What is the difference between direct and indirect dryers?

Direct dryers utilize heated air or inert gas that is continuously applied directly to the product. Indirect dryers, on the other hand, are typically run as a batch operation (though they can be continuous) and involve hot fluid circulating through a jacket surrounding a sealed vessel that contains the product. The indirect drying method is usually preferred for small or medium batches whose products risk contamination or degradation by exposure to the atmosphere or feature toxic and/or hazardous chemicals that should not be released into the environment.