Conventional APH Tubes are hot-rolled or cold drawn with ends that are either welded or not joined. They are designed based on the types of fuel, the temperatures and the pressures that they will be subjected to. The tubes are built from ERW (Electric Resistance Welded) or SAW (Seamless) tubing in a wide range of diameters and grades. The welded or not joined ends are then joined together to form tubes containing the air heater and flue gas heat transfer surfaces.

The APH Tubes metal matrix is heated in the process of rotation through the APH flue gas duct or, as the case of Rothemuhle-type APHs, through the flue gas hood. As a result, the temperature of the APH metal increases from the minimum value as it enters the flue gas duct to the maximum value as it leaves the APH metal matrix and reaches the cold air stream.

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A major factor that influences the thermal performance of an APH is the capacity rate ratio (CR) of the APH. CR is defined as the ratio of the capacity rates (product of the APH air and flue gas flow rate) to the APH air/flue gas flow rate. The higher CR, the greater the thermal performance of the APH.

In most APH installations the CR of an APH can be significantly reduced by the use of the CABP and/or HAR. However, in practice, such solutions are only moderately effective at acid condensation avoidance and they reduce the effectiveness of the APH through reduction of the Log Meant Temperature Difference (LMTD). This causes most operators to have to bypass their APHs on a number of days per year to prevent acid condensate damage and consequently lose some of their recoverable energy.