Vertical Mill Vertical Mill Layout, Alternatives.Vertical mill calculationsSymbolsPower consumption Grinding pressureMill internals Nozzle ringDam ringHeat balances, Example

Layout Alternatives• There are two basic layouts of the VRM

system; product collection in cyclone(s) or product collection in filter.

Product collection in cyclone(s):

• Product collection in filter:

Vertical mill calculations SymbolsSymbol Description Unit

A Roller projected area, one roller [m2]

Anozzle Nozzle ring area [m2]

Do Grinding table diameter [m]

DcyI Hydraulic cylinder diameter [m]

Dm Grinding track diameter [m]

Dpiston Hydraulic piston diameter [m]

Droller Roller diameter [m]

F Grinding force [kN]

FH: Hydraulic grinding force [kN]

FR: Roller grinding force [kN]

KT Specific grinding pressure [kN/m2]

MR Roller assembly weight, one roller [kg]

M Torque factor [-1

N Mill power uptake [kW]

n Grinding table speed [rpm]

Phyd Hydraulic grinding pressure [Bar]

v Grinding track speed [m/s]

Wroller Roller width [m]

Z Number of rollers [-]

Vertical mill calculationsPower consumption

• The theoretical power consumption of a vertical roller mill is expressed by the formula:

• N = KT • A • z • v • p [kW(net)] • For the vertical mill the following

applies:    KT: Typically 500 - 700 [kN/m2]

A = Droller • Wroller [m2]

Droller =0.6-Do [m]

Wroller = 0.2 • DO [m]

Do = Mill size/10 [m]

z = 3 [-]

v = n • 1/60 • Dm • TT [m/s]

Dm =0.8-Do [m]

Vertical mill calculations Grinding pressure

• The grinding pressure, F, consists of:• F = FR + FH [kN]

where • FR=MR . 9.81 /1000 [kN]

• FH = Phyd • ((Dcyl)2 - (Dpiston)2) • P/4 • 100 [kN]• The specific grinding pressure will then be:• KT = F / A

Mill internals _ Nozzle ring• The nozzle ring area is the free

area, perpendicular to the direction of the gas flow, as measured at the shortest distance between the table and the air guide cone:

• The gas velocity in the nozzle ring will normally be in the range:

35 - 50 m/s.The higher the velocity, the less

material falls through the nozzle ring.

Mill internals _ Dam ring

• The dam ring height is the height measured from the grinding table segment to the top of the dam ring.

• For vertical mills the dam ring height is often referred to as "% of table diameter". The optimum height normally being in the range 2.5 - 4% of table diameter.

• Example:• A dam ring of 120 mm in an Atox 32.5 will give

a relative height of: 120 mm / 3250 mm = 3.7%

Heat balance _ Example

• An example of a heat balance for a vertical mill Case: 200 t/h, 8 kWh/t(net), drying from 8% to 0.5% moisture

INFlow t/h Temp °C Cp kCal/kg°C Heat Mcal/h

Feed, dry 200 20 0.218 872

Water in feed 17.4 20 1 348

Recirc mat. {#)        

Grinding heat (a)       1376

Gas 302 252 0.242 18405

False air (*) 30 20 0.244 147

Water injection       0

TOTAL IN       21149

OUT Flow t/h Temp °C Cp kCal/kg°C Heat Mcal/h

Product 200 85 0.218 3706

Water in product 1 20 1 20

Water evap 16.4 85 0.453 10389

Gas, total 332 85 0.242 6833

Surface loss       200

Recirc mat. (#)        

TOTAL OUT       21149

(#): Recirculated material is not included in the heat balance since the inlet and outlet temperature of this is virtually the same.(a): 1600 kW * 3600 s/h / 4.186 J/Cal = 1376 MCal/h(*): False air from mill inlet to mill outlet is calculated based upon C02 and 02 measurements. Standard value is 10%