My guess would be 11" as your cylinder is only 36" high. If your element (like mine) is screwed onto the top (curved) surface I would imagine a 27 " long one would foul against the opposite side of the cylinder when fully inserted. My cylinder is approx 4 foot tall and that definately has a 27" element Is it possible for you to get a fairly accurate measurement following the line of insertion, if you see what I mean.
HTH
John