Why are induced draft fans typically larger than forced draft fans?

Induced draft fans are typically larger than forced draft fans primarily because they are designed to handle a mixture of air and combustion products, such as flue gases, which are generated during the combustion process. This design consideration necessitates a larger fan size to accommodate the increased volume and density of these gases, as well as to ensure that the system can efficiently remove them from the combustion area.

In contrast, forced draft fans are primarily responsible for supplying fresh air to the combustion process. These fans deal mostly with ambient air, which is less dense and requires less power to move compared to the mixture of gases handled by induced draft fans. Thus, while both types of fans are critical to combustion systems, the requirements for handling combustion byproducts result in the need for larger induced draft fans.

This fundamental difference in their operational purpose explains why the statement regarding the handling of a mixture of air and combustion products is the most accurate.