Vertical Roller Mill

Confederation of Indian Industry

VERTICAL

ROLLER MILLS


Vertical Roller Mills (VRM)

An advancement over ball mill

Roller mill Grinding by Compression

Rollers / Balls on ring

Presently for Raw meal, Coal &

Cement

More Energy efficient


VRM - Features

VRM Drive - 40% of ball mill

Dry moisture up to 15 %

More compact & Less space

Fan Power consumption - High

Separator - Integral Part of Mill


Features of VRM

Energy Efficient Grinding System

Mill drive 40% less consumption

Most suited for drying upto 15%

moisture

More compact & occupies less space

More amenable for Controls


Comparison of Ball Mill & VRM

Ball Mill VRM

Moisture (%) 3.0 7.0 3.0 7.0

Mill Drive (units/ton) 14.0 14.0 6.5 6.5

Fan + separator 1.9 5.0 7.5 7.5

Total 15.9 19.0 14.0 14.0

Ideal choice for high moisture materialIdeal choice for high moisture material


ENERGY SAVING IN VRMs

Fine tuning of louvres

Mechanical Recirculation system

High efficiency classifier

High Level Control system


Install Variable Area Louvre Ring

Air flow inside VRM not uniform

Material leaves in a definite pattern

around table

Max. material behind and in front of each

roller



Uniform area louvre ring

Uniform gas system

Uneven bed of material

Gas prefers path of least resistance

Higher gas flow rates essential



Variable area louvre ring

Larger area at points of larger

concentration

Gas flow directed to material

Potential open area higher

Lower Pr.Drop and lower gas flow

Reduced fan power consumption



Presently fixed area louvre ring

Pr.Drop across mill 870MM WG

Flow thro mill 3,40,000 M3 / HR

Mill output 155 TPH

Recommended to install variable area

louvre



Pr. Drop across mill 820 MM WG

Flow thro mill 3,10,000 M3/HR

Mill output 155 TPH

Annual savings 15000 USD


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C4 Roller 2 Roller

3 R

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Mill Door

Area 1.132 m2

Velocity 83 MPS

Rejects

RejectsGas

Rejects

RejectsGas


Fan Power Consumption in VRM

High in comparison to Ball Mill

Major Reason

Higher pressure drop

400 900 mm WG

Higher air flow rates


Pressure drop in a VRM

Ranges from 400 - 900 mm Wg

Pressure drop

75% - Mill louvre

25% - Housing & classifier


Why High Pressure Drop ?

High louvre velocity : 55 85 m/sec

Why high louvre velocity ?

Lift particles of higher sizes

Grinding table to separator

Reduction of louvre velocity

Reduction of mill - P

Higher quantity of rejects at mill bottom


Mechanical Recirculation System

Rejects handled mechanically

Recirculated back to mill through BE

Tremendous reduction in pressure

drop & energy consumption


Installation of Mechanical Recirculation


Louvre velocity vs. Mill power

Louvre velocity (m / sec) 75 50 35

Mill P (mm Wg) 900 630 450

Reject rate (% of fled) 0 5 15% 150%

Fan Power (units / ton) 6.9 5.5 4.7

Mill power (units / ton) 6.5 6.6 7.2

Total power 13.4 12.1 12.1*

* Includes power for mechanical recirculation


Louvre velocity How much reduction possible ?

Air flow through mill depends on

Louvre velocity

Drying requirement

Separator requirement

Reduction of louvre velocity to lower levels

Large material handling equipment

Large mill housing

Leads to pre-separation

Less stability & Table vibration

Optimum louvre velocities with mechanical

recirculation : 45 55 m/sec


Reduce Recirculation Air in VRM

Recirculation air 60 kM3/Hr

High recirculation air

High velocity At louvres

High P

High power consumption



Trial taken

Recirculation reduced by 20 kM3/HR

RPM of class red.

Same residue maintenance



R/M Fan power consumption

reduced by 150 kW

Savings- 48000 USD


Separators for VRMs

I stage - Static separator

II stage - Dynamic

conventional

separator

III stage - High efficiency

separator


High Efficiency Separators

Air requirement lower

High eff. - 0.6 0.8 kg matl / kg of air

Conv. - 0.4 0.5 kg matl. / kg of air

Better separation efficiency

Lower P across mill

Higher output & lower specific energy

consumption

High efficiencySeparator

OldSeparator


Features of high efficiency Separator

Minimum pre-separation in mill

housing

Maximum separation in the

separator

Coarse return drops exactly in the

centre of the table


Benefits

Increase in output 10 to 30%

Reduction in specific energy

1.5 to 3.5 units/ton

Very attractive financially

Features of high efficiency Separator


Replacement with High Efficiency Separator (Raw Mill)

Before After

Capacity 250 TPH 290 TPH

Residue (90 ) 11.5% 13.2%

Mill power (units / ton) 4.7 4.1

Fan power (units / ton) 8.5 6.2

Total Power 13.3 10.3


Effect of Roller to Table Dia


For the LM 38.4 the ratio

between diameter for roller and

table is 0.45, for the Ube LM 45.4

the ratio is 0.5, whereas for the

LM 56.4 the ratio is 0.40.



LM 38.4 LM 45.4(Ube) LM 56.4

Diameter of Grinding Table 3.80 m 4.50 m 5.60 m

Diameter of Rollers 1.70 m 2.24 m 2.24 m

Width of Rollers 0.67 m 0.85 m 0.85 m

Kwh/ts, Mill 6.31 5.99 7.00

Kwh/ts, Fan 6.23 5.51 7.00

Residue (90/212 microns) 12/0.8 11/0.8 11/0.9

Dust Load (g/cum) 610 630 595

Delta P, Mill 83 56 87

Vibration of Mill 3 - 5 2 - 4 5 - 8



Effect of Operating Pressure

Ube LM 40.4

Grinding Pressure

kN/m2

650 662 675 687 700

Capacity 290 295 300 305 310

Kwh/ts, Mill 9.0 9.0 9.0 9.0 9.0

Vibration 4-5 4-5 4-5 4-5 4- 4.5

Delta P of Mill 54 53.5 53 52.5 52


Effect of Operating Pressure

10 % increase in Grinding pressure

Capacity increase by 7%

Mill drive SEC no change

Fan power 0.5 kWh / MT reduction


Influence of Feed Size for an LM 40.4 Raw Mill

Max 90 mm Max 30 mm

Capacity 240 262

Kwh/ts, Mill 8.05 7.41

Delta P, Mill 76 73

Vibration 5 - 7 4 - 6


High Level Control System for Mills

Conventional Open loop controls

I Generation

P vs Feed controls

Latest control system

Min. 3 to 5% improvement

Production & Energy

More stable operation of VRM


High Level Control System for Mills

Latest control system

Feed Based on P, Load & Recirculation (B.E)

Mill flow kept constant

Mill Load, P, Recirculation load Fineness predicted

Separator speed varied

Prediction mechanism varied depending

on actual fineness


BEST RAW MILL VRM SEC

Sl

no

Parameter Plant 1 Plant 2

1 Output, TPH 403 320

2 Mill DP, mm WC 750 740

3 Sep Loading,

g/m3536 495

4 Mill drive SEC,

kW /MT Material

9.25 6.00

5 Mill fan SEC,

kW /MT Material

5.69 6.40


rotor

Vortex Rectifier

grit cone

Vortex rectifier for classifier

low dp leading to a reduced specific energy

consumption


Vortex kinetic

energy recuperation

Restores a linear

flow in the ductwork

Vortex rectifier for classifier


conventional design

high velocity

differences

LDC (new design)

homogeneous velocity

distribution

Install Vortex rectifier for classifier


To Sum-Up

VRM Developing into an excellent

choice

Raw mill, Coal, Cement & Slag

Further improvement opportunities

Mechanical recirculation

High efficiency separators

High level control system

Documents

Vertical Roller Mill