Abstract
General
guidelines for the dimensioning of dewatering at the Uhle box were
presented already in the 1970s and 1980s (see reference 2). It was
discovered that the dewatering model (DeCrosta) describes dewatering
relatively well on average, but differences to actual behavior took
place at extreme conditions (very dry and wet felt, high and low
dewatering requirements).
A
model and benchmarking data for Uhle box dewatering was developed
based on felt moisture measurements at several paper mills. Although
there are many unknown parameters, a relatively good correlation
between the estimation model and measurements could be developed.
1.
Background
Dewatering
requirements of the PM press section Uhle boxes vary widely depending
on the paper grade produced, machine speed, felt position, type and
age of the felt, vacuum system design, and many other parameters.
At
the design phase of the paper making line, it is, however, important
to dimension the Uhle box dewatering capacity properly, because it
has a strong effect on the runnability and energy consumption of the
line.
Increased
energy prices and the development of felts and press sections have
also given rise to new challenges in the dimensioning of felt
conditioning. In addition, the energy consumption of a PM vacuum
system should today be minimized without risking the runnability of
the paper making line.
Although
the felt moisture content should not always be minimized, a
sufficient low moisture level before the nip should be achieved,
especially with heavy paper and board grades.
2.
Development of the model and benchmarking data
2.1
Felt measurements
Measurement
data from about 30 machines producing different paper and board
grades were collected and analyzed. Data included scanpro
measurements, dewatering measurements, and basic data of the felts
(dry basis weight, air permeability, etc.). The results of this data
collection are shown at Picture 1.
Picture
1: Measured felt moisture before Uhle box and dewatering.
2.2
Analysis of the data, felt moisture
The
first observation was that there seems to be a practical limit for
the maximal dewatering depending on the felt moisture (Picture 1, red
line).
When
the felt operated relatively dry (red circle), the relative moisture
of the felt after the Uhle box was at a good level of 40% (35
....45%). Correspondingly, when the felt was running wet (blue
circle), the felt's relative moisture after the Uhle box was 50% or
more. Also, the mills operating at the area of the red circle were
satisfied with the operation.
2.3
Requirements for the Uhle box operation conditions
When
the data was further analyzed, it was observed that certain
conditions can be set for the operation conditions of the Uhle box to
achieve a good level of operation.
A
new parameter to describe the operation of the Uhle box was produced,
"VacuumTime". This is defined as a product of vacuum level
(kPa) and retention time (ms) at the Uhle box (later shortened to
VT).
It
looks obvious that there has to be either enough vacuum or retention
time available to achieve the required dewatering and felt moisture
level.
When
the dewatering requirements are high (heavy board grades), the VT
(VacuumTime) has to be more than 200 Pa*s. It could also be observed
that for the machines where the felt was running relatively wet, the
VT was smaller (130 ... 200 Pa*s), and the felt's relative moisture
after the Uhle box was higher (over 50%).
It
should also be observed that when the press section and felts are
designed for nip dewatering, the requirements for the Uhle box seems
to be very low, especially for paper grades and low basis weight
liner and fluting grades (see articles 2 and 3).
Based
on this data, a basic rule can be stated:
-
VT (Pa*s) should be more or less the same as the required dewatering (g/m2)
at the Uhle box.
2.4
Further tests and trials
More
tests were conducted to determine the effect of retention time and
vacuum on the felt dryness and dewatering at the Uhle box. These
tests were carried out at relatively low dewatering level (15 ....
50 g/m2),
thus the VT was also lower (see Picture 2).
Picture
2: Felt moisture before Uhle box as a function of VT (VacuumTime).
Felt
moisture decreases almost linearly as a function of VT when the felt
moisture level is over 700 g/m2,
but when the felt moisture further decreases (relative moisture under
45%), the curve saturates and the increase of VT no longer has any
effect at all.
It
can even be concluded that when the dewatering requirements at the
Uhle box are small (low basis weight paper grades or high nip
dewatering), vacuum time (VT) of 40...80 Pa's should always be
enough. The exception is pick-up felt where dewatering can be higher
and 100...120 Pa*s is needed at least with new open felt.
When
the felt compresses (open volume decreases) and nip dewatering
starts, the requirements for the Uhle box can be very low.
3.
Modeling of the dewatering at the Uhle box
3.1
KG-dewatering model
Because
a certain logic in the measurement data and trials could be observed,
a black box model for the estimation of dewatering at the Uhle box
was developed. Dewatering was estimated based on the following four
parameters:
1. Retention time (dwell) of the felt over open area of the Uhle box
(ms),
2.
Vacuum of the Uhle box (kPa),
3.Felt moisture before Uhle box (g/m2),
4.Felt air permeability (cfm).
Felt
moisture was selected as one parameter because dewatering looked to
correlate strongly with it. Also, felt air permeability was taken
into account mainly because it has a clear effect on the air flow
through the felt (see article 5), and thus it could also correlate
with dewatering.
3.2
Correlation of the model and measurements
The
estimated dewatering with the KG dewatering formula is plotted
against the measured data in Picture 3. The correlation coefficient
is 0.67.
The
average error is 42 g/m2, and the estimated dewatering is practically always inside +/- 75 g/m2 compared to the measurements.
Picture
3: Correlation of the estimate to the measurements.
4.
Discussion
As
the visual review demonstrates, the variation is still pretty large
due to, for example, the felt aging and other parameters, such as
felt type. However, it gives a rough estimate for the required VT
(Pa*s) as a function of the required dewatering and felt moisture
level.
It
can also be seen that the dwell time (ms) and vacuum level (kPa)
compensate for each other at a certain level. However, some
limitations exist:
-
The vacuum level cannot be decreased too much, and it should be over
25 kPa.
-
If the dewatering requirements are high, it is difficult to get the
relative moisture after the Uhle box much lower than 40%, which could
be used as a good target moisture level.
No
clear difference between slot sizes or cover geometry could be seen
in this study.
5.
Examples
Even
though the felt moisture or dewatering cannot be estimated exactly,
the required typical vacuum level and dwell time can be roughly
estimated with the KG dewatering formula.
For
example, if the dewatering level of 180 g/m2 is targeted at the felt moisture level of 900 g/m2 (before the Uhle box), the VT should be about 200 Pa*s (see Picture
4).
The
required dewatering can be achieved with several combinations of
vacuum level and dwell time (vacuum from 30 kPa to 50 kPa, and
retention time from 4 ms up to 7 ms).
Picture
4: Estimation of dewatering.
For
low basis weight products (or machines running mostly based on nip
dewatering), the requirements for the Uhle box are different. They
are mainly used for cleaning the felt and equalizing the moisture
profile (e.g. removing the moisture peaks of showers); the practical
minimum level of VT in these cases is about 40 Pa*s.
6.
Application possibilities
The
KG dewatering model provides one opportunity to do benchmarking
(check the operation conditions) of the operation of Uhle boxes. For
example, in the event where felts are running wet, it can be used to
check if the vacuum level and dwell time are at the required level.
Also,
in energy improvement projects, saving potential can preliminarily be
estimated based on the model; for example, if the felt conditioning
is over-dimensioned (see articles 1, 4 and 5), or the felts are runng too dry.
7.
Summary
Even
though felt moisture and dewatering can be roughly estimated, felt
aging and other parameters still have a significant effect on the
operation conditions.
For
this reason, the vacuum system should have a wide operation window,
and the vacuum level should be controllable without losing the energy
efficiency of the system.
In
the event that vacuum system efficiency and controllability are not
good, a vacuum system study (pre-engineering) can always be
recommended to determine the reasons for the efficiency losses and
improve the controllability of the system.
Related
articles and references:
1. O.
Kaapa, 18-19 / 2009 Wochenblatt für papierfabrikation:
Drastische Einsparungen in der Pressenpartie mit Auswirkungen in die Trockenpartie.
2. Edward
F. DeCrosta, TAPPI May 1980, Vol 63 No. 5:
Air flow requirements for conditioning press felts at suction pipes
3.
O. Kaapa and co , 2/2013, Wochenblatt für Papierfabrikation,
Vakuum – Kapazität in der Pressenpartie.
4. K.
Kokkonen, Results pulp&paper No 1/2011:
Energy savings through a
new vacuum system concept.
5. K.
Kokkonen, Results pulp&paper No 3/2012:
Curbing energy costs through
a blower system rebuild.
6. K.
Kokkonen, Wochenblatt fûr Papierfabrikazion 2/2012,
Energieeinspar-potentiale
bei der Filzkonditionierung,
