Process Duty versus Heat Liberation

The heat of combustion is a product of the amount of fuel consumed and the net heating value of the fuel. The heater s efficiency is a function of the flue-gas stack temperature, the excess air or oxygen, and the ambient-heat losses from the firebox and the convective-section structures. 

The heat absorbed by the tubes is the sum of the heat of vaporization of the process liquid to vapor, plus the increase in the sensible-heat content of the flowing process fluid. 

If the heat picked up from the combustion of the fuel does not equal the heat absorbed by the process fluid, then something is amiss with the data. Often, determining the cause of such an inconsistency will reveal several fundamental operating or measurement problems with a fired heater. Quite commonly, we may find that the metered fuel-gas rate is wrong, or that the ambient-heat losses are much greater than anticipated in our calculations. 

When fuel is burned in a fired heater, there are three major products of combustion  

The water, plus the C02, mix with the inert nitrogen in the combustion air to form flue gas. There is also some oxygen in this flue gas. This oxygen is called excess 02. A typical excess 02 content of flue gas is 2 to 6 percent. About 80 percent of the flue gas is N2. The rest is H20 plus C02. 

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