Dryer Section(12)Optimisation of the Air Conditions in the Dryer Section low

12Dryer Section Optimisation of the Air Conditionsin the Dryer SectionGeorgi Slawtschew (Dipl.-Ing.), Application and Technical Services, Heimbach GmbH & Co.KGgeorgi.slawtschew@OVERVIEW OF CONTENTSIntroduction1. Energy requirement for heating and evaporating the water from the sheet2. Removal of evaporated water – optimisation of hood ventilation3. Air flows in the pockets4. Optimisation of pocket ventilation4.1 Case studies: Pocket ventilation with blow boxes4.2 Pocket ventilation with air doctors4.2.1 Case study: Pocket ventilation with optimally functioning air doctors4.2.2 Case study: Pocket ventilation with non-optimally functioning air doctors4.3 Case study: Optimisation of a newsprint machine4.4 Zero point – air leakage4.4.1 Case study: Leakage – small “holes”, large effect5. Energy potential of exhaust airConclusionCaption “Measuring conditions”TASKINFO/46GB·97IntroductionIncreasing energy costs and demands on the envi-ronment require to an ever greater extent reduced energy consumption in the paper industry. Where else can the most energy be saved, if not where the most energy is consumed: in the dryer section.Paper drying in the dryer section can be divided into two parts of the process:1. Heating and evaporating the water, primarilyby means of steam-heated cylinders2. Removal of the evaporated water from thepockets and from the dryer hoodThe following contribution deals mainly with the often ignored second part of the process, the removal of the moisture from the sheet and from the machine.Previous TASK Information Booklets have consid-ered the significance of ventilation and heat exchanger systems for the functioning of the whole dryer section.Badly adjusted volumes and quality of supplyair and exhaust air (temperature / moisture) and inadequate energy recovery can lead to enormous losses of energy and / or production capacity as well as deterioration in paper quality.With the aid of case studies the following contri-bution highlights possibilities of how the air conditions in the dryer section can be optimised at relatively low cost.1. Energy requirement for heating andevaporating the water from the sheetIn order to warm (from 50 to 100 °C) and subse-quently evaporate 1 kg of water under ideal con-ditions ca. 2,400 kJ are required. That amounts to a condensation energy of ca. 1.1 kg steam at 1 bar pressure (or 1.2 kg steam at 7.0 bar pressure). Therefore the steam consumption can simply be reduced to 1.1 t steam / t evaporated water. In practice 1.2-1.6 t steam is needed for the heat-ing of the cylinders (not to be confused with the specific steam consumption per tonne of paper which depends on the dry content before / after the dryer section, paper grade etc.).These figures have come from examinations carried out by Heimbach in recent years.The required condensation energy for low pressure steam is higher than that for high pressure steam. The pressure of the steam determines the tempera-ture difference between the paper sheet and the cylinder surface and also the speed of heat transfer. Therefore, the bigger the temperature difference, the faster is the transfer of heat energy from the cylinder surface into the sheet.This temperature difference can, in the case of most board and packaging grades, be increased by raising the steam pressure – but with the disad-vantage of a higher specific steam consumption.On more sensitive grades such as writings and printings the higher cylinder temperatures can have negative effects on the sheet characteristics. In this case another route can be followed. In the open draws between the cylinders the sheet cools through vaporisation. The greater the vaporisation in this area, the greater will be the temperature difference between the paper sheet and the following cylinder. In this way heat transfer can be accelerated. The vaporisation is also augmented by a more effective pocket ventilation.2. Removal of evaporated water –optimisation of hood ventilation Immediately after the sheet leaves the hot cylinder surface the vaporisation of the water commences. For a rapid take-up of the resulting moisture into the surrounding air in the pockets between the cyl-inders, this air must be hot and dry. Experience has shown that a difference of 20 °C between the dry air temperature and the dew point temperaturewill ensure a rapid and complete take-up of the evaporated water from the sheet – the prerequisite being an appropriate temperature level. In order to ensure this process, dry air of at least 90-100°C is blown into the pockets. The initial moisture content of the supply air introduced is mostly that of the outside air and is on average 10-20 g water / kg dry air. The temperature of the supply air should be 110-120 °C so that it is no lower than 90-100 °C when blown into the pockets.Between the cylinders of conventional running sec-tions, the distance for augmented vaporisation of the water from the sheet is shorter than in Slalom sections. That is why the moisture in the pockets of conventional sections has to be removed in a shorter time.The exhaust air volume required for the removal of the evaporated water depends on the dew point in the hood. Conversely this means that the exhaust air can be carrying differing water volumes. In practice air consumption between8 and 20 kg dry air / kg evaporated water have been measured.The following table (Fig.1) shows the differences between the maximum possible absolute moisture content of the exhaust air in relation to the dew point of the hood.Example: A dew point temperature of 55 °C equates to a maximum absolute exhaust air mois-ture content of 120 g water / kg air. If the supply air content of 20 g water / kg air is subtracted, the result is a maximum water carrying capacity of the exhaust air of 100 g water / kg air. This means that the specific air consumption for the removal of the evaporated water is 10 kg air / kg water.An increase of the dew point temperature from 55 °C to 60 °C would increase the maximum absolute moisture content of the exhaust air from 120 g to 155 g water / kg air. Simultaneously the specific air requirement would reduce from 10 kg to 7.4 kg air / kg water – that means about 25 %. This reduced air consumption can lead to signifi-cant energy savings – subject to all air supply ven-tilators being frequency controlled. Alternatively the air volumes made available can be used to increase production.The figures in table 1 impressively confirm the above claims. At the same time they highlight the energy saving potential in the optimisationof the dew point by, for example, closing-up and insulating the hood.A further advantage of such optimisation can be found in the increased effectiveness of the heat exchangers. Briefly: the wetter the exhaust air, the greater is the gain from the heat exchangers.3. Air flows in the pocketsWith a number of examples it can be shown that large air supply and exhaust volumes alone are not sufficient for effective hood ventilation. It is important that not only air quality and quantityFig.1 Dewpoint –> Air moisture content –> Air consumptionare taken into consideration but also the manner in which it is introduced into the dryer section. In the following suggestions for improving air control are described.After the separation of the sheet and cylinder an underpressure occurs in the wedge-shaped separation zone – shown here in Figure 2 on a conventional dryer section. The result is that the air is drawn into the pockets on both sides, mainly affecting the edge zones. The result is dryer paper edges and a bowed moisture profile with a wetter middle. This air movement can be made visible in many dryer sections by means of smoke, as already described in earlier TASK Infor-mation booklets.at the exit from a cylinder in a Slalom section. The air flows in from the side and against the running direction into the separation wedge. After creating vortices within the wedge it is then taken from the paper surface – now in the running direction – to the outside. Similar conditions also apply in conventional sections.4. Optimisation of pocket ventilation From the above it can be seen that in order to even out the underpressure which occurs in the wedge between the exiting sheet and the cylinder, The air in the pockets moves evenly from the middle of the machine in the direction of front and drive sides (Fig.5). In this way a moisture block in the middle of the machine is prevented. The sheet is stabilised and sheet flutter signifi-cantly reduced.Fig.3 Air flow in a Slalom sectionAirFabricFig.4Optimal blowpipe positioning4.1 Case studies: Pocket ventilationwith blow boxes(Caption "Measuring conditions" see page 14)The possible effects of unevenly distributed pocket ventilation are documented in the following case study. It can be seen in Figure 6 that in the dryer sections measured, blow boxes are installed only in the pockets under cylinders 30, 33, 36 and 38. Result: The absolute moisture in these pockets is about 38 % lower than in the non-ventilated pockets. The water take-up capacity of the air in the ventilated pockets is appropriately higher,and lower in the non-ventilated pockets.The possible effects on the moisture profile have already been mentioned.In a comparable dryer section all the pockets were equipped with blow boxes (Fig.7).Result: Air flow from the middle of the machine in all pockets to front and drive side, lower mois-ture content of the air in all pockets and therefore higher water take-up capacity. In total a very even removal of the evaporated water from all pockets.Fig.6 Uneven moisture removal through uneven pocket ventilation[°C] 100 90 80 70 60 50 40 30 20 10 0800 700 600 500 400 300 200 100 0[g w a t e r / k g a i r ]28293031323334353637383940Cylinder temperature Dry air temperatureDewpointAbsolute moisture content pocket airWater take-up capacity pocket airFig.5 Optimal air flow out of the pockets4.2 Pocket ventilation with air doctors In the existing pocket ventilation in conventional sections the supply air must be blown through the dryer fabric. Depending on its condition (air permeability, degree of contamination etc) and also the positioning of the blow boxes, the pocket ventilation can be seriously impaired. Additionally, with increasing speed, by the dryer fabrics will be more air dragged along, which cannot be fullythe blow box.supply air – just as with Slalom sections – getsunhindered into the wedge between the exiting sheet and the cylinder (Fig.8). The underpressure created by the separation of sheet and cylinder is evened out. The very high moisture from the evaporation at this point is taken up significantly better by the air blown in.Besides higher drying capacity and lower specific air consumption the optimised pocket ventilation Fig.7 Even moisture removal through even pocket ventilation[°C] 100 90 80 70 60 50 40 30 20 10 0111213141516171819202122800 700 600 500 400 300 200 100 0[g w a t e r / k g a i r ]Cylinder temperature Dry air temperatureWater take-up capacity pocket airDewpointAbsolute moisture content pocket airFig.8Optimal functioning of air doctorsObviously Slalom sections can also be fitted with air doctors as is the case in some of the newer machines.Additionally sheet flutter in conventional sections can be reduced by repositioning the felt rolls in the direction of the headbox, so that the exiting sheet lies against the dryer fabric and is supported by it (Fig.8 to 10).4.2.1 Case study: Pocket ventilationwith optimally functioning air doctors (Caption "Measuring conditions" see page 14) Based on the above results a doctor beam was drilled across its width and connected to the air supply. Comparison between Figures 9 and 10 shows the significant reduction in sheet flutter as the supply air from this “air doctor” presses the sheet against the dryer fabric. The two measuring diagrams indicate the non-visible effects in each photo with closed air supply (Fig.9A) and with open air supply (Fig.10A) via an optimally functioning air doctor. The air conditions in the pocket are as follows:Figure 9A: Air supply closed:• Cold outside air flows laterally into the pocket • Moisture in the machine middle higher than at the edges• Lower dry air temperature in the pocket, particularly in the edge zones• Therefore lower water take-up capacity of the pocket air• Big temperature differences betweenmiddle and edges• Therefore bowed moisture profile• Severe sheet flutter2325 222426Figure 10A: Air supply open:• Hot air flows out of the pocket• More even moisture across the width (lower in machine middle than at the edges) • Significantly higher dry air temperature in the pocket• Resulting immense increase in water take-up capacity of the pocket air 4.2.2 Case study: Pocket ventilation withnon-optimally functioning air doctors (Caption "Measuring conditions" see page 14) On a machine with Slalom sections this example highlights the fact that air doctors do not always function optimally. Figure 11 shows the schematic sketch of an air doctor applicable to all cylinders on this machine, in which the supply air is blown100908070605040302010111213141516171819is equipped with such an air doctor. The air condi-tions were measured at both the inlet and outlet of each cylinder. The absolute moisture measured at the cylinder outlet is almost always double that measured at the inlet, and in fact at a relatively high level for Slalom sections. It is striking how precisely the absolute moisture and the water take-up capacity mirrors the function of the air doctors in consistent zig-zags.The following calculation shows the saving poten-tial of a relatively easily achievable improvement in the moisture profile – as described under 4.2.1. The figures apply to an 8 m wide machine running at 1300 m/min. An increase in the end moisture of 1 % (from 7.5 % to 8.5 %) at the same 2-Sigma values was the result of a more even moisture profile.Energy saving potential from profile improvementEnd moisture with “bad” profile 7.5 % End moisture with “good” profile 8.5 % Steam saving 1.5 t / h Steam costs30 EUR / tSaving potential in EUR per hour45 EURper day1,080 EUR per year (330 days) 356,400 EUR4.3 Case study:Optimisation of a newsprint machine(Caption "Measuring conditions" see page 14)Even quite small mechanical modifications can achieve energy saving and process relevant improvements, as shown in the example of a small newsprint machine with trim width of 3.2 m and a running speed of 920 m/min. Figure 13 shows the situation before optimisation. Figure 14 shows the measures carried out and documents the energy improvements. The energy savings are shown in the following calculation. A further stage of optimi-sation for cylinders from No.22 is planned. Measures carried out:1. The bottom cylinders of the2. and3. sections were separated from the steam and condensate systems (the 1. section had already been separated in the first stage of the optimisation).Fig.13 Before optimisation[°C] 120110100 90 80 70 60 50 40 30 20 10 0[g w a t e r / k g a i r ]800 700 600 500 400 300 200 100 0135791113151719212325Cylinder temperature Dry air temperatureDryer fabric temperature Sheet temperatureWater take-up capacity pocket airDewpointAbsolute moisture content pocket air2.1 The bottom cylinder blow boxes were repo- sitioned under the pockets so that the supply air could enter the pockets more easily. 2.2 Blow pipes were were installed in the pockets under cylinders 1 and3.3. The ventilators of the sheet stabilisers were replaced.Advantages of measures carried out: 1. The bottom cylinders of the 2. and 3. sections were no longer heated, with an appropriate energy saving. The dryer fabrics of the 2. and 3. sections were heated no more than necessary, reducing the risk of hydrolysis.2. Better pocket ventilation from machine middle to edges together with increased water take-up capacity of the pocket air3. Improved runnability – no sheet lifting and no edge creasingSaving from machine optimisation Steam saving 0.6 t / h Steam costs30 EUR / tEnergy saving in EUR per hour18 EURper day432 EURper year (330 days) 142,560 EUR 4.4 Zero point – air leakageThe “zero point” can be described as an imaginary surface within the hood which forms the barrier between underpressure and overpressure. The zero point should be at the same height at front and drive side in order to prevent a lateral air flow in the hood.Frequently a zero point height of about 2 m above machine floor, approximately at the level of the middle of the top cylinders, is recommended. This is substantiated that the pockets are in the underpressure zone from which the air can better escape. This only applies when the necessary ven-tilation of the pockets is lacking or is not function-ing adequately.The current contribution will show why the pock-ets should not be “evacuated” but positivelyFig.14 After optimisationCylinder temperature Dry air temperatureDryer fabric temperature Sheet temperatureWater take-up capacity pocket airDewpointAbsolute moisture content pocket air [°C] 120110100 90 80 70 60 50 40 30 20 10 0[g w a t e r / k g a i r ]800 700 600 500 400 300 200 100 0135791113151719212325be operated with a low zero point, which could be about the level of the bottom of the suction rolls or of the bottom cylinders.As a result almost the complete hood would be in overpressure which would be more favourable from an energy point of view. It also means that the lower the zero point, the less is the leakage that can penetrate the hood.This situation would have the result that the rela-tionship between exhaust air and supply air could be adjusted in the direction of 1:1 and thereby op-timised. The reduction of an incalculable amount of leakage would improve control of the total air conditions in the hood.The exhaust air should be dew point controlled and the supply air zero point controlled. This means that the exhaust air can be carrying a maxi-mum load of moisture so that the moisture can be transported from the pockets with a minimum exhaust volume.It also means that there is an optimal energy ad-vantage can be obtained by the heat exchangers (see chapter 2, final paragraph). through the wall there was sufficient space for the continuous inflow of cold outside air (see Fig.15).The air pulled in flowed onto the drive side, then through the rusty steel floor of the hood and later-ally to the sheet. Despite only moderate warming relative to the air in the hood, this flow of leakage air overdried the drive side edge zone of the sheet (see Fig.15).At the same time moist air built up towards the front side leading to a wetter front edge. In addition the zero point was way above the top cylinders which further encouraged the ingress of the cold outside leakage air.After closing the openings in the basement and sealing the rusted hood supports the 2-Sigma value fell from 0.33 (see Fig.15) to 0.11 (Fig.16).5. Energy potential of exhaust airIn optimising the dry end, measures for reducing energy consumption play a decisive part. The fol-lowing calculation indicates the volume of energy reserves to be found in the exhaust air (Fig.17A and 17B).ConclusionThe growing demands for economies on the whole paper machine require a higher effective-ness in the dryer section. Therefore a significantly more subtle approach to the question of “hot air” is needed than was the case in previous years. The air plays a very important role in paper drying. It is not just a question of how much air is available, but much more where and how itis applied. By the optimal use of supply air and exhaust air the specific air consumption can be much reduced and the speed of drying increased. Energy savings and also production increases can be achieved while bottlenecks in the dryers can be avoided or eliminated.In this case a part of the exhaust air from an 8 m wide machine was led direct to atmosphere – without any energy recovery by a heat exchanger (Fig.17A). The measurements showed residual heat in the exhaust air of 8,890 kW which can be utilised as demonstrated in Fig.17B. Energy saving potential of heat recovery Residual heat in exhaust8,890 kWResult with heat exchanger efficiency of 15 % 1,333 kW Price per kW 0.04 EUR Potential saving in EURper hour53 EURper day1,280 EUR per year (330 days)422,400 EURFig.15 Before optimisation:Effects of cold air ingress on drive side and blocked moist air at front sideFig.16 After optimisationFig.17A Existing situation:Energy loss Fig.17B Ideal situation:Using energy potential of exhaust airCaption "Measuring conditions"In the dryer section analyses the following para-meters were measured:Position 1. Cylinder temperaturePosition 2. Paper sheet temperaturePosition 3. Dry air and moist air temperature Position 4. Clothing temperatureThe measuring positions are shown in Figure 18. All measuring positions are located on the front side of the machine approx. 50 cm towards the machine middle from the front side of the sheet. The measurement of pocket air conditions is carried out with the hood closed to exclude the influence of the machine house climate. The dew point and absolute moisture figures are obtained from the dry air and moist air temperatures.The difference between dry air temperature and dew point temperature documents the water take-up capacity of the pocket air and is shown hatched within the measuring diagrams.Fig.18 Measuring positions。