OCR Text |
Show shows lower firing rates result in higher efficiencies. This trend is due to a reduced quantity of hot gas convecting with a fixed area of radiant tube. The smaller amount of gas loses more heat to the tube, per unit volume of gas, and therefore exhausts at a lower temperature. Another mechanism which, to a lesser extent, improves the tube's efficiency at low flows is the nature of the convection coeff i c i en t. Tube diameters were varied during the tests to determ i ne whether 1 arger tubes, wh i ch have greater mean beam lengths, would show more rapid improvements in efficiency (N) with increasing oxygen concentration (OX) than would small tubes. Greater beam lengths increase the gas emissivity which, in turn, increases the heat transfer rate and thermal efficiency. Although the absolute value of the thermal efficiencies of 1 arge tubes was low, due to the i r low heat transfer area, relative to the firing rate, the rate of increase of the eff i c i ency with oxygen concentration (dN/dOX) was greater. Table 3 describes the rate of efficiency improvement with increasing tube diameter. 1400~--------------------------~ u ~12000~f)-f) C> 1090 C -«S .... ~ ~ ~(!It) II 870 C E 1000 t)"""""'-- ::. 0/ Q) .0 :J I- 800 .::&. «S Q) a.. 600 0.2 0.3 Mole Fraction O2 in Oxidizer Figure 5 PEAK TUBE TEMPERATURE VS 0.8 MOLE FRACTION 02 IN OXIDIZER FOR VARIOUS FURNACE TEMPERATURES Tube Diameter 0.092 m (Ceramic) Fuel Input 5280 kW/m2 Table 3 Tube Diameter (m) 0.102 0.178 0.254 Oxygen Concentration OX1 OX2 0.210 0.210 0.210 0.659 0.400 0.400 52 The variation of the peak tube tem~erature with mole fraction 02 in the oxidizer 1S shown in Fi gure 5. Peak tube temperatures. are of interest because this temperature will 1m~act on the selection of materials of construct10n for radiant tubes. It can be seen that in~reasing the oxygen concentration results in an 1ncrease in the peak tube temperature, but to ~ lesser degree for hotter furnaces. Also, an 1 n~rease in the furnace temperature results 1n an increase in peak tube temperature of about the same magn i tude. Peak tube temperatures rarely exceeded 12°C above the average tube temperature and occurred at about one third of the tube length from the burner. This small temperature difference indicates that the use of oxygen enrichment in radiant tube systems does not put undue requirements on the materials of construction due to peak temperature accommodations. The variation of the peak burner cup temperature with mole fraction 02 is shown in Figure 6. As expected, peak cup temperature rises rapidly with increased oxygen concentration. Also seen in this figure is the -u Q) .... -:::J «..S.. Q) 1400~---------------------------, 12001- 0.092 m Dia. / (Metallic) a. 10001- E / A 0.185 m Dia. ./ A / (Metallic) Q) I-a. :::J U .::&. «S Q) a.. / / / 0.253 m Dia. / / /6 (Metallic) 800- / 6 ;4./ 600~/ I I I I I 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Mole Fraction O2 in Oxidizer Figure 6 PEAK CUP TEMPERATURE VS MOLE FRACTION O2 IN OXIDIZER FOR VARIOUS TUBE DIAMETERS Furnace Temperature 870 C Fuel Input 5280 to 5580 kW/m2 Thermal Efficiency N1 N2 0.311 0.277 0.154 0.645 0.508 0.509 dN/DOX 0.76 1.22 1.87 |