Located between Sichone and Masalai. Not a large waterfall but very nice.
Klongnai is the hidden waterfall between Noppitam and Phrom Khiri, and its not part of national park, so its a bit harder to find it than others.
You should drive left on the last sign, however my feeling was to drive right.
Waterfall itself is small (low water season), but its interesting to walk by its stream.