Heat Recovery from a Contaminated RTO Exhaust for a Coating Process

A technology provider delivering a regenerative thermal oxidiser package needed to recover waste heat from the RTO exhaust and turn it into 90°C process hot water for the receiving site.

The available duty was substantial: exhaust air at around 200°C at a mass flow of 9.4 kg/s, with roughly 0.84MW of recoverable energy.

The application was not a clean one. Carry through from a lacquer process meant the exhaust gas contained contaminants that deposited a white residue on heat transfer surfaces.

A finned tube heat exchanger supplied by others on an earlier project had fouled significantly in service, and the problem worsened as the exhaust gas began to condense across the unit.

The customer's own enquiry was explicit on the point. They asked for removable banks so the air side tubes could be cleaned, rather than asking simply for a thermal duty.

The unit was destined for a site in mainland Europe, so the design also had to satisfy a defined technical specification, a minimum 15-year operational life, and a fixed collection date for onward shipment.

Turnbull & Scott treated cleanability as a primary design driver alongside thermal performance, rather than as a secondary consideration.

Bare tubes were selected in place of finned tubes. The reduction of overall surface area was accepted in exchange for a geometry that is far easier to clean and less inclined to trap deposits in the first place.

The decision was made to split the heat exchanger into four cassette style coils housed within a common casing, so that every tube in the unit remains physically accessible for cleaning throughout its life.

The cassette style arrangement coupled with a clear set of instructions for safe removal & reinstallation of the cassettes further aided the ease of the cleaning process.

A minimum flue gas outlet temperature of 110°C was agreed. It would have been theoretically possible to extract more energy from the stream, but this would risk reaching the condensation point of the exhaust gas, significantly worsening the fouling issues. Capping the outlet temperature traded a small amount of theoretical recovery for a large gain in practical reliability.

Pillow plate construction was also presented as an alternative for cases of more extreme fouling, with an honest assessment that the application in hand did not warrant the additional cost of this technology.

Turnbull & Scott supplied a bare tube gas to water heat exchanger recovering approximately 0.84MW from a contaminated RTO exhaust, delivering 90°C process water to the receiving site.

Predicted pressure drops on both the tube side and the air side remained within the limits set by the customer's specification.

The end user has full access to every tube in the unit, so cleaning is a routine maintenance task rather than a strip down.

The controlled minimum outlet temperature addresses the condensation driven fouling that had shortened the life of the previous installation.

The project moved from first enquiry to purchase order in just over two months, on a technically demanding specification with a fixed shipping date.