Dryers

Table of Contents

What are Dryers?

Dryers are pieces of equipment which are used to vaporize or remove liquids from solid-liquid mixtures [1]. Dryers achieve this by utilizing a combination of complex heat and mass transfer. Liquids are vaporized from the mixture via the addition of heat. 

Types of Dryers

Drying can be achieved in several different ways, such as via direct, indirect or contact, radiant, or dielectric or microwave drying.

Direct

Direct or convection drying is a drying method, involving the transfer of heat through fluids directly onto the wet material. This type of drying is usually reserved for materials which are less likely to produce large amounts of dust, as the constant gas stream could take some of the product with it [2].

Convection dryers have two input feeds, one of which contains a wet solid and another containing a hot gas stream. In the dryer the two feeds are combined resulting in some or all the liquid entering to evaporate. The vapor and gas then exit the system as one output feed, taking most of the fluid in the wet solid, and the solid and any residual liquid exit the system as another output feed. The gas output is then occasionally run through an extra separation step involving a cyclone separator to remove any solid material which might have been taken during the drying process. This step is not necessary, however it does help to reduce lost material. The extracted solid material is then re-added into the product feed.

Indirect

Indirect or conduction drying is a drying method which utilizes the transfer of heat from fluids onto the containers walls, then onto the wet material. This means that the heated fluids and wet material never directly enter contact. This comes with the advantage of creating nearly no dust during heating, however it is not a feasible method of drying for large amounts of substances, or for materials which tend to stick onto the heated walls [2].  

Conduction drying operates in a very similar manner as Convection drying. The input feed is usually run through a cylindrical pipe, and the heated fluid is run along the outside of the pipe, without directly touching the input feed. The products then exit the cylinder, and most of the wet vapor leaves the system. This style of drying is usually recommended for materials with small particle sizes which could cause large amounts of dust to be produced, as it requires little to no airflow within the cylinder[3]. 

Radiant

Radiant, radiation, or infrared drying is a method of drying a substance involving the use of infrared and far infrared electromagnetic waves [4]. The infrared rays heat the material directly rather than the walls or the fluid like in conduction and convection dryers. This process is very power efficient compared to the other drying types, and thus is widely used for its cost-efficiency. The heat can also be very precisely positioned on the wet material [5]. 

Infrared drying operates very similarly to Conduction drying, however it utilizes even less air flow, and thus less air is heated, meaning it requires less energy. The input feed is run under the infrared light emitter, which causes the liquid to evaporate from the feed, and causes it to exit separately from the dryer. 

Dielectric or Microwave Drying

Dielectric or radio frequency (RF) drying is a method of drying a substance involving the use of non-ionizing electromagnetic radiation to dry the wet material. The dryer applies an alternating RF field, which causes the water molecules to consistently reorient themselves. This is done at around 40.68 million times per second, which generates heat via friction. The water will eventually evaporate due to the heat, and will exit the system.

It is commonly used in the food industry, but it is also present in other various industrial drying applications. RF drying is useful in low-moisture materials where other drying methods would be ineffective at removing the last percentages of water [7]. Some materials such as metals and other components which do not contain a dielectric component such as water are not compatible with RF drying.