Enter the total area of all the untubed lanes on the tubesheet. If there is no pass partition lane then this area is zero. See the figure below for a single pass exchanger, this area is UL1 * Do.
The maximum limiting value of AL is 4*Do*p.
Where,
Do = Equivalent diameter of outer limit circle.
p = tube pitch
UL1 = Distance between innermost tube hole centers (width of pass partition lane)