Paper: Two Devices For Improving The Thermal Efficiency Of Fired Equipment

It would be nice if elemental carbon and hydrogen could be burned and the entire heat of combustion could be obtained. But, handling raw elements is difficult, and hydrocarbons are by far the most available and convenient fuels. This means that the heat of formation of the hydrocarbon is lost. Fortunately, the heat of formation of fuel oils and natural gases is relatively small, e.g. 3 to 8 percent of the heat released. Combustion of hydrocarbons is complicated by the fact that the water produced may or may not be considered to be condensed. This is the difference between the higher (water condensed) and lower (water not condensed) heating values (HHV and LHV). In calculations of thermal efficiency, the heat obtained by condensing the water vapor may or may not be considered to be available. This is the difference between the gross (heat available) and net (heat not available) thermal efficiencies (GTE and NTE). The HHV and NTE are the more widely used terms, simply because they are higher values. These terms are used throughout this paper even though the respective bases are inconsistent. Figure 1 shows the NTE for burning natural gas (HHV of 1000 Btu/scf) in terms of the stack gas temperature and the excess air, which are the two parameters by which the performance of fired process equipment can be measured. The break in the curves at 100-130 F corresponds to the condensation of the water vapor produced during combustion. Above 135 F, the vapor pressure of water is greater than its partial pressure and condensation cannot occur. The NTE decreases with increasing stack gas temperature and the departure from linearity represents the change in the specific heat of the combustion gases with temperature. Below 135 F, the vapor pressure of the water vapor is less than the partial pressure and condensation occurs until it is essentially complete at 60 F. This corresponds to a GTE of 100 percent or a NTE of 111 percent. An efficiency over 100 percent means that heat is recovered which is not considered by the calculation procedure to be available. The decrease in the NTE with increasing excess air is the result of increased sensible heat losses in the stack gases. About 10 percent excess air is needed for complete combustion of the fuel. Any additional air acts as a diluent. It reduces the flame temperature and the heat-transfer rate in the firetube and increases the amount of stack gases. Methods for improving the NTE can be analyzed conveniently according to the function involved, that is, control of the excess air and recovery of more sensible heat from the stack gases.