What is a heat exchanger?
A heat exchanger is a technical device that can be found in your own home or apartment. The heating "battery", the condenser and evaporator of a household refrigerator or air conditioner, and a car radiator are the closest examples of heat exchange devices that a person encounters in everyday life. And much more often and more diverse than in household appliances, heat exchangers are used in various industries and economies.
Scientifically, a heat exchanger is a heat exchanger designed to organize a directed exchange of thermal energy between two different, necessarily mobile media (heat carriers), which have a temperature difference between them. Heat exchange can occur: between liquid and liquid, gas and gas, liquid and gas. The purpose of directed heat exchange is - heating by one heat carrier of another (heat supply in heaters) and cooling (heat removal in coolers). Accordingly, heat transfer fluids in the direction of heat transfer are distinguished into heating and heated or cooling and cooling media.
The overwhelming majority of heat exchangers with moving media operate on the principle of heat recovery, when both coolants participating in directed heat exchange are separated by a heat transfer surface. The principle of operation of recuperative heat exchangers is as follows: media moving through their separate channels exchange heat energy directed from a hotter heat carrier to a colder one through a common contact surface. In contrast to recuperative devices, there are also regenerative heat exchangers, where the heating (cooling) and heated (cooled) coolants are in contact with the heat transfer surface alternately.
The high efficiency of heat exchangers is ensured by the correct selection of their design, materials, manufacturing (thick-walled hardened, heat-resistant, chemically resistant, etc.), an increase in the heat exchange area due to the profiling and corrugation of the passage channels, etc. To increase the efficiency of heat exchangers allows the use of control automation devices, which are designed to ensure their operation at optimal conditions.
Areas of application of heat exchangers
The areas of application of heat exchangers are varied:
- heating, hot water supply and air or water conditioning systems used in everyday life, utilities, equipment and production;
- heavy industry and mechanical engineering - in systems for removing heat from the working area (zone) or vice versa, for heating it, in systems for recovering heat energy from waste gases, etc.;
- chemical industry - when creating the necessary temperature conditions for the passage of chemical reactions and phase transitions;
- food industry and storage facilities - when creating the temperature conditions necessary for both the production of products and their storage.
Heat exchanger types
By design, recuperative heat exchangers are of two main types:
- Shell and tube, in which the passage channels for the moving media involved in heat exchange are formed by tubular elements. In this case, a pipe or a group of pipes for the passage of one coolant is placed inside another pipe - a casing, through which another coolant passes. The advantages of shell-and-tube heat exchangers are ease of manufacture and low cost, the possibility of using thick-walled materials, to ensure operation at high operating pressures and temperatures, there are demountable models with the ability to dismantle the tube bundle for maintenance and repair. Among the shortcomings, it should be noted the relatively low heat transfer coefficients and, as a consequence, the large surface area of the heat exchange surface, due to which the shell-and-tube devices are distinguished by their large dimensions and weight.
- Lamellar, in which heat exchange between two media is carried out through contact surfaces - plates made of corrosion-resistant steels. Often, such plates, sealed with gaskets, soldering or welding, and form closed passage channels for heat transfer fluids. Plate heat exchangers, in comparison with shell-and-tube heat exchangers, are characterized by high turbulence of the media in the channels, high heat transfer coefficients; they are capable of transmitting large heat power with the same heat exchange area as shell-and-tube heat exchangers. The efficiency of plate heat exchangers is considered to be the highest today.
Varieties of shell-and-tube heat exchangers
- Classic shell -and-tube design - a group of heat exchange tubes for one coolant is located inside the shell, along which another coolant moves;
- Pipe-in-pipe is a simplified version where another conductive pipe is located inside one conductive pipe. The design is very simple and cheap to implement, but has a low heat transfer efficiency;
- helicoidal - improved (intensified) design, in which profiled (by rolling helical grooves) conductive tubes are used, as well as helicoidal ribs welded inside the casing. With the help of helicoid profiles, vortex flows are created inside the pipes, which improve the conditions for heat transfer. In terms of ego efficiency, helicoidal heat exchangers approach plate heat exchangers.
Varieties of plate heat exchangers
- Dismountable, consisting of a set of heat exchange plates together with polymer sealing gaskets, which form closed channels for heat transfer and heat transfer media. They combine high efficiency of heat transfer, accessibility for maintenance (cleaning), the ability to modify (by adding or reducing the number of plates).
- brazed , in which the plates are assembled in a single closed (sealed) case, where using thermal vacuum brazing (copper or nickel), they are sealed not with gaskets, but with brazed seams, and form a honeycomb structure with separate channels for the circulation of both heat carriers - heating and heated. Due to the absence of polymer gaskets, brazed heat exchangers have a wider range of operating temperatures, pressures, and working environments (including those that can work with freons). But, brazed heat exchangers are conditionally non-separable, which is why they can be cleaned exclusively by chemical washing. For this reason, they can only work with very clean media that do not allow the formation of internal deposits.
- welded (shell-and-plate). The principle of their structure and operation is approximately the same as that of brazed ones, but they are used for much higher thermal capacities, operating temperatures and pressures. Their lamellar-honeycomb structure is formed by corrugated-profiled plates assembled in packages, fastened and sealed with welded seams, which are significantly stronger than brazed ones. As a rule, a welded lamellar-honeycomb structure is placed in a collapsible sealed casing (casing), which serves to organize cross flows of working media and allows access for service.
For heavily contaminated heat carriers, other types of heat exchangers are widely used, such as:
- spiral , in these heat exchangers separate rectangular channels for the movement of media are formed by two long steel sheets, welded to the central manifold, and wound around it in a spiral. When forming a spiral, it is possible to change the width of the channel between the sheets for the passage of media, depending on the type of media involved in the heat transfer process. This whole spiral structure is placed in a collapsible cylindrical body - a casing. Spiral heat exchangers have the largest contact area of heat exchange, and are used in situations where heat transfer fluids have high viscosity, the ability of media to block channels of other types of heat exchangers with suspended soft particles or highly contaminated solid mechanical particles.
Separately, designs of heat exchangers are used in which only one medium circulates through a closed channel, and the second medium is openly washing or blowing it. These types of heat exchangers are distinguished by the simplicity of organizing heat exchange processes, but at the same time, they also have low efficiency of heat exchange and structural bulkiness. Such heat exchangers include:
- submersible , when the tube is heated - the coil is immersed in an open container (bath) with refrigerant;
- irrigated when the pipe system is irrigated with jets of free flowing liquid;
- duct heaters blown by an air stream. An ordinary heating or car radiator, evaporator and refrigerator condenser are just of this type.
Depending on the material , heat exchangers can be manufactured:
- from carbon steel - for chemically passive (non-aggressive) environments, such as oil products, or very dirty environments that require frequent maintenance, to reduce the cost of equipment;
- made of stainless steel - for chemically active environments, food industry, work in a wide range of operating temperatures and pressures. High alloy stainless steels can handle even concentrated acids.
- from aluminum - if necessary, to achieve structural lightness;
- from special heat-resistant or cold-resistant alloys - for use in high-temperature or cryogenic technology;
- from titanium - for use with chemically aggressive media of salt composition (for example, sea water);
- from graphite - for use with chemically aggressive media of acid composition (usually in technological processes for the production of industrial acids).
Main technical characteristics of heat exchangers
The main integral technical parameter of a heat exchanger is its thermal power, which is expressed in the ability, under given operating conditions, to transfer a certain amount of thermal energy from one heat carrier to another per unit of time. The heat output depends on the heat transfer coefficient, the heat exchange area and the average logarithmic temperature difference between the heat carriers. The higher the coefficient of thermal conductivity, the higher the efficiency of the heat exchange process. To increase the coefficient of thermal conductivity of the heat exchanger, design engineers go to all sorts of tricks: from the development of special profiles that provide high turbulence of the media inside the channels, to reduce their cross-sectional area, in order to increase the speed of the coolants.
Other important technical characteristics of heat exchangers :
- the difference between the inlet temperatures of the heat carriers , which, along with the heat exchange area, also determines the intensity of heat exchange in the device. The greater the difference between the inlet temperatures, the higher the intensity of heat transfer between them.
- working and maximum pressure of heat carriers . Its increase is achieved by using stronger and thicker-walled materials, strengthening joints and seals.
- operating and maximum temperature of heat carriers - determines the operating conditions and temperature resistance of the heat exchanger. To increase it, heat-resistant materials and seals are used.
- the rate of passage of the coolant through the heat exchanger . Being a function of the working pressure, the cross-sectional area of the conducting channels and the hydro- (gas-) dynamic resistance of the system, this indicator determines the contact time of the media for carrying out heat exchange, and, ultimately, its efficiency.
- the difference in temperature of the medium at the entrance to the heat exchanger and at the exit from it. This parameter is also integral, and characterizes the overall efficiency of the heat exchange process that takes place in the heat exchanger. The greater the difference in the temperatures of the coolant at the inlet to the device and at the outlet from it, the higher its efficiency;
- the degree of chemical (corrosion) resistance of the heat exchanger is determined by the chemical composition of the working media and the construction materials used.
Important performance characteristics of heat exchangers :
- hydraulic or gas-dynamic resistance - determines the level of energy consumption required for pumping the coolant through the heat exchanger.
- overall dimensions and weight - affect the possibility and conditions of transportation, installation, placement at the site and operation of heat exchange equipment.
- available access conditions for service and repair.
Calculation of heat exchangers
The production and economic conditions for the use of heat exchangers are very diverse. Depending on the specific conditions, they may differ:
- tasks to be solved - for the removal or supply of heat energy;
- the required amount of heat to be removed or supplied (thermal power);
- characteristics of heat carriers - state of aggregation (liquid or gas), density and viscosity, operating temperatures and pressure, their chemical activity.
Based on these differences, in each specific case of using heat exchange equipment, its engineering (heat engineering) calculation is required.
The calculation of heat exchangers and their selection from the available standard designs are performed by heat power engineers of our enterprise, using software systems that ensure high accuracy in determining all parameters of heat exchange equipment. To calculate any type of heat exchanger, the following initial data are required:
- mass flow rates of heat carriers;
- temperature of media at the inlet and outlet to the apparatus;
- hydraulic or aerodynamic resistance;
- maximum working pressure of heat carriers;
- maximum operating temperature
The result of calculating the heat exchanger is the determination of the required heat exchange area and weight and size characteristics . The heat exchange area can be taken with a surface margin of 10-15% to provide an additional power reserve.
Main manufacturers of heat exchangers
There is a fairly wide selection of heat exchangers for various purposes and designs on the market, which are manufactured by numerous manufacturers. But among them there are a number of manufacturers whose products have proven themselves from the best side and are popular with consumers. This list of the most popular manufacturers (brands) includes:
- OPEKS Energysystems is an international company with offices in different countries of the former Soviet Union, produces and supplies a wide range of heat exchangers - plate collapsible, welded, spiral, shell-and-tube, titanium and graphite, heaters, heat exchangers, etc .;
- SWEP is a Swedish company, manufacturer of high quality and very popular brazed heat exchangers;
- TRANTER is a global corporation headquartered in the USA, one of the largest manufacturers of gasketed plate, shell-and-plate, compact welded heat exchangers with factories located in different regions of the world, main production in the USA and Sweden;
- Alfa-Laval is one of the first developers of plate heat exchangers, produces various heat exchangers in the above-average price segment;
- Sondex is a Danish company that was recently acquired by another Danish company, Danfoss, to expand its presence in the heating segment;
- Funke is a German company, originally specialized in the production of tubular heat exchangers, in the early 2000s it began to produce plate heat exchangers;
- APV is a Danish company, manufacturer of plate heat exchangers and pumps for industrial applications;
- GEA is a German company that produces a wide range of heat exchange equipment, has a separate large division for the production of ventilation equipment.