Question 8 of 8: Fan Control

Question 8: Fan Control (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

All three methods move the operating point — the intersection of the fan and system curves — to a lower flow, but they act on different curves and differ greatly in efficiency.

(a)(i) / (b)(i) Duct dampers. Closing a damper adds resistance to the ducting, so the system curve steepens (rotates upward about the origin), with the new operating point where it re-crosses the unchanged fan curve. The reduction is real but wasteful: the extra head developed by the fan is dissipated as an irreversible pressure drop across the damper, so power is only modestly reduced.

(a)(ii) / (b)(ii) Inlet vanes (pre-whirl). Guide vanes at the fan inlet impart swirl to the incoming air in the direction of rotation. This reduces the change of whirl across the impeller and hence the Euler work, $w=U\,\Delta C_w$, so the whole fan characteristic is pushed downward. The operating point moves down the (unchanged) system curve to a lower flow. Because the input work itself is reduced — rather than dissipated downstream — this is significantly more efficient than damper control at part load. (A small inlet velocity triangle shows the pre-whirl reducing $C_{w1}$'s deficit and therefore $\Delta C_w$.)

(a)(iii) / (b)(iii) Fan speed. Reducing the motor speed scales the fan curve by the affinity laws, $Q\propto N$ and $H\propto N^{2}$, sliding the whole characteristic down and to the left. The operating point follows the fixed system curve to a lower flow. Since fan power scales as $N^{3}$, this is the most efficient method — no throttling loss and the work input drops steeply with flow — and is the preferred approach where a variable-speed drive is available.