tip 0404-63 paper machine energy conservation

TIP 0404-63

ISSUED – 2003 REVISED – 2006 REVISED – 2011 REVISED - 2016

2016 TAPPI

The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.

TIP Category: Automatically Periodically Reviewed (Five-year review)

TAPPI

Paper machine energy conservation Scope The paper machine area is a major energy consumer in most pulp and paper mills. The high cost of energy makes it important to implement energy management and conservation measures. Paper machine energy consumption represents 50-70% of purchased energy for an otherwise efficient integrated mill. If paper machines are inefficient in use of energy, the mill will be uncompetitive. Reductions in energy consumption reduce operating costs and increase profitability. Reducing paper machine energy consumption requires attention to details in design, operation, maintenance, and control of nearly all aspects of the papermaking process. This TIP discusses guidelines for monitoring, benchmarking, and optimizing energy-intensive unit operations to reduce paper machine energy consumption. Safety precautions Follow normal safety precautions when working around paper machinery, including use of personal protective equipment. Do not allow loose clothing or equipment to contact rotating machinery or ropes. Beware of overhead cranes and thermal and slip hazards around the dryer section. Avoid direct contact with hot surfaces. Use hearing protection in noisy areas. Eye protection should be worn in all production areas. Safety shoes and safety helmets should also be worn where required. Energy reduction strategy Efforts to improve paper machine energy efficiency center around five basic principles: • Minimize the amount of water to evaporate in the dryer section (and pressure of steam used to evaporate it). • Minimize the amount of steam condensed outside the dryers. • Maximize condensate return flow and condensate pressure to the powerhouse. • Minimize electrical consumption for key users. • Monitor and manage energy consumption and cost. Mill-wide energy savings require a multi-faceted approach, including purchasing smarter, using less, integrating processes from different parts of the mill, and generating more low-cost electricity. Human factors such as training, publicity, visibility, accountability, benchmarking, and targets can aid in achieving energy conservation goals. Paper companies with the best corporate energy performance have support from top management with capital funds allotted for energy reduction projects. Mill managers also must be committed to reducing energy use and encourage energy conservation mill wide.

TIP 0404-63 Paper machine energy conservation / 2

System monitoring Scottish mathematician and physicist Lord William Thomson Kelvin (1824-1907) said, “If you can’t measure it, you can’t improve it.” A key first step in energy conservation activities is monitoring energy consumption and making sure flow meters and cost information are accurate. Some mills have developed mill-wide system balances that can be used to check accuracy of individual flow meters. Assigning a person to be responsible for energy conservation in the mill and/or paper machine area can help increase visibility and accountability of conservation efforts. Steps for an effective monitoring program include: • Have an energy champion responsible for monitoring and reducing energy consumption on the machines. • Meter energy flows to each machine. • Establish key energy parameters. • Highlight variables that affect energy consumption. • Include energy parameters in operator rounds and centerlining efforts. • Provide information to operators, engineers, and managers to encourage continuous improvement. • Develop trouble, cause, and correction (TCC) procedures to troubleshoot issues contributing to high energy

consumption. • Discuss energy cost and conservation efforts in production meetings. • Include energy cost and consumption goals in personnel performance metrics. • Conduct periodic check-ups of key systems. • Benchmark machine operation with best in class and best achievable for the equipment installed. Utilities to be monitored include: • Pressure (kPa or psig), temperature (°C or °F), and flow (kg/hr or lb/hr) for each header supplying steam to the

machine. • Electrical consumption for each machine (MW). • Natural gas (m3/hr or scfm) • Water flows and temperatures – mill water, warm or hot water from other areas of the mill, and sewer (L/min or

gpm, °C or °F). • Compressed air pressure (kPa or psig) and flow (m3/hr or scfm). • Condensate return flow (l/min or gpm, kg/hr or lb/hr) and temperature (°C or °F). Based on these measurements and paper machine production rates, specific energy indices can be calculated and tracked: • Steam consumption (kg/tonne or lb steam/ton paper) • Electrical consumption (kWh/tonne or kWh/ton) • Natural gas consumption (m3/tonne or kscf/ton) • Total energy consumption (kWh/tonne or MMBtu/ton) • Water consumption (m3/tonne or gal/ton) • Compressed air consumption (m3/tonne or kscf/ton) • Condensate return (%) • Total energy cost ($/ton) Determination of energy unit costs typically requires assistance from mill accounting and powerhouse personnel. Understanding the relative cost of different energy sources can help papermakers minimize total energy costs. Note that the cost of various energy sources will change based on relative cost of corresponding raw materials. Cost components that should be included in evaluation of total costs include: • Net cost of steam to each paper mill supply header ($/kg or $/klb). One method is to determine fuel cost for

high-pressure steam minus the value of electricity generated by turbines. Marginal cost of steam (cost of the last steam generated) should be used to measure the value of steam savings. Marginal cost is usually higher than average cost since powerhouses use more expensive fuel to top off demand. Note that this method of calculation may be an over-simplification if pressure and flow in a low-pressure steam header are maintained by high-

3 / Paper machine energy conservation TIP 0404-63

pressure make-up steam supplied from a pressure-reducing valve in the powerhouse. There should also be a distinction in cost between direct steam usage and indirect steam usage where condensate is returned to the powerhouse.

• Net cost of natural gas (typically expressed in $/kcal, $/therm or $/MMBtu) • Electrical cost ($/MWh). Calculating $/kWh or $/hp-hr can assist in calculating electrical energy savings. • Water and sewer costs ($/M liter or $/MMgal). Both supply and sewer water treatment costs should be included

to determine true value of water conservation projects. Cost of fresh water introduced into the process should be determined and monitored. Cost is based on use of steam to heat incoming water to process operating temperatures and can be very significant. Some mills have determined the cost per gallon introduced to encourage reuse of clarified whitewater.

• The value of condensate returned to the powerhouse. This should include associated energy, water treatment costs, wastewater treatment costs, and raw water pumping costs to get it to the water treatment plant. Cost should be adjusted downward for condensate polishing costs. Value of condensate can be very significant and papermakers should be informed about the cost of wasting condensate.

The combination of production rates, energy consumption, and cost information can be used to determine energy cost per ton of product. It is also important to understand energy contracts. Generally managing energy savings downward is the correct move; however, with some peak energy contracts unless you are able to save off of peak there are no apparent savings and conversely if you can save off of peak there is an immediate benefit. Additional specific energy flows can also be useful, including dryer section steam, if it is metered separately from total steam to the machine. There are three areas that are typically poorly monitored that can help a mill identify steam waste: steam flow to the wire pit or silo, steam flow used to heat shower water, and the energy loss to the dryer vacuum condenser (water flow, temp in, temp out). Looking at valve position is one way of tracking these energy flows but does not tell the entire story. Most mills have no idea how much energy they are using in the silo or for shower water heating. The normal response from papermakers is "not much" but in reality it can be a significant use. Dryer drainage system vacuum condenser tracking is also recommended. It is a good way to assess and maintain the health of the dryer drainage system. The percent energy loss can be tracked and trended. This identifies bad vent valves, open vent valves, high wet end dryer losses, air leaks, high water flow, etc. The vacuum condenser is often a piece of equipment that is poorly controlled. Poor control often results in high water flow that dilutes and upsets the fresh warm or hot water system. Performance indices Performance indices can be used to benchmark energy consumption and identify opportunities for improvement. TAPPI TIP 0404-47 “Paper machine performance guidelines” (1) provides a broad range of indices for different grades of paper. Target values for key indices applicable to energy consumption are shown in Table 1 for various grades. Key factors Each machine typically has several key factors that influence energy consumption on the machine. Green/yellow/red indicators can be used for key process conditions that affect energy consumption to show whether values are in desired ranges. DCS and/or data historian trending can be used to track trends of key parameters. Sheet consistency out of the press section is often the primary variable affecting paper machine energy consumption. Regular grab samples (TAPPI TIP 0404-01 “Determination of water removal by wet presses” discusses the proper procedure) or the use of portable or fixed sheet moisture gauges specifically designed for use in the press section are recommended to track solids. Press solids can also be calculated based on press section and/or dryer section water balances. Typical additional key factors include: • Venting from dryer section thermocompressor or cascade sections • Condenser water valve output/condensate flow • Differential pressure (especially for lead dryers) and/or blowthrough steam flows • Wire pit steam and water heating steam valve positions • Mill water make-up into whitewater or warm water systems.


Table 1. Energy performance indices Grade Corru- Recycled Bleached gated Market Fluff paper- News- Kraft Index Fine board Liner medium pulp pulp board print LWC paper Uptime, % 95 93 95 95 95 95 93 93 93 94 First quality, % 93 90 97 97 99 97 93 98 85 97 Overall machine Efficiency, % 89 84 92 92 94 92 86.5 92 79 91 Total steam consumption lb/ton 4,000 4,000 2,800 2,750 2,000 2,500 2,800 2,800 3,000 5,000 kg/ton 2,000 2,000 1,400 1,400 1,000 1,250 1,400 1,400 1,500 2,500 Electrical consumption kWh/ton 350 350 300 300 150 150 300 300 400 400 kWh/tonne 385 385 330 330 165 165 330 330 440 440 Total energy cons. MMBtu/ton 6.0 7.0 5.0 5.0 4.0 4.5 6.0 5.0 5.5 6.0 GJ/tonne 7.0 8.1 5.8 5.8 4.6 5.2 7.0 5.8 6.4 7.0 Water consumption gal/ton 2,000 2,000 1,500 1,500 1,000 1,000 <1,000 2000 2000 1500 m3/ton 7.6 7.6 5.7 5.7 3.8 3.8 <3.8 7.6 7.6 5.7 Couch solids, % 22 25 27 27 28 28 NA 21/18 22/18 20/19 Press solids, % 42/45 42 42/50 42/50 50 45 48 43/48 43/49 42/46 Size press moisture, % 3.0 3.0 NA NA NA NA NA NA NA NA Reel moisture, % 5.0 5.0 7.5 10.5 10.0 7.5 >7.5 7.5 5.0 7.5 Drying steam lb steam / lb water evaporated 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 PV supply temperature °F <180 <180 <180 <180 NA NA <180 <180 <180 <180 °C <80 <80 <80 <80 NA NA <80 <80 <80 <80 Condensate return 75-80 75-80 75-80 75-80 75-80 75-80 75-80 75-80 75-80 75-80


• Basis weight versus standard • Press section weir flows • Size press starch solids and pick-up • Pocket ventilation temperature • Temperatures through hood exhaust heat recovery systems • Warm water flow, pressure, and temperature from pulp mill • Winter/summer operating strategy for machine room ventilation • Any additional steam venting Centerlining Centerlining is often used to help ensure consistent paper machine operation and quality. The tool can also be used to help monitor and control energy consumption. Centerlining of energy parameters can often be divided into two categories: process setpoints and factors reflective of system health. Examples of process setpoints that can be used in centerlining include: • Wire pit and other water heating temperatures • Pocket ventilation, blow box, roof supply, air make-up unit, and other air heating temperatures • Dryer section differential pressures (or blowthrough flows) • Press loads • Sheet moisture at the reel • Size press and coater solids • Refining kW, freeness, hpd/t and/or kWh/t Examples of factors reflective of system health include: • Overall consumption indices such as pounds steam/ton paper, kWh/ton, and energy cost/ton • Dryer section pounds steam/ton • Warm water flow and temperature from the pulp mill • Mill water flow • Silo and process heat exchanger valve positions • Warm water make-up valve positions • Mill water make-up valve positions into the white water or warm water systems • Venting from dryer sections (dp or blowthrough vent valve positions) • Pulper pump and agitator amps • Press section weir flows Operator rounds Operator rounds should be utilized to manage systems that are not visible in DCS or data historians. Examples of areas where operator rounds may be required include: • Roof or mezzanine rounds to check for leaking vent or safety-relief valves. • Roof supply and machine room ventilation temperatures. • Hydraulic cooling/heating systems. • Condenser systems. • Steam leaks in piping or through vent valves or safety-relief valves • No dumping of condensate. Some mills utilize an infrared temperature gun to check stock and water system temperatures and detect cold-water infiltration. Note that flat black spray paint should be used to mark areas on piping where infrared measurements are used to ensure uniform emissivity.


Energy surveys Energy audits can provide useful first steps to identify and prioritize opportunities to reduce paper machine energy consumption. Data can be collected from direct observation; data historians; discussions with mill operating; maintenance, and engineering personnel; and previous reports conducted on subsystems of the paper machine. A computer simulation of the papermaking process can help validate data and determine potential benefits from process changes. The United States Department of Energy funded development of paper machine energy scorecards. The scorecards use energy performance guidelines and other information in this TIP for benchmarking energy performance and identifying opportunities for reducing energy use. The scorecards are available for use by paper companies and others interested in reducing energy use. (3) Keys to successful implementation of recommendations from an energy audit include: • Obtaining buy-in from all parties involved • Focusing on optimal measures, but not forgetting incremental gains • Understanding the costs, risks, and benefits of potential projects • Considering life cycle costs in project evaluation • Thoroughly planning implementation • Training • Documenting results • Optimizing the system after the project • Sustaining results Additional surveys A detailed review of various paper machine systems can ensure that systems and equipment are operating efficiently. Some of these recommended surveys and suggested frequency are listed below. • Steam trap surveys (annual) • Compressed air system surveys (annual) • Refining optimization (on-going) and mechanical surveys (annual) • Saveall audit to check capacity and filtrate quality (annual) • Showering surveys (every 2 years) • Press section optimization (on-going) • Press section nip surveys (every 2-3 years) • Vacuum pump boroscope inspection or orifice plate testing (annual) • Vacuum system surveys/optimization (every 1-3 years) • Thermography to check for leaks and hot spots (annual) • Steam box surveys (annual) • Dryer steam and condensate system surveys (annual) • Hood air system surveys (annual) • Machine room ventilation studies (every 5 years) • Pulp dryer maintenance/capacity reviews (annual) • Tissue machine hood balances/inspections (annual) System optimization Key process areas to consider when in a program to reduce paper machine energy consumption are discussed below.


Reducing the amount of water to evaporate Drying steam represents the majority of energy consumption on a paper machine. A step in minimizing energy consumption is reducing the amount of water to evaporate in the dryers. Opportunities to do this include: • Increase press dryness (high-load, shoe presses, increasing couch solids) • Optimize press fabrics and roll cover designs (venting and hardness, nip dewatering vs. Uhle box dewatering) • Reduce basis weight (while meeting product specifications) • Trim the sheet at the wet end rather than at the dry end • Improve cross-machine moisture profile uniformity • Increase solids of starch and coating color used in the size press and coaters (metering size press) • Minimize water added to the sheet through rewet showers • Increase moisture content of the sheet at the reel (when sheet properties and profiles allow). Note that increasing sheet moisture content at an intermediate point through the dryer section such as into a size press, calender stack, or coater does not affect overall drying energy consumption. As long as size press, water box, or coater solids and pick-up do not change, the total amount of water that needs to be evaporated from the pre- and after-sections does not change. Changing sheet moisture content into these intermediate sections does affect the distribution of dryer steam – for example, increasing sheet moisture into the size press will reduce the main section steam requirement and increase after-size drying steam. Machine efficiency Increasing overall machine efficiency has a direct effect on specific energy consumption since it takes as much or more energy to produce a ton of broke as it does to make a ton of first-quality paper. Some steps to increase machine efficiency include: • Reduce sheet break and grade change times. • Shorten open press-to-dryer draws, provide direct sheet support. • Minimize trim losses with good edge control and coordination with business logistics. • Full machine threading – including features that minimize break recovery and thread times. • Optimize performance of trim squirts. • Utilize camera systems to identify and characterize breaks. • Optimize quality control system (QCS) performance to ensure good machine-direction (MD) and cross-machine

direction (CD) profiles. • Control sheet in open draws in the dryer section. • Utilize capability of distributive control systems (DCS) and data historians to impact efficiency and

troubleshooting. • Optimize process chemistry for runnability and maximizing ash content – closed loop control of retention,

charge, etc. • Manage broke to maintain stability. • Optimize whitewater saveall to maximize overall retention, to stabilize wet end during break conditions, and to

increase clear filtrate quality and quantity for replacement of mill water in showers. • Use camera systems to monitor sheet runs and identify causes of web breaks. Agitation Chest agitation is a significant contributor to paper machine electrical consumption. Opportunities to reduce energy consumption with design and operation of agitation include: • Do not overestimate consistency when designing systems • Design chests for the optimum dimensional ratios (cube is best) • Do not underestimate temperature • Allow for a larger manhole to install a larger impeller at low speed


• Keep flow impediments [ladders, etc] out of chest design • Only operate the number of pulper agitators necessary • Consider variable-speed or two-speed agitator motors • Utilize zone agitation where complete mixing is not required. • Consider top-entry instead of side-entry agitators to reduce drive requirements • Do not put pump suction behind the impeller • Slow down an agitator and reduce horsepower if operating consistency has dropped substantially from design. • Make sure there is not excessive motion when you look in a chest. • Replace marine propellers with modern designs to reduce drive requirements • Work with a supplier that understands the mixing process intimately Pump and motor systems U.S. Department of Energy (DOE) information indicates that average motor energy cost/mill/year is $1.7 MM for pulp mills, $4.6 MM for paper mills, and $3.0 MM for board mills. Average available motor savings opportunities per year are estimated to be $483,000 for pulp mills, $679,000 for paper mills, and $492,000 for board mills. The U.S. DOE Office of Industrial Technologies web site (3) includes information on pump and motor systems, compressed air systems, steam, and other opportunities to conserve energy. Approximately 30% of paper mill electrical energy consumption is by pumps, 20% by fans, 5% by compressors, and 45% by drive motors and other electrical equipment. Potential electrical energy savings opportunities are available through pumps and fans (53%), motor efficiency upgrades (23%), air compressors (6%), rewind improvements (6%), motor downsizing (6%), and other systems (6%). Pump-based systems represent the largest single group of energy-consuming equipment and offer greatest potential savings. DOE indicates that 80% of electrical consumption is by 10% of the motor population (motors greater than 50 hp). 200-500 hp motors typically have the largest percentage of savings opportunities. The primary reasons pumps waste energy are over-design, change in process conditions, or degradation. Over-design can be the result of overestimating design conditions, contingencies, safety factors, catch-up capability, “room to grow,” or design for a wide range of process conditions. Energy is wasted when a pump system is changed; resulting in a lower flow rate or lower head pressure requirements, but the pump, motor, and/or piping are not downsized to meet the change. Energy is also wasted when a larger pump than required is used for the purpose of commonality of spares. This also highlights the need to build to what will be required instead of building to some future incremental capacity. Pumps that operate in caustic or solids applications tend to experience impeller and wear ring degradation, causing a loss in pump efficiency. Routine inspection of pumps in these applications is recommended. Parts should be maintained and/or replaced as necessary. DOE promotes identifying motors with the greatest saving potential for further investigation. The greatest savings potential is typically centrifugal loads with a high duty cycle. These motors are referred to as the “vital few.” The following steps can identify them: 1. Categorize motors by size times operating time. Establish a threshold for more detailed consideration. (Should

be a one-day effort in most plants – a plant-wide motor inventory is not necessary). 2. Segregate by load type (focus on centrifugal loads) 3. Look for symptoms in pumping systems that indicate potential opportunity:

• Systems controlled by throttling valves • Recirculation line normally open • Systems with multiple parallel pumps with the same number of pumps always operating • Constant pump operation in a batch environment or frequent cycle batch operation in a continuous process. • Cavitation noise (at pump or elsewhere in the system) • High system maintenance


• Systems that have undergone a change in function.

4. Establish policies to replace seldom-used, small-load, and large, non-centrifugal systems with high-efficiency motors. The Pumping System Assessment Tool (PSAT) (4) can be used to quantify energy consumption and cost savings potential from a pump. The assessment requires flow rate, pressure, and motor current or power data. Note that cost to buy a pumping system is usually much less than operating cost. Life cycle cost should be used for evaluating pumps. Opportunities to reduce energy consumption by pumps and motor systems include: • Replace throttling valves with speed controls where appropriate • Reduce speed for fixed-load pumps • Install parallel system for highly variable loads • Equalize flows using surge vessels • Replace motors and/or pumps with more efficient models • Avoid recirculation control • Consider reducing impeller diameter for oversized pumps • Avoid incompatible duties on common pumps • Do not operate in startup configurations permanently • Design systems with proper line sizes • Avoid tanks where feasible • Optimize process configuration, consistency and pressure setpoints • Determine what can be shut off or bypassed during slow backs. Refining Refiners must be in good mechanical condition to minimize energy consumption and optimize fiber development. Effective life of refiners between rebuilds is typically 10-15 years. Mechanical condition can be estimated by checking no-load horsepower by backing off refiners while stock is running through them. Significant difference on stator circles between six o’clock and 12 o’clock positions is an indication that the refiner is out of alignment. Higher wear on the rotor/stator on the adjustment side than the motor side is generally an indication that the rotor is not floating evenly and there may be sticking. Higher wear on the motor end is generally an indication of too low flow for the type of ports on the rotor. Higher than normal no-load power indicates mechanical problems such as bad bearings, sticking quill, improperly greased slide coupling, etc. Lower than normal no-load horsepower indicates worn refiner plates. Poor mechanical condition can increase no-load horsepower by over 10%. Refiners should be inspected annually to check mechanical condition. Some questions to ask when evaluating a refining system include: • Are you running in specific energy control (either hpd/t or kWh/t)? Specific Energy control will minimize over-

refining and optimize energy usage. • Is the net specific energy applied within normal guidelines for the grade/pulp? Is there a well defined refining

strategy? • Is the refiner operating properly – alignment and no sticking (e.g., splined shaft conversions can prevent

sticking and alignment problems)? • Is plate design matched properly to the fiber and refiner to achieve effective fiber development (optimize

strength lift per unit of freeness loss)? • Is the impact of refining on water retention value (WRV) and dewatering understood, i.e., run just enough

refining? • Is the stock consistency to the refiners between 3.5-5.0% for best energy transfer and fiber development? Does

the consistency fluctuate to the refiners? A consistency that swings will cause fiber development to swing and lead to over-refining.


• Is the refiner run within proper flow limits? Opportunities to optimize refining energy include: • Select refiner type, size, speed, and plates to minimize pumping and no-load energy losses. • Operate refiners within design hydraulic flow range. Stocks flow above and below design capacity will reduce

refining efficiency. • Select refiner plate patterns to provide desired fiber property development with the lowest gross energy applied. • Operate with recommended refiner rpm. No-load horsepower increases exponentially with higher refiner rpm. • Operate with lowest plate diameter consistent with stock flow and refining intensity requirements. No-load

horsepower increases exponentially with refiner plate diameter. • Bypass and shut down unnecessary and underused refiners. Normal refiner operation is most energy efficient at

motor loads >80% of motor rating. • Check freeness drop per hpd/t regularly to monitor refining efficiency and determine whether refiners are

working correctly. Typical Canadian Standard Freeness (CSF) drops per net hpd/t are 25-60 for Southern bleached softwood Kraft and 50-60 CSF/net hpd/t for bleached hardwood.

• Rebuild double-disk refiners to utilize splined shafts. Energy consumption can typically be reduced by 10-15% compared to floating-shaft arrangements.

• Some new conical and cylindrical refiner designs have lower no-load horsepower and provide more uniform refining than conventional disk refiners. However, conventional disk refiners provide more options for optimizing refining since more plates patterns are available and there are multiple refiner plate suppliers.

Approach systems Opportunities to reduce energy consumption in the stock approach system include: • Determine whether cleaners are needed. Size system properly for machine wet end. • Utilize cleaners designed for low pressure drops (less than 207 kPa or 30 psi pressure drop). • Conduct flow balances and verify operating conditions (consistency, pressure drop, efficiency, and debris

removal) of cleaners. • Reduce flows to fiber recovery stages based on balancing the system properly. • Shut down cleaners when product quality permits. • Determine whether deaeration is needed. • Monitor pressure screen differential pressure and reject flows. • Minimize stuff box flow and recirculation. • Install variable-speed drives for machine chest pump (to eliminate stuff box), fan pumps, and other variable-

flow requirements. • Design for low friction losses in piping. • Consider installing compact stock approach systems offered by several suppliers. Some systems have reported

energy savings as much as 25% from elimination of tanks and pumps. Recycled fiber systems Opportunities to minimize energy consumption in recycled fiber systems include: • Install energy-efficient pulper rotor and extraction plate designs. • Install energy-efficient drum style screen rotors • Ensure that pumps are not oversized. • Install frequency control on motors to reduce energy waste. • Increase consistency as much as possible to reduce hydraulic volume for pumping and agitating. • Simplify process configuration. Run equipment at optimum operating point. • Make process stable and homogeneous. • Close water loops. • Refine and disperse pulp as little as necessary.


• Use latest development of machinery equipment to increase overall efficiency. Water heating Substantial savings in water consumption can be accomplished with limitations in retention, quality, and energy dissipation. The reduction in water usage will also lead to an equivalent saving in energy consumption. The most energy-efficient systems have no continuous usage of steam to the silo or warm water system. Basic rules for water conservation include “reduce, reuse, and recycle.” Reduce simply means reducing fresh water usage. A systematic approach is recommended with clear identification of every stream. Paper mill water usage varies between 0 and 60 ton of water per ton of paper produced. Approximately 4-6 tons per ton represent a practical minimum. Zero consumption is possible, but only with serious quality drawbacks on some grades depending on wet end chemistry. Zero discharge is generally only achievable with products such as recycled fiber grades. Paper odor is a concern on grades used for food packaging and can limit water system tightening. Simple water reduction possibilities are often overlooked, so it is sometimes possible to achieve reduction of water and wet end energy consumption by up to 50%. Wet end water heating can represent 20-45% of overall paper machine energy consumption. Reuse can require a systematic study of possibilities of substitution. New process equipment, such as filters, will be required to allow whitewater streams to be reused. Recycling can result in significant water and energy reduction, but extra equipment such as filters and/or evaporators may be required. Heat dissipation and chemical concentration can become issues as water systems are closed. Opportunities to minimize steam required for water heating include: • Maximize stock temperature from the pulp mill (at least 5°F, 3°C warmer than silo temperature). • Utilize waste heat from the pulp mill (water stream at least 5°F, 3°C warmer than silo temperature) and/or hood

exhaust heat recovery instead of steam to heat whitewater and warm water. • Return only warm/hot water streams to the warm/hot water systems. • Minimize mill water infiltration into whitewater and warm water systems. • Minimize flow and maximize temperature of water from condenser systems. • Maximize strained/polished whitewater reuse in paper machine showers. • Ensure proper saveall design, maintenance, and operation. • Utilize strainers and polishing filters after saveall clear legs to allow reuse in showers. • Circulate vacuum pump seal water using strainers and a cooling tower. • Utilize stock/whitewater or warm water instead of mill water for additive make-up and carrier water when

feasible. Some additive injection systems introduce chemicals into stock just ahead of the headbox. • Use warm (approximately 100 °F, 38°C) water instead of cold mill water for seals or utilize mechanical seals

that do not require seal water. Some packing materials permit elimination of seal water. Pumps with repellers do not require seal water but increase motor drive loads.

• Utilize dead-band control logic for emergency water make-up into whitewater storage chests. Minimize level set point when you start to add mill water into chests to maximize effective chest capacity.

• Determine optimum silo temperature for the machine. Minimize total steam consumption. Savealls Effective saveall design and operation are essential for minimizing material losses and reducing water consumption on the machine. Increasing capacity, improving maintenance, and/or installing post-saveall strainers and filters can improve filtrate water quality to allow saveall filtrate to be reused in place of fresh water. Key saveall parameters to evaluate include: • Installation and equipment, including size (number of installed discs and available blanked-off discs), drop legs

(diameter and layout), and sector type (cover type and condition).


• Operation, including proper sweetener type and quantity, well-tuned vat level control, dilution of recovered stock with rich white water bypass, and cloudy filtrate recycle. Saveall operation should be monitored regularly to avoid system upsets.

• Optimize split between cloudy and clear legs to match usage and prevent mill water make-up into the system. • Maintenance including sector cover condition, sector-to-rotor seals, and knock-off and oscillating cleaning

showers. Dissolved air flotation (DAF) savealls can be used in addition to or instead of disk or drum savealls to help improve whitewater quality. Showering Showering is a major source of fresh water consumption on many machines. Any shower water used on the former that is below whitewater temperature requires steam to return the silo to desired temperature. Cool showers in the press section can lead to deposits and reduced press solids. From an energy and water conservation perspective, showers should utilize filtered/polished whitewater wherever feasible. One approach to optimize shower performance is to assign a whitewater reuse risk factor for each shower based on: • Water filtered with current technology • Likelihood nozzles will plug • Potential fabric plugging from fines • Negative effect on papermaking process Typical low-risk showers include: • Breast roll showers • Knock-off showers • Fabric flooding showers Medium-risk showers typically include: • Lubrication showers • Wetting showers High-risk showers include: • High-pressure wire cleaning • High-pressure felt cleaning Steps for optimizing shower performance include: • Determine optimum shower flows, shower and nozzle design, and water quality requirements. • Calculate potential energy and fiber savings from utilizing whitewater instead of fresh/warm water. • Improve saveall and filtering to achieve water quality requirements. • Check shower nozzles for wear regularly. Worn nozzles can use 40% more water than they were designed for. • Some fabric cleaning chemicals can reduce required shower water volumes. • Rotary tank cleaning showers can reduce water usage if machines are using fill and drain methods of cleaning

chests. Chemistry Chemistry can impact paper machine energy consumption by affecting sheet properties and improving drainage. Make-down and introduction of chemicals into the system can also affect energy consumption. Opportunities to reduce energy consumption through chemical systems include:


• Utilize chemicals which can increase strength, such as synthetic anionic or cationic dry strength polyacrylamides. This can provide savings through reduced refining, reduced basis weight, increased couch and press solids, and /or reduced starch usage.

• Utilize enzymes for fiber modification to reduce refining needs. • Utilize silica and microparticles to improve drainage. • Utilize whitewater or warm water instead of mill water for chemical injection or use additive injection systems. • Maximize ash content in the sheet. Headboxes Basis weight profiles ultimately impact pressing, runnability, and dryer operation. Pressure drop through headboxes has increased with headbox design evolution. Turbulence level and nozzle convergence impact MD/CD ratio capability. Consistency profiled designs require lower flow from the cleaner system. Some areas where headboxes affect paper machine energy consumption include: • Minimize MD and CD basis weight variability to improve runnability and maximize dewatering and drying

efficiency • Improve moisture profile to allow maximum possible moisture content at the reel • Optimize turbulence level and nozzle convergence. The impact on MD/CD ratio capability can help optimize

required strength characteristics to allow for reduced basis weight or reduced refining levels • Optimize headbox contribution to formation and sheet uniformity to aid forming, pressing, and drying rates,

improve runnability, and to improve strength allowing the use of higher freeness furnishes. • Operate headbox within designed flow range. Over-designed flow capability generally has very poor results • Maintain cleanliness for efficiency. Formers Formers consume energy directly through drive load and vacuum systems. Formation and drainage affect performance of downstream processes. Areas where the former affects energy consumption include: • Utilize former type and headbox that provide optimum formation results at higher consistency • Match hardware to drainage needs • Avoid sealing the sheet early in the forming process. • Graduate vacuum down the table to reduce drag load and provide proper sheet consolidation. Number of boxes

and slot widths should be adjusted to achieve proper vacuum densities in the high vacuum area. • Utilize multi-compartment high-vacuum boxes. • Evaluate drainage element materials for impact on drag load. Minimize number of high vacuum elements. • Forming fabric design can affect drive load and there may be opportunities to change to a low drag design

without adversely affecting paper characteristics or machine performance. • Avoid couch re-wet (suction box orientation, double doctors, air doctors) • Optimize headbox and forming temperatures for impact on drainage and solids • Monitor former solids frequently, maintain high level of solids Paper machine clothing Properly designed clothing can have an impact on energy consumption that far exceeds the cost of the fabrics. Forming fabrics affect energy efficiency in much the same way as formers: • Consistency off the couch, with ~10% of solids improvement transferring to the dryers • Improved formation resulting in better pressing uniformity • Flatbox vacuum requirements • Reduced drive loads


Press fabrics are an important part of press section optimization. Opportunities include: • Pressure uniformity through micro-pressing • Increasing consistency into dryers • Minimizing sheet rewet • Nip dewatering • Opportunities to reduce Uhle box vacuum • Effective fabric cleaning is required so press section dewatering does not go down over time Dryer fabrics can affect capacity and energy efficiency through: • Fabric tension • Surface contact – heat transfer • Pocket ventilation – mass transfer • Resistance to contamination Vacuum systems The vacuum system is often the second largest process in the paper mill for electrical energy consumption (after paper machine drives), and is frequently one of the least understood parts of the papermaking process. Vacuum systems can have from 1,000 to 10,000 installed horsepower. Often vacuum systems can use 10−20% more horsepower than is necessary for paper production. Some of the most common vacuum system problems that can increase energy consumption and/or reduce system efficiency include: • Hot seal water. • High backpressure on vacuum pumps. • High seal water pressure, resulting in high seal water flow. • Use of synchronous versus induction motors can affect power factor for the entire paper mill. • Recirculated seal water system with no cooling, or poorly functioning cooling system. Usually this is done with

a cooling tower. • Worn or missing seal water orifices and nozzles. • Scale build-up in pumps and piping. • Worn pump rotor, casing, or lobes. • Old, obsolete, and less efficient vacuum pumps. • High piping losses and incorrect system design. Guidelines to minimize vacuum system energy consumption include: • Use fans or exhausters instead of vacuum pumps for low-vacuum applications such as vacuum foils. • Control vacuum level by bleeding air into the system instead of by throttling liquid ring pumps. Reduce vacuum

capacity if in bleed valves are always open. • Graduate flatbox vacuum to maximize dryness and minimize drag load. • Eliminate unnecessary vacuum boxes (remove or drop out of contact with the fabrics). In addition to requiring

additional vacuum pumps, sucking excessive air through the sheet can cool the sheet and cause press solids to drop more than the small amount of water that comes out with the air, especially on lightweight, open webs. Extra flatboxes also add drag load to the table. Proper flatbox setup can remove more water while reducing table drive load by as much as 10%.

• Ensure proper Uhle box slot size to provide required flow capacity and dwell time. • Evaluate number of Uhle boxes per fabric. One Uhle box provides adequate conditioning on some fabrics.

Some later press fabrics do not require Uhle boxes. • Ensure proper vacuum pump application (high-vacuum vs. low-vacuum pump design).


• Prevent carryover of process fluids from suction point. • Provide water/air separation ahead of the pump to prevent two-phase flow at the pump. • Use proper separator removal pump design. • Check vacuum pump internal clearances and/or capacity annually. Rebuild pumps operating at less than 80% of

design capacity. • Conduct routine maintenance of vacuum pumps and auxiliary equipment, including belt and gear drives and

motors. • Replace and calibrate gauges and process instrumentation (vacuum gauges, seal water pressure gauges, level

transmitters in vacuum pump sumps, amp meters for motors) • Remove old, inefficient vacuum pumps from service. Do not rebuild obsolete pumps with inefficient designs. • Modern blower systems consume much less electricity than liquid ring vacuum pumps, do not require seal

water, and can provide heat recovery opportunities. It is difficult to justify replacing liquid ring vacuum pumps with blowers unless electricity cost is high. Blower systems have been installed on several machines in Europe where electricity cost is relatively high. Some new machines in North America have installed blower systems. Some mills with hard water have installed blowers to avoid calcium carbonate buildup in conventional vacuum pumps.

System audits can be used to help reduce wasted energy. Replacing or calibrating gauges can ensure proper indication of vacuum levels. Key operating data should be monitored, reviewed and recorded. Sheet and fabric moisture should be checked regularly to ensure effective use of vacuum. One of the most effective ways to manage vacuum system energy is through EMBWA (Energy Management by Wandering Around). Additional information on vacuum system optimization is included in TAPPI TIP 0404-55 “Performance evaluation techniques for paper machine vacuum systems” (5). Press section On a typical paper machine with 0.5% headbox consistency, 20% couch solids, 40% press solids, and 5% reel moisture, 195 kg water is removed per kg fiber in the forming section, 2.5 kg water per kg fiber in the press section, and 1.45 kg water per kg fiber in the dryer section. However, the cost of water removal is significantly lower in the forming and pressing sections than in the dryer section. Removal of the water content after the press section represents more than 50% of the energy consumption in the paper machine system. Each one percentage-point improvement in solids out of the press section results in 3-5% less water that needs to be evaporated in the dryer section. Maximizing press performance is thus one of the most important aspects of paper machine energy conservation. Primary opportunities in the press section are increased water removal, dryer section steam savings, increased production, more efficient water removal, sheet property improvements, and fiber savings on bulk sensitive and strength grades. Factors influencing press water removal are furnish, time, temperature, and pressure. Press performance can be improved by increasing nip load and by increasing the time during which the press load is applied. Press impulse (press nip pressure times nip residence time) has been shown to be a good performance indicator for press water removal. Development of shoe presses has significantly increased time available in the nip, resulting in higher press impulse without the damaging effects of raising nip load. Press performance can also be improved by increasing temperature of the web during pressing. Experience indicates that solids content of the pressed web can be increased by one percentage point for each 10°C (18°F) increase in web temperature. Methods to increase temperature in the press section include increased stock temperature, steam shower applications on the sheet or on the fabric, and hot water flooded nip showers. Energy efficiency of heating the sheet in the press section should be compared with that in the dryer section (typically 1.3 kg steam per kg water evaporated). Operating felt showers with cool water (such as fresh water) cools press fabrics and reduces sheet dewatering. Trials have indicated that sheet dewatering can be increased by one percentage point by increasing shower water temperature by 10 oC. High-pressure and low-pressure shower water should be at least equal to the temperature of stock at the headbox. Shower water temperature of 54°C (130°F) or above is beneficial in maintaining fabric


temperatures. Shower water heating is an excellent application for direct or indirect heat recovery. Shower water temperature on the last press fabric should have priority for use of warm water on the wet end of paper machines. Uniformity of pressure applied to the sheet in the press is important, especially with modern shoe press technology, because of increased nip dwell times and lower peak nip pressures. Modern press fabric designs provide improved pressure uniformity and higher sheet solids content. Multi-axial laminated fabrics provide superior pressure uniformity, excellent bridging on vented/drilled rolls and more steady-state pressing compared to conventional fabrics. Flat batt fibers can offer contact area equal to round fine denier batt without sacrificing wear volume. TAPPI TIP 0404-52 “Press Section Optimization” (6) provides guidelines for evaluating and improving press section performance. The TAPPI Paper Machine Wet Press Manual (7) provides more complete coverage of press section optimization. Opportunities to optimize pressing include: • Shoe pressing increases dryness potential, and for bulk-sensitive grades, adds degree of freedom (bulk vs.

dryness). • Double felting improves dewatering on heavyweight grades. • Graduate press loads. • Maximize loading throughout the grade mix (within sheet quality limitations). • Steam boxes increase sheet temperature and increase exiting dryness; can also be used for profile improvement. • Felt heating will help clean the fabric as well as help maintain or increase sheet temperature but may not be

energy efficient. • Optimizing roll cover hardness and use of blind drilled or other cover designs can improve press dewatering. • Balance between nip and Uhle box dewatering over fabric life. • Maintain shower temperature at or above sheet temperature. • Nip dewatering efficiency, press geometry, fabric selection, and operations can result in improved profiles,

solids, and in vacuum for uhle boxes. • Felt and belt design optimization - press fabric design greatly impacts press efficiency, solids level. • Minimize rewet (fabric runs / sheet runs; sleeve doctors, double doctors, air doctors, use of catch pans on high

dewatering nips that generate water spray). • Minimize draw to maximize CD strength on grades requiring high CD strength properties. • Check nip profiles and optimize crowns, dubs, and fabric cleaning to improve moisture profiles. • Monitoring of pressing performance throughout fabric life—on-line monitoring of press water flows, frequent

CD and MD monitoring of fabric permeability, moisture, and temperature. • Check couch and press solids at least once every outage cycle. Maintain a database of results. Steam showers Steam shower efficiency depends on the product being made, where the steam box is installed and how it is operated. TAPPI TIP 0404-58 discusses steam shower applications in the forming and press sections. Steam showers are most energy efficient with low steam ratios on relatively cool systems with vacuum assist beneath the steam box. Better steam utilization efficiency occurs when steam showers are located ahead of the last press nip since there is less water to heat. For most applications, efficient steam flow ratios are 0.10 lb steam/lb paper for fourdrinier applications, 0.075 lb/lb for press section applications, and 0.05 lb/lb for Uhle box steam showers. Mills should determine the value of steam boxes for specific applications and operate accordingly. Some modern steam box designs can operate with much greater energy efficiency than some older models. Opportunities to optimize steam shower performance include: • Utilize low pressure “waste” or vented steam. • Reduce steam flow when producing grades that are not drying limited. • Operate steam box at clearance recommended by manufacturer. • Apply only as much steam as can be condensed in or on the sheet. • Lower steam supply to reduce excess “fog” in the machine room.


• Use profiling capability to apply steam only where needed. • Reduce vacuum to reduce sheet cooling and air infiltration under the steam shower. • Increase vacuum to improve steam penetration into sheet. • Control steam temperature to improve condensation rates. Typical recommended temperatures are 5-10°F (3-

6°C) of superheat above saturation temperature. • Provide proper mist elimination when utilizing flash steam. In many cases, some high-pressure make-up steam

is required to introduce a small amount of superheat. • Isolate non-profiling preheat section of profiling steam shower. • Extend and contain steam in “wedges” and “tunnels.” • Maintain pressure and temperature gauges. • Maintain profiling mechanisms in good working condition. • Eliminate pulp splatter from trim squirts. • Utilize Teflon and/or polished surfaces to minimize build-up and allow operation at design clearances. • Consider applying a little steam to multiple locations in the press section instead of a lot of steam in only one

location. • Elevate press fabric temperatures to the same as the sheet to encourage water movement in the press nip. Dryer section The dryer section represents the largest thermal energy consumer on the paper machine. Information on monitoring dryer section performance is included in TAPPI TIP 0404-33 “Dryer section performance monitoring” (8). The “10 Commandments” of energy efficient drying are: 1. Don’t dry any more than you must. 2. Don’t vent steam – anywhere. 3. Match the ventilation air flow to drying requirements. 4. Use steam from lowest header pressure possible. 5. Keep the machine running (minimize break times). 6. Improve the moisture profile. 7. Increase the heat transfer rate. 8. Measure what you must control. 9. Keep the steam system calibrated. 10. Don’t use motive steam when make-up steam can be used. Five rules for dryer steam system energy efficiency are: 1. Keep the system “tight.” 2. Efficiently utilize flash steam from high temperature condensate. 3. Maximize use of low pressure steam. 4. Minimize heat used for hood supply air heating. 5. Manage the steam system. Energy-efficient drying requires a combination of steam system design, equipment, operation, maintenance, and control. Dryer arrangement Dryer section arrangement primarily affects drying energy consumption by changing machine or heat transfer efficiency. Examples include: • Single-tier arrangements have high dryer-sheet wrap angles and short unsupported sheet lengths. Heat transfer

rates and threading efficiency are thereby improved. • Stacked dryer arrangements are less energy-efficient than conventional single or two tier dryer configurations.

Dryer fabrics generally cannot be applied with stacked dryer configurations. • Increased sheet restraint from wrap angles and fabric pressure improves thermal contact with dryers and reduces

CD shrinkage.


• Vacuum-assisted devices and/or blow boxes and placement and quantity of draw points affect total draw requirements. Draw reduction increases CD strength.

• Windage control impacts runnability • Fabric tension affects drying heat transfer by increasing sheet-dryer thermal contact. • Fabric design affects uniformity of sheet contact with dryer surfaces and heat transfer. • Fabric and dryer cleanliness impacts heat transfer performance. • Blow box systems can improve high speed runnability and machine efficiency. Thermocompressor systems Thermocompressor steam systems utilize high-pressure motive steam to recompress low-pressure blowthrough steam and reuse it in the same dryer section. Good steam separators, proper piping design, and adequate motive steam pressure are critical for efficient operation. Opportunities to minimize energy consumption using thermocompressor systems include: • Ensure no steam venting during normal operation. • Utilize blowthrough control and/or automatic pressure and differential-pressure letdown to minimize venting

during sheet breaks. • Optimize differential pressures for condensate evacuation and blow-through flows. • Utilize properly sized thermocompressors. (Note that thermocompressors, by design, are most effective over a

narrow operating range. Machines with wide variations in condensing loads may not be appropriate applications for thermocompressors.)

• High efficiency thermocompressor use less motive steam and have a broader operating range than conventional thermocompressors.

• Optimize motive steam pressure to minimize amount of motive steam flow required and net thermal energy cost.

• Utilize steam bleed in low-pressure dryers or other low-pressure steam user such as air pre-heat coils to purge non-condensable gases from the steam system.

Cascade systems Cascade steam systems reuse flash and blowthrough steam from a high-pressure dryer section in a different dryer section that operates at lower steam pressure. Opportunities to reduce energy consumption with cascade steam systems include: • Group dryer steam sections to minimize steam venting • Minimize number of dryers draining to a condenser and amount of blowthrough steam from these dryers. • Ensure proper section splits to prevent venting during normal operation. • Utilize blowthrough control and/or automatic pressure/differential-pressure letdown to minimize venting during

sheet breaks. • Provide make-up steam from the lowest available steam pressure header that will support section pressure

requirements. Steam system design There is no “one and only correct solution” for steam and condensate system design. The proper system design depends on the mill steam supply and condensate return systems and production requirements. Proper sizing of piping and equipment is critical, using well-established procedures and guidelines. Detailed piping design should be done and reviewed by a qualified party to ensure proper system operation. Considerations for energy-efficient steam system designs include: • Ensure no steam venting during normal operation. • Utilize low-pressure instead of high-pressure steam where appropriate.


• Utilize blowthrough control and/or automatic pressure letdown to minimize venting during sheet breaks. • Provide high steam separator efficiency, especially with blowthrough control. • Measure condenser water temperature at outlet rather than inlet. • Recover flash steam from separator tanks. • Return condensate to the boiler house at high temperature (> 230°F, 110 °C). • Do not valve off dryers to control drying capacity. Improve flexibility of the steam and condensate system

instead. Steam system hardware Proper syphon design is a key component in making the steam system energy efficient. Stationary syphons generally require less blowthrough steam (less than 10% of condensing load with stationary vs. 15-30% with rotary) and lower differential pressure (15-35 kPa or 2-5 psi with stationary vs. 40-95 kPa or 6-14 psi with rotary). In dryers draining directly to a condenser or heat exchanger, reduced blowthrough steam directly results in energy savings. In sections that cascade to lower pressure groups or in sections with thermocompressors, energy savings are still possible, but evaluation of energy savings is more complicated. In a thermocompressor section, energy savings will be achieved by converting to stationary syphons if the section was venting with rotary syphons. Lower differential pressures and lower blow through flows reduce the potential for venting. In a section that is not venting, savings opportunities depend on relative cost of motive and make-up steam. Reduced blow through and differential pressure will result in less motive steam and more make-up steam – but the total amount of steam will remain the same. If both motive and make-up steam are supplied from the same header, there will be no energy savings resulting from converting to stationary syphons. However, if the powerhouse is able to generate significantly more electricity from the lower-pressure make-up steam extraction than from the higher-pressure motive steam, energy cost savings can be significant. Turbines typically make the most electricity when most of the high-pressure steam goes through all of the stages. Likewise, in a cascade system, there is no net energy savings from simply converting to stationary syphons if the lower-pressure section condenses all of the blow through steam sent to it (with the exception of wet end sections). Dryer bars are recommended for all dryers operating above rimming speed to provide uniform heat transfer profile, high heat transfer rate, and correspondingly high drying rate. Rimming speed depends on dryer diameter and condensate layer thickness, but is typically around 300 meters/minute (1000 fpm). A dryer section will evaporate more water with dryer bars installed. Minor reductions in energy consumption are possible with dryer bars related to operation at lower steam pressures with improved heat transfer. However, it takes additional steam to evaporate this water, so the kg steam used per kg of water evaporated remains nearly the same. This same principle also applies to felting unfelted dryers or increasing dryer fabric tension. Drying rates will improve, but energy efficiency (as measured by kg of steam used per kg of water evaporated) will see little change. Additional information on dryer bars is included in TAPPI TIP 0404-35 “Application of dryer bars” (9). Guidelines for steam system hardware include: • Utilize stationary syphons where advantages can be realized from lower differential pressures and blowthrough

flows. • Install modern steam joints to reduce steam leaks and maintenance costs. • Install dryer bars (increase drying rates and improve moisture profiles in most cases) in all dryers operating

above rimming speed. • Size thermocompressors for current steam system operation. • Optimize thermocompressor design and operation to minimize motive steam use. • Check sizing of rotary syphons. • Utilize pilot-operated safety relief valves for applications that operate close to maximum allowable working

pressure. • Improve mechanical reliability of equipment to prevent leaks. • Utilize smart transmitters on all pressure, blowthrough, differential pressure, and level control loops.


Steam system operation A properly designed steam and condensate system with good equipment will still waste energy if not operated properly. Considerations for energy-efficient steam system operation include: • Operate dryer differential pressures at the proper set point for condensate evacuation and blow through flows. • Ramp warm-up dryers to maximize runnability • Maximize drying in sections that use low-pressure steam whenever possible • Minimize dryer surface contamination • Minimize steam venting to condensers and to atmosphere • Utilize proper dryer warm-up procedures to minimize steam joint seal failures and leaks and bearing failures. • Close separator tank drains • Verify functionality of separator tank level controls • Confirm functionality of vacuum tank level control • Avoid over drying – minimize number of cans in the falling rate zone • Conduct regular rounds to check for venting and leaking safety relief valves. • Regularly check vacuum systems for air leaks. Steam system maintenance Dryer section maintenance items that affect energy consumption include: • Ensure tight shut-off of steam vent, bypass, and blow-down valves • Calibrate pressure and differential-pressure transmitters • Disconnect steam to dryers that do not contact the sheet, such as bottom unorun dryers, felt dryers, and Feeney

dryers • Check that steam traps are functioning properly • Verify action of steam valves and thermocompressor actuators • Check calibration (annually) and zeroing (every outage) of dryer section pressure, differential-pressure,

blowthrough, and steam flow meters. • Fix steam leaks. A small pinhole, a leaking gasket, a trap that’s stuck open, or a leaky steam joint can easily

waste 150-250 lb/hr steam. Cost of maintenance is typically small compared to potential energy savings. • Ensure that there is no sewering of dryer or air system condensate. Steam system control Opportunities to utilize steam system control to minimize energy consumption include: • Control vacuum condenser to match differential pressure requirement. • Automate dryer warm-up and shutdown sequencing. • Automate dryer sheet break recovery response. • Automate grade change response. • Automate dryer steam system pressure and differential control. • Install thermocompressor cut-off control to prevent venting. • Install smart control valves to support preventive maintenance. • Trend major process variables. • Automated dryer management systems improve control of steam and condensate systems and improve energy

efficiency. Provide troubleshooting help. Dryer section condensers Condensers typically provide a link between paper machine steam and condensate systems and water systems, since heated water exiting the heat exchangers typically flows to paper machine warm water tanks. From the steam system standpoint, waste steam going to the condenser should be minimized. From the water system standpoint, condenser


water flows should typically be minimized (especially if an additional source of waste heat is used in the warm water tanks) and temperature maximized. Three types of condensing heat exchangers are typically used in paper machine steam and condensate systems: • Lead dryer condensers • Vent condensers • Flash tank condensers Three methods of control are typically used for condenser systems: • Cooling water outlet temperature (preferred from a water system standpoint). • Steam pressure or vacuum to the condenser (sometimes preferred for wet end dryer applications) • Condensate temperature (generally not recommended except in some flash condenser applications to prevent

venting to atmosphere. Monitoring inlet and outlet water temperatures and flow provides an indication of dryer section venting, heat loss, and energy efficiency. Proper condenser setpoints should be based on required differential pressure for lead dryers for wet end condensers, dryer section pressure for vent condensers, and required condensate temperature for flash condensers. One of the most common problems with condenser systems is air leaks into the system. Since air leaking into steam piping is not visible, it is often overlooked. Systems operating with pressure or condensate temperature control will typically open water valves in an attempt to obtain vacuum when air is leaking into the system. One method to identify air leaks is to pressurize the system that normally operates under vacuum and then conduct a round to check for steam leaks. A typical procedure follows. 1. Raise pressure set point in the individually controlled dryers to 10 psig (0.8 bar) (keep differential pressure in

automatic). 2. Increase vacuum condenser set point to +3 psig (+0.22 bar) 3. Turn off vacuum pump. 4. Inspect steam and condensate joints on all individually controlled dryers (looking for steam leaks). 5. Inspect piping, valves, and separators from dryers to heat exchangers. 6. When inspection is complete, return all settings to normal operation. Flash steam utilization High temperature condensate will generate flash steam as it is collected in a lower pressure tank. This flash steam is often wasted or poorly used. Often it is vented either at the machine or at the boiler house. Low pressure flash steam can be reused as make-up steam to wet end dryers, steam showers, water heating, or flash coils in the pocket ventilation system. If flash steam is used for steam showers, condensate carryover must be avoided through good separation, steam traps, and proper piping design. In some cases, small amounts of higher-pressure steam are required to provide a small amount of superheat to the line. Note that care should be taken in reusing flash steam. It is possible to distill pH-controlling amines from flash steam and end up with corrosive carbonic acid that will quickly eat through steam coils. In some cases it may be easier and more cost effective to pump hot condensate through air heating coils rather than utilizing a low-pressure flash tank. It is important to keep the condensate pressurized to prevent flashing and hammering before the coil, so level control valves must be positioned downstream of the coils. Pocket ventilation and hood supply systems Supply air temperatures of <80ºC (<180ºF) are generally optimal for pocket ventilation system performance. A general guideline is that supply air temperatures should be as low as possible as long as sweating does not occur in dryer hoods. Some machines operate supply air temperatures below 150oF (66oC) without hood sweating Poor air movement in dryer hoods can require higher supply air temperature on some machines if sweating occurs. Dryer air


systems operated at elevated temperatures increase energy consumption, but offer little or no improvement in drying capacity. There is typically no need for pocket ventilation temperature to be higher than sheet temperature. As an example, a machine with 3400 m3/min (120,000 cfm) of pocket ventilation air supplied to the dryers will utilize 8,220 kg/hr (18,100 lb/hr) of steam at 116ºC (240ºF), and only 6,040 kg/hr (13,300 lb/hr) steam at 93 ºC (200ºF). Operation at the lower temperature results in $230,000/yr energy savings at steam costs of $13.20/1000 kg steam ($6.00/1000 lb steam). Additional information on hood air systems is included in TAPPI TIP 0404-24 “Recommended operation of dryer section hood air systems” (10). Opportunities to reduce energy consumption with dryer section hoods and air systems include: • Operate pocket ventilation system at <180ºF (<80°C). • Optimize hood exhaust humidity. Adjust air exhaust to match drying load and hood capability. • Recover heat from hood exhaust streams to reduce steam used to heat process water and air streams. • Use air from inside the building rather than outside air for pocket ventilation. • Insulate dryer hoods and ductwork to maximize delivery of energy to the intended processes. • Replace damaged hood panels and panel seals to improve thermal head potential. • Monitor condition and cleanliness of air system filters and coils. • Change ventilation system filters and clean coils when necessary. • Check air heater coil tubes and headers for leaks • Check air heater coil finned surfaces for uniformity of temperature from top to bottom • Pocket humidity control impacts drying rate and profiles. • Monitor supply ventilation air temperature rise across heating coils • Monitor ventilation air exhaust temperature drop across heat recovery units • Replace existing hoods with high-thermal performance hood designs. • Use appropriate steam trap design for air heaters. • Monitor fan motor amps. • Monitor static pressure and vacuum levels of air system components. On some machines, significant savings can be achieved by varying hood exhaust, pocket ventilation air flows, and supply air temperature to match evaporation rate. With the lower cost of variable speed drives, it has become more practical to adjust hood exhaust and pocket ventilation flow rates to match the production (evaporation) rate. This results in steam savings, electrical drive savings, and optimized heat recovery. Control schemes can vary from a simple manual adjustment of the system by grade or automatic adjustment based on fans curves and production rates. Energy savings can be significant for machines that produce over a wide grade range; particularly when a significant portion of the production trends towards light weights and lower dryer pressures. Size press The size press offers opportunities to reduce energy consumption by reducing the amount of water evaporated, increasing machine efficiency, and optimizing sheet strength. Opportunities include: • Evaluate product need to determine appropriate application technique – surface application or heavy

penetration. • Film-type designs can minimize water load applied. • Maximize solids content of material applied. • Maximize ingoing moisture content. • Improve threading and spreading (efficiency). • Select early after dryer surfaces to improve runnability and minimize picking (air turns, dryer coatings,

alternative drying methods). • Optimize strength with size press application - to minimize fiber content and optimize filler content.


Reel Improvements to reel operation contribute to increased machine efficiency. Sheet defects near the spool and edges negatively increase slab losses and impact process efficiency. Spool deflection can contribute to defects. Opportunities include: • Rubber covered reel spools to minimize turn-up losses • Nip relieving and/or controlled primary/secondary arm transfers can reduce defects • Control winding parameters (torque, nip, tension) • Maximize turn-up efficiency (number of sets per reel) • Utilize efficient turn-up systems and reel brakes to minimize slab losses • Monitor and display slab losses, report results - control to maintain world class levels Miscellaneous steam systems The energy lost in steam lines from the powerhouse to the paper machine room and in condensate lines back to the powerhouse can be reduced by eliminating steam leaks, avoiding unnecessary pressure drops, ensuring proper operation of steam traps, and maximizing the amount of condensate that is returned. Opportunities to reduce energy consumption in the overall steam and condensate system include: • Repair steam leaks. • Insulate steam system piping and separators. • Utilize the lowest feasible steam pressure for miscellaneous steam users such as steam showers, water heating,

and air heating. • Conduct regular steam trap surveys and repair leaking or plugged traps. • Check for excessive pressure drops through flow meters, lines, etc. • Ensure that proper pressure and temperature compensation factors are used in steam flow meter calculations. • Utilize pilot-operated safety relief valves for applications that operate close to maximum allowable working

pressure. • Conduct regular rounds to check for venting and leaking safety relief valves. • Utilize “degrees of superheat” control for desuperheaters, where temperature setpoints are established based on

a given superheat level above saturated steam pressure. • Determine standard operating procedures for steam trap and drain line valving during warm-up and normal

operating conditions. • Clean heat exchanger and monitor overall heat transfer factors. • Maximize condensate return to the powerhouse. Compressed air systems Compressed air is one of the most inefficient sources of energy in the mill. It takes 5-6 kW (7-8 hp) of electricity to generate sufficient compressed air to drive a 0.75 kW (1-hp) air motor. A typical 56 kW (75 hp) compressor with 5-day/week, 2-shift operation will typically have $20,000 equipment cost, $20,000 maintenance cost, and $130,000 electrical cost over a 10-year life. Replacement of the air-driven motor with an efficient electric motor has the potential for significant savings over the life of the unit. Opportunities to minimize compressed air cost include: • Reduce compressed air system headers as much as possible. • Instrument air dew point should be 10ºC (18ºF) below the lowest temperature the system would see. • Utilize ultrasonic leak detectors to identify air system leaks. • Conduct annual air system audits. • Utilize dedicated compressor instead of mill air for headbox air pads and press section air doctors.


• Reclaim water from compressors where appropriate. • Install variable frequency drives on incremental capacity air compressors. Air system audits can typically identify energy savings of approximately 30% of compressor energy consumption. For a large mill, this can result in $250,000 - $1,000,000 in energy savings per year. Compressed air surveys typically involve: • Developing a block diagram of the system. • Measuring baseline conditions. • Implementing an appropriate control strategy. • Re-measuring after controls are adjusted. • Walking through the system to identify preventive maintenance and additional opportunities. • Identifying and fixing leaks and correcting inappropriate use. • Implementing awareness and continuing improvement plans and reporting results to management. Air-padded headboxes require relatively high volumes of compressed air (4.25 to 7 m3/min or 150 to 250 scfm) at low pressures (less than 100 kPa or 15 psig). These should utilize dedicated headbox compressors instead of bleeding off of mill air headers. Reclaiming water from air compressors can also provide energy and water savings. Additional information and references on compressed air systems are included in reference 3. Machine room ventilation Effective maintenance, proper temperature setpoints, and winter/summer operating strategies can be used to improve energy efficiency of machine room ventilation systems. Machine room ventilation is discussed more completely in TAPPI TIP 0404-50 “Machine room ventilation guidelines” (11). Opportunities to reduce energy consumption associated with machine room ventilation include: • Establish winter and summer operating conditions for machine room supply and exhaust fans. • Operate air make-up units at 21ºC (70ºF) set points and roof supply systems at 49ºC (120ºF). • Utilize water or glycol systems (with heat recovery) to heat make-up air. • Utilize air from inside the building instead of outside air for motor cooling, roof supply, and pocket ventilation. • Shut off steam coil or glycol systems to air make-up units when fans are shut off. Ensure that there is proper

freeze protection. • Shut outside doors in the winter time. Note that machine room ventilation air directly replaces the air removed from the machine room by process exhaust systems and general exhaust fans. Shutting these systems off as a means to reduce energy use can be counter-productive as the removed air will be replaced regardless. The replacement air will enter the machine room in an uncontrolled manner and can cause unintended product quality and housekeeping problems. Heat recovery An energy balance around the paper machine room shows that all thermal energy provided to the machine room exits with the sheet (very small amount), exhaust air streams, steam vents, condensate returns, and water streams. Opportunities for dryer hood heat recovery are typically limited to supply air preheating. Air-to-air economizers have limited potential to recapture energy from exhaust streams. The amount of energy recoverable in the drying section is limited due to the ratio of latent heat in the exhaust and the sensible heating of the dryer air. Overall energy content in the exhaust air is about 6-10 times greater than the potential heating of incoming air.


Air-to-liquid economizers used for heating fresh water, whitewater, or circulating water or glycol systems provide greater opportunity to improve the amount of recovered heat. More elaborate heat recovery systems could substantially improve the degree of energy saving, but these systems typically have increased cost, complexity, and maintenance. High humidity closed hoods require much less hood exhaust and offer much greater heat recovery potential. Areas with opportunity for heat recovery include: • Dryer section hood exhaust • Yankee hood exhaust • Pulp machine air dryer hood exhaust. • TMP steam • Vacuum blower exhaust • Waste heat from pulp mill and evaporators. • Sewer streams Tissue machines Many of the energy conservation practices discussed above also apply to tissue machines. Tissue and towel machines offer additional opportunities to optimize energy consumption. Most machines with conventional Yankee dryers utilize steam showers, suction pressure rolls, steam-heated Yankee dryers, and gas-fired hoods to remove water from the sheet. Energy conservation requires maximizing use of low-cost energy sources (typically low-pressure steam used in steam showers) and minimizing consumption of high-cost sources (typically natural gas used for hood burners). Increasing recirculation air and reducing make-up and exhaust air from the Yankee hood system will reduce energy consumption at the cost of drying rate. Good performance for tissue machine drying steam and gas usage is 5.2 GJ/tonne (6.0 MMBtu/ton). Low energy users utilize 3.4-4.3 GJ/tonne (4-5 MMBtu/ton), below average users are 4.3-5.2 GJ/tonne (5-6 MMBtu/ton), high-energy users are 5.2-6.0 GJ/tonne (6-7 MMBtu/ton), and very high-energy users are 6.0-6.9 GJ/tonne (7-8 MMBtu/ton). Through-air dried (TAD) machines typically use significantly more energy per kg of product than conventional Yankee machines. This is because more water is dried and none is mechanically pressed from the sheet. Additional information on TAD is included in TAPPI TIP 0404-25 “Through drying” (12). Opportunities to optimize energy consumption on tissue machine hood and air systems include: • Operate in cascade mode instead of parallel mode. • Optimize air system burner efficiency and stabilize static pressure to nozzles. • Set up air supply and exhaust dampers (or fan speeds) to optimize energy efficiency. Utilize hood humidity

sensors (0.40 – 0.45 lb/lb typically optimal). • Preheat burner combustion air. • Adjust air system fuel/air ratio. • Optimize hood impingement temperature vs. impingement velocity. • Optimize air cap gap (3/4”) to increase heat transfer from the nozzles. • Balance hood to minimize infiltration and leakage. • Maximize heat recovery from hood exhaust. • Preheat make-up and combustion air streams to minimize natural gas usage. • Ensure no leaks from hood, bypass dampers, or duct flange connections. • Conduct regular hood performance surveys. Additional opportunities to minimize energy consumption on tissue and towel machines include:


• Monitor and benchmark energy flows. • Optimize pressing to maximize sheet solids. Take regular sheet moisture samples after suction pressure rolls. • Optimize use of press section steam showers. • Maximize Yankee operating steam pressure (within limits of dryer rating, sheet quality, Yankee coating, and

thermocompressor venting issues) to minimize use of natural gas in heating hood air. • Maximize proportion of drying done by after-dryers on wet-crepe machines. • Utilize infrared cameras to check ductwork insulation for hot spots. • Optimize thermocompressor system operation to eliminate venting. • Monitor hood exhaust humidities and adjust dampers to minimize energy use. • Increase reel moisture when quality considerations allow. Conversions GJ/tonne × 0.8606 = MMBtu/ton kWh/tonne × 0.9072 = kWh/ton Keywords Energy, Paper Machines Literature cited 1. TAPPI TIP 0404-47 “Paper Machine Performance Guidelines”-2016 2. Energy Cost Reduction in the Pulp and Paper Industry, First Edition, PAPRICAN, November 1999 3. “How to Conduct a Paper Machine Energy Audit”, Richard A. Reese, TAPPI PaperCon 2013, Atlanta, GA, 4. U.S. Department of Energy Office of Industrial Technologies web site: http://www.oit.doe.gov/bestpractices 5. TAPPI TIP 0404-55 “Performance Evaluation Techniques for Paper Machine Vacuum Systems” 6. TAPPI TIP 0404-52 “Press Section Optimization” 7. Paper Machine Wet Press Manual, Fourth Edition, TAPPI PRESS, 1999. 8. TAPPI TIP 0404-33 “Dryer Section Performance Monitoring” 9. TAPPI TIP 0404-35 “Application of Dryer Bars” 10. TAPPI TIP 0404-24 “Recommended Operation of Dryer Section Hood Air Systems” 11. TAPPI TIP 0404-50 “Machine Room Ventilation Guidelines” 12. TAPPI TIP 0404-25 “Through Drying” Note that this TIP was originally developed from a panel discussion on “Paper Machine Energy Conservation” at the 2001 TAPPI Engineering Conference. Additional information Effective date of issue: September 9, 2016 Working Group: Dick Reese – Dick Reese and Associates, Chairman Ben Drummond – Spraying Systems Tim Hasbargen – Focus on Energy Ken Hill – Kadant Johnson Systems Jon Kerr – Miami University Dennis Kalberg – Retired Pekka Kormano – Deublin Steam Systems Jeff Reese – International Paper Tom Rodencal – Rodencal Associates


Arvind Singhal – Andritz Doug Sweet – Doug Sweet and Associates Greg Wedel – Kadant Johnson Philip Wells – Wells Enterprises Inc.

tip 0404-63 paper machine energy conservation

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