Several options exist for a supply of air for drying herbs. These systems can rely on natural air movement, or circulation can be assisted by convection or a fan.
Ambient Air: This is where drying relies solely on the evaporative capacity of the air under whatever conditions prevail at the time.
Ambient Air with Passive Solar Heat Gain: This system relies on ambient air but the design and materials used for the drying shed provide some solar heat gain to raise the temperature of the air inside the shed during sunny weather.
Solar-heated Air: Air is heated by flowing through a solar sollector and then flows through the drying facility.
Artificially Heated Air: Air is heated by flowing over an electric heater, heat pump, or a heat exchange burning wood or fossil fuels.
Direct Burner: The exhaust and hot air from a diesel or gas burner, mixed with additional air from outside, is directed through the herbs.
Freeze Drying: Frozen plant material is dried by a vaccuum process which volatilises its water content.
Dehumidifier: A condensation process removes the moisture from the air circulating over the herbs.