Plate heat exchangers for district heating and heating networks: network separation, substations, feed-in

In district heating and heating networks the plate heat exchanger is the standard unit for network separation. It couples networks with different pressure, temperature and water quality, transfers the heat duty with a small pressure drop and keeps both circuits hydraulically separate. This guide describes the typical locations in commercial, neighbourhood and industrial networks, the parameters relevant to sizing, and the suitable design.

Where does the plate heat exchanger separate networks?

In substations it separates a utility’s primary network from the secondary network of a neighbourhood, a business park or an industrial site. At the handover to commercial and industrial customers it forms the interface between the utility network and the customer’s plant; the network operator’s technical connection conditions set pressure rating, temperatures and often a maximum return temperature there. Within large sites it separates heating circuits with different pressure levels, for instance in high-rise buildings or widely branched site networks.

This separation is essential in practice because district heating networks often operate at working pressures of 8 -⁠ 16 bar, while secondary networks and customer plants are designed for lower pressures. Pressure level, water quality and any leaks then affect only one circuit.

How is heat from CHP plants and waste heat fed in?

The plate heat exchanger also works in the opposite direction: it feeds heat from the cooling water circuit of a combined heat and power plant, from industrial waste heat or from process cooling into a heating or district heating network. The separation protects the engine or process circuit from the pressure and water quality of the network; conversely, glycol, oil or process residues do not enter the network. The page Plate heat exchangers for CHP plants shows how this looks for a CHP plant.

For feed-in, the network-side return temperature matters most: it limits how far the heat source can be cooled, and therefore how much heat it can deliver.

What matters for temperature spread and pressure rating?

Temperature spread

The temperature spread is the temperature difference between the flow and return of a circuit. A large spread means that a lot of energy is transferred per unit volume of the heat transfer medium. In district heating, primary-side spreads of 30 -⁠ 60 K are typical (e.g. flow 90 °C, return 50 -⁠ 60 °C). On the secondary side spreads are often 20 -⁠ 30 K (e.g. flow 70 °C, return 50 °C).

A sufficiently large spread keeps flow rates small, which reduces pipe cross-sections and pump energy. At the same time the logarithmic mean temperature difference between the circuits must remain positive; in counterflow this is reliably achieved even with small mean temperature differences.

Pressure rating

Substations and network handovers are normally designed to PN 16 or PN 25. Primary-side operating pressure and secondary-side static pressure must be considered separately. For PN 16 the maximum allowable working pressure is 16 bar; for PN 25 it is 25 bar. The nominal pressure must satisfy the higher of the two circuit pressures.

Important: during pressure surges (water hammer) significantly higher peak pressures can occur briefly. The unit and its connecting piping must be designed for this.

Which design is recommended for district heating and heating networks?

For district heating and heating networks, gasketed plate heat exchangers are the standard choice:

• Pressure range: PN 16 to PN 25 achievable, suitable for district heating networks.
• Cleanability: the unit can be opened and mechanically cleaned, which is advantageous with magnetite and sludge from the network.
• Replacement gaskets: available, economical on-site replacement.
• Flexibility: plate count can be adjusted later if heat demand grows or the thermal task changes.

Welded units are an option for clean, chemically stable circuits where mechanical opening is not required and high pressures or temperatures favour the gasket-free design.

For a complete overview of designs see the design comparison page. Potable water heating has its own requirements for system separation; they are covered on the potable water heating page.

Which notes apply to practical operation?

• Strainers on the primary side protect the heat exchanger from particle ingress from the district heating network.
• Regulating valves and isolation fittings belong on both sides to allow the unit to be shut off conveniently for maintenance.
• Condensation and heat losses on the outer surface can be reduced by simple insulation of the unit shell.
• Sizing should include a fouling safety allowance; 10 -⁠ 20 % area reserve is typical for heating applications.

Detailed sizing steps are described in the guide Sizing a plate heat exchanger. The online calculator captures the key data of your plant; an overview of further application fields is provided on the applications page.

Summary

Plate heat exchangers in district heating and heating networks are proven standard units. The key factors are correct spread sizing, choosing the right pressure rating, and the network operator’s requirements. Gasketed designs provide the flexibility needed for maintenance and extension in operation.