Refractories
A
material in solid form which is capable of maintaining its shape at high tempo
(furnace tempo as high as 1650oC) have been recorded.
Purpose
i. To
protect blr casing from overheating and distortion and the possible resulting
leakage of gasses into the machinery space.
ii. To reduce
heat loss and ensure acceptable cold faced temperature for operating personnel
iii. To
protect exposed parts of drum and headers which would otherwise become
overheated. Some tubes are similarly protected.
iv. Act as a
heat reservoir.
v. To be
used to form baffles for protective purposes or for directing gas flow.
Properties
i. Must have
good insulating properties.
ii. Must be
able to withstand high tempo's
iii. Must have
the mechanical strength to resist the forces set up by the adjacent refractory.
iv. Must be
able to withstand vibration.
v. Must be
able to withstand the cutting and abrasive action of the flame and dust
vi. Must be
able to expand and contract without cracking Note: no one refractory can be
used economically throughout the boiler
Types
i. Acid
materials- clay, silica, quartz , sandstone etc
ii. Neutral
materials-chromite, graphite, plumbago, alumina
iii. Alkaline
or base materials- lime, magnesia, zirconia
Forms
i. Firebricks-
these are made from natural clay containing alumina , silica and quartz. They
are shaped into bricks and fired in a kiln
ii. Monolithic
refractories- These are supplied in the unfired state, installed in the boiler
and fired in situ when the boiler is commissioned.
iii. Mouldable
refractory- This is used where direct exposure to radiant heat takes place. It
must be pounded into place during installation . It is made from natural clay
with added calcided fire clay which has been chrushed and graded.
iv. Plastic
chrome ore- This is bonded with clay and used for studded walls. It has little
strength and hence stud provides the support.
v. Castable
refractory-This is placed over water walls and other parts of the boiler were
it is protected from radiant heat . It is installed in a manner similar to
concreteing in building
vi. Insulating
materials- Blocks, bricks , sheets and powder are usually second line
refractories. I.E. Behind the furnace refractory which is exposed to the flame.
Material; asbestos millboard, magnesia , calcined magnesia block, diatomite
blocks, vermiculite etc. all having very low heat conductivity.
