Heat exchangers are devices that are constructed with thermally conductive materials and are used in heat transfer between fluids. Depending on the model of the exchanger, the fluids could be in direct contact or separated. Heat exchangers, therefore, facilitate heat transfer for purposes such as cooling or heating of substances that are then utilized in other processes.
Heat exchangers are essential in industrial processes pertaining to the design, operation and maintenance of heating systems, power generation, air-conditioning systems, chemical processing, engineering and waste heat recovery systems.
History of Heat Exchangers
The first known forms of heat transfer were rocks that were used to warm up the homes during the earliest days of man’s evolution. The stones would be placed in the fire, and they would absorb the heat. They would then be transferred inside the hut to warm the interior. Romans later invented a central method of home heating. This was the hypocaust technology. It entailed a space being left to allow hot air to travel along the floor to warm the room. The floor was constructed from concrete materials that had the capability of holding heat waves within and transfer it along.
Later on, Koreans adopted a similar technology called Ondol heating. Through this technique, hot air and smoke from wood fires were channeled into pipes that run under the floors. In the years of 1700, a significant advance beyond the hypocaust happened whereby a Frenchman known as Jean Simon Bonnemain designed a system of water that could help incubate chicken eggs. In 1800’s a scientist by the name Marquis de Chabannes developed a heat exchanger that he used for growing grapes in a greenhouse. In 1829 the Price brothers worked to start steam heating in England. Hot water and steam inventions continued revolutionizing the world up to what we have today.
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Types of Heat Exchangers
In a bid to understand the working of heat exchangers, we have to take a closer look at the two primary categorizations. The two approaches which are considered include;
- Flow configuration
- Equipment classification mainly by construction
Classification by Flow Configuration
The four main types of flow configurations include;
- Cocurrent flow
In the counterflow heat exchanger configuration, there exist two types of fluids which flow in parallel but that takes place in different directions. The arrangement allows for a substantial temperature change of the fluids thus making it the most efficient.
In this heat exchanger design, the steam flows in parallel, but in this case, flow occurs in the same direction. However, the efficiency in this model of heat exchangers is low, but it provides uniformity in wall temperatures.
Crow flow model heat exchangers offer intermediate capabilities between cross and countercurrent flow. In this flow configuration, fluid flows at right angles.
This model is common in industrial heat exchangers whereby they combine counterflow/co-current flow with multipass flow heat exchangers.
Classification by Construction
Classification according to design results into two main groups namely;
- Recuperative Heat Exchangers
- Regenerative Heat Exchangers
Recuperative Heat Exchangers
In this type of heat exchangers, they contain separate flow paths for the fluids. These fluids maintain a continuous flow simultaneously within the exchanger and thereby enabling heat exchange across the wall separating the fluids.
Recuperative design offers a wide range of heat exchangers which can be grouped into three broad categories namely;
The Indirect contact exchangers
- The Direct Contact Exchangers
- The Specials
The Indirect Heat Exchangers
In this category of heat exchangers, the steam or fluids are separated by a wall which is usually made of metal. They basically include;
- Tubular Heat Exchangers
- The Plate Heat Exchangers
The Tubular Heat Exchangers
These tube heat exchangers make it possible for a wide variety of pressure and temperatures to be used hence, they are more popular due to the design flexibility. Tube heat exchangers have a number of categories, but the most common one is shell and tube exchanger model. Its design has gained a lot of preference in the modern industries.
The Shell-and-Tube Heat Exchanger
In the shell and tube design, it's made up of several tubes that are mounted on the inside of a cylindrical metal shell that facilitates heat exchange. The shell-and-tube heat exchanger allow for fluid to flow on the inside and outside of the tubes. Fluid flow happens in single phase or two-phase.
The exchanger comprises of four major parts namely;
The Tube Bundle
As a result of the popularity gained by the metal shell and tube, a standard has been initialized to monitor heat exchanger design and usage. This is known as the Tubular Exchanger Manufacturer's Association (TEMA) standard. The material used in the manufacture of shell and tube heat exchanger is metal. However, other types of materials are used, and they include glass, plastics and graphite. Manufacturers are keen to ensure that the materials are durable, corrosion resistant. The metals which fulfill these aspects are stainless steel, titanium, steel, copper and cast iron.
Some other examples of heat exchangers are;
The Tube in Plate
Their application is mainly in heat recovery and air conditioning. Plates act as the support and as well provide more surface area due to extensions.
Comprises of a fan system, bundle of tubes and supporting structures. Tubes have fins to increase surface area. A fan that is mounted above the bundle (induced draught) helps to suck air through the tube. These are mostly in application where there isn’t enough cooling water. This has similar operation in comparison to the air-to-air heat exchanger.
Air-to-Air Heat Exchangers
Brazed Plate Heat Exchangers
Are made up of specially formed plates, vacuum brazed together.
Flat Plate Heat Exchangers
Transfer heat through flat, corrugated plates.
Gasketed Plate Heat Exchangers
Are the most common. Elastomer gaskets are used in the plates, which contain the pressure and control the flow of each medium.
Mainly comprises of a working fluid as well as wack material. The fluid absorbs heat, then it evaporates and passes through the pipe to the end where it condenses and releases heat.
Marine Heat Exchangers
Are heat exchangers that are specifically designed for the transfer of heat, or thermal energy, by carrying the oil through cooling units in order to cool.
Play a crucial role in heating viscous fluids. These devices comprise of a vessel that contains tubes inside as well as an agitator, like the helical ribbon impeller or propeller. The tube is for transporting the hot liquid while the agitator ensures uniform heat distribution through the cold liquid.
Water to Air Heat Exchangers
Water to Water Heat Exchangers
Transfer heat energy from one liquid to another without bringing the two liquids into direct contact.
Welded Plate Heat Exchangers
Are fully welded and require no gaskets. These are usually constructed of one material, generally stainless steel.
Carbon Block Exchangers
Come in handy in heating or cooling of corrosive fluids. They comprise of carbon steel blocks in which holes are drilled to enable fluid passage. The blocks are finally bolted together.
Plate Heat Exchangers
Plate heat exchangers by design separate the fluids using plates. The plates have increased surface area either by using fins or embossing. More so, they can be bolted, brazed or welded together. The plate exchangers are common in food processing and cryogenic industries. Due to their desirable aspects such as high surface area to volume ratio, ability to handle more than two types of steam and low fluid inventory, plate heat exchangers are being incorporated in the manufacturing of chemicals.
Plate and Frame Heat Exchangers
Comprise of membranes to hold rectangular plates that have holes through which the fluid passes. A gasket functions to seal the plates together. They are very easy to disassemble for cleaning and for this reason, they are commonly used in the food industry. Also, the two plates can be welded together to eliminate the possibility of leakages. Brazed plate exchangers counter leakages through brazing.
Plate-Fin Heat Exchangers
They comprise of fins that are interlocked in between parallel joined plates. Fin arrangement is flexible to allow either parallel or cross-flow between adjacent plates. More so, they enable passing of around 12 fluid streams within a single exchanger by arranging the header rightfully. Plate-fin heat exchangers have gained preference in gas liquefaction since they can operate in close temperature limits.
Lamella Heat Exchangers
They bear a resemblance to the shell and tube model. They therefore comprise of tubes that are joined together forming a bundle. In this design, one of the fluids flows through the tube while the other flows through the gaps between the tubes. These exchangers are prevalent in paper industries.
Spiral Plate Exchangers
Developed through winding of two flat and parallel plates forming a coil. The terminals are either welded or sealed with a gasket. Their usage lies in viscous, heavy fouling fluids or fluids that contain particles or fibers.
Direct contact exchangers do not make use of heat transfer surface. However, to use the design with two fluids, they ought to be immiscible. For usage of one type of fluid, it has to undergo a phase change.
An example is the cooling system used in power generation. The cooling water is sprayed from the top onto the packing and in the meanwhile, air passes via the packing bottom. A disadvantage of this model is that the cooling water needs to be continuously replenished as a result of high evaporation. For maximum efficiency, water has to be maintained in a constant state.
Consists of two air heat exchangers that are;
The Wet Surface Exchanger
Here, water is used for cooling, then a fan is used to suck air and water through the bundle. The system is confined and therefore dump air is released to the surroundings.
Scraped Surface Exchangers
Comprises of a jacket like vessel that allows flow passage as well as a rotary scraper that clears deposits from the inner walls. These are prevalent in pharmaceutical and food industries since here deposits are formed on the heated walls.
The Regenerative Heat Exchangers
Here, the flow path is designed to contain a matrix that acquired heat through contact with hot fluid. The resultant heat is therefore passed along to the other fluid.
They are commonly used in gas heat recovery at power stations. They are further classified into two namely;
It is worth noting that regenerative heat exchangers pose the possibility of cross-contamination of streams of different temperature. For this reason, they are less preferred compared to recuperative exchangers.
This heat exchanger type, as the name suggests, contains no moving parts except for valves. Here, hot gas flows past the matrix, then change happens, whereby, flow is halted allowing the cold gas to pass through.
In this model, there is existent of rotating elements whereby a central packing rotates within. Therefore, there is a simultaneous flow of cold and hot gas.
In the design of heat exchangers, any model that tends to operate above the atmospheric pressure has to adhere to a code known as the ASME code (American Society of Mechanical Engineers). The stated codes are of essence in laying out the requirements for the pressure vessels.
- For plate and frame exchangers, they can be maintained by disassembling and doing cleaning periodically
- Monitoring to point out the heat transfer coefficient which tends to decrease with time as a result of fouling. Fouling is resultant of impurity deposition within the exchanger
- Tube exchangers are cleanable through some methods like bullet cleaning as well as high pressured water. The water jet will flush out the foreign objects
- Tubular heat exchangers are cleanable through some methods like bullet cleaning as well as high pressured water. The water jet will flush out the foreign objects
- Fouling of heat exchanger systems can be minimized through water treatment and addition of chemicals
- Water borne oscillations technology is being adapted to eliminate biofouling
How to Choose the Right Heat Exchanger Manufacturers
Well, before settling for a particular manufacturer, it's important to consider the below factors;
- How long has the manufacturer been in business?
- Do they have a good business reputation?
- What materials do they use in the manufacture of the heat exchangers?
- Do they have a good distribution network?
- Are they in a position to deliver your order promptly and with top quality assurance?
The above clarifications should help you make an informed choice about the right manufacturer that can effectively address your needs.
Heat Exchanger Terms
- The number of times, one, two or four, the liquid passes through the tube bundles of heat exchangers. Anything greater than a one pass is considered a multi-pass unit.
- Plate the tubes pass through for support that provides a blocked path for the shell side flow, which forces the flows across the tubes and improves the performance of heat transfers. These heat exchangers are shaped
in various ways, but are basically segmental.
- The space between the tube bundle baffle plates that is adjusted to maximize effectiveness of heat exchangers.
- Like a channel with straight tubes but without a removable cover. These heat exchangers do not have divider walls and are found at each end of heat exchangers.
- Manages the tube side liquid for circulation through heat exchanger tubes. This can also hold the tube side inlet and outlet connections and/or pass ribs.
- Tubes of heat exchangers parallel to each other from the inlet to the outlet manifold.
- The tubing assembly in removable bundle heat exchangers. This typically includes tubes, tube sheets, baffles, spacers and tie rods.
- A threaded bolt that holds the bonnet onto the core of some types of heat exchangers.
- A kind of front end with a removable cover from which the tube side flows in and out. A dividing wall separates the inlet and outlet flow.
- Same function as a bonnet assembly, except that the cover is removable and provides access to the ends of the tubes.
- What the tubes in heat exchangers drain into.
- The shell and tube assembly in fixed tube sheet heat exchangers.
- The parts that connect the piping to the heat exchangers, come in many varieties.
- Used to cover openings on heat exchangers. Covers are different from end plates because they can be removed to clean the interior of the tube side, without distressing any piping.
- The part used to support heat exchangers and to secure it to the mounting surface
when welded or strapped to the shell. Cradles may be fixed or moveable.
- Calculations of part thickness and design of heat exchangers based on the most severe conditions or highest operating pressures seen by heat exchangers, to make the pressure slightly higher.
- A type of nozzle connection that provides a larger nozzle opening between the pipe size and tube bundles of heat exchangers, typically to prevent tube erosion due to high inlet velocities.
- Covers welded to heat exchangers. The majority of end plates are used on bonnet assemblies.
- The first baffle space on a tube bundle, occurring between
the tube sheet and the first baffle plate. It is adjusted to maintain the baffle plates within the two shell side nozzles.
- A small copper or stainless steel piece of tubing that is crimped or squeezed onto the tie tube, up against the last baffle, and locks the baffles into position.
- A tube sheet that is an essential part of the core shell assembly of heat exchangers.
- Placed at one end of a removable tube bundle and allowed to move freely with the expansion and contraction of the tube bundle due to temperature changes in operation. It always has a smaller diameter than the immobile tube sheets.
- A device used between two parts that helps prevent leakage in heat exchangers.
- A small perforated-plate or bar assembly in the shell-side nozzle that can also be attached directly to the bundle. This protects and prolongs tube life by breaking up and slowing down the shell side fluid, which slows the erosion of the tubing.
- The side of heat exchangers that contains the tube side inlet and outlet connections in a multi-pass unit.
- A metal or nylon ring on some packed joint heat exchangers that holds the packing rings in place.
- The pressure of heat exchangers during operation and while in use.
- The end of heat exchangers, which contains the packed joint and the packing rings.
- A lane in a tube layout where there are no tubes and where the pass ribs mate.
- A separator plate inside a bonnet or channel that merges with the pass lane surface, used to form multi-pass heat exchangers. By arranging the ribs, a designer can direct the flow of the tube side substance.
- Protects the parts of heat exchangers (tubes, tube sheets and bonnets) from corrosion by acting as a sacrificial anode so that when water is flowing through the tube side it is consumed instead of other parts of heat exchangers.
- The temperature difference of a single fluid as it flows through heat exchangers.
- heat exchangers with a removable tube bundle from the shell casing. This provides easy cleaning of the shell side and also a more feasible way of replacing depleted tubes.
- The end of multi-pass heat exchangers where the tube side fluid reverses its flow. This usually contains only small vent and drain connections.
- The container where the tube bundle is placed and is the conduit for one of the fluids in heat exchangers.
- The assembly into which the tube bundle is placed. It also houses the shell side connections.
- A formed plate that is welded to the shell (or bonnet) pipe. It comes in many styles and shapes, including flanged and dished, elliptical, ellipsoidal and hemispherical.
- The part of heat exchangers where the fluid circulates around the tubes.
- Tubing that holds the baffle plate in place.
- Two or more heat exchangers connected together side by side or one on top of the other. Interconnecting piping hooks these heat exchangers together.
- The tube sheet at one end of a removable bundle that has a larger diameter than the floating tube sheet. The stationary tube sheet is held in a permanent position between the bonnet and shell flanges.
- A flange used at a packed end joint. When a packed joint is tightened, the packing ring is forced into this by the lantern ring/lantern gland.
- Bolted to heat exchangers using bonnet to shell flange bolting.
- Test that detects leaks on the joints of heat exchangers.
- Bars mounted between the tube sheets to support the baffles.
- A tie tube takes the place of the tie rod in small diameter heat exchangers, and serves
the same purpose.
- A flow channel for one of the fluids in heat exchangers. These heat exchangers are often parallel within the shell to provide a large surface area for heat transfers.
- Shows the positioning of the tubes inside heat exchangers and the locations of the tie rods.
- The apparatus that the tubes are affixed into that holds them in place. It also provides a seal between the tube-side and shell-side liquid.
- The fluid that circulates through the inside of the tubes of heat exchangers.