Softness
and reduced energy consumption – two requirements
which neatly sum up the aims of most tissue manufacturers
at the moment. The former sells the tissue, the latter
helps the selling price translate into a profit.
The press – in many ways the beating heart of a
tissue machine - is the piece of technology in the line
which, more than any other, can help the tissue maker
realise both these central ambitions.
Toscotec claims to be the first tissue machine supplier
to develop and install in tissue a conventional suction
press roll, but with considerably increased roll diameter
and specific linear load capability to produce a high
bulk web as well as energy-efficient tissue.
Toscotec says it is well known that wider nips provide
several benefits: lower peak pressures in the nip result
in less sheet densification or higher dewatering rates
at the same peak pressure; moreover reduced felt densification
leads directly to increased life.
Also a conventional suction press limits drawbacks in
terms of maintenance and investment costs typically involved
in the shoe press application. Yet this design still provides
significant performance improvement, while reducing maintenance
and rewetting, according to Toscotec.
Less downtime for maintenance and for felt change have
been additional drivers for the development of the large
nip press. Toscotec’s answer has been to create
a roll design with no internal bearing, the suction pipe
installed at drive side and a shaft mounted gearbox.
Reduced rewetting has been achieved by installing a double
doctoring system and through careful design of the felt
run.
Toscotec started its first large nip pres roll on a major
rebuild to a crescent former in 2004; the installation
has been successful and confirmed the expectations and
targets values of bulk and dryness. In 2006 one additional
large nip suction press was started up and the first 5.5m
press is on order.
Increasing the press diameter from standard values of
1000-1200 mm to over 1400 mm produces a nip width increase
up to 30 percent. Increasing linear load from the traditional
value of 90 kN/m to 120 kN/m further increases dewatering
capability, and coupling the effect of larger nip and
higher nip pressure brings up to 2 percent increase of
dryness after press - meeting customer expectations for
energy efficient products.
Further development can be achieved by innovation on press
cover pattern and softness, along with continuous improvement
on felt design, says Toscotec.
A NEW GENERATION FROM ANDRITZ
Andritz’s
latest innovation in pressing is the PrimePRESS XT, described
by the company as a new generation of shoe press technology
for tissue making.
The PrimePRESS XT allows a boost in paper quality and/or
post press dryness, implying higher production capacity
or lower energy consumption. With properties such as bulk,
handfeel and softness in mind, the PrimePRESS XT allows
gentle dewatering of the web, keeping the nip pressure
at a minimum throughout the press nip. With dewatering
performance in mind, the PrimePRESS XT is claimed to achieve
far higher post press dryness, compared to a conventional
suction pressure roll operated at the same line force.
This is due to the rapid release of the nip pressure at
the nip exit. What’s more, the PrimePress XT can
be operated at far higher line forces – respectively
press impulses – in order to achieve an even higher
dryness. As every percentage point increase in after press
dryness increase may decrease the energy consumption by
up to 5 percent, or alternatively increase the machine
capacity by up to 4 percent, the investment quickly pays
off.
Andritz says the new PrimePRESS XT features slim design
dimensions, allowing installation where space is an issue
and for rebuilds. It is engineered from durable allows
to prevent wear. The MD nip pressure profile can be changed
on the run, preventing operational disturbance. Shoe design
is optimized and edge zones have separate control. Andritz
says it has managed to reduce belt stress to a formerly
unachieved level thanks to FE simulations and pilot runs.
From a safety point of view, optimized belt clamping minimizes
the risk of belt injuries during belt removal. And optimized
oil handling allows neat belt changes, according to Andritz.
Voith has something up its sleeve for the middle of the
year. All we know is that it is a new, upgraded version
of TissueFlex. Watch this space.
