1、 Core principle: synergistic effect of atomization and heat exchange

1. Liquid material atomization
   Liquid materials (such as solutions, suspensions, emulsions, etc.) are dispersed into countless small droplets (usually with a diameter of 10-200 μ m) through atomizers, which exponentially increase the surface area of the material (for example, 1L of liquid can be atomized into about 10 ⁹ droplets), creating conditions for rapid drying.
2. Contact with hot air and evaporation of moisture
   The atomized droplets are thoroughly mixed and in contact with hot air (drying medium) in the drying chamber. Due to the significant temperature and humidity difference between the surface of the droplet and the hot air, moisture quickly evaporates from the droplet surface
   The surface evaporates and diffuses into the hot air, ultimately forming dry solid particles.
3. Dry product separation
   The dried particles are separated from the waste gas carrying moisture by a gas-solid separation system (such as a cyclone separator, bag filter, etc.), and the solid particles are collected. The waste gas is discharged or recycled after treatment.

2、 Principle differences of different types of spray dryers

1. Centrifugal spray dryer

• Atomization principle:
  The material is transported to the center of a high-speed rotating centrifugal atomizer (atomization disc) through a feed pump. Under the action of centrifugal force (usually 10000-30000 revolutions per minute), the material is thrown out along the edge of the atomization disc and split into small droplets, and the trajectory of the droplets is radial.

• Heat exchange characteristics:
  Hot air enters from the hot air distributor at the top of the drying chamber in a rotating or axial manner, and comes into contact with the mist droplets in reverse or parallel flow. It has high drying efficiency and is suitable for processing high viscosity materials.

2. Pressure spray dryer

• Atomization principle:
  The material is ejected at high speed through a nozzle (with a diameter of 0.5-2mm) under the action of a high-pressure pump (usually 5-20MPa). Due to the sudden drop in pressure and fluid shear force, the material splits into droplets.

• Heat exchange characteristics:
  Hot air and droplets often come into parallel contact, and the drying chamber has a compact structure, suitable for processing low viscosity and easily crystallizable materials (such as milk powder, dyes, etc.).

3. Airflow spray dryer

• Atomization principle:
  Using compressed air (or steam) to spray at high speed (300-1000m/s) from the airflow nozzle, mixing with the material inside or outside the nozzle, generating strong shear force through the velocity difference between the gas-liquid phase, and atomizing the material.

• Heat exchange characteristics:
  The atomization energy consumption is high, but the atomization effect is delicate (the droplet diameter can be as small as 5 μ m), suitable for processing high concentration, heat sensitive materials (such as biological products, pharmaceutical preparations, etc.).

3、 The influence of key process parameters on the principle

1. Atomization parameters:

   • The speed, pressure, or airflow velocity of the atomizer directly affect the size of the droplets, which in turn affects the drying time (the smaller the droplets, the faster the drying speed).

   • For example, as the speed of the centrifugal atomizer increases, the droplet diameter decreases, and the drying time can be shortened from a few seconds to milliseconds.

2. Hot air parameters:

   • The inlet temperature (usually 150-350 ℃) and flow rate determine the heat transfer, while the outlet temperature (about 80-120 ℃) needs to be controlled to avoid material overheating and deterioration.

   • The flow direction of hot air and droplets (co current, counter current, mixed flow) affects the uniformity of drying, such as co current being suitable for heat sensitive materials and counter current being suitable for high moisture content materials.

3. Material characteristics:

   • Viscosity, surface tension, solid content, and other factors affect the atomization effect. For example, high viscosity materials require higher atomization energy (such as centrifugal or pneumatic).

4、 Typical application scenarios and principle adaptation

• Food industry: When the milk powder is dried, the pressure spray dryer atomizes the milk through the high-pressure nozzle, and contacts with the hot air in parallel to quickly form instant milk powder particles.

• Pharmaceutical industry: Airflow spray dryers use low temperature airflow to atomize liquid medicine to avoid thermal damage to active ingredients of drugs. They are commonly used in the production of microcapsules.

• Chemical industry: When the centrifugal spray dryer processes the titanium dioxide powder solution, the high-speed atomization disk disperses the solution and cooperates with high-temperature hot air drying to obtain titanium dioxide powder with uniform particle size (such as the titanium dioxide drying case mentioned above).

5、 Summary: Core Principles and Advantages

• Extremely fast drying speed (completed within seconds), suitable for heat sensitive materials;

• The particle size of the product is controllable, and different forms such as powder and granules can be obtained through process adjustment;

• High degree of process automation, suitable for continuous large-scale production.