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By Robert N. Britz,
President
Part I: High Temperature Fans
Fans for heat treating furnaces are fundamentally divided
into two general classes - centrifugal and axial. In part I of this
article the characteristics of fans will be discussed, typical fan curves shown
and a table of basic fan laws provided to aid furnace designers or fan engineers
in the selection or operation of fans. Part II will cover furnace
considerations.
In selecting a fan, consideration is usually given to the cfm
or volume of flow in cu ft per minute, static pressure (S.P.), usually in inches
of water, which represents potential energy and the necessary horespower (H.P.).
The relationship of these factors is shown by typical performance charts, Figs. 1,
2 and 3,
and by basic fan laws, Table
I. Table
II presents some useful conversion data.
Centrifugal Wheels (Flow discharge at right angles to axis of rotation)
Multiblade forward curve wheel, Type "MI"
fans. This type of wheel is usually specified where
there is a necessity for large volumes of flow against low to medium static
pressure requirements. examination of the typical forward curve fan
diagram, Fig. 1,
shows a characteristic dip in the static pressure curve which could result in
the fan delivering different air volumes at the same static pressure. This
may cause a resulting air pulsation, if the total pressure requirement of the
system was estimated too low. (Type "MI" I.G.E. fan curve chart
data is based only on the part of the S.P. curve which has a stable range -
approximately 50% to 100% of the cfm).
The major advantage of the forward curve blade is that it
delivers the same volume of air as either the backward or radial blade wheel, at
a lower fan speed, which is important due to wheel stresses at elevated
temperatures. This fan is the most common type used in all types of heat
treatment operations except where a buildup of material on the blades could
occur, as wheel unbalance would result.
Radial Blade, paddle wheel type "PI" fans.
This type of wheel is usually specified where there is a necessity for
medium volumes of flow against medium to large static pressures.
Examination of the typical radial blade fan diagram, Fig.
2, shows a relatively stable static pressure curve. (Type
"PI" I.G.E. fan curve chart data is based only on the part of the S.P.
curve which is stable - approximately 40% to 100% of the cfm).
The paddle wheel is considered a self cleaning type of
wheel. It could be used where there are various materials or dust in the
air stream. Due to its structurally strong design, it is used in many heat
treatment applications at very high temperatures where maximum fan longevity is
desired.
Axial Blade Wheels (Flow discharge parallel to axis rotation) Type
"F" and "AX" fans
The axial blade wheel is usually specified where there is
a need for very large volumes of flows against low static pressure.
Examination of the typical axial fan diagram, Fig.
3, shows a slight dip in the static pressure curve, which like the
multi-blade forward curve type wheel could cause an axial fan to deliver
different air volumes at the same static pressure, if the total pressure
requirement of the fan system was underestimated. (Type "F"
I.G.E. fan curve chart data is based on a S.P. which is fairly stable throughout
its range, but when selecting "AX" type wheels from our fan flow
charts, the stable range is approximately from 55% to 100% of the cfm).
This type of fan is ideally suited where a large volume of
flow is required with a minimum of duct work and is used extensively for small
and large furnaces, such as aluminum annealing.
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