Challenge in furnace design

Transcription

1 Furnace

2 Definition and Uses The Oxford English Dictionary defines a furnace as an enclosed structure for intense heating by fire, esp. of metals, or water. A furnace is essentially a thermal enclosure and is employed to process materials at high temperatures both in solid state and liquid state. Furnaces are the basic building block of our industrial society. The principal objective of a furnace is to attain a higher processing temperature. Furnaces can be used to facilitate a wide range of chemical reactions, or in some cases simply for physical processes, such as annealing or drying. It can be used in several industries like iron and steel making, non ferrous metals production, glass making, manufacturing, ceramic processing, calcination in cement production etc.

3 Challenge in furnace design

4 Classification of furnace

5 Effectiveness of Furnace Thermal efficiency of furnace can be calculated by two methods. Direct method: Where, Furnace efficiency = η f = m C p t 2 t 1 m f GCV Heat input to the stock Heat supplied by fuel η f is furnace efficiency, m is weight of feed stock, m f is weight of fuel consumed, GCV is Gross Calorific value of fuel, t 1 and t 2 are initial and final temperature of stock respectively, Cp is heat capacity of stock.

6 Example based on direct method A furnace output is 5000 kg/hour of billets. Thermal efficiency is claimed to be 25% Specific heat of billet is 0.12 kcal/kg. o C. Billets enter the furnace at 40 o C and leave at 1200 o C. Calculate the hourly oil consumption in liter if GCV of oil is 9,200 kcal/liter. Answer: liter/hour

7 Effectiveness of Furnace Indirect method: Indirect method is based on calculation of various parameters such as fuel consumption, material input, excess air quantity, temperature of flue gases, temperature of furnace at various zones, skin temperature, combustion air temperature and ambient air temperature. To calculate efficiency by indirect method following losses are to be calculated. Dry flue gas loss Heat loss due to moisture from burning hydrogen Heat loss due to moisture in air Heat loss due to formation of carbon monoxide Heat loss through skin Heat loss due to unburnt in bottom ash and fly ash: only for solid fuels Heat loss due to sensible heat in bottom ash and fly ash: only for solid fuels Heat loss due to openings Unaccounted heat loss

8 Dry Flue gas loss (L fg ) Normally the major loss occurs through the flue gases, which escape at a high temperature. This heat loss depends on two factors. 1. Flue gas temperature 2. Quantity of flue gas generated (depend on the excess air level)

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12 Heat loss due to moisture in Fuel (L mf ) Depends on moisture content in fuel and final flue gas temperature. Moisture content in fuel oils and gas is generally insignificant.

13 Heat loss due to moisture from burning hydrogen in fuel (L hf ) Depends on hydrogen content in fuel and final flue gas temperature. Hydrogen content in fuel oils and gas is significant.

14 Heat loss due to moisture in air (L ma ) Depends on water vapor present in air, which in turn depends on DBT and WBT at prevailing time. (Beyond our control) It also depend upon amount of excess air supplied.

15 Heat loss due to CO in flue gas (L co ) This is due to CO formation due to either improper air/fuel mixing or insufficient combustion air in the burning chamber. These conditions lead to partial oxidation of carbon in the fuel and, thus, reduce the heat release.

16 Heat loss due to unburnt carbon in bottom ash (L ubb ) Only applicable for solid-fuel fired furnaces. A very small loss due to unburnt carbon would also occur while burning fuel oils, but this is generally ignored. M fb is factor dependent upon type of furnace.

17 Heat loss due to unburnt carbon in fly ash (L ubf ) Only applicable for solid-fuel fired furnaces. A very small loss due to unburnt carbon would also occur while burning fuel oils, but this is generally ignored. M ff is factor dependent upon type of furnace.

18 Heat loss due to sensible heat loss in bottom ash (L sb ) Only applicable for solid-fuel fired furnaces.

19 Heat loss due to sensible heat loss in fly ash (L sf ) Only applicable for solid-fuel fired furnaces.

20 Heat loss due to surface and convection (L rc ) also called as loss through skin. Depends on surface temperature and wind velocity.

21 Heat loss due to opening (L o ). Depends on shape and size of opening. These heat loss is calculated by black body radiation at furnace at furnace temperature, and multiplying it with emissivity, and factor of radiation through openings.

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23 Unaccounted losses, L u These are due to thermal storage in refractories, loss of furnace gases around charging door and cracks, loss due to formation of scale, heat transferred out of the furnace by load conveyers, fixtures, trays, and other hot exposed parts. Generally these losses are less than 6% of the total heat supplied to furnace.

24 Total heat loss, L hl = L fg + L mf + L hf + L ma + L CO + L ubb + L ubf + L sb + L sf + L o + L rc + L u Thermal Efficiency = L hl