Paper Drying Energy Tips

© 2016 Kadant Inc. or its affiliates. All rights reserved.

Are bigger thermocompressors always better?

Thermocompressors use high-pressure (“motive”) steam to raise the pressure of a low-pressure (“suction”) steam for subsequent use in the dryer section. They must be sized to match the application. Bigger is not always better. Properly sized thermocompressors minimize the amount of motive steam that is used to compress the suction steam.

A large thermocompressor requires significant motive flow to simply start the compression cycle. It is not uncommon to find thermocompressors that are so large that they operate with inadequate control, or, in the worst case, end up with a negative control response – opening when they should be closing. This control response results in excess motive steam flow and steam venting from the dryer section.

Will dryer bars affect energy consumption on low-speed machines?

Yes, dryer bars can affect energy consumption on low-speed dryers, specifically electrical energy consumption. Dryer bars (the Kadant Johnson trademark is "Turbulator bars") are normally installed in paper dryers running at high speeds, but they can also be installed in dryers that are running just below the normal condensate rimming speed. The rimming speed for most dryers is around 1000 fpm.

A dryer with Turbulator™ bars, however, has a lower rimming speed - closer to 600-700 fpm. Once the condensate is rimming, the drive power drops significantly. Therefore, the installation of dryer bars in slow-speed dryers can reduce energy consumption by reducing the drive load. The reduction in drive load also reduces the stress and wear on dryer drive gears.


Are high vacuum levels in the dryer section steam condensers better?Generally, no. The condensers in the dryer steam system are used to assist draining wet end, low-pressure dryers and in condensing excess blow-through steam from wet end and after-section dryers. Ideally, the condenser cooling water is heated to a level that is of value in other mill operations. Higher vacuum levels require more cooling water and result in a lower discharge water temperature. The vacuum condenser should be controlled to match the required dryer differential pressure. To save energy and water, do not run at a vacuum greater than is necessary to drain the dryers.

If you are not venting steam through the roof, do you have an efficient dryer section?

Not necessarily. You may be venting steam to a condenser. Although you have not lost the condensate, you have lost the energy in the steam to the water. This energy is wasted if the cooling water is not used elsewhere in the papermaking process. Balancing the dryer steam sections and reducing the amount of blow-through steam using managed dryer differential pressure control can greatly reduce the amount of steam that goes to condensers.


Are there opportunities to reduce energy costs when the machine is not producing paper?

Yes. Dryers that are idling during warm-up cycles, sheet breaks, and maintenance downs can consume as much as 20-30% of the steam used when paper is being produced. To reduce these losses, dryer steam pressures and differential pressures should be adjusted automatically during start-ups, shutdowns, grade changes, sheet breaks, and normal operation.

How can a condensate flash tank be used to reduce energy costs?

When high-pressure condensate is collected in an atmospheric tank, a portion of the condensate “flashes” back into steam. The temperature of the condensate drops to 212 F and energy is lost with the flash steam. This energy, however, can be recovered. By adding a separator (a condensate flash tank), the flash steam can be collected and used in steam showers, shower water heaters, air heaters, and stock heating systems. This can be a low-cost way to save a lot of energy.


What is the value of replacing older rotary steam joints and dryer syphons?

It is more than you may think. Leaking steam joints waste both energy and valuable condensate. Dryers that are valved off reduce the drying capacity of the machine. Older styles of steam joints have shorter seal life which increases the frequency for replacing parts, the labor to install them, and the downtime required for the work. If the syphons are oversized or eroded, the amount of steam that blows through the dryers will be greatly increased, often resulting in steam venting from the machine. If the dryer steam joints and syphons are over 20 years old, there is likely to be an opportunity for improvement.

How can the operation of a size press affect energy costs, production, and quality?

With a conventional puddle size press, the starch solids must be reduced to limit starch pick-up when the machine speed is increased. A film metering size press, however, is able to control starch pick-up independently of machine speed. This allows the starch solids to be increased which in turn greatly reduces the amount of water that must be evaporated in the after-dryer section. But this is only the start.

The starch pick-up is much less dependent on base sheet moisture profile, so it is no longer necessary to over-dry the sheet to produce a uniform pick-up. This further reduces the amount of energy required for production and significantly increases the drying capacity of the main dryer section. As an added bonus, more of the sizing remains on the surface of the sheet, increasing surface strength and sheet stiffness.


What is “choked flow” for a dryer section thermocompressor?

Thermocompressors use high-pressure (“motive”) steam to raise the pressure of low-pressure (“suction”) steam for re-use in the dryer section or for heating process water. As the actuator opens the thermocompressor, more motive steam enters the nozzle and more suction steam is entrained. Eventually, however, the motive steam fills the throat of the thermocompressor and restricts the amount of suction steam that is entrained. This condition is called, “choked flow”. Once the thermocompressor reaches this point, further increases in the motive flow actually reduces the flow of low-pressure steam.

With conventional thermocompressor control schemes, the actuator will increase the motive flow to the maximum with a corresponding loss of suction flow, pressure control, and efficiency. The Kadant Johnson Dryer Management System controls keep the actuator in range to prevent this from happening.

Energy costs give an indication of energy efficiency, right?

Not quite. As production (tons per day) increases, the amount of water that must be evaporated in the dryer section also increases. The best indication of energy efficiency is the ratio of the energy cost to the machine production. It is better to monitor steam flow per ton of paper produced than simply the steam flow rate.


Will dryer bars affect energy consumption on high-speed machines?

Not directly. Dryer bars can be installed in paper dryers running at any speed, but they have the greatest impact when the machine is operating at high speeds.

Dryer bars greatly increase the rate at which heat can be transferred to the paper when the condensate is rimming on the inside of the dryer. If the higher heat transfer capability is used to increase production, then the amount of water that must be evaporated will also be increased. This increases the amount of energy that is used in drying the paper.

If the production is not increased, then the higher heat transfer capability will result in lower operating steam pressures. Lower operating steam pressures could result in increased electrical production in a cogeneration facility. This can represent a very significant improvement in energy efficiency. There is another secondary benefit: Dryer bars will normally improve the cross-machine moisture profile. This often results in less over-drying. Less over-drying results in less drying and less energy.


What are some simple ways to monitor dryer section ventilation systems?

Two important dryer ventilation parameters are the supply air temperature rise across the heating coils and the exhaust air temperature drop across the heat recovery system. A change in these values often indicates that the filters are dirty, the heaters are plugged, or the steam coils are not draining properly. The KadantJohnson DMS™ control technology can monitor these parameters and send a maintenance alert when the parameters are out of normal operating range.

Does it make any difference whether the steam pressure used to dry paper is taken from a high-pressure header or a low-pressure header?

It might. Paper dries the same whether the steam was originally at 60 psi or at 160 psi. In a cogeneration plant, however, additional electrical energy can be generated from the high pressure steam if it is delivered to the dryer section at a lower pressure. It is much better to drop the steam pressure through a turbine-generator than across a pressure-reducing valve. Whenever possible, use the lowest source of steam pressure to meet the drying requirements.


What is the best steam pressure for dryers that do NOT contact the sheet?

Dryers that do not contact the sheet should be disconnected from steam service. Bottom unorun dryers, return run fabric dryers, and Feeney dryers (intermediate dryers that do not contact the sheet) contribute very little to drying. Modern fabrics do not carry water and do not have to be dried by separate dryers. Although condensing loads are very low in non-contacting dryers, a lot of steam blows through the dryers, only to be vented from the steam system.

Ideally, fabric and Feeney dryers should be removed from the machine or at least disconnected from the steam system and blanked off. This saves steam joint maintenance and reduces energy consumption.

Note: Unheated bottom unorun dryers will be smaller in diameter than the heated top dryers. To avoid stress on the drive gears, bottom unorun dryers should be disconnected from the gear train by removing idler gears. This can be done on many dryer configurations, leaving the bottom dryers to be driven through the fabric.


What affect does thermocompressorefficiency have on energy consumption in drying?

Thermocompressors use high-pressure (“motive”) steam to raise the pressure of low-pressure (“suction”) steam for subsequent use in the dryer section. Motive steam flow can be reduced by properly matching the size of the thermocompressorto the dryer syphon flow requirements.

It can be further reduced by using a high-efficiency thermocompressor. High-efficiency thermocompressors require less motive steam flow to boost the suction steam pressure. This reduces the potential for steam venting from the dryer section during normal operation as well as during sheet breaks. Reducing the flow rate of motive steam will also have a positive effect on mill-wide energy costs in a cogeneration facility. The powerhouse can often use the extra high-pressure motive steam to generate additional electrical energy.

Do you see steam venting through the roof of your paper mill?

If so, you have found an opportunity for reducing energy costs. Follow the vent line and determine its source. Improving the dryer steam system balance, using condensate flash steam, and installing high-efficiency thermocompressors can greatly reduce the likelihood (and cost) of venting steam. And you save the condensate at the same time.


Do high dryer hood temperatures increase drying capacity and energy efficiency?

No. The dryer hood air temperature must be high enough to prevent sweating and dripping, but higher air temperatures contribute very little to drying capacity. High air temperatures do, however, greatly increase the amount of energy that is consumed. In general, energy is being wasted if the dryer hood is operated at temperatures over 180-200 F (82 - 93 C). It takes only an adjustment of the temperature set point to begin saving money.

If the dryer internals are not making any noise, are they okay?

Not necessarily. Syphon shoes and syphon pipes can be badly eroded without the syphon components being loose inside the dryer. Eroded syphons result in high operating differential pressures, high blow-through steam flow rates, and flooded dryers. During annual dryer inspections, look specifically for eroded syphon pipes, syphon shoes, and syphon elbows (particularly those made of cast iron).


What are dryer “steam bleeds”?Dryer steam bleeds are valved pipes that vent steam from the inside of the dryer cylinders to the air in the dryer hood. Steam bleeds were common in the 1960's on dryers that were suffering from accumulation of air inside the dryers. Air accumulation would reduce the heating capacity of the dryers and the uniformity of the sheet moisture profiles.

Today, boiler make-up water treatment is greatly improved so there is less air in the steam. Further, there are fewer tendencies for air to accumulate in dryers with modern dryer syphons. In most cases, these steam bleeds can be eliminated. This reduces the loss of steam energy and the amount of ventilation air flow required to prevent the dryer hood from sweating.

In steam systems with a thermocompressor that recirculates blow-through steam back to the dryers, the steam system can be periodically purged of air. The KadantJohnson DMS® steam system control technology is programmed to provide periodic and automatic venting of air from dryers in thermocompressor loop systems.zzz

Does it make any difference where the sheet is trimmed?

Yes, it does. Whenever possible, trim the sheet at the wet end rather than at the dry end. This avoids drying paper that will not be sold. The best way to conserve energy is not to use the energy in the first place.


What is the impact of sheet breaks and grade changes on energy consumption?

Significant. A machine wastes 1 to 2 tons of steam for every ton of broke that it produces. Reducing sheet break time helps. Eliminating sheet breaks helps even more. And getting back on grade as quickly as possible is particularly important for machines that have frequent grade changes.

Unfortunately, getting back on grade is not easy. The thermal response of the dryer section is very slow. It takes a long time to change the temperature of 500 tons of iron. Solutions using Dryer Management System® control software can reduce sheet break recovery times up to 20% using feed-forward control and other strategies. By monitoring the drying process, the target steam pressures and differential pressures can be set even before the moisture scanner is back on line.

Are bigger dryer syphons always better?

Not necessarily. Dryer syphons can be too large as well as too small. The syphon size should be based on the dryer speed, condensing load, steam pressure, and style of syphon. Many rotating syphons that were installed in the 1970-1980’s had syphon pipes that were too large for their applications. Even when operating at recommended differential steam pressures, the high blow through flow rates coming from these syphons can result in an inefficient steam system operation. Check your syphon sizing to be sure it is correct for your current operation.


Should the dryer air exhaust be run at its design limit?

Generally not. Excessive air exhaust wastes energy. However, inadequate exhaust results in the hood panels sweating and water dripping on the sheet.

The exhaust should be adjusted to match the drying load. More exhaust is required when the drying load increases. Less exhaust is required when the drying load decreases. Very little exhaust is required during sheet breaks.

To minimize energy consumption and prevent sweating during heavy drying conditions (e.g., when producing higher basis weight grades), it is necessary to monitor drying loads, calculate the air exhaust requirements, and adjust the exhaust fans or dampers accordingly. The drying load can be calculated and the exhaust flow adjusted automatically using a Dryer Management System® control technology.

What is the best way to reduce energy costs?

The best way is not to use the energy in the first place. Increasing the dryness out of the last press nip by 1% can reduce the energy used in drying by 4%. Increasing press loads, optimizing press fabric design and operating life, installing shoe presses, and increasing the web temperature before the last nip will all help increase sheet dryness.


Can I do something to eliminate flash steam loss?

Flash steam is heat energy and treated water and occurs when pressurized steam condensate is vented to atmospheric pressure. Depending on the condensate pressure and the amount of condensate generated, flash steam loss could result in millions of BTUs of energy and treated water lost to atmosphere.

There are many methods to capture this energy and treated water loss. The key is to find a source to use the flash steam. One way is to use the flash steam to heat water. The flash steam could be directed to a water jet heater. The heated water could be used to preheat boiler make-up or other hot water needs in your facility.

Can the machine Silo/Wire Pit steam control valve position indicate excessive fresh water consumption?

Yes, as cooler fresh water (typically referred to as mill water) is added to the machine white water system, the silo/wire pit steam control valve must open to maintain the target system temperature. As the volume of cooler fresh water introduced into the white water system increases, the more the steam control valve must open and the greater the steam demand required to maintain system temperature. This is most often observed during machine startups or machine upset conditions, such as sheet breaks.


Is your dryer section ventilation system well-managed?

It might be, but only if the supply and exhaust air flows are adjusted based on the evaporation rate. Air flow adjustment is particularly important for paper dryer sections that operate over a wide range of steam pressures.

Properly controlled supply and exhaust air flows can provide large energy savings, but they must be actively managed to avoid operational problems. The exhaust humidity should vary with paper weight and air flows should be changed slowly to maintain machine direction and cross-machine direction uniformity.

The key to proper air flow control is the accurate measurement of the evaporation rate. It is difficult to reliably and accurately measure exhaust air humidity in the harsh operating environment of a dryer hood exhaust. An alternative to manually measuring exhaust air humidity is to use the Kadant Johnson DMS® control technology that features an advanced drying algorithm to continuously monitor, control, and optimize the dryer air system, relieving machine operators from this complex task and providing significant improvements in energy efficiency.


Can dryer bars cover only a portion of the dryer and improve the moisture profile?

In theory, yes. In practice, this can be very difficult. Dryer bars (the KadantJohnson trademark is "Turbulator® bars") are normally installed in paper dryers to cover the full width of the dryer. These bars increase both the capacity and the uniformity of heat transfer. This alone will often improve the moisture profile. If, however, the non-uniform moisture profile is being caused by something external to the dryer, correcting the profiles by making the heat transfer non-uniform is not a best practice.

For example, if the basis weight profile is non-uniform, or the press nip profile is non-uniform, or the press felts are streaky, it may be possible to improve the profile with partial-width dryer bars, but once the external problems are solved, the poor moisture profile returns. Unless the moisture profile non-uniformity is consistent and persistent (over many years), it is best to maintain uniform heat transfer from the dryers with full-width bars.

How does the sheet moisture at the reel effect energy consumption?

The moisture profile and moisture level both have direct effects on each other and on the resulting energy consumption. One of the often-used tools used to fight a poor moisture profile is lowering the moisture content. This levels the moisture profile, but it also increases the amount of water that must be evaporated. It increases the amount of fiber that is required to achieve the specified reel weight and it reduces the overall drying capacity (the drying rate for a dry sheet is very low).

It is more cost-effective to achieve a uniform cross-machine moisture profile without drying below the average moisture required to achieve the finished sheet properties. As a bonus, basis weight profiling systems, profiling steam boxes, moisturizing showers, and caliper profile control devices can be used to optimize sheet properties and not used to correct for non-uniformities.


Is there a cost associated with introducing fresh water into the white water system?

Yes, white water systems are typically controlled to a targeted temperature. When fresh water is introduced into the white water system, this water must be heated to maintain the system at the desired temperature.

To calculate the annual cost (350 operational days) to heat fresh water to process temperature with low pressure steam, one can use the following equation:

Steam Costs for 1 Hour $ = GPM*8.25*ΔT*60*$MMBTU/1,000,000

Steam Costs for 24 Hours $ = (GPM*8.25*ΔT*60*$MMBTU/1,000,000)*24

Steam Costs for 350 Days $ = (GPM*8.25*ΔT*60*$MMBTU/1,000,000)*24*350

Where:

GPM = Gallons per Minute

8.25 = Conversion Factor GPM to lbs/min

ΔT = Delta T of Inlet Temperature to Process Temperature

$MMBTU = Dollars per Million BTU


Can an energy monitoring system reduce energy consumption?

Yes, but not directly. By definition, a monitoring system does not do anything, it just watches what is going on. However, the first step in energy conservation activities is monitoring energy consumption. "If you can't measure it, you can't improve it." To properly monitor the energy consumption, flow meters, temperature transmitters, and cost information need to be accurate.

An effective monitoring program requires the measurement of all major energy flows (steam, condensate, water, electricity, gas, compressed air, and vacuum) and their associated costs and the associated production. The best monitoring programs include active benchmarking against best-in-class machines. Assigning a person to be responsible for energy conservation in the mill can help increase visibility and accountability of conservation efforts.

Can steam boxes reduce the amount of energy required for making paper?

In theory, yes. In practice, maybe. Heating the sheet in the wet end or in the press section will increase the dewatering capability and deliver a drier sheet to the dryer section. This will reduce the amount of energy consumed in the dryer section to achieve the desired sheet moisture.

The amount of energy saved in the drying process is often less than the amount of energy used in heating the sheet. As a general rule, the press section will increase the sheet dryness by one percentage point for every 10°C increase in sheet temperature. The sheet strength, however, also increases with increasing dryness and this helps to improve sheet runnability. An added bonus!


Can the mill DCS be used to monitor energy consumption?

Yes. The mill Distributed Control System and process historian (process database) are very powerful tools for monitoring and trouble-shooting machine operations. But there may be more to the story. Not all process flows and valve positions are measured, displayed, and recorded by the DCS. Routine operator rounds can fill in the gaps. Operators can see when safety relief valves or vent valves are open or leaking, when rotary steam joints are blowing steam, when condensate is being dumped into the sewer, when compressed air lines are left open, when ventilation systems are out of balance, where dump valves and by-pass valves are inadvertently left open, and when seal pits are flooded over. Open drains result in not only lost energy, but also in lost condensate and lost boiler feed water treatment chemicals.

Thermography using infrared camera systems can be a very effective tool in finding energy losses that are not visible to the eye: steam leaks, hot spots in insulated panels, pipes, separators, and valves, and the ubiquitous defective steam traps. An observant operator, DCS displays, process historian data, and an infrared camera are all integral parts of a complete energy management program.


How do stationary syphons help to reduce papermaking energy consumption?

Stationary syphons can be used to drain condensate from steam-heated paper dryers. They do this with lower differential steam pressures and lower "blow-through" steam flows than that required for draining dryers with a rotating syphon. In a cascade steam system, this results in less blow-through steam being vented to atmosphere or to a condenser.

In a thermocompressor system, this results in less high-pressure (motive) steam to recompress the steam for re-use. When properly sized and installed, they can also greatly reduce the tendency for the dryers to load up with condensate ("flood"). Stationary syphons also reduce the amount of upset that occurs during a sheet break and correspondingly reduces the tendency for venting.

Does the type of rotary steam joint affect energy consumption?

Yes, the type of rotary joint does affect the consumption of energy. Older style rotary joints, self-supported joints and many lug-supported joints, use internal steam pressure to apply load to the carbon sealing ring. It takes high torque to rotate these "pressure joints", particularly when the steam pressure is high.

Modern compensated steam joints, such as the Kadant Johnson PTX® steam joint, require much less torque. For example, the drive power for a machine running at 1750 fpm with (80) 60" diameter dryers could be reduced by 80 hp simply by converting from pressure joints to PTX steam joints. At an electrical energy cost of $0.07 per kWhr, this would be a savings of over $35,000 per year. The elimination of steam leaks and rotary joint maintenance makes the savings even larger.


Does machine clothing selection affect energy consumption?

Yes! Forming fabric design has a direct effect on drainage rate, sheet formation, and sheet properties. The fabric permeability may affect the vacuum load and air flow requirements- and perhaps the frictional drag over stationary forming elements, but these are typically smaller effects. Dryer fabric tension, permeability, and (to a lesser degree) fabric construction can affect the drying rate and drying capacity, but dryer fabrics do not have a direct effect on the amount of energy required for drying.

Press fabrics, on the other hand, have a direct and significant effect on energy consumption. The press fabric construction, wet permeability, and re-wet control directly affect the dryness of the sheet leaving the press. A one percentage point change in the moisture content of the sheet leaving the last press nip can result in a 4 to 5% reduction in the amount of water that must be removed in the drying process.


Does machine runnability affect energy consumption?

Most people would say that runnability does not have a direct effect on energy consumption. But they are only partially right. If the press draw is long or the sheet edges are curling or there are signs of sheet wrinkles, folds, tears, or edge cracks, the machine speed is reduced to avoid the resulting sheet breaks. As long as there are no sheet breaks, there is no energy loss associated with sheet breaks. But when there is a sheet break, the energy consumption remains high and there is no resulting production.

During a dry end sheet break, for example, pumps, drives, fans, showers, and heaters continue to run, using the same amount of energy, and the dryers waste 1-2 tons of steam energy for every ton of broke. Anything that can be done to improve machine runnability (unorun dryer sections, blow boxes, suction pick-up rolls, ventilation air control, threading devices, supervisory control systems) will help to reduce the amount of energy consumption.

Is a dryer fabric cleaning system required for high energy efficiency?

Clean dryer fabrics do not directly increase or decrease the amount of energy required to dry the paper web. However, dryer fabrics that have a lot of build-up of fiber, filler, and stickies will reduce the uniformity of heat transfer, the effectiveness of the dryer pocket ventilation system, and sheet runnability. These lead indirectly to more demands on the cross-machine profiling devices, reduced drying capacity, and more sheet breaks. These all result in reduced energy efficiency.

Fabrics can be cleaned by a number of methods, either during a shut-down or on-the-run. On-the-run devices use a series of high-pressure water jets and an integral system for capturing and removing the debris cleaned off of the dryer fabrics.


Vacuum Condenser Energy Monitoring

Many dryer drainage systems discharge wet end dryers and differential vent valves to a vacuum condenser. Water is circulated through the heat exchanger to condense the discharged steam and create a vacuum. Monitoring the vacuum condenser is an excellent way to track dryer section energy efficiency. Any steam vented to the vacuum condenser is a loss from the system and should be considered a waste. A well designed dryer drainage system will have losses to the condenser less than 3% of the total steam flow to the dryer section. Many machines have losses that are in the range of 1% to 2%.

How is energy loss to the vacuum condenser monitored? By measuring the heat gained by the cooling water. The heat gain can be calculated from the cooling water flow, water temperature into the condenser, and water temperature out of the condenser. These measurements can be made with a portable water flow meter and a thermometer. Many machines are installing permanent devices to measure and record the information in a data historian. The trend line will quickly show when the energy efficiency of the system is changing. Low energy efficiency can be caused by something as simple as a poorly maintained vent valve from one of the steam sections or something more complicated like mis-sized syphons or thermocompressors.


Piping Insulation - The Rodney Dangerfield of Energy Conservation

Many paper mills will have hundreds of feet of exposed steam and condensate pipes. Insulation may have been removed for maintenance work or is in such poor condition that it provides little insulation value. A perception exists that the energy loss is not significant and the lack of good pipe insulation is often overlooked when assessing energy conservation opportunities. Like Rodney, piping insulation gets no respect.

For example, a ten foot length of 6" pipe handling 100 psig steam will have a heat loss of 14,100 Btu/hr. This can be reduced to 865 Btu/hr with proper insulation. If the energy cost is $6 per million Btu, the savings would be $600 per year. Insulation of exposed piping can quickly add up to significant savings. Insulation has the added benefit of improving the machine room operating temperature and providing a safer working environment.

The Department of Energy has free software available called 3E Plus. This software can be used to assess the value of insulation of pipes and tanks. It can be accessed at the Dept. of Energy website. Look for the Steam System Tool Suite. There are other useful free tools at this site to help with energy conservation efforts.


Are cooling towers the best approach to reduce temperature of recirculated seal water for liquid ring vacuum pumps?

Low temperature seal water improves liquid ring vacuum pump efficiency which can improve the paper machine performance.

Many paper mills use cooling towers to lower seal water temperature. Cooling towers remove heat from the seal water through evaporative cooling. This valuable heat is then lost to atmosphere. Cooling towers also require electrical energy to maintain air movement, fresh water to maintain level, chemicals to prevent biological growth, and maintenance to keep the towers clean and functioning.

Heat exchangers can be an energy efficient alternative for reducing vacuum pump seal water temperature and heating process water. Modern spiral heat exchangers can even operate with lean white water without plugging or extensive maintenance. With the right heat exchanger configuration, the energy (heat) that was previously lost to atmosphere can be recovered and the costs of operating a cooling tower are eliminated.


Can cold pump, agitator, and refiner seal water impact paper machine wet end steam use?

Yes, it can. Typically, cool fresh water is used for paper machine seal water systems. Over 30% of seal water used for pumps, agitators, and refiners enters the papermaking process. Altogether, this represents a significant amount of fresh water that must eventually be heated up to process water temperature.

For example, a seal water system that supplies 400 gpm to pumps, agitators, and refiners will add over 170,000 gallons per day of cool fresh water to the whitewater system. And this amount can easily be much more.

Many seal water lines have only a manually adjusted ball valve to regulate seal water flow. These valves are often found operating completely open when only a fraction of the flow rate is really required for the seal.

A much better approach is to use flow meters on the seal water lines to each pump, agitator, and refiner. Maintenance can use these flow meters to adjust the seal water flow to each device to the manufacturers' design specifications. This will have a significant impact on overall water and energy consumption.


What is the optimum operating tension for a dryer fabric?

It depends. Dryer fabrics hold paper in tight contact with dryer surfaces to increase the drying rate and heat transfer uniformity. On some paper machines, dryer fabrics are also used to drive dryer cylinders. Dryer fabrics can also reduce the cross-directional shrinkage of the paper web. Despite these positive effects, there is a point of diminishing return. Excessive tension can damage the fabrics and paper machinery with no practical gain in drying capacity, drivability, sheet handling, or sheet restraint. Fabric tensions should never exceed those recommended by the fabric supplier or the equipment manufacturer. That said, the minimum fabric tension is about 4 pli and felt tensions in the range of 10-20 pli would normally give high drying rates, stable fabric guiding, and good machine runnability. Slightly higher values may be required on machines with runnability components such as blow boxes and vacuum boxes.

What is the correct operating differential pressure for a dryer section?

The correct operating differential pressure for maintaining dryer drainage depends on a number of parameters. Some of the more important variables are: dryer section steam pressure, steam condensing rate, type of dryer syphon (rotating, stationary, or scoop), syphon pipe sizes, machine speed, and dryer diameter. When it comes to setting the operating differential pressure, unlike the Ron Popeil 4000 rotisserie, operators cannot just "set it and forget it". The operating differential pressures must be adjusted to reflect changes in operating conditions that come with start-ups, shut-downs, grade changes, and sheet breaks. Operators can make these changes manually, or they can use a supervisory control system, to make the changes automatically.


What is the recommended frequency for dryer inspections?

There may not be pressure vessel code, local jurisdictional, or other regulatory requirements for periodic dryer inspection, but it is a good practice to formally inspect dryers every five years. To make this process more manageable, mills and their insurance carriers will often adopt a program in which 20% of the dryers are inspected each year so that all dryers are inspected on a five-year cycle. These inspections include magnetic particle testing of dryer heads, ultrasonic testing of head bolts, dryer shell thickness measurements, acoustic emissions examination, and external and internal visual inspections.

Internal visual inspections should identify grooves in the shell, erosion under syphon shoes, loose dryer bars, loose dryer balance weights, visible casting flaws, and erosion or damage to syphon piping. Eroded syphon shoes, eroded syphon elbows, and leaks past syphon flange gaskets can lead to poor condensate evacuation, increased operating differential pressures, and high blow-through steam flows. These in turn reduce the energy efficiency of the drying process.


Why are dryer surface temperatures measured in a dryer study?

The surface temperature of a dryer is a good indicator of dryer performance. The temperature of the surface is compared to the temperature of supply steam (the "saturation" temperature that corresponds to the dryer steam pressure). The difference between the dryer surface temperature and the steam temperature is an indication of heat transfer efficiency from steam through the condensate in the dryer, the cast iron dryer shell, contamination on the inside and outside of the dryer, and any other resistance between the steam and sheet.

Interpretation of these results, however, requires some care. High dryer surface temperatures, for example, can be the result of very good heat transfer from the steam to the dryer shell, but they can also be the result of poor thermal contact between the dryer shell and the sheet. When used in conjunction with other measurements (machine speed, trim width, grade, steam pressures, condensing load, fabric tensions, etc.), the dryer surface temperature measurements can provide valuable information on areas for potential improvement.


What is the difference between sensible heat and latent heat?

"Sensible" heat is energy that results in a change in temperature, that is, energy transfer that can be sensed. "Latent" heat is energy that is required for a change in state, for example, to evaporate water without any change in the water temperature. Latent heat is also known as "heat of evaporation" or "heat of condensation". When drying paper, some of the drying energy is required to increase the temperature of the sheet, without evaporating any water. This is called sensible heating. The majority of the energy required for drying paper is the energy required to evaporate the water. This is called the latent heat.

It takes about 30 Btu (British Thermal Units) of energy to increase the temperature of one pound of water from 182°F to 212°F. It takes 970 Btu to evaporate that same pound of water with no change in its temperature. The least amount of energy is required to dry the paper when the wet web is not over-dried (that is, when it is not over-heated) and when the amount of water that must be evaporated is minimized, for example, by having higher exiting press dryness or a higher reel moisture content.


What is the difference between sensible heat and Does the grouping of dryers affect energy efficiency? heat?

The grouping of dryer cylinders in separate steam groups does not directly increase or decrease the amount of energy required to dry the paper web. The grouping of dryers does, however, have a direct effect on the design and ultimately on the performance of the dryer steam system. In a thermocompressor steam system, for example, each thermocompressor must be sized to match the number of dryers in the section, their operating steam pressures, the differential steam pressures, and the resulting blow-through steam flow.

In a cascade steam system, the down-stream dryer groups must be sized to use the blow-through steam flows from the up-stream dryer groups. The last of the steam groups must also be properly sized so that its blow-through steam is either used in a low-pressure process or low enough in quantity to be discharged to a condenser. Without a proper grouping of dryers, the steam energy will be lost, either through vent valves to the atmosphere or to condensers.


Is it important that differential pressure transmitters be calibrated?

Proper calibration of instrumentation is at the heart of any efficient dryer section operation. If a process parameter cannot be properly measured, it cannot be properly controlled. Differential pressure transmitters are particularly important. Dryer drainage depends on it. Dryers with rotary syphons will flood if the differential pressure is not high enough and they will pass excessive amounts of blow-through steam if the differential pressure is too high. Even with modern stationary syphons, accurate differential pressure measurement is very important. Only 3-4 psi differential pressure is needed to drain the dryers with a stationary syphon, but with a measurement error of only 2 psi, the blow-through flow will easily be either inadequate or excessive.

It does not end there, the differential pressure transmitters may be properly calibrated, but the pressure sensing lines may not be delivering the right pressure to the transmitter. Over half of the dryer sections surveyed by Kadant Johnson had sensing lines that were improperly designed, installed, and maintained.


Where do you start in optimizing a dryer section?

The dryer section consumes large amounts of energy and it can have a dramatic impact on both sheet quality and profitability. The drying process, like other papermaking unit operations, is a complex matrix of interacting systems: Pocket ventilation, steam system, drive system, tail threading, sheet handling, energy recovery, and fabric conditioning, guiding, and tensioning.

A good way to start an evaluation of the dryer section is to contact a drying systems supplier, consultant, or engineering firm for a dryer audit. A professional dryer audit should include a complete set of dryer section measurements, an analysis of equipment sizing, recommendations for operational improvements, recommendations for equipment upgrades, lists of short-term and long-term improvement projects, and an analysis of the expected return for each project.

Typical returns come from energy savings, increased drying capacity, reliable dryer drainage, reliable dryer drainage equipment, improved dryer section runnability, and enhanced drying uniformity. A comprehensive report identifies improvement opportunities and quantifies the associated benefits.


What is the role of monitoring in optimizing a dryer section?

Optimizing a dryer section begins with monitoring the dryer section operation and developing a baseline for dryer section performance. The baseline can be compared to the performance of similar machines producing similar grades. This baseline can also be used to evaluate the impact of changes made on the machine. Improvements, however, require an active program that follows and acts on the data. Daily production meetings should include a review of energy efficiency readings. This will help to identify losses as they occur.

For example, an increase in steam going to a condenser may indicate the differential steam pressures are too high. A vent valve opening up may indicate a thermocompressor is running in a choked flow condition. An increase in the dryer steam flow (per ton of product) may indicate the exiting press moisture is increasing and it is time to change one of the press felts. An increase in steam pressure to the air heaters may indicate air filters are plugged and need to be replaced. Again, the first step is monitoring. The next steps involve acting on the results.


How do condensate traps impact energy efficiency?

Traps indeed impact the energy efficiency of a steam system. Condensate traps are applied to the drain lines of various types of steam heating equipment such as steam coils, heat exchangers, and unit heaters. They are also frequently applied to keep steam mains free of condensate.

The purpose of condensate traps is to allow condensate to drain to a lower-pressure condensate collection system while disallowing the flow of steam to the lower-pressure system. There are many types of condensate traps, but nearly all are designed such that if they fail, they fail open. If a trap fails open, the equipment served by the trap will normally continue to function trouble-free. But, the failed trap will allow live steam to flow to the downstream condensate receiving system. This normally has negative impact on energy efficiency because the live steam going to the receiving system usually is not put to good use. It might be vented to atmosphere or, worse yet, it might cause the condensate receiving system to pressurize which will potentially cause drainage problems for other users leading to various operational problems.

To ensure condensate traps are functioning properly, they should be surveyed every two to four years by a trained professional. Most of the trap suppliers offer trap survey services.


Are the non-condensable bleeds on thermocompressor sections necessary?

Most thermocompressor sections are equipped with a non-condensable bleed (NCB) which are are applied to thermocompressor sections to prevent accumulation of non-condensable gases in the dryers. If allowed to accumulate, the non-condensable gases will diminish heat transfer rates for paper dryers in the steam section and cause non-uniform cross direction dryer surface temperature profile. This will lead to non-uniform cross direction sheet moisture profile.

Today, most boilers generate steam with extremely low levels of non-condensable gases and some producers seem to "get by" with no NCB's on their thermocompressor sections. Still, most steam system suppliers advocate having a NCB to safeguard against costly operational problems.

The discharge from NCB's is usually directed to atmosphere or a condenser. In this case, the presence of the bleed has negative effect on energy efficiency. In some cases the need for a NCB is avoided by arranging an individually controlled dryer (that drains to a condenser) to draw its steam from a thermocompressor section. Where allowable, this is an advantageous concept.

The most prevalent improvement opportunity for NCB's is to gain an appropriate flow rate. For a typical thermocompressor section, an NCB flow of 100 to 200 lbs/hris generally considered adequate to protect against accumulation of non-condensable gases. To ensure the desired flow rate is achieved, the bleed should be equipped with a properly sized stainless steel orifice plate.

When it comes to NCB's, avoiding excessive flow rate will save steam. Your mill might benefit from a review of your NCB's.


What common metrics are used for measuring paper drying efficiency?

Two common metrics for measuring paper drying efficiency are drying load (amount of water evaporated per hour) and steam consumption (amount of steam used per hour). The Technical Association of the Pulp and Paper Industry (TAPPI) recommends these metrics be indexed by the dryer surface area (resulting in pounds of water evaporated per hour per square feet of dryer surface area) and by the amount of water evaporated (resulting in pounds of steam per hour per pound of water evaporated). The indexed values are better indicators of dryer performance and overall energy efficiency of the drying process. Indexed values can be compared to similar machines as a benchmark of performance.

Is there an accurate, reliable, and safe way to determine the out-going press dryness?

Yes, there is. The out-going press moisture can be determined from a mass and energy balance of the dryer section. By knowing the paper weight, width, speed, and reel moisture and the amount of steam used for drying, the out-going press moisture can be calculated using an analytical model of the dryer section. This is much simpler than a water balance that requires accurate measurement of many water flow rates around the press section. It is also much safer than attempting to get a "grab" sample of the sheet on the front side of the press section. It is also less costly than a cross-machine moisture scanner. And the result is more accurate than all of these alternatives. For a "tight" well-managed dryer steam system, the sheet dryness can be determined within 0.1-0.2 dryness percent.


Is there a better way of measuring energy efficiency in paper drying?

Yes, there is. Rather than measuring the amount of steam that you use, measure the amount of steam that you lose. The steam consumption (amount of steam used per hour) is, by itself, an inadequate measure. A high value could be the result of either drying a lot of paper or venting a lot of steam. One is good. The other is bad. Indexing the amount of steam used with amount of water evaporated (pounds of steam per hour per pound of water evaporated) is a better metric, but significant amount of energy can easily be lost in the "noise" of this metric.

A better approach is to simply look for the steam losses. Specifically, look for steam that is vented to the atmosphere or vented to condenser tanks. Look for condensate that is dumped to a sewer and look for hot air that is exhausted from the dryer hood to the atmosphere. These are direct measurements of energy losses and they are direct indications of potential areas for energy recovery.


Evaluating Dryer Drainage System Energy Use

Steam flows are commonly measured to monitor dryer section energy consumption. The total steam flow, however, does not provide enough information to assess energy efficiency. Measures that consider steam consumption in terms of production or output can be used to evaluate the efficiency of steam use in the dryer section. There are three common metrics to evaluate steam use efficiency: pounds of steam per pound of paper produced, pounds of steam per pound of water evaporated, and percent of energy loss from the system.

Pounds of steam per pound of paper produced is not a good metric to use when attempting to benchmark energy efficiency. Dryer energy consumption is directly related to the amount of water evaporated. Differences in press moisture entering the dryer section will affect the benchmark. Each 1% difference in press moisture will change the dryer section energy consumption by approximately 4%. Machines with different pressing efficiencies will have different steam consumption benchmarks. Therefore, it is difficult to benchmark energy efficiency against other machines using this metric.

Pounds of steam per pound of water evaporated is a better way to assess energy efficiency. This metric requires an accurate value for the press moisture to calculate the water evaporation for a given production condition. Obtaining an accurate press moisture can be difficult. Grab samples and press section water balances are often used. The press moisture will also vary depending on grade, speed, furnish, and other unique characteristics of the machine. A good value for this metric is 1.2 lbs of steam per pound of water evaporated.

Energy losses from the system is a highly usable and appropriate metric to use to assess steam utilization and evaluate energy efficiency. Any steam that is not condensed in the dryers to dry paper should be considered a loss from the system. The losses typically consist of the steam that is vented to the condenser or atmosphere. This can be tracked by valve position in a process historian. The loss can also be calculated by measuring the water flow and temperature around the vacuum condenser. A well designed dryer drainage system will have an energy loss of 1% to 3% of the total steam supplied to the dryer section.


Tracking Energy Losses From A Dryer Drainage System

One method to evaluate dryer section energy efficiency is to track the losses from the dryer drainage system. Any steam not condensed in the dryers to dry paper should be considered a loss. A well designed system will have a loss of no more than 1% to 3% of the total steam supply to the dryer section.

Many dryer drainage systems are designed with a vacuum system that condenses steam vented from the system. The energy consumed by the vacuum condenser represents the loss from the system. Measuring water flow, the water temperature in and out of the heat exchanger allows the energy loss to be calculated. This can be done with three field instruments and provides a valuable online energy monitoring system. The heat loss from the system can be trended against the steam flow to the dryer section to calculate the percent loss from the system.

This method of energy monitoring will quickly show any change in efficiency of the dryer drainage system. A leaking vent valve or improper system setting will show up as an increase in the percent loss from the system.


Do Turbulator bars improve the energy efficiency of a dryer section?

The amount of energy required to dry a paper web depends on the amount of moisture entering and leaving the dryer section, machine speed, sheet width, and sheet weight. Turbulator bars do not directly affect the amount of energy required to dry the paper, but they can have a major impact on other operating parameters:

Turbulator bars increase the heat transfer capacity. This can be used either to reduce the dryer steam pressures or increase the amount of paper produced.

Turbulator bars improve the uniformity of heat transfer. This improves the cross-directional moisture profile.

Turbulator bars increase the responsiveness to changes in dryer steam pressure. This results in less out-of-spec paper being produced during grade changes.

The increased uniformity and drying response improve sheet runnability and machine efficiency.

Turbulator bars force the condensate in the dryers to rim at a lower speed. This reduces the power required to drive the dryer cylinders.


Do high-efficiency thermocompressors save steam?

High-efficiency thermocompressors do offer a higher entrainment ratio which means a lower motive steam (high-pressure) flow is required to recompress the same amount of suction steam (low-pressure) flow or conversely pull a higher amount of suction flow for the same amount of motive flow for a given set of pressure condition.

A high-efficiency thermocompressor will offer an operating cost advantage if the source of the motive steam bypasses a cogeneration turbine or if the source of motive steam is produced on a high-pressure boiler separate from the source of low-pressure steam. In these cases, the motive steam will have a higher cost than the make-up steam. Minimizing the motive steam required will maximize the make-up steam flow and the power generation.

For tissue machine applications where the motive flow is limited to the Yankee's condensing rate, the higher entrainment ratio provides the benefit of increased blowthrough flow for the same amount of motive steam flow and for this reason, high-efficiency thermocompressors have become the industry standard for Yankee dryers.

On booster thermocompressor applications, a high-efficiency thermocompressorwill reduce the ratio of motive to suction steam flow for a fixed discharge flow.


Is separator tank size and design critical to the proper operation of dryer cans?

Absolutely! Undersized separator tanks generally result in high velocities of steam and condensate flow through the inlet separator nozzle, through the shell of tank and through the outlet blowthrough steam nozzle. High velocities do not provide the basis for good separation of the steam and condensate and as a result, the blowthrough steam will tend to carryover some condensate. High velocities will also erode the baffles and impingement plates inside the tank much quicker, especially with 2-phase steam and condensate flow potentially causing short-circuiting between the inlet and outlet nozzles. Condensate carryover will accelerate the erosion of downstream piping and thermocompressors and increase the condensate load for the syphons to remove from the dryers. Undersized separator tanks also tend to have poor level control. Separator tanks must be sized based on achieving a low vapor velocity through the tank and adequate liquid retention time.

The issues associated with undersized separator tanks will cause poor operation of the thermocompressor and dryer control equipment. Venting and steam waste is common when condensate separator tanks are undersized.


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