Chemical Engineering - Heat Transfer - Discussion

The equivalent diameter of a double-pipe heat exchanger with an annular flow path is a theoretical diameter that represents the flow area of the annulus in terms of a circular pipe of equivalent cross-sectional area. The equivalent diameter for an annulus can be calculated as:
Deq = (4 * A) / P, Where A is the cross-sectional area of the annulus, and P is the wetted perimeter of the annulus.

If the inner pipe has fins on the outside, the cross-sectional area of the annulus will be reduced due to the presence of the fins.

This will result in a smaller equivalent diameter compared to the same size pipes without fins.

Therefore, the equivalent diameter for the annulus of a double pipe heat exchanger, whose inner pipe has fins on the outside, will be smaller than the equivalent diameter of the same size pipes without fins.

Equivalent diameter = 4 * hydraulic radius.
De= D2^2-D1^2/D1.
If it has fins inner diameter is larger.
Consider, Dout= 5cm ; Din = 4cm for with fins,
Dout= 5cm; Din = 3cm for without fins.,
Subs in the De formula.
ANS: B is the right option.

The answer should be A because the fins increase the cross sectional area and perimeter for flow is constant.