b) Medium Speed Pulverizers. In this category, there are two
major types; the ball-and-race type and the roller-type. Both types use the
same principle of pulverizing by a combination of crushing, impact, and
attrition between grinding surfaces and material. The ball-and-race type
consists of a row of balls with one race below them and one race above them.
As the lower race is rotated, coal is crushed between the balls and both
races. The roller type consists of a series of rollers running close to an
outer grinding ring. This type of pulverizer can be of two designs; the ring
can be stationary with moving rollers, or the ring can rotate around
stationary rollers. This type of pulverizer has a somewhat higher initial
cost than the high speed type, but advantages include less maintenance,
reduced outage time, and maintained product quality over the entire operating
c) Slow Speed Pulverizers. This type of pulverizer is usually
referred to as the ball or ball tube mill. These mills are essentially slow
turning horizontal cylinders filled with various sized steel balls. As the
tube rotates, the coal and steel balls are mixed together. Impact of the
falling balls and attrition pulverize the coal which is carried out by
preheated air. High initial cost, high power input, and large floor-space
requirements are some of the disadvantages of this type of mill. Advantages
Oil and Natural Gas Firing. Gas and oil burners are furnished for
various services in a multiplicity of types and arrangements. Combination
burners may also be obtained which will burn either gas or oil alone or in a
specialized class to burn gas and oil simultaneously. The functions of a
burner are to deliver fuel and air to the combustion space, mix the fuel and
air and provide for continuous ignition of the mixture. Burners should be
considered as a part of the boiler and should be subjected to the tests,
performance guarantees and warranty requirements of the boilers.
Gas Burners. Gas burners are generally classified as premixing and
nozzle mixing. In the premixing type, which includes inspirator, aspirator,
and fan mix types, the primary air and gas are mixed at the source point
upstream of the burner ports. In the nozzle mixing type, gas and air do not
mix until they leave the burner ports. Burners of the nozzle mixing type are
recommended. Types of burners are listed below.
a) Inspirator. The inspirator type uses a jet of gas which has
sufficient energy to induce the proper quantity of primary air in proportion
to the fuel.
b) Aspirator. The aspirator type uses air as the fluid which has
the necessary energy to induce the gas.
c) Fan Mixing.
The fan mixing type admits air and fuel to the
blower inlet for mixing.