VSD control versus IGV control for a Centrifugal Fan

Centrifugal fan characteristic curves are shown in the Fig 2.1 below. Original design and operating
conditions are tabulated. The fan is currently required to operate only at 50% capacity most of the
time. It is provided with IGV for capacity control. Find out appropriate IGV % opening for this
condition.
2(a) Read pressure (head) and power consumption at this capacity. (2 marks)
2(b) Calculate head and power using fan laws. (2 marks)
2(c) Compare numbers with estimated power using fan laws. (2 marks)
Several literature plot show variations of fan power with different control mechanism. Two such
plots are shown in Fig 2.2.
2(d) Plot your estimates of % power on these two plots and compare it with VSD power value
from these plots (using the graphs, estimate the %power input(or % full load) for IGV, VSD
and fan law based on the capacity mentioned above) using figures 2.2.
(2 marks)
Variation in the power consumption with VSD may not match with the estimates from fan laws
which does not account for variation in efficiency because part load efficiency of fan as well as VSD
varies significantly as seen from the plots below (statement and graph just below is for your
reference).