WO2000045102A1

WO2000045102A1 - Flat oval tube

Info

Publication number: WO2000045102A1
Application number: PCT/EP1999/000657
Authority: WO
Inventors: Bjorn Vestergaard
Original assignee: Norsk Hydro Asa
Priority date: 1999-01-28
Filing date: 1999-01-28
Publication date: 2000-08-03
Also published as: AU2721999A

Abstract

Flat oval tube, to be used as heat exchanger tube, which the tube is internally provided with grooves and/or ribs. Preferably the lead angle of the grooves and/or ribs is between 12 - 30 degrees, and their depth is between 15 - 65 % of the nominal wall thickness.

Description

Flat oval tube

The invention relates to a flat oval tube to be used as heat exchanger tube.

Such a tube is generally known, see e.g. EP-A- (B-63.11)

In standard heat exchanger, the heat exchanging tubes are arranged in parallel between two headers. Inside the tubes a firstheat transfer medium in flowing, while heat exchanging with a heat transfer medium flowing around the outer wall of the heat exchanging tubes. The efficiency of the heat exchanging process is defined by a number of parameters, which can be summarized, as the K-value, the surface area of the contact wall between the two fluids and the Δ t, the temperature differences between the two fluids. Normally the K-value is the ability to transfer heat through the wall, which is dependent upon the wall thickness, the wall material, the nature of the two fluids, as well as their physical condition (gas, fluid, vapor) and their flow characteristics (laminar or turbulent).

I the case of tubes, round or oval the surface area inside is somewhat smaller than outside, but as a rule this is insignificant. However with more sophisticated shapes such as MPE-tubes (multi-part-extrusion tubes) the inside wall area is significantly bigger than the outside wall area.

Mechanically assembled heat exchangers are often made of round aluminium or copper tubing, which is mechanically expanded into the surrounding fins. The limitation of the performance here is very often the lack of sufficient internal surface. Especially at high air speeds, the tube cannot transfer the heat fast enough.

To increase performance, round tubing with internal enhancements, such as a number of grooves in the longitudinal direction, has been used. Round tubing with spiral grooves has been used as well, especially in connection with copper tube. As a supplement, internal turbulators made from aluminium, copper or plastic can change the laminar flow to turbulent flow characteristics, and thus extra performance is gained.

To increase performance round tubing can be rendered into Flat Oval Tubing. Compared to a round tube, the flat oval tube surface is bigger in relationship to the cross flow area of the internal media. Due to reduced internal cross flow area, flow velocity is higher, and turbolar flow is achieved. The extended tube surface of the tube also increases the heat exchange to the fins and the fin area will be used more effectively.

C ^* Multi Port Extruded aluminium tubing can be made with a multiple number of webs and internal enhancements, such as small fins from the sides. The increased internal surface provides very good heat transfer coefficient between internal heat transfer medium and the tube wall. Heat exchangers designed with multi port extruded tubing are usually compact, which results in limited fin surface and therefore less external surface compared to mechanical expanded solutions. Furthermore, multiport extruded tubing is an expensive solution which require brazing in assembly. Mechanically assembled heat exchanger using round or oval tubing is a more cost efficient manufacturing alternative, at lower air speeds, the mechanical assembled heat exchanger appears more efficient whereas the multi port extruded and brazed solution is superior at higher air speeds.

It is therefor an object to the invention to provide a flat oval tube with increased performance at higher air speeds. This object is achieved when the flat oval tube is internally provided with grooves and / or ribs.

In this way it is achieved that the heat exchanging performance of the flat oval tube heat exhanger becomes comparable with the performance derived from designs based on

MPE-tube at high speed of the external heat transfer medium and comparable to standard flat oval tube at low speed of the external fluidium.

In fig. 1 there is shown the performance of different types of heat exchanging tubes in relation to the air speed i.e. the speed of the external heat transfer medium. The performance is along the y-axis, whereas the air speed is along the x-axis.

The typical performance of a so-called MPE-tube is shown by a curve 1 , in dotted lines. The typical performance of a standard flat oval tube is shown by a curve 2, in dots and dash line. As becomes clear from this figure there is a better performance of the MPE-tube in the high air speed area and a better performance of the standard flat oval tube in the low air speed area. In solid line there is shown the performance of the flat oval tube according to the invention. In the low air speed area its performance is slightly better than the standard flat oval tube, but with increasing air speed this difference is increasing as well and at the intersection of the curves 1 and 2, the performance of the tubes according to the invention is somewhat higher than both known tube, and with increasing air speed the performance of the tube according to the invention is following substantially the performance of the MPE-tubes. Flat oval tubes provided with internal grooves can be made in different ways. In CH-A-588 311 there is described a method for providing round tubes with integral grooves or fins. In order to make flat oval tube with internal grooves and / or ribs it is possible to make round tubes which internal grooves and / or ribs and thereupon to reshape the round tubes into flat oval tubes by use of external forces. Preferably the tubes are made out of aluminium or aluminium alloys, but other materials having good heat transfer characteristics may be used as well, such as copper or copper alloys. The oval tubing with internal grooves is extruded, drawn and ovalised which gives the advantage of no welding zones in which the material characteristics are different from the rest of the material. Furthermore, the temper of the oval tubing with internal grooves can be varied from annealed to full hard. This new oval tubing with internal grooves can be mechanically assembled by expension, due to the fact that the material properties are symmetrically around the axis, and the oval tube with internal grooves does not contain of any zones with different surface characteristics or mechanical properties.

Flat tubing with internal grooves are known from welded tubing where the grooves are introduced into the material before forming into tube. The strip material is now formed to a flat (oval) tube and welded. This solution is difficult to use in mechanically assembled heat exchangers because of the differences in material properties around the welding zone so it is mainly used for brazed heat exchangers.

The size of the tubes might be equal to the standard flat oval tubes having a relation between the major and minor dimension between 1 ,50 and 6,00 , preferably between 3,50 and 4,50.

The depth of the grooves or the height of the ribs must be selected in such a way that an sufficient increase in contact area is obtained. It is known from literature on round tubing (e.g. Halcor. S.A. brochures) that the depth or height must be equal to 15 - 65 % of the nominal wall thickness i.e. the average thickness of the wall, whereby a sufficient strength of the wall together with an efficient increase of internal contact area is obtained. Preferably the depth or height is equalto 20 - 55 % of the nominal wall thickness and more preferably equal to 25 - 45 %.

In order to have a turbulent flow inside the tubes it is preferred that the grooves and / or ribs are positioned under an angle with respect to the longitudinal axis of the tube and hence heat transfer medium flow within the tube. Good results are obtained when the angle is between 12 - 30° with respect to the longitudinal axis of the tube. Preferably however this angle is 12 - 25° and more preferably 18 - 25°.

Claims

1. Flat oval tube, to be used as heat exchanger tube, characterized in that the tube is internally provided with grooves and / or ribs.

2. Flat oval tube according to claim ^characterized in that the internal grooves and / or ribs extend in spiral way along the longitudinal axis of the tube.

3. Flat oval tube according to claim 2, characterized in that the lead angle of the grooves and / or ribs ranges from 12-30 degrees.

4. Flat oval tube according to claim 2, c h a r a c t e r i z e d in t h a t the lead angle of the grooves and / or ribs ranges from 12-25 degrees.

5. Flat oval tube according to claim 2, characterized in that the lead angle of the grooves and / or ribs ranges from 18 - 25 degrees.

6. Flat oval tube according to any one of claims 1 -5, characterized in that the depth of the groove is between 15 and 65 % of the nominal wall thickness.

7. Flat oval tube according to any one of claims 1 -5, characterized in that the depth of the groove is between 20 and 55 % of the nominal wall thickness.

8. Flat oval tube according to any one of claims 1 -5, characterized in that the depth of the groove is between 25 and 45 % of the nominal wall thickness.

9. Flat oval tube according to any one of the claims 1 -5, characterized in that the height of the ribs is between 15 and 45 % of the nominal wall thickness.

10. Flat oval tube according to any one of the claims 1 -5, characterized in that the height of the ribs is between 20 and 55 % of the nominal wall thickness.

11. Flat oval tube according to any one of the claims 1 -5, characterized in that the height of the ribs is between 25 and 45 % of the nominal wall thickness.

12. Flat oval tube according to any one of the preceeding claims, characterized in that a number of parallel grooves and / or ribs is used, said number being 20 -65.

13. Flat oval tube according to any one of the preceeding claims, characterized in that the top angle of the ribs lies between 50 and 110 degrees.