Note that the motor was nominally rated at 50 Hz A/C to run 1440 rpm. We adopt
the null hypothesis and must say there is no proof that paddlewheel A performed
better than B, although the standard errors of Table 4 do not quite overlap. The
null hypothesis is strictly adopted because t-tests of Table 5 show no significant
difference. Conversely, all indications are that there certainly is a significant
improvement in SOTR comparing paddlewheel C to either of paddlewheels A or B
operating at full speed.
Adjusting the frequency of A/C power effectively controlled the paddlewheel
speed and resulted in significant differences in SOTR performance as tabulated in
Table 5. There was generally an increase in oxygen transfer rate with speed, but
paddlewheel A displayed a most distinct bump when paddling was increased from
76 to 82 rpm. This particular aerator was most efficient paddling at 87 rpm, with
an apparent drop as paddling increased to 103 rpm (50 Hz A/C).
It is hypothesised in the present paper that paddlewheels work better when
backsplash is avoided, which was visually observed when running faster than 76
rpm. Backsplash could be avoided by A/C frequency control or by fitting an 18:1
gearbox instead of the standard 14:1. It is believed that backsplashing caused
paddlewheels A and C to display a flattening of the SAE performance curves above
76 rpm.
Paddlewheels B also increased oxygen transfer rate with speed, but efficiency did
not display a significant trend. Perhaps this particular model reduced the backsplash
effect when operating above the critical speed. Perhaps the elastic behaviour
of paddlewheels A, B, and C are somehow different to such an extent that droplets are flicked off in different ways. But it is the opinion of the authors that some
experimental error caused paddlewheel B to behave strangely, especially during test
c3 at 87 rpm.