This article presents all the information you need to know about heat exchangers.
Read further and learn more about:
- What are heat exchangers?
- Thermodynamics of heat exchangers
- Flow configuration of heat exchangers
- And much more…
Chapter One – What Are Heat Exchangers?
Heat exchangers are pieces of equipment used to transfer heat between two or more fluids. This process usually involves abundant working or utility media such as water or air that rejects or absorbs heat from a more valuable fluid such as crude oil, petrochemical feedstocks, and fluidized products. There are many types and designs of heat exchangers. The hot and cold fluids may be separated by a wall with high thermal conductivity (usually made of steel or aluminum tube), or they may have direct contact with each other. The fluids may have the same or different phases (e.g., liquid-to-liquid, vapor-to-liquid). Changes in the phases of both fluids are considered by the manufacturer during the design phase.
Heat exchangers differentiate themselves from fuel, electrical, or nuclear-powered heat transfer equipment, such as boilers. The heat source and receiving medium must be both fluids. Fluids are defined as any substance that flows under applied shear stress or external force, which encompasses liquids, gases, and vapors.
Heat exchangers are widely used in many industries such as food, pharmaceutical, bioprocessing, and chemical manufacturing, where heating or cooling is the final or an intermediate step to prepare the fluids for further processing. They can also be used in the sterilization of microorganisms in food and pharmaceutical products. There are many instances where the use of heat exchangers is deemed practical. For instance, high-temperature exhaust gases from power plants and engines contain a large amount of heat which can be recovered by installing a heat exchanger before the smokestack.
Chapter Two – Thermodynamics of Heat Exchangers
All types of heat exchangers operate using the same thermodynamic principles and mechanism of heat transfer. These principles basically describe how thermal energy is transferred at the macroscopic level. Three bodies are interacting in a heat exchanger system: the hot fluid, the cold fluid, and the wall separating the two fluids. Energy flows from the hot fluid, through the wall or barrier, and then into the cold fluid. The following are some thermodynamic principles that are useful to understand how heat exchangers work:
First Law of Thermodynamics: The first law is referred to as the Law of Conservation of Energy, which states that energy (in the form of heat and work) can neither be created nor destroyed. It can only be transferred to another system or converted to one form or another. In heat exchangers, this statement is translated by the heat balance equation written as:
(Heat In) + (Generation of Heat) = (Heat Out) + (Accumulation of Heat)
Assuming it operates in a steady-state flow that means that the thermal properties remain constant at all points as time changes, and the system is adiabatic (perfectly insulated), the heat balance equation simplifies to Heat In = Heat Out. This is one of the most basic equations which is used in the design and operation of heat exchangers.
- Second Law of Thermodynamics: The second law introduces the concept of entropy, the degree of disorderliness and randomness of a system. The entropy of the universe is constantly increasing and can never decrease. It tells us the direction of the flow of energy between two interacting systems in which the highest entropy is generated. Heat is always transferred from a body with higher temperatures to lower temperatures which is the natural tendency of all systems. For heat exchangers, the cold fluid gains heat and increases its temperature and the hot fluid loses heat and decreases its temperature.
Mechanism of Heat Transfer
The mechanism involved in the transfer of heat in heat exchangers is a combination of both conduction and convection. The driving force of heat transfer is the temperature difference between two or more regions.
Conduction: This is the transfer of heat energy by direct collisions of adjacent molecules. A molecule with higher kinetic energy will transfer thermal energy to a molecule with lower kinetic energy. It occurs more readily in solids. For heat exchangers, it takes place on the wall separating the two fluids. Fourier’s Law of Heat Conduction states that the rate of heat transfer normal to the material’s cross-section is proportional to the negative temperature gradient. The proportionality constant is the material’s thermal conductivity.
Q = -k A
Where Q is the rate of heat transfer, k is the material’s thermal conductivity, A is the area normal to the direction of the flow of heat, and dT/dx is the temperature gradient.
Convection: Convection in heat exchangers occurs through the bulk motion of the fluid against the surface of the wall, thus transferring thermal energy. This phenomenon is represented by Newton’s Law of Cooling which states the rate of heat loss is proportional of a body to the difference of the temperature of the body and its surroundings (for this instance, the wall and the fluid).
Q = h A ΔT
Where Q is the rate of heat transfer, A is the area normal to the direction of the flow of heat, and ΔT is the temperature difference between the wall and bulk fluid. The convective heat transfer coefficient, denoted by h, is evaluated based on the wall dimensions, physical properties of the fluid, and fluid flow characteristics.
During an operation of a heat exchanger with conductive partition, heat is transferred from the hot fluid to the cold fluid in this sequence:
- From the hot fluid to the adjacent surface of the wall by convection.
- Through the wall surface side by conduction.
- From the wall to the cold fluid by convection.
Chapter Three – Flow Configuration of Heat Exchangers
So far, the two fluids were differentiated as hot and cold and their roles in heat exchange. In industrial processes, process owners distinguish which is the process fluid and which is the utility fluid. The process fluid is the more valuable and expensive fluid which can be raw materials, products, or by-products. The utility fluid, which is usually water, air, or steam, acts as the heating or cooling agent to the process fluid.
The following are flow configurations of the process and the utility fluid in heat exchangers:
Countercurrent Flow: In countercurrent flow heat exchangers, the process and utility fluid streams flow in opposite directions. Countercurrent flow in heat exchangers is the most efficient and the most utilized flow pattern. A large temperature difference of the fluids is almost maintained constant across the length of the heat exchanger. This provides a more uniform heat transfer rate and minimizes the thermal stress. It is also possible for the cold fluid to have an outlet temperature close to the inlet temperature of the hot fluid (highest temperature). This configuration requires less surface area compared to its co-current flow counterpart.
Co-current or Parallel Flow: In co-current or parallel-flow heat exchangers, the process and utility fluid streams flow in parallel directions. It is suitable if the outlet temperatures of the two fluids are nearly the same temperature. The temperature difference of the fluids is very large at the inlet and drastically decreases across the length of the heat exchanger, which causes large thermal stress and eventual material failure. This configuration has less efficiency compared to countercurrent flow.
Cross Flow: In cross flow heat exchangers, the process and the utility fluids flow perpendicular to each other. They are commonly used on systems with gas-liquid or vapor-liquid heat exchange, wherein the gas or vapor is the process fluid. The liquid is contained in a tube and the gas flows outside those tubes. Examples of a cross flow heat exchanger are steam condensers, radiators, and air conditioner evaporator coils.
Hybrid Flow: 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.
Chapter Four – Types of Heat Exchangers
A heat exchanger is a broad class of heat transfer equipment. They are mainly classified into two groups: recuperative and regenerative exchangers.
Recuperative Heat Exchangers
These types of heat exchangers 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:
Double-pipe Heat Exchangers: Double-pipe heat exchangers, also known as a hairpin or jacketed pipe exchanger, are the simplest type among the heat transfer equipment. They are made of two concentric pipes with different diameters. 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 countercurrent flow pattern is the most utilized, though it may be configured to co-current flow.
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. However, they only operate at lower heating duties compared to other heat exchanger equipment.
Shell and Tube Heat Exchangers: 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.
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.
Aside from the shell and tubes, other 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. The tubes are protruded on the tube sheet to guide the inlet and outlet flow of the process fluid. The pitch is the distance between the tubes which is normally 1.25 times the outside diameter of the tube and may be arranged in a triangular or square pitch.
- 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.
- Baffles: Baffles installed in a shell and tube heat exchanger serve many functions. They guide the shell fluid flow across the shell and increase its turbulence. The space in which the fluid is permitted to flow from one baffle space to another is called baffle cut, wherein it experiences countercurrent flow. Also, 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.
Turbulator: The turbulator is a device that induces high velocity of the tube fluid and subsequently prevents fouling of the tubes while at the same time increases heat transfer capacity.
- Impingement Plate: This plate is located directly below the shell fluid inlet. It absorbs shock and vibration to protect the top row tubes as the shell fluid is introduced with a high initial velocity.
Plate Heat Exchangers: These types of heat exchangers utilize conductive plates to transfer heat between two fluids. The most common types are:
Plate and Frame Heat Exchangers: These use a series of corrugated plates that are joined by either a gasket, weld or braze to ensure that the two 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 and are arranged in alternating hot-cold-hot-cold fluid streams. Fluids flow in a countercurrent flow configuration; the hot fluid flows down the plates while cold fluid flows up.
The design of the plate and frame heat exchanger creates a large heat transfer area, high turbulence, and high fouling resistance. The overall heat transfer coefficient and efficiency are higher compared to tubular heat exchangers. However, the high-pressure drop is encountered by the fluids due to high wall shear stress that makes pumping costs expensive. It is also not advisable to be used if the fluids have high-temperature differences.
Plate and frame heat exchangers are classified depending on how the plates are joined.
Gasketed Plate Heat Exchangers: These types use 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.
- Welded Plate Heat Exchangers: Welded plate heat exchangers reduce the possibility of leakage. They are also similar to a gasketed plate heat exchanger, except that the plates are welded. They can handle higher temperatures, higher pressures, and more corrosive fluids since the operating temperature is not limited by the gasket seals. They are also more durable than gasketed plate heat exchangers. Since the plates are permanently fixed, manual cleaning is not possible.
- Brazed Plate Heat Exchangers: These heat exchangers have plates joined by a method called brazing. Brazing is a soldering method that uses filler metals such as copper or zinc. These types are used in chillers, pumps, evaporators, and condensers. Brazed plate heat exchangers are efficient, compact (consumes smaller floor space), and have long service life even under continuous exposure to high pressures.
- Gasketed Plate Heat Exchangers: These types use 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.
Plate Fin Heat Exchangers: These types consist of alternating layers of corrugated metal fins and flat metal plates called parting sheets. The fluid streams pass through the interface created by the fin and parting sheets. The parting sheets are the primary heat transfer surface. The fins create a secondary heat transfer surface, and they serve as the mechanical support of the plates against high internal pressures. The sidebars are also fixed to prevent the mixing of the two fluid streams. All components are bonded by brazing. Countercurrent flow configuration is incorporated in most designs.
Plate fin heat exchangers are valued for their compactness, the ratio of the heat transfer area to heat exchanger volume. Hence, they consume small floor spaces and are lightweight. Its efficiency is also above 95%. This type of heat exchanger is used in aerospace, cryogenic air separation, and refrigeration.
- Plate and Frame Heat Exchangers: These use a series of corrugated plates that are joined by either a gasket, weld or braze to ensure that the two 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 and are arranged in alternating hot-cold-hot-cold fluid streams. Fluids flow in a countercurrent flow configuration; the hot fluid flows down the plates while cold fluid flows up.
- Tube Sheet: The tubes are held in place by inserting them into the holes of a plate called a tube sheet. The tubes are protruded on the tube sheet to guide the inlet and outlet flow of the process fluid. The pitch is the distance between the tubes which is normally 1.25 times the outside diameter of the tube and may be arranged in a triangular or square pitch.
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.
Regenerative Heat Exchangers
These are also known as regenerators or capacitive heat exchangers. Regenerative heat exchangers are types of heat exchanger equipment that utilizes a heat storage medium that is made to contact with the hot and cold fluids. The two fluids are usually gases. They are used in power plants, glass and steel making, and heat recovery systems. However, 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: 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. 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.
Operation of static regenerators is semi-batch since the flow of the fluids is intermittent. To achieve a continuous operation, two channels must be used.
Dynamic Regenerators: These heat exchangers have a rotating element that 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.
- Heat exchangers are pieces of equipment used to transfer heat from a hot fluid to a cold fluid. They are essential as they carry out the task of changing the temperature of the more valuable fluid which is used later on in the process. The fluids may be of different phases, which may be transformed to another phase. They may be separated by a conductive wall or have direct contact.
- Heat exchangers operate using the same set of thermodynamic principles. Thermal energy is always conserved in heat exchangers. The direction of heat flow is always from the fluid with higher temperature to lower temperature.
- The mechanism of heat transfer in a heat exchanger is a combination of conduction and convection.
- Flow configuration of heat exchangers is countercurrent, co-current or parallel flow, cross flow, and hybrid flow.
- The two main classes of heat exchangers are recuperative and regenerative heat exchangers. Recuperative heat exchangers are designed to have separate flow paths for the two fluids. Regenerative heat exchangers utilize a heat storage medium that has direct contact on both fluids.
- Recuperative heat exchangers are divided into two groups, the indirect contact, and direct contact heat exchangers. Indirect contact heat exchangers utilize a conductive wall to separate the two fluids. Examples are double-pipe heat exchangers, shell and tube heat exchangers, and plate heat exchangers. Direct contact heat exchangers do not have a partition and heat transfer relies on the direct contact of both fluids.
- Regenerative heat exchangers are also divided into two groups, static and dynamic regenerators. Static regenerators have a fixed bed of heat storage medium in which the hot and cold fluids flow intermittently. Dynamic regenerators have a rotating element that contains the heat storage medium.