Heat exchanger

To understand what a heat exchanger is and how it works, we first invite you to familiarise yourself with the theory of heat transfer.

Heat transfer theory

The laws of physics make heat energy move through a system until temperature equilibrium is reached. Heat can be transferred in the following ways:

• Radiation. Heat energy is transferred by electromagnetic radiation;
• Conduction. Heat transfer through a solid material;
• Convection. Heat is transferred when one medium comes into contact with another. Convection can be:
• • natural (free). The movement of the medium depends entirely on the difference in its density and temperature, which equalise during the heat transfer process;
  • forced convection. The movement of the medium depends partly or fully on an external influence on the medium, for example the operation of a pump that moves a liquid.

It is worth knowing that:

• because of the temperature difference, heat always passes from the warmer medium to the colder medium
• for heat transfer to take place, there must be a temperature difference between the two media
• the amount of heat given up by the hot medium equals the amount of heat received by the cold medium, excluding external losses.

Heat carrier

A heat carrier is a working substance that transfers heat from a hotter environment to a colder one. Heat carriers can be: liquid - water, oil, organic compounds, liquid metals, molten salts; gaseous - air, water vapour, gases, combustion products. The state of aggregation of a heat carrier may or may not change. Heat carriers are characterised by intensive heat transfer, a high boiling point, high thermal resistance and low chemical and corrosive activity. Of the liquid heat carriers, water is the most commonly used. Its thermal conductivity coefficient is higher than that of gases, but it forms deposits and causes corrosion of heat exchangers.

Heat exchanger

A heat exchanger is a device in which heat is exchanged between two or more heat carriers, or between a heat carrier and the surface of a solid body. By construction, heat exchangers can be coil, plate, pipe-in-pipe, block, shell-and-tube and other types, and by the direction of heat carrier flow - parallel-flow, counter-flow and cross-flow. By operating principle they are recuperative, regenerative, contact (mixing) and internal heat source types. In a recuperative heat exchanger, the heat of one heat carrier is used to change the temperature of another by convection. In regenerative heat exchangers, the heating and the heated heat carriers flow alternately over the same surfaces. In contact heat exchangers, heat is exchanged by contact (mixing) of both heat carriers. In an internal heat source heat exchanger, the heat source is a chemical reaction taking place inside it or an electric heater.

The most commonly used types of heat exchangers are:

• Plate heat exchangers: made of corrugated plates, with two liquids flowing between them. Heat is transferred through the plate wall. Plate heat exchangers are compact and easy to clean;
• Tubular heat exchangers: made of tubes, with one liquid flowing inside the tubes and the other flowing around them. Heat is transferred through the tube wall. Tubular heat exchangers are robust and resistant to high pressure;
• Air heaters: used to heat air. Air heaters can be of various designs, for example plate, tubular and regenerative;
• Condensers and evaporators: used to condense vapours and evaporate liquids. Condensers and evaporators are often used in refrigeration systems.

In heat exchangers, heat is transferred until the temperatures equalise. The efficiency of heat transfer is determined by the thermal conductivity of the heat carrier material, the wall thickness and the temperature difference between the liquids. It is important to note that heat exchangers can heat or cool liquids (gases) even when the temperature difference is minimal.

Heat exchangers are used in various industries, for example in heating and cooling systems, the food industry, the chemical industry and power generation.

Plate heat exchanger

In many applications the plate heat exchanger is considered the most efficient. The use of this type of heat exchanger usually means optimal heat transfer over a wide range of operating pressures and temperatures. It is also often the solution within the limits that the production equipment in use sets for these parameters.

The main advantages of a plate heat exchanger:

• Compact design. This type of heat exchanger needs far less space than a traditional tubular heat exchanger;
• High turbulence of the medium flow creates good conditions for efficient heat transfer between the liquids, and the use of thin sheet metal for the heat transfer plates gives a higher heat transfer coefficient per unit of heat transfer surface area, which results in a heat exchanger that is not only exceptionally compact but also efficient;
• Lower heat losses to the environment thanks to the small size of the heat exchanger.
• High turbulence of the medium flow creates a self-cleaning effect. In addition, compared with a conventional tubular heat exchanger, the heat transfer surfaces of a plate heat exchanger are much less prone to fouling (formation of deposits). This means that a plate heat exchanger can run much longer between flushing cycles;
• Simple operation. A plate heat exchanger consists of a frame and a certain number of individual plates. By adding extra plates, the capacity of this type of heat exchanger is easy to increase. It is also easy to dismantle for cleaning (this applies to gasketed plate heat exchangers).

A plate heat exchanger has a variable thermal length. Many plate heat exchangers made by well-known manufacturers such as Alfa Laval, Hexonic, Kelvion, SWEP and others use plates with different pressing patterns. For example, if a plate with "H" channels is used, the heat exchanger has a higher pressure drop and works more efficiently, because it has a long thermal channel and therefore a long heat transfer time. When an "L" plate is used, the heat exchanger works with a small pressure loss and, accordingly, a somewhat lower heat transfer coefficient. Such a heat exchanger has a short thermal channel. A compromise between "H" and "L" (narrow and wide) channels, and between pressure loss and heat exchanger efficiency, can be achieved by using plates with different pressing patterns during assembly.

Applications of plate heat exchangers

The heat exchangers offered on the Amegata.lt supply platform can solve a wide variety of process tasks. They can be used for both heating and cooling, for air conditioning, and for cooling in various process applications. They also make possible the sustainable use of surplus energy for secondary functions such as preparing additional domestic hot water or heating or cooling buildings.

Plate heat exchangers are most commonly used in the following industry sectors:

• District heating;
• District cooling;
• Heating of tap water;
• Heating of water in various pools (swimming, fish farming and so on);
• Cooling of various engines;
• Cooling of various furnaces used in industry;
• Food, power and chemical industries where heating and/or cooling is required during the process;
• Solar heating systems.

In different processes, plate heat exchangers are used for cooling hydraulic fluid, cooling various oils (for example engine oil), as the intermediate cooling unit of a heat pump, for cooling air in ventilation systems, in solar heating systems and so on.

The following heat carrier types are most commonly used in plate heat exchangers:

• Water/water. Most of the heat exchangers we make solve the heat transfer task when the medium in both circuits is water (water/water mode). The solutions can vary:

  • Water heating. In this case, water at a higher temperature is used, for example from a district heating system, a boiler or a hot water preparation pipeline.

  • Water cooling. In this case, water at a lower temperature is used, for example from a cooling tower, a river or the sea.

• Water/oil. In some industries oil has to be cooled with water. This water can then be directed to a waste heat recovery system to recover the heat obtained from cooling the oil, which can then be used for various purposes.
• Water/glycol. Glycol is used to prevent possible freezing of the water. It is important to note that
  the heat capacity of glycol differs from that of water, so heat exchangers with a larger heat transfer surface area are required. The physical properties of different glycols are almost the same. Glycols include, for example: ethylene glycol (mono-, di- or tri-), propylene glycol.

When working with synthetic oils, special gaskets are used. Plate heat exchangers can work with oil whose viscosity reaches 2 500 cP. In plate heat exchangers, an emulsion with a concentration of less than 5 % can be used instead of water.

The range of plate heat exchangers is very wide. They can be very large units with a maximum heat transfer surface of as much as 2 000 m2 and a flow of 3600 m3/h, down to the smallest heat exchangers with a heat transfer surface of less than 1 m2 and a flow of 0.18 m3/h.

Construction of a plate heat exchanger

Plate heat exchangers can be brazed or gasketed.

A gasketed plate heat exchanger consists of a number of heat transfer plates, determined by the parameters, which are fixed between the exchanger's support bars and held in place by the frame plate and the pressure plate, forming a single assembly with them. All the plates have gaskets, which ensure the tightness of the channels through which the working liquids flow.

The gasket system ensures that the media flow only through the channels intended for them, so the liquids of the circuits flow continuously in opposite directions. The gasket design ensures that these liquids do not mix with each other.

The heat transfer plates have a corrugated (ribbed) surface on both sides, which makes the flow of each liquid through the channels turbulent. The highly turbulent liquid flow, together with a suitable ratio of medium volume to heat exchanger size, ensures a high heat transfer coefficient.

The same construction principle is used in brazed heat exchangers. In this type of heat exchanger, however, special brazing technology is used instead of elastomer gaskets to seal the medium channels, delivering the same result.

Plate heat exchanger components

The main components of a plate heat exchanger are: heat transfer plates, the fixed (stationary) frame plate and the movable pressure plate, connecting elements and support bars. The heat transfer plates hang on the upper bar and rest on the lower support bar. The support bars also serve to centre the heat transfer plates.

These plates are placed one after another between the frame plate and the pressure plate, forming a pack, which is then tightened with bolts into a monolithic assembly.

Gaskets

Materials used:

Glue-free "Clip-on" sealing gaskets are fixed to the edge of the heat transfer plate with special clips.

Assembly

Manufacturers supply pre-assembled, pressure-tested heat exchangers.
Gasketed plate heat exchangers can be opened easily for inspection and cleaning.
If the capacity of a gasketed heat exchanger needs to be increased, this can be done quite simply - just install additional heat transfer plates.
The illustrations show the assembly sequence of a gasketed plate heat exchanger.

Installation

All brazed and gasketed plate heat exchangers have connections on the main plate. For convenience they are labelled, for example, S1, S2, S3 and S4.

A gasketed plate heat exchanger can stand directly on the floor at the place of manufacture. If possible, always fix it to the floor with fixing bolts for reliability. Remember that a plate heat exchanger takes up less space than traditional heat exchangers. When planning the installation of a plate heat exchanger, you only need to leave free space on one side of it. Pipe connections can be threaded or flanged. This depends on the type of heat exchanger chosen.

Brazed plate heat exchangers

A brazed plate heat exchanger is small and light, it cannot be dismantled and it is inexpensive. This type of heat exchanger uses no gaskets; instead, brazing joins the plates into a single strong, sealed structure.

Brazed plate heat exchangers are particularly suitable where the working pressure reaches 50 bar and the ambient temperature varies from -196 to +550 °C.

A brazed plate heat exchanger is usually installed in the pipework or mounted on a small bracket or stand.
The connection for the inlet of one medium is next to the connection for the outlet of the other medium. If S1 is the inlet of medium 1, then S4 will be the outlet of medium 2.
Each heat exchanger is supplied by the manufacturer with a manual describing how to use the connections for the inlet and outlet of each medium.

Depending on the chosen pipe connection method, prepare the pipe ends for threading, flange mounting or welding.
"Alfa Laval" supplies accessories for its plate heat exchangers, such as drip trays and heat-insulating sheet materials.

Heat exchanger selection

To solve a heat transfer task with a heat exchanger, certain parameters must be known. Knowing them, you can determine what kind of heat exchanger is needed for the specific task. The six most important parameters are:

1. The amount of heat to be transferred (thermal capacity);
2. Inlet and outlet temperatures on the primary and secondary sides of the heat exchanger;
3. Maximum permissible pressure drop in the primary and secondary circuits;
4. Maximum operating temperature;
5. Maximum operating pressure;
6. Medium flow rate on the primary and secondary circuit sides.

This article is based on Amegata.lt's hands-on experience. See it differently or have something to add? Write to us – we're happy to discuss, and if you share new, well-founded information, we will update the article. Updated October 2026.