air preheater problems

[pradeep4u]

Dear All
My boiler is gas fired boiler. Natural gas is fuel. It contains no sulphur. Is it advisable to use carbon steel heating elements in cold end portion as there is no sulphur so there is no risk of Acid dew point corrosion. Please guide me

Regards
Pradeep

 

[athomas236]

There is usually some sulphur compound in natural gas called mercaptan to produce a smell for safety reasons. In UK this is about 25ppm so this causes an acid dew point.
Regards

 

[pradeep4u]

But in our boiler natural gas we use is free of mercaptan. In that case can we use carbon steel heating elements in place of corten steel. I am attaching our natural gas analysis for information.

Regards
Pradeep

 

[FoxRox]

I'm no boiler expert, but I agree with you that acid would not be a problem with such clean gas. Is oxidation potentially a problem? Natural gas exhaust would certainly include H2O and O2. What material would you typically use if acid was a risk? Is it significantly more expensive?

 

[chicopee]

There will be acidic formation from the CO2 in the event condensation form on the cold end and that is known as carbonic acid. Same acid that attacks mufflers and tailpipes in car exhausts.

 

[EdStainless]

The exhaust will also include NOx, and form nitric acid condensate.
Typically low grade stainless alloys are used. Where it is hot enough that there is no risk of condensation they use CS, then move to 409 as it cools, and then in cold areas where they will frequently be wet they may move to 439.

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P.E. Metallurgy, Plymouth Tube

 

[pradeep4u]

Dear All
Thanks for your very very valuable answers. I have one more doubt if you all can answer then I will be very grateful
In our air preheater, design circumference strips clearance was 3mm but now clearance increased to 30mm. I want to know that apart from heat loss( Hot flue gas will bypass heating elements and go through increased clearance thereby not giving heat to heating elements)which other parameters will be disturbed like draft and Id fan load etc.

Regards
Pradeep

 

[pradeep4u]

Dear All

I have not got ny reply did I asked wrong question?

regards
pradeep

 

[racookpe1978]

Your added question was not clear.
Sketch the problem, to scale, and label the change you described.