Air to Air Heat Recovery for Granular Bulk Materials

A minerals and construction materials producer were developing a waste heat recovery scheme for the trommel dryers on its drying plants, as part of a wider decarbonisation programme.

The intent was gas to air recovery: extract energy from the dryer flue gas and feed preheated air back into the burner, reducing burner duty and fuel consumption.

Flue gas was available at approximately 170°C at a mass flow in the region of 10.5 kg/s.

The exhaust stream was heavily dust laden and carried abrasive particulate a challenging environment for any heat exchanger to withstand.

Two hard process constraints framed the design. Exhaust gas temperature had to stay above 85°C to retain buoyancy in the exhaust and protect upstream filter media. Preheated air returning to the burner had to stay below a defined ceiling to protect internal burner components.

Pressure drops on the exhaust side had to be minimised. Any meaningful increase would have forced replacement of the larger exhaust handling fans and risked the negative pressure the dryer relies on.

An obvious route was to place the heat exchanger downstream of the filters, where the gas is clean and abrasion ceases to be an issue. However, upon review it became clear that by this point much of the recoverable energy had already left the flue. Taking the recovery upstream was the only way to deliver a meaningful return.

That decision meant the heat exchanger had to be built to survive the dust and abrasion rather than avoid it.

A thick walled, all carbon steel construction was specified, giving the unit the material thickness to tolerate abrasive particulate over a long service life.

Finned tubes were used, but at a deliberately low fin density of three fins per inch, striking the balance between cost effectiveness for the customer and manageable external fouling.

The design was developed with an exhaust side pressure drop low enough to leave the existing fan arrangement and dryer pressure regime intact.

The thermal design held the flue gas above the minimum filter entry temperature and kept the preheated air within the burner's protection limit.

Support ran from initial site visit and survey through thermal design, quotation with payback modelling against the site's own gas unit cost, drawing approval and manufacture.

Turnbull & Scott supplied a thick-walled carbon steel finned tube heat exchanger for gas to air recovery on a drying plant, returning preheated air to the dryer burner and directly reducing gas consumption.

The unit recovers energy upstream of the filter, where high grade heat is available, while holding flue gas temperatures above the minimum entry temperature the filters and stack require.

Low fin density construction keeps external fouling manageable and the cleaning requirement practical on a highly polluted exhaust stream.