APV Dryer Handbook - Umbc

The pulsation rate is determined by the geometry of the combustor and has a
natural frequency of approximately 45 to 50 cycles per second in a 3.5 to 4.0
mBTU burner. A single detonation cycle causes the pressure to rise in the burner,
shutting off the fuel flow and air supply momentarily, while exiting the burner
through the exhaust system. As the pressure wave falls after the first cycle, it is
followed by a negative pressure wave, which draws in more air and fuel. The
next cycle is initiated either by an igniter, or residual heat in the combustion
chamber. In a continuously operating pulse burner, the divergent exhaust nozzle
amplifies these discreet detonations into a 250 Hz wave form in the downstream
hot exhaust gasses. This combination of pressure waves and heat is used for
drying free-flowing, high moisture and low solids liquid feed streams.
The wave form atomizes the raw feed liquid into many finely divided slurry and
water particles, apparently acting to shear the bond that the water has to the
solids. Since this is simultaneously done in a 2,300°F (1260°C) heat zone,
flash evaporation takes place. The residence in the drying section is only a few
milliseconds and the continuing evaporation keeps the product particle temperature
very low, typically 125° to 135°F (52° to 57°C). The process is continuous.
From a cold start, production drying can take place within 3 to 5 minutes. Pulse
drying has been used for organic, inorganic, process and waste streams. It can
handle a wide range of feeds, from sub-micron to 5/16 in (8 mm) particle size,
and can be fed with any positive displacement pump, diaphragm pump or screw
auger. The smallest system can evaporate one ton/hour of water and the largest
single system module evaporates four tons/hour.
Production Pulse Dryers have been in use since 1985 to dry organic fish meal,
hazardous plating wastes and radioactive depleted uranium oxides. It also
is specified to dry a food grade, cold water, swelling starch product.
Energy consumption ranges from 1,475 to 1,675 BTU/s (370 to 420 Kcal/s)
to remove one pound of water, depending on whether the feed is organic
or inorganic. The evaporation rate is improved by the pulse combustion burner,
which increases the heat and mass transfer rates so that there is less time required
for drying.
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