Effect of the Dryer Fabric on Energy Consumption in the Drying Section

Transcription

1 T66 Effect of the Dryer Fabric on Energy Consumption in the Drying Section By I. Lang Abstract: The dryer fabric plays a role in both heat and mass transfer in the paper drying process. Cylinder drying relies on conduction heat transfer to the web. Acting through fabric tension, the dryer fabric exerts a pressure on the web holding it in intimate contact with the cylinder. Increased fabric tension improves contact heat transfer resulting in an increased overall heat transfer coefficient, steam to paper. The dryer fabric exerts an influence on mass transfer through its ventilating effect. Owing to the entrainment of boundary layer air, a moving fabric will contribute to dryer pocket ventilation resulting in lower humidity levels, which increases the driving force for evaporation. This effect will be felt more on machines having no pocket ventilation systems or systems which are inefficient. Higher heat transfer coefficients and improved mass transfer allow the papermaker to operate the dryer section at lower steam pressure. This provides for steam savings owing to reduced heat losses through the unwrapped part of the dryer shell and heads. Compared to the energy transferred to the paper web, heat losses from the dryers to the surrounds are small, in the order of 5%. Energy savings from operation at reduced pressure will be smaller still, but given the large consumption of energy by the drying process they are still significant. E nergy consumption in the dryer section accounts for more than half of the energy consumed by the entire paper machine, as can be seen in Fig. 1. Rising energy costs have, once again, put the focus back on energy consumption in the drying section. Heat transfer from the condensing steam to the web is limited by a number of factors; namely, thickness of the condensate layer, dryer scale, cylinder resistance, and contact resistance between the web and cylinder. This can be represented by the following simplified equation: R total = R stm + R cyl + R cont According to some researchers, the contact resistance accounts for 35 to 70% of the overall heat transfer resistance [1]. The role of the dryer fabric is to convey the web through the dryer section and maintain intimate contact between the web and cylinder. Fabric tension acts on the paper web through contact pressure at the fabric-web-cylinder interface and has a direct influence on contact resistance. The contact pressure (P) is determined by the following equation: P = t r (A) Heat transfer from the cylinder to the web can be given by the following equation: Q = Uoa * A * (Tstm Tweb) (B) where Uoa, the overall heat transfer coefficient is equal to: Uoa = 1 (C) R stm + R cyl + R cont Reducing contact resistance increases the overall heat transfer coefficient. Previous work by the author [2, 3] has reported on the effect of dryer fabric tension on contact heat transfer resistance from measurements on laboratory drying apparatus as well as paper machines. For a fixed machine speed condition a higher Uoa value will allow operation with reduced steam temperature. This has an influence on heat losses in the dryer section. Another aspect of the dryer fabric influence on heat transfer is the effect of the fabric on convection heat transfer. Heat transfer by convection may occur from the surrounding air to the web or from the web to the surrounding air depending on the temperature difference. The fabric plays a role in the convection coefficient which is largely permeability dependent. In general, however, convection heat transfer in conventional cylinder drying is quite small owing to the relatively low air speeds and temperature differences. One exception would be in the case of a convection dryer (impingement hood) blowing through a fabric however this case is not examined in this work. The second area of impact of the drying fabric on the drying process relates to its effect on mass transfer. In a simple air/water drying process the I. Lang Asten Johnson, Kanata, Ont. pulpandpapercanada.com Pulp & Paper Canada May/June

2 T67 drying performance driving force for evaporation is the difference in the partial pressure of water vapour at the web surface and the surrounding air, as shown below by the Stefan equation: K * M * Ptot * A Ptot Pa m = 1n [ ] (D) R * T Ptot Ps Reducing the humidity of the surrounding air (air in pocket) increases the driving force for evaporation. The increase in evaporative cooling in the draw leads to cooler sheet in the subsequent heating cycle and increased temperature difference. The net effect is that for constant machine speed (evaporation), operating at lower pocket humidity allows for operation at lower steam pressures. The dryer fabric exerts an influence on mass transfer in two ways. This can be examined by considering the paper drying process as 4 separate phases, as described by Nissan [4], shown for the double felted case in Fig. 2. Phase I and III correspond to the sheet on the dryer with no covering fabric, Phase II consists of the fabric covered part of the sheet wrapping the cylinder and phase IV the open draw. In the fabric-covered part of the sheet run the fabric will act as a barrier to the migration of water vapour to the surrounds. The second way that the fabric influences mass transfer (and drying) is due to its air pumping ability. Dryer fabrics carry air into the dryer pocket due to the effect of entrained boundary layers. This effect is well known. For the case of machines with no pocket ventilation system or poorly operating systems, the pumping of entrained boundary layer air into the pocket may reduce moisture levels and consequently improve evaporation rates. Compared to the dry air delivered by the machine ventilation system, which has an absolute humidity of 0.01 kg/kg, the humidity of the air entrained by the fabric will be quite high, in the range of 0.1 to 0.15 kg/kg. Consequently, about 10 to 15 times the mass flow of air must be pumped by the fabric into the pocket to achieve the same ventilating effect as a well designed ventilation system. The air pumping effect is of no benefit on machines with well designed ventilation systems and may lead to undesirable effects such as sheet flutter and breaks. With the exception of very slow machines or machines producing heavyweight grades the upper limit for fabric permeability is always determined by runnability requirements. ENERGY CONSUMPTION IN THE DRYER SECTION Energy is consumed in the drying process to heat the web and its associated water, evaporate the water, and heat the process air used to evacuate the water vapour. In addition energy is consumed by heat loss through the dryer heads and the unwrapped part of the shell. Energy consumption in the drying process can be broken down as follows: Sensible heating of web and water 7-8% Latent heat to evaporate water 75% Losses through dryer head and shell 4-5% Heat to PM Hood Supply Air Systems 12-14% The amount of energy consumed to dry a unit mass of paper is dependent on a number of factors, the most significant one being the incoming solids content from the press. This can be seen clearly in Table I. TABLE I. Energy consumed in the drying process at differing sheet solids Energy Consumption (kj/kg 92% Reel Solids 40% 45% 50% Solids In Solids In Solids In Sensible Heating of Water and Fibre Latent Heat of Evaporation Dryer Head and Shell Losses Dryer Air Systems TOTAL table ii. Machine conditions Base No. of Dryers 41 No. of Steam Groups 4 Dryers by Group: #1, 1-4, #2, 5-10, #3, 11-21, #4, Dryer Diameter (m) 1.8 Width (m) 9.1 Solids In % 43 Solids Out % 91 Single Felted Dryers 1-19 Condensing Coef.(W/m²/ C) 1000 Basis Weight (g/m2) 45 Speed base case (m/min) 965 Contact Coefficient, x=1.0 (W/m²/ C) 1431 Contact Coefficient, x=0 (W/m²/ C) 344 table iii. Operating Parameters Base Case II Case III Speed (m/min) Steam Pressure (kpa) Group #1 Steam Pressure (kpa) Group #2 Steam Pressure (kpa) Group #3 Steam Pressure (kpa) Group #4 The values indicated above can be considered as minimum values as they do not include other miscellaneous heat losses such as blow-through steam or leaking steam joints. Energy carried away by the dryer via blow-through steam, may be substantial depending on the design of the steam and condensate system and siphon type. Venting steam to a condenser is a highly wasteful use of blow-through steam. As mentioned previously the dryer fabric exerts an influence on drying performance through its effect on contact heat transfer coefficient or mass transfer coefficient. To help demonstrate these fabric effects on energy consumption, a dryer simulation program was employed to carry out the simulation of different drying scenarios. The model was developed by the Institute of Paper Science and Technology (IPST) at Georgia Tech and has been described in detail by Ahrens [5]. The model is a lumped parameter model which solves for 34 May/June 2009 Pulp & Paper Canada pulpandpapercanada.com

3 T68 Fig. 1. Energy consumption in the paper machine. Fig. 2. Four phases of paper drying. Heat Transfer Coefficient (W/m 2 C) Temperature (C) Fig. 3. Heat transfer coefficients vs. contact pressure - Newsprint, dryer group #4. transient one-dimensional heat transfer using a partial difference method. A new feature added to the program described above is a calculation to determine the heat loss through the dryer head and shell. Head losses were based on the equations developed by Chance [6]. Losses through the unwrapped part of the dryer shell are based on simple convection equations for flow over a flat plate. DRYER MODELING Two conditions were examined in this study. The first examined the effect of increasing the contact heat transfer coefficient simulating increased dryer fabric tension. The second examined the effect of a step change in pocket humidity owing to increased fabric permeability. Increasing Contact Heat Transfer Coefficient Previous studies on fabric tension have shown the relationship between fabric tension and heat transfer coefficient [3]. The result of one measurement on a lightweight sheet can be seen in Fig. 3. In this case a 50% increase in contact pressure, corresponding to a rise in fabric tension from 1.5 to 2.7 kn/m, yielded a 14% increase in overall coefficient. Although this is within the range of operation that is realistic for many machines it would be strongly suggested that the design limits of the machine be considered before making such a change. In carrying out the simulation of a dryer section a suitable model must first be developed. The basic machine considerations are listed in Table II. Fig. 4. Steam, sheet, and cylinder temperatures for the base case. The condensate heat transfer coefficient used in the simulation was assumed for a machine running at approx m/min, without spoiler bars. It is well known that the contact coefficient varies with moisture content. Numerous authors have shown this although the precise relationship appears to be elusive given the range in results reported. For the model used here it was assumed to be linearly increasing with moisture content, an assumption based on the results for lighter grades as reported by Wilhelmsson [1]. The values for the contact heat transfer coefficient were determined by trial and error based on the machine speed and steam pressures for the base case. Figure 4 shows the development of the steam, cylinder, and web temperatures through the drying section for the base case simulation. Subsequently, two additional conditions were run. In case II a step increase in contact coefficient of 10% was assumed and the machine speed held constant. The steam pressure in the third dryer group was reduced to maintain constant speed. In case III the steam pressures of the base case were maintained and the speed allowed to increase. Steam pressure and steam data is shown in Table III. Comparing the base case with case II it was observed that steam pressure in the third group could be reduced by approximately 45 kpa as a result of the increase in contact coefficient. Maintaining constant steam pressure for case III resulted in an increase in machine speed of approximately 2%. Of particular interest to this study was the determination of pulpandpapercanada.com Pulp & Paper Canada May/June

4 T69 drying performance table iv. Shell, head loss and heat to sheet Base Case Case II Case III Speed (m/min) Shell Losses (kw) Head Losses (kw) Heat to Sheet (kw) TOTAL (kw) Difference (kw) Difference (%) the change in contact coefficient on energy consumption. From a heat balance the energy consumption was determined for each individual dryer, including heat loss through the dryer head and shell, and heat to sheet. The results are shown in Table IV. For the base case it was seen that heat consumption was approximately 1.5%, 5.5%, and 93% for the head loss, shell loss, and heat to the sheet, respectively. Comparing the base case with case II it is seen that the reduction in heat loss due to increase in heat transfer coefficient is small. Compared to the base case, overall heat consumption is reduced by less than 0.5%. The majority of the savings are the result of the reduction of head and shell loss due to reduced steam pressure in the third steam group. as well as the reduction in cylinder temperature due to the increased heat transfer coefficient. Comparing case III with the base case it can be seen that heat consumption increased, by an amount approximately equal to the increase in production, due to the increase in the sensible heating and drying load. Losses through the dryer head are essentially unchanged - not surprising as steam and ambient temperatures are unchanged. Losses through the unwrapped part of the dryer shell are reduced slightly - the result in the drop in shell temperature with the improved contact coefficient. Effect of Pocket Humidity Levels As mentioned earlier, operation with high humidity levels in the dryer pocket results in a reduction in the driving force for mass transfer and drying rate. For simplicity s sake, pocket humidity was assumed to be constant through the dryer section a gross simplification compared to practice but useful to explain the effect of a humidity change. The same machine geometry was employed as in the previous example, likewise the same base case conditions, with a pocket humidity of x = 0.2 kg/kg. The first alternate scenario, case II A looked at a slight increase in pocket humidity from 0.2 kg/kg to 0.3 kg/kg, reflecting a change in pocket humidity due to a reduction in ventilation or air pumping. In Case II-A no change was made to the steam pressures and consequently speed dropped. Case III-A shows the results with higher humidity but with steam pressure increased in the fourth dryer group to maintain the same machine speed (and evaporation) as in the base case. Steam pressure and steam data are shown in Table V. A step increase in humidity resulted in a drop of machine speed of approximately 4%. To maintain constant speed it was necessary to increase steam pressure in the third group by approximately 25 kpa. The results of the heat balance are shown in Table VI. Compared with the base case the energy consumption in case II-A was reduced by slightly more than 3%, roughly the amount table V. Machine conditions variable pocket humidity Base Case II-A Case III-A Speed (m/min) Steam Pressure (kpa) Group #1 Steam Pressure (kpa) Group #2 Steam Pressure (kpa) Group #3 Steam Pressure (kpa) Group #4 table VI. Shell and head loss and heat to sheet Base Case Case II-A Case III-A Speed (m/min) Shell Losses (kw) Head Losses (kw) Heat to Sheet (kw) TOTAL (kw) Difference (kw) Difference (%) by which the speed was reduced. Comparing case III-A with the base case, it was seen that the overall energy consumption increased, in part due to the slight increase in shell and head losses as well as an increase in sensible heating of the sheet, largely the result of the higher web temperatures in the drying process, the result of the increased humidity. The High Cost of Drying When Things Don t Go Well From the two conditions examined, increase in heat transfer coefficient and increase in humidity, it was observed that drying energy consumption stayed essentially the same (at constant speed), within 1%, the difference being due to change in head and shell losses or heat to the sheet. In practice however, it is not uncommon to see changes in energy consumption in conjunction with a dryer fabric change that are well in excess of those determined from the simulation. One frequently observed condition that affects energy consumption is dryer fabric contamination. This condition may result in non-uniform drying across the width of the paper machine and uneven moisture profiles. This inevitably results in over drying of the sheet in some areas to correct for wet streaks and an increase in steam consumption. The increase in drying energy is further compounded by the increase in the amount of energy required to evaporate a unit mass of water owing to the increase in sorption heat with solids content. A very important factor in determining energy consumption in the drying process is the design and operation of the steam and condensate system. Efficient removal of condensate by the siphon requires maintaining pressure differential across the dryer can in a specific range. Stationary siphons will generally operate over the range of machine speeds found in practice with a differential of 30 kpa. Rotary siphons, on the other hand, require increasing pressure differential to maintain condensate removal as machine speeds increase. The pressure differential required for condensate removal with rotary siphons may vary from a low of 30 kpa at 36 May/June 2009 Pulp & Paper Canada pulpandpapercanada.com

5 T70 speeds of 300 m/min to 100 kpa or more at 1200 m/min [7]. Siphon type also has a significant effect on blow-through steam requirements (steam consumed in the dryer section not contributing to evaporation). Blow-through steam consumption is generally lower with stationary siphons, from 8 to 12% of total steam consumption, while rotary siphons may require 20 to 25% blow-through steam to adequately remove condensate. For best energy efficiency blow-through steam should be used in the low pressure dryers. On a typical dryer section with rotary joints, with a cascade steam system, when operated at or near the limits of its design capacity, it may not be unusual to see a dryer group vent directly to a condenser in order to maintain the necessary differentials to keep up with drying demand. In such a case energy consumption by the dryers as measured by total steam consumption will be high. Any improvement in drying performance, be it from improved heat transfer coefficient, a reduction in pocket humidity, or uniform moisture profiles, that leads to a reduction in steam pressure can have significant effect on reducing dryer section energy consumption far in excess of what may result from the improvement of drying conditions alone. CONCLUSIONS Fabric design or operation that contributes to increased contact heat transfer coefficient and/or reduced pocket humidity yields an increase in drying capacity. At constant speed operation this leads to a reduction in energy consumption. A numerical model was employed to determine the effect of 1) a step change in heat transfer coefficient and 2) a step change in pocket humidity. The calculations show that a 10% improvement in contact coefficient (at constant machine speed) yields a reduction in drying energy consumption of less than 1%. Operation with pocket humidity of 0.2 kg/kg versus 0.3 kg/kg results in a reduction of drying energy consumption of less than 1%. Operating the dryer section at less than ideal conditions can result in energy consumption far in excess of the theoretical minimum. Operation with plugged felts causes moisture profile problems and operation with excessively high pocket humidity will lead to higher energy consumption. Poor design of the steam and condensate system or operation at or beyond the limits of system design can lead to high energy consumption, far in excess of what is theoretically required for the drying process. NOMENCLATURE H = area (m²) K = mass transfer coefficient (m/s) m = mass transfer rate (kg/s) M = molecular weight (kg/mol) P = pressure (kpa) Q = heat transfer (W) r = radius (m) R = gas constant (J/mol/ K) t = tension (kn/m) T = temperature ( C) U = heat transfer coefficient (W/m²/ C) Subscripts a = air cont = contact cyl = cylinder oa = overall s = sheet stm = steam tot = total LITERATURE 1. WILHELMSSON, B., FAGERHOLM, L., NILSSON, L., STENSTROM, S. An Experimental Study of Contact Coefficients in Paper Drying, TAPPI Journal, 77(5), (1994). 2. LANG, I. Dryer Fabric Tension Revisited, Proceedings, 88th Annual Meeting, Pulp and Paper Technical Association of Canada, Preprint A, A , (2002). 3. LANG, I. Drying Performance and Fabric Tension: Mill Trials, Pulp & Paper Canada, 105:11(2004). 4. NISSAN, A. H., HANSEN, D. Heat and Mass Transfer Transients in Cylinder Drying Part II Felted Cylinders, A I Ch E Journal 7 (4), (1961). 5. AHRENS, F., RUDMAN, I. The Impact of Dryer Surface Deposits and Temperature Graduation in The First Dryer Section on Drying Productivity, Preprints, TAPPI Spring Technical Conference, CHANCE, J. L. Dryer Head Heat Losses, Proceedings TAPPI Engineering Conference, , (1981). 7. HILL, K. Five Rules for Energy Efficiency to Improve Dryer Operations, Pulp and Paper, 52-57, September Résumé: La toile sécheuse joue un rôle important dans le transfert de la chaleur et le transfert de masse lors du séchage du papier. Le séchage sur cylindres mise sur le transfert de la chaleur par conduction à la feuille. Grâce à la tension de la feuille, la toile sécheuse exerce une pression sur la feuille et la maintient contre le cylindre. La tension améliore le transfert de la chaleur lors du contact, ce qui permet d augmenter le coefficient de transfert de la chaleur dans toute la feuille, de la vapeur au papier. La toile sécheuse exerce une influence sur le transfert de masse par un effet de ventilation. En raison de l entraînement de la couche d air, la toile en mouvement contribue à ventiler les poches d air et fait diminuer l humidité, ce qui améliore l évaporation. Cet effet sera constaté davantage sur les machines non dotées d un système de ventilation des poches ou dont le système est inefficace. Un coefficient de transfert de chaleur plus élevé et un transfert de masse amélioré permettent de faire fonctionner la sécherie à une plus faible pression de vapeur. Il est ainsi possible d économiser la vapeur parce qu on réduit les pertes de chaleur dans la partie non enveloppée de la virole et de la tête des sécheurs. Comparativement à l énergie transférée à la feuille en continu, les pertes de chaleur des sécheurs dans l environnement sont faibles, soit environ 5 pour cent. Les économies d énergie réalisées lorsqu on fonctionne à pression réduite seront moindres, mais, étant donné la forte consommation d énergie lors le séchage, elles sont encore appréciables. Reference: LANG, I. Effect of the Dryer Fabric on Energy Consumption in the Drying Section, Pulp & Paper Canada 110(5): T66-T70 (May/June 2009). Paper presented at the 93rd Annual Meeting of PAPTAC in Montreal, Que., February 5-9, Not to be reproduced without permission of PAPTAC. Manuscript received February 21, Revised manuscript approved for publication by the Review Panel November Keywords: DRYER FABRICS, DRYER SECTION, DRYING, ENERGY CONSUMPTION, DRYING PERFORMANCE, HEAT TRANSFER. pulpandpapercanada.com Pulp & Paper Canada May/June