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Hard-working features and components of vertical roller mill

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Hard-working features and components of vertical roller mill

April 7, 2023 nflg 0 Comments

Easy to accommodate force variations

Since the grinding track is horizontal and the centre of gravity of the roller system only moves vertically, all impact forces and dynamic reactions are mainly vertical and thus easy to accommodate. The extremely low mass of the roller suspension system means low total inertia and thereby low dynamic forces.

In the event of excessive vibrations of the roller system, the lower inertia also limits the dynamic loads on the grinding table and thrust bearing in the gear box as well as on the surroundings.

The variation in hydraulic force is greatly limited by means of the nitrogen-filled accumulators connected to the cylinders, which act as springs. Even though the variation in the grinding bed takes place during operation, the hydraulic force varies within only a few bars.

The horizontal force into the mill housing normally accounts for 10-15% of the hydraulic force. The required raw meal quality and capacity are obtained by easily adjusting the grinding pressure, separator speed, gas flow and dam ring height.

Great wear resistance

Segmentation of wear parts means low risk of cracking. Therefore, wear-resistant but relatively brittle segments in HiCr for both the rollers and table can be used. Furthermore, the HiCr segments are suitable for repetitive hardfacing, resulting in double life of the hardfaced layer. In case of very abrasive materials, HiCr segments with embedded ceramic inserts ensure considerably increased life.

The segments must be replaced when worn out. Roller segments can be reversed, enabling high material utilisation, also in case of uneven wear.

The ATOX mill’s rollers are larger in diameter than most other vertical roller mills, compared to the diameter of the table. This larger roller diameter means that operation with a proportionally thicker grinding bed is less critical as regards uneven wear and foreign bodies in the mill feed. The large diameter rollers further allow a proportionally coarser mill feed, which reduces the requirements of pre-crushing.

No auxiliary mill drive required

The rollers are lifted hydraulically before start, so no auxiliary mill drive is required. Starting torque requirements to the mill are modest, since it starts unloaded.

Low gas speed saves fan power

The roller suspension system permits the hot gas inlet section to be designed virtually, without restrictions to the gas flow below the grinding table. The nozzle ring area is adjusted by inserting or removing shims on top of the vanes. Thereby the gas speed can be reduced to a minimum allowed by the amount of external circulation. The benefit of the low gas speed in the nozzle ring is a significant reduction in pressure loss across the nozzle ring and thus a significant saving in power for the mill fan.

Maintain mill size for easy materials

The beneficial design of the hot gas inlet with low pressure loss enables the mill to be fitted with an oversize nozzle ring. That makes the standard ATOX mill operate with the extra amount of gas required by specially wet or easyto- grind materials.

It is not necessary to involve a larger mill size just to accommodate the gas flow – only the nozzle ring and the separator must be adapted to the higher gas flow. Due to the relative large diameter grinding rollers the ATOX can handle up to 200 mm feed lumps. This feature reduces energy and wear in the crusher.

Optimum mill performance depends on optimum separator performance. Therefore the ATOX raw mill is equipped with the most updated version of the dynamic separator, which features:

– Cutsize and fineness adjusted by change in rotor speed

– Low bypass to reject ensures high separation efficiency

– Sharp separation curve ensures steep particle size distribution

– Effective top seal ensures raw meal without oversized particles

This highly efficient separator results in the following benefits:

– Lowest specific power consumption for the mill motor

– Highest mill capacity

– Low sieve residue for 200 micron and good burnability of raw meal

– High grinding bed thickness and low vibrations

– Lowest pressure loss across the nozzle ring due to low reject bypass The required fineness of the raw meal is obtained by adjusting the rotor speed.

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