Baffle Thickness for NTIW Type Baffle

It is "unsupported tube length".

For segmental baffles (single, double, etc...) this will be two baffle spaces, as the tubes in the window (or cutout) area only are supported by every other baffle.

For NTIW baffles, each baffle or support plate in between baffles supports all of the tubes. Therefore the support plates in between baffles count. The unsupported tube length would be the space between support plates.

In general, every tube span should be considered. These are tubesheet to baffle, baffle to baffle, baffle to support plate, and support plate to support plate. All of the baffles in the heat exchanger don't have to be the same thickness.