Paper Drying Energy Tips from Kadant

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.

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 Kadant Johnson DMS® steam system control technology is programmed to provide periodic and automatic venting of air from dryers in thermocompressor loop systems.

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 Kadant Johnson DMS™ control technology can monitor these parameters and send a maintenance alert when the parameters are out of normal operating range.

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.

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.

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 affect does thermocompressor efficiency 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 thermocompressor to 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.

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.

How do you evaluate the performance of a dryer section?

A complete dryer section audit requires a review of the drying rate, energy losses, control system response, air handling system, mechanical reliability, and steam system operation. The steam system review includes a check of syphon sizing, line sizing, separator tank sizing, thermocompressor performance, steam joint performance, and the balance between high-pressure and low-pressure steam flows. Tappi provides benchmarks for some of these parameters (for example, Tappi TIP 0404-33 “Dryer Section Performance Monitoring”, Tappi 0404-63 “Paper Machine Energy Conservation”, Tappi 0404-47 “Paper Machine Performance Guidelines”, Tappi TIP 0404-07 “Paper Machine Drying Rates”). Benchmark evaluation is key. You cannot hit a target if you cannot see it.

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.

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.

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.

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.

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.

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

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.

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.

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).

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.

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.

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.

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.

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.