BALL AND TUBE
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
6. GRINDING PRINCIPLE IN TUBE MILLS
MOST EASILY AVAILABLE
CHEAP FUEL SOURCE
GRADE OF COAL
BASED ON CALORIFIC VALUE, GRADING OF COALGRADE
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.
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
Sufficient temperature is maintained for drying of coal
and moving the Pulverised Coal to the furnace without
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
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
FUNCTIONAL REQUIREMENTS OF
DRYING OF COAL
GRINDING OF COAL
TRANSPORTATION OF PULVERIZED COAL
MECHANISM OF PULVERISATION
BASED ON PRINCIPLES OF PARTICLE SIZE
PULVERISERS USE ONE, TWO OR ALL THE
TYPES OF PULVERISERS
10 to 20 RPM
Ball tube mills
(16 rpm in
40 to 70 RPM 900 to 1000
Ball and Race
or Fan Mill.
TYPES OF MILL
Tube or ball mills: These are low-speed machines that
grind the coal with steel balls in a rotating horizontal
• 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.
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
These are slow speed Mills. The Mills are
designated as BBD-4772
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.
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.
CONSTRUCTION OF TUBE MILL
CYLINDRICAL SHELL LINER
SHORT CONICAL LINER
LONG CONICAL LINER
SCREW CONVEYOR RIBBON
HOT AIR TUBE
•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
40 Nos. Trunion to Cheek
Joint Bolts in each end
DETAILS FOR SCREW CONVEYOR ASSEMBLY
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
MILL LUBRICATION SYSTEM
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.
DETAILS OF MAIN REDUCER
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.
• 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
• 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
• After the filter the oil goes to the reducer.
Schematic Diagram of Main Reducer Lube Oil System
PUMP DISCHARGE/ OIL INLET TO REDUCER
PUMP SUCTION/OIL OUTLET TO REDUCER
OIL COOLER ASSEMBLY
OIL FILTER ASSEMBLY
VIEW OF SEAL AIR FAN
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.
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
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
MILL CLASSIFIER ARRANGEMENT
HEX. CONE ASSEMBLY
P.F. DISCHARGE TO CLASSIFIER
PRINCIPLES OF GRINDING
Raw Coal is fed into the Mill for Grinding. Grinding
takes place by Impact and attrition.
• 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.
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