VAV-107 - page 11

11
Figure2
Figure3
Figure4
adjustable frequency and even as lowas
60%of full load, the savings is 5%.
It should be noted that since most vari-
able volume systems operate at their de-
sign point only a few hours per year,it is
common, and good practice to select the
controllable pitch fan to the right of its
most efficient operatingarea.As the sys-
temturnsdown, the fanwill run in thenor-
mal system operating range at a high
efficiency level.
Also,most variablevolumesystemshave
fixed resistances that donot varywith the
volume of air being supplied.When the
system requires a high turn down, care
must be taken in fan selection when ad-
justable speed control is used so the fan
is not forced into an unstable operating
point at lowair flow.
Themost commonuseof thecontrollable
pitch fan ison thesystemcurvewhere the
systempressure varies as the square of
thevolume.Figure2 indicatessuchasys-
tem. The orange line indicates horse-
power consumed as the fan operates
along the system.Note the selectionwas
madeso thedesignpoint is to the right of
themost efficient operating area so that
evenat 75%of flow, the fanwill operateat
equal or higher efficiency than it does at
thedesign flow.
With a controllable pitch fan, you are not
limited to operating on the system as
notedabove.Figure3 indicates theoper-
ation of controllable pitch fan constant
static pressure system.With the fan de-
picted, you can operate on a constant
staticpressuredown to25%of thedesign
flow. Since the systempressure remains
constant, thehorsepower savingsarenot
asgreat as forFigure2, but still amount to
a substantial energy savings.
You can also apply the controllable pitch
fan to a systemwhen a constant volume
of air is requiredat varyingpressures.Fig-
ure4 is representativeof this typeof sys-
temand indicates theenergy that canbe
savedby using the controllablepitch fan.
1...,2,3,4,5,6,7,8,9,10 12,13,14,15,16
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