PROJECT ON

3-FLUID HEAT EXCHANGERABSTRACT

The detailed behavior of three -fluid, parallel flow heat exchangers has been investigated. The equations governing the two -dimensional temperature distributions of the three fluids have been derived and nondimentionalized. Performance characteristics have been determined for a wide range of operating parameters for single-pass heat exchangers. The performance of two-pass heat exchangers for both concurrent and countercurrent flow has been studied for selected operating conditions. Results have been presented graphically in terms of the temperature effectiveness of the two outer fluids as functions of heat-exchanger size for sets of fixed operating conditions. Nondimensional operating parameters have been defined which allow an efficient presentation of the large volume of performance data required to represent a practical range of operating conditions. Sample problems are included to illustrate the use of the performance graphs for design applications

A heat exchanger has a first tube bundle for circulating a first fluid, a second tube bundle for circulating a second fluid, and a shell which accommodates the tube bundles arranged in series in the shell, so that when a third fluid is circulated through the shell it successively contacts the tube bundles for a successive heat exchange between the third fluid and a respective one of the two first-mentioned fluids, to provide a heat transfer between three fluids

INTRODUCTION:

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. There are four primary classifications of heat exchangers according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side.

Countercurrent (A) and parallel (B) flowsIn counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is the most efficient, in that it can transfer the most heat from the heat (transfer) medium due to the fact that the average temperature difference along any unit length is greater. See countercurrent exchange. In a cross-

An interchangeable plate heat exchanger Tubular heat exchanger flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger. For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while

minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence. The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "log mean temperature difference" (LMTD). Sometimes direct

knowledge of the LMTD is not available and the NTU method is used.

Fig. 1: Shell and tube heat exchanger , single pass

1

Fig. 2: Shell and tube heat , 2-pass tube side Fig. 3: Shell and tube heat ,

exchanger, 2-pass shell side

Types of heat exchangers

Double pipe heat exchanger

Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers are cheap for both design and maintenance, making them a good choice for small industries. But on the other hand, low efficiency of them beside high space occupied for such exchangers in large scales, has led modern industries to use more efficient heat exchanger like shell and tube or other ones. But yet, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basic to students and as the basic rules for modern and normal heat exchangers are the same, students can understand the design techniques much easier. To start the design of a double pipe heat exchanger, the first step is to calculate the heat duty of the heat exchanger. It must be noted that for easier design, its better to ignore heat loss in heat exchanger for primary design. The heat duty can be defined as the heat gained by cold fluid which is equal to the heat loss of the hot fluid.

Shell and tube heat exchanger

A Shell and Tube heat exchanger Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The

second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater

than 30 bar and temperatures greater than 260 C).[2] This is because the shell and tube heat exchangers are robust due to their shape. Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers: Tube diameter: Using a small tube diameter makes the heat exchanger both economical And compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered.

Tube thickness: The thickness of the wall of the tubes is usually determined to ensure:

There is enough room for corrosion

That flow-induced vibration has resistance

Axial strength

Availability of spare parts

Hoop strength (to withstand internal tube pressure)

Buckling strength (to withstand overpressure in the shell)

Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including space available at the installation site and the need to ensure tubes are available in lengths that are twice the required length (so they can be

withdrawn and replaced). Also, long, thin tubes are difficult to take out and replace.

Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch (i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter, which leads to a more expensive heat exchanger.

Tube corrugation: this type of tubes, mainly used for the inner tubes, increases the turbulence of the fluids and the effect is very important in the heat transfer giving a better performance. Tube Layout: refers to how tubes are positioned within the shell. There are four main types of tube layout, whichare, triangular (30), rotated triangular (60), square (90) and rotated square (45). The triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping. Square patterns are employed where high fouling is experienced and cleaning is more regular.

Baffle Design: baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles forcing the fluid to flow upward and downwards between the tube bundle. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shells inner diameter. Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle, which consists of two concentric baffles. An outer, wider baffle looks like a donut, whilst the inner baffle is shaped like a disk. This type of baffle forces the fluid to pass around each side of the disk then through the donut baffle generating a different type of fluid flow. Fixed tube liquid-cooled heat exchangers especially suitable for marine and harsh applications can be assembled with brass shells, copper tubes, brass baffles, and forged brass integral end hubs. heat transfer which includes a source of a two-phase flow, for example a boiler. A tank separator 2 is connected to the source of the two-phase flow, in which the flow is subdivided into two fluids having different phase states, in particular into liquid and vapor. If the source 1 is a boiler, the tank separator 2 subdivides the liquid supplied from the boiler into a blow down water and a flush steam. The system is provided with a heat exchanger which is formed in accordance with the present invention and identified as a whole with reference numeral 3. The liquid (the blow down water) is supplied from the tank separator into a left part 3' of the heat exchanger which is provided with a first tube bundle, and flows through the tube bundle so as to be discharged at the end, for example into a sewage. The fluid supplied in the left tube bundle can be a fluid which does not change its phase state, and in particular is liquid. The vapor (flush steam) is supplied to a right portion 3" of the heat exchanger provided with a second tube bundle and flows through the second tube bundle in which it condenses. The fluid in the tube bundle in the right portion 3" of the heat exchanger 3 is a fluid which changes its phase state. A third fluid which is a cold flow to be heated in this case can be a make up water, is supplied into a shell which surrounds both tube portions located in series with one another, so that the cold flow first flows around the left tube bundle located in the left part 3' of the heat exchanger, then flows around the right tube bundle arranged in the right part 3" of the heat exchanger, and then is withdrawn from the shell. In the example with the heat exchanger from the boiler, the heated flow or the make up water supplied for example with a temperature 40 heated in the left part 3' of the heat exchanger by heat exchange with the hot blow down water supplied for example with temperature of 230 C. that the make up water is heated for example to 60 thereafter the make up water flows in the right part 3" of the heat exchanger and a heat transfer is performed with the flush stream, for example with temperature of 230 further.

FIG. 2 shows details of the heat exchanger in accordance with the present invention. Here, the left tube bundle is identified as a whole with reference numeral 11 and has a fluid inlet 12 and a fluid outlet 13, the right tube bundle is identified with reference numeral 14 and has a fluid inlet 15 and a fluid outlet 16, and the shell is identified with reference numeral 17 and has a fluid inlet 18 and a fluid outlet 19.

It should be mentioned that the fluid which changes its phase state can be utilized further. In particular, the condensate produced from the vapor in the right tube bundle can be not only discharged, but also can be supplied back to a line leading to the source 1 of the two-phase flow or to another line in which the liquid which does not change its phase state flows.

The heat exchanger shown in FIG. 3 substantially corresponds to the first embodiment of the present invention. In this embodiment, however, the heat exchanger is arranged directly in the tank separator 2. This simplifies the overall construction of the heat exchanger in which the heat exchanger of the present invention is used.

While in the embodiment of FIG. 1 the fluid which passes through the left part 3' of the heat exchanger and does not change its phase state and the fluid which passes through the right part 3" of the heat exchanger and changes its phase state are the fluids produced from the same source, in particular from the two-phase flow, FIG. 4 shows the system in accordance with another embodiment. In the system shown in this figure, vapor which is a fluid which changes its phase state, is supplied into the tube bundle 11 located in the left part 3' of the heat exchanger. The vapor is condensed in the tube bundle 11, and then as a liquid which does not change its phase state, is supplied into the tube bundle 14 located in the right part 3" of the heat exchanger and is cooled in the tube bundle 14. In all above described embodiments, the third fluid is a cold fluid to be heated which is circulated through the heat exchanger to cool the other two fluids and to be heated. In the embodiment of FIG. 4, similarly to the previous embodiments, the third, cold fluid is circulated inside the shell 17 so that again it is first brought in a heat transfer with the fluid which does not change its phase state and thereafter is brought into heat exchange with the fluid which changes its phase state

In the system shown in FIG. 5 the third fluid is a heating fluid which is circulated inside the shell 17 so as to heat the other two fluids and to be cooled. In this heat exchanger the third fluid is brought into a heat transfer first with a fluid which changes its phase state and thereafter is brought into a heat transfer with a fluid which does not change its phase state. An initial flow through the tube bundles is provided by a liquid which is first supplied into the tube bundle 11 located in the left part 3' of the heat exchanger and is heated into the tube bundle 11 to evaporate. The vapor is then supplied into the tube bundle 14 located in the right part 3" of the heat exchanger and is superheated there.

In the embodiments of FIGS. 6 and 7, the first fluid and the second fluid flow independently from one another. The first fluid is supplied into and withdrawn from the tube bundle 14 located in the right part 3" of the heat exchanger, while the second fluid is supplied into and withdrawn from the tube bundle 11 located in the left part 3' of the heat exchanger. The third fluid cools or heats the fluid in one tube bundle and in the other tube bundle. In addition, in the embodiment of FIG. 7 the third fluid is recirculated for example, by a recirculating pump to cool the fluid in one bundle and to heat in the other by heat transfer between the fluids in the bundles.

The heat exchanger shown in FIG. 8 has a first tube bundle 11" and a second tube bundle 14" which are arranged one after the other or in other words in series with one another in the parts 3a' and 3a" of the heat exchanger 3a. The third fluid is circulated through the interior of the shell 17". Here, however, the heat exchanger 3a is U-shaped. More particularly, its shell 17' is bent in a U-shaped manner, and the tube bundles 11' and 14' are located in the corresponding legs of the U-shape. In this construction the fluid inlets and outlets of the tube bundles and the shell are located at one side of the heat exchanger, and therefore servicing of the heat exchanger as well as its repair and maintenance are facilitated.

Finally, the embodiment of FIG. 9 shows a heat exchanger which substantially corresponds to the heat exchanger shown in FIG. 3, but is provided with a heat exchanger of FIG. 8. In particular, the heat exchanger 3a here is U-shaped and arranged in the tank separator 2'. Also, here a pump 4 is provided for recirculating of the liquids back into a liquid line of this system.

It should be mentioned that the two fluids which are circulated in the two bundles can be fluids of the same chemical substance, for example a water flow and a steam flow. On the other hand, these two fluids can be formed by flows of different chemical substances, for example an ammonia vapor flow and a water flow, etc.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in heat exchanger, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

Accordingly, it is an object of the present invention to provide a heat exchanger which has an improved intensification of a heat exchange between the fluids.

In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention of the resides, briefly stated, in a heat exchanger provided with two tube bundles for circulation of a first fluid and a second fluid, and a shell which accommodates the tube bundles in series with one another and through which a third fluid is circulated to be brought into a heat transfer with the first mentioned two fluids, so that a heat transferbetween three fluids is performed.

When the heat exchanger is designed in accordance with the present invention, it provides for a substantially intensified heat exchange between the fluids.

The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

1. A heat recovery system comprising a source of a first fluid; a first tube bundle having a first inlet and a first outlet arranged so that said first fluid is introduced from said source of said first fluid into said first inlet, passes through said first tube bundle, and is then withdrawn from said first outlet; a source of the second fluid; a second tube bundle having a second inlet and a second outlet arranged so that said second fluid is introduced from said source of said second fluid into said second inlet, passes through said second tube bundle is then withdrawn from said second outlet; a source of a third fluid; and a shell which accommodates said first and said second tube bundles and has a shell inlet and a shell outlet arranged so that said third fluid is introduced from said third source of said third fluid into said shell inlet, passes through said shell in contact with said first tube bundle and second tube bundle for a successive heat transfer between said third fluid and said first and second fluids, and thereafter is withdrawn from said shell outlet, so that all said three fluids are supplied from external thermal sources to conduct a heat transfer between said three fluids and thereafter all said three fluids are withdrawn for heat recovery.

2. A heat recovery system as defined in claim 1, wherein said shell has an axis and extends substantially in an axial direction and has two axial ends, said tube bundles being arranged substantially in said axial ends of said shell and spaced from one another in an axial direction.

3. A heat recovery system as defined in claim 1, wherein said shell is substantially U-shaped and has two leg portions connected with one another, said tube bundles being arranged in said leg portions and each being provided with a fluid inlet and a fluid outlet located at one side of the heat exchanger.