To define the necessary thermal energy for paper drying, one can refer to
the Tappi formula (code TIP 0404-05) that considers the total evaporation (Rw
= Rwhood + RwYD) determined by the drying balance between hood and yankee.
Mainly the total supply for evaporation is tied to the value of dryness at
the hood inlet determined by the press action. The greater this value, the
lesser is the total evaporation value and therefore the 'work' of hood and
yankee. In a conventional installation it is clear that the two presses solution,
which guarantees dryness at the hood outlet of approximately 42%, is advantageous
in terms of thermal energy saving compared to the single press solution, which
can guarantee an average value of 38%. However, this has an impact on the paper
quality and increases electricity consumption.
Furthermore, considering both drying supplies at hood and yankee, it can
be said that the yankee system is, in energy terms, more efficient that the
hood one. Toscotec has been studying for some time now an interesting solution
which allows for a reliable energy saving. This technology consists in the
combination of a large diameter press (TT SPR 1430) with the steel yankee cylinder
(TT SYD).
The large diameter press, having the possibility of working with an increased
nip, both in terms of length and linear pressure, is able to reach values of
dryness at the hood outlet comparable to those of a double press, whereas the
steel yankee cylinder is able to supply a greater evaporation coefficient compared
to the conventional cast-iron cylinder, enhancing the drying of the paper and
making it more economic.
As previously mentioned, the concept of TT SPR 1430 press is to fit the growing
need for tissue products, with their high thickness and softness, with energy-saving
demands. This new concept of suction press roll has the characteristic of generating
a wider nip compared to conventional press rolls.
The approximate value of this increase of nip is 30%. Maintaining the advantages
of traditional press rolls, such as low maintenance cost, reduced investment
cost and quick installation, the suction press roll allows the increase in
paper softness and bulk .This press is designed to operate with a maximum nip
of 120 kN/m instead of the 90-95 kN/m typical of conventional press rolls,
thus making it possible to choose between operating at full power for an economic
product line and reduced pressure to enhance softness and thickness in the
higher quality products. The increase in dryness at the press outlet with greater
nip is at least 2-3% compared to a conventional press roll with consequent
energy saving. Today Toscotec has three functioning applications of such technology.
A fourth press roll will be started up in mid-2009 together with a complete
new line managed by Toscotec.
The other important tool proposed by Toscotec is the steel yankee dryer TT
SYD. To date, Toscotec is the only company in the world to have grooved cylinders
working on tissue machines, entirely made of steel both with ASME and PED codes.
The use of steel plates instead of cast-iron fusions allows for the significant
reduction of the shell thickness and therefore resistance to heat transfer.
The result is an increase in drying capacity achieved through the steam condensation
inside the yankee, which is the most efficient paper drying system and often
the least expensive. The energetic yield of a cylinder is further improved
by the possibility of operating at a higher steam pressure; the development
of an efficient system of condensate extraction allows further reductions in
steam pressure differential necessary for the correct running of the cylinder
On the reference running YDs it was possible to verify that the heat transfer
coefficient and the evaporation capacity of the cylinder are on average at
least 30% higher compared to a cast-iron cylinder with the same dimensions
and operating pressure. Alternatively the same productive capacity can be obtained
by increasing the contribution of the cylinder with obvious benefits in terms
of energy.
The first yankee cylinder of 2500 mm diameter was installed in Italy in 2000.
Two years later the first 12ft yankee was built. Then in 2006 the first
15ft cylinder was started up in a paper machine reconstruction for the
Wepa group, to which a second and a third 15ft yankee followed in 2008. Toscotec
has sold a total of 35 cylinders up to now, 26 of which are actually
running. Two more 15ft cylinders will be started up in the second half of 2009.
The maximum face of the cylinder actually working is 5000 mm. TW
For further information contact : davide.mainardi@toscotec.com
