I assume that you are considering replacing your ESP with an FFP to save on maintenance costs; however, they are not generally swap-out swap-in replacements for each other unless specifically designed to be so due to pressure drop and mass flow rate considerations. Contact the manufacturers of the boiler, ID fan and FFP as a start since each case is different.
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“Tell me and I forget. Teach me and I remember. Involve me and I learn.” Ben Franklin.