Air Preheat System is usually applied to increase a fired heater’s efficiency, and the economics of air preheating should be compared with other forms of flue gas heat recovery. Air preheat systems become more profitable with increasing fuel costs, with increasing process inlet temperature (i.e., higher stack flue gas temperature), and with increasing fired duty.
Economic analysis of preheater
The economic analysis should account for the APH system’s capital costs, operating costs, maintenance costs, fuel savings and the value (if any) of increased capacity. In the case of a system retrofit, the economic analysis should include the cost of incremental heater downtime for the preheat system installation. In addition to economics, the system’s impact on the heater’s operations and maintenance should also be considered. Compared to natural draft systems, air preheat systems typically provide the following operational advantages:
a. Reduced fuel consumption.
b. Improved control of combustion air flow.
c. Reduced oil burner fouling.
d. Better flame pattern control.
e. More complete combustion of difficult fuels.
Air preheat systems typically have the following operational disadvantages (vs. natural draft systems):
a. Increased radiant section operating temperatures (coil, film, supports, etc.).
b. Increased potential for corrosion of flue gas wetted components downstream of the preheat exchanger, from sulfuric acid condensation.
c. Formation of acid mists, resulting in stack plume, if fuel sulfur content is high.
d. Increased maintenance requirements for mechanical equipment.
e. Increased nitrogen oxide concentration in the flue gas.
f. Reduced stack effluent velocity and dispersion of the flue gases.
In all applications, the use of an air preheat system will increase both the heater’s firebox temperature and radiant flux rate(s). Because of these hotter operating conditions, a thorough review of the heater’s mechanical and process design under APH operations should be performed on all retrofit applications. The hotter firebox temperatures could result in overheated tube supports, guides, tubes, and/or unacceptably high process film temperatures.
In some cases, an air preheat system may provide an increase in fired heater capacity or duty. For example, when a fired heater’s operation is limited by a large flame envelope or poor flame shape (flame impingement on tubes) or by inadequate draft (flue gas removal limitations), the addition of an air preheat system may increase the heater’s capacity.
Based on the flue gas and air flow through the system, the three system types are:
a. Balanced Draft APH Systems (most common type).
b. Forced Draft APH Systems.
c. Induced Draft APH Systems.
The common “balanced draft” system has both a forced draft (FD) fan and an induced draft (ID) fan. The system is balanced because the combustion air charge, provided by the forced draft fan, is balanced by the flue gas removal of the induced draft fan. In most applications, the forced draft fan is controlled by a “duty controller” that is reset by the heater’s O2 analyzer and the induced draft fan is controlled by an arch pressure controller.
In comparison, the simpler “forced draft” system has only a forced draft fan to provide the heater’s combustion air requirements. All flue gases are removed by stack draft. Because of the low draft generation capabilities of a stack, the exchanger’s flue gas side pressure drop must be kept very low, thus increasing the size and cost of the APH exchanger.
The third and last designation based on fluid flow design is the “induced draft” system, which has only an induced draft fan to remove flue gases from the heater and maintain the appropriate system draft. Combustion air flow is induced by the sub-atmospheric pressure of the heater. In this application, the exchanger must be carefully designed to minimize the combustion air pressure drop while providing the necessary heat transfer.
A typical balanced draft APH system, employing a direct exchanger, is illustrated in figures below.
Air Preheat System Using Regenerative, Recuperative, or Heat Pipe Air Preheater
Air Preheat System Using an Indirect Closed Air Preheater and Mechanical Circulation
