Plate heat exchangers for heating and district heating: sizing and design recommendations

In heating and district heating applications the plate heat exchanger is a standard piece of equipment. It hydraulically separates the district heating network from the building heating circuit, transfers heat duty with minimal pressure drop, and keeps both circuits operating independently. This guide describes the typical tasks, the parameters relevant to sizing, and the recommended designs.

Which tasks does the heat exchanger handle in a district heating connection?

In a district heating substation the heat exchanger connects the primary district heating network to the secondary building heating circuit. Both circuits remain hydraulically separate: pressure level, water quality and any leaks affect only one circuit. This separation is essential in practice because district heating networks often operate at working pressures of 8 - 16 bar, while building heating circuits are designed for 2 - 6 bar.

Beyond district heating substations, plate heat exchangers are used in heating systems as heating circuit separation stations, cascade stations and instantaneous domestic hot water heaters (fresh water stations). All these applications require careful sizing with respect to duty, temperature spread, and the respective pressure ratings of both circuits.

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 (building heating circuit) 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

District heating substations 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 heating and district heating?

For heating and district heating applications, 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 where occasional sludging in the heating circuit is a risk.
• Replacement gaskets: available, economical on-site replacement.
• Flexibility: plate count can be adjusted later if the thermal task changes.

Welded units are an option for clean, chemically stable heating 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.

What applies to hygiene and domestic hot water?

When the plate heat exchanger is used for domestic hot water heating (fresh water station), enhanced hygiene requirements apply. The drinking-water-side surface must be made of a food-grade material. Stainless steel 1.4404 (316L) is standard; for chloride-laden water a higher-grade steel or titanium may be needed.

The operating temperature is critical for hygiene: to prevent Legionella growth the water in the unit must reach at least 60 °C. In fresh water stations drinking water is always heated instantaneously and not stored, which significantly reduces Legionella risk compared to storage solutions. Sizing temperatures must ensure this.

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. An overview of further application fields is provided on the applications page.

Summary

Plate heat exchangers in heating and district heating are proven standard units. The key factors are correct spread sizing, choosing the right pressure rating, and - for domestic hot water heating - compliance with hygiene requirements. Gasketed designs provide the flexibility needed for maintenance and adjustment in operation.