Heat Exchangers

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Introduction

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 devices engineered to transfer heat between two fluids without allowing them to mix. This separation is achieved by a wall with high thermal conductivity, carefully designed to prevent direct contact between the fluids while facilitating efficient heat transfer. The working medium in the heat exchanger absorbs or rejects heat from the processed fluid, resulting in the cooling or heating of the fluid stream. As technology advances and the properties of materials improve, an ever-growing variety of heat exchangers continue to be developed, meeting increasingly diverse and specialized needs.

Heat transfer in a heat exchanger involves both convection in the fluids and thermal conduction through the separating wall. The design process begins with the heat transfer coefficient, or U-factor, which is derived from Newton's law of cooling. Engineers also use the logarithmic mean temperature difference (LMTD) to determine the temperature driving force for heat transfer. Additionally, they consider whether the fluids involved are in the same phase or different phases, such as liquid-to-liquid or vapor-to-liquid, as these factors influence the heat exchanger's design and performance.

The hot and cold fluids may be separated by a wall with high thermal conductivity, typically made of materials like steel or aluminum, or they may come into direct contact with each other.

Heat exchangers differentiate themselves from fuel, electrical, or nuclear-powered The hot and cold fluids can be separated by a wall with high thermal conductivity, typically made of steel or aluminum tubing, or they may come into direct contact with each other.

Heat transfer equipment, such as boilers, requires both the heat source and the receiving medium to be fluids. Fluids are defined as substances that flow when subjected to shear stress or external force, which includes liquids, gases, and vapors.

Chapter Two - What are the Thermodynamics of Heat Exchangers

All heat exchangers function based on the same fundamental thermodynamic principles and mechanisms of heat transfer, which describe how thermal energy is transferred on a macroscopic scale. In a heat exchanger system, three elements interact: the hot fluid, the cold fluid, and the separating wall between them. Energy flows from the hot fluid, through the barrier, and into the cold fluid. Understanding the following thermodynamic principles is key to grasping how heat exchangers operate:

• First Law of Thermodynamics: The first law, known as the Law of Conservation of Energy, states that energy, whether in the form of heat or work, cannot be created or destroyed. It can only be transferred to another system or converted into different forms. In heat exchangers, this principle is represented by the heat balance equation, which can be expressed as:

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

  The first law, known as the Law of Conservation of Energy, states that energy, whether in the form of heat or work, cannot be created or destroyed. It can only be transferred to another system or converted into different forms. In heat exchangers, this principle is represented by the heat balance equation, which can be expressed as:

  
  
• Second Law of Thermodynamics: The second law introduces the concept of entropy, which measures the disorder and randomness within a system. According to this law, the entropy of the universe is always increasing and never decreases. It dictates the direction of energy flow between two interacting systems, where energy naturally moves in a way that maximizes entropy. Specifically, heat always transfers from a body with a higher temperature to one with a lower temperature, reflecting the natural tendency of all systems. In heat exchangers, this principle means that the cold fluid absorbs heat and its temperature rises, while the hot fluid releases heat and its temperature decreases.

Mechanism of Heat Transfer

The heat transfer mechanism in heat exchangers involves a combination of conduction and convection. The driving force behind this transfer is the temperature difference between the inlet and outlet temperatures of the heat exchanger and the corresponding temperatures of the process stream.

• Approach Temperature:The heat transfer mechanism in heat exchangers involves both conduction and convection. The driving force for heat transfer is the temperature difference between the inlet and outlet temperatures of the fluids, minus the temperature difference of the process stream.

  All heat exchangers have an optimal approach temperature that must be considered when selecting one for a particular application. Misjudging this approach temperature can result in choosing an inappropriate heat exchanger for the process.

  
  

• 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

  In this equation, QQ represents the rate of heat transfer, AA is the area perpendicular to the direction of heat flow, and \Delta TΔT is the temperature difference between the wall and the bulk fluid. The convective heat transfer coefficient, hh, is determined by the wall dimensions, the physical properties of the fluid, and the characteristics of the fluid flow.

  
  

  In a heat exchanger with a conductive partition, heat is transferred from the hot fluid to the cold fluid in the following sequence:

  1. From the hot fluid to the adjacent surface of the wall by convection.
  2. Through the wall surface side by conduction.
  3. From the wall to the cold fluid by convection.

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Chapter Three &#; What is the Flow Configuration of Heat Exchangers

Up to this point, we've referred to the two fluids in a heat exchanger as hot and cold based on their roles in heat transfer. In industrial processes, however, these fluids are classified as process fluids and utility fluids. The process fluid is typically the more valuable and costly fluid, which may include raw materials, products, or by-products. In contrast, the utility fluid&#;often water, air, or steam&#;serves as the heating or cooling agent for the process fluid.

The following are the flow configurations used for the process and utility fluids in heat exchangers:

• Countercurrent Flow: In countercurrent flow heat exchangers, the process and utility fluids move in opposite directions. This configuration is the most efficient and widely used because it maintains a large temperature difference between the fluids across the length of the heat exchanger, leading to more uniform heat transfer and reduced thermal stress. Additionally, it allows the cold fluid's outlet temperature to approach the hot fluid's inlet temperature (the highest temperature). Countercurrent flow also requires less surface area compared to the co-current flow configuration.

  
  

• Co-current or Parallel Flow: In co-current or parallel-flow heat exchangers, the process and utility fluids flow in the same direction. This configuration is suitable when the outlet temperatures of both fluids are nearly equal. However, the temperature difference between the fluids is initially large at the inlet and decreases significantly along the length of the heat exchanger, leading to increased thermal stress and potential material failure. As a result, co-current flow is less efficient compared to countercurrent flow.

  
  

• Cross Flow: In co-current or parallel-flow heat exchangers, the process and utility fluids flow in the same direction. This configuration is suitable when the outlet temperatures of both fluids are nearly equal. However, the temperature difference between the fluids is initially large at the inlet and decreases significantly along the length of the heat exchanger, leading to increased thermal stress and potential material failure. As a result, co-current flow is less efficient compared to countercurrent flow.

  
  

• 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.

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Chapter Four &#; What are the Types of Heat Exchangers

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

Recuperative Heat Exchangers

These heat exchangers are designed with separate flow paths for the two fluids, allowing them to exchange heat simultaneously. They are further divided 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:

• Also known as hairpin or jacketed pipe exchangers, double-pipe heat exchangers are among the simplest heat transfer devices. They consist of two concentric pipes of different diameters. The process fluid flows through the smaller inner pipe, while the utility fluid circulates through the annular space between the two pipes. The wall of the inner pipe serves as the conductive barrier for heat transfer. While the countercurrent flow pattern is most commonly used, these exchangers can also be configured for co-current flow.

  
  

  Double-pipe heat exchangers are ideal for heating or cooling small flow rates of fluids. They are cost-effective, flexible in design, and easy to maintain. Typically constructed from pipes of the same length interconnected with fittings at the ends, they can be arranged to optimize floor space. However, they are less effective for higher heating duties compared to other types of heat exchangers.

• Shell and tube heat exchangers consist of a bundle of tubes enclosed within a large cylindrical vessel, known as the shell. Like double-pipe heat exchangers, the wall of the tubes serves as the conductive barrier. The process fluid flows through the tubes, while the utility fluid circulates around the outside of the tubes within the shell.

  
  

  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 secured by inserting them into holes in a plate known as a tube sheet. The tubes extend through the tube sheet to direct the inlet and outlet flow of the process fluid. The pitch, which is the distance between the tubes, is typically 1.25 times the tube's outer diameter and can be arranged in either a triangular or square pattern.

    
    
  • 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.
  • The tubes are secured by inserting them into holes in a plate known as a tube sheet. The tubes extend through the tube sheet to direct the inlet and outlet flow of the process fluid. The spacing between the tubes, known as the pitch, is typically 1.25 times the outside diameter of the tube and can be arranged in either a triangular or square pattern.

  • 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 increasing 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. They have a counter-current flow that allows for lower approach temperature differences, high temperature exchanges, and improved efficiency.

• Plate and Frame Heat Exchangers: Plate and frame heat exchangers utilize corrugated plates joined by gaskets, welding, or brazing to prevent fluid mixing. The plates feature inlet and outlet ports at the corners for fluid passage. The flow paths for the fluids are the spaces between the plates, arranged in alternating hot-cold-hot-cold streams. The fluids flow in a countercurrent configuration, with the hot fluid moving downward through the plates and the cold fluid flowing upward.

  
  

  The design of plate and frame heat exchangers offers a large heat transfer area, high turbulence, and increased resistance to fouling. As a result, they achieve higher overall heat transfer coefficients and efficiencies compared to tubular heat exchangers. However, the fluids experience a high-pressure drop due to elevated wall shear stress, which increases pumping costs. Additionally, this type of heat exchanger is not recommended for fluids with significant temperature differences.

  A crucial component of a plate and frame heat exchanger is the frame, which compresses the plates together to create a series of parallel flow channels alternating between hot and cold fluids. The corrugated metal plates are assembled into packs and secured to the frame with bolts.rubber gaskets between the plates to prevent fluids from mixing or leaking.

  Plate and frame heat exchangers are classified depending on how the plates are joined.

  • Gasketed Plate Heat Exchangers: These heat exchangers use gaskets to connect and seal the plates. They are commonly used in industries requiring frequent sanitation, such as food and beverage processing. Gasketed plates help reduce maintenance costs due to their ease of cleaning, disassembly, and reassembly. Additional plates can be added to enhance the heat exchanger's capacity and throughput. However, a drawback of this design is the potential for leakage.

    
    
  • Welded Plate Heat Exchangers: Welded plate heat exchangers minimize the risk of leakage and are similar to gasketed plate heat exchangers, except that the plates are welded together. This design allows them to handle higher temperatures, pressures, and more corrosive fluids, as they are not constrained by gasket seals. Welded plate heat exchangers are also more durable than their gasketed counterparts. However, because the plates are permanently fixed, they cannot be manually cleaned.
  • Brazed Plate Heat Exchangers: These heat exchangers feature plates joined through a process called brazing, where two metal pieces are fused using a molten filler metal. This brazing process creates a low thermal resistance joint, which contributes to the high efficiency of brazed plate heat exchangers. They are commonly used in chillers, pumps, evaporators, and condensers. Brazed plate heat exchangers are known for their efficiency, compact size (requiring less floor space), and long service life, even under continuous exposure to high pressures.

• Plate Fin Heat Exchangers: These heat exchangers feature alternating layers of corrugated metal fins and flat metal plates known as parting sheets. The fluid streams flow through the spaces created between the fins and parting sheets. The parting sheets serve as the primary heat transfer surface, while the fins provide a secondary heat transfer surface and support the plates against high internal pressures. Sidebar components are included to prevent mixing of the two fluid streams. All parts are bonded through brazing, and most designs use a countercurrent flow configuration.

  
  

  Plate fin heat exchangers are highly regarded for their compact design and high ratio of heat transfer area to heat exchanger volume, which makes them space-efficient and lightweight. They also achieve efficiencies greater than 95%. These heat exchangers are commonly used in aerospace, cryogenic air separation, and refrigeration applications.

• Plate and Shell Heat Exchangers:Plate and shell heat exchangers merge the advantages of shell and tube heat exchangers with those of plate heat exchangers. In this design, a fully welded plate is inserted into the shell to distribute stress and eliminate the need for gaskets. Fluid A flows through the plate side flow channels, while Fluid B circulates through the shell flow channel. This configuration results in a high heat transfer rate.

Direct Contact Heat Exchangers Direct contact heat exchangers do not use a conductive partition and instead rely on direct contact between the fluids for heat exchange. They are ideal for situations involving two immiscible fluids or when one of the fluids undergoes a phase change. Due to their simpler design, they are more cost-effective. These exchangers are commonly used in seawater desalination, refrigeration systems, and waste heat recovery. Examples of direct contact heat exchangers include direct contact condensers and natural draft cooling towers. cooling towers, driers, and steam injection.

Regenerative Heat Exchangers

Also known as regenerators or capacitive heat exchangers, regenerative heat exchangers use a heat storage medium that comes into contact with both the hot and cold fluids, which are typically gases. They are commonly used in power plants, glass and steel manufacturing, and heat recovery systems. However, there is a risk of contamination since the same medium interacts with both fluid streams.

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 feature a rotating element that contains the heat storage medium. The hot and cold fluid streams flow simultaneously on opposite sides of the rotating wheel, aligned parallel to its axis of rotation. As the wheel turns, it absorbs heat from the hot fluid stream and releases it to the cold fluid stream as it rotates.

  
  

Adiabatic Wheel Heat Exchangers

Adiabatic wheel heat exchangers feature a rotating wheel and an intermediate fluid that aids in the heat exchange process. The wheel is designed with threaded surfaces to enhance its heat transfer area and rotates through the fluid where the heat exchange occurs. This distinctive structure enables effective heat transfer between gases. Adiabatic wheel heat exchangers are highly efficient and are particularly valuable in processes that require minimal heat exchange.

Pillow Plate Heat Exchangers

Pillow plate heat exchangers are known for their low pressure loss and exceptionally high heat transfer coefficient. Their unique design, which differs significantly from conventional heat exchangers, has contributed to their widespread use and popularity. Central to their design are three-dimensional, lightweight, wavy plates&#;hence the term "pillow." These plates are stacked into plate banks and include an inlet distributor and outlet collector. Each plate consists of laser-welded metal sheets stacked on top of one another.

During the manufacturing of the plates, after welding, they undergo a hydroforming process where they are inflated using pressures ranging from 60 to 80 bar. This process creates channels that are hermetically sealed. For the hydroforming to be effective, the laser-welded plates must have uniform thickness.

Pillow heat exchangers combine the pressure and temperature resistance of shell and tube heat exchangers with the cost-effectiveness and compact design of plate heat exchangers. Their reduced heat transfer surface contributes to process stability. The wavy pattern of the plates is crucial for their thermohydraulic performance, promoting turbulent flow of the heat carriers within.

Conclusion

• 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 that is used later on in the process. The fluids may be of different phases, which may be transformed into 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 with both fluids.
• Recuperative heat exchangers are divided into two groups: 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.

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7. The correct sequence for different types of heat exchangers in the decreasing order of effectiveness is Heat (c) The two fluids should flow nomn other Tmal (a) parallel-flow, counter flow, shell & tube (ESE-12) (d) The directions of flow of are of no consequence tWo and cross-flow f1o b) cross-flow, counter flow, shell&tube and 41. A counter flow shell and tube heat (c) counter-flow, shell & tube, cross-flow and The water (c (d) counter-flow, cross-flow, shell &tube and K) flow at the rate of 5 kg/s. is used to heat water with hot exhaus r parallel-flow parallel-flow parallel-flow = J/kg K) flows at the of 2 kg/s and the exhaust gases (c= If the he transfer surface area is 32 m and the heat transfer coefficient is 200 Win'k NTU of the heat exchanger is (ESE-L4 (a) 4.5 (b) 2.4(c)8.6 ( 1.2 38. What does NTU indicate? (ESE-12) (a) Effectiveness heat exchange (b) Efficiency of heat exchanger (c) size of heat exchanger (d) temperature drop in heat exchanger 42. In a two fluid heat exchanger, the inlet and outlet temperatures of the hot fluid are 6SC and 40°C respectively. For the cold fhuid these are 15°C and 43°C. The heat exchangr 39. In a heat exchanger, the hot gases enter with a (ESE-14) temperature of 150 oC and leave at 75 °C. The cold fluid enters at 25 °C and leaves at 125 °C. The capacity ratio of the exchanger is is a (a) Parallel flow heat exchanger (b) Counter flow heat exchanger (c) Mixed flow heat exchanger (d) Phase -change heat exchanger (ESE-12) (b) 0.75 (c) 0.85(d) 0.95 (a) 0.65 40. In a concentric double-pipe heat exchanger 43. In a double pipe heat exchanger, the s phasefluid is water with inlet temperature 20°C where one of the fluids undergoes mass flow rate 20 kg/s and the hot fluid w flow rat (ESE-14)

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