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Optimisation of the Air Conditions in the Dryer Section

By Georgi Slawtschew (Dipl.-Ing.), Application and Technical Services, Heimbach GmbH & Co. KG

Increasing energy costs and demands on the environment require to an ever greater extent reduced energy consumption in the paper industry. Where else can the most energy be saved, if not where the most energy is consumed: in the dryer section.

Paper drying in the dryer section can be divided into two parts of the process:
1. Heating and evaporating the water, primarily by means of steam-heated cylinders
2. Removal of the evaporated water from the pockets and from the dryer hood

The following contribution deals mainly with the often ignored second part of the process, the removal of the moisture from the sheet and from the machine. Previous TASK Information Booklets have considered the significance of ventilation and heat exchanger systems for the functioning of the whole dryer section. Badly adjusted volumes and quality of supply air and exhaust air (temperature / moisture) and inadequate energy recovery can lead to enormous losses of energy and / or production capacity as well as deterioration in paper quality. With the aid of case studies the following contribution highlights possibilities of how the air conditions in the dryer section can be optimised at relatively low cost.

Energy requirement for heating and evaporating the water from the sheet

In order to warm (from 50 to 100 0C) and subsequently evaporate 1 kg of water under ideal conditions ca. 2,400 kJ are required. That amounts to a condensation energy of ca. 1.1 kg steam at 1 bar pressure (or 1.2 kg steam at 7.0 bar pressure). Therefore the steam consumption can simply be reduced to 1.1 t steam / t evaporated water.

In practice 1.2-1.6 t steam is needed for the heating of the cylinders (not to be confused with the specific steam consumption per tonne of paper which depends on the dry content before / after the dryer section, paper grade etc.). These figures have come from examinations carried out by Heimbach in recent years.

The required condensation energy for low pressure steam is higher than that for high pressure steam. The pressure of the steam determines the temperature difference between the paper sheet and the cylinder surface and also the speed of heat transfer. Therefore, the bigger the temperature difference, the faster is the transfer of heat energy from the cylinder surface into the sheet.

This temperature difference can, in the case of most board and packaging grades, be increased by raising the steam pressure – but with the disadvantage of a higher specific steam consumption.

On more sensitive grades such as writings and printings the higher cylinder temperatures can have negative effects on the sheet characteristics. In this case another route can be followed. In the open draws between the cylinders the sheet cools through vaporisation. The greater the vaporisation in this area, the greater will be the temperature difference between the paper sheet and the following cylinder. In this way heat transfer can be accelerated. The vaporisation is also augmented by a more effective pocket ventilation.

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