Heat Exchangers

What Is It?

A heat exchanger is a device that transfers heat between two fluids without mixing them. It is one of the most common pieces of equipment in industry — used in heating, cooling, condensation, evaporation, and heat recovery applications.

How Does It Work?

Heat exchangers work by bringing two fluids into close thermal contact through a separating wall (tube wall, plate). Heat flows from the hotter fluid to the colder fluid by conduction through the wall and convection at each surface. The effectiveness depends on the heat transfer area, the temperature difference, and the heat transfer coefficient.

Common Failures

• Tube/plate fouling: Scale, biofilm, or deposits on heat transfer surfaces reduce efficiency.
• Tube leakage: Corrosion, erosion, or vibration causes tube perforation — cross-contamination of fluids.
• Gasket failure (plate type): Gaskets age, harden, or are damaged — causes external or internal leakage.
• Corrosion: Oxygen pitting, chloride SCC, or galvanic corrosion thins tubes and shells.
• Erosion: High-velocity flow or particles erode tube inlets and U-bends.

Maintenance

• Cleaning: Clean tubes or plates on schedule based on fouling rate — chemical, mechanical, or hydroblasting.
• Tube inspection: Perform eddy current testing annually on critical tubes to detect wall thinning.
• Gasket replacement: Replace all gaskets when reassembling plate heat exchangers — never reuse old gaskets.
• Water treatment: Maintain scale inhibitor, corrosion inhibitor, and biocide on the water side.
• Performance monitoring: Trend approach temperature — an increase indicates fouling.

Typical Lifespan

Shell-and-tube heat exchangers can last 20–30+ years. The shell typically outlasts the tubes — retubing extends the life indefinitely. Plate heat exchangers typically last 15–20 years; plates may need replacement after 10–15 years. Gaskets need replacement every 3–5 years.

Selection Criteria

• Duty: Determine the heat transfer requirement (kW) and the LMTD (log mean temperature difference).
• Type: Shell-and-tube for high pressure/temperature/fouling. Plate for compact, high-efficiency, clean fluids.
• Fluids: Select tube/plate material compatible with both fluids and temperatures.
• Fouling factor: Add a fouling factor to the design to account for expected scaling.
• Pressure drop: Balance heat transfer area against allowable pressure drop on both sides.