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Anonymous Poster #1

Gas Fired Boiler Gas Consumption at Different Exhaust Temperatures After Economiser

02/13/2019 5:28 AM

Hello,

I want to calculate gas fired boiler gas consumption at different stack temperatures after economiser,

at 150,130,100,80, and 60.

please suggest any formula.

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#1

Re: Gas Fired Boiler Gas Consumption at different exhaust temperatures after economiser

02/13/2019 7:50 AM

Measure first then calculate your results from the measurements. Your boiler laboratory should already have all of the instrumentation you need.

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#2

Re: Gas Fired Boiler Gas Consumption at Different Exhaust Temperatures After Economiser

02/13/2019 2:52 PM

Carry out a combined mass balance and energy balance calculation on the equipment. However it is inadvisable to let the stack temperature fall below the dew point of sulphuric acid as the stack and the equipment immediately upstream of it are at risk of accelerated corrosion rates. 135degC is a rule-of-thumb minimum.

However a telephone call to the equipment manufacturer may be worthwhile as the performance of the equipment may already be documented and such a call could save a substantial effort. It is always worth making such a call.

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#3
In reply to #2

Re: Gas Fired Boiler Gas Consumption at Different Exhaust Temperatures After Economiser

02/13/2019 2:58 PM

That's why the laboratory stack uses Pyrex® glass piping. Acids are not a problem.

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#4
In reply to #3

Re: Gas Fired Boiler Gas Consumption at Different Exhaust Temperatures After Economiser

02/13/2019 3:14 PM

A glass stack is a rare beast.

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#5
In reply to #4

Re: Gas Fired Boiler Gas Consumption at Different Exhaust Temperatures After Economiser

02/14/2019 9:50 AM

I expect them to be in many Chemistry laboratories.

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#6

Re: Gas Fired Boiler Gas Consumption at Different Exhaust Temperatures After Economiser

02/15/2019 4:45 PM

It is important to know exactly what you are changing to affect the stack temperature...

There is more than one formula, and you need to account for all of the energy supplied to the incoming combustion air as well as what is taken from the flue gas.

As the flue gas temperature varies, the heat you transfer to the economizer/combustion air will change, significantly. The heat exchanger was designed to be most efficient at a certain flow rate and temperature differential, outside air to flue gas.

A very rough idea of the range of the savings for running close to ideal combustion air supply would be to assume you have complete combustion, O2 sensor showing a little excess air, and the stack temperature varies due to you deliberately supplying excess air to the combustion. The BTU savings is similar to the BTU required to heat the excess air, using your LHV BTUH value for your gas/fuel. Convert your combustion air flow to mass, lb/hr, and use a number similar to 0.24 BTUH/lb Mass per degree rise.

That should be similar to the cost to heat air blowing through the furnace that is only cooling it off, sending heat up the stack.

PWSlack advice on flue gas temperature is good counsel, especially if your fuel has sulphur content, and your breeching and stack is carbon steel. Given different materials, you just need to be sure you handle the condensate from the stack at the lower temperatures.

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#7
In reply to #6

Re: Gas Fired Boiler Gas Consumption at Different Exhaust Temperatures After Economiser

02/16/2019 2:03 AM

thank you

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