Questions regarding convective heat transfer

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There is a problem related to convective heat transfer that remains unclear, and I hope everyone can discuss it: 1. In the design of the coil in the convection chamber of tubular heating furnaces, a counterflow arrangement is generally used, meaning that the flue gas enters from the bottom of the convection chamber and exits from the top, while the material to be heated enters from the top and exits from the bottom, thus enabling counterflow heat exchange； 2. If the convection chamber is designed for heating boiler water to generate steam, heat transfer occurs in a co-current manner, that is, the boiler water enters from the bottom of the chamber while steam exits from the top. A typical design is that of a waste heat boiler at the top of a reforming heater. My question is: 1) From the perspective of heat exchange principles, counterflow heat exchange should be the most efficient, so why is forward flow heat exchange used when generating steam? Is it to prevent the coil from overheating? 2) Some designs of tubular air preheaters also employ cross-flow heat exchange; for example, Figure 14-15 on page 558 of the latest edition of \"Tubular Heaters\" illustrates this. I’m even more baffled. I hope everyone can discuss it.