Calculations

[John Mauney]

Gas fired burner with combustion air fan producing 589 cfm. trying to find btu/hr.
scfm x 1.1 (T2-T1)/available heat= btu/hr
If available heat is T2-T1/100, is the number extrapolated the low fire btu/hr?

 

[Snickster]

scfm x 1.1 (T2-T1) equals the heat input in BTU/hr into air to raise the temperature from T1 TO T2. In other words

btu/hr = scfm x 1.1 (T2-T1)

Your statement that:

scfm x 1.1 (T2-T1)/available heat= btu/hr

Does not make sense. Please explain what you mean by this.

 

[John Mauney]

That is a formula I received from Midco International on try to determine the temperature rise of the Midco UniPower V3 burner.
I'm trying to understand this myself.

 

[Snickster]

What are you trying figure out. The BTU/hr capacity of the burner is listed in the attached as 1890 mbtu/hr or 2000 mbtu/hr with blower rated at 360 CFM.

 

[John Mauney]

Discharge temperature

 

[Snickster]

For flow of air the equation is:

BTU/hr = 1.1 CFM (T2-T1)

So if you have a heat input of a certain BTU/hr and you know the CFM (cu ft/min) and the inlet temperature T1 ( deg F) you can determine the outlet temperature T2 using the above equation.

In your case the BTU/hr would I guess be based on the inefficiency of the furnace such that

Btu/hr = ((100-Eff)/100)*(Rated BTU/hr of furnace)

This would give you the heat input to the flue gas

However the flue gas is the product of combustion which is mainly CO2. CO2 has a similar specific heat as air of 0.21 BTU/lb F versus air of 0.24 so the above equation will still apply close enough, but to be correct multiply the right side of the equation by 0.21/0.24.

So if the 589 CFM is the actual flowrate of the flue gas then this would be the CFM to plug into the above equation. If the 589 CFM is the supply air to the burner then you would need to estimate the flue gas flowrate base on stochiometric combustion of the fuel and combustion efficiency.