Probably depends on what kind of fuel you're using and for what type of equipment. Look for any minimum and maximum acceptable temperatures in your equipment manual.
If you are asking for fired heater air preheater, right question is: What is minimum temperature of flue gas leaving air preheater? If that temperature is lower then sulphuric acid dew point severe corrosion will develop on exposed metal surfaces.
This temperature will set preheated air temperature.
Let me give you more details. The Air outlet temperature has dropped to 370degF from 500degF , reason being fouling. I want to find the fuel expended in preheating the air. In this regard, I want to know the minimum temperature of combustion air from an air preheater to the burners.
rokkam,
I think you are asking the wrong question and that Milutin has already provided you with the "best" answer. Even if you are firing a difficult to burn fuel, I can't imagine it being necessary to achieve some minimum air temperature in order to sustain the combustion. Is that the question you are asking? There are a variety of factors, such as efficiency, that would suggest certain operating ranges be maintained. But you could likely operate outside those ranges if you acceptt he associated economic penalty. There could even perhaps be environmental constraints that could drive your operations into a constrained range.
HTH,
Doug
You are not giving enough details for anyone to help you. What you are seeking is basically found in your heat balance. What amount of heat that your APH fails to put into the combustion air, the burners now must, no matter what type of APH you have.
So find out your air flow and fuel flow and do the calculations.
One comment; since you have to run with some excess air, the more the burners have to make up for the inadequacies of the APH, the more air they will need to support combustion, all of which will have to be heated with more fuel.
Assuming what you want to do is maximize the thermal efficiency of the combustion furnace (or steam generator) so as to decrease the furnace fuel consumption, then the combustion air preheater exit temperature should be as high as:
(a) technically possible to achieve and
(b) can be economically justifed.
If you are preheating the combustion air by heat exchange with the furnace flue gas, then you must not lower the flue gas temperature to below the sulfuric acid dewpoint (as pointed out above by milutin) to avoid serious corrosion.
Milton Beychok
(Visit me at www.air-dispersion.com)
.
Just to get a rough idea, the temperature loss (500oF[→]370oF) equals about 17.5 kcal/kg air.
Assuming you are burning a 10,000 kcal/kg NCV fuel, and are using about 17.1 kg air per kg of fuel, the loss is about 300 kcal/kg fuel.
Thanks rmw, mbeychok and 25362.
The maximum possible (technicaly and economically) outlet temp for combustion air is available. Whatever the APH fails to do is now done by the burners. I can now calculate the extra fuel expended.
