either circumferentially or in a helical form. The enhanced pattern thus obtained is shown in Fig. 3. The strip with this enhanced pattern is then fed to the tube forming section.
In another enhanced pattern on the strip, first stage embossing is not carried out. The second stage embossing here gives three dimensional raised projections or fins of varying shapes having cross-sections, which are circular, oval, rectangular, and trapezoidal, among others. The heights or lengths of
-11 -
these raised projections or fins are in the range of 0.1 to 0.7 mm. In this invention it is found that the percentage reduction in drag coefficients, which include both form drag and skin friction, ranges from 6% to 60% depending on these raised portions or the three-dimensional fins. This thus improves "efficiency index" because of reduced pressure drop. The underlining principle with the bending of fins lies with the common or general understanding as stated by Tavoularis (2002) that "As a rule of thumb, one may compare the estimated widths of the corresponding wakes and infer that drag coefficient will be higher for the case with the wider wake". Fig. 4 shows in schematic form this enhanced surface pattern before and after the cold drawing operation.
The enhanced pattern forming tunnels and pores have similar height dimensions in the range of 0.1 to 0.7 mm. Another enhanced pattern is employed in this invention, in which the first embossing stage as the three dimensional raised projections or fins pattern is not carried out. The second embossing stage however is used as for enhanced patterns giving tunnels, with the height dimensions reduced to provide continuous opening for the tunnels. US 4, 059, 147 A1 discuss such an enhanced heat transfer pattern for an evaporator tube. The embossed or finned strip or plate with integral finned surface is formed into tube with finned surface of strip (or plate) on the inside of the tube using rolls.
- 12-
During this forming process, few fins (depending on the specific fin pattern used) may come in contact with the rolls (on the inside of the strip) and may show slight deformation, which for such fin patterns can be suitably accounted as the element of the fin patterns themselves (of the finned tube).
This is altogether different from the fin bending process as disclosed in US patent number 6,067,712, where all fins are bent using rolls by passing through rolling mill before the tube forming operation.
The tube is welded using high frequency welding and bead cutting operation is carried out to obtain an ERW tube.
The fins are modified and structured surface is obtained using tube drawing operation, as shown in Figure 1. The drawing operation using smooth surface plug gives the desired fin modifications (bending and other changes) and structured surfaces for the tubeside (inside) enhancement. The modified fins, which may be bent at an angle can take any shape and size, and can form structured surfaces like reentrant cavities and tunnel like voids.
- 13-
In conclusion, the enhanced tube of the present invention has fins (integral), which are provided by processes of metal forming such as embossing such that these fins, which may be of any dimension (up to and including third dimension) do not extend in the longitudinal direction of the tube as has been disclosed in US Patent Number 4, 476, 704.
By providing such fin patterns (profiles), better turbulence is expected in the flow. The tube is formed from these finned strips, such that finned surface is obtained on the tubeside (inside). The fined tube is then drawn over a smooth (i.e., not patterned) plug of suitable size(s), which bends and modifies the fins. The advantage of fin bending during tube drawing is that while the tube is reduced in its size, improved structured surfaces can be obtained on the inside with the fins bending and getting modified (in their shapes and sizes). In addition, reentrant cavities and tunnels can be produced on the tube inside surface. With the present invention, reentrant cavities (the small openings in the voids in JP 54044256 present structures like mouths of reentrant cavities) and tunnels can be formed in single tube drawing operation using suitable finned pattern on the strip. This provides considerable advantage over more than one processes (processing using drawing plug and processing discs) adopted in JP 54044256.
- 14-
The tubes of the present invention described above have applications in the areas of heat exchange equipments.
Reference
1. Bergles, A.E., 1998, "Techniques to Enhance Heat Transfer", in Handbook of Heat Transfer, Eds. W.M. Rohsenow, et al., 3rd Ed., McGraw Hill, New York, Ch. 11.
2. Manglik, R.M., 2003, "Heat Transfer Enhancement", in Heat Transfer Handbook, Eds. A. Bejan and A.D. Kraus, John Wiley & Sons, New Jersey, Ch. 14.
3. Hage, M. Schinkoth, G., 1984, "METHOD FOR PRODUCING FINNED TUBES", US 4476704.
4. Randlett, M.R., Webb, R.L. and Chamra, L.M., 2000, "HEAT EXCHANGE TUBE WITH EMBOSSED ENHANCEMENT", US 6067712.
- 15-
5. Satou, T., 1979, "HEAT TRANSFER TUBE AND METHOD FOR FORMING THE SAME", JP 54044256.
6. Thome, J.R., 1990, Enhanced Boiling Heat Transfer, Hemisphere, New York.
7. Webb, R.L., 1994, Principles of Enhanced Heat Transfer, John Wiley & Sons, Inc., New York.
8. Tavoularis, S., 2002, "Flow Past Immersed Objects", in Fluid Flow Handbook, Ed. J. Saleh, McGraw Hill, New York, pp. 20.23.
- 16-
WE CLAIM
1. A method for producing finned and structured surface heat transfer tubes comprising the steps of:
- obtaining fin patterns on a strip by embossing;
- forming said finned strip with integral finned surface into a tube, having the finned surface of the strip on the inside of the tube, using rolls;
- obtaining an ERW tube; and
- drawing the finned tube using smooth surface plugs, thereby modifying the fins and obtaining a structured surface like reentrant cavities and tunnel like voids.
-17-
2. The method as claimed in claim 1, wherein said embossing step is carried out such that the fins formed may be of any dimension and do not extend in the longitudinal direction of formed tube.
3. The method as claimed in claim 1, wherein said ERW tube is obtained by using high frequency welding.
4. The method as claimed in claim 1, wherein said bending operation of the fins on the inside surface of the finned tubes is carried out after forming the tube by drawing the finned tube using a die and plug.
5. The method as claimed in claim 1, wherein the reentrant cavities and tunnels are produced on the tube inside surface in a single tube drawing operation using suitable finned pattern on the strip.
- 18-
6. A method for producing finned and structured surface heat transfer tubes, substantially as herein described and illustrated in the accompanying drawings.