Heat exchangers are vital components in district heating systems, enabling the efficient transfer of heat from central sources to the distribution network. Their design, selection, and maintenance are crucial factors in the overall efficiency and effectiveness of district heating systems, contributing to energy savings and environmental sustainability.
What is the purpose and function of heat exchangers?
Heat exchangers are designed to transfer thermal energy from a heat source (such as a power plant, Combined Heat and Power (CHP) plant, or geothermal source) to the circulating fluid (typically hot water) in the district heating network.
Types of Heat Exchangers
- Plate Heat Exchangers: These are commonly used in district heating due to their high efficiency and compact design. They consist of multiple thin plates with channels for the hot and cold fluids. The close proximity of the plates allows for efficient heat transfer.
- Shell and Tube Heat Exchangers: These consist of a shell (a larger outer vessel) with a bundle of tubes inside. Hot fluid flows through the tubes, while the cold fluid surrounds the tubes in the shell.
- Brazed Plate Heat Exchangers: These are compact and efficient heat exchangers with plates that are brazed together. They are often used in smaller-scale district heating systems.
- Finned Tube Heat Exchangers: These heat exchangers have tubes with fins on the outside to increase the surface area for heat transfer.
The heat transfer process of heat exchangers
- Heat exchangers work based on the principle of thermal conduction, where heat flows from the hot fluid to the cold fluid without direct mixing of the two fluids.
- The hot fluid (e.g., steam or hot water from the central heat source) passes through one side of the heat exchanger, while the cold fluid (water from the district heating network) passes through the other side.
- The heat is transferred from the hot fluid to the cold fluid through the heat exchanger’s walls, raising the temperature of the cold fluid.
Efficiency and Energy Recovery
Efficient heat exchangers maximize the transfer of thermal energy, ensuring that the heat source’s energy is effectively utilized in heating the district.
Energy recovery is a key benefit of heat exchangers, as they allow the capture and reuse of waste heat from industrial processes, power generation, or other sources, contributing to the overall efficiency and sustainability of district heating systems.
Materials and Maintenance
Heat exchangers are typically constructed from materials that can withstand the temperature and pressure conditions of the district heating system. Regular maintenance, including cleaning and inspection, is essential to keep heat exchangers operating efficiently and to extend their lifespan.
Size and Capacity
The size and capacity of heat exchangers are selected based on the specific requirements of the district heating system, including the heat load, flow rates, and temperature differentials.
Integration with Control Systems
Modern district heating systems often incorporate sophisticated control systems that optimize the operation of heat exchangers based on real-time demand and supply conditions.
Plate heat exchangers are widely used in Denmark to separate circulating water in different networks. It is an example of compromising investment and operating costs, with the objective of minimising pressure loss and simultaneously maximising the heat transfer coefficient.
Plate heat exchangers have the additional advantages of demanding less space than other types and being built in modular form. The capacity of a plate heat exchanger can thus be increased by adding more plates – an operation that can proceed in parallel with the expansion of the network. In this way, investments in heat exchanger capacity can be scheduled to follow demand.