Effect of Fin Thickness and Conductivity

Effect of Fin Thickness and Conductivity

The thickness of fins and the thermal conductivity of fins used in boiler tubes affect the heat transfer, duty, and tube wall and fin tip temperatures and hence should be chosen with care. Thermal conductivity of alloy steels is lower than that of carbon steels as seen in Table E.15. Hence, when we use alloy fins in carbon steel evaporators, then the K value of fin material reduces decreasing the U value and increases the fin tip temperature as shown later. Note that the K value of stainless fins will be much lower than that of carbon steel. When gas inlet temperature is high, say, above 700°C, the boiler is designed with a few rows of plain tubes, followed by a few rows of tubes with alloy steel fins and then fol- lowed by several rows of carbon steel fins. As the gas cools, carbon steel fins are used. In finned superheaters, it may be necessary to use the alloy steel fins with alloy steel tubes.

Example E.9

A boiler evaporator uses the following geometry: 50.8 × 44.6 tubes, 36 tubes/row, 15 deep, 5 m long, 101 mm S T ,S L , staggered arrangement, 216 fins/m, 19 mm high, 1.5 mm thick,

4 mm serration, 40 kg/cm 2 a steam pressure, 105°C feed water. Gas flow = 150,000 kg/h. Study the effect of using alloy steel material for fins and thicker fins.

440 Appendix E: Calculations with Finned Tubes

TABLE E.15

Effect of Fin Thickness and Thermal Conductivity Gas inlet temperature, °C

800 Exit gas temperature, °C

23.96 24.1 24.0 23.73 Gas pressure drop, mm wc

37.74 38.9 34.0 31.9 Steam generation, kg/h

U o , kcal/mm 2 h °C

5,386 Fin conductivity, kcal/m h °C

Surface area, m 2 5,386

30 30 15 15 Fin thickness, mm

1.5 2.67 2.67 1.5 Max tube wall temperature, °C

354 Max fin tip temperature, °C

The results from the computer program are shown as follows. The differences in surface areas are due to the fin thickness though the number of tubes remains unchanged.

The following points may be noted. 1. As fin conductivity decreases for the same fin thickness, duty decreases

slightly. 2. As the fin conductivity decreases, the tube wall temperature decreases while the fin tip temperature increases. 3. Flue gas pressure drop increases significantly if fin thickness is increased due to an increase in obstruction area A o . 4. As the fin thickness increases, the fin tip temperature decreases while the tube wall temperature increases.

Hence, it is not prudent to increase the fin thickness as the operating cost in the form of gas pressure drop increases significantly. If we can keep the tube wall and fin tip tem- peratures within limits, using a nominal fin thickness of 1.25–1.5 mm is fine; else, we are unnecessarily increasing the weight and cost of the boiler tubes. The boiler duty also does not improve much due to the use of thicker fins. One should also note the impact of using alloy steel fins with low thermal conductivity.

The purpose of this example is to make plant engineers aware of the effect of fin thickness on performance and discuss intelligently with the HRSG suppliers if they use thick fins in their design. The operating costs in the form of gas pressure drop can be reduced by using nominal fin thickness. Thick fins may be required to lower the fin tip temperature in some instances to avoid the use of alloy steel fins.