Scraper Thin Film Evaporator

Scraper thin film evaporator is a new and efficient evaporator, that forced material into thin film by rotary scraper, and concentrate the material under vacuum condition. It has characteristic of large heat transfer coefficient, high evaporation capacity, short flowing time and large operation elasticity, especially for evaporation concentration, deaerization and distillation and purification of thermosensitive materials, high viscosity materials and materials containing particles which easy to crystallization. Therefore, it has been widely used in chemical, petrochemical, pharmaceutical, pesticide, daily chemical, food, fine chemical and other industries.

Structure and Configuration

1.Motor and reducer

The motor and reducer are the driving device of rotary scraper-type thin film evaporator. The rotation speed will determined by the form of the scraper, the viscosity of material and inner diameter of evaporator; the suitable linear velocity selecting of the scraper is one of the important parameters to ensure the stable and reliable operation and satisfactory evaporation effect.

2. Separating Cylinder

The material enters tangentially into the evaporator through the inlet at the lower side of the separating cylinder, and is distributed uniformly on the inner wall of evaporating cylinder with the distributor in the separating cylinder. The secondary steam evaporated from the evaporating cylinder flows upwards to the separating cylinder, and it passes through the built-in vapor-liquor separator to remove entrained liquid drop or foam removed, and then it is escapes from the outlet at the top of evaporator. The separating cylinder is designed properly according to the resistance inside the evaporator, is one of the key factors to avoid ‘short circuit’ of material. (The ‘short circuit’ is that, the material entered the evaporator and left the evaporator from the secondary steam outlet without finishing the evaporation procedure. )

3. Material Distributor

The material distributor is installed on the rotor. It is designed reasonably to ensure material entering the evaporator in tangential direction, and is splashed uniformly and continuously on the inner wall of evaporating cylinder in thin film.

4. Vapor-liquid separator

The rotary-vane type vapor-liquid separator is mounted above the separating cylinder, to catch the entrained drops or foam from the rising secondary steam, and which falls back to the evaporating surface.  

5. Evaporating Cylinder

The evaporating cylinder is known as heating cylinder too. It is the evaporation surface to exchange heat between the thin-film material and the heating medium in jacket. The inner diameter and length of the evaporating cylinder are determined by the evaporation area and the appropriate length-diameter ratio. The inner wall of the heating cylinder is processed and polished by a special machine tool, and the connecting surface with the flange at both ends to ensure the overall integrity of the equipment. The inner wall after polishing (mirror polishing if necessary) is smooth and bright, not easy to sticking or scaling, effectively ensuring the high heat transfer coefficient.

5.1 If the heating medium is steam, jacket type heating cylinder is adopted.  

5.2 If the heating medium is conduction oil or high-pressure steam, semi-tube type of the heating cylinder is adopted.

6. Rotor

The rotor installed in the evaporator consists of a rotating shaft and rotating frame. The rotor is driven by the motor and the reducer, and drives the scraper moving in a circle. The rotating frame is made of stainless steel and precision casting, ensuring the strength, geometric size and stability.

7. Scraper  

Because of the movement of the scraper, the material is distributed into thin film on the evaporating surface continuously to achieve the effect of evaporation. According to the viscosity of the material, the following three alternative forms of scraper:

(1) Sliding scraper, Slide scraper is the most basic and common form of scraper. The scraper is mounted in the four guides of the rotor, and is thrown radially to the inner wall by the centrifugal force of the rotating rotor rotation while moving in a circle together with the rotor. The movement of the scraper makes the material keeping a turbulence state and thin film on the evaporation wall, greatly improves the heat transfer coefficient. Meanwhile, the continuous scraping effectively prevent the material from overheating, drying wall and scaling. Generally, the scraper is made of filled PTFE in case the evaporation temperature is lower than 150℃; and it is made of carbon fiber when the evaporation temperature is higher than 150. There is guide cell with a certain angle on the end surface of the scraper.

(2) Fixed scraper, the fixed scraper is made of metallic material, and it is rigidly connected to the rotor, the length of the scraper is equal with the evaporating cylinder. The gap between the rotating scraper and the inner wall of the evaporating cylinder  is 1-2mm, which requires high processing and installation accuracy. It is suitable for evaporation and concentration, dissociation or purification of material which is of high viscosity and easy to foam.

(3) Hinged scraper, hinged scraper is suitable for materials which are easy to scale on the heating surface. The scraper is usually made of metal parts and is mounted on the rotating frame by movable hinge. When the rotor rotates, under the action of the centrifugal force, the scraper is pressed on the inner wall of evaporating cylinder, is sliding on the wall at a certain angle,  to scrape the material into film state and to prevent the material from scaling.

8. Bottom Head

The W-shaped bottom head is equipped with high-temperature resistant and  self-lubricating bearing, which facilitates the material output and maintenance.

Performance characteristics

    The rotary scraper-type thin film evaporator has the following unique advantages than the conventional film evaporator:

    1.Minimal pressure loss

    In the rotating scraper film evaporator, the material flow and the secondary steam flow are independent: the material flows downwards along the inner wall of the evaporating cylinder while the secondary steam generated from the evaporating surface flowing through the center of the cylinder unrestricted, so the pressure loss (or resistance drop) is minimal.

    2. Operation under real vacuum condition 

    Because of the minimal resistance of the secondary vapor from the evaporating surface to the condenser, the evaporating surface can maintain a high vacuum degree (up to-750mmHg), which is almost equal to the vacuum degree at the outlet of the vacuum system. And the boiling point of the treated material can be reduced effectively through the improvement of the vacuum degree.

    3. High heat transfer coefficient and high evaporation intensity

The reduction of boiling point increases the temperature difference between the material and the heating medium; the state of turbulence reduces the heat resistance; the coking and scaling of material is suppressed which increases the heat transfer coefficient of the evaporating cylinder; the heat transfer coefficient of the rotary scraper thin film evaporator can be up to 8000KJ/h.㎡.℃.

    4.Strong adaptability and convenient operation

    The unique structure ensures the equipment able to process difficult materials, eg. high viscosity, containing particles, thermal sensitivity and easy to crystallization.

    Large operation elasticity, stable operation conditions, and small maintenance workload and convenient maintenance