Patent Publication Number: US-6220344-B1

Title: Two-passage heat-exchanger tube

Description:
FIELD OF THE INVENTION 
     The present invention relates to a two-passage heat-exchanger tube. More particularly this invention concerns such a tube forming at least two separate passages through which fluids at different temperatures can flow for heat exchange between them. 
     BACKGROUND OF THE INVENTION 
     A heat-exchanger tube forming a pair of passages one of which extends helically is used in many different applications as described in German patent application 45,544, German patents 31,764 of Hocking and 804,502 of Sommer as well as in U.S. Pat. No. 3,566,615 of Roeder and U.S. Pat. No. 3,777,343 of D&#39;Onofrio. A standard such system has an inner tube wall formed with a radially outwardly projecting helical ridge and an outer tube wall formed with a radially inwardly projecting helical ridge. The two tube walls are assembled together so the two ridges fit in direct contact with each other and create a helical passage. An outer passage can be provided around the two joined walls by fitting them with play in an outer pipe, and an inner pipe can be run coaxially through the inner wall to form an inner chamber. One fluid is flowed through the helical passage and another in the same direction or countercurrent through the outer and/or inner passage. 
     Such a heat-exchanger tube is fairly hard to manufacture. The inner and outer tube walls must be made to very exacting tolerances so that when they are fitted together the outer surface of the outwardly projecting ridge snugly engages the inner surface of the inwardly projecting ridge. Even with very accurate construction, assembling the system is quite difficult. Furthermore after some use, adhesions on the tube walls often make it impossible to pull the inner and outer walls apart for cleaning. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved heat-exchanger tube assembly. 
     Another object is the provision of such an improved heat-exchanger tube assembly which overcomes the above-given disadvantages, that is which is of simple construction and which is easy to assemble and disassemble. 
     SUMMARY OF THE INVENTION 
     A heat-exchanger tube has according to the invention an inner tube wall centered on an axis, formed with an outwardly projecting helical ridge having a helical outer surface, and forming an outwardly open helical groove delimited by the ridge and an outer tube wall centered on an axis, formed with an inwardly projecting helical ridge of the same hand as the inner-tube ridge, having a helical inner surface, and forming an inwardly open helical groove delimited by the outer-tube ridge. The inner tube is coaxially received in the outer tube with the grooves together forming a helical passage having a plurality of turns and the surfaces radially confronting and spaced from each other to form an axial passage for fluid communication between adjacent turns of the helical passage. A fluid is fed to one end of the helical passage and withdrawn from an opposite end with flow of the fluid helically along the helical passage and axially through the axial passage. 
     It has surprisingly been found that leaving a space between the inner-ridge outer surface and the outer-ridge inner surface does not appreciably affect the heat-exchange efficiency of the tube according to the invention. The bulk of the flow is still helical with the axial leakage having little effect on overall residence time of the fluid between the tube walls. On the other hand the spacing of the inner and outer ridge surfaces makes it much easier to manufacture the two tube walls, since if they are a tiny bit too big or small they will still fit together, and makes it very easy to assemble and disassemble the tube according to the invention. 
     The helical passage in accordance with the invention has a flow cross section that is substantially greater than a flow cross section of the axial passage. This is typically done by making the grooves axially longer (that is wider) than the ridges and making the helical passage substantially deeper measured radially than the space between the ridge surfaces. The grooves are of generally rectangular section and the surfaces extend parallel to the respective axes. 
     According to the invention an inner pipe spaced radially coaxially inside the inner wall therewith an inner passage, and an outer pipe spaced radially coaxially outside the outer wall forms therewith an outer passage. The inner and outer pipes can be cylindrical or helically corrugated. 
     Since with the system of this invention the loose fit makes it difficult to determine if the inner-tube ridge is radially aligned with the outer-tube ridge, the inner and outer tubes are formed with markings that are aligned axially when the inner-tube ridge is radially aligned with the outer-tube ridge. These markings can be bumps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which: 
     FIG. 1 is a small-scale axial section through a heat-exchanger assembly according to the invention; 
     FIGS. 2 and 3 are axial sections through the inner and outer walls of the tube; 
     FIG. 4 is an axial section through the assembled tube; 
     FIG. 5 is a larger-scale view of the tube according to the invention combined with inner and outer pipes; 
     FIG. 6 is a partly axial section through an end of the tube in accordance with the invention; 
     FIG. 7 is an end view taken in the direction of arrow VII of FIG. 6; and 
     FIG. 8 is a view like FIG. 5 but showing an alternative tube assembly. 
    
    
     SPECIFIC DESCRIPTION 
     As seen in FIG. 1 a heat-exchanger tube  10  extends between two manifold fittings  11 . The tube  10  is centered on an axis A and forms as described below a helical passage  18  extending from an inlet P 1  to an outlet P 2  and a second passages  23  and  25  outside and inside the passage  18  and extending from an inlet P 3  to an outlet P 4 , or vice versa from P 4  to P 3 . Respective fluids in the passages  18  and  23  are at different temperatures and exchange heat through the walls between the passages  18 ,  23 , and  25 . 
     As shown in FIGS. 2 through 5, the tube  10  comprises a tubular inner wall  12  (FIG. 2) and a tubular outer wall  13  (FIG. 3) of uniform wall thickness and fitted coaxially together (FIG. 4) to form the passage  18 . The inner tubular wall  12  has a radially outwardly projecting rectangular-section helical ridge  15  having an outer surface  14  and forming a rectangular-section helical outwardly open groove  19 . The tubular outer wall  13  has a radially inwardly projecting rectangular-section helical ridge  16  having an inner surface  17  and forming a rectangular-section helical inwardly open groove  20 . The ridges  15  and  16  are of the same axial dimension and pitch so the grooves  19  and  20  will also be of the same axial dimension and pitch. 
     The two tubular walls  12  and  13  are assembled coaxially with the outer face  14  of the ridge  15  directly confronting but separated by an axial space or passage  24  (FIG. 5) from the inner face  17  of the ridge  16 . Thus the total radial heights of the ridges  15  and  16  is less than the radial dimension of the passage  18 . The inner wall  12  is formed with a marking  30  (see FIGS. 6 and 7) in the form of an inwardly directed circular dimple and the outer tube  13  is similarly formed with another such dimple  31  that are axially aligned when the tube wall  12  is properly inserted into the wall  13  with the ridges  15  and  16  aligned. 
     Thus the two grooves  19  and  20  combine to form the helical passage  18  which is of considerably greater radial and axial dimension than the space  24  between the faces  14  and  17 . The flow from input P 1  to output P 2  will therefore primarily exist as a helically flowing stream along the helical passage  18  formed by the grooves  19  and  20 . There will be some axial leakage through the space  24  between adjacent turns of the helical flow in the passage  18  that will serve to keep the space  24  clear without significantly impairing heat-exchange effect. In addition the space  24  makes fitting the two tube walls  12  and  13  together and taking them apart a relatively simple operation. Even minor accretions on the tube walls  12  and  13  will not prevent them from being separated for cleaning. 
     The tube  10  further includes as shown in FIG. 5 an outer cylindrical pipe  22  spacedly surrounding the outer wall  13  and an inner cylindrical pipe  21  received with spacing inside the inner wall  12 . The outer pipe  22  therefore forms the outer wall of the passage  23  and the inner pipe  21  the inner wall of the passage  25 . Normally the passages  23  and  25  are connected together at their ends so the same fluid is flowed outside and inside the passage  18  formed by the walls  12  and  13 . 
     In the system of FIG. 8 helically corrugated inner and outer pipes  21 ′ and  22 ′ are used. The circularly arcuate corrugations make the flow in the outer and inner passage  23  and  25  substantially more turbulent for better heat-exchange effect.