Fuel and Combustion (Sir Fassi) Fassi)
2012
Lobo Evan method If we want to design a furnace, we want to design a pipe still then these are some of the rules which are very important, i mportant, these are the features of the furnace. 1. We should be using either fuel or the cracked gas as heating media. It means that this method is not recommended recommended for electrically heated furnace. 2. The radiant rate should be in between 5000-30000 Btu/hrft 2. It is based on the outer tube diameter. The amount amount of radiant rate shows that we can’t use this method for very small furnaces. 3. Percentage Percentage of excess air should be between 5 to 80%. 4. Tube skin temperature must not be closer
Suppose you have a furnace, then hot gasses would be rising up and there are tubes also. When the temperature temperature between the the tubes and the flue gasses gasses is closest? closest? Tube skin temperature (temperature at the surface of the tubes), under which condition the difference between the flue gasses and the tube skin temperature would be minimum? This would be when excess air is less or minimum, if the amount of flue gas is less than the temperature difference difference would be less because the flue gasses is going to cool, the flue gasses would be getting the heat out so if the difference between the tubes and the flue gas is less it means that the amount of the flue gas is less and we will get the less amount of the flue gas when the excess air would be less.
Prepared by: Syed Saad Ahmed Ahmed | B.E Chemical Engineering
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2012
Fuel and Combustion (Sir Fassi) Fassi)
If the amount of the excess air is less, the flue gasses would be less, and the temperature between the tubes and the flue gas would be closer to 400oC. If amount of excess air is more it means that the flue gasses would be rising up in the greater quantity, then the difference between the tube skin temperature and the flue gas would be more than 400oC. There has to be excess air and if there is excess air there would be difference in temperature and that difference in temperature must not be less than 400 0C. There was a time when the amount of excess air would generally be considered as 30%, but now this has been reduced to 10% because at the flame temperature there is a possibility of (No)x formation because the temperature is very high and at that temperature oxygen and nitrogen combines otherwise the nitrogen is generally taken as inert. So that’s why the excess air is reduced, but with the reduction there is another problem, the problem problem is that if the amount of air is less then then there will be incomplete incomplete combustion which means that the formation of CO that’s why the modern trend is to restrict it to 10%. 5. The mean length of radiant beam should not be less than 15 ft. This is defined in terms of length, width and height if it is rectangular and if it is a circular then it is defined in terms of diameter and height. Rectangular furnaces:
Length of beam (L, ft)
1-1-1 to 1-1-3 and 1-2-1 to 1-2-4
⁄
Suppose the dimension of the furnace is 15-30-40 ft 3, then we divide all with the smallest valve
and we get 1-2-2.6 then the length of the radiant beam will be
⁄ √
Rectangular furnaces: 1-1-4 to 1-1-
Length of beam (L, ft) 1.0 times smallest dimension
Suppose the dimension of the furnace is 15-15-70 ft3, then for ratio
And we get 1-1-4.6 then the the length of the radiant radiant beam will be 1*15=15 ft Circular furnaces:
length of beam (L, ft)
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Fuel and Combustion (Sir Fassi) Fassi)
2012
Distribution of radiant heat
Heat and Electromagnetic waves generates in radiant section. They may be reflected and absorbed. It may be reflected for the wall and the tubes. Once it reflected its intensity decreases. For example:
We have the flame in the furnace and the first raw of tubes in the convection section would be receiving heat both by radiation and convection, here we just now considering the receiving heat by radiation. About the total amount of heat which is developed by the flame, only 40-45% of the heat is being absorbed by the radiation section. The remaining heat is carried by hot flue gases so this heat is falling on these tubes. The tube center to center distance is twice the tube diameter so it means that maximum amount of heat is falling on the first tube and some of the heat is falling on the second raw also this may be deflected by many sources. The first raw would be getting 100% of the radiation, the second raw would be receiving 56%, third raw receives 19%, the forth raw would be receiving 4% and fifth raw receives 1.5%. These percentages percentages are only by radiation but these tubes also getting heat by convection.
