Each of us came across the simplest heat exchangers. A striking example of this is the “pipe in pipe” design 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. Between themselves, they differ 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 are constantly trying to minimize heat loss during its transfer to another medium. Today you can not boast of 100% efficiency, but about the efficiency of 90-95% can be safely said. Operational as well as technical characteristics of the product are enhanced through the use of specially prepared materials, as well as coolant. Of course, all this slightly increases the price of equipment, but it's worth it.
When designing, engineers are constantly faced with conflicting requirements that must 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 appearance of many types of heat exchangers. Depending on the situation, the one that is best suited is used.
Heat Exchanger Classification
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 this kind of apparatus heat transfer occurs during forced movement without changing the phase state. But this applies only to permanent heat exchangers. If we talk about units with a periodic mode of operation, then for a certain period of time heating, evaporation, as well as cooling, are carried out, and all this in a 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 consider several types. But the classification of heat exchangers does not end there.
Regenerative units and electric heating
In this case, just like in the previous one, a heat exchange surface is used to transfer thermal energy. However, this surface is a kind of nozzle. It acts as 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 along the surface of the nozzle. This happens when changing coolant flows. At this stage, the nozzle is gradually cooled, and the accumulated heat is transferred to the heated environment, which may be your room.
Regenerators belong to non-stationary units. The nozzle is often stationary, and thermal processes are synchronously repeated. Devices of this type are often called scrubbers or cooling towers.
The essence of electrically heated heat exchangers is that electricity is used as the main source of heat. To convert electric energy into heat , electric arc plants are used. They can be either direct or indirect heating. The most common heat exchangers in the industry are induction and resistance heaters. As you can see, heat exchange equipment can be different, now we will consider in detail each type, its scope and design features.
Spiral heat exchangers
The device is a pair of spiral channels. Usually they are wound around a central partition. To do this, they are made from roll material. It is worth noting that spiral heat exchangers are well suited for heating and cooling liquids having a high viscosity coefficient.
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 of rectangular cross section, made in the form of a spiral. The end of the spiral (inner) has a dividing wall and is fixed with pins. Heat exchangers can produce both vertical and horizontal. If it is not possible to establish one view due to insufficient space or a complex configuration of the room, then a second, more preferred one is used. It is also interesting that the consumer can choose spiral heat exchangers with different spiral widths, from 20 to 150 centimeters. Moreover, 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, this is a reduced hydraulic resistance. Secondly, compactness and high efficiency and heat transfer rate. But all this contributed to the fact that shortcomings arose in the form of complex construction and repair.
About plate heat exchangers
Collapsible and non-collapsible plate heat exchangers are currently being manufactured. Naturally, the first type is more preferable due to many reasons. The first is ease of maintenance. Such equipment is very quickly disassembled and assembled, so any breakdown is eliminated in a short time. Non-separable models usually do not repair, and if this is done, then much longer.
Actually, the name suggests that this equipment consists of a package of prefabricated plates. They can be made of various materials, such as copper, titanium, graphite, etc. Almost always, to improve the performance properties of the plate are corrugated. In plate heat exchangers, flows of cold and hot heat carrier pass in layers.
The equipment itself is good in that it has a competent layout. This made it possible to increase the heat-exchange surface area and fit all this into relatively small dimensions. In any case, before buying, heat exchangers are calculated, which allows you to get data on how much power the device needs in a particular case. You need to understand that all the plates that are pulled together in a package, due to the same shape, form channels between themselves. Liquid flows through them. Well, now we will look at some more interesting details that relate to this equipment.
Using gaskets
As already noted above, plates serve as the main element of heat transfer. They are made by cold stamping. 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 pressed tightly against one another. In this case, small slotted channels are formed. There is a special groove on the front side, a rubber gasket (seal) is installed there. In addition, there are openings in the gaskets that are necessary for supplying and discharging liquid. In the event of a breakthrough in one of the holes, a drainage groove system is provided that excludes mixing of cold and hot media.
Due to the creation of a counterflow 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 will not be amiss to say that the basic principle of operation is based on a countercurrent, that is, the movement of heating and heating fluids in different directions. To avoid mixing, a double rubber seal or a metal plate is installed. The number of plates and channels may vary depending on the operational requirements for the equipment. Before creation, the heat calculation of heat exchangers is carried out, which allows you 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
RHTs are used to transfer heat in non-aggressive and gaseous media over a wide temperature range, from -270 to +200 degrees Celsius. In this case, the pressure in the system can reach 100 atmospheres and begin with 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, due to which heat transfer between the media is carried out. It is worth noting that it is the fin-plate heat exchanger that has a wide variety of rib shapes. This allows you to slightly change the operational and technical characteristics. Most often, you can see continuous and wavy ribs. But besides these, there are more exotic ones, such as perforated and scaly. As a material, thin sheet metals are usually used. Their thickness is regulated depending on the pressure in the system and the fluid used.
Often, these types of heat exchangers are made with various types of flow. The countercurrent is most often used, but there are direct-flow and cross-circuits. If we talk briefly about the strengths of such equipment, then there are a lot of them. Firstly, these are operational properties, such as fast and intense heat transfer. Secondly, these are small dimensions. Today, many say that finned heat exchangers 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 liquids and pressures in the system.
Shell-and-tube heat exchanger: design and features
The surface-type heat exchange equipment that we have already examined 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 to ensure 100% tightness. The housing is equipped with additional nozzles. Some are needed for supplying steam, others for condensate drainage. In addition, there are transverse grids in the casing, which are used to support heat transfer tubes along the entire length of the unit. It is interesting that shell-and-tube heat exchangers are used at temperatures from 190 degrees Celsius or saturated vapor pressures of more than 15 Bar.
Any system involving fluid movement can be subject to water hammer. This phenomenon is able to partially or completely bring the equipment out of operation. To prevent this from happening, various types 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 a clean environment. A significant minus 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 is 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 use, others are used in individual industries and are not suitable for 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 - plates. In principle, regardless of the type, the heat exchanger for the gas column 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 and steel. The latter option is less preferable, since such a metal has a large weight, which reduces the efficiency. In any situation, the heat exchanger for the geyser should last at least 5 years.
Conclusion
So we have examined with you the main types of heat exchangers. Such species as shell-lamellar were ignored. In principle, they differ slightly from the classical lamellar or ribbed. But often you can find a furnace for a bath with a heat exchanger having a casing. However, a key feature is that the equipment is resistant to high temperatures and operating pressures. The case can be made of materials such as titanium, stainless or carbon steel. It is interesting that the furnace for a bath with a shell and plate heat exchanger is well regulated by steam or condensate, which, undoubtedly, is a significant advantage. In principle, the story can be completed on this, since now you know everything you need about heat exchangers.