Tube Wall Temperature Estimation at Economizer Inlet
Tube Wall Temperature Estimation at Economizer Inlet
The method of calculating water and acid dew points was presented in Chapter 1. When the flue gas drops below the water dew point, water starts condensing on the heating surfaces. Similarly, when the tube wall temperature drops below the acid dew point in air heater or economizer, acid vapor, if present, starts condensing on the surfaces leading to corrosion and material loss. The tube wall temperature in an economizer is very much dictated by the feed water entering temperature. Similar calculations for minimum tube wall temperature are presented in Chapter 4 for tubular air heaters.
150 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers
Example 3.10
The performance results of a carbon steel plain tube and finned tube economizer in par- allel- and counter-flow arrangement are shown in Table 3.15. Gas flow = 100,000 kg/h at 350°C. At 105°C, 70,000 kg/h water flows inside the tubes. Flue gas is natural gas products of combustion; 38 × 32 mm tubes, 24 tubes/row, 30 deep, 4 m long are used with S T = 75, S L = 80 mm in inline arrangement for plain tubes.
For the finned tube option, same tube size is used with 24 tubes/row, 12 rows deep, 3 m long tubes, S T = 100, and S L = 90 mm in staggered arrangement. Ratio of external to inter-
nal surface is 13.44. Inside and outside fouling factors are 0.0002 and 0.0004 m 2 h °C/kcal,
respectively. Estimate the tube wall temperature at the cold end. Assume acid dew point temperature is 130°C.
Solution
The U values are computed as explained in Appendices B through E. Then the perfor- mance is obtained using either the NTU method or the conventional method of equat- ing assumed duty with transferred duty. Results are shown in Table 3.15. The U o is the average overall heat transfer coefficient.
Case 1: Finned tubes, parallel-flow
Heat flux inside tubes = 37 × (350 − 105) × 13.44 = 121,833 kcal/m 2 h.
Tube wall resistance = [0.038/(2 × 35)] ln(38/32) = 0.0000933 m 2 h °C/kcal. Resistance due to tube-side heat transfer film = 1/6800 = 0.000147 m 2 h °C/kcal. Tube wall temperature = 105 + 121,833 × (0.000147 + 0.0002 + 0.0000933) = 159°C. The minimum tube wall temperature is above acid dew point temperature, and hence,
this is a viable option provided we find the tube wall temperature above 130°C at low loads also.
Case 2: Finned tube, counter-flow
Heat flux inside tubes = 37 × (170 − 105) × 13.44 = 32,323 kcal/m 2 h.
Tube wall resistance = [0.038/(2 × 35)] ln(38/32) = 0.0000933 m 2 h °C/kcal. Resistance due to tube-side heat transfer film = 1/6800 = 0.000147 m 2 h °C/kcal (due to difference in average water temperature, h i is different). Tube wall temperature = 105 + 32,323 × (0.000147 + 0.0002 + 0.0000933) = 119°C. This does not solve the acid vapor condensation problem as the minimum tube wall
temperature is below 130°C.
TABLE 3.15
Plain Tube and Finned Economizer Performance Results
Finned-CF Plain-CF
Gas temp. in, °C
350 350 Gas temp. out, °C
188 223 Duty, MM kcal/h
4.23 4.79 3.51 3.75 4.32 3.39 Water temp. in, °C
130 130 Water temp. out, °C
190 177 U o , kcal/m 2 h °C
37 37 76.5 76.5 37 77 h i, kcal/m 2 h °C
1167 343 Min tube wall, °C
Surface area, m 2 1167
Steam Generators 151
Case 3: Plain tubes, parallel-flow Heat flux inside tubes = 76.5 × (350 − 105) × 38/32 = 22,257 kcal/m 2 h. Tube wall resistance = [0.038/(2 × 35)] ln(38/32) = 0.0000933 m 2 h °C/kcal.
Resistance due to tube-side heat transfer film = 1/6600 = 0.0001515 m 2 h °C/kcal. Tube wall temperature = 105 + 22,257 × (0.0001515 + 0.0002 + 0.0000933) = 115°C. This does not solve the acid vapor condensation problem as the minimum tube wall
temperature is below 130°C. Case 4: Plain tubes, counter-flow
Heat flux inside tubes = 76.5 × (209 − 105) × 38/32 = 9448 kcal/m 2 h.
Tube wall resistance = [0.038/(2 × 35)] ln(38/32) = 0.0000933 m 2 h °C/kcal. Resistance due to tube-side heat transfer film = 1/6600 = 0.0001515 m 2 h °C/kcal. Tube wall temperature = 105 + 9448 × (0.0001515 + 0.0002 + 0.0000933) = 109°C. This does not solve the acid vapor condensation problem as the minimum tube wall
temperature is below 130°C. Case 5: Finned tube option, counter-flow with feed water inlet at 130 °C using a heat exchanger Case 6: Plain tube option using a feed water temperature of 130°C and counter-flow.
Calculations for cases 5 and 6 are similar. Therefore, only the results are shown.
Strictly speaking, one should compute the U o values using the heat transfer coefficients at the inlet/exit gas temperatures (parallel-/counter-flow) and inlet water temperature. However, the objective of this exercise is to show the effect of parallel- versus counter- flow arrangement and the effect of finned and plain tubes. The following points may
be noted: 1. With plain tube economizer, there is not much of a difference between parallel-
and counter-flow arrangements as the difference in tube wall temperatures is hardly 6°C, which will reduce at lower loads. The tube-side coefficient governs the tube wall temperature. Hence, this method may not avoid condensation of acid vapor. We have to preheat the feed water to a temperature close to the acid dew point as shown in case 6.
2. With finned tubes, due to the higher heat flux with parallel-flow configura- tion, the tube wall temperature is much higher at the feed water inlet end, 159°C versus 119°C. One should perform these calculations at part load also. It is possible to avoid acid condensation in case the dew point temperature is below the tube wall temperature at the lowest load. The effect of tube-side coefficient is somewhat reduced due to finning. The counter-flow arrange- ment is not helpful as the tube wall temperature is below the dew point.
3. Raising the feed water temperature to 130°C in finned and plain tube options avoids the condensation concern. The duty with the finned bundle is also slightly more compared to the parallel-flow option of case 2. Hence, one should consider this option rather than going in for parallel-flow configuration, as at low loads too, this option works.