An engineer wants to estimate the annual inflation–free cost of owning an aerobic digester
system with a capacity of 195 million gallons per day (MGD) for the first 8 years of
operation. Company records show that the cost of a similar system with a capacity of 75
MGD was $8 million five years ago. The equipment cost index has increased 26% per year
since then, and the future general inflation rate is forecast to be 2% per year. He estimates
that the annual operating expenses of the new system would be $440,000 per year for the
first two years and increase to $441,500 per year thereafter, due to the increase in
maintenance costs. Calculate the annual inflation–free cost of owning the 295–MGD
system for the first 8 years. Use a cost–capacity exponent of 0.14 for the system and a
market–based MARR of 8% per year.
Using the power sizing technique with an adjustment for the price increase in
equipment costs, the initial cost of the system now is:
CA=CBSA
SB
x(1 + ec)8
= (8,000,000) 195
75 0.14 (1.26)5
=$29,042,833.25
PW(8%) = –29,042,833.25 –440,000(P/A, 8%, 2) –441,500(P/A, 8%, 6)(P/F, 8%, 2)
= –29,042,833.25 –440,000(1.7833) –441,500(4.6229)(0.8573)
= –29,042,833.25 –784,652.00 –1,749,758.17
= –$31,577,243.42
im=8% per year; f =2% per year
im=0.08 =ir+0.02 + (0.02)ir
1.0200r =0.0600,
ir=0.0588 or 5.88%,
If we assume the base year to be the present (b = 0), then the annual
inflation–free cost of owning the 195–MGD system for the first 8 years is:
AW(5.88%) = –$31,577,243.42 (A/P, 5.88%,8)
= –$31,577,243.42(0.1603)
= –$5,061,832.12 per year
f =36.75% per year; ir=7% per year
im=0.07 +0.3675 + (0.07)(0.3675)
=0.4632, or 46.32% per year