This document provides a summary of a presentation on ball and tube mills. It discusses the types of coal and reasons for pulverizing coal. It then describes the construction and operating principles of ball and tube mills, including their slow speed of rotation, steel ball grinding mechanism, and use of impact and attrition to pulverize coal. Maintenance practices for the mills are also summarized such as ball charging schedules and preventative maintenance procedures.

1. FTA
PRSENTATION
ON
BALL AND TUBE
MILL
RHITESH KUMAR

2. Presentation Outline
1. COAL – TYPES OF COAL
2. WHY PULVERISATION OF COAL?
3. FIRING – DIRECT FIRING, INDIRECT FIRING
4. PULVERISERS – TYPES, CLASSIFICATION
5. BALL & TUBE MILLS – CONSTRUCTIONAL
FEATURES
6. GRINDING PRINCIPLE IN TUBE MILLS

3. COAL
 MOST EASILY AVAILABLE
 ABUNDANTLY AVAILABLE
 CHEAP FUEL SOURCE
 EASILY TRANSPORTABLE

4. GRADE OF COAL
BASED ON CALORIFIC VALUE, GRADING OF COALGRADE
KCAL/KG
A
>6200
B
5600-6200
C
4490-5600
D
4200-4490
E
3360-4200
F
2400-3360
G
1300-2400

5. WHY PULVERISED FORM OF COAL
 IT HAS MADE MODERN STEAM GENERATING UNITS
HIGHLY THERMAL EFFICIENT, RELIABLE AND SAFE.
 IT HAS MADE EFFICIENT USE OF LOW GRADE COALS.
 IT PROVIDES GOOD CONTROL OVER COAL AIR
MIXTURE PROPORTION, FLAME STABILITY WITH
MINIMUM LEVEL OF UNBURNT CARBON.

6. FIRING
DIRECT FIRING
Coal is ground and is directly carried from the
Pulverizer to the Furnace by use of Pressurized air.
Pressurized Air from Primary Air Fans goes to the
Pulverizers.
Sufficient temperature is maintained for drying of coal
and moving the Pulverised Coal to the furnace without
settling.

7. FIRING
INDIRECT FIRING
Here coal is ground and is carried from Pulveriser to
an Intermediate Bunker. Based on the Unit load
demand pulverised coal moves from the Intermediate
Bunker to the Furnace.
A suction fan helps in sucking the pulverized fuel to
the furnace.
This system is suitable, where the boiler load varies
and the mills consume huge power. But, the
disadvantage is that the capital cost is high due to extra
bunkers and fans. Also, there is danger of Fire in the
Intermediate Bunkers

8. FUNCTIONAL REQUIREMENTS OF
PULVERISER
 DRYING OF COAL
 GRINDING OF COAL
 TRANSPORTATION OF PULVERIZED COAL

9. MECHANISM OF PULVERISATION
 BASED ON PRINCIPLES OF PARTICLE SIZE
REDUCTION
- IMPACT
- ATTRITION
- CRUSHING
PULVERISERS USE ONE, TWO OR ALL THE
THREE PRINCIPLES.

10. TYPES OF PULVERISERS
Speed
Type
Low
10 to 20 RPM
Ball tube mills
(16 rpm in
Vindhyachal)
Medium
High
40 to 70 RPM 900 to 1000
RPM
Bowl Mill,
Impact or
Hammer
MPS Mill,
Mill.
Ball and Race
Beater Mill
mill
or Fan Mill.

11. TYPES OF MILL
•
Tube or ball mills: These are low-speed machines that
grind the coal with steel balls in a rotating horizontal
cylinder.
• Vertical spindle mill: These are medium-speed
machines that include bowl mills, ring roll mills, and ring
and ball mills. The bowl mills are further divided into deep
bowl or shallow bowl mills.
• Impact mill: These are high-speed impact machines
that use beater wheels to crush the coal.

12. Ball & Tube Mills
Tube mills: The tube mills are low-speed machines that grind the coal
with steel balls in a rotating horizontal cylinder. Due to its shape only,
people call it as Tube Mill and due to use of Grinding Balls for
crushing, it is called Ball Mill. Hence, is the name Ball Tube Mill

13. TUBE MILL
These are slow speed Mills. The Mills are
designated as BBD-4772
Where,
B – Broyer (Name of inventor).
B – Boulet (French word for Balls).
D – Direct firing.
47 – Diameter of shell (in Decimeters) i.e. 4.7m dia.
72 – Length of shell (in Decimeters) i.e. 7.2 m length.

14. BALL & TUBE MILL
Tube mills are employed to pulverize the precrushed raw coal to the required fineness before it
is admitted into the boiler furnace for combustion.
The operating characteristics i.e. Faster
response to Load variation of tube mills make it
the best mill available for the direct firing system
of the coal.

15. CONSTRUCTION OF TUBE MILL
CYLINDRICAL SHELL LINER
SHORT CONICAL LINER
LONG CONICAL LINER
INLET LINER
SCREW CONVEYOR RIBBON
HOT AIR TUBE
CONNECTING ROD

16. •The Shells which contain ball and liner are of 7.2 mtr. length.
•The Shells are of three pieces. The Intermediate shell connects to the end shells
by flange joints.
60 Nos. Bolts in each flange
CHEEK
TRUNION
40 Nos. Trunion to Cheek
Joint Bolts in each end
SHELL FLANGE
GIRTH GEAR

17. DETAILS FOR SCREW CONVEYOR ASSEMBLY
RIBBON
SHAFT
ANGLE
CHAIN
&
CHAIN LINK

18. MILL DETAILS
•
•
•
•
•
•
•
•
Design coal capacity- 83.15 Tons/Hr
Ball charged- 90 Tons (50 mm, 40 mm, 30 mm)
Speed – 15 to 16 rpm
Screw assembly- Right hand helix(DE side)
Left hand helix (NDE side)
Direction of rotation of Shell from NDE end-AntiClockwise.
Shell door opening- two numbers.
Shell liners- 600 nos.
weight of liners- 60.26 MT

19. MILL LUBRICATION SYSTEM
TRUNION
SUPPORT BEARING
H
P
OIL
TANK
BS
LP
FILTER
COOLER
Pum
p

20. MILL LUBRICATION DETAILS
•
•
•
•
•
•
•
•
•
•
LP PUMP FLOW – 137 Ltrs/min.
LP PUMP PR.- 10 Bar.
LP PUMP SPEED – 750 rpm.
B&S PUMP FLOW – 6 Ltrs/min.
B&S PUMP PR.- 70 Bar.
B&S PUMP SPEED – 1450 rpm.
HP PUMP FLOW – 40 Ltrs/min.
H PUMP PR.- 80 Bar.
HP PUMP SPEED – 965 rpm.
TANK CAPACITY- 1800 Ltrs.

21. MAIN REDUCER
Output
Shaft

22. DETAILS OF MAIN REDUCER
LOW SPEED
SHAFT 118.98
rpm
LUB.
OIL
LINES
HIGH SPEED
SHAFT
1487 rpm

23. MAIN REDUCER DETAILS
•
•
•
•
•
•
CAPACITY- 300 Ltrs.
OIL- SERVO MESH SP 460.
HIGH SPEED SHAFT- 1487 rpm
LOW SPEED SHAFT- 118.98 rpm.
DE SIDE- RESILIENT (BIBBY) COUPLING
NDE SIDE – GEARED COUPLING.

24. REDUCER LUBRICATION
• This is closed cycle lubrication system
• This is required for the lubrication of gears and bearings
• The oil is pumped to the different points in the reducer by
motorized pumps.
• Oil is sprayed to the gears by means of nozzles in the gearbox
at the mating plane.
• The bearings get oil through nozzles inside the gearbox
• The pumps are screw pump.
• Oil coolers are used to cool the lubricant coming out of the
reducer.
• After the filter the oil goes to the reducer.

25. Schematic Diagram of Main Reducer Lube Oil System
MILL
FILTER
MAIN
REDUCER
P
COOLING
WATER
OIL
COOLER
M

26. PUMP DISCHARGE/ OIL INLET TO REDUCER
PUMP SUCTION/OIL OUTLET TO REDUCER
OIL COOLER ASSEMBLY
OIL FILTER ASSEMBLY
LUB.OIL PUMP

27. MAIN REDUCER LUB. OIL DETAILS
•
SCREW PUMP.
• SPEED- 1500 rpm
• FLOW RATE- 79 Liters/Min.
• PRESSURE- 2 Bars.
• WATER FLOW THROUGH COOLERS- 60-65 LPM
• COOLER WATER I/L & O/L TEMP.- 35 & 45 deg.
• FILTER RATING- 160 Microns.

28. GIRTH GEAR GREASING
NOZZLE
NOZZLE
GREASE
DISTRIBUTOR
GREAS PUMP
M
GREASE
BARREL
ELECTRICAL
SHUT OFF
VALVE
COMPRESSED AIR
SUPPLY
AIR FILTER

29. GIRTH GEAR GREASE PUMP
• NO.OF DISCHARGE: 06
• SPEED- 1400 rpm.
• FLOW RATE- 0-18M3 /24 Hrs.
• MAX. PRESSURE- 100 Bars.
• OIL: Servo Tak-40
• SPRAY INTERVAL- 30Sec.

30. VIEW OF SEAL AIR FAN
MOTOR
GEARED BEARING
COUPLING HOUSING
IMPELLER
HOUSING

31. TRUNION SEAL AIR FAN
• CAPACITY – 3.9 m3/sec.
• SPEED- 2880 rpm.
• HEAD DEVELOPED- 1650 mmWCL.
• MEDIUM HANDLING- Atmospheric air.
• ORIENTATION- Axial suction & vertical delivery.

32. VISCOUS FILTER
DRIVING
GEAR
COARSE MAT
FILTER
OIL SUMP
Behind the Coarse mat
filter, the fine mesh filter is
present. It is also called Oil
drop eliminator. This is
present to prevent carry
over of oil drops to the
Impeller.
Both the Coarse mat filter and
the Oil drop eliminator screens
rotate intermittently as per the
Timer settings. Normally, it is
30 sec. rotation after every 20
minutes

33. MILL CLASSIFIER ARRANGEMENT
MULTIPORT OUTLET
CLASSIFIER CONE
INTERNAL CONE
HEX. CONE ASSEMBLY
REFUSAL DUCT
P.F. DISCHARGE TO CLASSIFIER

34. BALL FEEDING DETAILS

36. PRINCIPLES OF GRINDING
Raw Coal is fed into the Mill for Grinding. Grinding
takes place by Impact and attrition.

37. MAINTENANCE PRACTICES
• Ball Charging when current goes below 95
Amps. for which current data is collected
from UCB on First day of every week.
• Monthly Preventive Maintenance of Mills, R
C Feeders and Seal Air fans
• Minor Overhaul of Mills in 4-6 months in
which P/M works, Servicing & Replacement
of CADs, Girth Gear Grease Pumps
servicing is carried out.

38. Problems faced in the Mills at Vindhyachal
•Failure of Liners
• Explosion in the Mills
• Girth Gear Greasing trouble
• Frequent Jamming of CADs
• Frequent Filter Choking
• Firing of Wet coal in the Mills
• Non-operation of Viscous filters