IndustrialHeating.com ■ MAY 2015 41Typically, vacuum systems are combinations consisting of
fore-vacuum pumps (rotary piston or vane-style) and roots-type
vacuum blowers (rotary-lobe boosters). While roots blowers
are mostly unaffected by furnace outgassing, higher attention
must be given to the fore-vacuum pumps. These pumps have
to compress to atmosphere, therefore, they face the risk of
potential condensation inside the compression room. The
main differentiator to adjust and optimize a vacuum system for
specific applications is the fore-vacuum pump.
Dry-Compression Pumps
Dry pumps have the general advantage of an oil-free gas path.
They lack oily surfaces inside the compression room, have
relatively warm operating temperatures and excel, therefore, in
handling condensable vapors and particles. Today’s standard in
industrial markets are screw-type pumps with a variable-pitch
rotor design, which widely replaced older technologies such as
multi-stage roots and claw-type dry pumps because of their
higher robustness.
Two screw-pump principles are promoted.
Simply Supported Rotor Design
The rotor is supported with bearings on both ends (Fig. 1).
This range of pumps includes the most modern pumps on the
market today, which excel with high compactness and low power
consumption and noise level.
Such pumps are the best choice for medium applications.
Acidic f lux vapors extracted during brazing processes can quickly
degrade pump oil, while dry pumps can simply transfer and pump
them out without any increased need for maintenance or service.
Cantilevered Rotor Design
The supporting bearings are on one side of the rotor only,
demanding a more stable shaft design and relatively long
construction (Fig. 2). The main advantage is the lack of sensitive
shaft seals and bearings on the vacuum side.
These pumps have proved their value for even the dirtiest
applications. Internally cooled rotors ensure a moderately
warm compression room, still avoiding condensation but
reducing the cracking tendency of temperature-sensitive vapors.
Buildup of layers inside the compression room is minimized.
The cantilevered design enables a user to manually clean the
compression room easily.
These pumps are the best choice for harsh furnace
applications such as carburizing or sintering, where hydrocarbon
vapors enter the pumps. The pumps tend to build up layers,
requiring the related maintenance demand of periodic cleaning.Maintenance Strategies
Vacuum pumps are valuable assets of a production company and
require strategies for protection and maintenance.
The event-oriented maintenance strategy is of a very reactive na-
ture. Any maintenance activity will be triggered by failures that
already occurred. This requires sufficient well-trained resources
to recover the system in time during a 365/24/7 production
mode. In addition, an adequate number of spare pumps for po-
tential defective parts must be readily available in inventory. The
recovery time of an unscheduled downtime is typically higher
than a scheduled downtime, and the related costs correspond too.
The time-oriented maintenance strategy proactively initiates
maintenance work on a fixed time interval, independent of real
pump conditions. This reduces risk on unscheduled failures
and downtime caused by missed maintenance, but it can lead
to premature exchange of pumps or components. Considering
the full production life cycle, this tends to result in a higher cost
of ownership. Compared with the event-oriented maintenance
strategy, this offers advantages in better allocation of
maintenance resources and reduced risk to unplanned downtime.Fig. 2. Layout of the SCREWLINE screw pump featuring rotors in cantilevered design, enabling cleaning of the compression room on site