FR2623893A1 - HEAT EXCHANGER HAVING TUBES HAVING INNER FINS - Google Patents

Abstract

The invention relates to a heat exchanger. It includes a tube 10 having inner fins 12 which form a minimum THETA helix angle with a longitudinal axis 14 of the tube. This angle is preferably zero. It gives tube 10 a higher heat transmission rate than that obtained with other tubes having larger helix angles. Field of application: heat exchangers, especially for refrigeration systems, etc.

Description

The invention relates generally to heat exchanger tubes

  heat, and more particularly

  tubes with inner fins.

  Refrigeration systems, such as air conditioners and heat pumps, usually include a compressor, a first evaporator heat exchanger, a pressure reducer device and a second condenser heat exchanger, all of which are mounted in a refrigerator. series so as to circulate a refrigerant in a closed circuit. The two heat exchangers each comprise at least one heat exchanger tube for transmitting heat to or receiving refrigerant from the refrigerant. The speed of heat transmission is increased by the presence of fins inside the tubes

heat exchangers.

  In the design of inner finned tubes, many interrelated factors, such as fin height, spacing, helix angle, heat flow, flow rate and various properties, must be considered. fluid transported in the tube. The fact of varying each of these factors gives an infinite combination of factors, which makes it difficult and expensive to extensively study all the possibilities. As a result, some seemingly contradictory conclusions have been drawn from

  different tests carried out in the laboratory.

  U.S. Patent No. 4,044,797 suggests using a height of 0.02 to 0.2 mm and a helix angle of 4 to 15 for the inner fins, while the United States patent United States of America N 4 118 944 concludes that the most effective helix angles are greater than 20. U.S. Patent No. 4,545,428 suggests a height of 0.1 to 0.6 mm and a helix angle of 7 to 35 for the fins, but it should be noted that none of the three patents

  cited recommends a lower helix angle & 4.

  From the sole patents above, it is difficult, if not impossible, to determine the optimum height and helix angle for the fins. Therefore, in developing the present invention, laboratory tests were conducted to determine the best design for the inner fins. Unexpected results of the tests have shown that straight inner fins, i.e. having a zero helix angle, provide a surprisingly effective interior heat transfer surface. The results were so surprising that additional, more extensive tests were conducted to verify the validity of previous tests. The larger tests confirmed the

surprising results.

  Since a heat exchanger tube having inner helical fins is more difficult and more expensive to manufacture than a straight finned tube, an object of the invention is to provide a heat exchanger tube having inner fins.

  substantially straight, which offers a transmission coefficient

  higher heat than a similar tube

equipped with helicoidal fins.

  Another object of the invention is to provide an inner finned heat exchanger tube

  substantially straight, having an optimal height of

  your. Another object of the invention is to provide a tube with substantially straight inner fins, which are circumferentially spaced at optimum intervals in order to avoid the presence of excessively thin and fragile fins and to avoid the presence of a space

excessive between the fins.

  Another object of the invention is to propose an efficient heat exchanger tube for the transport of

  refrigerants of varying quality.

  Another object of the invention is to provide a flow rate of the refrigerant which takes full advantage of the substantially straight inner fins. Finally, another object of the invention is to

  to propose an internally finned heat exchanger tube

  which has an effective transmission of the

  heat by transporting a hydrocarbon refrigerant

fluoride bure.

  The invention relates to a heat exchanger comprising a heat exchanger tube with inner fins. The fins extend substantially in line

  right along the tube and are distributed circumferentially

  on the inner surface of the tube at intervals of 0.25 to 1.02 mm. The height of the fins is 0.25. at 0.51 mm. The very small dimension and the configuration of the fins increase the heat transfer during the transport of a refrigerant of variable quality, to a

  flow rate less than 182 kg / hour.

  The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting examples and in which: - Figure 1 is a perspective view, with axial half-section, of a preferred embodiment of the invention; FIG. 2 is a perspective view, with an axial half section, of a heat exchanger tube having a helix angle greater than zero; FIG. 3 is a graph showing how a heat transmission multiplier varies in

  depending on the height and the helix angle of the

  your; FIG. 4 is a graph showing how a heat transmission multiplier varies as a function of the helix angle for a given height of the fins; FIG. 5 is a graph showing how a heat transmission multiplier varies as a function of the height of the fins for a given helix angle of the fins; and FIG. 6 is a graph showing how a heat transmission multiplier varies as a function of the mass flow rate in one form.

  preferred embodiment of the invention.

  A heat exchanger tube 10, shown on the

  Figure 1, constitutes an embodiment of the invention.

  tion. The tube 10 comprises inner fins 12,

  generally straight, extending longitudinally, that is,

  say whose helix angle e is 0 with respect to a longitudinal axis 14 of the tube. The fins 12 have a height "h" of 0.394 mm and are distributed circumferentially along the inner surface of the tube 10 at intervals of 0.43 mm. The interval 16 is defined as the distance between a center point of a fin and the point

central to an adjacent fin.

  Under equivalent test conditions, a tube 10 is found to be superior to various other inner finned tubes having vanes of varying heights and helix angles, such as the tube 18 shown in FIG. various other tubes which have been tested and compared to tube 10 are generally suitable for refrigeration systems. This means that the tubes have a nominal outside diameter of about 2.54 cm or less and that the inner surface of the tubes has a high concentration (range 0.25 to 1.02 mm) of relatively small fins (height of

  less than 0.89 mm) to increase transmission

  heat flow while minimizing the flow resistance or pressure drop in the tube. Pressure drop minimization is particularly important when transporting a refrigerant, because the temperature and vapor / liquid quality (ratio of the mass of vapor to the mass of liquid) of a refrigerant fluid changes significantly with the pressure. which, as a result, affects the rate of heat transfer through the tube. The fins are distributed closely

  along the inner circumference of the tube, at intervals

  the 16 from 0.33 to 0.84 mm, to maximize the surface area of the fins while avoiding the use

  finely thin and fragile fins.

  Each tube tested had a nominal outside diameter of 9.5 mm and was tested by transporting a coolant inside the tube. The particular refrigerant that was used in the tests was

  "FREON", which is a trademark for a hydrocarbon

  fluoride bure. More particularly, the refrigerant was "FREON R-22", which is a trademark for difluoromonochloromethane. The temperature of the outer surface of the tube was adjusted to establish a constant heat flux of 15,775 W / m2. The inlet and outlet temperatures of the refrigerant were adjustably varied to test the conduction, by each tube, of coolants of different vapor / liquid qualities. The quality of five incremental values ranging from 0.15 to 0.85 has been varied, and the results shown in Figures 3 to 6 are based on a

  average quality of 0.6, the tubes working in evaporation

  tor. Figures 3, 4 and 5 are based on a throughput

  mass flow rate of the refrigerant of 90.8 kg / h.

  The tests gave the heat transfer coefficient (W / m 2 C) of various tubes with inner fins. The coefficients were compared to those of smooth tubes without inner fins. By way of comparison, a dimensionless improvement factor, hereafter referred to as a heat transfer multiplier, or simply a multiplier, was determined by dividing the heat transfer coefficient of the tube to

  fins by the coefficient of a comparable smooth tube.

  The results of the tests are grouped together in Figure 3. Eight tubes with different inner fins are represented by points "A-H" which have been plotted as a function of the fin height h and the helix angle

  e. As indicated in point B, each point A-H is accom-

  accompanied by its heat transfer multiplier 20 as determined by real tests. Below each multiplier 20, in parentheses, is indicated a calculated multiplier "Z" based on an empirically established equation 22, having a correlation of 96% with the measured multipliers 20. Equation 22 defines the multiplier Z as a function the height h and the angle e of the fins, h being expressed in units of 0.025 mm and e being expressed in degrees relative to the longitudinal axis of the tube. Zones 24 and 26 of FIG. 3 represent combinations of a height h and a helix angle e of the vanes which give a multiplier greater than 2 on the basis of equation 22. In other words, a double heat transfer velocity of a smooth tube could be provided if it was modified to have inner fins having a combination of height h and helix angle e included in the zones 24 or 26. It is interesting to note that the aforementioned patents Nos. 4 545 428 and 4 118 944 have emphasized the importance of zones 28 and 30, respectively, which coincide globally with zone 26; however, the importance of the relatively narrow area was not noticed. Zone 24 identifies a particular set of tubes that represent the preferred embodiment of the invention. Figure 4 shows how a change in helix angle e affects the heat transfer for a given fin height of about 0.20 mm. The tubes represented by points C, D, G and H have a fin height of 0.203, 0.190, 0.216 and 0.203 mm, respectively. A V-shape curve 32 represents the multiplier Z as a function of the helix angle e on the basis of Equation 22 for a constant pitch height of 0.203 mm. Curve 32 shows how the multiplier increases as the helix angle increases or

  decreases from a low point 34 of 12.

  The effect of the fin height for a given helix angle is illustrated in FIG. 5. The curve 36 represents the multiplier Z as a function of the height of the vanes on the basis of equation 22, with an angle of constant helix of 6. Inner finned tubes represented by points B, C, D and E have a helix angle of 7, 7, 5 and 6, respectively. Point 38 represents a comparable smooth tube without inner fins. Figure 5 shows that for a helix angle of 6, optimum heat transmission is obtained at a fin height of at least 0.178 mm. However, it is better to limit the height to less, 0.762 mm to facilitate the realization of the fins. This generally limits the height of the fins to a value not exceeding a nominal thickness 40 (FIG. 2) of readily available tube having a nominal wall thickness, front fin, of between 0.305 and 0.838 mm. In addition, when using a tube with an outside diameter of 9.53 mm, having a nominal wall thickness of -0.686 mm, an inner fin having a height of 0.508 mm, for example, protrudes only 7% on the diameter

  inside the tube to withstand the flow.

  The tests also showed that the heat transfer multiplier Z is highest at the lower flow rates. This is illustrated by curve 42, shown in FIG. 6, which is based on actual data points 44 and 46 and empirically established points 48. Curve 42 clearly shows that a mass flow rate of less than 181.6 kg / h is the optimum flow rate for a tube 10 which represents an embodiment of the invention. Line 43 represents a smooth tube which, by definition, has a multiplier equal to one. A comparison of the curve 42 to the line 43 shows that the tube 10 is greater than a smooth tube comparable to flow rates less than 295.1 kg / h. For a tube 10 having a nominal outside diameter of 9.53 mm and an internal diameter of 8.153 mm, a value of 295.1 kg / h gives a mass flow rate per cross-sectional area (mass flow).

of 564.7 kg / h.cm2.

  The tube 10 was also tested in a condensation mode. The tests were similar to those in the evaporative mode except that the refrigerant carried in the tube was cooled instead of heated. The tests showed that the multiplier of the tube 10 only decreased from 2.12 in the evaporative mode to 2.02 in the condensation mode. The slight decrease of 4.7% indicates that the tube 10 is suitable for

  function in both evaporator and condenser.

  It goes without saying that many modifications can be made to the described heat exchanger and

  shown without departing from the scope of the invention.

Claims (17)

  1. Heat exchanger, characterized in that   it comprises a heat-exchange tube (10) for transporting   a fluid with a mass flow of less than 562.5 kg / h.cm2, said tube having a plurality of longitudinally extending inner fins (12) disposed along an inner surface thereof at a helix angle e less than 12 with a longitudinal axis   (14) of the tube, the fins being circumferentially spaced apart   on the inner surface, at intervals (16) of   0.25 to 1.02 mm, and having a height h of 0.23 to 0.76 mm.   2. Heat exchanger according to claim 1, characterized in that the helix angle is less than 4. 3. Heat exchanger according to claim 1, characterized in that the mass flow establishes a flow rate   mass flow rate below 181.6 kg / h.   4. Heat exchanger according to claim   1, characterized in that the fluid is a refrigerant   variable vapor / liquid quality.   5. Heat exchanger, characterized in that   it comprises a heat-exchange tube (10) for transporting   a coolant having a mass flow of less than 562.5 kg / h.cm2, said tube having a plurality of longitudinally extending inner fins (12) disposed along the inner surface of the tube at a lower helix angle e at 4 "with the longitudinal axis (14) of the tube, the fins being spaced circumferentially on said inner surface at intervals (16) of 0.254 to 1,016 mm.   6. Heat exchanger according to one of   Claims 2 and 5, characterized in that the fins   extending longitudinally are substantially rectilinear fins, so that said helix angle is substantially 0 '.   7. Heat exchanger according to one of the   Claims 1 and 5, characterized in that the fins   are circumferentially spaced on the surface   inside at intervals of 0.330 to 0.838 mm.   8. Heat exchanger according to claim, characterized in that the height of the fins is less than 0.762 mm.   9. Heat exchanger according to claim 8, characterized in that the height of the fins is greater than 0.127 mm.   10. Heat exchanger according to claim 9, characterized in that the height of the fins is greater than 0.178 mm.   11. Heat exchanger according to one of   claims 1 and 10, characterized in that the height   fins is 0.254 to 0.508 mm. -   12. Heat exchanger according to one of   1 and 5, characterized in that the fluid   refrigerant is a fluorinated hydrocarbon.   Heat exchanger according to claim 1, characterized in that the coolant has a vapor / liquid quality which varies during the transport of the fluid in the tube and in that the flow establishes a flow rate.   mass flow rate below 181.6 kg / h.   14. Heat exchanger according to claim 13, characterized in that the vapor / liquid quality changes between two values, inside the tube, a value being   greater than 0.6 and the other value being less than 0.6.   15. Heat exchanger, characterized in that   it comprises a heat-exchange tube (10) for transporting   at a mass flow rate of less than 181.6 kg / h, a refrigerant consisting of a fluorinated hydrocarbon whose vapor / liquid quality changes between two values inside the tube, a value being greater than 0, 6 and the other value being less than 0.6, said tube having a plurality of longitudinally extending inner fins (12) disposed along an inner surface thereof at a helix angle e substantially equal to zero degree with a longitudinal axis (14) of the tube, the fins being spaced circumferentially on said inner surface, at intervals (16) of 0.330 to 0.838 mm, and having a height h of 0.254 to 0.508 mm. 16. Heat exchanger according to any one of   claims 1, 5 and 15, characterized in that   comprises an evaporator and is connected to a refrigeration system having a second heat exchanger such that   defined by any one of claims 1, 5 and 15 and operating as a condenser.   17. Heat exchanger according to one of   claims 12 and 15, characterized in that the fluid   refrigerant is difluoromonochloromethane.