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 Current
energy prices are encouraging design engineers to propose
ways optimizing
the use of the energy in drying systems.
The optimization must involve the design phase but also
the operational phase. Once the machine is running we cannot
leave to the operators to decide how to use the differen
drying potentials available in the machine.
Tissue drying offers good opportunities for optimization
as the specific amount of energy use per ton of paper on
the reel is quite significant. The potential savings are
thus also very significant and the effect of the drying
parameters (namely, the impingement velocity and temperature,
the moisture of the exhaust gases and the steam pressure
inside the Yankee cylinder) have a direct effect on the
different energy consumptions: gas consumption in hood
burners, electricity consumption in different hood fans
and steam consumption in the Yankee cylinder0.\
The design phase of a Yankee hood must involve the minimization
of heat, internal and external, and pressure losses in
different parts of the hood and air systems by means of
the application of new and modern techniques (CFD and FEA).
It must also incorporate mechanical solutions designed
to limit limit hall air infiltrations and hot gas spillage
to the hall.
The operational phase for the optimization must include
all the necessary automation controls to ensure the correct
balance of the hood halves and the best conditions for
the gas combustion.
In addition to the conventional concepts described above,
we must also focus, both in design and operational phases,
on optimizing the total operational cost, defined as
Cop
= cgas • Vgas + celec • Welec + csteam • msteam
where Cop = operational cost (€/ton paper), cgas =
gas unit cost (€/Nm3 gas), celec = electricity unit
cost (€/kWh electricity), celec = steam generation
unit cost (€'/ton
steam) Vgas = specific gas consumption (Nm3/ton paper),
Welec = specific electricity consumption (kWh/ton paper),
msteam = specific steam consumption (ton/ton paper), and
where all specific consumptions are functions strongly
dependant on the main drying parameters, ie.
Vgas = fcn1(tsupply, vsupply, Wexhaust),
Welec = fcn2(tsupply, vsupply, Wexhaust), and
msteam = fcn3(psteam, tsupply, Wexhaust),
with,
tsupply = supply or impingement temperature in the hoods
(ºC),
vsupply = supply or impingement velocity in the hoods (m/s),
Wexhaust = moisture or water content in exhaust gases (kgH2O/kg
dry gases), and
psteam = steam pressure in Yankee cylinder (barg).
In design phase an optimization study
must be carried out to define the best combination of operating
conditions (tsupply, vsupply, Wexhaust, and psteam) at
the current levels. Even better, studies shouuld be carried
out based on estimated future levels of unit costs (cgas,
celec, and csteam).
Some practical examples of this short series of studies
for the optimized determination of the drying parameters
are shown the charts reproduced here.
This study can be (and should be) applied not only to new
installations but to existing installations where we must
face different constraints function derived from the re-use
of existing equipment (for instance, the re-use of existing
circulation fan or motor, or the re-use of existing Yankee
hoods with mechanical limitations in operating temperatures
related to the sort of materials utilised in their fabrication).
In the operational phase the optimization
of the drying parameters must follow the fluctuations of
the unit cost of the energy (cgas, celec, and csteam) gradually
updated (monthly, for instance).
This optimization will be the result of a permanent evaluation
of the minimum operational cost, Cop, by means of an algorithm
that estimates instantaneously the operational cost as
a function of its historical evolution taking into consideration
the trends of the last reading of the main drying parameters,
obviously submitted to the convenient restrictions always
compatible with the equipment and the different controls
interacting in the air systems.
This algorithm so defined
would fix the appropriate speed of rotation of the circulation
fans. This way, the supply temperature of the hood, defined
by the moisture of the paper at the QCS scanner, would
depend of the rotation speed of the circulation fans determined
by the minimum operational cost (again, always compatible
with the normal restrictions of the equipment and the drying
process). The only parameters still to be defined are the
moisture of the exhaust gases (Wexhaust) and the steam
pressure (psteam) of the Yankee cylinder.
Nowadays the moisture of the exhaust gases is controlled
independently from the other parameters, as it contributes
in a smoother way to the drying capacity (graph below):
The
steam pressure, on the other hand, is normally manually
operated by the papermakers depending on different factors
(some of them rather subjective).
We are currently working in the integration of the steam
pressure into the optimization algorithm to provide a complete
tool able to optimize all the main drying parameters in
the tissue machine.
This integration would also consider the common restrictions
in steam pressure variation compatible with the operation
and quality of the final product to be achieved. TW
Mr Lopez
is R&D Manager-with Brunnschweiler SA based
in Munguia, Spain |
