The function of a plate heat exchanger is to transfer heat between two fluids of different temperatures, without them touching or mixing. These fluids may be liquids or gases. Heat exchange takes place in separate chambers. We find a hot fluid and a cold fluid. Plate heat exchangers offer optimum energy efficiency, even though they are small devices. Find out more about the role of a plate heat exchanger, its structure, how it works and the different models available on the market.
Principle and function of a plate heat exchanger
The principle and function of a plate heat exchanger is to enable the transfer of thermal energy from one fluid to another, both at different temperatures. In effect, a cold fluid and a hot fluid circulate without touching or mixing. This type of heat exchanger is small in size, but very energy-efficient. The role of the plate heat exchanger is to cool hot water and heat cold water. Such an exchanger may be expandable. It’s a device found in stratified accumulators, air conditioners, refrigerators and boilers for producing domestic hot water. When plate heat exchangers become clogged, this leads to significant internal pressure losses.
Types of heat exchange
Several types of heat exchange exist with these devices:
- Single-phase exchange: this involves an exchange of heat flow between two fluids, but without any change of state. This is sensible heat. This is the type of exchange found in a radiator. Heat is exchanged by raising or lowering the temperature;
- Two-phase exchange: unlike the single-phase model, two-phase heat transfer between the two fluids involves a change of state. Here, latent heat is involved. This type of exchange is found inside refrigeration system evaporators or condensers.
Composition of the plate heat exchanger
Heat exchangers are made up of several plates in stainless steel, aluminum or synthetic materials. They are either herringbone or ribbed. The plates are brazed, welded or pressed together in a gasketed frame. The plates measure between 0.1mm and 0.8mm, making them very thin. Spaces between exchanger plates are between 5mm and 10mm. Channels are placed between each plate, so that the fluids circulate in parallel. This allows the cold fluid to be heated and the hot fluid to be cooled. One of the two fluids flows through the plate heat exchanger’s odd-numbered channels, and the other through the even-numbered channels.
By design, the role of a plate heat exchanger is to create turbulence inside the device. This simplifies heat exchange and improves conductivity. Heat energy is transferred to the surface of the plates. This surface is called the heat transfer zone.
What are the different types of plate heat exchangers?
The different types of plate heat exchangers are as follows:
- Brazed plate heat exchangers: this type of heat exchanger is compact. Its construction makes it remarkably efficient. Most heat transfer plates are made of stainless steel. However, nickel alloy plates are also available, enabling the plate heat exchanger to be used with corrosive fluids. Fluids can be exchanged without damaging the exchanger. Brazed stainless steel plate heat exchangers can withstand pressures of up to 30 bar. Nickel-based plate heat exchangers can withstand up to 10 bar;
- Gasketed plate heat exchangers: this heat exchanger is the most complicated in terms of assembly. The package containing transfer plates and gaskets is surrounded by a frame consisting of tie rods. When the exchanger is to be used with seawater, the plates are made of titanium. This ensures resistance to salt attack.
Role and operation of a plate heat exchanger
The role and operation of a plate heat exchanger are simple. The primary and secondary fluids inside a plate heat exchanger are opposed, parallel or crossed. Each fluid flows through separate channels. There is an even and an odd channel, both close to each other. This allows the two liquids or gases to heat and cool each other. The desired temperature can be maintained permanently and stably.
All plate heat exchangers feature the same technology and operation:
- Conduction ;
- Convection ;
- Radiation.
There are several types of circulation:
- Co-current or anti-methodic circulation with parallel flow of the two fluids going in an identical direction. The fluids enter on the same side of the exchanger;
- Countercurrent or methodical circulation with parallel flow of fluids not going in the same direction (reverse fluid circulation). This type of configuration is the best for optimizing heat exchanger performance;
- Cross-current circulation with perpendicular fluid flow.
Advantages and disadvantages of plate heat exchangers
Having explained the functions of the plate heat exchanger, here are its advantages and disadvantages.
Advantages
- Robust, resistant and compact;
- Small footprint;
- Wide choice of plate materials;
- High thermal performance;
- Low pressure drop;
- Optimized fluid velocity;
- Easy to maintain and clean, thanks to 4-sided disassembly;
- Operation at high temperatures and pressures.
Disadvantages
- Limited temperature variations;
- Regular maintenance requiring the unit to be shut down to clean the channels;
- Risk of leaks.
