WO2004051168A2

WO2004051168A2 - Tube-tube heat exchangers

Info

Publication number: WO2004051168A2
Application number: PCT/IN2003/000377
Authority: WO
Inventors: Milind V. Rane; Madhukar S. Tandale
Original assignee: Rane Milind V; Tandale Madhukar S
Priority date: 2002-12-03
Filing date: 2003-12-03
Publication date: 2004-06-17
Also published as: WO2004051168A3; AU2003302703A1; AU2003302703A8

Abstract

Tube-Tube Heat Exchangers (TTHE) comprising a plurality of thermally bonded conduits (1, 2, 3) with combination of bends, straight lengths, tube diameter, tube material, thermal bonding in multi-stream multi-phase heat transfer applications. The secondary flows induced in bend leads to enhancement in bend as well as in straight length downstream of bend without significant increase in pressure drop and pumping power.

Description

TUBE-TUBE HEAT EXCHANGERS

Field of the Invention

The present invention relates to Tube-Tube Heat Exchangers (TTHE), wherein combination of bends, straight lengths, tube diameter, tube material, thermal bonding enables effective transfer of heat in multi-stream multi-phase heat transfer applications.

Background of the Invention

Heat exchangers find a wide variety of applications in the chemical industry. In the case of heat exchangers employing fluids as heat transfer means, it is desirable to have a heat exchanger with low pressure drop on all fluid streams, leading to low pumping power, exchanging heat with each other for specified design conditions with respect to heat duty, inlet-outlet temperatures, size, cost a nd maintainability. The fluid passing through a nnulus for example i n tube-in-tube heat exchangers experience pressure drop due to viscous drag at the inner tube outer surface, which is the active heat transfer surface, and outer tube inner surface which usually does not participate in heat transfer. While designing for specified annular side pressure drop, the velocity of fluid in annulus is limited which results in lower heat transfer coefficient on that side leading to increase in heat transfer area, weight, volume, cost of heat exchanger and increase in inventory of fluids exchanging the heat. Developments are therefore necessary to provide heat exchanger for effective transfer of heat with respect to heat duty, pressure drop, size, cost, accessibility for repair, fluid hold-up, etc. US Patent 5,238,058; "Spiral Flighted Double Walled Heat Exchanger" discloses a heat exchanger in which a piece of tube is wound helically over a straight tube. Helically coiled tube is then internally pressurized non-elastically to change the cross-sectional area of the passage while maintaining intimate physical contact between the two tubes. This straight tube may then be formed into a helical, spiral, or serpentine shape in order to make it more compact. Both, inner and outer coils are made out of the formable material such as copper with the wall thickness of outer coil thinner than inner coil. Outer-coiled tube may be made up of a single tube or plurality of tubes to minimize the pressure drop on refrigerant side. The outer tube is wound over a straight tube in helically coiled shape that reduces the pressure bearing capacity of outer tube due to its non-circular shape. The two tubes exchanging heat are not bonded using TBM. Absence of TBM reduces the efficacy of heat transfer when cold fluid is used in straight tube side of heat exchanger. This design results in increase in pressure drop thereby increasing pumping power to pass the fluids through tubes due to non-circular cross-section as well as due to continuous coiling of helically wound tube/s resulting in ineffective heat transfer.

US Patent 4,785,878; "Double-Spiral Heat Exchanger" discloses a spiral heat exchanger in which t wo s ets o f c opper o r e opper b ased a Hoy t ubes, s piral i n s hape, a re a rranged o ne above other. These alternate spiral tubes are connected to a common header, one inside and the other outside of spiral tube bundle. Thermal contact between the fluid conduits is achieved by soldering, dipping or casting of metals such as Zn, Sn, Pb, Al etc. or application of any other suitable thermal bonding material (TBM). Using round pipes for tubing, the pressure bearing capacity increases by about 10 folds as compared to that obtained by flattened pipes. In case of any leakage, the fluid is discharged into space between the pipes, where it can be observed by various methods. The diameter of two-pipe system can be same or different. The limitations of this system include difficulty in headering, complexity in fabrication of multi-spiral systems. The system design is restricted only to two fluid streams without accessibility of the tubes for repair in case of leakage. This design does not offer flexibility of using multiplicity of tubes for balancing pressure drop to handle different flow rates in a heat exchanger. In this design, continuous spiral shapes of tubes causes increase in pressure drop thereby increasing pumping power resulting in ineffective heat transfer.

US Patent 4,602,674; "Two-Circuit Heat Exchanger" describes a TTHE in which each circuit is formed from at least one non-circular tube of spiral shape. These two circuits are in thermal contact with each other. Water-soluble film between spirals enhances heat transfer and assists in detecting leakage of the circulating fluids. The tubes of wall thickness 0.5 to 0.7 mm and diameter 10 mm on both sides, made from a choice of materials such as Cu, bronze, Cu alloy, steel or plastic are arranged in a spiral alternately one above the other. Flattening the circular tubes by pressing the tube bundle between two plates shapes the two circuits. During flattening, the heat exchange tubes are subjected to internal pressure of 50 bar thereby eliminating the indentations and constrictions. This construction has several limitations such as cyclic temperature changes result in poor heat transfer surface contact over time resulting in less heat transfer, reduction in pressure bearing capacity as compared to that offered by circular tubes, and difficulty in repair of heat exchanger on leakage. Continuous coiled shape of the tubes in this design increases the pressure drop thereby increasing pumping power without commensurate enhancement in heat transfer.

US Patent 4,479,533, "Tertiary Heat Exchanger" discloses a tertiary type of TTHE having three sets of adjacent ducting for three fluids like radiator water, freon and tap water. Two sets of spirally coiled tubing are stacked within the third set of ducting. Two sets of tubing are alternating layers one above the other and pressed against each other eliminating the need of a baffle. There is no thermal bonding material used between two sets of tubes. There is provision for changing effective length of flows in two circuits so as to optimize heat exchanger for heat duty, pressure drop, and temperature levels. One of the shortcomings of this system is poor heat transfer between the fluids in a bundle due to absence of thermal bonding material. If tubes are not coiled and aligned accurately, it will result in less effective heat transfer due to absence of physical bonding. Continuous coiling of t ubes in spirally coiled TTHE increases pressure drop thereby increasing pumping power without commensurate enhancement in heat transfer.

US Patent 4,411 ,307, "Wound Tube Heat Exchanger" discloses a double walled heat exchanger in which tubes (d = 22.2 mm, copper) through which fluids exchanges heat are flattened on one side with the flattened areas held adjacent to each other. Wrapping conductive wire (width = 1.27 to 3.2 mm, thickness = 0.5 to 1.0 mm, 2 to 8 turns per inch) around the tubes holds the flattened tubes and establishing the contact. Then tubes are expanded after winding and coiling into desired shape. Conductive water-soluble compounds used between flattened surfaces of two tubes serves the dual purpose of leak detection and enhancement of heat transfer. A major weakness of this arrangement is difficulty in coiling of the tube bundle due to D shape tube cross-section. Placement of conductive tube at the center of the two conduits will reduce contact surface for heat transfer affecting heat transfer. The non-circular cross-section of tubes reduces the pressure bearing capacity of the tubes.

US Patent 4,380,912, "Double Wall Tube Assembly for Use in Heat Exchangers" discloses double walled heat exchangers made of copper or steel alloy used for recovering superheat from hot compressed refrigerant gases discharged by the compressor before releasing into condenser in a refrigerant circuit. This patent describes heat exchangers in which two tubes in spiral form are constructed with one tube circular in cross-section and the other of matching contour. The coil is welded at two points to give intimate contact. Star shaped conductive material in the circular cross-section tubes throughout its length provides double surface area per unit of cross-sectional area and enhances the heat transfer efficiency. Application of high-pressure fluid to the inner tube squeezes out the air/voids trapped between contacting tube s urfaces. C ontinuous coiling of tubes i n this s pirally coiled TTHE i ncreases pressure drop and pumping power without commensurate enhancement in heat transfer. This design does not offer flexibility of using different number of tubes for balancing pressure drop while handling different flow rates on two sides. The non-circular cross-section of one of the conduits causes i ncreased p ressure d rap for a particular flow rate as compared to that i n a circular conduit. The effectiveness of heat transfer will be reduced due to absence of thermal bonding material between two tubes. US Patent 4,287,724, "Air Chiller/Drier" discloses TTHE of spiral shape for separation of moisture from air to produce dry air. A pair of steel or copper conduits is spirally wound in a horizontal plane to establish intimate contact. The conduits in the heat exchanger are complementarily contoured to provide significant area of contact, wrapped tightly and brazed at one or two points to establish good contact. Generally, this device has an upper portion, which contains a pre-cooling section in which air from the compressor is partially cooled before entering the chillier located at lower portion of the device. The TTHE uses heat sink material throughout the chillier to transfer heat adequately to outgoing air from incoming air in the pre- cooling section. Larger capacities can be achieved by increasing the number of spirals one above other and connected in parallel. Although, this air drier system represents significant improvement over the 'Shell and Tube' systems, it possesses several disadvantages. Thermal bonding material is not used between the two conduits. It employs heat sink material, which is usually an expensive mixture of aluminium particles dispersed in oil. The aluminium particles have strong tendency to settle down resulting in concentration difference of aluminium in upper and lower portion of drier. This design is also limited to conduits of same size. Continuous coiling of tubes in this spirally coiled TTHE increases pressure drop and pumping power without commensurate enhancement in heat transfer resulting in ineffective heat transfer. This design does not offer flexibility of using multiplicity of tubes for balancing pressure drop while handling different flow rates on two sides

US Patent 3,739,842; "Water Cooled Heat Exchanger" describes a heat exchanger in which water is chilled in the reservoir by refrigerant flowing through a tube encircling a tank. The tube is flat from inside and concave from outside. The flat side of refrigerant coil engages with reservoir and concave side with matching convolutions of water tube encircles it from outside. It also pre-cools the water flowing through circular tube encircling the refrigerant coil. Each convolution is brazed to the tank for enhancing heat transfer. Continuous coiling and non- circular cross-section of refrigerant tube results in higher pressure drop thereby increase in pumping power in refrigerant tube without commensurate enhancement in heat transfer resulting in ineffective heat transfer.

US Patent 1,799,081 , "Condenser" describes a heat exchanger in which two conduits one inside the other is arranged in a helical form. The inner helical conduit of smaller diameter is screwed into the outer conduit of larger diameter. The conduits are constructed of copper or any other material on water side and steel or other suitable material on refrigerant side. The material of construction of the conduits may vary based on their compatibility with the fluids in use. The coil is dipped in to the molten metal with low melting point such as solder to improve heat transfer and is used in applications involving heat transfer between water and refrigerant. This design with the helical form of the coil is not suitable for applications needing higher capacity, as it is difficult to integrate multiple coils. -The pressure drop in such coils is also high without commensurate enhancement in heat transfer due to their continuous coiling. This leads to ineffective heat transfer.

A study of the prior art related to TTHE reveals several technological gaps like increase in pressure drop due to continuous coil in spiral and helical forms, inability to handle more than three fluids through conduits, restricted choice of conduit materials, conduit sizes and number of conduits, poor accessibility of conduits for repair in case of leakage, inability for partial or complete extraction or introduction of fluids at intermediate temperatures, absence of flexibility to increase or decrease the capacity of heat exchanger during use by adding or removing number of conduit-sets in a single heat exchanger. These designs does not offer flexibility of using multiplicity of tubes for balancing pressure drop while handling different flow rates on two sides. Continuous coiling of tubes in these spirally coiled TTHE increases pressure drop without significant enhancement in heat transfer. Increase in pressure drop causes increase in pumping power. This results in ineffective heat transfer. The judicious selection of number of bends, straight lengths and other design parameters such as conduit diameter, conduit material, thermal bonding for the TTHE needs to be done to achieve effective heat transfer. Use of bends and straight lengths in TTHE results in significant enhancement in heat transfer due to secondary flows induced in the bends. The secondary flows induced in bend leads to enhancement in bend as well as in straight length downstream of bend without significant increase in pressure drop.

Summary of the Invention

The main object of the invention is to provide TTHE with optimized combination of bends, straight lengths, tube diameter, tube material, thermal bonding to achieve effective transfer of heat in multi-stream multi-phase heat transfer applications

Yet another object of the invention is to achieve heat transfer higher than that achieved in TTHE with continuous or only straight conduits for a given pressure drop and pumping power

Yet another object of the invention is to facilitate variation of mass flow rate of fluid/s stream/s at intermediate stages in a single heat exchanger.

Yet another object of the invention is to facilitate heating/cooling of fluid stream/s to different temperature levels in a single heat exchanger. Yet another object of the invention is to provide TTHE for effective transfer of heat from a single fluid stream to multiple fluid streams in a single heat exchanger

Yet another object of the invention is to provide TTHE for effective transfer of heat with a vented design for fluids flowing through separate conduits eliminating problem of mixing of fluid streams in case of leakage Yet another object of the invention is to provide TTHE for effective transfer of heat with the flexibility of utilizing diverse tube materials that are compatible with the fluids

Thus in accordance with the invention the TTHE for multi-stream multi-phase heat transfer comprises single or plurality of conduit-sets with or without headers for fluid inflow and out flow wherein a conduit-set comprises of plurality of thermally bonded conduits of any cross sectional shapes with optimized combination of bends, straight lengths, diameter, material, thermal bonding for effective transfer of heat with option of linking multiplicity of such conduit- sets with common or separate headers. Further, the ratio of heat transfer to pumping power is higher than TTHE using continuous coiled or only straight conduits without bends.

Detailed Description of The Invention Features and advantages of this invention will become apparent in the following detailed description and the preferred embodiments with reference to the accompanying drawings.

Figure 1 , 2, and 3 are various sections of conduit-set (Sheet 1 )

Figure 4 is the Trombone shape TTHE (Sheet 2)

Figure 5 is the Serpentine TTHE (Sheet 3) Figure 6 is the Squeezed Serpentine TTHE (Sheet 3)

Figure 7 is the Serpentine TTHE with partial extraction / introduction (Sheet 4)

Figure 8 is the Serpentine TTHE with two fluids on one side (Sheet 4)

Figure 9 is sectional view of T ube-in-Tube Heat Exchanger and conduit-set of TTHE

(Sheet 5) Figure 10, Figure 1 1 , and Figure 12 are sectional views of Trombone, Serpentine, and

Squeezed Serpentine TTHE respectively (Sheet 6) •

Definitions and Nomenclature

For the purpose of the description contained herein, the definition of the following terms is relevant:

Thermal Bonding: It is mechanical or physical bonding with or without medium, between the conduits with bends and straight lengths containing f luids exchanging heat. Some non- limiting examples of bonding medium are metallic or non-metallic solids, liquids, suspensions etc. Conduit-set: A part or entire TTHE comprises conduits with bends and straight lengths, defining internal spaces, which are mutually isolated with respect to fluid communication, with or without thermal bonding material with or without headers.

Nomenclature I _en_d Length of bend, m l_pt. e Heat exchange length per conduit, m l_st Straight section length of TTHE, m

TBM Thermal bonding material

Uj Overall heat transfer coefficient based on inside surface, W/m² K W_pp.to_t Total pumping power, W ø Semi-cone angle subtended at conduit centre by section of a conduit which is in contact with TBM, degree

One of the embodiments of the conduit-set, with bends and straight lengths, consisting of three conduits as represented in Figures 1, 2 and 3 wherein heat exchange is between two fluids. Figure 1a shows three conduits for heat transfer between two fluids; one of the fluids passes through the conduit 2 and the other fluid through conduits 1 and 3. Heat exchange between the two fluids take place by heat first transferring through the first fluid conduit then passing t hrough the thermal bonding material 4, followed by its transfer through the conduit material of the conduit carrying the other fluid. Various configurations are possible, with one fluid passing through the conduit 2 and another fluid passing through the other two conduits 1 and 3. Another configuration is possible wherein heat exchange takes place amongst three different fluids flowing through separate conduits 1, 2 and 3. This design offers flexibility for choosing the diameter and thickness of the conduits appropriate to the application; further the material of construction of the conduits may vary and could be different for the two conduits. Figure 1b shows the section of conduit-set wherein conduit 92 is with larger diameter than other two conduits 91 and 93. These conduits 91, 92, and 93 are bonded using TBM 94. The TBM is selected from those materials that have good bonding characteristics and thermal conductivity to thermally bond the two conduits. For example conduits made of copper and its alloy, carbon steel, mild steel, stainless steel, etc. may be thermally bonded with materials like copper brazing alloys, zinc, tin, silver, or their alloys. The bonding process may be executed by brazing, soldering, hot metal dipping in molten metal, metallizing using molten metal sprays, welding, etc. Various techniques like spot welding, seam welding, ultrasonic welding, etc. or other means of metallic bonding may be employed. Another method of forming the heat exchanger conduits is by extrusion process. The different sections of conduit-sets formed by using extrusion process are shown in figures 2 and 3. Figure 2a shows extruded sections wherein conduits 6, 7, 8 are formed within the extruded material 5. Figure 2b shows another section of TTHE wherein conduits 10, 11, and 12 are embedded within the extruded material 9. Alternately, these conduits may be wrapped by using mesh made up of conductive material and then covered with thermal bonding material like thermal paste, paints, suspensions etc. The conduits 10, 11 12 may be made up of same or different materials depending on compatibility with fluids. Figure 2c shows conduits 14, 16 formed in the extrusion while conduit 15 is embedded in the extruded material 13. Figure 2d shows conduit 19 formed in the extrusion while conduits 18, 20 are embedded in the extruded material 17. Figure 3a, shows rectangular conduits 21 , 22, 23 formed by extrusion or other forming process. Figure 3b, shows rectangular conduits 24, 25, 26, 27, 28 for two or more fluids flowing through alternate conduits. While the heat exchangers are usually used for transfer of heat between two fluids, it may also be used to exchange heat between three fluids using three-conduit design. One can extend this design wherein more than three fluids can exchange heat by increasing the number of conduits, with bends and straight lengths, brought in thermal contact. Figure 4 shows the Trombone TTHE design, wherein a conduit-set with conduits 29, 30, and 31 which are thermally bonded and coiled in a Trombone shape where straight sections 33 and semi-circular bends 32 are arranged alternating about an imaginary central axis by bending conduits by an angle 180°. It offers advantages of coiled tube heat exchangers, namely enhanced heat transfer due to secondary flows induced by the bends. The secondary flows also lead to increased pressure drop. But compared to continuous spiral TTHE, pressure drop in Trombone TTHE is lower. The secondary flows induced in the bend leads to enhancement in heat transfer even in the straight section. Thus, enhancement in heat transfer in straight section is achieved without significant increase in pressure drop. The conduit-set in Trombone TTHE may be squeezed during its return after every bend by bending the conduits by an angle greater than 1 80°, s o that the straight s ections a re n ot parallel, to g ive Squeezed Trombone TTHE. This increases the surface area density up to about 50% compared to Trombone TTHE. Thus, Squeezed Trombone TTHE with bends and straight lengths results in compact size than Trombone TTHE.

In Serpentine TTHE as shown in Figure 5, conduit-set, with bends and straight lengths, is arranged wherein a Serpentine shape is formed by the combination of bends 40 and straight sections 41, by bending conduits by an angle 180° and changing the direction after each bending. These bends and straight sections are arranged alternately wherein all straight portions of a conduit in a conduit-set are parallel to each other. A conduit-set is formed by thermally bonding the conduits 34, 35. The conduits on respective sides of conduit-sets in TTHE are connected to the respective headers 37, 38 and 36, 39 with provision for inlet and outlet nozzles for entry and exit of fluids. These designs offer flexibility of using different number of conduits for balancing pressure drop while handling different flow rates on two sides. Use of bends and straight sections in Serpentine TTHE will result in significant enhancement in heat transfer due to secondary flows induced in the bends. The capacity of Serpentine TTHE can be increased or decreased during use by adding or removing the conduit-sets. Accordingly, the headers can be designed accordingly for accommodating the change in capacity.

In Squeezed Serpentine TTHE as shown in Figure 6 conduit-set with bends and straight lengths, is coiled wherein a Squeezed Serpentine shape is formed by combination of bends 112 and straight lengths 113 in which bends and straight lengths are arranged alternately by bending the conduits by an angle greater than 1 80° with bends on opposite sides of TTHE brought closer to each other. This shape is more compact as compared to that possible from the Serpentine shape. This arrangement is suitable for higher capacity wherein number of conduit-sets can be integrated by arranging these conduit-sets side-by-side resulting in compact heat exchanger and yet accessible easily for repair.

A TTHE with bends and straight lengths in which fluid at required temperature can be selectively extracted at some intermediate point is shown in Figure 7. Fluid A at 101 enters header 48 and leaves header 43 at 102 while passing through conduits 44 which are thermally bonded to conduits 42. Fluid B at 103 enters TTHE through header 45 and exchanges heat with fluid A while it flows through conduits. It is then extracted selectively through intermediate header 46 at 105. The rest of the Fluid B continues to flow further while exchanging heat with fluid A. It is then extracted completely through header 47 at 104. TTHE can be designed with one or more extraction/s and/or introduction/s for fluid stream/s, if required.

A TTHE with bends and straight lengths in which one fluid is exchanging heat with two fluids is shown in Figure 8. Fluid C is exchanging heat with fluids A and B. Fluid C enters in heat exchanger through header 56 at 106 and leaves heat exchanger through header 51 at 107 and on its way it exchange heat with fluid B and then with fluid A. Fluid B enters TTHE through header 54 at 110 and leaves through header 55 at 111. Similarly, fluid A enters TTHE through header 52 at 108 and leaves through header 53 at 109. Similarly one can design TTHE with bends and straight sections wherein one or more fluids delivers heat with one or more fluids receiving heat. Sectional views of TTHE and Tube-in-Tube Heat Exchanger (T-in-THE) are shown in

Figure 9. A sectional view of TTHE in Figure 9a shows two conduits 59 and 60 thermally bonded using thermal bonding material 61. Figure 9b shows a sectional view of T-in-THE with inner conduit 57 and outer conduit 58. Angle ø is semi-cone angle subtended at the centre of the conduit by part of the conduit which is in contact with TBM. The quantity of T BM with respect to conduit size is proportional to this angle. For the same pressure bearing capacity, thickness of outer conduit 58 is higher than inner conduit 57 which results in increase in weight and cost of heat exchanger compared with TTHE having conduit size same as inner conduit of T-in-THE.

In Trombone, Serpentine and Squeezed Serpentine TTHE; conduits in a conduit-set may be placed side-by-side wherein the longitudinal axes of the conduits are in different planes/curved surfaces parallel to each other. Alternatively the conduits in a conduit-set may be placed side-by-side wherein the longitudinal axes of the conduits are in one plane/curved surface. Thus two configurations, configurations a in which conduits with bends and straight lengths in a conduit-set may be placed side-by-side wherein the longitudinal axes of the conduits are in different planes parallel to each other and configuration b in which the conduits with bends and straight lengths in a conduit-set may be placed side-by-side wherein the longitudinal axes of the conduits are in one plane, are possible in Trombone, Serpentine and Squeezed Serpentine TTHE as shown in figures 10, 11 and 12 respectively.

In one of the embodiments, conduit-set/s of TTHE consist of two or plurality of conduits with bends and straight lengths wherein two fluids flow in alternate conduits that are bonded thermally. Alternately a conduit-set consist of three or plurality of conduits wherein two or three fluids flow in alternate conduits that are bonded thermally. Alternately a conduit-set consist of plurality of conduits wherein plurality of fluids flow in thermally bonded conduits. Alternately, a TTHE consist of a conduit-set or a plurality of conduit-sets with or without headers including the case wherein an additional fluid exchanges heat from the external surface of the conduits. The conduits referred in a conduit-set can be of various cross-sections like circular, elliptical, square or any other suitable shape that f acilitates heat transfer between the fluids exchanging the heat. The conduits may be enhanced internally or externally to facilitate the heat transfer. Alternately two, three or multiple conduits may be co-extruded using a single material or a set of materials. In one of the embodiments, TTHE consists of conduit-set/s with or without headers.

EXAMPLE 1

This example illustrates how effective heat transfer is achieved by selecting bends, straight lengths for a particular combination of conduit material, number of conduits, conduit diameter, thermal bonding etc. in two-phase heat transfer in steam condensation application.

Six sets of TTHE each with different geometrical layouts viz. Straight, Helical and Serpentine with 3, 7, 8, 9 bends, were experimentally tested for performance. Each set comprising of two conduits each 2 m length. These conduits are thermally bonded using TBM. Semi-cone angle of TBM was same in all six sets of TTHE. The performance tests were conducted on six sets of TTHE. During tests, inlet temperature of cooling water was kept constant and mass flow rate was varied from about 4 to 9.5 LPM. Condensation of steam was carried out at near atmospheric pressure. Heat duty and pumping power were measured. The variation of heat duty with pumping power for six sets of TTHE is shown in Graph 1. Higher value of heat duty for same pumping power or less pumping power for same heat duty indicate achievement of effective heat transfer.

4 6

Pumping Power, W

Graph 1 : The graph showing variation in heat duty with pumping power in

Serpentine TTHE with 3, 7, 8, 9 bends, Helical TTHE and Straight TTHE

As seen from Graph 1 , Serpentine TTHE with 7 bends offered most effective heat transfer (highest heat duty when compared for same pumping power). It is also seen that not all combinations of bends and straight lengths in a Serpentine TTHE are better than Straight or Helical TTHE. Between Helical and Straight TTHE, Helical TTHE is better.

Hence, for a particular conduit material, conduit size, TBM and size of TBM (angle ø ), there exist a combination of bends and straight lengths, which result in effective heat transfer.

EXAMPLE 2

This example illustrates the effect of conduit material, TBM on performance of TTHE in single-phase water-to-water heat transfer application. The configuration selected is 1-1. Each set in a configuration 1-1 comprising of two conduits, for two fluids, that are thermally bonded using TBM. Terminal temperatures, mass flow rate on two sides are held constant to design TTHE for a fixed duty and application. The effect of conduit material, thermal bonding on design parameters of TTHE for particular value of φ, is shown in Table 1.

Table 1 Effect of conduit material and thermal bonding on design parameters of TTHE

Table 1 shows that TTHE with low thermal conductivity Stainless Steel conduits and high thermal conductivity TBM like Silver, Copper or Zinc have variation in overall heat transfer coefficient, Uj, of less than 3% while the variation in pumping power is within 2.48% of each other although the thermal conductivity of Zinc is about 25% of that of Silver. However, TTHE with low thermal conductivity TBM, like Thermal Paste, has about 25% lower overall heat transfer coefficient, Uj, and 26.2% higher pumping power requirement, when compared with TTHE with high thermal conductivity TBMs like Silver, Copper or Zinc.

Table 1 also shows that TTHE with high thermal conductivity Copper conduits and high thermal conductivity TBM like Silver, Copper or Zinc have variation in overall heat transfer coefficient, U|, of less than 7.4% while the variation in pumping power is within 4.5% of each other although the thermal conductivity of Zinc is about 25% of that of Silver. However, TTHE with low thermal conductivity TBM, like Thermal Paste, has about 44% lower overall heat transfer coefficient, U|, and 48% higher pumping power requirement, when compared with TTHE with high thermal conductivity TBMs like Silver, Copper or Zinc.

The above variation is for a fixed value of φ. Heat transfer performance of TTHE further depend on volume of TBM, which increases with of φ, and conduit size. Therefore it is necessary to optimise the performance of TTHE for a particular application, with respect to bends and straight lengths, tube material, tube size, thermal bonding, etc.

Claims

CLAIMSWhat is claimed is:

1. A TTHE for multi-stream multi-phase effective heat transfer comprising single or plurality of conduit-sets with or without headers for fluid inflow and out flow wherein a conduit-sets comprises of plurality of thermally bonded conduits of any cross-sectional shape/s with optimised combination of bends, straight lengths, diameter, material, thermal bonding for effective transfer of heat with option of linking multiplicity of such conduit-sets with common or separate headers

2. A TTHE as claimed in Claim 1 wherein fluid/s stream/s are partially/completely extracted/introduced at desired intermediate location/s of the conduit-set to provide heating/cooling of fluid stream/s to desired temperature levels

3. A TTHE as claimed in Claims 1-2 with a single fluid stream exchanges heat effectively to multiple fluid streams with option to provide vented double wall

4. A TTHE as claimed in Claim 1-3 wherein conduit-set/s is/are in the form of a helix with straight lengths and bends

5. A TTHE as claimed in Claims 1-3 wherein the longitudinal axes of the conduits in a conduit-set/s are in same or different planes

6. A TTHE as claimed i n Claims 1 -3, 5 wherein l ongitudinal axes of straight I engths of a conduit in a conduit-set are in same or different planes

7. A TTHE as claimed in Claims 1-6 with flexibility of utilizing diverse tube materials that are compatible with the fluids

8. A TTHE as claimed in Claims 1-7 wherein the ratio of heat transfer to pumping power is higher than TTHE using continuous coiled or straight conduits without bends

9. A TTHE as claimed in Claims 1-8 wherein one or multiple fluid stream/s exchanges heat with one or multiple fluid stream/s in a conduit-set 10. A TTHE as claimed in Claims 1-9 wherein fluid stream/s in conduit/s exchange heat with other fluid/s outside conduits.