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PULSE DRIED PAPER

EXISTING TECHNOLOGY
Paper drying consumes 1.5% of Earth's total energy use, but steam cylinders (the most common system) average 2400 BTUs for each pound of water they evaporate (40% efficient), and hot air hoods average only 4000 BTU/# (25% efficient). 
STEAM CYLINDER HOT AIR HOOD



  AIR DRYING
The inefficiency of hot air drying results from its flow pattern.  Nozzles efficiently dry the paper directly underneath, but then traps cold wet air next to the surface (see diagram), keeping hot dry air away from the paper. The graph (from Martin [4]) shows excellent drying under the nozzle (r/D=2), but poor efficiency further out.



AIR DRYING THERMODYNAMIC BALANCE

Hot air hoods use 900F impinging air with a 10K Reynolds number, and 3% nozzle area.  Installed units are 24% efficient, and exhaust gas of 621F, so heat recycling is required.  Recirculation fans typically consume 2000 horsepower.





PULSE DRYING HISTORY
In 2001, only two major studies had measured pulse drying.  Azevedo [1] showed poor efficiency using mild flow, but Eibeck [2] showed excellent efficiency using strongly reversing flows, and discovered that toroids make pulsing flows efficient.



TOROID DISCOVERY
Eibeck modeled pulsed flows and discovered toroids (like smoke rings).  Toroids don't dilute as they travel to the drying surface.  They attach to the surface, forcing hot dry gas down while lifting cold wet air up.  Cold wet air is no longer trapped near the surface, as with steady nozzles.



PULSED NUSSELT NUMBER
In 2001, PCE received a grant [5] to study pulse drying, and invented the "Wave Number", or "Wn", a way to match steady flow, Wn=0 (Martin's model), with oscillating flow.  At Wn=1, gas oscillates from double to stopped, and flows above Wn=1 are reversing.  Eibeck discovered efficient drying at Wn=7 (impractical), but PCE discovered better efficiencies at Wn=3.5, that produce 61% efficient drying hoods (or better).


PULSED VARIABLE SPACE
PCE discovered that efficiencies are predictable at Wn<2, but when Wn>2.5, drying is unpredictable, containing local peaks and valleys. These resonances are unexplored, but important to commercial use of pulse drying, so more study is needed.




PULSED AIR DRYING PROCESS
At Wn=3.5, a pulsed drying hood removes 145#/Hr from wet paper, or 70X the drying rate of a steam cylinder.  This productivity explosion is worth more to the paper industry than the energy savings.  Exhaust air is now 319F, so heat recovery is simple. 



PULSE DRYER RETROFIT

When one steam cylinder is retrofit with pulse drying, it evaporates the
moisture of twenty normal steam cylinders, effectively doubling the entire paper machine's drying capacity. Capital investment is $10M (50% margin).  Commercial value is about $100M, so payback is 10 months.  Hot air retrofits (right) have similar payback.  





PULSE BURNER
Pulse burners power the dryer.  They burn fuel in an enclosed chamber, similar to a car engine.  Its chamber and tail pipe form a resonating pair which controls the burner's firing frequency. The air valve (the dryer's only moving part) reacts explosion forces internally, protecting the motor bearings from combustion forces.  Pulse burners are now used in the powder drying market and are highly reliable.
  PCE has engineered eight such burners.



Proctor and Gamble's pulse drying trials [3] failed because they didn't know about PCE's Wn  discoveries.  Before further trials, pulse burners (Wn=0.5) must be redesigned to produce Wn=3.5 (low risk).  

Pulse dryer introduction into the paper industry is a $10M investment that starts paying back in year three, and exits in year six for $300M (est).  The global market is $20B. 

 REFERENCES
[1] Azevedo, et. al, "Pulsed Air Jet...", Exper. Therm. Fluid Science, 8:206-213 (1994)
[2] Eibeck, et. al., "Pulse Combustion...", Combustion Sci. & Tech., 94:147-165, (1993)
[3] Patterson, et. al., "High Perf...Jet Heat Transfer...", 2003 TAPPI Confer., Session 62
[4] Martin, H., "Heat and Mass Transfer...", Advan. in Heat Transfer, P1:60, V13, (1977)
[5] States, R., "Pulse Drying Experiment and Burner Construction", Department of Energy, DE-FG36-03GO92008





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