In a shell and tube heat exchanger having square pitch, the shell side equivalent diameter is given by $($where$,$ P $=$ pitch, d $=$ outside diameter of the tube$)$

$4($P$2-$ #$960$;d$2/4)/$d

$($P$2-$ rd$2/4)/$rd

$4$P$2/$d

d$/4$P$2$