How can I get the convergent factor ( 360 ) for the minimum column diameter to design iron sponge bed ? using q = VA , A = pi/4 d^2 , V equal to 10 ft/s

dmia2=360PQzTZ where dmin= minimum required vessel diameter, in. Q5= gas flow rate, MMsefd T = operating temperature, R Z= compressibility factor P= operating pressare, psia