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Gas Infrared System |
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Gas Infrared Burner (MFB IR BURNER) |
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MFB BURNER |
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The metal fibre burners represent the state-of-the technology in gas infrared. It combines the low operating cost advantage of gas with unique features such as
- Unbreakable construction, with very high life.
- Short heat-up and cool down time (3-4 seconds) when usedin the fan driven mode.
- Absolute uniformity of radiation over the emitting surfaceeven in case of 2-3 m long burners.
- Flexibility of shape
- Thermal & mechanical shock-proof.
There are two basic types:
- MFB Cloth 100.
- Sintered Mat 200/ Knitted Mat 250.
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Construction of MFB Burner
The cloth does not have strength of its own. A thin perforated sheet supports it. Housing is made of SS304 and is fully welded with diverters inside for equal distribution of premix. The mat is fully welded along its periphery. The housing can be made with or without collar.
Gas entry is either from back or from side (height increases in case of side entry).
MFB Cloth 100 is low density, 1.5Kg/m2 and is used mainly for blue flame applications.
Sintered Mat 200 is medium density, 2.5Kg/m2 and is used mainly for radiant applications.
Construction of Sintered Mat Burner
Knitted Mat 250 is high density, 4Kg/m2 and is used mainly for applications where the burner surface temperatures are consistently high.
As the mat has strength of its own, it can be welded between the housing and a flange. Rest of the construction is as per the MFB design.
Operation
The burner can be fired facing sideways, up or down (with some care). Also it can be fired vertically with no loss of uniformity till a heated length of 1000mm. Atmospheric firing is inexpensive but possible only if a pressure of 120 mbar is available. Also it has less modulation capacity.
Firing gas-air premix requires paraphernalia but it has higher modulation range and can work with lower pressures. Also with the addition of gas train it can be made reliable for long-term usage. |
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| GAS COMBUSTION |
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Important note:
Composition of gas is the most important parameter in designing a safe system. An installation is always set for one particular type of gas. Variation in this can cause safety hazards, for example if butane is used in a system originally designed for propane or natural gas, flashback and explosion can occur because of lower self ignition temperature of butane (450 ºC) as compared to 550 ºC for propane and 750 ºC for natural gas and also due to higher calorific value of butane.
Chemistry of Combustion
Generally CO2 and H2O are formed as byproducts 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. |
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| Gas / air ratios. |
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λ = stoichiometric ratio = 1 if air is just adequate for combustion
λ > 1 if air is excess
λ < 1 if gas is excess |
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| Radiant / Blue flame modes |
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With premix flow such that heat intensity is 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, λ = 1.05 to 1.1
For blue flame mode, λ = 1.1 to 1.2 |
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| Efficiencies in Radiant mode |
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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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