Removing water from the paper web is the most energy-intensive stage of the entire production cycle, with thermal energy consumption exceeding 1 MWh per tonne of paper produced. Given that final thermal drying involves extremely high specific costs, the primary objective is to maximize the efficiency of vacuum system and mechanical pressing, to minimize the residual water for final thermal evaporation.
A key parameter in press performance is water viscosity, i.e., the internal friction between molecules that resists flow from the web to the suction roll. Increasing the water temperature facilitates this process. For instance, as temperature rises from 30°C to 40°C and 50°C, viscosity decreases from 0.8 to 0.65 and 0.55 mPa·s, respectively.
An ideal solution for both new installations and existing mills, the TT TurboDryer utilizes exhaust gases from the hood before they are released into the atmosphere. By transferring this heat to the paper web, it facilitates water removal thanks to reduced viscosity. A portion of the hood exhaust, after being used to preheat fresh air, is drawn by a fan and directed toward the suction roll or suction press.
Process air is extracted downstream of the air-to-air heat recovery unit (dedicated to pre-heating combustion and make-up air for the air system) via a dedicated fan.
An automatic control system regulates the air temperature at the TT TurboDryer inlet to prevent excessive thermal stress from damaging the felt. High temperatures (typically 150-200°C) combined with the air’s inherent humidity provide a significant energy input to the web, increasing the post-press dryness content to a great extent.
A major portion of this thermal power originates from the humidity in the exhaust fumes: upon contact with the paper (usually at 35-45°C), this moisture condenses, transferring its latent heat of condensation directly to the web.
Data from operational plants demonstrates a water reduction of approximately 10%, which corresponds to an increase in dryness of over two percentage points.
The system is equipped with the necessary temperature, flow, and pressure controls to maintain the blow box in a balanced state. This prevents cold air infiltration, which would compromise performance and prevents hot air from escaping, thereby ensuring the safety of both the operators and the machinery. Typically, only the box and its piping are positioned within the machine frame, while all remaining components are positioned in the air system room.
Unlike similar systems, the TT TurboDryer design prevents fiber buildup on the surface facing the web and thus eliminates the need for frequent cleaning. Any stock droplets projected toward the box by centrifugal forces are deposited internally, maintaining a significant distance from the web itself. Furthermore, any deposits that may accumulate within the TT TurboDryer can be easily removed.
Inspections and any necessary washing can be performed in seconds during a standard reel change, without requiring any adjustment to the machine’s operating parameters. A system of pneumatic cylinders retracts the TT TurboDryer from its operating position to allow for visual inspection and cleaning via water hose. During this movement, the exhaust ducting remains connected through flexible piping.
TOSCOTEC has validated these results across several machines in operation, both in terms of reduced thermal consumption at a constant production rate, and as an increase in machine speed under the same operating parameters.
The table illustrates performance benchmarks for the TT TurboDryer on a machine with an 18’ Yankee dryer and a 5600 mm web width.
It also highlights the tangible impact of increasing after-press dryness by over 2%, which can be leveraged either as a reduction in thermal consumption (gas or steam) or as an increase in machine speed. In addition, the system’s reliability and minimal maintenance requirements ensure consistent performance over the long term.
Results from running machines prove that it is no longer necessary to compromise between high production output and resource conservation. The TT TurboDryer meets both these objectives, demonstrating that significant energy savings can be achieved without adding complexity to daily mill operations.
With its ease of integration and minimal maintenance needs, it has become an essential component for any tissue machine facing the energy challenges of today.
The TT TurboDryer stands out for having one of the fastest ROI (Return on Investment) periods in the industry, thanks to the balance between low capital expenditure and immediate energy savings. Based on the current average gas costs, with an overall production of 60,000 tpy the total saving is more than 500,000 euros per year.
Toscotec
Since its foundation in 1948, Toscotec specializes in the design and manufacture of turnkey projects, complete machines, plant rebuilds and single equipment for the tissue and paper & board industries.
Headquartered in Lucca, with affiliates and service centres in China and North America, Toscotec provides state-of-the-art technology and customized solutions to leading paper producers around the world.
Our technology aims at maximum production efficiency, reduced energy consumption and operating costs, finished products of the highest quality and respect for the environment.
From 30th April 2020, Toscotec is part of the Voith Group.
Products
Drying cylinders, Drying hoods, Dust extractors, Energy Saving, Heat recovery systems, Paper machines, Paperboard machines, Presses, Rewinders, Spare parts, Steam and condensate systems, Stock preparation systems, Winders, Yankee cylinders, Yankee hoods .
