Page 220 - Boiler plant and distribution system optimization manual

Boiler Plant Calculations 205

Time to
Payback Discount Recoup
Period Rate Investment
————————————————————————————————————————
Option 1. 1.5 Yr. 20% ————— (Ans.: <2 yr.)
Option 2. 2.6 Yr. 20% ————— (Ans.: <3 yr.)
Option 3. 5.0 Yr. 20% ————— (Ans.: >25 yr.)
Option 4. 5.0 Yr. 15% ————— (Ans.: 10 yr.)
Option 5. 5.0 Yr. 10% ————— (Ans.: 8 yr.)
Option 6. 5.0 Yr. 5% ————— (Ans.: 6 yr.)
————————————————————————————————————————

replacing an Old Inefficient Boiler
Size: 20 Million Btu/hr (20,000 PPH) (10 Ton/hr)
Fuel: No. 2 Distillate
Cost: $1.00/Gallon
250,000 Gallons Fuel per year
137,000 Btu/Gallon
Net Stack Temperature 750F, The economizer needs replacement .
Oxygen 9.5%, Excess Air 76%
Stack losses 30%
Surface losses 2.5% of full rating—(500,000 Btu/hr)

Table 11-17 is calculated from the following equation:

1 – (1 + D) –EL
PWF ————————
D

where D is discount rate expressed as a fraction and EL is the expected lifetime of the
project in years.

1. The boiler is fired for 6,000 hours a year, what is its utilization rate?

20 MBtu/hr x 6,000 hr/YR
Theoretical fuel usage = ———————————— = 896,000 gallons/YR
137,000 Btu/gallon

Actual Fuel Usage 250,000 gallons
—————————— = ———————— = about 30% (Utilization)
Theoretical Fuel Usage 896,000 gallons

2.5%
2. Corrected surface losses —— = 8.3%
30%

3. How much fuel is being wasted each year with this old boiler?

(30% Stack Loss + 8.3% Surface Loss) x 250,000 gallons/yr = $95,750 gallons or
($95,750/yr)