A heat exchanger is a device that allows heat to be transferred quickly and efficiently from one medium to another. It is used to heat or cool one medium while simultaneously heating or cooling another. The technique is based on the basic physics of heat movement from a hot media to a cold one. While anybody may change the temperature of a material by making direct contact with it or combining it with another liquid. A heat exchanger allows heat to be transferred without physical touch.
It is made up of separated components with high thermal conductivity that operate as a heat transmission element. They keep the two fluids apart while yet allowing for effective heat transmission. The real heat transfer occurs in reaction to the relative flow of liquid in these segregated parts, regardless of the and form of the exchanger. For optimal heat exchange between the medium, a heat exchanger might have a contemporaneous, counter, or cross flow configuration.
The two mediums flow in opposite directions in separate tubes or segments in a counterflow configuration. It is also known as parallel flow type because of its opposing yet parallel flow direction. While the technique is well recognised for its excellent efficiency, it is utilised when a substantial change in temperature is required in a short period of time. A concurrent flow, on the other hand, has a counterflow with one medium going left to right and the other top to bottom.
It is also known as a counter flow type due to the direction of its flow. Similarly, in a heat exchanger, the two mediums exchange heat by crossing each other at 90 degrees. The flow technique is referred to as cross flow type, with an efficiency that falls between the other two approaches.
Types of Heat Exchangers and How They Work
A heat exchanger can be of many sizes, shapes, and sorts. While it may be classified as shell and tube, plate, direct contact type, heat produced, stem injection type, wet surface type with rotary and reciprocating regeneration heat exchanger. However, only three types are commonly employed in diverse industries: shell and tube, plate type, and regenerative heat exchanger.
1) Heat exchanger of the Plate Type
A plate type heat exchanger transfers heat from one medium to another using identical thin titanium or stainless steel plates. They are kept together by a fine clearance maintained by the rubber and asbestos fiber gasket material. They are substantially more compact and have the extra benefit of changeable capacity.
It is easier to clean and operates more efficiently than shell and tube heat exchangers due to its simple design and huge surface area. It is made up of six major components: the pressure plate, the frame plate, the care bar, the guiding bar, the plates pack, and the support post. When coupled, they create a plate type heat exchanger, which consists of a plate pack constructed of corrugated metal plates with holes for fluid medium flow.
The plates pack is kept in place by a pair of plates; the permanent plate is known as the frame plate, while the moveable plate is known as the pressure plate. Even the number of plates in a plates pack is variable, depending on criteria such as flow rate, pressure differential, operating temperature, fluid type, and installation cost.
A single metal plate can have a high or low tetra pattern, which facilitates the effective exchange of heat between the liquids. Turbulence in the flow is created by these corrugated structure types, resulting in substantially improved heat exchange between the two mediums. The full plate pack assembly is designed such that hot medium flows on one side of a plate and cold medium flows on the other. To improve heat exchange between the medium and the metal plate, the fluids flow in opposite directions.
Principle of Operation
The basic concept of thermal conductivity and the second law of thermodynamics govern plate type heat exchangers. It is essentially a pack of plates with four apertures for warm and cold medium entry and exit. Warm and cold media travel via various channels, with differing mediums on opposing sides of the plate. The gasket along the length of the corrugated plate construction keeps the medium from mixing or flowing to the wrong plate side.
The two fluid mediums in the system flow in opposite directions, with one entering from the top and leaving from the bottom, and the other entering from the bottom and departing from the top. The system regulates the flow rate to avoid the harmful impacts of turbulent flow, such as erosion. The type and length of the plate are chosen based on the needs, since the rate of heat exchange and its efficiency are determined by the size and thickness of the metal plate.
A thin layer of fluid occurs on either side of the metal plate when a sequence of plates are jammed together with an extremely narrow clearance. This gives you a lot of surface area for heat exchange. Depending on the flow rate efficiency required and the temperature differential between the fluids, a plate might have a variable corrugated structure. With correct operation, a plate heat exchanger may accomplish adequate heat transfer with temperature differences as small as one degree.
2) Heat Exchanger with Shell and Tube
A shell and tube heat exchanger is the most often used equipment in businesses since it is compatible with all types of fluids, including gas. It is made up of two basic parts: a big spherical shell casing and a number of tubes flowing through it. Tubes in a standard shell and tube exchanger can run from one side to the other or curve within to form a U-shaped route.
The design with tubes welded to the shell is the most basic in the building of a shell and tube exchanger. It is the most cost-effective method of moving fluid from one side of the shell to the other. Furthermore, it enables physical cleaning of the tubes’ interiors in addition to standard chemical cleaning techniques.
The two fluids flow through the tubes and the surrounding shell, with heat passing from the medium in the tubes to the medium in the shell or vice versa. The input and outflow sites for shell medium are referred to as shell inlet and outlet nozzles. In contrast, the input and outlet ports for tubes are referred to as the front and rear headers, respectively.
The tubes used in its construction must be thermally conductive and able to endure thermal stress caused by temperature variations across the breadth of the tube surface. Furthermore, the tube must tolerate thermal expansion due to temperature changes. Furthermore, the tube must be robust, corrosion resistant, and fluid media suitable. Under normal conditions, the material of these tubes should not react with these fluids.
Principle of Operation
A heat exchanger operates on the simple premise of the second law of thermodynamics, which states that heat flows from one body to another based on their temperature differences. Heat will naturally transfer from a heated body to a colder one. The cooling medium, whether water, steam, ethanol, or polypropylene glycol, is transferred via the tubes within the shell construction of a shell and tube heat exchanger. The medium should be cooled around these tubes within the shell construction, on the other hand.
In most cases, the cooling medium, say sea water, comes from the bottom or rear header and exits from the top or front header via aluminum brass tubes. Similarly, the cooling medium, such as lub oil, comes through the intake nozzle and exits through baffles within the shell construction. These baffles boost efficiency by producing turbulence in the flow, which prevents the formation of hot and cold pockets inside the medium.
Depending on the design and requirements, it can also have a concurrent, counter concurrent, or cross flow configuration. A bypass valve controls the temperature of the output fluid medium by raising or lowering the flow of the cooling medium. Similarly, the pressure of the cooling medium is kept lower than that of the fluid medium to be cooled to prevent intermixing owing to leakage. This protects the fluid, such as lub oil, from contamination even if there is a leak.
3) A Heat Exchanger with Regenerative Capability
Heat is transmitted indirectly from one medium to another via a heat storage medium in a regenerative type heat exchanger, which can be rotary or fixed matrix type. This enables the use of the same media as both a hot and cold medium. This type of heat exchanger is only utilised when the two mediums are a heterogeneous combination or have a high concentration of dissolved contaminants. The most prevalent applications for such heat exchangers are ballast furnaces.
To transmit heat from one medium to another, it is a considerably simpler design with a larger surface area. The entire setup is simple to build, inexpensive, and requires little maintenance. The main issue with such a design is that it is difficult to quantify the precise amount of heat transported or its true efficiency. This alone makes it unattractive in modern sectors that operate on razor-thin margins.
It resembles a bygone era in terms of low cost, convenience of use, and ability to handle vast volumes of fluid at once. It provides a rapid return on investment while dealing with the difficulties of sustaining performance in extremely cold circumstances. In most cases, the system is utilised to heat air at a flow rate ranging from 400 to 85,000 m3 per hour. A rotor, its shaft, galvanized steel frames, and bearings that support the shaft comprise the entire assembly.
The heat storage medium of a regenerative heat exchanger can be built of ceramic, aluminum, or steel. Bricks, honey comb, and spherical particles are also utilised in low-cost, inefficient designs. The technology is frequently employed as a waste heat recovery technique in many modern businesses to boost the overall efficiency of the facility.
Principle of Operation
It features an indirect form of heat transmission, as opposed to plate or shell and tube heat exchangers. A hot medium fluid is initially transported to a temporary storage location known as the head storage area. They are often filled with packing and heat-absorbing material. The heated medium loses some of its heat here and transfers it to the walls or surroundings.
The medium is now flushed and replaced with another cold medium. The time period during which the hot medium is provided and kept is referred to as the hot period. Similarly, the time when cold fluid is drained and stored is referred to as the cold phase. The heat received during the hot time is released and transferred to the cool medium during this cold period.
The continuous process of alternating hot and cold periods steadily raises the temperature of the medium. When extra steps or process reversal are required to preserve efficiency after numerous rounds of such activity. A regenerative type can also have concurrent flow, as opposed to a regular heat exchanger, which has cross or counter concurrent flow.
A regenerative heat exchanger has higher efficacy, lower exchange volume, lower pressure drop, and is easier to adapt as needed at a cheap cost. This makes it more cost effective and simple to run than a plate or shell and tube heat exchanger. With these properties and the ability to pick any flow type, it is a viable alternative for air-to-air heat exchange.
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