State-Of-The-Art Developments to Save Energy in Coating Drying Bob Bates, P.Eng Metso Paper
Overview Two (2) case studies are presented to illustrate two (2) new developments in coating drying, with a focus toward saving energy while maintaining high sheet quality and machine performance (efficiency) Case Study #1 looks at new floatation bar (nozzle) technology for air dryers Case Study #2 presents new air dryer technology aimed at drying the sheet at a high evaporation rate 2
Case Study #1- Air Dryer Rebuild 3
Case Study #1- Air Dryer Nozzle Development Early nozzles were relatively simple foils with a single air slot. Later nozzles typically included a double slot (float) arrangement to improve sheet floatation and drying performance. The most recent, state-of-the-art designs, include a much larger nozzle profile that supports and dries the sheet over a greater area. This improved design has a significant effect on the dryer s ability to deliver energy to the sheet, more effectively; thus saving energy. 4
Nozzle Development More drying capacity Nozzle designs have evolved to deliver increased energy per unit area thus increasing drying effectiveness This in turn enhances drying performance (+25%) and overall runnability Float Float Plus Foil Impingement 5
Hi Performance Float Nozzles Operating principles Wide nozzles give more active drying surface Direct jets of hot air create high turbulence and high heat transfer coefficient Stable air pad in the middle of the nozzle gives stability to the runnability 6
Nozzle Performance Drying Comparison (heat transfer coefficient) 2.5 Relative Heat transfer coeff 2.0 1.5 1.0 0.5 +32% 0 Typical Double Slotted Nozzle New Extended Area Nozzle Over approximately the same (MD) cross section, with the nozzles operating at the same temperature (200C) and blowing velocities (50 m/s), the new nozzle design performs much better (20-32%) in terms of average heat transfer coefficient. 7
Case Study #1- Methodology Based on success of lab results and field trials, new nozzles arrangements were designed to fit an existing production machine. Prior to rebuilding existing dryers, benchmark testing was done to develop an accurate drying model and establish current drying rates and energy consumption. Drying simulations were run to confirm drying scenarios and predict energy savings. Supported by simulation results, two (2) existing dryers were rebuilt with new nozzles. Before and after comparisons analyzed to confirm performance and energy savings 8
Case Study #1- Machine Data Table 1 Machine Data Machine Coater #1 (bottom) Coater #2 (top) Coater #3 (top) Coater #4 (bottom) Fine Paper (LWC) Coat wt: 7.97#/ream @ 63% solids Coat wt: 8.19#/ream @ 63% solids Coat wt: 7.16#/ream @ 63% solids Coat wt: 7.56#/ream @ 63% solids 3700 fpm Gas IR Gas IR Gas IR Gas IR 150 trim 2 Air Dryers 2 Air Dryers 1 Air Dryer 1 Air Dryer 67.95 #/ream 2 steam cylinders 2 steam cylinders 4 steam cylinders 7 steam cylinders 3.28% moisture (base) 5.35% moisture (final) Drying Simulation 9
Energy Calculation Summary -- Simulation projection - All Dryers with Existing Air Bars (data taken 22 April 09) Speed 3,701 ft/min Operating days per year= 350 Trim 152.8 in % Operating efficiency=85 Coater 1 Gas IR Dryer 1 Dryer 2 Abs. Energy 792 MBH Abs. Energy 2,065 MBH Abs. Energy 1,993 MBH Evaporation 238 lb/hr Evaporation 1,217 lb/hr Evaporation 1,655 lb/hr Imp Temp 498 F Imp Temp 511 F Imp Vel 9,820 ft/min Imp Vel 9,940 ft/min Gas Cons. 1,898 MBH Gas Cons. 3,430 MBH Gas Cons. 3,404 MBH Elect Cons 86 HP Elect Cons 86 HP Coater 2 Gas IR Dryer 3 Dryer 4 Abs. Energy 793 MBH Abs. Energy 1,939 MBH Abs. Energy 1,768 MBH Evaporation 325 lb/hr Evaporation 1,114 lb/hr Evaporation 1,699 lb/hr Imp Temp 505 F Imp Temp 519 F Imp Vel 8,840 ft/min Imp Vel 8,070 ft/min Gas Cons. 1,898 MBH Gas Cons. 2,973 MBH Gas Cons. 2,846 MBH Elect Cons 71 HP Elect Cons 57 HP Coater 3 Coater 4 Gas IR Dryer 5 Gas IR Dryer 6 Abs. Energy 443 MBH Abs. Energy 1,488 MBH Abs. Energy 221 MBH Abs. Energy 1,635 MBH Evaporation 201 lb/hr Evaporation 967 lb/hr Evaporation 157 lb/hr Evaporation 1,348 lb/hr Imp Temp 463 F Imp Temp 517 F Imp Vel 8,720 ft/min Imp Vel 8,050 ft/min Gas Cons. 1,253 MBH Gas Cons. 3,006 MBH Gas Cons. 633 MBH Gas Cons. 3,831 MBH Elect Cons 59 HP Elect Cons 45 HP Total Gas Consumption Air Dryer Gas Consumption Total Water Evaporated Gas Energy Consumption 25,173 MBH 19,490 MBH 8,922 lb/hr 2,821 BTU/ lb H 2 O 1 0
Energy Calculation Projection -- Dryers 5 & 6 with New Hi Performance Nozzles (From Simulation) Coater 1 Gas IR Dryer 1 Dryer 2 Abs. Energy 792 MBH Abs. Energy 2,065 MBH Abs. Energy 1,993 MBH Evaporation 238 lb/hr Evaporation 1,217 lb/hr Evaporation 1,655 lb/hr Imp Temp 498 F Imp Temp 511 F Imp Vel 9,820 ft/min Imp Vel 9,940 ft/min Gas Cons. 1,898 MBH Gas Cons. 3,430 MBH Gas Cons. 3,404 MBH Elect Cons 86 HP Elect Cons 86 HP Coater 2 Gas IR Dryer 3 Dryer 4 Abs. Energy 793 MBH Abs. Energy 1,939 MBH Abs. Energy 1,768 MBH Evaporation 325 lb/hr Evaporation 1,114 lb/hr Evaporation 1,699 lb/hr Imp Temp 505 F Imp Temp 519 F Imp Vel 8,840 ft/min Imp Vel 8,070 ft/min Gas Cons. 1,898 MBH Gas Cons. 2,973 MBH Gas Cons. 2,846 MBH Elect Cons 71 HP Elect Cons 57 HP Coater 3 Coater 4 Gas IR Dryer 5 Gas IR Dryer 6 Abs. Energy 443 MBH Abs. Energy 1,355 MBH Abs. Energy 223 MBH Abs. Energy 1,474 MBH Evaporation 201 lb/hr Evaporation 1,010 lb/hr Evaporation 151 lb/hr Evaporation 1,337 lb/hr Imp Temp 311 F Imp Temp 342 F Imp Vel 8,720 ft/min Imp Vel 8,050 ft/min Gas Cons. 1,253 MBH Gas Cons. 2,159 MBH Gas Cons. 633 MBH Gas Cons. 2,753 MBH Elect Cons 80 HP Elect Cons 54 HP Total Gas Consumption 23,248 MBH Air Dryer Gas Consumption 17,566 MBH Total Water Evaporated 8,947 lb/hr Gas Energy Consumption per lb 2,598 BTU/ lb H 2 O 1 1 Dryer 5 Before After Abs. Energy 1,488 1,355 MBH Evaporation 967 1,010 lb/hr Imp Temp 463 311 F Imp Vel 8,720 8,720 ft/min Gas Cons. 3,006 2,159 MBH Elect Cons 59 80 HP Dryer 6 Before After Abs. Energy 1,635 1,474 MBH Evaporation 1,348 1,337 lb/hr Imp Temp 517 342 F Imp Vel 8,050 8,050 ft/min Gas Cons. 3,831 2,753 MBH Elect Cons 45 54 HP = 9.9% decrease in Air dryer energy = 7.9% in total gas energy
Case Study #1- Rebuild Objectives 8% reduction in natural gas consumption Equal or better finished sheet quality Equal or better runnability and overall machine efficiency Good return on investment (ROI<2yrs) 1 2
Nozzle Details Beforeslotted bars (wide spacing) and large screens Afternew bars and return screens 1 3
Case Study #1- Rebuild Results Total OMC Natural Gas Utilization MBH 26000 25000 24000 23000 22000 21000 20000 19000 Pre-Rebuild Post Rebuild (Estimate) Post Rebuild (Actual) 7% Approx. 10% due to new nozzles Approx. 5% due to system optimization 10% reduction in natural gas consumption realized due to improved nozzle performance plus additional 5% achieved due to optimization of existing equipment (e.g. dryer alignment, air system balancing, instrument tuning, etc.) Excellent finished sheet quality Equal or better runnability and overall machine efficiency <1yr ROI 1 4
Case Study #1- Conclusions New hi performance air bars can deliver energy significantly better than traditional nozzles to promote more efficient drying, while maintaining runnability and sheet quality Potential for significant cost savings and reduction in carbon foot print 1 5
Case Study #2- Hi Intensity Drying 1 6
Case Study #2- Hi Intensity Drying The most common drying strategy used today is the so-called high-low-high theory. Given the limitations associated with the earlier studies, a new study [1] was undertaken to see the effects in coating quality and binder gradients if the coating is dried using a single-sided high intensity dryer. Utilizing newly developed nozzle designs (see Case Study #1 above), and a hi-intensity dryer system, it is possible to test the all-high drying hypothesis using an all air dryer, in place of traditional IR. 1 7
Coating Drying Consolidation 73-85% Pigment matrix starts to format 3D Matrix is compact Drying of Coating Layer Drying Phase Coated Paper The principal quality parameters that are affected by drying are: Mottling (uneven quality of a printed image, e.g. ink absorption) Gloss and smoothness, also printed gloss Surface strength. 1 8
Drying Strategies- Traditional High-Low- High Introduced in the 1980 s If part of the coating is consolidated in the early stages (e.g. at a low evaporation rate), then the rest of the coating has to be consolidated the same way (e.g. at low evap rate). This limits the drying strategy in that the low drying rate established Relatively low evaporation rates 73-85% Need to maintain Relatively low evaporation to prevent mottle at the beginning needs to be maintained later in the drying section to dry evenly and preserve the coating structure [2] 1 9
Effect of the evaporation rate on mottling at 77% average solids [2] Summary: - Traditional strategy is correct if part of the coating has been consolidated in the IR dryer or free draw; the rest of the coating also has to be consolidated at the same conditions (evap rate) to reach the same coating structure to prevent mottling. Max evap. rate +/- to prevent mottle 2 0
Drying Strategies- New All High Alternatively, a new drying strategy allows for high evaporation rates immediately following the coater, and continuing throughout the drying process. Very even drying throughout critical phase, resulting in good finish quality Progressively Higher/Even Evaporation Rates Critical Phase 2 1
Case Study #2- Methodology A broad cross-section of grades and coating colors were tested as part of the coating/drying trials. Study criteria: Sheet quality Drying capacity and efficiency Runnability 2 2
Trial Results The pilot trials on LWC, WF and board were conclusive in confirming that a single hi-intensity air dryer (400C, 60 m/s), in place of IR, could net excellent results including: - higher drying (evaporation) rates - mottle, gloss and smoothness improved when effective air drying is included immediately following the coating station - air drying is equal or superior to IR in terms of paper quality Reduced Mottle 2 3
Test Results: Mottle, Gloss, Smoothness Reference value = Gas IR 25, 40, 60 m/s nozzle velocity Less mottle Improved gloss Better smoothness This series of pilot trials confirmed that a very uniform surface porosity distribution was possible using extremely high evaporation rates and a single dryer, resulting in better print characteristics. From this, it was possible to prove that a high-high-high air drying strategy could produce very even porosities resulting in less mottling, better gloss, and increased smoothness. 24
Case Study #2- Rebuild Building on the success of the pilot trials, this new high-high-high drying strategy was implemented on a production machine. Two (2) existing gas fired IR dryers were replaced with two (2) new hiintensity air dryers. Utilizing new nozzle designs, the single sided hi-intensity dryer system is very similar to a traditional air dryer configuration including, a supply fan, burner, air dryer, combustion fan, make-up air and exhaust (heat recovery optional), 2 5
Case Study #2- Results Operating at approximately 450deg C, 60 m/s (840F, 11800 fpm) the hi-performance dryers can deliver a significant amount of energy to the sheet. This increase results in approximately 20% more evaporation per square meter compared to traditional gas IR dryers; while maintaining runnability and sheet quality. In terms of drying efficiency (e.g. energy costs), the hiperformance air dryer operates at close to 80% efficiency, compared to traditional IR at approximately 35% (BTU/#H 2 O evaporated). 2 6
Case Study #2- Conclusions Equal or better paper quality (mottle, gloss, smoothness) compared to IR - Lower web temperature - Less water penetration - Higher binder content on the paper surface Higher drying capacity (more production potential) Energy savings e.g. 60-80% drying efficiency with air drying, 25-40% with IR drying and reduced carbon footprint (50% less fuel burned= 50% less CO 2 formed) Cont d.. 2 7
Case Study #2- Conclusions (cont d) Low maintenance costs Improved operating conditions, e.g. lower machine hall moisture and heat loads Low investment costs/evaporation ($/H 2 O/ft 2 h) compared to IR Greatly reduced operating hazards (e.g. fires) 2 8
Summary Two (2) new developments in air drying technology were presented offering opportunities to improve coating drying efficiency. New air bar designs provide increased evaporation rates leading to higher drying capacity. Coupled with these new nozzles, new hi-intensity dryers have proven successful in providing an all high drying strategy, resulting in increased drying with less mottling, better gloss, and increased smoothness. 2 9
Acknowledgements & References I wish to acknowledge Messrs Pertti T Heikkila and Richard Solin for their research in coating drying and the development of new nozzle and dryer technologies. Also thanks to Don Cesario and Tom Puukila for their work in dryer rebuilds (Case Study #1). 1. Heikkila, P, Rajala, P (2004), The effect of high temperature air drying on evaporation, runnability and coated paper properties, Proceedings 14 th International Drying Symposium (IDS 2004), Sao Paulo Brazil, pp 1295-1302 2. Heikkila, P, Rajala, P (2002), International Ph.D Programme in Pulp and Paper Science and Technology (PaPSaT), Pigment Coating Technology 3 0