In a double pipe (concentric) heat exchanger, the hydraulic radius for heat transfer (for a fluid flowing through the annulus) would be
A
Same as that for fluid flow
B
Less than that for fluid flow
C
More than that for fluid flow
D
D<sub>2</sub> - D<sub>1</sub> (D<sub>1</sub> and D<sub>2</sub> are I.D. of inner and outer pipes respectively)
Correct Answer: More than that for fluid flow