Dresser Roots DVJ WHISPAIR, Installation Operation & Maintenance

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DVJ WHISPAIR
™
Dry Exhauster | 3
Operating Characteristics
DVJ WHISPAIR
™
dry vacuum exhausters are covered in this
manual. In size they range from 10 inches through 20 inches
gear diameter. All are equipped with an effective splash oil
lubrication system. From a principle usage standpoint they
are designated as air blowers.
The DVJ rotary lobe blower is a positive displacement type
unit, whose pumping capacity is determined by size, operat-
ing speed and inlet conditions. It employs 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, therefore,
depends on the resistance of the connected systems.
Effective sealing of the blower inlet area from the discharge
area is accomplished by use of very small operating clear-
ances. (This feature also eliminates rubbing contact between
rotating and stationary parts, hence, internal lubrication is not
required.) Clearances between the impellers during rotation
are maintained by a pair of accurately machined timing
gears, mounted on the two shafts extending outside the air
chamber.
The DVJ blower is used in vacuum service and uses the
same principle as the RAS WHISPAIR
™
design except the
discharge and slots feeding air back into the cylinder closed
pocket areas all have individual plenums. In vacuum opera-
tion, the discharge pressure differs from atmospheric only
by the backpressure in the discharge system. Atmospheric
pressure air flows into one cylinder slot plenum supplying air
to the closed Pocket B as shown in Position 2 of figure 1 and
through the cross-over pipe (DVJ jet air flow diagram-above
figures) to Pocket A. The atmospheric air, flowing to the
closed pockets, is at a much lower temperature than the
discharge air returned to the pockets in RAS WHISPAIR
design. The cooling effect allows blower operation at higher
vacuum levels without exceeding the blower’s maximum
temperature rise. For closed loop DVJ applications, as shown
in Fig. 4, a portion of discharge gas is recycled back through
a cooler to the cylinder slot plenum to give similar cooling
effects.
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. Likewise, if a possibility exists
that flow to the blower inlet may be cut off during normal
operation of a process, then an adequate vacuum relief valve
should be installed in the inlet line near the blower. A pressure
type 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 desire capacity by changing the diameter
of one or both sheaves. In a direct coupled arrangement, a
variable speed motor or transmission is required, or air may
be vented through a manually controlled unloading valve and
silencer. If discharge air is returned to the blower inlet, it must
go through a cooling by-pass arrangement.
Before making any change in blower capacity or
operating conditions
, contact the nearest Sales Office for
specific information applying to your particular blower. In all
cases, operating conditions must be maintained within the
approved range or pressures, temperatures and speeds as
stated under LIMITATIONS. Also, the blower
must not be
used to handle air containing liquids or solids . Serious
damage to the rotating parts will result.
Figure 1 - DVJ WHISPAIR
™
Operating Principle
Position 1
A
B
Position 2 Position 3