What Are Heat Exchangers & How Do They Work?

[Chapter One] What Are Heat Exchangers?

Heat exchangers are pieces of equipment that transfer heat from one fluid to another. This process often includes a large amount of working or utility media, such as water or air, rejecting or absorbing heat from a more valuable fluid, such as crude oil, petrochemical feedstocks, or fluidized products. Heat exchangers come in a variety of shapes and sizes. The hot and cold fluids may be divided by a high-thermal-conductivity wall (often composed of steel or aluminium tube), or they may come into direct contact. The fluids might be in the same phase or have distinct phases (e.g., liquid-to-liquid, vapour-to-liquid). During the design process, the manufacturer considers changes in the phases of both fluids.

Heat exchangers are distinct from heat transfer devices driven by fuel, electricity, or nuclear power, such as boilers. Fluids must be used as both the heat source and the receiving medium. Any substance that flows under applied shear stress or an external force is referred to as a fluid, which includes liquids, gases, and vapours.

Heat exchangers are utilised in a variety of sectors, including food, pharmaceuticals, bioprocessing, and chemical manufacture, where heating or cooling is the last or intermediate stage in preparing fluids for future processing. Microorganisms in food and medicinal items can also be sterilised using them. The usage of heat exchangers is appropriate in a variety of situations. High-temperature exhaust gases from power plants and engines, for example, contain a significant quantity of heat that may be recovered by placing a heat exchanger before the chimney.

[Chapter Two] Thermodynamics of Heat Exchangers

Thermodynamic concepts and heat transfer mechanisms are used in all sorts of heat exchangers. These concepts essentially explain the transport of thermal energy at the macroscopic level. In a heat exchanger system, three entities interact the hot fluid, the cold fluid, and the wall that separates the two fluids. Energy is transferred from the hot fluid to the cold fluid via a wall or barrier. Some thermodynamic ideas that can help you understand how heat exchangers function are as follows:

• The first law of thermodynamics, sometimes known as the Law of Conservation of Energy, asserts that energy (in the form of heat and work) cannot be generated or destroyed. It can only be moved to another system or converted into a different format. This assertion is converted into heat exchangers by the heat balance equation, which is written as:

(Heat In) + (Heat Generation) = (Heat Out) + (Heat Generation) (Accumulation of Heat)

Assume it runs in a steady-state flow, which implies the thermal parameters of the system stay constant as time passes and it is adiabatic (perfectly insulated), Heat In = Heat Out simplifies the heat balance equation. This is one of the most fundamental equations used in heat exchanger construction and operation.

• The second law of thermodynamics introduces the idea of entropy or a system's degree of disorderliness and unpredictability. The universe's entropy is always expanding and will never diminish. It indicates the direction of energy flow between two interacting systems that generates the most entropy. The inherent inclination of all systems is to transfer heat from a body with higher temperatures to a body with lower temperatures. The cold fluid acquires heat and raises its temperature through heat exchangers, whereas the hot fluid loses heat and lowers its temperature.

Heat Transfer Mechanism

Heat transmission in heat exchangers is accomplished by a mixture of conduction and convection. The temperature differential between two or more locations is the driving force of heat transmission.

• Conduction: It is the transmission of heat energy between nearby molecules through direct collisions. A higher-kinetic-energy molecule will transmit thermal energy to a lower-kinetic-energy molecule. Solids are more likely to contain it. It happens on the wall separating the two fluids in heat exchangers. The rate of heat transmission normal to the material's cross-section is proportional to the negative temperature gradient, according to Fourier's Law of Heat Conduction. The thermal conductivity of the substance is the proportionality constant. Where Q denotes the rate of heat transfer, k denotes the thermal conductivity of the medium, A is the area corresponding to the direction of heat flow, and dT/dx denotes the temperature gradient.

Q = -k A

• Convection: Occurs in heat exchangers when the fluid moves in a bulk motion across the surface of the wall, exchanging thermal energy. Newton's Law of Cooling describes this phenomenon, stating that the rate of heat loss is proportional to the difference in temperature between the body and its surroundings (for this instance, the wall and the fluid). Where Q is the heat transfer rate, A is the area normal to the heat flow direction, and T is the temperature differential between the wall and the bulk fluid. The convective heat transfer coefficient, abbreviated as h, is calculated using wall dimensions, fluid physical qualities, and fluid flow characteristics.

Q = h A Δ T Q

When a heat exchanger with conductive partition is in operation, In this order, heat is transmitted from the hot fluid to the cold fluid:

1. Convection transports the heated fluid to the wall's neighbouring surface.

2. By conduction, through the wall surface side.

3. Convection from the wall to the cold fluid.

[Chapter Three] Flow Configuration of Heat Exchangers

So far, the two fluids have been distinguished as hot and cold, as well as their functions in heat transfer. Process owners distinguish between the process fluid and the utility fluid in industrial operations. The process fluid, which might be raw materials, products, or by-products, is the more precious and expensive fluid. The process fluid is heated or cooled by the utility fluid, which is often water, air, or steam.

The process and utility fluid flow configurations in heat exchangers are as follows:

• Countercurrent Move: The process and utility fluid streams flow in opposing directions in countercurrent flow heat exchangers. In heat exchangers, countercurrent flow is the most efficient and widely used flow pattern. The temperature differential between the fluids is nearly constant throughout the heat exchanger's length. This ensures a more consistent heat transmission rate and reduces thermal stress. It's also feasible for the cold fluid's exit temperature to be close to the hot fluid's entrance temperature (highest temperature). When compared to its co-current flow equivalent, this design requires less surface area.

• Co-current or Parallel Move: The process and utility fluid streams flow in opposite directions in co-current or parallel-flow heat exchangers. It is acceptable if the two fluids' output temperatures are about the same temperature. The temperature differential between the fluids is quite considerable at the input and rapidly diminishes throughout the heat exchanger, causing significant thermal stress and eventual material failure. When compared to countercurrent flow, this setup is less efficient.

• Cross Flow: The process and utility fluids flow perpendicular to each other in cross-flow heat exchangers. They're frequently employed in systems with gas-liquid or vapour-liquid heat exchange, where the process fluid is a gas or vapour. The liquid is held within a tube, while the gas escapes via the tubes. Steam condensers, radiators, and air conditioner evaporator coils are examples of cross flow heat exchangers.

• Hybrid Flow: Manufacturers produce hybrid flow heat exchangers to combine the properties of the above-mentioned flow arrangements. Shell-and-tube heat exchangers, cross flow-counter flow, and multi-pass flow heat exchangers are examples of hybrid flow patterns. 

[Chapter Four] Types of Heat Exchangers

A heat exchanger is a type of heat transfer equipment that falls into several categories. There are two types of exchangers: recuperative and regenerative exchangers.

Heat Exchangers for Recuperation

These heat exchangers are made with distinct flow channels for the two fluids, allowing them to exchange heat at the same time. Heat exchangers are further divided into two types: indirect contact and direct contact.

The two fluids are separated by a conductive wall in indirect contact heat exchangers. The following are the most often used heat exchangers:

1. Double-pipe Heat Exchangers: The simplest form of heat transfer equipment is the double-pipe heat exchanger, often known as a hairpin or jacketed pipe exchanger. They are constructed from two concentric pipes of varying diameters. The utility fluid runs through the annular space between the two pipes while the process fluid travels through the smaller inner pipe. The inner pipe's wall serves as a heat-transmitting conductive barrier between the two fluids. Although it may be adjusted to co-current flow, the countercurrent flow pattern is the most common.

2. Heat exchangers with two pipes: Can be used to heat or cool fluids with modest flow rates. They're inexpensive, have a versatile design, and are simple to keep up with. To save floor area, they can be made from pipes of similar lengths that are joined at the ends by fittings. In comparison to conventional heat exchanger equipment, they can only handle lesser heating loads.

3. Shell and Tube Heat Exchangers: Shell and tube heat exchangers are made up of a bundle of tubes that are contained in a huge cylindrical vessel known as a shell. The inner pipe wall works as a conductive barrier, similar to the twin-pipe heat exchanger. On the tube side, the process fluid flows, while on the shell side, the utility fluid flows. 

Heat exchangers with shell and tube construction are suitable for heating and cooling liquids at high flow rates, temperatures, and pressures. They can be constructed to have many passages where one fluid comes into touch with the other several times to boost operating efficiency.

Aside from the shell and tubes, a shell and tube heat exchanger must include the following components:

- Tube Sheet: The tubes are maintained in place by putting them through the perforations in a tube sheet plate. The tubes protrude from the tube sheet to direct the process fluid's intake and outflow flow. The pitch is the space between the tubes, which is usually 1.25 times the tube's outer diameter and can be triangular or square in shape.

- Plenums: Can be found in both the entrance and outflow of tubes. It's a container that collects tube fluid before loading and discharging.

- Baffles: Baffles in a shell and tube heat exchanger serve a variety of purposes. They augment the turbulence of the shell fluid flow by guiding it over the shell. The gap where the fluid is allowed to pass from one baffle space to another and undergoes countercurrent flow is known as the baffle cut. They also keep the tubes in place during operation, as they are prone to sagging caused by flow eddies. Tie rods keep the baffles in place and guarantee their spacing.

- Turbulator: A turbulator is a device that causes the tube fluid to move at a high velocity, preventing fouling and increasing heat transfer capacity at the same time.

- Impingement Plate: The impingement plate is situated just below the fluid entrance of the shell. As the shell fluid is supplied with a high initial velocity, it absorbs stress and vibration to protect the top row tubes. 

- Plate Heat Exchangers: These heat exchangers transmit heat between two fluids using conductive plates. The following are the most prevalent types:

• Plate and Frame Heat Exchangers: These are made up of a series of corrugated plates that are linked together using a gasket, weld, or braze to keep the two fluids separate. The plates include input and exit apertures on the corners that allow the fluid streams to flow through. The fluid flow channels are the gaps between the plates, and they are organised in hot-cold-hot-cold fluid streams. The heated fluid flows down the plates while the cool fluid flows up in a countercurrent flow pattern. The plate and frame heat exchanger's design allows for a wide heat transfer area, high turbulence, and low fouling. When compared to tubular heat exchangers, the total heat transfer coefficient and efficiency are greater. Fluids, on the other hand, have a high-pressure drop owing to high wall shear stress, which increases pumping costs. It's also not a good idea to utilise it if the fluids have a lot of temperature changes between them.

Plate and frame heat exchangers are divided into two categories based on how the plates are connected.

• Plate Heat Exchangers with Gaskets: These heat exchangers employ gaskets to connect and seal the plates together. They're commonly employed in sectors like food and beverage manufacturers that demand frequent cleanliness. Because gasketed plates are simple to clean, remove, and reassemble, they save money on maintenance. To expand the heat exchanger's capabilities and throughput, more plates can be added. The risk of leaking is a downside of this design.

• Welded Plate Heat Exchangers: Welded plate heat exchangers help to prevent leaks. They're comparable to gasketed plate heat exchangers, however, the plates are welded instead of gasketed. Because the working temperature is not restricted by the gasket seals, they can manage greater temperatures, higher pressures, and more corrosive fluids. They're also more durable than plate heat exchangers with gaskets. Manual cleaning is not feasible because the plates are permanently attached. Brazed Plate Heat Exchangers: These heat exchangers are made up of plates that are brazed together. Brazing is a soldering technique that employs copper or zinc as a filler metal. Chillers, pumps, evaporators, and condensers all employ these kinds. 
• Brazed Plate Heat Exchangers: These heat exchangers are made up of plates that are brazed together. Brazing is a soldering technique that uses copper or zinc as a filler metal. Chillers, pumps, evaporators, and condensers all use these types. Brazed plate heat exchangers are efficient, compact (take up less floor area), and have a long service life even when exposed to high pressures on a continuous basis.

- Plate Fin Heat Exchangers: These are made up of alternating layers of corrugated metal fins and parting sheets, which are flat metal plates. The fluid streams flow via the fin and parting sheets-created interface. The principal heat transfer surface is the separating sheets. The fins serve as an additional heat transfer surface and mechanical support for the plates in the event of high internal pressures. The sidebars are likewise fixed to prevent the two fluid streams from mingling. Brazing is used to join all of the parts together. Most designs have a countercurrent flow arrangement. 

The compactness of plate-fin heat exchangers, or the ratio of heat transfer area to heat exchanger volume, is highly desired. As a result, they take up little room on the floor and are light. It also has high efficiency of above 95%. Aerospace, cryogenic air separation, and refrigeration all employ this sort of heat exchanger. 

Direct Contact Heat Exchangers are heat exchangers that do not use a conductive barrier and rely on direct contact for heat transfer. They are suited for two immiscible fluids or a phase transition in one of the fluids. Because of their basic design, they are less expensive. It's utilised in seawater desalination, refrigeration, and waste heat recovery systems, among other things. Direct contact condensers, natural draught cooling towers, driers, and steam injection are examples of direct contact heat exchangers.

Regenerative Heat Exchangers (RHE)

This type of heat exchanger is also known as a regenerator or a capacitive heat exchanger. Regenerative heat exchangers are heat exchangers that make use of a heat storage medium that comes into touch with both hot and cold fluids. Regenerative heat exchangers come in two varieties:

• Static regenerators: is known as fixed bed regenerators, lack mechanical components that allow hot and cold fluids to flow freely. A system of pipes and ducts, equipped with valves that operate as a "switch" during the separate release of hot and cold fluids, allows the fluids to travel through the channel. The heated fluid is allowed to flow for a set period of time before being cooled. The valve connecting the reservoir of the hot fluid is turned off when the heat storage medium has accumulated enough heat. Following that, the cool fluid is allowed to flow through the channel, which absorbs the heat from the hot fluid. Because the fluid flow is intermittent, static regenerators are operated in a semi-batch mode. Two channels must be employed to achieve continuous functioning.

• Dynamic Regenerators: Heat exchangers with a revolving element that includes the heat storage medium are known as dynamic regenerators. The hot and cold fluid streams run in opposing directions, parallel to the axis of rotation, on opposite sides of the revolving wheel. As the wheel turns in the hot fluid stream, heat is transferred to the heat storage medium, which is then released once it reaches the cool fluid streams.

Conclusion

• Heat exchangers are devices that allow heat to be transferred from a hot to a cool fluid. They are necessary because they change the temperature of the more precious fluid that will be utilised later in the process. The fluids might be in many stages, each of which could be changed into another. A conductive wall may divide them or they may be in close touch.
• Thermodynamic principles govern the operation of heat exchangers. Heat exchangers constantly preserve thermal energy. The direction of heat movement is always from the higher temperature fluid to the lower temperature fluid.
• In a heat exchanger, heat transmission is accomplished by a mixture of conduction and convection.
• Heat exchangers can be configured for countercurrent, co-current or parallel flow, cross-flow, or hybrid flow. Recuperative and regenerative heat exchangers are the two basic types of heat exchangers. Separate flow channels for the two fluids are designed into regenerative heat exchangers. The heat storage medium in regenerative heat exchangers is in direct contact with both fluids.
• Indirect contact and direct contact heat exchangers are the two types of recuperative heat exchangers. The two fluids are separated by a conductive wall in indirect contact heat exchangers. Double-pipe heat exchangers, shell and tube heat exchangers, and plate heat exchangers are all examples of heat exchangers. Heat transmission is based on direct contact of both fluids in direct contact heat exchangers, which do not have a partition.
• Static and dynamic regenerators are the two types of regenerative heat exchangers. The hot and cold fluids run intermittently through a fixed bed of heat storage material in static regenerators. The heat storage medium is contained in a revolving element in dynamic regenerators.