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Acid Dew Point Temperature for 5 Typical Acid Gases

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Thermal efficiency of combustion systems is increased with the reduction in the exit Flue Gas temperature, however to low of an exit temperature could result in the condensation of Acid Gases that could corrode metallic surfaces in contact with those acids. This occurred in the 1990's with some of the early versions of High Efficiency Condensing Natural Gas Furnaces that failed due to corrosion of the heat exchanger caused by the formation of Sulfuric Acid. Some of these furnaces did not have stainless steel heat exchangers that could stand up to the Sulfuric Acid. This computer program ACID5C calculates the dew point temperature for the following Acid Gases:

1. Sulfuric Acid (H2SO4)
2. Sulfurous Acid (SO2)
3. Hydrobromic Acid (HBr)
4. Hydrochloric Acid (HCl)
5. Nitric Acid (HNO3)

To perform the Acid Dew Point calculation the engineer needs to input the following data which he can get from a combustion analysis program:

1. Total Pressure of the Flue Gas: Ptotal (in Hg) = Pbar + Pgage

2. Percent of Water Vapor in the Wet Flue Gas Volume Basis:
(ft3 water vapor/ft3 wet flue gas) x 100


3. Percent of Acid Gas in the Wet Flue Gas Volume Basis:
(ft3 acid gas/ft3 wet flue gas) x 100

With the above data the program calculates the Partial Pressure of the Water Vapor (mmHg) and the Partial Pressure of the Acid Gas (mmHg) which is used to calculate the Dew Point Temperature.


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Example Analysis for Sulfuric Acid (H2SO4) Dew Point Temperature

In this particular example the fuel is Standard Natural Gas consumed at 100,000 Btu/hr using combustion air at 70 F, 35% Relative Humidity at 10 % Excess Air. The Barometric Pressure is 29.9210 in Hg and the Total Flue Gas Pressure is 29.9210 in Hg. The Natural Gas has 0.000337 % Sulfur on a volume basis and it is assumed that 2.0 % of the SO2 is converted to SO3 in the formation of H2SO4. The following data came from the combustion analysis:

1. Percent Volume of Water Vapor in the Wet Flue Gas: 17.7198 %

2. Percent Volume of Sulfuric Acid in the Wet Flue Gas: 0.000000544955 %

3. The program calculated Sulfuric Acid (H2SO4) Dew Point Temperature = 178.9587 F

ACID5C.txt

Complete Energy Analysis for a Lower Efficiency Induced Draft Natural Gas Furnace (PDF)




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