Specific mill volume and power demand
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Fig. 11.1. shows the ratio of the specific tube mill volume expressed in cubic feet to the mill drive horse power. The specific mill volume, i. e. the cubic foot per unit of mill power demand decreases with increasing mill drive power. A 6000 HP mill e. g. shows a specific mill volume of approximately 1.26 cubic foot per HP, whereas the corresponding figure for a 1000 HP mill is about 2 cubic feet per HP.
Mill size and specific mill capacity.
Large tube mills work more economically; this was proved by two years production records of two differ ent size cement finish mills, installed side by side and grinding the same feed to about the same product size and specifications. One 4400 HP mill (3.95 x 12.4 m) showed a 12 % higher capacity per unit of power than a 1500 HP mill (2.9 x 10.9 m) . The increase in mill capacity is compared and shown graphically in Fig. 11.2.
L /D -ratio of tube mills and the mill power demand
Mill diameter and the required mill drive HP
Fig. 11.3. shows the ratio between tube mill diameter (measured on mill lining) and the required mill drive HP per foot of mill length. The diagram is based on dry grinding with a mill load of 40 % and 75 % of critical mill speed.
For other conditions, corresponding multipliers are applied. For 35 % mill load the mill drive HP is multi plied by 0.954, and for 32% mill load by 0.916. Also the kind of mill drive is taken into consideration.
If the mill is operated with 70 % of the critical speed,
Investment cost of tube mills
Fig. 11.4. shows a graph of investment costs for tube mills per mill drive horsepower. The specific investment cost per horsepower decreases with increasing mill drive power i. e. with the size of the mill.
If the unit price per 1 HP of a 1000 HP tube mill equals 100 %, than the price per 1 HP of a 7000 HP mill equals only about 65 % of that of the 1000 HP mill. Not included are: the electrical equipment, the grinding media and the erection cost.
Technical data of grinding plants
Survey of characteristic grinding plants erected in America (closed circuit grinding); see table 11.4.1.
Tube mill dimensions