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A fairy tale about tissue drying
By Frank Sorrels

Once upon a time in never never land of Fairy Ville, the fairy owner of paper company Alfa was celebrating the return home of his son, who had been away at college for eight years and had obtained a doctorate with highest honors in Paper Science. Some months before, Dad had ordered a new high speed tissue machine and the installation was just being completed. Everyone was guessing what might come about with the new machine and the knowledge the son possessed. Dad had been, and still was, a bit puzzled when earlier his son told him "Be sure the hoods over the Yankee cylinder are connected in parallel." Many hood suppliers promote connecting them in series, claiming that is a way to save air side drying energy.

When the son arrived, he quickly became acquainted with all aspects of the machine. The air side was a dream, loaded with smart transducers, meters, gauges and a sophisticated software program; it had more instruments than any previous hot air system. The quantities of many process variables that had been previously unknown became known (humidity ratios, mass flows, etc) and they were all controllable from the DCS. Additionally, the software program performed an optimization analysis for the entire drying spectrum and, for any given energy costs for natural gas and electricity, it provided graphs showing the optimum settings for drying air temperature, humidity ratio and velocity. No wonder Dad was so happy.

But paper company Omega, a fierce competitor in the same town was also gifted with good fairies and it too was starting a new high speed tissue machine. In fact, its machine was identical to the new machine at Alfa except for two things: the air side drying hoods were connected in series, and they did not have the instruments or knowledge program. Because they did not know how the instruments and program would aid Alfa in their production, they found a bad fairy and encouraged him to become employed at Alfa and then bring them secrets. He would be rewarded with a few pieces of silver. Surely that and having the hoods in series would give Omega the 'up' and make it the lowest cost producer ever in Fairy Ville.

Now Alfa started its machine and entered set points into the DCS starting with a dry end supply air humidity of 0.17 which, for the grade being made, resulted in an exhaust air humidity of 0.246. The wet end supply air humidity was set to 0.25 with a resultant exhaust air humidity of 0.341. The energy costs for the natural gas to the burners and electricity to the supply fans was equivalent to $270/hr. Omega also started up making the same grade at the same speed, etc. Now the bad fairy had observed the positions of the dampers on the dry end hood and gave the information to Omega, which then set its dampers to the same angles. Omega's dry end supply air humidity was 0.17 giving an exhaust air humidity of 0.246, the same as Alfa's machine. Because all of the hot exhaust air from the dry end hood was sent to the wet end section as make-up air, it caused the exhaust air humidity to increase to 0.45, and because of the heat energy it reduced the burner requirement. So the drying cost was less than Alfa's; it was $266/hr. The Omega fairies were thrilled. Yes, even though the savings were slight, they had proved that hoods connected in series cost less to operate! So they thought.

When Dad came to work Monday morning he immediately called Son into his office, demanding an explanation. Son said: "Dad, I understand your concern, but be assured that things are okay. I elected to start up the air hoods at a low humidity level so that the operators could see and understand the effects of operating at a higher humidity. In fact, yesterday we entered new set points into the DCS increasing the supply air humidity on the dry end hood to 0.3 and on the wet end to 0.29 and that brought the exhaust air humidity to 0.38 for both hood sections and our energy costs is now $245/hr, which is lower than Omega. And we can do better." Dad was a happy man.

When Omega learned of this change it decreased its dry end makeup air and the wet end exhaust air humidity went way up. Now we all remember the song that goes "rain drops keep falling on my head . . ."; it has great music and cool lyrics. Well, the fairies had the wet end hood humidity so high they were singing "rain drops keep falling on my sheet." They found that they needed to control the wet end exhaust air to a maximum humidity of 0.55 to safely produce the product. For this operation, the dry end supply air humidity was 0.225 with an exhaust of 0.303. The energy costs was $245/hr.

"Wait a minute," screamed the lead fairy. "This is the same cost that Alfa has, yet they are running with much lower exhaust humidity. Can I believe these cost numbers?" So a second bad fairy found his way inside Alfa and returned to Omega saying "Alfa's costs are exactly $245/hr. We are at our limit, but Alfa can increase its humidity and reduce its costs further."

The machine with hoods connected in series had peaked out. The machine at Alfa with parallel hoods was then optimized. The supply air humidity was increased to 0.45 which raised the exhaust air humidity to 0.54. The costs dropped to $221/hr. Alfa's cost between start and finish fell from $270/hr to $221/hr. Over the year, with 24-hour operation for 365 days/yr and an operating rate of 93%, that represented $399,193. It gave Alfa a cost advantage over Omega, which operated at the same levels but with costs per hour falling from $245 to $221 amounted to $195,523.

So the fairy tale has ended.

My article in the October/November 2006 issue of Tissue World magazine discussed the laws of thermodynamics and showed that series connected hoods do not use less drying energy than parallel connected hoods. As demonstrated above, when all operational characteristics are equal, the energy costs can be different amounts or the same amount. And my article in the June/July 2007 issue of Tissue World magazine pointed out that there is a limit to the amount of humidity that can be sent to the wet end hood when connected in series. Simple logic reveals that the independently connected parallel hoods, dry end and wet end, operate at a much lower humidity ratio.

I ask all readers to indulge my humor of fairy land. I hope this method conveys my message which is "Help save our planet for future generations. Please do your share and operate your hoods at the highest humidity ratio possible." You will save natural gas and reduce air pollution; then you can take your savings to the bank. To achieve this level of operation the tissue manufacturer must measure, analyze and then control the air side variables. Instruments and meters are required; a sophisticated computer program is a must. These exist and are available, and the cost is generally a fraction of the first year's energy savings. TW