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[Deep24]

in almost every stack, there is a helical type metal frame kept in the outside of the stack. why this is given?

In forced draft furnace stack, VFD is mounted at the bottom of stack. Then how stack height is decided?

[smalawi]

Hi,

this is called a " wind breaker" and it should break and wind force to ease the stress on the stack structure and foundation. it reduces the drage force similar to the golf ball concept. somtimes a stiffing ring is also used for adding stregnth to the structure.

not every stack needs it and in some high columns its also installed for the same reason. you can check with mechanical form to see thow these are desinged / adopted.

the stack hight is designed based on the hot gas bounacy theory. hot gas go up and develops low pressure in the bottom (called draft pressure measued in in water) as it raises up. the hotter the gas the lower density it has and higher it goes. the higher the stack the higher the gas maines it bouancy and more draft (slight negtive pressure) it achives. the stack hight is calcualted to achive the required stack bottom pressure. for a furance this is back calcualted to get the fruance arch pressure.

it should be noted that there are other factors that affect stack hgiht, the gas dispresion / minimum stack hight also sould be looked at.

the stack dia is calcuated from the quantity (mass flow) of hot gas to get a specifc target Dp/ per meter of stack hight, usually calcualtedfrom the econonics of dia / hight cost.

hope this helps,

sm