Paper Machine Energy Conservation

8/12/2019 Paper Machine Energy Conservation

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TIP 0404-63ISSUED 2003

REVISED 2006REVISED - 2011

2011 TAPPI

The information and data contained in this document wereprepared by a technical committee of the Association. The

committee and the Association assume no liability or responsibilityin connection with the use of such information or data, including

but not limited to any liability under patent, copyright, or tradesecret laws. The user is responsible for determining that thisdocument 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 itimportant to implement energy management and conservation measures. Paper machine energy consumptionrepresents 50-70% of purchased energy for an otherwise efficient integrated mill. If the paper machine is inefficientin its 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 formonitoring, benchmarking, and optimizing energy-intensive unit operations to reduce paper machine energyconsumption.

Safety precautions

Follow normal safety precautions when working around paper machinery, including use of personal protectiveequipment. Do not allow loose clothing or equipment to contact rotating machinery or ropes. Beware of overheadcranes and thermal and slip hazards around the dryer section. Avoid direct contact with hot surfaces. Use hearingprotection in noisy areas. Eye protection should be worn in all production areas. Safety shoes and safety helmetsshould 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, integratingprocesses 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.

System monitoring

Scottish mathematician and physicist Lord William Thomson Kelvin (1824-1907) said, If you cant measure it, youcant improve it. A key first step in energy conservation activities is monitoring energy consumption and makingsure flowmeters and cost information are accurate. Some mills have developed mill-wide system balances that canbe used to check accuracy of individual flowmeters. Assigning a person to be responsible for energy conservation inthe mill and/or paper machine area can help increase visibility and accountability of conservation efforts. Steps foran effective monitoring program include:


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TIP 0404-63 Paper machine energy conservation / 2

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 energyconsumption.

Discuss energy cost and conservation efforts in production meetings.

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 themachine.

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 andtracked:

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. Notethat the cost of various energy sources will change based on relative cost of corresponding raw materials. Costcomponents 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 forhigh-pressure steam minus the value of electricity generated by turbines. Marginal cost of steam (cost of the laststeam generated) should be used to measure the value of steam savings. Marginal cost is usually higher thanaverage cost since powerhouses use more expensive fuel to top off demand. Note that this method of calculationmay be an over-simplification if pressure and flow in a low-pressure steam header are maintained by high-

pressure make-up steam supplied from a pressure-reducing valve in the powerhouse. Net cost of natural gas cost (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 includedto determine true value of water conservation projects.

The value of condensate returned to the powerhouse. This should include associated energy, water treatmentcosts, wastewater treatment costs, and raw water pumping costs to get it to the water treatment plant. Costshould be adjusted downward for condensate polishing costs.


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3 / Paper machine energy conservation TIP 0404-63

The combination of production rates, energy consumption, and cost information can be used to determine energycost per ton of product. It is also important to understand energy contracts. Generally managing energy savingsdownward is the correct move; however, with some peak energy contracts unless you are able to save off of peakthere 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 fromtotal steam to the machine. There are three areas that are typically poorly monitored that can help a mill identifysteam waste; the steam flow to the wire pit or silo, the steam flow used to heat shower water, and the energy loss tothe dryer vacuum condenser (water flow, temp in, temp out). Looking at valve position is one way of tracking theseenergy flows but does not tell the entire story. Most mills have no idea how much energy they are using in the siloor for shower water heating. The normal response from papermakers is "not much" but in reality it can be asignificant use. Dryer drainage system vacuum condenser tracking is also recommended. It is a sure way to assessand maintain the health of the dryer drainage system. The percent energy loss can be tracked and trended. Thisidentifies bad vent valves, open vent valves, high wet end dryer losses, air leaks, high water flow, etc. The vacuumcondenser is often a piece of equipment that is poorly controlled. Poor control often results in high water flow thatdilutes and upsets the fresh 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 differentgrades of paper. Target values for key indices applicable to energy consumption are shown in Table 1 for variousgrades.

Key factors

Each machine typically has several key factors that influence energy consumption on the machine. Green/yellow/redindicators can be used for key process conditions that affect energy consumption to show whether values are indesired 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 properprocedure) or the use of portable or fixed sheet moisture gauges specifically designed for use in the press section arerecommended 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)

Wire pit steam water heating steam valve positions

Mill water make-up into whitewater or warm water systems.

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 a tool used to help ensure consistent paper machine operation and quality. The tool can also beused to help monitor and control energy consumption. Centerlining of energy parameters can often be divided intotwo categories: process setpoints and factors reflective of system health.


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Table 1. Energy performance indices

GradeCorru- Recycled

Bleached gated Market Fluff paper- News- KraftIndex Fine board Liner medium pulp pulp board print LWC paper

Uptime, % 95 93 94 94 95 95 93 93 93 94

First quality, % 93 90 97 97 99 97 93 98 85 97

Overall machineEfficiency, % 89 84 91 91 94 92 86.5 92 79 91

Total steamconsumptionlb/ton 4,000 4,000 2,800 2,750 2,000 2,500 2,800 2,800 3,000 5,000kg/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 400kWh/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 6.0GJ/tonne 7.0 8.1 5.8 5.8 4.6 5.2 7.0 5.8 6.4 7.0

Water consumptiongal/ton 2,000 2,000 1,500 1,500 1,000 1,000


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Examples of process setpoints that can be used in Centerlining include:

Wire pit and other water heating temperatures

Pocket ventilation, blow box, and other air heating temperatures

Dryer section differential pressures (or blowthrough flows)

Press loads

Sheet moisture at the reel Refining kW, freeness, hpd/t and/or kWh/t

Examples of factors reflective of system health include:

Overall consumption indices such as ton steam/ton paper, kWh/ton, and energy cost/ton

Dryer section ton 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 ofareas where operator round 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.

No dumping of condensate.

Some mills utilize an infrared temperature gun to check stock and water system temperatures and detect cold-waterinfiltration. Note that flat black spray paint should be used to mark areas on piping where infrared measurements areused to ensure uniform emissivity.

Energy surveys

Energy audits can provide useful first steps to identify and prioritize opportunities to reduce paper machine energyconsumption.

Data can be collected from direct observation; data historians; discussions with mill operating; maintenance, andengineering 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.

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


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Documenting results

Optimizing the system after the project

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 boroscopes or orifice plate testing (annual)

Vacuum system surveys/optimization (every 3 years)

Thermography to check for leaks and hot spots (annual)

Steambox 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)

Optimize press fabrics and roll cover designs (venting and hardness, nip dewatering vs. Uhle box dewatering)

Reduce basis weight (while meeting the product specifications)

Trim the sheet at the wet end rather than at the dry end

Improve cross-machine moisture profile uniformity

Increase starch solids used in size press (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).

Machine efficiency

Increasing overall machine efficiency has a direct effect on specific energy consumption since it takes as much ormore energy to produce a ton of broke as it does to make a ton of first-quality paper. Some steps to increase machineefficiency 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.


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Utilize camera systems to identify and characterize breaks.

Optimize quality control system (QCS) performance to ensure good machine-direction (MD) and cross-machinedirection (CD) profiles.

Control sheet in open draws in the dryer section.

Utilize capability of distributive control systems (DCS) and data historians to impact efficiency andtroubleshooting.

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 toincrease clear filtrate quality and quantity for replacement of mill water in showers.

Agitation

Chest agitation is a significant contributor to paper machine electrical consumption. Opportunities to reduce energyconsumption 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

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.

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 forpulp mills, $4.6 MM for paper mills, and $3.0 MM for board mills. Average available motor savings opportunitiesper year are estimated to be $483,000 for pulp mills, $679,000 for paper mills, and $492,000 for board mills. TheU.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, and45% by drive motors and other electrical equipment. Potential electrical energy savings opportunities are availablethrough 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 ofenergy-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 pressurerequirements, but the pump, motor, and/or piping are not downsized to meet the change. Energy is also wasted when


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a larger pump than required is used for the purpose of commonality of spares. This also highlights the need to buildto 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 aloss in pump efficiency. Routine inspection of pumps in these applications is recommended. Parts should bemaintained and/or replaced as necessary.

DOE promotes identifying motors with the greatest saving potential for further investigation. The greatest savingspotential is typically centrifugal loads with a high duty cycle. These motors are referred to as the vital few. Thefollowing steps can identify them:

1. Categorize motors by size times operating time. Establish a threshold for more detailed consideration. (Shouldbe 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-efficiencymotors.

The Pumping System Assessment Tool (PSAT) (3) can be used to quantify energy consumption and cost savingspotential 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 its operating cost. Life cycle cost should be usedfor 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

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 bychecking no-load horsepower by backing off refiners while stock is running through them. 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 increaseno-load horsepower by over 10%. Refiners should be inspected annually to check mechanical condition.


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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 minimizeover-refining and optimize energy usage.

Is the Net Specific Energy applied within normal guidelines for the grade/pulp?

Is the refiner operating properly alignment and no sticking (e.g., splined shaft conversions can preventsticking and alignment problems)?

Is plate design matched properly to the fiber and refiner to achieve effective compression index and number offiber treatments (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 enoughrefining?

Is the stock consistency to the refiners between 3.5-5.0% for best energy transfer and fiber development? Doesthe consistency fluctuate to the refiners? A consistency that swings will cause fiber development to swing andlead to over-refining.

Is the hardware 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 net 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-loadhorsepower increases exponentially with refiner plate diameter.

Bypass and shut down unnecessary and underused refiners. Normal refiner operation is most energy efficient atmotor loads >80% of motor rating.

Check freeness drop per hpd/t regularly to monitor refining efficiency and determine whether refiners areworking correctly. Typical Canadian Standard Freeness (CSF) drops per net hpd/t are 25-60 for Southernbleached 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.

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 debrisremoval) of cleaners.

Reduce flows to fiber recovery stages based on balancing the system properly.

Shut down cleaners where 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 reportedenergy savings as much as 25% from elimination of tanks and pumps.


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Recycled fiber systems

Opportunities to minimize energy consumption in recycled fiber systems include:

Install energy-efficient pulper rotor and extraction plate designs.

Ensure that pumps are not oversized. Install frequency control on motors to reduce energy waste.

Reduce pulp and water volume.

Increase consistency as much as possible to reduce hydraulic volume for pumping and agitating.

Simplify process configuration.

Run equipment at optimum operation point.

Make process stable and homogeneous.

Increase process temperature to gain additional production, being careful not to exceed the stickies activationtemperature.

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 energydissipation. The reduction in water-usage will also lead to an equivalent saving in energy consumption. The mostenergy-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 waterusage. A systematic approach is recommended with clear identification of every stream. Paper mill water usagevaries between 0 and 60 ton of water per ton of paper produced. Approximately 4-6 tons per ton represent a practicalminimum. Zero consumption is possible, but only with serious quality drawbacks on some grades depending on wetend chemistry. Zero discharge is generally only achievable with products such as recycled fiber grades. Simplewater reduction possibilities are often overlooked, so it is sometimes possible to achieve reduction of water and wetend energy consumption by up to 50%. Wet end water consumption 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 berequired to allow whitewater streams to be reused.

Recycling can result in significant water and energy reduction, but extra equipment such as filters and/or evaporatorsmay 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 5F, 3C warmer than silo temperature).

Utilize waste heat from the pulp mill (water stream at least 5F, 3C warmer than silo temperature) and/or hoodexhaust 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 whenfeasible.


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Use warm (approximately 10 F, 38C) water instead of cold mill water for seals.

Utilize dead-band control logic for emergency water make-up into whitewater storage chests.

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 consumptionon the machine. Increasing capacity, improving maintenance, and/or installing post-saveall strainers and filters canimprove filtrate water quality to allow saveall filtrate to be reused in place of fresh water. Key saveall parameters toevaluate include:

Installation and equipment, including size (number of installed discs and available blanked-off discs), droplegs(diameter and layout), and sector type (cover type and condition).

Operation, including proper sweetener type and quantity, well-tuned vat level control, dilution of recoveredstock with rich white water bypass, and cloudy filtrate recycle.

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 cleaningshowers.

Dissolved air flotation (DAF) savealls can be used in addition to or instead of disk or drum savealls to help improvewhitewater quality.

Showering

Showering is a major source of fresh water consumption on many machines. Any shower water used on the formerthat is below whitewater temperature requires steam to return the silo to desired temperature. Cool showers in thepress 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 showerperformance 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 paper making process

Typical low-risk showers include:

Breast roll showers

Knock-off 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.


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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 toreduce energy consumption through chemical systems include:

Utilize polyamine products to increase strength. This can provide savings through reduced refining, reducedbasis 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 instead of mill water for chemical injection.

Maximize ash content in the sheet.

Headboxes

Basis weight profiles ultimately impact pressing, runnability, and dryer operation. Pressure drop through headboxeshave increased with headbox design evolution. Turbulence level and nozzle convergence impact MD/CD ratiocapability. 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 dryingefficiency

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 optimizerequired strength characteristics to allow for reduced basis weight or reduced refining levels

Impact MD/CD ratio capability

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 affectperformance 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.

Utilize multi-compartment high-vacuum boxes.

Evaluate drainage element materials for impact on drag load

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:


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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

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 (afterpaper machine drives), and is frequently one of the least understood parts of the papermaking process. Vacuumsystems can have from 1,000 to 10,000 installed horsepower. Often vacuum systems can use 1020% morehorsepower than is necessary for paper production.

Some of the most common vacuum system problems that can increase energy consumption and/or reduce systemefficiency 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 witha 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. 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 requiringadditional vacuum pumps, sucking excessive air through the sheet can cool the sheet and cause press solids todrop 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 reducingtable drive load by as much as 10%.

Ensure proper Uhle box slot size to provide required flow capacity and dwell time.

Ensure proper vacuum pump application (high-vacuum vs. low-vacuum pump design).


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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% ofdesign 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 and do not require sealwater. Some mills with hard water have installed blowers to avoid calcium carbonate buildup in conventionalvacuum pumps.

System audits can be used to help reduce wasted energy. Replacing or calibrating gauges can ensure properindication of vacuum levels. Key operating data should be monitored, reviewed and recorded. Sheet and fabricmoisture should be checked regularly to ensure effective use of vacuum. One of the most effective ways to managevacuum system energy is through EMBWA (Energy Management By Wandering Around). Additional informationon vacuum system optimization is included in TAPPI TIP 0404-55 Performance evaluation techniques for paper

machine vacuum systems (4).

Press section

On a typical paper machine with 0.5% headbox consistency, 20% couch solids, 40% press solids, and 5% reelmoisture, 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 theforming and pressing sections than in the dryer section. Removal of the water content after the press sectionrepresents more than 50% of the energy consumption in the paper machine system. Each one percentage-pointimprovement in solids out of the press section results in 3-5% less water that needs to be evaporated in the dryersection. Maximizing press performance is thus one of the most important aspects of paper machine energyconservation.

Primary opportunities in the press section are increased water removal, dryer section steam savings, increasedproduction, more efficient water removal, sheet property improvements, and fiber savings on bulk sensitive andstrength 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 isapplied. Press impulse (press nip pressure x nip residence time) has been shown to be a good performance indicatorfor press water removal. Development of shoe presses has significantly increased time available in the nip, resultingin 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 indicatesthat solids content of the pressed web can be increased by one percentage point for each 10C (18F) increase inweb temperature. Methods to increase temperature in the press section include increased stock temperature, steamshower applications on the sheet or on the fabric, heated press rolls, or hot water flooded nip showers. Energyefficiency 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. Trialshave indicated that sheet dewatering can be increased by one percentage point by increasing shower watertemperature by 10 oC. High-pressure and low-pressure shower water should be at least equal to the temperature ofstock at the headbox. Shower water temperature of 54C (130F) or above is beneficial in maintaining fabrictemperatures. Shower water heating is an excellent application for direct or indirect heat recovery. Shower watertemperature on the last press fabric should have priority for use of warm water on the wet end of paper machines.


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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 improvedpressure uniformity and higher sheet solids content. Multi-axial laminated fabrics provide superior pressureuniformity, excellent bridging on vented/drilled rolls, and more steady-state pressing compared to conventionalfabrics. Flat batt fibers can offer contact area equal to round fine denier batt without sacrificing wear volume.

TAPPI TIP 0404-52 Press Section Optimization (5) provides guidelines for evaluating and improving presssection performance. The TAPPI Paper Machine Wet Press Manual (6) provides more complete coverage of presssection 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.

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 highdewatering 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 lifeon-line monitoring of press water flows, frequentCD 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 steambox is installed and how it is operated.TAPPI TIP 0404-58 discusses steam shower applications in the forming and press sections. Steam showers are mostenergy efficient with low steam ratios on relatively cool systems with vacuum assist beneath the steambox. Bettersteam utilization efficiency occurs when steam showers are located ahead of the last press nip since there is lesswater to heat. For most applications, efficient steam flow ratios are 0.10 lb steam/lb paper for fourdrinierapplications, 0.075 lb/lb for press section applications, and 0.05 lb/lb for Uhle box steam showers. Mills shoulddetermine the value of steamboxes for specific applications and operate accordingly. Some modern steamboxdesigns 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 steambox 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.


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Control steam temperature to improve condensation rates. Typical recommended temperatures are 5-10F (3-6C) of superheat above saturation temperature.

Provide proper mist elimination when utilizing flash steam. In many cases, some high-pressure make-up steamis 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 onelocation.

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 monitoringdryer section performance is included in TAPPI TIP 0404-33 Dryer section performance monitoring (7).

The 10 Commandments of energy efficient drying are:1. Dont dry any more than you must2. Dont vent steam anywhere3. Match the ventilation air flow to drying requirements4. Use steam from lowest header pressure possible5. Keep the machine running (minimize break times)6. Improve the moisture profile7. Increase the heat transfer rate8. Measure what you must control9. Keep the steam system calibrated10. Dont use motive steam when make-up steam can be used

Five rules for dryer steam system energy efficiency are:1. Keep the system tight2. Efficiently utilize flash steam from high temperature condensate3. Maximize use of low pressure steam4. Minimize heat used for hood supply air heating5. Manage the steam system

Energy efficient drying requires a combination of steam system design, equipment, operation, maintenance, andcontrol.

Dryer arrangement

Dryer section arrangement primarily affects drying energy consumption by changing machine or heat transferefficiency. Examples include:

Single tier arrangements have high dryer-sheet wrap angles and short unsupported sheet lengths. Heat transferrates and threading efficiency are thereby improved.

Increased sheet restraint from wrap angles and fabric pressure improves thermal contact with dryers and reducesCD shrinkage.

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

Windage control impacts runnability

Fabric tension affects drying heat transfer by increasing sheet-dryer thermal contact.

Felt design affects uniformity of sheet contact with dryer surface and heat transfer.


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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 motivesteam pressure are critical for efficient operation.

Opportunities to minimize energy consumption using thermocompressor systems include:

Ensure no steam venting during normal operation.

Utilize blow-through control and/or automatic pressure and differential-pressure letdown to minimize ventingduring 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 anarrow operating range. Machines with wide variations in condensing loads may not be appropriate applicationsfor 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 dryersection that operates at lower steam pressure. Opportunities to reduce energy consumption with cascade steamsystems 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 blow-through control and/or automatic pressure/differential-pressure letdown to minimize ventingduring sheet breaks.

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

Steam system design

There is no one and only correct solution for steam and condensate system design. The proper system designdepends on the mill steam supply and condensate return systems and production requirements. Proper sizing ofpiping and equipment are critical, using well-established procedures and guidelines. Detailed piping design shouldbe 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 blow-through control and/or automatic pressure letdown to minimize venting during sheet breaks.

Provide high steam separator efficiency, especially with blow through 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 (> 230F, 110 C).

Do not valve off dryers to control drying capacity. Improve flexibility of the steam and condensate systeminstead.


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Steam system hardware

Proper syphon design is a key component in making the steam system energy efficient. Stationary syphons generallyrequire less blow through steam (less than 10% of condensing load with stationary vs. 15-30% with rotary) andlower differential pressure (15-35 kPa or 2-5 psi with stationary vs. 40-95 kPa or 6-14 psi with rotary). In dryersdraining directly to a condenser or heat exchanger, reduced blowthrough steam directly results in energy savings. Insections that cascade to lower pressure groups or in sections with thermocompressors, energy savings are stillpossible, but evaluation of energy savings is more complicated.

In a thermocompressor section, energy savings will be achieved by converting to stationary syphons if the sectionwas venting with rotary syphons. Lower differential pressures and lower blow through flows reduce the potential forventing. 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 thetotal 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 ableto generate significantly more electricity from the lower-pressure make-up steam extraction than from the higher-pressure motive steam, energy savings can be significant. Turbines typically make the most electricity when most ofthe 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 thelower-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 andcondensate layer thickness, but is typically around 300 meters/minute (1000 fpm). A dryer section will evaporatemore water with dryer bars installed. Minor reductions in energy consumption are possible with dryer bars related tooperation at lower steam pressures with improved heat transfer. However, it takes additional steam to evaporate thiswater, so the kg steam used per kg of water evaporated remains nearly the same. This same principle also applies tofelting unfelted dryers or increasing dryer fabric tension. Drying rates will improve, but energy efficiency (asmeasured by kg of steam used per kg of water evaporated) will see little change. Additional information on dryerbars is included in TAPPI TIP 0404-35 Application of dryer bars (8).

Guidelines for steam system hardware include:

Utilize stationary syphons where advantages can be realized from lower differential pressures and blow-throughflows.

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 operatingabove 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 workingpressure.

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 operatedproperly. Considerations for energy-efficient steam system operation include:

Operate dryer differential pressures at the proper setpoint for condensate evacuation and blow through flows.

Ramp warm-up dryers to maximize runnability


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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 preventventing 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 dryersfor wet end condensers, dryer section pressure for vent condensers, and required condensate temperature for flashcondensers.

One of the most common problems with condenser systems is air leaks into the system. Since air leaking into steampiping is not visible, it is often overlooked. Systems operating with pressure or condensate temperature control willtypically open water valves in an attempt to obtain vacuum when air is leaking into the system. One method toidentify air leaks is to pressurize the system that normally operates under vacuum and then conduct a round to checkfor steam leaks. A typical procedure follows.

1. Raise pressure setpoint in the individually controlled dryers to 10 psig (0.8 bar) (keep differential pressure inautomatic).

2. Increase vacuum condenser setpoint 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 isoften wasted or poorly used. Often it is vented either at the machine or at the boiler house. Low pressure flash steamcan be reused as make-up steam to wet end dryers, steam showers, water heating, or flash coils in the pocketventilation system. If flash steam is used for steam showers, condensate carryover must be avoided through goodseparation, steam traps, and proper piping design. In some cases, small amounts of higher-pressure steam arerequired 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 willquickly eat through steam coils.

In some cases it may be easier and more cost effective to pump hot condensate through air heating coils rather thanutilizing a low-pressure flash tank. It is important to keep the condensate pressurized to prevent flashing andhammering before the coil, so level control valves must be positioned downstream of the coils.

Pocket ventilation and hood supply systems

Supply air temperatures of


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Operate pocket ventilation system at


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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 properoperation 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 flowmeters, lines, etc.

Ensure that proper pressure and temperature compensation factors are used in steam flowmeter 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 ona given superheat level above saturated steam pressure.

Determine standard operating procedures for steam trap and drain line valving during warm-up and normaloperating 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 togenerate 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 $130,000electrical cost over a 10-year life. Replacement of the air-driven motor with an efficient electric motor has thepotential for significant savings over the life of the unit.

Opportunities to minimize compressed air cost include:

Instrument air dew point should be 10C (18F) 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.

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.


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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 m 3/min or 150 to 250 scfm) atlow pressures (less than 100 kPa or 15 psig). These should utilize dedicated headbox compressors instead ofbleeding off of mill air headers.

Reclaiming water from air compressors can also provide energy and water savings. Additional information andreferences 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 improveenergy efficiency of machine room ventilation systems. Machine room ventilation is discussed more completely inTAPPI TIP 0404-50 Machine room ventilation guidelines (10).

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 21C (70F) set points and roof supply systems at 49C (120F).

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 properfreeze 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 exhaustsystems 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 anuncontrolled 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 roomexits 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 economizershave limited potential to recapture energy from exhaust streams. The amount of energy recoverable in the dryingsection is limited due to the ratio of latent heat in the exhaust and the sensible heating of the dryer air. Overallenergy 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 providegreater opportunity to improve the amount of recovered heat. More elaborate heat recovery systems couldsubstantially 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 recoverypotential.

Areas with opportunity for heat recovery include:

Dryer section hood exhaust

Yankee hood exhaust

Pulp machine air dryer hood exhaust.

TMP steam


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Vacuum blower exhaust

Waste heat from pulp mill and evaporators.

Sewer streams

Tissue machines

Tissue and towel machines offer additional opportunities to optimize energy consumption. Most machines withconventional Yankee dryers utilize steam showers, suction pressure rolls, steam-heated Yankee dryers, and gas-firedhoods 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 (typicallynatural gas used for hood burners).

Increasing recirculation air and reducing make-up and exhaust air from the Yankee hood system will reduce energyconsumption 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 energyusers 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 (7-8MMBtu/ton). Through-air dried (TAD) machines typically use significantly more energy per kg of product thanconventional Yankee machines. This is because more water is dried and none is mechanically pressed from thesheet. Additional information on TAD is included in TAPPI TIP 0404-25 Through drying (11).

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 humiditysensors (0.40 0.45 lb/lb typically optimal).

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 system 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, andthermocompressor 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.

Increase reel moisture when quality considerations allow.


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Conversions

GJ/tonne 0.8606 = MMBtu/tonkWh/tonne 0.9072 = kWh/ton

Keywords

Energy, Paper Machines

Literature cited

1. TAPPI TIP 0404-47 Paper Machine Performance Guidelines2. Energy Cost Reduction in the Pulp and Paper Industry, First Edition, PAPRICAN, November 19993. U.S. Department of Energy Office of Industrial Technologies web site: http://www.oit.doe.gov/bestpractices4. TAPPI TIP 0404-55 Performance Evaluation Techniques for Paper Machine Vacuum Systems5. TAPPI TIP 0404-52 Press Section Optimization6. Paper Machine Wet Press Manual, Fourth Edition, TAPPI PRESS, 1999.7. TAPPI TIP 0404-33 Dryer Section Performance Monitoring

8. TAPPI TIP 0404-35 Application of Dryer Bars9. TAPPI TIP 0404-24 Recommended Operation of Dryer Section Hood Air Systems10. TAPPI TIP 0404-50 Machine Room Ventilation Guidelines11. TAPPI TIP 0404-25 Through Drying

Note that this TIP was originally developed from a panel discussion on Paper Machine Energy Conservation at the2001 TAPPI Engineering Conference. The 2006 revision utilized material and discussions from the EnergyConservation Track at the 2006 TAPPI Papermakers Conference.

Addi tional information

Effective date of issue: January 5, 2011

Working Group:Jeff Reese, Chairman, International PaperMark Harrison, MetsoKen Hill, Kadant Johnson SystemsJon Kerr, AndritzPekka Kormano, Deublin Steam SystemsDick Reese, Dick Reese and AssociatesDoug Sweet, Doug Sweet and AssociatesGreg Wedel, Kadant JohnsonPhilip Wells, Wells Enterprises Inc.

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Paper Machine Energy Conservation