Cabin/Box heaters (either with vertical coil arrangement or helical coil arrangement) can be provided with an additional heat recovery system (to preheat the combustion air) to improve overall efficiency thus reducing the fuel consumption.
The most common application of air preheating system is based on the use of a heat exchanger (commonly called air preheater) to transfer heat from the flue gas exiting from the fired heater to the combustion air. The increase of the heater efficiency is achieved by lowering the flue gas temperature leaving the system.
In addition to the air preheater the system requires different components: 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 the 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 system are typically applied to heaters with duties higher than 20 MW. Almost all the existing heaters can also be fitted with air preheat systems as soon as a fuel saving analysis justifies the investment and an adequate process feasibility study is carried out.