APV Dryer Handbook - Umbc

54
The drying chamber would be maintained at a pressure slightly higher than
ambient using a pressurized nitrogen purge (17). The heater (5) would use either
a steam coil or a thermal fluid system with an external heater.
A less expensive alternative to the closed cycle approach is a “Lo-Ox” system, where
a low excess air burner is used in a direct fired heater. The products of combustion
are recirculated through the condenser and the surplus gas is vented to atmosphere.
The oxygen level in such a system can be controlled to less than 3%.
STEAM ATMOSPHERE SPIN FLASH DRYING
A recent development of the Spin Flash drying technology is it’s operation with
an atmosphere consisting solely of superheated steam. The plant configuration
is very similar to the closed circuit system described above, but the primary
condenser is omitted, and replaced with a vent duct. The dryer system is operated
at a positive pressure throughout the loop. Before start-up, the loop is purged with
auxiliary steam, either from the factory supply, or from a packaged boiler adjacent
to the dryer. The process heater is a two-pass fired tubular super-heater, which can
heat the steam atmosphere up to 1100°F entering the inlet distributor of the dryer.
Once the dryer loop has been thoroughly warmed up, and the feed system has
been started, the moisture evaporated supplies the necessary vapor to maintain
a positive pressure and the boiler is no longer required. Modulating the vent
valve downstream from the baghouse, controls the actual system pressure. A dry
superheated condition is maintained throughout by controlling the feed rate to hold
a dryer outlet temperature of around 250°F.
Compared to other alternatives the APV Steam Spin Flash concept offers the
following significant advantages:
Lower Operating Cost
Compared with conventional dryers operating at the same temperature span,
there is a reduction of up to 30% on the actual fuel cost for the dryer itself. This
results from only heating the drying gas from the dryer outlet temperature back up
to the inlet temperature.