Energy efficient vacuum solutions for industrial furnaces

Energy efficient vacuum solutions
for industrial furnaces
Uwe Zoellig, Klaus Buhlmann
Fighting against global warming and increasing greenhouse gas concentrations,
the regimentation of CO2 emission are the justification for
the European Commission to set up the European Directive 2005/32/EC.
This and several amending directives describe a framework of eco-design
requirements for energy using products asking for increased efficiencies.
New norms have been written and others have been updated which now
need to be considered in new product engineering projects. Beside this
energy costs are rising constantly and will continue to rise in the future.
Considering these aspects the design of modern furnaces has changed
recently. New designed vacuum furnaces are operating much more
efficient as older furnaces. In consequence, also the vacuum pumps and
pump systems must be designed accordingly to support these energy saving
attempts. For sure first priority for the vacuum pumps and systems is always to
reliably provide the required vacuum level; trouble-free operation is a
must. The need of energy saving does not allow uptime reduction of the
furnace. High reliability can be reached by using traditional technologies
as e.g. oil-sealed rotary piston or rotary-vane pumps, roots blowers and
diffusion pumps, but next to these also more modern technologies such
as dry screw pumps already have a proven track record to work most
reliable even under harshest conditions. But, are all these pumps also
optimized in view of energy consumption?
The article will demonstrate that modern
vacuum solutions with dry-compressing
screw type vacuum pumps and
new innovative roots blower designs can
help meeting the goals in energy saving.
Further on it describes measures taken
at conventional vacuum components as
for example rotary-vane pumps or even
diffusion pumps support fulfillment of
the new energy-saving requirements.
Standard vacuum systems used
on furnaces today
Oil-sealed vacuum pumps, combined
with roots-type booster pumps are
today’s standard equipment for most
industrial vacuum furnaces. These vacuum
pumping systems do have relatively
low investment costs and are still
the norm for a broad field in heat-treatments.
Mostly depending on regional preferences,
two concepts for oil-sealed forevacuum
pumps compete against each
other; rotary-vane pumps and rotarypiston
pumps. Rotary piston pumps are
for sure more robust as vane pumps, but
they become more and more obsolete.
For applications such as annealing, hardening
and tempering, which cause only
a benign load with negligible impact on
the vacuum system, oil-sealed vacuum
pumps offer cost-effective and very reliable
performance. Compared with piston
pumps, rotary vane pumps offer the
same sufficient process stability, have
a lower capital cost and consume app.
5 to 10 % less energy, simply as the
rotating mass is lighter weighted. But,
is there additional potential for even
higher energy savings?
For more harsh applications as e.g.
brazing, carburizing or sintering more
and more end-users choose dry screw
pumps as backing pumps, simply as
these are more capable and reliable to
withstand the higher process requirements
as any oil-sealed pump. Ten-
dentious, even in standard applications
many oil-sealed pumps are substituted
by dry screw pumps because of the
significantly lower cost-of-ownership of
these pumps.
But, today’s dry pumps are typically
more energy consuming as oil-sealed
pumps of the same size. Is this a pig in
a poke? Does the user have to accept
a higher power demand as trade-in for
the higher robustness?
Roots-blowers are used on nearly
each furnace and have a more or less
unchanged design since decades. Their
power consumption is minor compared
to fore-vacuum pumps but can their
power demand even be further reduced
by modern designs or intelligent controls?
Most high-vacuum furnaces use oil-diffusion
pumps to reach pressure ranges
below 10-3 mbar. These type of pumps
are proven “working-horses” and very
robust, but their power consumption is
high and does not change even if the
furnace is in idle mode. Can this also be
changed by innovative designs or controls?
The following chapters will give answers
to each of the above raised questions.
It will be demonstrated that by using
modern solutions higher efficiency-rates
and reduced power consumption can
be achieved. “The most environmental
friendly and cost saving kWh is the one
which is never consumed – it pays off”.
Detail view on rotary-vane
pumps
The principle of an up-to-date rotaryvane
pump is already quite optimal
for low power consumption. Rotating
masses (including oil) are minimized,
internal friction is low and by principle
the pumps do the most efficient adiabatic
compression. The power consumption
of such pumps is clearly less than
the nominal motor power. A SOGEVAC
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SV630B for example (Fig. 1) is equipped
with a 15 kW flange motor. The effective
power consumption of this pump
at a typical operation pressure for a
furnace below 1 mbar is approximately
7.4 kW.
New pump deliveries will include IE2
motors, which meet latest standards.
Motors declared IE2 have a higher
efficiency factor, all kind of losses, such
as iron- and copper-losses, are minimized
to achieve a higher efficiency.
Realistically additional energy saving can
only be reached by operating the pumps
motor with a matched variable speed
drive (converter). Thus an optimum in
performance can be achieved. Ramp up
function and torque optimization are
two features among other provided by
modern variable speed drives. Next to
this frequency drives offers extended
possibilities for process control, e.g.
a suction speed reduction during the
boost phase of a carburizing process to
reduce HC-gas consumption.
Detail view on screw-type dry
vacuum pumps
Today’s standard dry pumps are screw
pumps with variable pitch rotor design.
Fig. 2: Layout of the hermetically sealed DRYVAC screw pumps
featuring 2 rotors with progressive pitch profile and build-in
frequency-converter driven high-efficiency motor
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Fig. 1: Energy efficient standard pump for
benign applications: SOGEVAC SV300B
Continues compression along the rotor
length minimizes the energy demand.
Older technologies with constant screw
pitch or even dry pumps based on multiple
stages of roots- or claw type rotors,
have a significantly higher power consumption
due to design and pumping
principle.
But even the plurality of today’s screwpumps
with variable pitch, differ a lot
from each other. Most pumps of the
600 m³/h class demand app. 10 kW
power at typical furnace operation pressures
below 1 mbar, which is a clearly
higher value as those of a comparable
rotary-vane pump.
During the development of DRYVAC
(Fig. 2), reduction of energy consumption
was a mayor focus. Optimizing the
mechanical rotor design, the electrical
motor concept and by selection of a
perfectly matched build-in frequency
converter the achieved result is overwhelming.
The DRYVAC 650S (Fig. 3) does only
consume 6.9 kW at 1 mbar, it is even
more energy efficient than a rotary-vane
pump and by this becomes the new
bench mark for power consumption in
the market.
The build-in frequency converter also
offers potential for additional savings
and more process control. Many process
steps do not require “full-power” suction
speed, especially during operation
at rougher pressures (e.g. during carburizing).
Soft start and ramping functionality
can be realized with the variable
frequency drive. Chamber pressure can
be controlled by varying the rotational
speed. The customer can even realize
a process specific “standby condition”
considering certain valve positions to
save for example the volume of supplied
Nitrogen.
Next to these environmental issues, for
sure the modern design of the DRYVAC
does eliminate sensitive components as
shaft-seals or couplings which clearly
increase robustness and reliability of the
pump.
Detail view on roots-type
vacuum blowers
Most roots pumps used today are standard
designs with flanged motors and
lip-type shaft sealing. For most applications
this design is sufficient, but it
includes some weaknesses:
• Shaft-sealing, couplings and motorbearings
wear down over time
• Shaft-seals do not allow an operation
at significantly increased rotary
speed.
• The motor is mostly a standard motor
not really matched to the pump for
optimal usage.
The first two points could be addressed
by usage of the well known “cannedmotor”
pumps, such as the RUVAC
WS2001 – since years available on the
market. However, when looking at
energy-efficiency this is a step back-
Fig. 3: Picture: DRYVAC Enduro 650S, the new
most energy efficient standard for demanding heattreatment
applications

Fig. 4: Motor concepts of a standard roots-pump (left) and a RUVAC WH roots-type booster
pump (right)
wards, as the canned motors typically
operate by far less efficient than standard
flange motors.
More promising are the most modern
developments, using build-in, potted
motor designs. Here the advantages of
the canned-motor design are combined
with reduced power consumption and
offers improved compactness. In addition,
the potted motors used for RUVAC
WH roots-type booster pumps (Fig. 4)
Fig. 5:
RUVAC WH4400, most
compact and power saving
design on market
are high efficiency IE2 motors without
external shaft seal or couplings. Further
on the pumps are hermetically tight, a
benefit for many processes.
At typical furnace operation pressures a
RUVAC WH (Fig. 5) can be sped up by
a frequency converter to rotary speeds
up to 120 Hz (varying with pump-size).
Thus a small pump can often substitute
a much bigger pump, which reduces the
installed and consumed motor power.
Fig. 6:
Pumping principle
of a diffusion pump
Detail view on diffusion pumps
Diffusion pumps work in the high-vacuum
field and require therefore a complete
different pumping principle as the
before described displacement pumps.
Inside a diffusion pump, oil (Fig. 6) is
evaporated, guided through multiple
jet stages to build a steam “umbrella”
which traps gas molecules flying into it.
The oil steam is condensed when reaching
the outer wall of the pump and
recirculated.
Being a very simple, very robust and
proven principle on the one hand side,
a diffusion-pump on the other hand
requires lots of energy. Most energy is
consumed to evaporate the oil and not
to compress the gas. All this evaporation
energy is then removed by the coolant
again as all oil is condensed again. Obviously,
this is not a pump-principle with
low energy demand.
Poorly the industry still need to stick to
this energy wasting solution for most
applications, as alternative high-vacuum
pumps which are much more energy
efficient (e.g. Turbomolecular pumps
or Cryo pumps) are simply not robust
enough to handle most furnace applications
and next to this their initial costs
are much higher.
But there are ways to minimize the
power consumption of a diffusion
pump. A major intuence is given by the
heater design. Most producers use heating-plates
which are positioned below
the oil-reservoir (boiler), see Fig. 7. This
is not very energy efficient as there are
high losses during the heat transfer into
the oil, e.g. caused by the air-gap in
between the heater element itself and
Fig. 7: Heater elements positioned inside
the oil reservoir of a diffusion pump
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Fig. 8: Oil-diffusion pump LEYBOJET 630, improved for
high throughput and lowest power-consumption
the bottom of the diffusion pump. Better
designs use heating elements positioned
directly inside the oil-reservoir, by
this clearly improving the heat transmission,
bringing all heating energy directly
into the oil.
Most energy is used during the heating
phase of the diffusion pump. To shorten
the heating time, it is important to
minimize the mass of the pump which
needs to be heated. Energy only used
to heat the pumps material is generally
a wasted energy. Intelligent designed
diffusion pumps as those from the DIP
or LEYBOJET type (Fig. 8) have a total
mass around 25 % less as that of most
competitors, therefore offering a much
shorter heating-up time and lots of
energy saving.
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The full heater power is only necessary
during the pumps heating phase. Compared
with the total operation time of
a typical furnace process, this time is
relatively short. Most time the furnace is
constantly maintained at high-vacuum
or the diffusion pump is idling behind
a closed valve. During all this time, a
diffusion pump does not have such
high power demand. Despite this fact,
most diffusion pumps are continuously
operated with full heating power, wasting
lots of energy. Innovative controlsystems
can identify the actual power
demand and regulate the power supply
of the heaters accordingly. By these
measures a reduction in power consumption
of more than 30 % can be
realized.
Conclusions
There are many possibilities to safe
energy. For sure vacuum pumps
demand only a smaller part of the total
energy consumption in a heat treatment
process, but the responsible user should
take all useful measures to reduce
energy consumption, helping to reach
our all environmental goals.
To use modern dry-compression screw
type vacuum pumps or roots-pump
clearly helps to minimize the environmental
footprint of a vacuum pumps
and pump-systems. Along with this, the
implementation of latest vacuum pump
designs, even for more traditional technologies
as diffusion pumps or rotaryvane
pumps will further reduce the CO2
footprint. Intelligent controls will play
a more and more important role and
definitely help to meet the environmental
goals.
Users should evaluate all options and
not just stick to conventional solutions.
Modern pump technology is robust
and reliable, but most environmental
friendly, too. It helps to safe operation
costs and to protect our green environment.
Uwe Zoellig
Oerlikon Leybold Vacuum
GmbH
Cologne (Germany)
Tel.: +49 (0) 221 / 347 1375
uwe.zoellig@oerlikon.com
Klaus Buhlmann
Oerlikon Leybold Vacuum
GmbH
Cologne (Germany)
klaus.buhlmann@
oerlikon.com