18 ANDRITZ
Three-dimensional representation of an ANDRITZ PrimeLineTAD tissue machine
Pulp Paper & Logistics
properly mix the air and optimise the supply air temperature and pressure profile uniformity to the TAD drum. For this purpose, we developed specially-designed static mixers, guide vanes and perforated screens that are incorporated into the ductwork and the TAD supply hoods. Our extensive experience in air systems engineering, both from field installations and from using CFD analysis for optimisation, has allowed us to excel in this area. The air system layout must not only be carefully designed with minimum space consideration, it must also address energy efficiency, air flow simplicity and component accessibility. All these factors have a direct impact on installation and operating costs. The process air is transported
through the large insulated ductwork by the variable speed supply fans that deliver the required air mass to the sheet. A natural-gas-fired burner is installed after each TAD supply fan to heat the air supplied to the sheet and to increase its drying potential. The burners are selected to operate with low CO and NOx emissions throughout their design firing rate. The PLC- based control logic monitors the burner firing rate, and ultimately the TAD supply air temperature,
July/August 2021
for the various system operating modes.
The supply air ducts are
connected to the TAD hood modules with articulated joints that permit the TAD hoods to retract, allowing access during fabric change, sheet breaks or cleaning and inspections during maintenance shutdowns. The wet and dry end TAD hoods are constructed of two independent, retractable halves, with perforated nozzle plates that allow proper airflow distribution into the space between the TAD hood and TAD drum. The supply air is ‘pulled through’ the sheet by negative pressure within the TAD drum. Moisture removed from the
sheet is then partially evacuated to the atmosphere using the TAD exhaust fan. The air exhausted to the atmosphere is replaced with fresh air make-up. The TAD supply fan pushes the TAD return air back into the pre-dryer supply air duct system in this continuous drying cycle. By controlling the TAD supply air properties such as mass flow, temperature and absolute humidity, we optimise the drying capacity of the system. With the proper control of the exhaust air pressure, moisture and temperature, the optimum energy use can be achieved together with
extended life of your TAD fabrics, both which represent a significant operating cost. Of course, various types of heat recovery can be implemented to reduce overall energy consumption. We have implemented several types of heat recovery options, such as air-to-air, air-to-water, turbine and vacuum blower exhaust for pre-heating air, and several others heat recovery solutions. Our goal is to make the highest quality tissue and towel products with reduced energy input.
Can you talk more about energy savings, and provide some numbers? Paul Richards: TAD by its very nature is extremely energy intensive, which is one of the key reasons the market has developed so much in the USA, where energy is traditionally so much cheaper than in other regions. What we are seeing today, however, is an interest from regions where energy is expensive, but where a market can still be developed if a way can be found to reduce the energy. Historically, TAD was made with
a dryness of around 95 per cent – one of the reasons that led to the development of UCTAD, or the Un-Creped TAD, process. The
Yankee, a traditional part of any tissue machine, was only really a carrier, allowing a percentage of dry crepe to be put into the end product. Today, depending on the technology being used and the manufacturer, the dryness after the TAD can be significantly lower. In fact, we and our customers are constantly looking to push these boundaries and are evolving with the chemistry suppliers to allow higher moisture to the Yankee. This movement of the drying balance is a major step in reducing the energy demand for drying. From the time of the
development of our first TAD machine to the current offering, we have reduced the drying energy demand by 15 to 20 per cent by the simple use of a second TAD drum, albeit smaller. This allows a much more controlled and targeted use of the hot air in the drying process. We can use much hotter air for the first air system, which also has a lower permeability of the tissue sheet, where the pressure drop is considerably higher. Then in the second system, where the sheet is drier and more open, we can use lower temperatures and higher volumes. All of this protects the other system components, such as the TAD fabric, whilst allowing smaller overall system components and commonality between the system sizes. Taking this line a little further, our drive is now to further push the balance of the drying towards the Yankee, with a target around 65 per cent max solids to the cylinder. This can only be achieved in conjunction with our customers and our partners in the chemical side of our industry. Financially, however, if such a movement can be realised, then the energy efficiency of
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