hello

im modeling a shell and tube heat exchanger (it's an economizer hot gas-air).i consider the mass flow for both air and fumes as condition of entry but for the outlet i dont know what to put. which pressure i have to consider? what i know is that the fumes will be extracted to the chimeny and the air (since its heated) will be distributed to the thermal oven in the level of burner

thanks in advance, have a nice day