For the readers of my last two blogs who slogged through the boring subject matter of government regulations, I hope to move along to something less sterile as well as being more helpful. In our business, we get so used to calculating various formulas in our day-to-day work that we somehow lose sight of the fact not everyone involved knows the intricacies of our field. Earlier this week through my LinkedIn connection someone asked about calculating paybacks. I gave an answer and found that this led to more questions so I hope a few simple equations here will be helpful to all.
First, how do we calculate heat recovery in an economizer?
Q=(m) (Cp) (Tin-Tout)
Where:
Q = heat recovered (Btu/hr)
m = mass flow rate of flue gas (lb/hr)
Cp = Specific heat of the flue gas (Btu/Hr °F)
T = temperature of the flue gas (°F)
Example:
50,000 PPH Boiler with 54,000 PPH flue gas
2,916,000 Btu/hr = (54,000 PPH)(.27)(500-300)
Note that this holds true for both the water-side as well as the gas side.
Another way to calculate Q is:
Q= (U) (A) (LMTD)
Where:
Q= Heat recovered (Btu/hr)
U= Heat Transfer Coefficient (Btu/hr • ft²)
A= Effective Surface Area (sq. ft)
LMTD = Log Mean Temperature Difference across the heating surface
Note in this formula the most efficient way to lower the amount of surface area is by increasing the heat transfer coefficient.
Next, how do we calculate fuel savings?
S=
Where:
S = Savings $/yr
Fuel = cost of fuel in $/MMBtu
Q = heat recovered Btu/hr
H = hours of annual operation
E = Boiler efficiency w/o economizer
Example:
$142,155= ($6) (2,916,000 Btu) (6500) / (.8) (1,000,000)
My blog quote for the day is about our mortality, something especially poignant as I grow older.
Eternity is really long, especially near the end. ~ Woody Allen
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