Quality Assurance checks
Special Test Facilities
Services
Current Projects
HRD
 
 
INTRODUCTION TO BOWL MILLS

Coal is the one of the most dominating energy source because of its cost, availability and transportability. Pulverized form of coal has made modern steam generating units highly thermal efficient, reliable, safe and able to efficiently use of low-grade coals. It provides coal-air fuel control, flame stability, effective utilization of carbon and thus reduction in cost and flexibility of operation. The efficiency of combustion depends upon efficiency of pulverizing.

The machine used for particle size reduction is called pulverizes or mill. In 1919 at combustion Engineering U.S.A., Raymond Mill was developed. The early mills were built with maximum capacity of 6 tons per hour. By 1930 the maximum capability of this mill design had been increased to 15 and then to 25 tons per hour.

Grinding mills use one , two or all three of the basic principles of particle size reduction namely impact, attrition and crushing. With respect to speed these machines may be classified as low speed, medium speed and high speed. The characteristics of commonly used pulverizes are shown in the table below

Speed

Low

10 to 20 RPM

Medium

40 to 70 RPM

High

900 TO 1000 RPM

Type

Ball Tube Mill

Ring Roll Mill

Ball and Race Mill

Bowl Mill

Impact or Hammer Mill

Predominant

Principle

Attrition

Crushing

Impact

BOWL MILL

This is a medium speed vertical mill with conical grinding rolls 3no.s per mill. The race will be either deep or shallow like saucer covered with liner called bullring segment. The bowl is rotated by a gearbox have reducing gear like worm gear set or bevel helical gears.

CE Raymond Mills were first developed for pulverizing coal for cement and mining industry. Subsequently, they found extensive use in the Power Plants for Pulverizing coal. BHEL entered in to technical collaboration with Combustion Engineering, USA for the manufacture of these mills called XRP mills in India.

Subsequently technology for HP mills with withdraw-able was also acquired from CE. Upto 1978 the Bowl Mills were manufactured at BHEL-Tiruchirapalli. From 1978 to 1984 these were manufactured at both BHEL-Tiruchirapalli and BHEL-Hyderabad. From 1984 onwards total product has been with BHEL-Hyderabad.

Nomenclature

583 XRS/803 XRP Bowl Mills.
58,80 Stands for Bowl diameter in inches.
         If the number is even then its Shallow Bowl Mill.
         If the number is odd then its Deep Bowl Mill
3-Number of Rollers
X- Frequency of Power supply(50 cycles).
R- Raymond Name of the inventor
S- Suction type with Exhauster after mill
P- pressurized type with P.A. FAN before mill
The above mills have saucer type bowl (approx. 20 angle to horizontal).There are few Bowl Mills initially supplied by CE in india. They have been discontinued.

Range:
BHEL manufactures Bowl Mills in the following Ranges (corresponding to Bowl diameter in inches):
1. 60" Series : 583, 603 & 623 XRP/XRS
2 66" Series : 663 XRP
3 70" Series : 703 XRP, 703 HP
4 80" Series : 763, 783,803 XRP, 803 HP
5 90" Series : 883 XRP
6 100"Series : 1003 XRP
Principle of Operation

Bowl Mill is driven by AC motor, coupled with flexible coupling to the worm shaft. The worm shaft drives worm gear, mounted on the Main Vertical Shaft. A bowl mounted on the top of the shaft rotates at a speed of 40 to 65 rpm. For a particular mill the speed is constant .Integral coolers are provided in the mill base with circulating water for removing heat from the oil. External lube oil systems are also used for cooling the oil in the gear box, which consists of a motor driven pump, cooler and filter. To prevent coal entering in to the Gear case, an air sealing system is provided.

Hot primary air for drying and carrying pulverized coal, enters through insulated Mill side and Liner Assembly. Tramp Iron, pyrites and stones coming in the mill with raw coal move out of the bowl and being heavy, fall into the mill side and linear assembly. These are scraped and dumped in the pyrite hopper assembly through the Tramp Iron and Spout valve.

The journal assembly consisting of Grinding Rolls is mounted on Separator Body. Grinding force is applied by pressure spring adjustment. The roller assembly can be moved up and down depending upon the roll condition and coal bed. Separator Body consisting of linear assemblies deflects the air stream to the center of bowl.

Mill is equipped with the stationery classification with adjustable vanes for adjustment of coal fineness. The coarser particles in the in the pulverised coal are separated in the classifier and returned back to the bowl for further grinding.

Mill Discharge Valves mounted on the Separator top consist of Flap type closure plates operated by pneumatic Cylinders, which can be tilted out of the path of coal stream. This is used to isolate the mill from Boiler during maintenance.

 
 
QA Checks
Steam turbies
Gas turbines
Compressors
Boiler Feed Pumps
Turbo Genarators
Gear boxes
Bowl Mills
Oil Rigs