Title:
HEAT EXCHANGER WITH VARYING TUBE DESIGN
Kind Code:
A1


Abstract:
A shell and tube heat exchanger includes a plurality of tubes positioned inside a shell. A first type of tube has a first outer surface, and a second type of tube has a second outer surface, where the first outer surface is different than the second outer surface. The inner surface of the first type of tube is in liquid communication with the inner surface of the second type of tube, so fluid can flow between the inner surfaces of the different tube types.



Inventors:
Luo, Zhong (Shanghai, CN)
Qiu, Yalin (Shanghai, CN)
Application Number:
12/341989
Publication Date:
07/16/2009
Filing Date:
12/22/2008
Assignee:
Wolverine Tube, Inc. (Huntsville, AL, US)
Primary Class:
International Classes:
F28D7/00
View Patent Images:



Primary Examiner:
THOMPSON, JASON N
Attorney, Agent or Firm:
BRADLEY ARANT BOULT CUMMINGS LLP (200 CLINTON AVE. WEST SUITE 900, HUNTSVILLE, AL, 35801, US)
Claims:
1. A horizontal shell and tube heat exchanger comprising: a shell having a bottom; a process fluid outlet penetrating the bottom of the shell; a first tube positioned inside the shell, the first tube having a first outer surface; and a second tube positioned inside the shell, the second tube having a second outer surface different than the first outer surface.

2. The shell and tube heat exchanger of claim 1 wherein the first tube is positioned above the second tube.

3. The shell and tube heat exchanger of claim 1 where the first outer surface is a two dimensional outer surface and the second outer surface is a three dimensional outer surface.

4. The shell and tube heat exchanger of claim 3 where the second outer surface is a three dimensional outer surface.

5. The shell and tube heat exchanger of claim 1 where the process fluid outlet is in liquid communication with the tube outer surfaces, the shell and tube heat exchanger further comprising a heat transfer fluid inlet, where each tube has an inner surface in liquid communication with the heat transfer fluid inlet.

6. The shell and tube heat exchanger of claim 1 where the first tube includes a plurality of first tubes and the second tube includes a plurality of second tubes.

7. The shell and tube heat exchanger of claim 1 further comprising a horizontal baffle positioned inside the shell.

8. The shell and tube heat exchanger of claim 1 where the first tube has an inner surface, the first tube further comprising a ridge defined on the first tube inner surface.

9. A shell and tube heat exchanger comprising: a shell; a first tube positioned inside the shell, the first tube having a first outer surface and an inner surface; and a second tube positioned inside the shell, the second tube having a second outer surface different than the first outer surface, and the second tube having an inner surface in liquid communication with the first tube inner surface.

10. The shell and tube heat exchanger of claim 9 further comprising a process fluid outlet in liquid communication with the first outer surface and the second outer surface, where the shell includes a bottom and the process fluid outlet penetrates the shell bottom.

11. The shell and tube heat exchanger of claim 9 where the first outer surface is a two dimensional outer surface.

12. The shell and tube heat exchanger of claim 9 where the first tube includes a plurality of first tubes defining a first heat exchanger section and where the second tube includes a plurality of second tubes defining a second heat exchanger section below the first heat exchanger section.

13. The shell and tube heat exchanger of claim 9 further comprising a third tube having a third outer surface different than the first outer surface and the second outer surface, where the third tube is positioned below both the first tube and the second tube.

14. The shell and tube heat exchanger of claim 9 further comprising a helical ridge defined on the first tube inner surface.

15. A shell and tube heat exchanger comprising: a shell; a first tube positioned inside the shell, the first tube having a first outer surface and an inner surface; a second tube positioned inside the shell, the second tube having a second outer surface different than the first outer surface, and the second tube having an inner surface; a heat transfer fluid outlet penetrating the shell, the heat transfer fluid outlet in liquid communication with the first and second tube inner surfaces; and a process fluid outlet penetrating the shell, the process fluid outlet in liquid communication with the first and second tube outer surfaces.

16. The shell and tube heat exchanger of claim 15 further comprising a tube sheet positioned inside the shell, where every tube penetrates the tube sheet, and the tube sheet provides a barrier between the process fluid outlet and the heat transfer fluid outlet.

17. The shell and tube heat exchanger of claim 15 where the tubes are approximately horizontal inside the shell, the shell includes a bottom, and the process fluid outlet penetrates the shell bottom.

18. The shell and tube heat exchanger of claim 15 further comprising a longitudinal baffle positioned over the process fluid outlet.

19. The shell and tube heat exchanger of claim 15 where one of the first and second tubes is a two dimensional tube, and the other of the first and second tubes is a three dimensional tube.

20. A shell and tube condenser comprising: Means for increasing heat transfer rates by varying the heat transfer surfaces in the heat exchanger.

Description:

This patent claims priority to Chinese patent application number 200810032586, which was filed on Jan. 11, 2008, the full contents of which are incorporated by reference.

BACKGROUND OF THE INVENTION

A. Field of the Invention

This invention relates to shell and tube heat exchangers.

B. Description of the Related Art

Heat exchangers are devices which are used to transfer heat from one material to another. Frequently the heat exchanger will involve the transfer of heat between two fluids, and often these fluids will both be liquids. On common type of heat exchanger is a shell and tube heat exchanger. The shell and tube heat exchanger has a shell on the outside with a plurality of tubes inside the shell.

Typically, a shell and tube heat exchanger has two types of fluids present. The first is a heat transfer fluid, which is frequently water but can also be other substances, including oil, brine, glycol solutions, steam, and molten metal. The second fluid is a process fluid. The process fluid is generally heated or cooled by the heat transfer fluid. Some process fluids include chlorofluorocarbons or hydrochlorofluorocarbons used in air conditioning or refrigeration, reaction masses in chemical plants, and crude oil which is heated before pumping. In some cases, heat is transferred between two process fluids, such as when heat is recovered from a reaction mass. Heat can also be transferred between two heat transfer fluids, such as when water is pre-heated by steam condensate. It is also possible in some shell and tube heat exchanger designs to have more than two fluids present.

Generally a shell and tube heat exchanger has two basic fluid compartments which are referred to as the tube side and the shell side. A fluid present in the tube side of a shell and tube heat exchanger will not be able to directly contact or intermix with the fluid present on the shell side. They are separated by physical boundaries such as a tube wall or a tube sheet. The tube side refers to the portion of the heat exchanger which has access to the inside of the tubes within the shell. The shell side refers to the portion of the heat exchanger which has access to the outside of the tubes.

In most instances a shell and tube heat exchanger will include one single type of tube throughout the heat exchanger. There are also other types of heat exchangers which are used in many places. Examples of other heat exchangers include plate and frame heat exchangers and spiral heat exchangers.

Some heat exchangers are used as condensers. A condenser is a heat exchanger which is used to condense a gas into a liquid. One example of a condenser is a heat exchanger which is used with air conditioners to condense the refrigerant. Chlorofluorocarbons and hydrochlorofluorocarbons are often used as the refrigerant, but other compounds can also be used, including ammonia and light hydrocarbons. At a separate portion of the air conditioner there is an evaporator where the refrigerant is evaporated from the liquid state to the vapor state. Condensers are used in many air conditioning units, but there are multiple other uses for condensers. For example, most distillation columns have an overhead condenser for condensing the vaporous material from the distillation column and returning the condensation back to the column. There are also reflux condensers which are positioned over reactors so the reactors can be kept at the boiling point with the vaporous material condensed and returned to the reactor. There are steam condensers used with power generation and other purposes, and there are also condensers used in a wide variety of other applications.

Many types of tubes have been developed to improve the performance of heat exchangers, especially heat exchangers used for condensation or evaporation. One type of tube improvement involves machining or forming a predetermined surface structure on a tube. Tubes can have a surface structure formed on the outer surface of the tube to improve the rate of heat transfer. Tubes can also have a surface structure formed on the inner surface of the tubes, and sometimes the tubes will have a surface structure formed on both the outer surface and the inner surface. Different designs for this surface structure are better suited for various purposes. Some types of surfaces are particularly suited for condensation of vapors whereas other surfaces are particularly suited for evaporation of liquids into vapors. Other surface structures are suited to improving heat transfer to a fluid flowing through the tube.

BRIEF SUMMARY OF THE INVENTION

A shell and tube heat exchanger includes a shell with at least two separate types of tubes positioned inside the shell. The first tube type has a first outer surface and the second tube type has a second outer surface where the second outer surface is different from the first outer surface. The two tube types are in liquid communication such that a liquid contacting the first tube type inner surface can flow into contact with the second tube type inner surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a tube with a one dimensional surface outer surface.

FIG. 2 is a perspective view of a portion of a tube wall having a two dimensional outer surface.

FIG. 3 is a perspective view of a portion of a tube wall having a three dimensional outer surface.

FIG. 4 is a perspective view of a portion of a tube wall having a different three dimensional outer surface.

FIG. 5 is a sectional end view of a shell and tube heat exchanger.

FIG. 6 is a sectional side view of a shell and tube heat exchanger.

FIG. 7 is graph of the heat transfer coefficient in the gas cooling section.

FIG. 8 is a graph of the heat transfer coefficient in the upper condensation section.

FIG. 9 is a graph of the heat transfer coefficient in the lower condensation section.

FIG. 10 is a sectional side view of a shell and tube heat exchanger with a horizontal baffle above the condensate collection section.

FIG. 11 is a graph of the heat transfer coefficient in the condensate collection section.

DETAILED DESCRIPTION

Tube Types

The tubes used in shell and tube heat exchangers are typically cylindrical. Fluid can flow through the interior of the tube or across the outside surface, but fluid cannot pass through the tube wall and go from the interior to the exterior of the tube. There are several different types of tube outer surfaces 10 available, with a few examples shown in FIGS. 1 through 4. FIG. 1 shows a portion of the tube body 12 of one type of tube 50. The tube body 12 has a tube outer surface 10 and a tube inner surface 14. The tube outer surface 10 is a smooth outer surface and is referred to as a one dimensional surface in this disclosure. The one dimensional outer surface 16 is often used as a raw material for making tubes with more elaborate outer surfaces 10. The one dimensional outer 16 has a relatively low surface area compared to other tube outer surfaces 10.

FIG. 2 depicts what is referred to as a two dimensional outer surface 18 in this disclosure, and tubes with the two dimensional outer surface 18 are referred to as T1 tubes in this disclosure. The two dimensional outer surface 18 includes fins 20 with channels 22 between the fins 20, where the fins 20 stand straight up from the tube outer surface 10 and the fin top 24 is essentially smooth. The fin top 24 is the portion of the fin 20 furthest from the tube body 12. The fin based 26 is where the fin 20 joins the tube body 12. A fin height 28 is defined as the distance between the fin base 26 and the fin top 24. In the tube 50 with a two dimensional outer surface 18, the fin height 28 is essentially constant as a result of the fin top 24 being smooth.

A third type of tube 50 is shown in FIG. 3. The tube in FIG. 3 has an outer surface 10 which is referred to as a 3 dimensional outer surface 30 in this disclosure. The 2 dimensional outer surface 18 of FIG. 2 can be used as a starting material to produce a 3 dimensional outer surface 30, such as the surface depicted in FIG. 3. A 3 dimensional outer surface 30 is a finned outer surface 10 where the fins 20 are not simply standing straight up from the tube outer surface 10, such as where the fins 20 are bent, serrated, or partially flattened. This 3 dimensional outer surface 30 includes fins 20 having peaks 32 and depressions 34 defined along the fin top 24. The alternating peaks 32 and depressions 34 along the fin top 24 add an extra layer of contour to the 3 dimensional outer surface 30. There are many types of 3 dimensional outer surfaces 30 which are currently available. These types include saw-tooth fins, petal fins, external nural fins, etc. These fins 20 can be bent over, they can include platforms at the fin top 24, and they can include a wide variety of other shapes and geometries. In this disclosure, the 3 dimensional outer surface 30 shown in FIG. 3 is referred to as a 3D peak surface 35, and tubes having the 3D peak surface 35 are referred to as T2 tubes.

Another type of 3 dimensional outer surface 30 is depicted in FIG. 4. This surface includes fin pins 36. The fin pins 36 include peaks 32 and depressions 34 where the depressions 34 extend all way through the fin 20 to the fin-based 26, so the fin pins 36 are essentially free standing. This design allows for fluid to flow across the tube outer surface 10 through the fin pins 36 because the depressions 34 extend all the way to the tube outer surface 10. In this disclosure, this surface is referred to as the 3D pin surface 37 and tubes with the 3D pin surface 37 are referred to as T3 tubes.

Other types of tube outer surfaces 10 can include tube surfaces designed for evaporation, making reference now to FIGS. 1-4. Typically these are similar to the 3 dimensional outer surfaces 30 with a channel or pocket established below a roof. This can be done in a variety of ways, such as bending the fins 20 over such that one fin top 24 extends over and touches or nearly touches the neighboring fin 20. The channel 22 between the two fins 20 is then covered by the bent over fin 20, which forms the roof. Often the fins 20 will be notched before they are bent over such that the roof includes holes. Typically the outer surfaces 10 which are most advantageous for condensation have different structure than those outer surfaces 10 which are most advantageous for evaporation.

The tubes 50 can also have structure formed on the tube inner surface 14. There can be a wide variety of different structures present on the tube inner surfaces 14. One example is a helical ridge 39 which spirals through the tube inner surface 14. This helical ridge 39 defined on the tube inner surface 10 causes a swirling of the fluid flowing through the tube 50, which can promote better heat transfer. The tubes 50 can also have varying internal diameters. Different tubes 50 can have different resistances to flow. Many factors can affect a tube's resistance to flow, including the tube diameter, the tube length, and the structure on the tube inner surfaces 14. There are also other factors which can affect the resistance to flow through a tube. There are a wide variety of tube types available for a multitude of specialized purposes. Of course, some tubes are more expensive to produce than others, and cost is a factor in heat exchanger design.

Heat Exchangers Sections

The shell and tube heat exchanger with varying tube designs is particularly effective for heat transfer involving a phase change, such as the condensation of a vapor or the evaporation of a liquid. This description will focus on the condensation of a vapor, but it should be understood that this invention is also applicable to an evaporator. A typical shell and tube heat exchanger 38 is depicted in FIGS. 5 and 6, where FIG. 5 shows a sectional end view and FIG. 6 shows a sectional side view, with further reference to FIG. 1. The heat exchanger 38 includes a heat transfer fluid inlet 40 and a heat transfer fluid outlet 42, which provide ingress and egress for a first fluid which is commonly a heat transfer fluid. The heat exchanger 38 also includes a process fluid inlet 44 and a process fluid outlet 46, which provide ingress and egress for a second fluid which is commonly a process fluid. The heat exchanger 38 includes a shell 48, a plurality of tubes 50, tube sheets 52, tube side headers 54, and the heat exchanger 38 may include one or more pass partitions 56. The plurality of tubes 50 is often referred to as a tube bundle. The tubes 50, tube sheets 52, tube side headers 54, and any pass partitions 56 are positioned inside the shell 48. The heat transfer fluid inlet and outlet 40, 42 and the process fluid inlet and outlet 44, 46 penetrate the shell.

The tubes 50 penetrate the tube sheets 52, and there is a seal such that fluids do not pass between the tube sheet 52 and a tube 50, or between a tube sheet 52 and the shell 48. The tube sheets 52 separate the heat exchanger shell side 58 from the tube side 59, where the shell side 58 refers to those areas in liquid communication with the tube outer surface 10, and the tube side 59 refers to those portions in liquid communication with the tube inner surface 14. Generally, all the tube inner surfaces 14 in a heat exchanger 38 are in liquid communication, so a fluid can flow from inside one tube 50 to the inside of any other tube 50 in the heat exchanger 38 without passing through any walls or barriers, such as a tube body 12 or a tube sheet 52. The tube inner surfaces 14 are also in liquid communication with the heat transfer fluid inlet and outlet 40, 42.

When used as a condenser, the heat exchanger 38 will typically have vapor introduced to the shell side 58 of the heat exchanger 38 through the process fluid inlet 44. The condensate exits the heat exchanger 38 through the process fluid outlet 46. The process fluid outlet 46 typically penetrates the shell bottom 57, because condensate flows downwards due to gravitational pull. Commonly, the process fluid inlet 44 will be positioned at the top of the heat exchanger shell 48, but it is also possible for the process fluid inlet 44 to be positioned on the side of the heat exchanger 38 or in other locations. The process fluid inlet and outlet 44, 46 allow fluids to enter and exit the heat exchanger shell side 58, so the process fluid inlet and outlet 44, 46 and in liquid communication with the heat exchanger shell side 58, including the tube outer surfaces 10.

The examples shown in this disclosure are horizontal heat exchangers 38, but other inclinations are possible. The tubes 50 in the heat exchangers 38 are essentially horizontal with gravity pulling primarily perpendicular to the tube axis, so the heat exchangers 38 are considered horizontal heat exchangers 38. Some tube designs can be optimized for horizontal use, or for other inclinations, so the inclination of the heat exchanger 38 can impact performance.

The heat exchanger 38 shown in FIG. 6 is a double pass heat exchanger 38. This is because the heat transfer fluid passes through tubes 50 twice before exiting the heat exchanger 38. The heat transfer fluid enters the tube side header 54 connected to the heat transfer fluid inlet 40 and flows through a plurality of tubes 50 to the tube side header 54 on the opposite side of the heat exchanger 38. The heat transfer fluid then flows through another set of tubes back to the tube side header 54 connected to the heat transfer fluid outlet 42. The pass partition 56 separates the tube side header 54 between the heat transfer fluid inlet 40 and the heat transfer fluid outlet 42.

The heat exchanger 38 shown in FIGS. 5 and 6 depict the process fluid inlet 44 positioned at the top of the heat exchanger 38 and the process fluid outlet 46 positioned at the shell bottom 57. In this example, the condenser has several sections which are determined by the type of heat transfer occurring in those sections. When the vapor enters the heat exchanger 38 through the process fluid inlet 44, it contacts tubes 50 in the gas cooling section 60 of the heat exchanger. In the gas cooling section 60 the gas is cooled from a super heated gas to the saturation point where condensation can begin. In the gas cooling section 60 there is typically no phase change of the vapor, or at least very little phase change. The gas cooling section 60 begins at the process fluid inlet 44, and extends some distance into the tube bundle. The gas cooling section 60 extends into the tube bundle to the point where the heat transfer transitions from the sensible cooling of the incoming gas to the latent condensation of the gas into a liquid. As with the transfer between each section in the heat exchanger 38, there may be no clear demarcation indicating a transfer from one section to the next.

In the gas cooling section 60 it has been found that the two dimensional outer surface 18 has a higher heat transfer efficiency than the three dimensional peak surface 35, as shown in FIG. 7, with continuing reference to FIGS. 2, 3, 5 and 6. In this graph, {h0} stands for the surface heat transfer coefficient, expressed as kilowatts per square meter—degree Kelvin. The horizontal scale depicts the velocity of the gas flowing past the tubes in meters per second. As can be seen, the two-dimensional outer surface 18 has a higher heat transfer coefficient than the three-dimensional peak surface 35, when measured at the same gas flow rate. Therefore, in order to maximize the efficiency of the heat exchanger 38, one would select T1 tubes with the two-dimensional outer surface 18 for the gas cooling section 60 of the heat exchanger 38.

Once the gas has cooled to the saturation point it enters the upper condensation section 62 of the heat exchanger 38. In the upper condensation section 62, the gas begins to condense to a liquid. As the gas condenses, it forms drops on the tubes 50 and these drops begin to rain down from the upper tubes 50 onto the lower tubes 50. As one proceeds downward through the tube bundle, one passes from the upper condensation section 62 to the lower condensation section 64. The transition between the upper condensation section 62 into the lower condensation section 64 depends on the amount of condensate which has rained down on the tubes 50.

In the lower condensation section 64, a relatively thick layer of condensate has formed on the tubes 50. In the upper condensation layer 62, the condensate is either not on the tubes 50, present on the tubes 50 as intermittent drops, or present on the tubes 50 as a thin layer. A condensate layer on the tube outer surface 10 serves to insulate the tube 50, and has a significant effect on heat transfer through the tube 50. There is no bright line or clear demarcation which indicates when one transitions from the upper to the lower condensation section 62, 64. It is even possible, depending on the heat exchanger design, for the upper and lower condensation sections 62, 64 to be merged into one single condensation section 66.

Heat transfer in the upper condensation section 62 tends to be different than heat transfer in the lower condensation section 64 because of the thickness of the condensate layer on the outside of the tubes 50. FIG. 8 depicts the surface heat transfer coefficient Ho for the upper condensation section 62 relative to the heat flow rate per square meter (m2), which is indicated by the letter q. FIG. 9 depicts the surface heat transfer coefficient ho relative to the heat flow rate per square meter q in the lower condensation section 64. It can be seen by referring to FIGS. 2, 3, 5, 6, 8, and 9 that the 3D peak surface 35 has a higher surface heat transfer coefficient than the two-dimensional outer surface 18 in the upper condensation section 62, but the two-dimensional outer surface 18 has a higher surface heat transfer coefficient than the 3D peak surface 30 in the lower condensation section 64. In order to maximize efficiency of the heat exchanger 38, T2 tubes 50 with a 3D peak surface 35 can be installed in the upper condensation section 62, but T1 tubes 50 with a two dimensional outer surface 18 can be installed in the lower condensation section 64.

Other design considerations can be used to further maximize the efficiency of a heat exchanger 38. For example, one can further increase the heat transfer coefficient in the upper condensation section 62 by minimizing the heat flow rate per unit area. The highest heat transfer coefficient for the T2 tubes 50 is at the lowest heat flow rate per unit area. Also, increasing the heat flow rate per unit area does not result in a significant deterioration of the heat transfer coefficient for the T1 tubes 50 in the lower condensation section 64. By examining the graph, one can determine the T1 tube heat transfer coefficient decreases from approximately 16 kilowatts per square meter degree Kelvin (kw/m2-K) to approximately 15 kw/m2-K for a heat transfer rate per unit area increase from approximately 10 kilowatts per square meter (kw/m2) to approximately 60 kw/m2. Heat exchanger designs which shift heat transfer per unit area from the upper condensation section 62 to the lower condensation section 64 may further increase overall efficiency.

As the condensate rains down and gathers, it can collect at the bottom of the heat exchanger 38 in what is referred to as the condensate collection section 68. In the condensate collection section 68, the condensate is cooled below the saturation point to a lower temperature. This is a sensible heat transfer because there is no phase change, and different types of tubes have different efficiencies. The factors affecting heat transfer rates in the condensate collection section 68 are different than in the condensation section 66, and should be considered separately.

FIGS. 6 and 10 depict different heat exchangers 38 where the condensate flows past the tubes 50 in the condensate collection section 68 differently. In the heat exchanger 38 shown in FIG. 6, the condensate flow is generally perpendicular to the axis of the tubes 50 in the condensate collection section 68, because the condensate rains down from above the tubes 50 and drains out of the heat exchanger 38 from below the tubes 50. Condensate flows from where it drips down directly to the process fluid outlet 46.

Condensate flow is different in the alternative design depicted in FIG. 10, where a horizontal baffle 70 directs the flow of the condensate. The horizontal baffle 70 is positioned inside the shell 48, over the process fluid outlet 46, and generally separates the condensation section 66 from the condensate collection section 68. Therefore, the horizontal baffle 70 would be within the tube bundle because all the tubes 50 form the tube bundle. The horizontal baffle 70 is not perfectly horizontal, but instead is nearly horizontal with a slope in at least one direction to direct fluid flow. The condensate rains down on the horizontal baffle 70 and is directed by the horizontal baffle 70 towards one end of the heat exchanger 38. The process fluid outlet 46 is positioned at the opposite end of the heat exchanger shell 48 from the point where the horizontal baffle 70 directs the condensate. Because of this, the condensate flows essentially parallel with the axis of the tubes 50 in the condensate collection section 68 in the heat exchanger design depicted in FIG. 10.

FIG. 11 shows the surface heat transfer coefficient of various tubes designs in the condensate collection section 68. In FIG. 11, the vertical axis shows the condensate flow rate parallel to the axis of the tubes 50 in the condensate collection section 68, with continuing reference to FIGS. 2, 3, 4, and 6. This graph indicates use of T3 tubes with a 3D pin surface 37 provides the highest efficiency when the condensate flows along the axis of the tube 50. However, efficiency of T1 , T2, or T3 tubes with a two dimensional surface 18, a 3D peak surface 35, or a 3D pin surface 37 respectively are approximately comparable as the flow rate parallel to the axis of the tube 50 slows. Therefore, if the heat exchanger 38 has a design with significant condensate flow parallel to the tube axis, such as a design with a horizontal baffle 70 as shown, the 3D pin surface 37 provides greater efficiency. However, if the heat exchanger 38 does not have significant flow along the tube axis, the efficiencies of the different tubes 50 are roughly comparable and other factors, such as tube price, should determine which tube 50 is used.

It should be noted that tubes 50 with different outside surfaces 10 than those shown in this disclosure may perform differently than the particular examples depicted herein. In such a case, testing can be conducted to determine which particular tube outer surfaces 10 should be positioned in the respective portions of the heat exchanger 38. Essentially the same principles can be applied to other heat exchanger applications. One divides a heat exchanger 38 into different sections based on the type of heat transfer occurring, and then determines a tube type which is efficient for that section of the heat exchanger 38. The heat exchanger 38 is then built with the tubes 50 determined to be more efficient in the appropriate location in the heat exchanger 38. This produces a heat exchanger 38 with varying tubes 50 and improved overall efficiency. Besides tube efficiency, other factors such as tube cost can be evaluated so a less expensive heat exchanger 38 may be built which has acceptable overall efficiency. In some cases, it may be possible to build a heat exchanger 38 with varying tubes 50 which is less expensive, but just as efficient, as a heat exchanger 38 having only one type of tube 50.

Heat Exchanger Design

The shell and tube heat exchanger 38 as described can have several different possible designs. These designs include a two-pass heat exchanger with the condensate flowing perpendicular to the axis of the tubes 50 in the condensate collection section 68 as depicted in FIG. 6. These designs can also include the condensate flowing parallel to the axis of the tubes 50 in the condensate section 68 as depicted in FIG. 10. Other designs with the process fluid inlet 44 entering the shell 48 at the side of the heat exchanger 38 are possible, or the process fluid inlet 44 could be positioned to one side of the heat exchanger shell 48. It is possible to have multiple passes of the process fluid through the shell side 58 of the heat exchanger 38. It is possible for the heat exchanger 38 to have a single pass for the heat transfer fluid, where there is no pass partition 56 and the heat transfer fluid inlet 40 is on the opposite side of the heat exchanger from the heat transfer outlet 42. It is also possible for the heat exchanger 38 to have 2, 3, or more passes for the heat transfer fluid. Other designs not specifically mentioned are also possible.

Often times, the heat transfer fluid will pass through the heat exchanger 38 an even number of times so the heat transfer fluid inlet and outlet 40, 42 can be on the same side of the heat exchanger. This can simplify access and piping associated with the heat exchanger 38. The heat exchanger 38 includes a plurality of tubes 50, where there are more than one type of tube outer surface 10, as seen by referring to FIGS. 1, 2, 3, 4, 5, 6, and 10. There is at least a first tube 50 with a first outer surface 10 and a second tube 50 with a second outer surface 10 where the first outer surface 10 is different than the second outer surface 10. The varying outer surfaces 10 can result in the heat transfer efficiency being improved throughout the heat exchanger 38 by matching the tube outer surface 10 with the type of heat transfer occurring in the appropriate section of the heat exchanger 38. There can also be a third tube 50 with a third outer surface 10, where the third outer surface 10 is different than either the first or second outer surface 10, and so on. Because condensate flows downward due to gravity, the different tube outer surfaces 10 can be segregated vertically within the heat exchanger 38. Therefore, the first, second and third tubes 50 can each be a plurality of tubes 50 defining different heat exchanger sections, where the first section is above the second section, and the second section is above the third section.

The number of tubes 50 used in each of the sections within the heat exchanger 38 can be varied, and different factors can affect the number of these tubes 50. One possible factor to be considered is the resistance to flow inside the tube 50. If a 3D peak surface 35 is used in the upper condensate section 62 and a two dimensional surface 18 is used in the lower condensate section 64, it is possible that the upper and lower condensate sections 62, 64 will have a different resistance to flow inside the tubes 50. For example, the tube 50 with the two dimensional outer surface 18 might have lower resistance to flow than the tube 50 with the 3D peak outer surface 35. In such a case, the number of tubes 50 for each section might be adjusted such that the total resistance to flow through the tubes 50 with the two dimensional outer surface 18 is approximately equal to the total resistance to flow with the tubes 50 having the 3D peak surface 35. This can maximize the flow rate through the heat exchanger tubes 50, and higher flow rates tend to result in higher heat transfer rates.

An example of varying the number of tubes in the upper and lower condensation sections 62, 64 is indicated in the table below, with continuing reference to FIGS. 1, 2, 3, 4, 5, 6, and 10. Test results are based on the Chinese National Standard GB/T 18430.1-2001 (Water chilling (heat pump) packages using the vapor compression cycle—Water chilling (heat pump) packages for industrial and commercial and similar applications.) Tests were conducted on a 1758 kilowatt refrigerating unit using refrigerant R134a. Three plans are shown, and in each plan there are a total of 500 tubes 50, and each tube 50 is 3,660 millimeters long with an external diameter of 19 millimeters. Plan 1 shows the use of 500 tubes having a 3D peak surface 35. Plan 2 shows the use of 250 tubes having the 3D peak surface 35 positioned above 250 tubes having a two dimensional outer surfaces 18. In Plan 3 the heat exchanger 18 uses 280 tubes with the 3D peak outer surface 35 positioned above 220 tubes with the two dimensional outer surface 18. In these three plans, the gas cooling section 60 was merged with the upper condensation section 62, and the condensate collection section 68 was merged with the lower condensation section 64.

ItemPlan 1Plan 2Plan 3
Saturation Temperature36.8° C.36.25° C.35.95° C.
Water side resistance56.6 kpa50.9 kpa56.5 kpa

In this example, the tube side resistance to water flow for 220 T1 tubes having the two dimensional outer surface 18 is approximately equal to the tube side resistance to water flow for 280 T2 tubes having the 3D peak surface 35. This resistance to flow, listed above in kilopascals (kpa), depends on the inner surface of the tubes and other factors, and is not necessarily a factor of the outer surface.

The lower the saturation temperature, the higher the efficiency of the overall heat exchanger 38. By utilizing the proper ratio of the different types of tubes 50, the efficiency of the heat exchanger 38 can be improved. The saturation temperature was lowered by using 250 tubes 50 of each type in the appropriate section of the heat exchanger 38, but the saturation temperature was lowered even more by approximately balancing the tube side resistance to flow in the two sections of the heat exchanger 38. The degree of super cooling of the condensate was increased from 2.5 degrees centigrade for plan 1 to 4.3 degrees centigrade for plan 3. The higher the degree of super cooling of the condensate, the more efficient the heat exchanger 38. These tests demonstrate the increased efficiencies possible by varying the tube design in a heat exchanger 38.