Heat exchangers (shell and tube heat exchanger)

A heat exchanger transfers thermal energy between two fluid streams without allowing them to mix. The fluids are separated by a metal wall — tubes, plates, or coils — across which heat flows from the hotter stream to the cooler one. In CBG plants and tyre pyrolysis operations heat exchangers are everywhere: heating incoming slurry to digester temperature, recovering heat from outgoing digestate, cooling pyrolysis vapours into liquid oil, and conditioning compressed biogas before storage.

Three construction types dominate Indian practice. Shell-and-tube exchangers route one fluid through a bundle of metal tubes and the other through the shell around them; they handle high pressures and temperatures and are the default for hot-oil heating in pyrolysis condensers. Plate heat exchangers consist of corrugated stainless-steel plates stacked with thin gaskets and alternating flow channels; they are compact, easy to clean, and well-suited to slurry-to-water heat transfer on biogas plants because plate spacing can be opened up to handle fibrous material. Spiral and double-pipe exchangers serve niche applications with viscous or fouling-prone fluids.

Performance is governed by the heat-transfer coefficient (U-value, typically 500-1,500 W/m²K for slurry-to-water service), the temperature difference between streams (Log Mean Temperature Difference), and the total heat-transfer area. A digester heating loop on a 5 TPD CBG plant typically uses 30-60 m² of plate-area to lift incoming slurry from ambient 15-25°C to mesophilic 35-40°C using waste heat from a CHP engine's jacket water at 85-90°C.

The two operational trade-offs are fouling and pressure drop. Slurry-side fouling from biofilm, struvite scale and undigested fibres can drop U-value by 30-50% within 3-6 months, requiring quarterly cleaning. Pressure drop through tight plate channels rises sharply with viscosity and limits throughput; oversizing the exchanger reduces fouling risk but raises capex. In tyre pyrolysis, condenser exchangers fouling with tar and waxes is a chronic issue, often requiring removable straight-tube designs that can be steam-cleaned in place. Specifying easy-clean, removable, oversized plate exchangers at the design stage typically saves multiples of the capex difference in operational reliability and uptime over the plant's life.