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.
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.
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.
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.
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,