Energy savings are of prime concern nowadays and interesting optimization opportunities exist for tissue mills, particularily on the Yankee dryer.
Air side drying with the hood, typically using natural gas, can yield significant cost savings while reducing CO2 emissions which can be of interest from an environmental standpoint. Inversely, if the Yankee controls drying with the steam side (i.e. the Yankee dryer cylinder) then significant steam savings can be obtained.
A case example of the latter is presented at the end of this article to show how important a savings that can be obtained with a simple hood exhaust optimization trial. On the air side of the Yankee dryer the absolute humidity of the hood exhaust can be increased by optimizing its volume, that is, reducing it to the point just before it spills out of the hood. In so doing, if the exhaust volume is reduced then consequently the required make-up air volume is also reduced by a proportional amount. With less make-up air the requirement for natural gas also drops in proportion.
A qualified hood supplier can assist in conducting such an optimization, especially since hood operating conditions need to be monitored as the exhaust flow is optimized. That is, hood spill conditions have to be closely monitored during the optimization trials and a qualified technician should not only be able to quantify the spill using process measurements but also have a good feel for what limits need to be observed. Before the trial, the spill conditions need to be evaluated including the feedback of the machine operators which usually can provide valuable insight with regards to optimal operating conditions. The gas and steam consumption of the Yankee need to be noted at the pre-trial conditions. Likewise, absolute humidity of the hood exhaust also needs to be measured in order calculate the savings once the trial is complete. The latter measurement can be very difficult and unreliable without the proper measurement tools. Certain patented humidity samplers and humidity sampling methods exist which permit the accurate determination of wet bulb temperature.
Such readings are difficult simply because the temperatures at which Yankee hoods operate are so high, compared to regular paper machines, that the latent energy plateau of the temperature curve is often passed in a fraction of a second and definitely not observable to the naked eye! Commercial automated humidity sensors can be very expensive since reliability is very difficult under harsh operating environments such as those of the Yankee hood. Continuous maintenance becomes crucial to protect the investment, sometimes requiring the need to send the unit back to the manufacturer for recalibration on a regular basis; at which point some mills simply abandon their use. The good news is that very reliable “manual” measurement methods exist (as discussed above) and a qualified technician can provide very accurate readings. In terms of spill measurement, a determination of volume and temperature is recommended at predetermined points along the hood perimeter both along the tending and drive side of the machine. Once these parameters are measured we can then proceed with the trial by taking small incremental steps; for example taking each exhaust fan speed down by 10% at a time. At each step we re-evaluate the spill conditions mentioned above and continue in such a manner until spill increases in an appreciable way. This maximum condition can also be confirmed by mill personnel who will be operating under these conditions on a daily basis. Once the maximum is obtained we then go back one step to insure a certain operational comfort zone.
At this time, we measure the post trial exhaust humidity and gas consumption so as to quantify the savings. To illustrate the importance of these savings, an actual case example of such an optimization exercise was conducted by the author at a US tissue mill, below.
CASE EXAMPLE: TISSUE MILL “A” - NORTHEAST U.S.
The hood exhaust optimization at this mill was conducted over the course of two individual trials, in order to confirm the results, during the production of 11 lb tissue with 130” trim. Usually the mill operates the drive on the exhaust fan continuously at 100% yielding an exhaust humidity of 0.43 lb H2O/ lb DA.
As a result of the optimization the VFD on the exhaust fan was turned down to 87% in order to maintain a comfortable operating zone for the hood with regards to acceptable hood spill.
Exhaust humidity increased to 0.50 lb H2O/ lb DA. Gas consumption remained steady since they do not control overall drying at the hood but on the yankee cylinder by varying steam flow to maintain an outlet sheet moisture setpoint of 6.0%. No noticeable changes in process operation on the air side drying: the air/gas MRV valve settings remained unchanged before, during and after trial. The make-up air damper is usually kept closed on this machine (all make-up from infiltration).
On the other hand, a significant reduction in steam consumption was seen at the Yankee dryer cylinder from 3450 lbs of steam per ton of paper produced down to 3100 lbs/ton, or 10% reduction. Right after the trial the steam consumption went back to its pre-trial rate.
This spill was quantified before the trial as follows: the wet end hood perimeter did not show signs of spilling at the tail (ambient temp 110°F) but a very slight “spill spot” just about 2ft up (showing 220°F and 200 fpm airflow). No spill detected at the dry end (98°F and 50 fpm airflow).
At the end of the optimization there was still no spill at the W/E hood tail but a slight increase at the “spill spot” was seen going to 350°F and airflow of 450 fpm. On the other hand a slight reduction of spill was seen at the D/E (88°F, 30 fpm).
In conclusion, we can estimate the value of this steam savings as follows:
Savings = (3450 - 3100)lbs steam / ton paper x 3.5 ton/hr paper = 1225 lbs/hr steam
Using a steam cost of $9/1000 lbs of steam we obtain the following:
Savings = 1225 lbs/hr x $9/1000lbs steam x 24 hrs/day x 250 days/yr = $92,600/ yr.
Inversely, if a particular mill controls drying on the air side (i.e. Yankee Hood) then the savings are realizable on the gas consumption.
Alfredo Sarli, ing.
PremiAir Technology Inc. / Voith Paper / Air Systems Group