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Heat exchangers for heating

• Capacity in kW: design, calculation
• Water/water: DHW heating
• Corrosion
• Limescale deposits
• Condensing and non-condensing boilers
• Central heating, district heating, heating stations
• Solar heating
• Cogeneration (combustion engines)

SWEP brazed plate heat exchangers offer a compact and reliable solution for applications such as steam exchangers, domestic hot water (DHW) heating, space heating, biogas and district heating stations, and central heating systems.

Performance table of SWEP water exchangers

The performance table provides indicative capacities for SWEP brazed plate heat exchangers in typical water-to-water applications. If one of the media is an antifreeze mixture (e.g. ethylene glycol), the heat exchanger must be oversized by approximately 30 % to compensate for the reduced thermal conductivity.

Water-to-Water: Domestic Hot Water (DHW) Heating

The wide range of SWEP heat exchanger sizes allows for an economical solution for domestic hot water heating, drinking water systems, and space heating. Typical heat sources include boilers, heat pumps, or solar panels. DHW can be heated using either a flow-through (instantaneous) system or in combination with a hot water storage tank. In flow-through systems, the water flow rate varies based on consumption, and the pressure drop must be calculated for peak flow conditions. Pressure losses increase roughly with the square of the flow rate — higher flow means significantly higher resistance in the heat exchanger.

DHW Flow Heater at home

A standard shower uses approximately 12–15 liters per minute of warm water, while energy-saving showers use 6–8 l/min. A comfortable showering temperature is around 44 °C. This means that a boiler with an output of at least 15 kW is required for proper instantaneous DHW heating. The table below provides an indicative selection of heat exchangers based on various output requirements. All SWEP exchangers listed use ISO G external thread connections.

It is not recommended to heat DHW above 60 °C in a plate heat exchanger, as this increases the risk of limescale buildup, which may clog the exchanger and reduce its performance, or even lead to pitting corrosion.

Charging the DHW tank

The tables below show the recommended SWEP heat exchanger models for charging domestic hot water (DHW) storage tanks. When the heat source temperature is high, plate heat exchangers can sometimes be too efficient, which may lead to excessive temperature gradients and limescale formation. To avoid this, it is advisable to use a co-current flow connection (in the tables, this is indicated with a cross: #).

# This heat exchanger is designed for co-current flow. In countercurrent operation, it would be too efficient, potentially causing limescale deposits to form on the plates due to high local temperatures.

Replacing Existing Heat Exchangers

SWEP heat exchangers are commonly used to replace outdated units from other manufacturers. All models are made from stainless steel (AISI 316) for durability and corrosion resistance. In addition to standard models, multi-circuit versions are also available — these contain two internal circuits connected either in series or in parallel, and feature multiple connections within a single compact unit.

Plate Heat Exchanger Corrosion

The typical service life of brazed plate heat exchangers is 8 to 12 years. If a unit begins to leak within 5 years or less, the most common cause is corrosion due to an aggressive medium.

• In central heating systems, the heat supplier usually monitors the water quality.
• In a boiler-based closed circuit, the risk of aggressive substances is lower.
• Corrosion most often originates on the heated (secondary) side of the exchanger.

A detailed chemical resistance table is available via the link.

Signs and Causes of Corrosion

• A greenish-blue tint on the exchanger surface signals copper solder corrosion, typically caused by rusty water with free iron and manganese. The iron forms a galvanic cell with dissimilar metals (e.g., copper and stainless steel), leading to gradual copper leaching into the water.
• Open expansion systems accelerate corrosion by allowing oxygen into the circuit.
• Galvanic corrosion can also occur at pipe joints where dissimilar metals (e.g., copper and carbon steel or reactive brass) are in contact. If these joints are not properly insulated — for example, using a flat non-conductive gasket — reactive metals like iron or zinc may leach into the system water. These ions can accelerate corrosion of copper solder within the heat exchanger.
• Distilled water (also called “hungry water” due to its low mineral content) is aggressive to copper solder and should be avoided.
• Material fatigue due to frequent and large temperature changes is another risk. A common example is instantaneous DHW heating, where cold water is rapidly heated by a high-temperature source (hot water or steam), putting mechanical stress on the exchanger.
• Corrosion of AISI 304/316 stainless steel is rare but can occur in the presence of high concentrations of chlorides (typically above 300 ppm). In some cases, limescale deposits can trap corrosive agents and lead to pitting corrosion.

Recommended Measures

If corrosion is observed on other components or laboratory water analysis is available, it is essential to identify and eliminate the root cause. Otherwise, any replacement heat exchanger will suffer the same early failure. Preventive actions include:

• Identifying and removing the corrosive factor.
• Replacing or treating the medium (e.g., adding a corrosion inhibitor).
• Installing a magnetic filter.
• Converting to a closed expansion system.

If system conditions cannot be corrected, consider selecting a more corrosion-resistant heat exchanger, such as an all-stainless steel model (without copper solder), or a gasketed plate heat exchanger, which is serviceable and better suited for harsh conditions.

Limescale Deposits

To prevent limescale build-up, the system should operate at lower temperatures, ideally around 60 °C. Lower temperatures not only reduce scaling risk but also improve system efficiency. In contrast, higher temperatures accelerate limescale formation, which can clog the heat exchanger and reduce its performance. In addition, limescale in contact with stainless steel may lead to pitting corrosion.

Water can also be chemically treated by dosing polyphosphates, which bind to calcium ions and prevent them from depositing on the plates. Polyphosphates are not suitable for drinking water. Instructions for cleaning the heat exchanger can be found in the user manual or on the home page.

Design Tips to Prevent Scaling

• Design for higher flow rates and lower temperatures to transfer the same heat load more efficiently.
• Higher flow creates turbulent flow inside the plate channels, which provides a self-cleaning effect.
• For DHW systems, cocurrent flow is often recommended. This may require a slightly larger heat exchanger, but it helps prevent overheating of the domestic hot water.
• Connect the DHW inlet to the lower port of the exchanger. The upward water flow helps minimize sediment buildup in the channels.

SiO2 based protection to prevent limescale deposits

Sealix is a new generation of brazed plate heat exchangers featuring a thin SiO2 coating on internal surfaces. It offers advanced protection against:

• Corrosion from high chloride or fluoride levels in water.
• Copper solder degradation due to free iron (e.g., from rusty radiators).
• Limescale formation on the plates.

This protective layer helps extend the life of the heat exchanger and maintain consistent performance.

Condensing and Non-Condensing Boilers

SWEP heat exchangers from the E5, E6, and E8 series are suitable for both condensing and non-condensing boilers. They are compact, with low pressure drop, making them ideal for modern heating systems.

Central Heating, District Heating, and Heating Stations

SWEP heat exchangers are widely used in central heat supply stations and district heating systems due to the following advantages:

• Compact design – they take up minimal space, making them ideal for installation in tight or hard-to-access areas.
• High reliability – every unit is pressure tested before leaving the factory to ensure long-term performance and safety.
• Versatile use – while primarily designed for water-to-water systems, they can also be installed as steam exchangers in suitable applications.
• Low maintenance – SWEP exchangers are gasket-free, which significantly reduces service requirements compared to gasketed models.
• Additionally, turbulent flow between the plates provides a self-cleaning effect, minimizing fouling.
• Exceptional efficiency – thanks to their compact structure, nearly the entire surface area is active in heat transfer, making them extremely efficient and cost-effective.

Biogas Stations

Biogas plants generate biogas, which is then used as a source of energy. The main component of biogas is methane, produced through the breakdown of organic material by bacteria and other microorganisms. The biogas is burned in an engine to produce electricity, with typical station outputs ranging from 200 to 2000 kW

The electrical efficiency of gas combustion is generally 30 to 45 . As the engine heats up during operation, it must be cooled—usually with water or a glycol mixture. The resulting waste heat is recovered using a water-to-water plate heat exchanger, allowing it to be reused. This approach significantly increases the overall system efficiency. The recovered heat is used to:

• maintain the optimal temperature in the fermenter,
• provide space heating,
• or heat greenhouses and other thermal applications.

Solar Heating

During the summer months, solar radiation of up to 1000 W/m2 can be utilized. Solar panels typically operate with an efficiency of 60–70 %, allowing a well-designed system to capture approximately 500–600  W/m2 under favorable conditions. Output is naturally lower during winter.

SWEP plate heat exchangers play a key role in many solar thermal systems — transferring heat to hot water storage tanks, swimming pools, or domestic hot water systems. A circulation pump is essential for operation. Higher flow rates improve solar panel efficiency. To prevent damage during freezing conditions, the system should be filled with an antifreeze solution, most commonly ethylene glycol at 30–40 % concentration.

Conventional vs. Plate Heat Exchanger Design

In conventional systems with an internal coil heat exchanger, the water in the tank barely circulates. Over time, algae, rust, or biological contamination can accumulate, impairing heat transfer and hygiene. This reduces system performance and may raise concerns about water safety, especially in domestic hot water applications. Periodic cleaning of the tank is required.

In contrast, adding an external SWEP plate heat exchanger ensures turbulent water flow, which greatly improves heat transfer efficiency and offers a self-cleaning effect. The bulky internal coil is no longer necessary, and deposits are minimized.

Recommended Model Series

SWEP heat exchangers offer high thermal performance in compact dimensions, making them ideal for solar thermal applications. For heat transfer from solar panels, we recommend the SWEP E8 series. The number of plates is selected based on system flow rate. These models feature 3/4" ISO G external thread connections.

Cogeneration

Internal combustion engines typically operate at an efficiency of less than 40 %, with a significant portion of energy lost as waste heat via the exhaust. Cogeneration units recover this thermal energy and use it to produce domestic hot water (DHW) or space heating, significantly improving overall system efficiency. This is achieved by installing a heat exchanger, which transfers heat from the primary circuit (the engine) to the secondary circuit (DHW or heating system). With this setup, total thermal efficiency can reach 80 % or higher.

Heat Recovery

Industrial and commercial air conditioning systems often operate at pressures close to atmospheric, making small, compact heat exchangers less suitable than larger, specially designed units tailored for such applications. Despite this, SWEP brazed plate heat exchangers maintain a strong position in the HVAC industry, particularly in roles such as:

• Economizers
• Internal Heat Exchangers (IHX)
• Sub-coolers