Howden Roots RAS Whispair, Installation, Operation & Maintenance Manual

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ILRB-3000 GEA20261
RAS and RAS-J WHISPAIR Blowers
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Operating Characteristics
The Roots air blowers described in this manual represent
both the basic type of Roots Anti-friction System (RAS)
rotary lobe arrangement and the proprietary Roots RAS
WHISPAIR design, with gear diameters ranging from 10 to
20 inches. All units are equipped with an effective splash oil
lubrication system.
The Roots RAS Blower
The Roots RAS rotary lobe blower is a positive
displacement unit; its pumping capacity is determined by
size, operating speed, and inlet conditions. It has two
double-lobe impellers mounted on parallel shafts and
rotating in opposite directions within a cylinder closed at the
ends by headplates. As the impellers rotate, air is drawn
into one side of the cylinder and forced out the opposite
side against the existing pressure. The differential pressure
developed depends on the resistance of the connected
systems.
Effective sealing of the blower inlet area from the discharge
area is accomplished using very small operating
clearances. This feature significantly reduces rubbing
contact between rotating and stationary parts, removing the
need for internal lubrication. Clearances between the
impellers during rotation are maintained by a pair of
accurately machined timing gears mounted on the two
shafts that extend outside the air chamber.
Operation of the RAS rotary lobe blower is illustrated in
Figure 1, where air flow is from right to left from inlet to
discharge, with the lower impeller rotating clockwise. In
Position 1, it delivers a known volume (A) to the discharge,
while the space (B) between the upper impeller and
cylinder wall is being filled. Counterclockwise rotation of
this impeller then traps an equal volume (B) in Position 2,
and further rotation delivers it to the discharge in Position 3.
At the same time, another similar volume is forming under
the lower impeller, and will be discharged when rotation
reaches Position 1 again.
One complete revolution of the driving shaft alternately
traps four equal and known volumes of air (two by each
impeller) and pushes them through to the discharge. The
pumping capacity of a lobe blower operating at a constant
speed therefore remains relatively independent of
reasonable inlet or discharge pressure variations. To
change the capacity it is necessary to change the speed of
rotation or vent some of the air.
The Roots RAS WHISPAIR
Operation of the proprietary Roots RAS WHISPAIR blower
is represented in Figure 2. Just as in the standard Roots
RAS blower, the air flow is from right to left, with the lower
shaft rotating clockwise. This design differs from the basic
RAS rotary blower in that it provides a chamber on the
discharge side of the cylinder.
From this chamber, two or more slots open back into the
two alternately closed pocket areas of the cylinder (shown
as A and B). These slots, at certain impeller positions,
allow discharge pressure to bleed into the normally low
pressure pockets. They also provide a jet action on the
impellers in the direction of rotation. Gradual pressure
build-up in the pockets, to a level almost equal to the
discharge pressure, reduces the backflow rate at the
instant of pocket discharge, effectively minimizing pulsing
and shock noise.
As shown in Position 1 in Figure 2, the known volume (A) is
delivered to the discharge chamber while the space (B) is
filled with an equal volume at inlet pressure. In Position 2
the space has been sealed from an inlet, but some
discharge pressure is entering the space (B) through the
slot passage, as indicated by the small arrows. The force of
the jet assists the rotation of the impeller, while also
building pressure in the sealed space (B). Rotation
continues to Position 3, where the volume (B) is delivered
to the discharge chamber in the same manner as the
volume (A) in Position 1. Because of the almost complete
pressure equalization through the slot, shock is minimized.
Notes and Warnings
With either blower design, no attempt should ever be made
to control capacity by means of a throttle valve in the intake
or discharge piping. This increases the power load on the
driver and may seriously damage the blower. Similarly, if a
possibility exists that flow to the blower inlet may be cut off
during normal operations, then an adequate vacuum relief
valve should be installed in the inlet line near the blower. A
pressure relief valve in the discharge line near the blower is
also strongly recommended for protection against cut-off or
blocking in this line.
When a belt drive is employed, blower speed can usually
be adjusted to obtain the required capacity by changing the
diameter of one or both sheaves. In a direct-coupled
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