Patent Application
20120298340
Not Applicable
1. Field of the Invention
This invention relates to tubular heat exchangers, and in particular to turbulence-inducing devices positioned in the tubes of the tubular heat exchanger that minimize or prevent fouling caused by the heat transfer fluids and enhance or maintain the overall heat transfer coefficient over the operational life of the tubular members.
2. Description of Related Art
Heat exchangers are found in many industrial and commercial applications. In the design of heat transfer equipment, an important factor includes the footprint of the exchanger relative to the capacity of fluid that is to be heated or cooled (the “receiving fluid”), as well as the requisite flow of the heating or cooling fluid (the “transferring fluid”). The heat transfer coefficient between the transferring fluid and the receiving fluid should be maximized to achieve the smallest allowable footprint of the heat exchanger.
Another factor that must be considered in designing heat exchangers is the tendency of heating or cooling fluids to foul in the tubes through which they pass. One detrimental effect of fouling is a lowering of the heat transfer coefficient. The thermal conductivity of the fouling layer is less than that of the tube material, which increases the heat transfer resistance, reduces the efficacy of the heat exchanger, and increases the tube skin temperature. Another negative effect of fouling is that the formation of depositions on the interior surface of the tubes reduces their cross-sectional area, causing increased resistance to the fluid flow and an increase in the pressure drop across the unit.
In refinery and petrochemical plants, problems caused by tube fouling are very expensive to remedy. Capital expenditures are higher due to the increased size of the heat exchanger (e.g., selecting heat exchangers with 10-50% greater surface area to accommodate conventional fouling expectations), the associated increase in requisite area within the plant, the higher strength and size foundations, and the extra transport and installation costs. Furthermore, the cost of operating the unit is increased due to additional fuel, electricity or process steam requirements. In addition, production losses occur during planned and emergency plant shutdowns due to fouling and associated system failures.
Various attempts to minimize or prevent fouling problems have been advanced. One common prevention technique is to use a fouling factor in the design phase of a heat transfer unit that includes increasing the heat transfer surface area, either by increasing the number of tubes or the tube length. Such a fouling factor is considered a necessary aspect of heat exchanger design, based on acceptance of the fact that fouling is inevitable. In addition to the aforementioned costs associated with selecting a larger heat exchanger, an additional concern is that the excess surface area calculated with a fouling factor can result in start-up complications and actually encourage more fouling. That is, it is common that at start-up, sludge and dirt migrate into dead zones and low velocity locations. The effect of increasing the number of tubes is to decrease the fluid flow velocity, thereby increasing the likelihood of fouling. Similarly, increasing the tube length results in lower fluid pressure, also increasing the likelihood of fouling.
Other known attempts to mitigate fouling problems involve the use of in-line mechanical cleaning devices to remove fouling build-up inside the tubes. These devices, which generally require direct physical contact with the inner tube surface, have not been especially successful in preventing fouling.
Deflection insertions are also another general category of fouling prevention or mitigation devices. For instance, U.S. Pat. No. 1,015,831 to Pielock et al. discloses a device that is inserted in a pipe to deflect the central and peripheral flow of liquid. Fluid along the side walls is directed toward the center of the pipe, and fluid moving along the longitudinal center line is directed towards the side walls. The device is constructed as a ring installed on the pipe's inner surface having a diametrically disposed web or a plurality of webs that form an apex pointed against the direction of fluid flow. However, the device described in Pielock et al. is mainly intended to diffuse central flow in multiphase fluid for equal distribution. Furthermore, in the context of a heat exchanger's transferring tube, fouling will predictably occur at the interface of the Pielock device and the tube's inner surface.
U.S. Pat. No. 3,995,663 to Perry describes a ferrule for insertion at the inlet of a vertical shell-and-tube heat exchanger, including a flange and shoulder to seat upon the tube sheet, a bore and a cylindrical portion as an extension of the bore to facilitate formation of a solid column of liquid entering the tube. The ferrule also includes an outwardly extending connecting wall that distributes fluid towards the apex of a conical member. Fluid entering the bore is directed to the side walls due to the shape of the conical member. Apparently, the purpose of the device is to distribute liquid to the walls of the ferrule rather than to the tube walls to provide liquid in the form of a falling film on the inner surfaces of the vertical tubes for evaporation. Therefore, application of this structure is necessarily limited to vertical shell-and-tube heat exchangers.
U.S. Pat. No. 5,311,929 to Verret and U.S. Pat. No. 4,794,980 to Raisanaen both disclose air-to-air heat exchangers that include cone-shaped elements disposed in each tube along a central rod. The cones serve as deflectors to create turbulence in the gases flowing through the tube. The elements disclosed in Verret are attached using a twisted strip of material bent inside the tubes to provide contact with the tube's internal surface. The conical elements described in Raisanaen are open on the downstream end, thus allowing fouling and sludge accumulation inside the cone.
The above-described references each have drawbacks that render them unsuitable for minimizing or preventing fouling. Additional known attempts to prevent fouling rely upon inserts fixed to the inner wall of the tube. However, fouling will eventually accumulate at, and proximate to the attachment points, which hinders removal of the inserts and thus complicates cleaning the inner surface of the tube.
Therefore, it is an object of the present invention to provide an apparatus for use in the tubes of heat exchangers that eliminates or minimizes fouling of the interior surfaces of the tubes.
It is another object of the present invention to provide an apparatus for use in tubes of heat exchangers that maintains the heat transfer coefficient over the operational life of the tubes.
It is still another object of the present invention to provide an apparatus for use in the tubes of heat exchangers that permits the designer to utilize the minimum theoretical heat exchanger size or capacity for a given application.
The above objects and further advantages are provided by the apparatus of the present invention for promoting turbulence in the tubes of a heat exchanger conveying the heat transfer fluid that in one embodiment comprehends a turbulence-inducing element formed with a conical upstream portion, from the base of which a second portion extends downstream. In one embodiment, the second portion is convex or hemi-spheroid in shape. In another embodiment, the second portion is conical in shape. In yet another embodiment, the second portion is shaped as a conical frustum. In yet another embodiment, the second portion is shaped as a truncated convex shape with a rounded edge surface. In another aspect of the present invention, longitudinal grooves and/or protrusions are formed on the exterior surfaces of the turbulence-inducing elements. The solid or closed downstream ends of the elements prevent accumulation of deposits.
A plurality of these turbulence-inducing elements are secured to a structural support member that is centrally positioned along the longitudinal axis of the tube. In a preferred embodiment, a plurality of the turbulence-inducing elements extend along substantially the entire length of the tube. The centrally-positioned support member can be a rigid member, such as a rod, or a flexible material, such as a solid or stranded wire or cable. Alternatively, a plurality of centrally-positioned links can be used to join the turbulence-inducing elements.
In a further aspect of the invention, springs can be provided at both ends of the centrally-positioned support member, to maintain the system in tension and absorb sudden load variations.
In the practice of the method of the invention, the apparatus including a plurality of turbulence-inducing elements mounted on the supporting member is inserted into one or more of the tubes of tube-type heat exchangers to induce turbulent fluid flow inside the tube, particularly at the inner wall of the tube. The supporting member is attached to the ends of the tube. The supported elements are dimensioned and configured so that they do not touch the adjacent inner wall of the tube in which they are mounted. During operation, the fluid in the tube flows across the symmetrically-shaped surfaces of the turbulence-inducing elements, which in turn applies tension to the supporting member and which thereby maintains the elements along the center of the tube.
Preventing formation of a quiescent boundary layer enhances the heat transfer coefficient and breaks down or prevents formation of the stagnant film on the inner surface of the tubes associated with the boundary layer. The apparatus and method of the invention also result in a thorough mixing of the heat transfer fluid as it passes through the tube, thereby enhancing its efficiency.
The invention will be described in further detail below and with reference to the attached drawings in which the same or similar elements are referred to by the same reference numerals, and in which:
Referring to
Exchanger 20 includes a shell 22 and a tube set 24 having a plurality of tubes 26. The tubes 26 are supported at their ends by tube sheets 28, also known as end plates. In the typical construction of a bundled tube heat exchanger, a series of baffles 30 are provided through which the plurality of parallel tubes 26 pass.
In operation, heat transfer fluid is introduced via a tube set inlet 38 proximate to the first end 34 of the shell-and-tube heat exchanger 20, passes through the tubes 26, and is discharged from a tube set outlet 40 proximate to the opposite end 36 of the heat exchanger 20. While heat transfer fluid is passing through tubes 24, receiving fluid is introduced into the shell inlet 42 proximate the end portion 36. Receiving fluid contacts the outer surfaces of the tubes 26 as it passes over them and around the baffles 30, thereby undergoing a temperature change. Heated or cooled fluid from the shell 22 is discharged via the shell outlet 44 proximate to the first end 34.
As noted above, a common problem encountered in the tubes of shell-and-tube and other tubular heat exchangers is fouling of the inner walls and plugging of the tubes carrying the heat transfer fluid. This fouling leads to decreased cross-sectional area of the tubes, thus increasing the pressure drop across the tubes, and also causing decreased thermal conductivity. This phenomenon is schematically illustrated in
Heat transfer fluids can be gases or liquids, including high viscosity lube oil. The selection of the number, size and shape of turbulent-inducing elements depends on the allowable pressure; type of need; enhancement of heat transfer; and need for fouling mitigation. For example, if the pressure drop of a specific heat exchanger is small and more turbulence is required, a preferred embodiment would be to use a large number of turbulent-inducing elements, of relatively small size.
As will be apparent to one of ordinary skill in the art, although a shell-and-tube heat exchanger is depicted in
The dimensions and spacing of the turbulence-inducing elements 150 relative to the size of the tube 126 are described according to the following formulas and with reference to
A minimum gap (g) is maintained between the inside diameter (ID) of the tube and the outer diameter (d) of the turbulence-inducing element, according to the following formula:
g≧0.25*ID (1)
The diameter of the turbulence-inducing element (d) is determined relative to the inside diameter (ID) of the tube, according to the following formula:
d=ID−2g (2)
The length (L) of the turbulence-inducing element is determined relative to the inside diameter (ID) of the tube, according to the following formula:
1.25(ID)<=L<=1.5(ID) (3)
The space (S) between adjacent turbulence-inducing elements is determined relative to the diameter (d) of the turbulence-inducing element and the gap (g) (described above), according to the following formula:
S=3.5*d/g (4)
The depth (h) of the second portion extending towards the downstream end of the tube is determined relative to the diameter (d) of the turbulence-inducing element, according to the following formula:
0≦h≦0.25d (5)
The above formulas used for calculating the dimensions and spacing of the turbulence-inducing elements are provided by way of example. In general, the relative dimensions and spacing of the turbulence-inducing elements can be modified in order to strike a balance between preventing or minimizing the formation of a boundary layer and the potential for erosion of the inner surface of the tube due to increased fluid flow rate against the inner surface walls.
Materials of construction suitable for the turbulence-inducing elements and the structural support element include: plastics, including PTFE (Teflon) and nylon; natural or synthetic rubbers; wood or wood-based composites; or relatively soft metals such as aluminum, titanium, and copper.
The ends 184 and 186 of the structural support element 152 are attached at the upstream end 154 and the downstream end 156, respectively. The ends 184, 186 can include, for example, ball stops that are attached to a linking wire 155 at the upstream end 154 and a linking wire 157 at the downstream end 156 of the tube.
In one embodiment, as shown in
In a another embodiment shown in
In a further embodiment shown in
The turbulence-inducing elements 150 are configured and dimensioned to direct the flowing heat transfer fluid towards the inner surface of the tube wall. For example,
In addition,
Furthermore, in preferred embodiments of the present invention, the turbulence-inducing elements are symmetrical about their longitudinal axes, i.e., from the upstream portion to the downstream portion. Such an arrangement permits a balanced distribution of the transferring fluid within the tube and along the inner wall of the cylindrical tube as shown in
The first portion 260 of the turbulence-inducing element 250 is configured generally in the shape of a conical frustum, with the distal end 262 formed as a truncated apex or a truncated curved or rounded apex. In certain embodiments, the truncation can be minimized such that the distal end approaches an apex or rounded apex, depending on the diameter of the structural support element 252. In a preferred embodiment, the distal end 262 is configured so as to minimize any energy loss associated with localized pockets of turbulence, which would otherwise deleteriously increase the pressure drop along the tube.
The turbulence-inducing element 250 can be attached to the structural support element 252 by any of a number of means. In a preferred embodiment, the turbulence-inducing element 250 can be cast on the wire or rod of the structural support element 252. Alternatively, the turbulence-inducing element can be hollow or have a light-weight core between the distal end and the center of the convex second portion so that the rod can be inserted through and welded in place. Other examples include attaching the turbulence-inducing element 250 to structural support element 252 by crimping or pinning
In one preferred embodiment, the shape of the second portion 270 facing the downstream end of the tube is generally convex. The edges 266 of the interface 264 between the imaginary transverse plane of the base of the first portion 260, e.g., a plane characterized by a plurality of circumferential lines of a cone-shaped structure, and the imaginary base of the second portion 270 (shown in broken lines) are preferably rounded or partially rounded.
The configuration of the second portion can be any suitable shape that minimizes or eliminates edges, as this will minimize or eliminate the accumulation of material that can promote surface fouling of the second portion.
In preferred embodiments, the configuration of the second portion includes a closed outer surface to prevent heat transfer fluid from accumulating within the turbulence-inducing elements.
As shown in
One of ordinary skill in the art will appreciate that other configurations can be applied to the second portion of the turbulence-inducing elements according to the present invention, including a cross-sectional area that generally decreases in the direction of fluid flow.
The first portion of the turbulence-inducing elements can also be one of many shapes that have a cross-sectional area that generally increases along the direction of fluid flow, with the exception of embodiments shown in
One of ordinary skill in the art will appreciate that other configurations can be applied to the first portion of the turbulence-inducing elements according to the present invention that have a cross-sectional area that generally increases in the direction of fluid flow.
In further embodiments, and referring to
In another embodiment, with reference to
In a further embodiment, as shown in
The arrangement of the turbulence-inducing elements within a tube can follow the general configuration shown and described above with respect to
The spring 1380 is preferably formed as helical extension spring having coils that are suitably dimensioned and spaced apart so as to minimize or prevent the likelihood of fouling inside the spring and/or on the tube's inner wall surface proximate the spring. In particular, the outer coil diameter is smaller than the inside tube diameter, with sufficient clearance to prevent scraping of the inner tube wall. Further, the coil spacing, known as the “pitch” of a spring, is sufficiently large to allow fluid to flow through the spring without substantial resistance to minimize or prevent the likelihood of fouling inside the spring. For example, each spring element 1380 can have an outer diameter one-half of the tube's inside diameter, and the spacing between coils of the spring can be between the tube's inside diameter and the tube's outer diameter. It will be appreciated that the spacing between coils depends upon the tension and the coil factor, in addition to any stop ball that may be in place.
Advantageously, including one or more spring elements on the turbulence-inducing apparatus of the present invention facilitates installation of the apparatus, allows for stresses to be absorbed thereby reducing the stress load on the structural support element and the end connections, and maintains tension in the apparatus 1348 even under conditions of transferring fluid flow surge. In addition, spring elements can minimize the tendency of the structural support element 152 to expand longitudinally during operation due to high temperatures, and also to minimize the tendency of the turbulence inducing devices 150 to sag toward the bottom surface of the tube due to gravity. The use of the spring elements can such sag and maintain the turbulence-inducing elements the longitudinal centerline of the tube.
Referring now to
Referring now to
Referring now to
The outer cone 1620 is hollow. At the upstream end, the wall of the outer cone 1620 is relatively thin. At the downstream end, the wall of the outer cone 1620 is relatively thick. The inner cone 1630 includes a substantially closed outer surface and is affixed to the central wire in the same manner as described in the earlier embodiments.
The inner cone 1630 is connected to the outer cone 1620 via a plurality of longitudinal strips 1640 that are plate welded. It is preferable to use an even number of longitudinal strips to provide a symmetrical load which helps to maintain the cone assembly at the tube center. In a preferred embodiment, four longitudinal strips are utilized.
This embodiment will provide for more turbulence during fluid flow for more fluid mixing. In addition, because this configuration allows a portion of the transferring fluid to flow through the annulus gap between the two cones, it creates less erosion to the pipe's inner surface compared with the previously described embodiments. This embodiment is useful in situations where a large cone diameter is required for generating additional turbulence, which would otherwise cause erosion to the pipe's inner surface if some fluid was not permitted to flow through the inside of the cone assembly as described above.
Referring to
A gap (g1) is maintained between the inside tube diameter (ID) and the outer diameter of the outer cone 1620, according to the following formula:
g1=0.1*ID (6)
A gap (g2) is maintained between the outer cone 1620 and the inner cone 1630, according to the following formula:
g2=0.1*ID (7)
The thickness (t) of the base of the outer cone 1620 is determined according to the following formula:
t=0.15*ID (8)
The diameter (d) of the base of the inner cone 1630 is determined according to the following formula:
d=ID−2*g1−2*g2−2*t (9)
The length (L1) of the outer cone 1620 is the same as the length (L2) of the inner cone 1630 and is determined by the following formula:
L=1.5*ID (10)
The spacing (S) between adjacent cone assemblies is determined by the following formula:
S=3.5*d/(g1+g2) (11)
The cone assembly of this embodiment does not include a second portion extending towards the downstream end of the tube as was described with some of the above embodiments.
Advantageously, the apparatus of the present invention can be integrated in new or existing heat transfer devices. Unlike prior art systems that attempt to impart turbulence to fluid flowing in a heat transfer device, the apparatus of the present invention can be installed in clean or fouled existing tubes of a heat transfer device.
In addition, the turbulence-inducing devices of the present invention are not designed to contact the tube's inner surface during operation, unlike prior art systems that attempt to impart turbulence to fluid flowing in a heat transfer device.
In an alternative embodiment one or more radial supporting devices are installed on, and extend radially from the longitudinal support element to contact the adjacent wall of the tube. The supporting device can be constructed from one or more pieces of wire tubing or other rigid material to provide three or four points of contact with the inner surface of the tube to thereby maintain the structural support element aligned with the longitudinal axis of the tube. The arms can be spaced from each other at intervals of 120° or 90°. The radial supporting devices can also be fabricated by casting metal or plastic materials with radial arms extending from a central hub.
Referring again to
The special geometry and studs serve to center the device in the tube during operation, preventing or reducing build-up of deposits inside the tubes. Existing turbulent devices that are held in position by contacting the tube inner surface lead to fouling at the contact points with the tube surface. This complicates maintenance because of the difficulty of first removing the turbulent devices without damaging them and then cleaning the tubes.
The method and apparatus of the present invention have been described above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art and the scope of protection for the invention is to be defined by the claims that follow.
