The drum dryer is an internal-conduction, rotary, conduction-type drying equipment. Wet material receives heat conducted through the drum wall, removing moisture and achieving drying. Liquid material completes the entire process of film application, dehydration, and scraping, yielding the dried product within a single drum rotation. Drum dryers are widely used in the food industry to dry liquid or paste-like materials.
1 Characteristics of Drum Drying
(a) Large operational flexibility and wide adaptability. The main factors affecting drum drying operations—heat medium temperature, material properties, film thickness, and drum speed—can be adjusted independently and easily to suit different materials and production capacities.
(b) High thermal efficiency, approximately 80%. Because heat transfer occurs in a stable direction, heat loss is limited to end-cover heat dissipation and radiation.
(c) High drying rate and short drying time. Material is dried in a thin film of 0.5–1.5 mm thickness, with the entire drying cycle taking only 10–15 seconds, making it particularly suitable for drying slurry-type food materials.
(d) The high surface temperature of the drum can easily cause protein structural changes, making the product less soluble and resulting in poorer product quality. Therefore, it is unsuitable for materials that require high product quality and solubility.
(e) The drum body and scraper blades are prone to wear, leading to a short service life.
2 Drum Drying Process
Figure 13-18 shows a typical drum drying process, comprising a drum dryer, liquid feed equipment, a fine powder collector, conveying equipment, and a filling/packaging machine. The slurry feed flows from a head tank into the drum dryer’s feed trough. A film-forming device applies the material as a thin film onto the drum surface. As the drum rotates, the film absorbs heat conducted through the drum, evaporating moisture and drying. The dried material is scraped off with blades, yielding a mixture of flakes and powder. This is conveyed by a screw conveyor to the product bin. Some can be directly ground by a crusher to obtain a powdered product, then filled and packaged. The steam generated during drying and the fine powder from the scraper area are drawn into a cyclone separator via an enclosed hood for fine-powder recovery or direct discharge. The entire process operates continuously.
Drum Drying Process Diagram
1—Steam Trap 2—Belt Conveyor 3—Screw Conveyor 4—Double Drum Dryer
5—Feed Head Tank 6—Wet Air Heater 7—Cut-off Valve 8—Fine Powder Collector
9—Elevator 10—Fan 11—Finished Product Hopper 12—Filling/Packaging Machine
Drum Drying Equipment Process Diagram
3 Drum Dryer
The main working component of a drum dryer is a hollow-structured drum, through which heating steam is passed. They are classified by the number of drums into single drum, double drum, and multi-drum dryers; by operating pressure into atmospheric and vacuum types (vacuum types typically use intermittent discharge, have complex structures, and higher drying costs, suitable for recovering high-value solvent vapors or drying highly heat-sensitive materials like fruit juices and infant foods); and by the method of applying the liquid film onto the drum into immersion type, splash type, roller application type, top trough type, and spray type.
3.1 Single Drum Dryer
As shown in Figure 13-19, an atmospheric single drum dryer mainly consists of the drying drum (5), heating steam supply device, condensate discharge pipe (4), curved liquid trough (12), liquid film scraper, discharge blade assembly, discharge screw (11), and drive mechanism. The drum body (5) is made of cast iron or welded steel/stainless steel plates, with a diameter D = 0.6–1.6 m and a length-to-diameter ratio L/D = 0.8–2. The drum is supported by hollow shafts, with one end serving as the steam-heating passage and containing a condensate syphon discharge pipe. The drum operates at 4–6 rpm, adjustable via the drive mechanism. The internal heating steam pressure is 0.2–0.6 MPa. A curved liquid trough is positioned below the drum for bottom-feeding and immersion-type film application. The liquid level is monitored and controlled by two level gauges to prevent dry drum walls and liquid overflow. The film thickness is controlled by a liquid film scraper, which removes excess liquid to form a uniform film. The distance between the scraper and the drum surface can be adjusted by a handwheel to control film thickness, typically 0.5–1.5 mm. The discharge device consists of blades, supports, and adjustment mechanisms. The blade edge is aligned along the drum’s generatrix. To ensure uniform clearance and ease of adjustment, the blade, its supporting frame, and adjustment mechanism are divided into several groups along the drum axis, allowing individual adjustment. The discharge device is a horizontal screw conveyor arranged axially below the blade, where the scraped dried product falls directly and is discharged from one end. Above the drum is a sealed hood that collects and discharges the moist air generated during drying.
3.2 Double Drum Dryer
A double drum dryer uses two drums of the same diameter rotating at equal speed in opposite directions. The drum diameter is smaller than that of a single drum, typically Φ0.5–2 m, with L/D = 1.5–2. A screw conveyor can be set in the middle at the bottom for centralized discharge, or separate dischargers can be set on both sides of the two drums.
a. Immersion-Type Double Drum Dryer: There are top trough immersion and the same trough immersion types. In the top-trough immersion double-drum dryer (Figure 13-20), the liquid is held in the concave area between the two drums, with weir plates at both ends to retain the material. The gap between the two drum surfaces is adjustable, typically 0.5–1 mm, and liquid leakage is not allowed. The film thickness is controlled by the gap between the drums, with some interference between them. This dryer is suitable for dense or high-viscosity slurry materials. In a trough-immersion double-drum dryer, the gap between the drums is larger, and both drums are immersed in the same liquid trough for film application without interference, suitable for solutions, emulsions, etc.
b. Spray-Type Double Drum Dryer (Figure 13-21): Unlike immersion types, it uses a spray method for film application, resulting in higher thermal utilization of the heating surface.
Figure 13-19 Single Drum Dryer
1—Exhaust Pipe 2—Condensate Discharge Pipe 3—Heating Steam Inlet
4—Heating Steam Condensate Discharge Pipe 5—Drying Drum
6—Liquid Outlet 7—Liquid Film Thickness Adjustment Handwheel 8—Upper Level Gauge
9—Lower Level Gauge 10—Frame 11—Discharge Screw 12—Curved Liquid Trough
13—Blade Adjustment Handwheel 14—Blade 15—Blade Sealed Hood
16—Sealed Hood 17—Feed Trough Exhaust Pipe
