Energy cost reduction at an Automotive factory

Operational has joined forces with one of the world’s leading automotive companies with plants across the world to improve their energy efficiency.

In their factory in the Northwest of Spain, they operate 5 painting lines, each with a thermal oxidiser, for the treatment of solvents which must be eliminated prior to their discharge to atmosphere.

The opportunity was recognised to recover heat from the thermal oxidiser exhaust gases and to recycle & reuse this recovered heat in order to reduce overall gas consumption within the factory.

Operational Engineers identified that the recovered heat could be used in another ‘drying’ stage of the painting process where 17 air handling drying cabinets were currently using gas burners to dry car surfaces after painting at a controlled temperature of 24°C.

Operational designed and installed a Heat Recovery System that linked the exhaust stacks from the oxidisers with the fresh air inlets of the drying cabinets, by installing air to water heat exchangers into both sides of the process. The energy is transferred from the hot exhaust gases leaving the thermal oxidisers to the inlet make-up air entering the air handling cabinets – via a closed loop, hot water circuit.

The system was fitted with control dampers in order to modulate the airflow to and from the heat exchangers and 3 way valves in the water circuit controlled by the PLC system and designed to accurately control water temperatures dependent upon the air temperature requirements in the air handling drying cabinets.

During high ambient, daytime temperatures the Operational Heat Recovery System produces excessive energy which cannot fully be reutilised; whilst the lower ambient, night-time temperatures produce only part of the required process energy requirements. To ensure the System provides the optimum energy recovery & reutilisation, despite these temperature cycles, Operational have incorporated a water tank accumulator system of 50,000 litres capacity where the excess hot water recovered during the daytime period is stored and subsequently reutilised during the night-time.

A centralised PLC unit controls the complete system, where key function process details, such as air and water temperatures and flowrates, can be entered and system operations monitored and controlled.

Summarising the process operating conditions: – The exhaust gases leaving each thermal oxidiser have a flowrate of 15,000 Nm3/h and a temperature of 300oC. The heat is transferred from these hot exhaust gases to a hot water circuit with a flowrate of 75m3/h flowrate and a supply temperature of up to 95oC. This hot water is used to heat the supply air, of approximately 75,000 Nm3/h entering each of 17 air handling cabinets, to a temperature of 24oC, prior to introduction into a drying stage of the painting process.

The overall result is a saving of 12,000 MWh of gas per year, representing a reduction of 2,500 tonnes/year in CO2 emissions to the atmosphere.

The General Manger of the Plant says “we have achieved the objectives initially set for Energy Savings along with a significant positive Impact on the Environment, whilst ensuring the required temperature stability in our drying cabinets and painting process.”

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