Heat Exchangers & its types & classifications

Heat exchanger definition & its classification based on flow arrangement & classification based on construction its advantages, disadvantages and applications

1. HEAT EXCHANGERS PRESENTED BY M. JEEVAN PRASAD Assistant Production Engineer

2. CONTENTS Introduction Classification based on flow arrangement Classification based on Construction Advantages Disadvantages Applications

3. INTRODUCTION Heat exchanger is a device which is used for transferring the heat from one fluid to another through the separating wall. 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.

4. Classification of Heat Exchangers by Flow Arrangement There are four basic flow configurations: Counter current Flow Co-current Flow Cross Flow Hybrids such as Cross Counter Flow and Multi Pass Flow

5. Counter Flow Heat Exchangers Figure-1 illustrates an idealized counter flow exchanger in which the two fluids flow parallel to each other but in opposite directions. This type of flow arrangement allows the largest change in temperature of both fluids and is therefore most efficient (where efficiency is the amount of actual heat transferred compared with the theoretical maximum amount of heat that can be transferred).

6. Co-current Flow Heat Exchangers In co-current flow heat exchangers, the stream flow parallel to each other and in the same direction as shown in Figure. This is less efficient than counter current flow but does provide more uniform wall temperatures.

7. Cross Flow Heat Exchangers Cross flow heat exchangers are intermediate in efficiency between counter current flow and parallel flow exchangers. In these units, the streams flow at right angles to each other as shown in Fig.

8. Hybrid flow Flow Heat Exchangers Hybrid flow heat exchangers are created by manufacturers to combine the characteristics of the above-mentioned flow configurations. Examples of hybrid flow patterns are shell-and-tube heat exchangers, cross flow-counter flow , and multi-pass flow heat exchangers.

9. Classification of Heat Exchangers by Construction

10. Recuperative Heat Exchangers A Recuperative Heat Exchanger are designed to have separate flow paths for the two fluids , wherein they exchange heat simultaneously. They are further classified into two categories: indirect contact and direct contact heat exchangers. Indirect Contact Heat Exchangers utilize a conductive wall to separate the two fluids. They are the most employed heat exchangers. Direct Contact Heat Exchangers do not involve a conductive partition and rely on direct contact for the heat exchange to take place. They are suitable for two immiscible fluids, or if one of the fluids will undergo a phase change . They are cheaper due to their simpler design. It is commonly used in seawater desalination, refrigeration systems, and waste heat recovery systems . Examples of direct contact heat exchangers are direct contact condensers, natural draft cooling towers, driers, and steam injection.

11. Double-pipe Heat Exchangers The process fluid flows through the smaller inner pipe , and the utility fluid flows through the annular space between the two pipes. The wall of the inner pipe acts as the conductive barrier between the two fluids wherein heat is transmitted. The counter current flow pattern is the most utilized, though it may be configured to co-current flow. Double-pipe heat exchangers, also known as a pipe in pipe or hairpin or jacketed pipe exchangers, are the simplest type of heat transfer equipment. They are made of two concentric pipes with different diameters.

12. Advantages & Dis-advantages Advantages: Double pipe heat exchangers are suitable for heating or cooling small flow rates of fluids. They are cheap , have a flexible design, and are easy to maintain . They can be constructed from pipes of the same lengths interconnected with fittings at the ends to maximize floor space. Dis-advantages: They only operate at lower heating duties compared to other heat exchanger equipment.

13. Shell and Tube Heat Exchangers Advantages: Shell and tube heat exchangers are ideal for heating and cooling liquids with high flow rates, temperatures, and pressures . To increase operational efficiency, they can be designed to have multiple passes wherein one fluid comes in contact with the other several times. Shell and tube heat exchangers are composed of tubes arranged in a bundle that is housed in a large cylindrical vessel called a shell. Similar to the double pipe heat exchanger, the wall of the inner pipe acts as the conductive barrier . The process fluid flows in the tube side, and the utility fluid flows on the shell side .

14. Shell and Tube Heat Exchangers

15. Shell and Tube Heat Exchangers 4.Baffles: They guide the shell fluid flow across the shell and increase its turbulence. They hold the tubes in proper position during operation since they are prone to sag induced by the flow eddies . Tie rods secure the baffle spacing and position. Essential components of a shell and tube heat exchanger are: Tube Sheet: The tubes are held in place by inserting them into the holes of a plate called a tube sheet. Plenums: Plenums are located in both tube fluid inlet and outlet. It is a container wherein the tube fluid is gathered before loading and discharge.

16. Plate Heat Exchangers These types of heat exchangers utilize conductive plates (corrugated) to transfer heat between two fluids. They have a counter-current flow that allows for lower approach temperature differences, high temperature exchanges , and improved efficiency.

17. Plate and Frame Heat Exchangers Plate and frame heat exchangers use corrugated plates that are joined by a gasket, weld, or braze to ensure that the fluids do not mix. The plates have inlet and outlet ports on the corner to allow passage of the fluid streams. The flow paths of the fluids are the spaces between the plates that are arranged in alternating hot-cold-hot-cold fluid streams. Fluids flow in a countercurrent flow configuration with the hot fluid flowing down the plates while the cold fluid flows up the plates.

18. Plate and Frame Heat Exchangers Advantages of Plate Type Heat Exchanger : Low cost of operation Low cost of maintenance Easy to clean Highly efficient heat transfer Future changes are possible by fitting extra heat transfer plates Less floor space required Applications of Plate type Heat Exchanger: 1.Power generation applications 2.In food, Dairy and brewing industries 3.Refrigerants in cooling systems

19. Gasketed Plate Heat Exchangers These type uses gaskets to connect and seal the plates together. They are widely used in industries that require frequent sanitation, like food and beverage processing. Gasketed plates reduce maintenance costs since they are easy to clean, dismantle, and assemble. More plates may be added to increase the heat exchanger‘s capability and throughput. The disadvantage of this type is its potential for leakage.

20. Welded & Brazed Plate Heat Exchangers Welded Plate Heat Exchangers: Welded plate heat exchangers reduce the possibility of leakage . They can handle higher temperatures , higher pressures, and more corrosive fluids since the operating temperature is not limited by the gasket seals Brazed Plate Heat Exchangers: These heat exchangers have plates joined by a process called brazing, where two pieces of metal are joined by a molten filter metal. They are used in chillers, pumps, evaporators, and condensers.

21. Regenerative Heat Exchangers Regenerative heat exchangers are also known as regenerators or capacitive heat exchangers. Regenerative heat exchangers are types of heat exchanger equipment that utilize a heat storage medium that is made to contact with the hot and cold fluids. The two fluids are usually gasses. They are used in power plants, glass and steel making, and heat recovery systems. There is potential contamination since the same medium is used to interact with the hot and cold fluids. There are two types of regenerative heat exchangers: Static Regenerators Dynamic Regenerators

22. Static Regenerators The hot fluid is made to flow first at a certain length of time. Once the heat storage medium accumulates enough heat, the valve connecting the reservoir of the hot fluid is switched off. The cold fluid is then allowed to flow through the channel, which absorbs the heat coming from the hot fluid. Static regenerators, or fixed bed regenerators, do not have mechanical parts that facilitate the flow of hot and cold fluids. The fluids are made to pass through the channel by a system of pipes and ducts, fitted with valves that act as a "switch" during the separate release of the hot and cold fluids.

23. Dynamic Regenerators Dynamic Regenerators are heat exchangers that have a rotating element which contains the heat storage medium. The hot and cold fluid streams flow simultaneously and are placed on opposite sides of the rotating wheel, parallel to the axis of rotation. Heat is transferred on the heat storage medium as the wheel rotates on the hot fluid stream, and is released once it reaches the cold fluid streams.

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