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Double value of the gap between circular concentric pipes:

d_h = d_{out} - d _{in}

where

	inner diameter of outer pipe
	outer diameter of inner pipe

It equals to Hydraulic diameter of the annular area.

d_h = \frac{4 \, A}{P} = \frac{\pi \, d_{out}^2 - \pi \, d_{in}^2}{\pi \, d_{out} + \pi \, d_{in}} = d_{out} - d_{in}