Paper Drying Energy Tips from Kadant

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.