Cylindrical heaters – either with vertical or helical coil arrangement – can be provided with an additional heat recovery system, to preheat the combustion air and improve the overall efficiency by reducing fuel consumption.
The most common application of an air preheating system is based on the use of a heat exchanger, commonly called air preheater (APH), to transfer the flue gas’ heat exiting the fired heater to the combustion air. The increase of the heater’s efficiency is achieved by lowering the flue gas temperature exiting the system.
The system requires different components such as: forced- and induced-draft fans; air and flue gas ducting; guillotines and dampers, along with special safety controls and instrumentation.
The use of an APH system to increase efficiency has to be evaluated in consideration of different aspects. Among those, the most important is the fuel cost that typically can justify the addition of an APH system. For this reason, APH systems are typically paired with heaters with duties higher than 20 MW.
Almost all the existing heaters can be paired with APH systems, whenever a fuel-saving analysis justifies the investment. However, this decision is subject to an adequate process feasibility study to properly verify the impacts that an APH may have on the entire system.