GB2212899A

GB2212899A - Heat exchanger tube having minute internal fins

Info

Publication number: GB2212899A
Application number: GB8808863A
Authority: GB
Inventors: Carl Bergt
Original assignee: American Standard Inc
Current assignee: Trane US Inc
Priority date: 1987-11-30
Filing date: 1988-04-14
Publication date: 1989-08-02
Anticipated expiration: 2008-04-14
Also published as: CA1302395C; GB8808863D0; FR2623893A1; JPH01150797A; DE3815095C2; GB2212899B; FR2623893B1; DE3815095A1

Description

1 1 D E S C R I P T 1 0 N Title HEAT EXCHANGER TUBE HAVING MINUTE INTERNAL

FINS

Technical Field

The subject invention generally pertains to heat exchanger tubes, and more specifically to tubes having internal fins.

Background of the Invention

Refrigeration systems, such as air conditioners and heat pumps, typically include a compressor, one heat exchanger functioning as an evaporator, an expansion device, and a second heat exchanger functioning as a condenser, all of which are connected in series to circulate a refrigerant in a closed loop. The two heat exchangers each include at least one heat exchanger tube for transferring heat either to or from the refrigerant. The rate of heat transfer is enhanced by providing the heat exchanger tubes with internal fins.

In designing tubes having internal fins, many interrelated factors need to be considered such as fin height, fin spacing, helix angle, heat flux, flow rate, and various properties of the fluid being conveyed through the tube. Varying each of these factors provides an infinite combination of factors, making it difficult and costly to extensively study all the possibilities. As a result, some apparently conflicting conclusions have been drawn based on different lab tests.

22112899 2 U.S. Patent 4,044,797 suggests using an internal fin height of.02 to.2 mm (.0008 to.0079 inches) and a helix angle of 4 to 15', while U.S. Patent 4.118,944 concludes that the most effective helix angles are greater than 20'. U.S. Patent 4,545,428 suggests a fin height of.1 to.6 mm (.0039 to.0236 inches) and a helix angle of 16' to 35'. It should be noted, however, that none of the three patents recommends a helix angle of less than 4.

In view of the above patents alone, it is difficult, if not impossible, to determine the optimum fin height and helix angle. Therefore, in developing the present invention, lab tests were performed to determine the best internal fin design. The unexpected results of the tests showed that straight internal fins, i.e., a zero degree helix angle, provide a surprisingly effective internal heat transfer surface. The results were so surprising that additional, more extensive tests were performed to cheek the validity of the earlier tests. The more extensive tests confirmed the surprising results.

Since a heat exchanger tube having internal helical fins is more difficult and more costly to manufacture than a straight finned tube, it is an object of the invention to provide a heat exchanger tube with substantially straight internal fins that provides a greater heat transfer coefficient than that of a similar tube with helical fins.

Another object of the invention is to provide a heat exchanger tube with substantially straight internal fins having an optimum fin height.

A further object is to provide a tube with substantially straight internal fins that are circumferentially spaced apart at optimum intervals to avoid excessively thin, fragile fins and to avoid excess space between the fins.

1 3 A still further object is to Provide an effective heat exchanger tube for conveying refrigerant of varying quality.

Yet another object is to provide a refrigerant flow rate that takes full advantage of substantially straight internal fins.

And another object is to provide an internally finned heat exchanger tube that provides effective heat transfer when conveying a fluorinated hydrocarbon refrigerant.

These and other objects of the invention will be apparent from the attached drawings and the description of the preferred embodiment which follows hereinbelow.

This embodiment comprises a heat exchanger having a heat exchanger tube with internal fins. The fins run substantially straight along the length of the tube and are distributed circumferentially around the inner surface of the tube at.010 to inch intervals. The height of the fins is.010 to.020 inches. The minute size and configuration of the fins enhance heat transfer when conveying refrigerant of vary quality at a mass flow rate below 400 pounds per hour.

Brief. Description of the Drawings

Figure 1 illustrates a half-section of a preferred embodiment of the invention.

Figure 2 il.lustrate a half-section of a heat exchanger tube having a helix angle that is greater -than zero.

Figure 3 shows how a heat transfer multiplier varies as a function of fin height and helix angle.

4 Figure 4 shows how a heat transfer multiplier varies as function of helix angle for a given fin height.

Fi gure 5 shows how a heat transfer multiplier varies as function of fin height for a given helix angle.

Figure 6 shows how a heat transfer multiplier varies as function of mass flow rate for a preferred embodiment of the invention.

Description of the Preferred Embodiment

Heat exchanger tube 10, shown in Figure 1, is one embodiment of the subject invention. Tube 10 includes generally straight longitudinal running internal fins 12, i.e., its helix 1 1. Z angle 6 is zero degrees with respect to a long.Ltudina'L axis 14 of the tube. Fins 12 have a height "h" of.0155 inches and are distributed circumferentially around the inner surface of tube 10 at.017 inch intervals 16. Interval 16 is defined as the distance between a center point of one:fin to the center point of an adjacent fin.

Under equivalent test conditions, tube 10 proves to be superior to a variety of other internally finned tubes having various fin heights and helix angles, such as tube 18 shown in Figure 2. The variety of other tubes that have been tested and compared to tube 10 are generally suitable in refrigeration systems. This means that the tubes have a nominal outside diameter of approximately one inch or less and the internal surface of the tubes have a high concentration (intervals of.010 to.040 inches) of relatively minute fins (fin height below.035 inches) to enhance heat transfer while minimizing flow resistance, or pressure drop through the tube. Minimizing pressure drop is especially important when conveying refrigerant, because the refrigerant's temperature and vapor/liquid quality 1 (ratio of vapor to liquid mass) changes significantly with pressure which, in turn, affects the rate of heat transfer across the tube. The fins are closely distributed around the tube's inner circumference at.013 to.033 inch intervals 16 to maximize fin surface area while avoiding the use of excessively thin, fragile fins.

Each tube that was tested had a nominal outer diameter of 3/8 inches and was tested by conveying a refrigerant through the interior of the tube. The specific refrigerant that was used in the tests was "FREON", which is a trademark for a fluorinated hydrocarbon. More specifically, the refrigerant was "FREON R-22" which is a trademark for difluoromonochloromethane. The temperature of the external surface of the tube was controlled to provide a constant heat flux of 5,000 Btu/hr-ft2. The inlet and outlet temperature of the refrigerant was controllably changed to test each tube as it conveyed refrigerant of different vapor/liquid qualities. The quality was varied at five incremental values ranging from.15 to.85, and the results that are shown in Figures 3 through 6 are based on an average quality of.6 with the tubes functioning as an evaporator. Figures 3, 4 and 5 are based on a refrigerant mass flow rate of 200 lbs/hr.

The tests provided the heat transfer coefficient (Btu/hr-ft2-'F) of various tubes having internal fins. The coefficients were compared to those of smooth tubes having no internal fins. From the comparison, a dimensionless improvement' factor, referred to hereinbelow as a heat transfer multiplier or simply a multiplier, was determined by dividing. the heat transfer coefficient of the finned tube by the coefficient a comparable smooth tube.

1 6 The results of the tests are summarized in Figure 3. Eight different internally finned tubes are represented by points "A-H" which have been plotted according to fin height h and helix angle 6. As indicated at point B, each point A-H is accompanied by its heat transfer multiplier 20 as determined by actual tests. Below each multiplier 20, in parentheses, is a calculated multiplier "Z" based on an empirically derived equation 22 having a 96% coorelation with the measured multipliers 20. Equation 22 defines multiplier Z as a function of fin height h and helix angle e, with h being expressed in mils (1 mil =.001 inches) and e expressed in degrees with respect to a longitudinal axis of the tube.

Regions 24 and 26 of Figure 3 represent combinations of fin height h and helix angle e that provide a multiplier greater than two based on equation 22. In other words, the heat transfer rate of a smooth tube could be expected to double if it were modified to include internal fins having a combination height h and helix angle 0 that lies within regions 24 or 26. It is worth noting that U.S. Patents 4,545,428 and 4,118,944 have pointed out the importance of regions 28 and 30 respectively which generally coincides with region 26; however, the importance of the relatively narrow region 24 has not been appreciated. Region 24 identifies a specific set of tubes that represent the preferred embodiment of the invention.

Figure 4 illustrates how varying the helix angle affects heat transfer for given a fin height of approximately.008 inches (8 mils). Tubes represented by poi.nts C, D, G and H have a fin height of.008,.0075,.0085, and.008 inches respectively. A V-shape curve 32 represents multiplier Z as a function of helix angle & based on equation 22 for a constant fin height of.008 inches. Curve 32 illustrates how the multiplier increases as the helix angle increases or decreases from a low point 34 of 12 degrees.

7 The effect of fin height for a given helix angle is illustrated in Figure 5. Curve 36 represents multiplier Z as a function of fin height based on equation 22 with a constant helix angle of 6. Internally finned tubes represented by points B, C, D and E have a helix angle of 7', 7', S' and 6' respectively. Point 38 represents a comparable smooth tube having no internal fins. Figure 5 shows that for a helix angle of C, optimum heat transfer is obtained at a fin height of at least.007 inches. However, its best to limit the height to less than.030 inches to facilitate fin forming. This generally limits the fin height to no more than a nominal thickness 40 (Figure 2) of readily available tubing having a nominal pre-finned wall thickness ranging from.012 to.033 inches. Moreover, when using 3/8 inch O.D. tubing having a nominal wall thickness of.027 inches, an internal fin having a fin height of.020 inches, for example, will only project 7% across the internal diameter of the tube to provide minimal flow restriction.

The tests also show that the heat transfer multiplier Z is highest at lower flow rates. This is illustrated by curve 42, shown in Figure 6, which is based on actual. data points 44 and 46 and empirically derived points 48. Curve 42 clearly shows that a mass flow rate below 400 lbs/hr is the optimum flow rate for tube which represents one embodiment of the invention. Line 43 represents a smooth tube which, by definition, has a multiplier equal to one. Comparing curve 42 to line 43 shows that tube 10 is superior to a comparable smooth tube at flow rates below 650 lbs/hr. For a tube 10 having a nominal 3/8 inch ouside diameter and a.321 inch inside diameter, 650 lbs/hr provides a mass flow per cross-sectional area (mass flux) of 8,032 lbs/hr-in2.

8 Tube 10 was also tested in a condensing mode. The tests were similar to the tests performed in the evaporating mode, except the refrigerant being conveyed through the tube was cooled instead of heated. The tests showed that multiplier 20 of tube 10 only decreased from 2.12 in the evaporating mode to 2.02 in the condensing mode. The small 4.7% decrease indicates that tube 10 is suitable to function as both an evaporator and a condenser.

Although the invention is described wih respect to a preferred embodiment, modifications thereto will be apparent to those skilled in the art. Therefore, the scope of the invention is to be determined by reference to the claims which follow.

1 L 1 9

Claims

C L A I M S

1 1. A heat exchanger comprising a heat exchanger tube conveying a fluid at a mass flux of less than 8,000 lbs/hr-in2, said tube having a plurality of longitudinally running internal fins disposed along an inner surface of said tube at a helix angle of less than 12 degrees with respect to a longitudinal axis of said tube, said fins being circumferentially spaced around said inner surface at intervals of.010 to.040 inches and having a fin height of.009 to.030 inches.

2. The heat exchanger as recited in claim 1, wherein said helix angle is less than 4 degrees.

3. The heat exchanger as recited in claim 2, wherein said longitudinally running fins are generally straight running fins, whereby said helix angle is substantially zero degrees.

4. The heat exchanger as recited in claim 1, wherein said fins are cirdumferentially spaced around said inner surface at intervals of.013 to. 033 inches.

5. The heat exchanger as recited in claim 1, wherein said fin height is. 010 to.020 inches.

6. The heat exchanger as recited in claim 1, wherein said heat exchanger functions as an evaporator and is connected to a refrigeration system having a second heat exchanger, said second heat exchanger being defined by claim 1 and functioning as a condenser.

7. The heat exchanger as recited in claim 1, wherein said mass flux provides a mass flow rate of less than 400 lbs/hr.

8. The heat exchanger as recited in claim 1, wherein said fluid is a refrigerant of varying vapor/liquid quality.

CL

9. The heat exchanger as recited in claim 1, wherein said refrigerant is a fluorinated hydrocarbon.

10. The heat exchanger as recited in claim 9, wherein said refrigerant is difluoromonochloromethane.

11. A heat exchanger comprising a heat exchanger tube conveying a refrigerant at a mass flux of less than 8,000 lbs/hr-in2, said tube having a plurality of longitudinally running internal fins disposed along an inner surface of said tube at a helix angle of less than 4 degrees with respect to a longitudinal axis of said tube, said fins being circumferentially spaced around said inner surface at intervals of.010 to. 040 inches.

12. The heat exchanger as recited in claim 11, whereinsaid longitudinally running fins are generally straight running fins, whereby said helix angle is substantially zero degrees.

12

13. The heat exchanger as recited in claim 11, wherein said fins are circumferentially spaced around said inner surface at intervals- of.013 to.033 inches -

14. The heat exchanger as recited in claim 11, wherein said fin height is less than.030 inches.

15. The heat exchanger as recited in claim 14, wherein said fin height is greater than.005 inches.

16. The heat exchanger as recited in claim 15, wherein said fin height is greater than.007 inches.

17. The heat exchanger as recited in claim 16, wherein said fin height is. 010 to.020 inches.

k S.

r.

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18. The heat exchanger as recited in claim 11, wherein said heat exchanger functions as an evaporator and is connected to a refrigera tion system having a second heat exchanger, said second heat exchanger being defined by claim 9 and functioning as a condenser.

19. The heat exchanger as recited in claim 11, wherein said refrigerant is a fluorinated hydrocarbon.

20. The heat exchanger as recited in claim 19, wherein said refrigerant is difluoromonochloromethane.

21. The heat exchanger as recited in claim 11, wherein said refrigerant has a vapor/liquid quality that varies as said refrigerant is conveyed through said tube and said flux provides a mass flow rate of less than 400 Ibs/hr.

22. The heat exchanger as recited in claim 21, wherein said vapor/liquid quality changes between two values within said tube with one value being greater than.6 and tle other value being less than.6.

14

23. A heat exchanger comprising a heat exchanger tube conveying, at a mass flow rate of less than 400 lbs/hr, a fluorinated_hydrocarbon refrigerant whose vapor/liquid quality changes between two values within said tube with one value being greater than.6 and the other value being less than.6, said tube having a plurality of longitudinally running internal fins disposed along an inner surface of said tube at a helix angle substantially equal to zero degrees with respect to a longitudinal axis of said tube, said fins being circumferentially spaced around said inner surface at intervals of.013 to.033 inches and having a fin height of.010 to.020 inches.

24. The heat exchanger as recited in claim 23, wherein said heat exchanger functions as an evaporator and is connected to a refrigeration system having a second heat exchanger, said second heat exchanger being defined by claim 19 and functioning as a condenser.

1

25. The heat exchanger as recited in claim 23, wherein said refrigerant is difluoromonochloromethane.

26. A heat exchanger tube as defined in any one of the preceding claims for a heat exchanger as claimed therein.

27. A heat exchanger tube substantially as herein described with reference to Figure 1 of the accompanying drawings.

Published 1989 at The Patent Offloe. State House,68171 High Holbom London WCIR 4TP. Further copies maybe obtainedfromThe Patentofnze. Wes Branch, St Mary Cray, Orpington, Kent BR5 3F.D. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con- 1/87 RI c