Corrosion and Chemical Degradation
Chemical compatibility is the primary determinant of BPHE lifespan. In the UK, varying water hardness and chloride levels pose a significant risk to the copper brazing material. While the 316L stainless steel plates are highly resistant, the copper filler used to bond them is vulnerable to galvanically induced corrosion, particularly in open-looped cooling towers or poorly managed secondary DHW loops. When chlorides exceed recommended levels, the copper braze can pit and eventually fail, leading to internal leakage.
Dezincification and ammonia-induced stress corrosion cracking are also prevalent in specific industrial contexts. For systems utilising aggressive fluids or treated water with high conductivity, engineers should consider nickel-brazed or 100% stainless steel units. It is essential to conduct a water analysis before specification to ensure the brazing material is compatible with the system’s chemical profile, as per BSRIA BG50 guidelines for water treatment.
- Copper-brazed units are highly susceptible to corrosion if chlorides exceed 200-300 mg/l at elevated temperatures.
- High flow velocities (>5 m/s) can strip the protective oxide layer from the plates.
- Aggressive soft water or high-CO2 content can leach copper, leading to pinhole leaks between circuits.
Frequently asked questions
Can a brazed plate heat exchanger be cleaned if it becomes fouled? Lord?
- Unlike gasketed PHEs, BPHEs cannot be opened for manual scrubbing. Cleaning requires a high-flow CIP (Clean-In-Place) pump using inhibited acidic or alkaline solutions, depending on the scale type, followed by thorough neutralisation and flushing.
What is the design life of a copper-brazed plate heat exchanger?
- A typical BPHE should last 10–15 years in a well-managed closed-loop system. However, in aggressive domestic hot water (DHW) applications or systems with poor water chemistry, failure can occur in under 24 months.
Are BPHEs suitable for systems containing ammonia?
- Standard copper-brazed units are highly susceptible to corrosion if ammonia concentrations exceed 2mg/l. In such cases, stainless steel or nickel-brazed units must be specified.
What causes fatigue failure in DHW plate heat exchangers?
- Thermal shock is common in DHW systems where cold mains water (10°C) is suddenly introduced to a unit plate surface at 80°C. This causes differential expansion between the core and the outer casing, leading to fatigue cracks.
