Magnetic vs Depth Side Stream Filtration

Magnetic filtration relies on the principle of high-intensity neodymium magnets positioned within the flow path to attract and retain ferromagnetic particles. In UK heating systems, the primary constituent of 'sludge' is magnetite (Fe3O4), a byproduct of internal corrosion. Because magnetite is highly magnetic, these filters are exceptionally efficient at capturing even sub-micron particles that would pass through standard strainers. The primary advantage here is the negligible pressure drop (typically <0.1 bar) and the fact that the filter does not 'blind' or block in a way that restricts system flow.

However, the limitation of a purely magnetic approach is its specificity. While it addresses the most common cause of heat exchanger fouling, it remains passive toward non-magnetic materials. Elements such as calcium carbonate scale, pipe scale, aluminium oxides, and biological debris will remain in suspension. For this reason, magnetic units are often used as an initial stage of protection or in conjunction with other technologies to provide a comprehensive water quality solution.

Depth filtration involves passing a portion of the system water through a porous medium—typically a wound yarn cartridge, a pleated element, or a sand/glass media bed. This mechanism captures particles throughout the thickness of the medium, not just on the surface. Unlike magnetic units, depth filters provide 'absolute' or 'nominal' filtration based on micron size, meaning they are capable of removing any suspended solid larger than the pore size, regardless of its magnetic properties. This is vital in new builds where construction debris, such as solder and silica, is prevalent.

The operational challenge with depth filtration is the progressive increase in differential pressure. As the filter captures more debris, the resistance to flow increases, eventually requiring a cartridge change or a backwash cycle. In a side stream configuration, this is managed by a dedicated pump and controller on a skid, ensuring that the main system flow is never compromised. These systems are often specified in accordance with BSRIA BG29/21 for pre-commission cleaning and BG50 for ongoing maintenance to achieve the required water clarity.

For large-scale commercial plant rooms, the debate between magnetic and depth filtration is often resolved by utilising an integrated side stream filtration skid. These units typically combine a high-intensity magnetic rod with a secondary fine-filtration cartridge or bag. By placing these on a dedicated skid with its own pump, engineers can ensure a constant 'polishing' of the water, independent of the main system's varying flow rates. This is particularly important in variable volume systems (VVT) where low flow periods might otherwise lead to particle settlement in dead legs.

The performance of these skids is measured by their ability to achieve 'the five volumes rule'—circulating the equivalent of the total system volume through the filter five times every 24 hours. A UKGP side stream filtration skid is designed to meet these requirements, providing a robust, plug-and-play solution that encompasses air separation, magnetic capture, and fine particle removal. This multi-stage approach ensures that the water meets the stringent conductivity and turbidity levels required by modern boiler and chiller manufacturers.

The ultimate goal of side stream filtration is the protection of the most vulnerable components in the system: the heat exchangers. Modern UK plant rooms frequently utilise a UKGP plate heat exchanger to separate primary plant from secondary distribution. These units feature extremely narrow channels to maximise turbulent flow and heat transfer efficiency. However, these same channels act as a filter for system debris. Even a thin layer of magnetite (0.5mm) can reduce heat transfer efficiency by up to 10%, leading to increased burner cycling and higher energy costs.

By implementing rigorous side stream filtration, FM managers can prevent the settlement of solids within the plate packs. This not only maintains the design temperature differential (Delta T) but also prevents under-deposit corrosion—a phenomenon where oxygen-depleted zones beneath a layer of sludge lead to rapid pitting of the stainless steel plates. Comparing the cost of a filtration skid against the cost of stripping and acid-cleaning a large plate heat exchanger demonstrates a clear ROI, typically within 18 to 24 months of operation.

The selection between magnetic and depth filtration, or a combination of both, should be dictated by the system's age, metallurgy, and water volume. For an older, existing steel-piped system with high levels of established corrosion, a magnetic-heavy approach is required to handle the high volume of magnetite. Conversely, in a new build with plastic or copper pipework and aluminium heat exchangers, depth filtration is the priority to remove non-ferrous manufacturing residues and protect delicate components.

Engineers must also consider the 'human element' of maintenance. A standalone magnetic filter that is never cleaned becomes a dead asset. An automated skid with a BMS interface provides the necessary oversight, alerting the FM team when a filter change is required based on actual debris loading (differential pressure) rather than a calendar date. For critical infrastructure like data centres or hospitals, the redundancy and monitoring capabilities of a formal side stream skid are mandatory.

Compliance with BSRIA BG50 is the benchmark for water quality in the UK. This standard emphasizes that water treatment is a multi-faceted process involving mechanical filtration, chemical inhibiton, and regular monitoring. While side stream filtration removes the physical contaminants, it must be supported by a UKGP chemical dosing pot to ensure that the chemical balance—specifically molybdate or nitrite levels and pH—remains within the recommended parameters. Without proper chemical inhibition, no amount of filtration can stop the ongoing generation of new corrosion products.

In summary, while magnetic filters are a highly effective tool for capturing the most common form of debris in UK systems, they are not a universal panacea. For a system to remain efficient, reliable, and compliant with modern standards, a holistic approach that combines the strengths of magnetic capture with the broad-spectrum removal capabilities of depth filtration is the technical gold standard. This ensures the protection of high-efficiency plant and the long-term integrity of the hydronic circuit.