Optimising Plate Heat Exchanger Specification for Process Cooling

The selection of a plate heat exchanger for process cooling is fundamentally driven by the Number of Transfer Units (NTU). In systems where the temperature cross is tight—such as free-cooling loops or chilled water buffers—high-NTU plates with a shallow chevron angle are specified to increase residence time and thermal length. Conversely, low-NTU plates with steep angles are utilised where low pressure drop is prioritised over extreme temperature shifts.

In UK industrial applications, designers must balance the 'thermal length' against the available pumping head. A common error is over-specifying the surface area without accounting for the resulting pressure drop, which can exceed the capacity of standard secondary pumps. Engineers should target an optimal pressure drop across the PHE of between 30 and 70 kPa for most process cooling loops to ensure efficient flow rates without excessive energy consumption from the pumps.

The choice between gasketed and brazed units often dictates the long-term O&M (Operation and Maintenance) costs of the plant room. Brazed units are the industry standard for heat pump evaporators and small-scale process cooling due to their compact footprint and high pressure rating (often up to 40 bar). However, they are a 'consumable' item; once fouled internally, restore-to-prime condition is difficult to achieve.

For larger district cooling or heavy industrial processes, gasketed units are preferred. The ability to add or remove plates allows the system to 'evolve' if production demands increase. Furthermore, the use of EPDM gaskets (up to 160°C) or NBR (for oil-based processes) provides a reliable seal that can be replaced during scheduled shutdowns, significantly extending the lifecycle of the asset compared to brazed alternatives.

Plate heat exchangers are highly sensitive to particulate matter. Because the fluid channels are narrow, even minor debris can cause significant 'dead zones' on the plate surface, leading to localised overheating and reduced kW output. To mitigate this, UKGP Industrial recommends the installation of high-velocity air and dirt separators on both the primary and secondary circuits.

In process cooling loops where the primary side is an open cooling tower, the risk of scaling and organic fouling is high. Here, the PHE acts as a protective barrier for the process equipment (such as expensive moulding tools or server jackets). However, the PHE itself must be protected. Incorporating side-stream filtration and automated chemical dosing ensures that the heat transfer surfaces remain clean, maintaining the designed approach temperature and preventing the 'creep' in energy costs associated with fouled exchangers.

Corrosion is the primary failure mode for PHEs in process cooling. Pitting corrosion, often caused by high chloride levels in the fill water, can lead to internal cross-contamination between the process and cooling fluids. Engineers must consult the site water analysis before specifying plate materials. While 304 stainless steel is common in HVAC, it is generally avoided in industrial process cooling in favour of 316L, which offers superior resistance to chloride-induced stress corrosion cracking.

The gasket material is equally critical. For most cooling applications involving water or ethylene/propylene glycol, EPDM is the standard choice due to its excellent resilience and temperature range. However, if the process fluid contains lubricants, oils, or certain fatty acids, NBR gaskets must be used to prevent swelling and eventual seal failure. If the system involves high-purity water (DI water), the gaskets must be peroxide-cured to prevent leaching of carbon into the process stream.

Correct installation is critical for the performance of a PHE in a process cooling environment. Units should be isolated with full-bore valves to allow for maintenance without draining the entire system. Pressure gauges and temperature sensors (PT100 or 4-20mA) should be installed on all four ports. Monitoring the differential pressure (ΔP) across the PHE is the most reliable way for Facilities Managers to identify fouling before it impacts production.

Maintenance of gasketed units involves periodic tightening to the manufacturer's minimum dimension. Over-tightening beyond the specified nameplate 'A' dimension will deform the plates and damage the gaskets. For brazed units, a periodic chemical CIP (Clean-In-Place) using a mild acidic de-scaler can help maintain efficiency, provided the chemicals are compatible with the copper or nickel braze material. If efficiency cannot be restored, the compact nature of UKGP Industrial's PHEs allows for rapid replacement with minimal downtime.

To accurately size a PHE for a UK process cooling application, engineers must provide five key variables: the heat load (kW), the inlet/outlet temperatures for both the hot and cold sides, and the allowable pressure drop. Any uncertainty in these figures typically leads to 'safety factors' that result in oversized heat exchangers. An oversized PHE can be just as problematic as an undersized one, as low flow velocities lead to increased sedimentation and fouling.

In modern heat pump-led process cooling (simultaneous heating and cooling), the PHE must be sized for the most arduous condition—typically the summer peak load. Utilizing variable speed drives (VSD) on the secondary pumps ensures that flow rates match the cooling demand, but the PHE must still be robust enough to handle the maximum design flow without exceeding the 5 m/s velocity limit at the ports, which would otherwise lead to erosion of the plate material.