Energy Optimization Package for Yankee Hoods

Power and Energy
Typography

Energy optimization is priority today in all sectors of fabrication including tissue and towel. The Yankee drying system on the tissue machine represents an interesting savings opportunity since both natural gas and steam are utilized therefore significantly impacting energy costs.

On the Yankee Dryer Hood (aka the « air side ») the absolute humidity of the exhaust air can be maximized by minimizing the exhaust volume, that is, by reducing it to the point at which the pressure inside the hood brings it to a slight spill condition. Too much spill creates undesireable operating conditions in the machine room.

Energy optimization is priority today in all sectors of fabrication including tissue and towel. The Yankee drying system on the tissue machine represents an interesting savings opportunity since both natural gas and steam are utilized therefore significantly impacting energy costs.

On the Yankee Dryer Hood (aka the « air side ») the absolute humidity of the exhaust air can be maximized by minimizing the exhaust volume, that is, by reducing it to the point at which the pressure inside the hood brings it to a slight spill condition. Too much spill creates undesireable operating conditions in the machine room.

On the other hand if there is no spill one cannot be sure if the hood is at optimum operating condition or if instead there is excessive air infiltration into the hood. In the case of high infiltration the exhaust volume is unnecessarily high and results in high electrical power costs at the exhaust fan and possibly also the supply fans (if make-up consequently also high).

So if exhaust volume is lowered then make-up air requirement is consequently also lowered in proportion therefore lowering the consumption of natural gas required to heat this air.

The resutling savings can be significant if for example we consider a 100 mtpd machine operating at an absolute exhaust humidity of 0.30 kilogram of water per kilogram of dry air which is then optimized to say 0.50 kg H2O /kg DA. Based on our study of commercial machines we note the savings in the graph below as being approximately 1.45 - 1.3 = 0.15 MM$ or $150,000 per year. Please see previous articles in this section for examples of detailed calculation of such a savings.

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A recommended control system for such an optimisation is the Voith « Energy Package ». This kit allows an operator to directly affect the performance of the Yankee hood via remote manual control, optimize exhaust humidity and maintain optimum hood spill conditions.

The impact is almost instantaneous allowing the operator to directly lower energy costs at the turn of a button. This type of optimisation can be done at each new grade run if desired.

The Energy Package consists of a humidity sensor in the main exhaust duct which sends its measurement reading directly to the mill DCS. Based on this reading the operator can adjust the exhaust balancing damper or better still the variable frequency drive (« VFD ») on the exhaust fan if it is so equipped.

The advantage of the VFD is the possibility to fine tune the exhaust flow and minimize electrical cost. On the other hand the benefit of damper control is the possibility to affect both wet end and dry end exhaust streams independently.

The kit can be tailored accordingly depending on the number of sensors and damper controllers required.

Finally, the level of acceptable spill is monitored by spill sensors installed on the hood perimeter, i.e. along the gap between the Yankee hood and the Yankee cylinder as shown in the figure below:

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Although it is possible to fully automate this control loop our experience has shown that doing so produces counter-acting effects in certain process air systems. It is therefore more prudent and just as easy to use remote manual control via the mill DCS.

In any event it is just as easy to use remote manual control since the actual steps required by the operator are as simple as one turn of a knob once every few days. Even at once a month it is better than never doing it all; each application requires appropriate evaluation.

And so the remote manual control strategy recommended is depicted in the figure below:

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With an annual savings of approximately $150,000 described in the example above an Energy Package can end up having an ROI payback period of about 3 months. A more complex kit with multiple humidity sensors (one for each wet end and dry end exhaust streams) and multiple damper controllers can yield an ROI of 1 to 1.5 years in the worst of cases.

Regular scheduled maintenance is primordial for the proper upkeep of the humidity sensors and protecting the investment. For a small maintenance fee it is recommended to send the unit back to the manufacturer for calibration on an annual basis. Very few solutions can yield such significant gas savings at so low an ROI payback period!


alfredo-sarliAlfredo Sarli, Eng., Voith Paper Air Systems

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