Each of us has encountered the simplest heat exchangers. A striking example of this is the design of "pipe in pipe" or something like that. It would be difficult to imagine our life if the heat exchanger had not been invented. Today there are a huge number of heat exchangers. They differ from each other not only in technical characteristics, but also in the scope, design, etc. Let's talk in more detail on this topic and deal with interesting points.
Some general information
A heat exchanger is a device that is used to transfer heat from one medium to another. At the same time, you need to understand that the heat exchanger itself, without heating equipment, is completely useless, but in the complex you can get wonderful results and successfully heat even very large and cold rooms. In addition, scientists have constantly tried to minimize the loss of heat when it is transferred to another environment. Today it is not possibleboast of 100% efficiency, but we can safely talk about the efficiency of 90-95%. Operational, as well as technical characteristics of the product are increased through the use of specially prepared materials, as well as a coolant. Of course, all this increases the price of equipment somewhat, but it's worth it.
When designing, engineers are constantly faced with conflicting requirements that need to be combined into one bottle. For example, it is necessary to reduce the hydraulic resistance and at the same time increase the heat transfer coefficient. The heat exchanger must be resistant to corrosion, but not too difficult to maintain. All this led to the fact that many types of heat exchangers appeared. The one that best suits the situation is used.
Classification of heat exchangers
As noted above, there are currently a huge number of heat exchangers. First of all, they must be separated according to the method of heat transfer to the medium. Here the heat exchangers are divided into the following groups:
- recuperative;
- regenerative;
- mixing;
- electrically heated.
Let's take a closer look at recuperative heat exchangers. The design of the product implies the presence of a single-layer or multi-layer wall through which heat is transferred. Usually this happens already in steady motion. It is interesting that in such devices, heat transfer is carried out with forced movement without changing the phasestates. But this applies only to permanently operating heat exchangers. If we talk about units with a periodic mode of operation, then for a certain period of time heating, evaporation, and cooling are carried out, and all this is in sequential mode. Such devices belong to heat exchangers with unsteady thermal motion. This is due to the fact that the temperature of the coolant at the inlet and outlet is significantly different. Often, such aggregates are found in the form of coils and are lamellar, ribbed and other forms. A little later we will look at several types. But the classification of heat exchangers does not end there.
Regeneration units and electrical heating
In this case, just like in the previous one, the heat exchange surface is used to transfer heat energy. However, this surface is a kind of nozzle. It plays the role of an intermediate accumulative means that accumulates heat. By and large, the whole process can be divided into several stages. At the first stage, the nozzle perceives a certain amount of heat. Then there is a transition to the second stage, and the coolant is transferred over the surface of the nozzle. This happens when changing the flow of coolants. At this stage, the nozzle gradually cools, and the accumulated heat is released into the heated environment, which may be your room.
Regenerators are non-stationary units. The nozzle is often motionless, and thermal processes are synchronously repeated. Devices of this type are often called scrubbers orcooling towers.
The essence of electrically heated heat exchangers is that electricity is used as the main source of heat. Electric arc installations are used to convert electrical energy into thermal energy. They can be both direct and indirect heating. The most common heat exchangers in industry are induction and resistance heaters. As you can see, heat exchange equipment can be different, now we will take a closer look at each type, its scope and design features.
Spiral heat exchangers
The device is a pair of spiral channels. They usually wind around the central partition. To do this, they are made from rolled material. It is worth noting that spiral heat exchangers are well suited for heating and cooling fluids with a high viscosity.
By and large, the heating surface is formed by two sheets of metal, which are attached to the core by means of a weld. The unit itself consists of only 2 channels, usually rectangular, made in the form of a spiral. The end of the spiral (internal) has a dividing wall and is fixed with pins. Heat exchangers can be made both vertical and horizontal. If it is not possible to install one type due to insufficient space or a complex configuration of the room, then the second, more preferable, is used. It is also interesting that the consumer can choose spiralheat exchangers with different spiral widths, from 20 to 150 centimeters. At the same time, the heating surface can vary from 3.2 to 100 square meters with a maximum system pressure of 1 MPa.
It should be noted that this heat exchange equipment has a number of significant advantages. Firstly, it is a reduced hydraulic resistance. Secondly, compactness and high efficiency and heat transfer intensity. But all this contributed to the fact that there were shortcomings in the form of a complex design and repair.
About plate heat exchangers
Currently, collapsible and non-separable plate heat exchangers are manufactured. Naturally, the first type is more preferable due to many reasons. First, it is ease of maintenance. Such equipment is very quickly disassembled and assembled, so any breakdown is eliminated in a short time. Non-separable models are usually not repaired, and if they are, then much longer.
Actually, the name suggests that this equipment consists of a package of prefabricated plates. They can be made from various materials such as copper, titanium, graphite, etc. Almost always, to improve the performance properties, the plates are made corrugated. In plate heat exchangers, the flows of cold and hot coolant pass in layers.
The equipment itself is good because it has a competent layout. This made it possible to increase the area of the heat exchange surface and fit all this into relatively small dimensions. In any case, before buying, a calculation of heat exchangers is carried out, which allows you to obtain data on how much power the device needs in a particular case. It must be understood that all the plates that are pulled together in a package form channels between themselves due to the same shape. Fluid flows through them. Well, now we will look at some more interesting details that relate to this equipment.
Using gaskets
As noted above, the main element of heat transfer are plates. They are cold stamped. For this, corrosion-resistant alloys are used, which can significantly increase the durability and efficiency of the unit. The thickness of the plates, depending on the model, can vary from 0.4 to 1.0 mm. In the working position, the plates are tightly pressed against one another. In this case, small slotted channels are formed. On the front side there is a special groove, a rubber gasket (seal) is installed there. In addition, the gaskets have holes that are necessary for the supply and removal of fluid. In case one of the holes breaks through, a system of drainage grooves is provided to prevent mixing of cold and hot media.
Due to the creation of a countercurrent between the two media, it was possible to achieve not only an improvement in the temperature set, but also faster heat transfer with relatively small hydraulic resistances. It would not be superfluous to say that the basic principle of operation is based on countercurrent, that is, the movement of heating andheating fluid in different directions. To prevent mixing, a double rubber seal or a metal plate is installed. The number of plates and channels may vary depending on the operational requirements of the equipment. Before creation, a thermal calculation of heat exchangers is carried out, which makes it possible to determine the optimal mode of operation. Sometimes expensive alloys are used that are not afraid of long-term operation in an aggressive environment.
Plate-Fin Heat Exchangers
PRT are used to transfer heat in non-aggressive and gaseous media in a wide temperature range, from -270 to +200 degrees Celsius. In this case, the pressure in the system can reach 100 atmospheres and start from a vacuum. The design is based on the idea of applying a ribbed surface on both sides of the plates. The product itself consists of several ribs, thanks to which heat transfer between media is carried out. It is worth noting that it is the finned-plate heat exchanger that has a wide variety of fin shapes. This allows you to slightly change the operational and technical characteristics. Most often you can see continuous and wavy ribs. But in addition to these, there are also more exotic ones, such as perforated and scaly ones. Sheet metal is usually used as the material. Their thickness is adjustable depending on the pressure in the system and the fluid used.
Often these types of heat exchangers are made with different types of flow. Most often, counterflow is used, but there are alsostraight-through, and cross circuits. If we briefly talk about the strengths of such equipment, then there are a lot of them. Firstly, these are operational properties, such as fast and intensive heat transfer. Secondly, it is small in size. Today, many say that it is finned heat exchangers that are the most advanced. Most often, PRT is used in industries such as energy, oil refining, chemical and aviation industries. All this is due to a large number of advantages, as well as a wide range of fluids and pressures used in the system.
Shell-and-tube heat exchanger: design and features
Surface-type heat exchange equipment, which we have already reviewed, is not as popular as shell and tube units. These are just the devices that were mentioned at the very beginning, in the simplest version - this is the "pipe in pipe" system. A heat exchanger of this type is a system (bundle) of tubes that are placed in a casing. The tubes are rolled and welded to the body of the product. In some cases, they are additionally scalded. This is done to ensure 100% tightness. The body is supplied with additional nozzles. Some are needed for supplying steam, others for condensate removal. In addition, there are transverse gratings in the casing, which are used to support the heat exchange tubes along the entire length of the unit. Interestingly, shell and tube heat exchangers are used at temperatures from 190 degrees Celsius orsaturated steam pressures over 15 bar.
Any system that involves fluid movement can be subject to water hammer. This phenomenon is capable of partially or completely deactivating the equipment. To prevent this from happening, various kinds of storage elements are used, the so-called expansion tanks. But in our case, this is not necessary, because shell-and-tube heat exchangers are very resistant to them. In addition, there are no strict requirements for the cleanliness of the environment. A significant disadvantage of such equipment is that all types of heat exchangers of this type are very metal-intensive, which affects the final cost and dimensions.
Heat exchangers for gas equipment
It's no secret that any solid fuel or gas boiler has a heat exchanger in its design, they are also called heaters. We have already considered the main types. As you probably noticed, these or those types are used in various industries. Some devices have found wider application, others are used in certain industries and do not fit others. In our case, the use of tubular and plate heat exchangers takes place. In the first case, we are dealing with a system of tubes, in the second - with plates. In principle, regardless of the type, a heat exchanger for a geyser must meet a number of requirements. Firstly, to have a high heat transfer coefficient, and secondly, to be durable and resistant to high temperatures. The most popular materials are copper, aluminum andsteel. The latter option is less preferable, since such a metal is heavy, which reduces efficiency. In any case, the heat exchanger for the geyser must serve at least 5 years.
Conclusion
So we reviewed the main types of heat exchangers. Such species as shell-and-plate were left without attention. In principle, they differ slightly from the classic lamellar or ribbed. But you can often find bath stoves with a heat exchanger with a casing. However, the key feature is that the equipment is resistant to high temperatures and operating pressures. The housing can be made of materials such as titanium, stainless steel or carbon steel. It is interesting that bath stoves with a shell-and-plate heat exchanger are well regulated for steam or condensate, which, undoubtedly, is a significant advantage. In principle, this can complete the story, since now you know everything you need about heat exchangers.
