Coal is the one of the very dominating energy source due to its cost, availability and transportability. Pulverized kind of coal has created modern steam generating units highly thermal efficient, reliable, safe capable to efficiently use of low-grade coals. It gives coal-air fuel control, flame stability, effective usage of carbon and so decrease in cost and suppleness of operation. The efficiency of combustion is determined by efficiency of pulverizing.
The appliance utilized for particle size reduction is called vertical mill. In 1919 at combustion Engineering U.S.A., Raymond Mill was designed. The earlier mills were built with maximum capacity of 6 tons an hour. By 1930 the most ability to this mill design ended up being increased to 15 and then to 25 tons hourly.
Grinding mills use one, two or all 3 in the basic principles of particle size reduction namely impact, attrition and crushing. When it comes to speed these appliances might be considered low speed, medium speed and speed. The characteristics of popular pulverizes are shown in the table below
This is a medium speed vertical mill with conical grinding rolls 3no.s per mill. The race is going to 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 produced for pulverizing coal for cement and mining industry. Subsequently, they found extensive utilize in the energy Plants for Pulverizing coal. BHEL entered directly into technical collaboration with Combustion Engineering, USA for the output of these mills called XRP mills in India.
Subsequently technology for HP mills with withdraw-able had also been 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 continues to be with BHEL-Hyderabad.
Bowl Mill is driven by AC motor, in addition to flexible coupling towards the worm shaft. The worm shaft drives worm gear, installed on the key Vertical Shaft. A bowl mounted on the top of the shaft rotates in a speed of 40 to 65 rpm. For the mill the rate is constant .Integral coolers are provided within the Coal Vertical Mill base with circulating water for removing heat in the oil. External lube oil systems are also used for cooling the oil in the gear box, which includes a motor driven pump, cooler and filter. In order to avoid coal entering within the Gear case, an aura sealing technique is provided.
Hot primary air for drying and carrying pulverized coal, enters through insulated Mill side and Liner Assembly. Tramp Iron, pyrites and stones arriving the mill with raw coal move out of your 98devhpky and being heavy, fall into the mill side and linear assembly. These are typically scraped and dumped from the pyrite hopper assembly throughout the Tramp Iron and Spout valve.
The journal assembly composed of Grinding Rolls is placed on Separator Body. Grinding force is applied by pressure spring adjustment. The roller assembly could be moved up and down depending on the roll condition and coal bed. Separator Body consisting of linear assemblies deflects the environment stream to the center of bowl.
Mill comes with the stationery classification with adjustable vanes for adjustment of coal fineness. The coarser particles inside the inside the pulverised coal are separated inside the classifier and returned back to the bowl for more grinding.
Mill Discharge Valves mounted on the Separator top comprise of Flap type closure plates operated by pneumatic Cylinders, which is often tilted out of the path of coal stream. This is used to isolate the Calcium Carbonate mill from Boiler during maintenance.