Brazed plate heat exchangers are designed for highly efficient transfer of energy between liquid, vapor and gaseous environments.
Brazed plate heat exchangers have the following advantages:
- High reliability due to design features and advanced technology manufacturing;
- High efficiency plate heat exchanger;
- Wide range of operating temperatures;
- High working pressure;
- High corrosion resistance of plate heat exchanger;
- Compactness and light weight plate heat exchanger;
- Small internal volume;
- Wide range of capacities and dimensions;
- Ease of installation and maintenance of plate heat exchanger;
- Low cost of plate heat exchanger.
Applications of brazed plate heat exchangers:
- Heating and hot water (steam and water heaters);
- Ventilation systems;
- Air conditioning in rooms and buildings;
- Refrigeration: Evaporators, Condensers;
- Consisting of heat pumps: evaporators, condensers, intermediate heaters and coolers;
- For different technological needs (coolers, heaters);
- Water heaters in pools, etc.
The design of brazed plate heat exchangers
Brazed Heat Exchangers consist of high quality steel plates that are vacuum brazed into one compact, pressure resistant block. For the solder are widely used copper or nickel. In assembling each second plate is rotated 180 degrees, forming a channel separation for the heat transfer medium.
For special applications (depending on type) may create a parallel stream media. Some profile plates or extra built-in turbulence plate provides a high degree of turbulence, which ensures efficient heat transfer even at low volume and reduces costs to a minimum the risk of contamination.
Heat exchangers for chiller applications
Understanding the thermodynamic and transport properties of fluids - combined with simple calculations to define a specific heat transfer problem - will help you select the appropriate heat exchanger for your liquid chiller application.
Numerous types of heat exchangers are used in chiller applications. They serve the specific purpose of controlling a system’s temperature by removing thermal energy. Although there are numerous sizes, levels of complexity and types of heat exchangers, they all use a thermally conducting element, typically in the form of a tube or plate, to separate two fluids so that one can transfer energy to the other.
When selecting the proper type of heat exchanger, one faces the fundamental challenge of fully defining the problem to be solved, which requires an understanding of thermodynamic and transport properties of fluids. This knowledge can be combined with simple calculations to define a specific heat transfer problem and to select the appropriate heat exchanger to use.
Fluid Flow Properties
Fluid flow inside the heat exchanger is a major consideration when selecting what type of exchanger is the best choice in a specific application. Fluid flow will be either turbulent or laminar. Laminar flow heat transfer relies entirely on the thermal conductivity of fluid to transfer heat to the heat exchanger surface. Laminar flows have lower film coefficients than turbulent flows.
Turbulent flows rely not only on thermal conduction but also thermal convection due to the increased fluid movement created in this type flow, thus producing better heat transfer. The higher film coefficients create less resistance to heat transfer.
The heat exchanger’s fluid flow can be determined from its Reynolds number. If the Reynolds number is less than 2,300, the fluid flow will be laminar. Fully turbulent fluid flow has a Reynolds number greater than 10,000. The transition region between laminar and turbulent flow produces higher thermal performance as the Reynolds number increases.
The type of flow determines how much pressure a fluid loses as it moves through the heat exchanger. This factor is important because higher pressure drops require greater pumping requirements. Laminar flow produces less pressure drop and increases linearly with the flow velocity.
This heat exchanger consists of a vertical set of plates welded together to form a cavity through which the colder fluid flows while the hotter fluid flows over the outside of the plates. The hot fluid cools as the fluid film flows down the plates. Most falling-film plates are embossed with intermittent welds placed throughout the plate surface. They can be single (top right) or double (bottom right) embossed.
Many types of heat exchangers are utilized in chiller applications. These range from shell and tube, brazed plate, semi-welded plate, welded plate and vertical falling-film plate. Each has specific characteristics that should be considered during the engineering selection process of a chiller system.
Shell-and-tube heat exchangers are used in applications where high temperatures and pressure demands are of great consequence. This type of design consists of a bundle of parallel tubes typically in a U-tube configuration. The bundle is supported by a series of baffles, which also helps to direct the flow across the tubes. Tubesheets close the ends and separate the two fluids.
The process fluid typically flows through the tubes to take advantage of the higher pressure capabilities inside the tubes and ease cleaning. The thermal performance of the shell-and-tube design generally is less than a plate design but the pressure rating is generally higher.
Brazed plate heat exchangers, like other plate heat exchangers, provide higher turbulent flow and heat transfer coefficients in a much smaller footprint. The plate material is typically AISI 316 type stainless steel. The herringbone plates are vacuum brazed to form the heat exchanger.
Brazed plate heat exchangers provide a highly efficient compact unit that will conserve space and reduce fluid volume requirements. Dual-circuit and double-wall models allow for numerous design options. The major factor to take into consideration is the fouling factor of the smaller channels. Because these units cannot be dismantled, filtration should be used on these heat exchangers.
Another variation of plate heat exchangers is the semi-welded plate heat exchanger. This type of heat exchanger utilizes the chevron-plate design to increase turbulent flow within the plate channels. The semi-welded heat exchanger consists of two plates laser-welded together into what is called a cassette. Plate gaskets seal between each cassette, and the cassettes are bolted together between end frames to retain the complete cassette pack. One fluid flows in the welded channel while the other flows through the gasketed channel.
Semi-welded plate heat exchangers have the same inherent advantages as all plate designs: higher turbulent flows, greater heat transfer coefficients and reduced fluid volume requirements. The largest difference with this design is the opportunity for expansion and ease of opening the unit for repair or cleaning. Cassettes can be added to increase the capacity of the heat exchanger.
The vertical falling-film plate heat exchanger design takes advantage of a large surface area for heat and mass transfer at the boundary of the two fluid flows. This design utilizes a vertical set of plates welded together to form a cavity through which the colder fluid flows. The hotter fluid flows over the external sides of the plates and is cooled when the film of fluid flows down the plate length. Typically, the plates are made of stainless steel for compatibility with sanitary fluids. An upper pan controls the external fluid flow with holes located over the plates.
This type heat exchanger allows closer approach temperatures between the fluids. The internal design of the plate cavity is critical. Most of these type of plates have an embossed design with intermittent welds throughout the plate surface. This helps to increase the turbulent flow inside the plates for higher heat transfer coefficients.
By considering the characteristics of the different types of heat exchangers at the beginning of a chiller selection, a more efficient system can be achieved.
Numerous types of heat exchangers are used in chiller applications. They serve the specific purpose of controlling a system’s temperature by removing thermal energy. Although there are numerous sizes, levels of complexity and types of heat exchangers, they all use a thermally conducting element, typically in the form of a tube or plate, to separate two fluids so that one can transfer energy to the other.
When selecting the proper type of heat exchanger, one faces the fundamental challenge of fully defining the problem to be solved, which requires an understanding of thermodynamic and transport properties of fluids. This knowledge can be combined with simple calculations to define a specific heat transfer problem and to select the appropriate heat exchanger to use.
Fluid Flow Properties
Fluid flow inside the heat exchanger is a major consideration when selecting what type of exchanger is the best choice in a specific application. Fluid flow will be either turbulent or laminar. Laminar flow heat transfer relies entirely on the thermal conductivity of fluid to transfer heat to the heat exchanger surface. Laminar flows have lower film coefficients than turbulent flows.
Turbulent flows rely not only on thermal conduction but also thermal convection due to the increased fluid movement created in this type flow, thus producing better heat transfer. The higher film coefficients create less resistance to heat transfer.
The heat exchanger’s fluid flow can be determined from its Reynolds number. If the Reynolds number is less than 2,300, the fluid flow will be laminar. Fully turbulent fluid flow has a Reynolds number greater than 10,000. The transition region between laminar and turbulent flow produces higher thermal performance as the Reynolds number increases.
The type of flow determines how much pressure a fluid loses as it moves through the heat exchanger. This factor is important because higher pressure drops require greater pumping requirements. Laminar flow produces less pressure drop and increases linearly with the flow velocity.
This heat exchanger consists of a vertical set of plates welded together to form a cavity through which the colder fluid flows while the hotter fluid flows over the outside of the plates. The hot fluid cools as the fluid film flows down the plates. Most falling-film plates are embossed with intermittent welds placed throughout the plate surface. They can be single (top right) or double (bottom right) embossed.
Many types of heat exchangers are utilized in chiller applications. These range from shell and tube, brazed plate, semi-welded plate, welded plate and vertical falling-film plate. Each has specific characteristics that should be considered during the engineering selection process of a chiller system.
Shell-and-tube heat exchangers are used in applications where high temperatures and pressure demands are of great consequence. This type of design consists of a bundle of parallel tubes typically in a U-tube configuration. The bundle is supported by a series of baffles, which also helps to direct the flow across the tubes. Tubesheets close the ends and separate the two fluids.
The process fluid typically flows through the tubes to take advantage of the higher pressure capabilities inside the tubes and ease cleaning. The thermal performance of the shell-and-tube design generally is less than a plate design but the pressure rating is generally higher.
Brazed plate heat exchangers, like other plate heat exchangers, provide higher turbulent flow and heat transfer coefficients in a much smaller footprint. The plate material is typically AISI 316 type stainless steel. The herringbone plates are vacuum brazed to form the heat exchanger.
Brazed plate heat exchangers provide a highly efficient compact unit that will conserve space and reduce fluid volume requirements. Dual-circuit and double-wall models allow for numerous design options. The major factor to take into consideration is the fouling factor of the smaller channels. Because these units cannot be dismantled, filtration should be used on these heat exchangers.
Another variation of plate heat exchangers is the semi-welded plate heat exchanger. This type of heat exchanger utilizes the chevron-plate design to increase turbulent flow within the plate channels. The semi-welded heat exchanger consists of two plates laser-welded together into what is called a cassette. Plate gaskets seal between each cassette, and the cassettes are bolted together between end frames to retain the complete cassette pack. One fluid flows in the welded channel while the other flows through the gasketed channel.
Semi-welded plate heat exchangers have the same inherent advantages as all plate designs: higher turbulent flows, greater heat transfer coefficients and reduced fluid volume requirements. The largest difference with this design is the opportunity for expansion and ease of opening the unit for repair or cleaning. Cassettes can be added to increase the capacity of the heat exchanger.
The vertical falling-film plate heat exchanger design takes advantage of a large surface area for heat and mass transfer at the boundary of the two fluid flows. This design utilizes a vertical set of plates welded together to form a cavity through which the colder fluid flows. The hotter fluid flows over the external sides of the plates and is cooled when the film of fluid flows down the plate length. Typically, the plates are made of stainless steel for compatibility with sanitary fluids. An upper pan controls the external fluid flow with holes located over the plates.
This type heat exchanger allows closer approach temperatures between the fluids. The internal design of the plate cavity is critical. Most of these type of plates have an embossed design with intermittent welds throughout the plate surface. This helps to increase the turbulent flow inside the plates for higher heat transfer coefficients.
By considering the characteristics of the different types of heat exchangers at the beginning of a chiller selection, a more efficient system can be achieved.
Biological Fouling
The attachment of microorganisms (bacteria, algae, and fungi) and macroorganisms (barnacles, sponges, fishes, seaweed, etc.) on heat-transfer surfaces where the cooling water is used in as drawn condition from river, lake, sea and coastal water, etc., is commonly referred to as biological fouling. On contact with heat-transfer surfaces, these organisms can attach and breed, sometimes completely clogging the fluid passages, as well as entrapping silt or other suspended solids and giving rise to deposit corrosion. Concentration of microorganisms in cooling-water systems may be relatively low before problems of biofouling are initiated. Corrosion due to biological attachment to heat transfer surfaces is known as microbiologically influenced corrosion.
The techniques that can be effective in controlling biological fouling include the following:
1. Select materials that posses good biocidal properties.
2. Mechanical cleaning techniques like upstream filtration, air bumping, back flushing, passing brushes, sponge rubber balls, grit coated rubber balls, and scrapers.
3. Chemical cleaning techniques that employ biocides such as chlorine, chlorine dioxide, bromine, ozone, surfactants, pH changes, and/or salt additions.
4. Thermal shock treatment by application of heat, or deslugging with steam or hot water.
5. Ultraviolet radiation.
The techniques that can be effective in controlling biological fouling include the following:
1. Select materials that posses good biocidal properties.
2. Mechanical cleaning techniques like upstream filtration, air bumping, back flushing, passing brushes, sponge rubber balls, grit coated rubber balls, and scrapers.
3. Chemical cleaning techniques that employ biocides such as chlorine, chlorine dioxide, bromine, ozone, surfactants, pH changes, and/or salt additions.
4. Thermal shock treatment by application of heat, or deslugging with steam or hot water.
5. Ultraviolet radiation.
Fouling Resistance - Impurities
Fluids are rarely pure. Intrusion of minute amounts of impurities can initiate or substantially increase fouling. They can either deposit as a fouling layer or acts as catalysts to the fouling processes.
In crystallization fouling, the presence of small particles of impurities may initiate the deposition process by seeding. Sometimes impurities such as sand or other suspended particles in cooling water may have a scouring action, which will reduce or remove deposits.
Read the full article about fouling resistance.
In crystallization fouling, the presence of small particles of impurities may initiate the deposition process by seeding. Sometimes impurities such as sand or other suspended particles in cooling water may have a scouring action, which will reduce or remove deposits.
Read the full article about fouling resistance.
Fouling Resistance -Velocity and Hydrodynamic Effects
Some of the parameters that known to influence fouling resistance are:
Velocity and Hydrodynamic Effects
Hydrodynamic effects, such as flow velocity and shear stress at the surface, influence fouling. Within the pressure drop considerations, the higher the velocity, higher will be the thermal performance of the exchanger and less will be the fouling. Uniform and constant flow of process fluids past the heat exchanger favors less fouling. Foulants suspended in the process fluids will deposit in low-velocity regions. Higher shear stress promotes dislodging of deposits from surfaces. Maintain relatively uniform velocities across the heat exchanger to reduce the incidence of sedimentation and accumulation of deposits.
Read the full article about fouling resistance.
Velocity and Hydrodynamic Effects
Hydrodynamic effects, such as flow velocity and shear stress at the surface, influence fouling. Within the pressure drop considerations, the higher the velocity, higher will be the thermal performance of the exchanger and less will be the fouling. Uniform and constant flow of process fluids past the heat exchanger favors less fouling. Foulants suspended in the process fluids will deposit in low-velocity regions. Higher shear stress promotes dislodging of deposits from surfaces. Maintain relatively uniform velocities across the heat exchanger to reduce the incidence of sedimentation and accumulation of deposits.
Read the full article about fouling resistance.
Parameters that influence fouling resistances
Many operational and design variables have been identified as having well-defined effect on fouling. One of those parameters is Fluid Temperature.
A good practical rule to follow is to expect more fouling as the temperature rises. This is due to a “baking on” effect, scaling tendencies, increased corrosion rate, faster reactions, crystal formation and polymerization, and loss in activity by some antifoulants.
Lower temperatures produce slower fouling buildup, and usually deposits that are easily removable. However, for some process fluids, low surface temperature promotes crystallization and solidification fouling. For those applications, it is better to use an optimum surface temperature to overcome these problems.
Biological fouling is a strong function of temperature. At higher temperatures, chemical and enzyme reactions proceed at a higher rate with a consequent increase in cell growth rate.
A good practical rule to follow is to expect more fouling as the temperature rises. This is due to a “baking on” effect, scaling tendencies, increased corrosion rate, faster reactions, crystal formation and polymerization, and loss in activity by some antifoulants.
Lower temperatures produce slower fouling buildup, and usually deposits that are easily removable. However, for some process fluids, low surface temperature promotes crystallization and solidification fouling. For those applications, it is better to use an optimum surface temperature to overcome these problems.
Biological fouling is a strong function of temperature. At higher temperatures, chemical and enzyme reactions proceed at a higher rate with a consequent increase in cell growth rate.
Heat Exchanger Fouling
Fouling is defined as the formation of undesired deposits on heat transfer surfaces, which increase the resistance to fluid flow, resulting in higher pressure drop and reduced heat transfer. The growth of deposits causes the thermohydraulic performance of heat exchanger to degrade over time. Fouling affects the energy consumption and therefore increases the amount of extra material or fuel required to generate the required amount of heat transfer.
Principles of Heat Transfer
To understand how heat losses occur and how they can be minimized needed to understand the principles of heat transfer. Heat transfer finds application in equipment sizing as well. For instance, a heat exchanger is used to transfer heat load from one fluid to another. Thus, heat transfer applications are involved with energy transfer in equipment, piping systems, and building design.
Heat transfer is determined by the effects of conduction, radiation and convection.
Conduction - heat transfer is based on one space surrendering heat while another one gains it by the ability of the dividing surface to conduct heat. Metals are the best conductors of heat, while wood, asbestos, and felt are the poorer ones.
Radiation - heat transfer is based on the properties of light, where no surface or fluid needed to carry heat from one object to another
Convection - heat transfer is based on the exchange of heat between a fluid, gas, or liquid as it transverses a conducting surface.
Heat transfer is determined by the effects of conduction, radiation and convection.
Conduction - heat transfer is based on one space surrendering heat while another one gains it by the ability of the dividing surface to conduct heat. Metals are the best conductors of heat, while wood, asbestos, and felt are the poorer ones.
Radiation - heat transfer is based on the properties of light, where no surface or fluid needed to carry heat from one object to another
Convection - heat transfer is based on the exchange of heat between a fluid, gas, or liquid as it transverses a conducting surface.
Wort Cooling Systems
Wort cooling systems are employed to bring the wort to a temperature suitable for fermentation. Closed systems with plate heat exchangers have been used for several decades to prevent the danger of infection and energy loss.
Wort cooling systems extract heat from the wort and generate hot water, brine, and propylene glycol solutions, as well as direct expansion of ammonia.
Wort coolers can be classified into: single stage (chilled water only) or multiple stage (ambient water, brine). Dimensions of the wort cooler depend on the amount of hot water required in the brewery for the needed fermentation temperature.
The cooling process is quite simple. Wort enters the heat exchanger and cools to a pitching temperature.
Heat exchangers require scheduled cleaning and proper maintenance for optimal heat transfer. The wort should be properly clarified before entering the cooler to reduce fouling.
Wort cooling systems extract heat from the wort and generate hot water, brine, and propylene glycol solutions, as well as direct expansion of ammonia.
Wort coolers can be classified into: single stage (chilled water only) or multiple stage (ambient water, brine). Dimensions of the wort cooler depend on the amount of hot water required in the brewery for the needed fermentation temperature.
The cooling process is quite simple. Wort enters the heat exchanger and cools to a pitching temperature.
Heat exchangers require scheduled cleaning and proper maintenance for optimal heat transfer. The wort should be properly clarified before entering the cooler to reduce fouling.
Pressure drop in heat exchangers
Fluids need to be pumped through the heat exchanger in most applications. It is essential to determine the fluid pumping power required as part of the system design and operating cost analysis.
The fluid pumping power is proportional to the fluid pressure drop, which is associated with fluid friction and other pressure drop contributions along the fluid flow path. The fluid pressure drop has a direct relationship with heat transfer, operation, size mechanical characteristics, and other factors including economic considerations.
The fluid pumping power is proportional to the fluid pressure drop, which is associated with fluid friction and other pressure drop contributions along the fluid flow path. The fluid pressure drop has a direct relationship with heat transfer, operation, size mechanical characteristics, and other factors including economic considerations.
Evolution of plate heat exchangers
Since the introduction in the 1920s for commercial usage plate-and-frame heat exchangers has evolved over the last several decades and various modifications were developed.
Some of these modifications were driven by new strategies for making more compact equipment, some focused on overcoming disadvantages of PHEs, others on expanding the applications spectrum. That resulted mostly in variations of corrugation patterns of plate’s surfaces and altered construction.
Brazed plate heat exchangers are the most compact type of heat exchangers available on a market today. And it is the most efficient one.
Brazed plate heat exchangers are made of a pack of thin corrugated plates that are brazed together to form a durable, self-contained unit. Brazing eliminates the need of frames and gaskets, and results in a unit able to withstand higher pressure and temperatures compared to PHEs. They are compact and lightweight due to absence of frames.
Typical applications of brazed plate heat exchangers include heating and cooling in the process industry, evaporation and condensation in refrigeration systems, and other HVAC installations.
Some of these modifications were driven by new strategies for making more compact equipment, some focused on overcoming disadvantages of PHEs, others on expanding the applications spectrum. That resulted mostly in variations of corrugation patterns of plate’s surfaces and altered construction.
Brazed plate heat exchangers are the most compact type of heat exchangers available on a market today. And it is the most efficient one.
Brazed plate heat exchangers are made of a pack of thin corrugated plates that are brazed together to form a durable, self-contained unit. Brazing eliminates the need of frames and gaskets, and results in a unit able to withstand higher pressure and temperatures compared to PHEs. They are compact and lightweight due to absence of frames.
Typical applications of brazed plate heat exchangers include heating and cooling in the process industry, evaporation and condensation in refrigeration systems, and other HVAC installations.
Classification of heat exchangers according to transfer process
There are 2 major categories:
1) Indirect contact type;
2) Direct contact type.
Indirect Contact Type Heat Exchangers
In this type of heat exchangers, the fluid streams remain separate, and the heat transfer takes place continuously through a separating wall. There is no direct mixing of the fluids because each fluid flows in separate fluid passages.
Direct Contact Type Heat Exchangers
In this type of heat exchangers, the two fluids are not separated by a wall. Here, closer temperature approaches are attained and the heat transfer process is also accompanied by a mass transfer.
1) Indirect contact type;
2) Direct contact type.
Indirect Contact Type Heat Exchangers
In this type of heat exchangers, the fluid streams remain separate, and the heat transfer takes place continuously through a separating wall. There is no direct mixing of the fluids because each fluid flows in separate fluid passages.
Direct Contact Type Heat Exchangers
In this type of heat exchangers, the two fluids are not separated by a wall. Here, closer temperature approaches are attained and the heat transfer process is also accompanied by a mass transfer.
Compact heat exchangers
Compact heat exchangers are used in a wide variety of applications. Typical among them are the heat exchangers used in air conditioning, beer and wort chilling, solar and geothermal systems, waste and process heat recovery. The need for light-weight, space-saving, and economical heat exchangers has driven the development of compact surfaces.
Specific characteristics of compact heat exchangers include the following:
- a high heat-transfer surface area per unit volume;
- fluids must be clean and relatively non-fouling because of relatively small flow passages and difficulty in cleaning;
- pressure drop consideration;
- operating pressures and temperatures are limited to a certain extent compared to shell and tube exchangers due to joining of the plates by brazing;
etc.
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