The rotary drums are the most popular system to dry cool, maturate, compost, very different stuff.
Chemical industry: dye pigments, phosphates, metal sludge, ...
Fertilizer industry: to dry cool granulate or coat the product.
Environmental: incinerate or compost waste products
Non metallic minerals industry: limestone , clay, plaster, sand, slag, coal, ...
Food production: to dry agricultural products (grain, beet pulp, ...)
The product and the air flow are crossing the drum counter current or parallel current. Each application is to be studied carefully in consideration of the most suitable temperature of the product, of the inlet air/gas and of the exhaust air.
A fabricated drum is driven on 2 rolling stations by a girth gear, a chain transmission or a friction system.
There are so many possible alternative due to heat transfer process, product dispersion in the drum itself (sectional, tube bundle, lifter devices, ...)
Product discharge system, feeding device, ...
Each case is to be specifically designed.