Handbook of air conditioning and refrigeration / Shan K

20.38 CHAPTER TWENTY
FIGURE 20.15 (Continued)
4. For the same fan-duct system, from the fan law, the fan speed required at reduced volume flow
of 10,000 cfm (4719 L/s) is
Fan power input at a fan speed n 2 � 850 rpm is
V˙
2
n 2 � n 1� V˙ � 1 � 1700 � 10,000
� 850 rpm
20,000�
n 2
P2 � P1� n � 1 3
� 21 � 10,000
20,000� 3
� 2.63 hp (1.96 kW)
The difference in fan power inputs between using damper and variable-speed drive fan
modulation is the power consumption at the damper: 15.2 � 2.63 � 12.5 hp (9.32 kW).
At reduced fan speeds, fan total efficiency may be reduced. Because of the degradation of
motor efficiency and ac inverter loss at part-load operations actual power input at 50 percent
of part-load operation for variable-speed drive modulation is far higher than 2.63 hp
(1.96 kW).