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Fig.6-2 TRIPLE PASS ROTARY DRUM DRYER (Courtesy of Baker Rullman Co.)
ByDRUM, DRUM, 6)SEALS, 7JAIR-S0UDS SEPARATOR, 8JDRIVE BASE, 9)FUGHTS,
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Numerous other single and triple pass dryers are used in Canada, Europe and Japan for biosolids drying. The rotary drum dryer is the most widely used design with the lowest capital cost.

Disadvantages of direct rotary dryers, especially ones without drying air recirculation (so called open-cycle system), include: heat losses in the dryer's exhaust; use of expensive systems to clean large amounts of air from particulate and gaseous pollutants including odors often with application of costly afterburners; potential for dust fire and explosion.