The
Metal Fibre Burner has been designed to be the state-of-technology
in gas infrared. It has the advantage of low operating cost
with unique features such as
- A long life due to its unbreakable construction.
- Very low heat-up and cool down time (3 to 4 seconds),
when used in the fan driven mode.
- Absolute uniformity of radiation on throughout the
emitting surface even if the burner is 2 to 3 meters long.
- Flexible shape.
- Mechanical and thermal shock proof.
There are two basic types:
- MFB cloth 100.
- Knitted Mat 250 and Sintered Mat 200.
Construction of MFB Burner
A thin perforated sheet supports the burner as the cloth does not have strength
of its own. There is equal distribution of premix as it is fully
welded with diverters inside and the housing is made up of SS304.
The housing comes with or without collar. The gas can be made
to enter from the back or its side. The height will increase
in case of side entry of the gas. MFB Cloth 100 is used for
blue applications for its low density - 1.5 KG/M2.
Construction of Sintered Mat Burner
Sintered Mat 200 is used for radiant application for its medium density - 2.5
KG/M2 and Knitted Mat 250 is high density - 4 KG/M2, used for
applications where the surface temperature of the burner is
consistently high. It can be welded between the housing and
a flange as the mat has the strength of its own. The other construction
is similar to the MFB design.
Operation
The burner can be fired facing sideways, up or down. The vertical firing is also
possible as long as the heated length is 1000 mm, without any
loss in its uniformity. The atmospheric firing is possible only
with a pressure of 120 mbar, although being inexpensive and
has less modulation capacity. Firing gas-air premix has higher
modulation range and works with low pressure but it requires
a set of other equipments and parts. The addition of gas train
makes it reliable for long term.
GAS COMBUSTION
A safe system design is known by its composition of gas. Any design is always
set for a particular type of gas and variation in which will
lead to safety hazard. A system designed for Propane or natural
gas cannot have Butane due to its lower self ignition temperature
(450 0C), will cause flashback and explosion, as the temperature
of Propane is 550 0C and natural gas is 750 0C.
Chemistry of Combustion
Generally, CO2 and H2O are formed as by products of combustion of hydrocarbons.
For example,
CH4 + 1/2O2 = CH3OH
CH3OH + 1/2O2 = HCHO*+H2O
HCHO = CO+H2
H2+1/2O2=H2O
CO+1/2O2=CO2
* This is formaldehyde. If the mixture hits cold wall, the process stops here.
This is why formaldehyde smell is observed in poor heat exchanger
designs.
Gas / air ratios.
? = stoichiometric ratio = 1 if air is just adequate for
combustion
? > 1 if air is excess
? < 1 if gas is excess
Radiant/ Blue flame modes
With premix flow leading to heat intensity in the range of 100- 500KW/m2, most
of the combustion takes place within the surface of the burner
itself. This makes the surface heat, glow and transfer the heat
in radiant form. This is infrared mode. Here the surface temperature
is highest at 1050 ºC.
If premix quantity is increased, the burner actually cools down
because of excess nitrogen flow. Combustion takes place outside
the burner. This is blue flame mode. Here the heat transfer
is in convection mode. Intensities as high as 20 MW/m2 are possible.
In both the cases flue gases are released in the air.
For radiant operation, ? = 1.05 to 1.1
For blue flame mode, ? = 1.1 to 1.2
Efficiencies in Radiant mode
At lower intensities higher amount of combustion takes place within the surface.
At around 125KW/m2 the radiant efficiency is highest at approx.
55-60%. Below 100KW/m2 the flame cannot be sustained.
Efficiency is high in face down position. It can be increased
in any position by adding a grid in front.
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