Page 227 - Handbook of Energy Engineering Calculations

engine at full load. Local laws, insurance requirements, or the NBFU may
limit the quantity of oil that can be stored in the engine room or a day tank.
One day tank is usually used for each engine.
Assume that a 4-hour supply will be suitable for each engine. Then the
day-tank capacity for a 1000-hp (746-kW) engine = (1000 hp) [0.35 lb/(bhp ·

h) fuel] (4 hours) = 1400 lb (630 kg), or 1400/7.387 = 189.6 gal (717.6 L),
given the lighter-weight fuel, step 1. Thus, one 200-gal (757-L) day tank
would be suitable for each of the 1000-hp (746-kW) engines.

For the 600-hp (448-kW) engines, the day-tank capacity should be (600
hp)[0.35 lb/(bhp · h) fuel] (4 hours) = 840 lb (378 kg), or 840/7.387 = 113.8
gal (430.7 L). Thus, one 125-gal (473-L) day tank would be suitable for each
of the 600-hp (448-kW) engines.

4. Determine which is the better fuel buy

Compute the higher heating value HHV of each fuel from HHV = 17,645 +
54(°API), or for 24° fuel, HHV = 17,645 + 54(24) = 18,941 Btu/lb (44,057
kJ/kg). For the 28° fuel, HHV = 17,645 + 54(28) = 19,157 Btu/lb (44,559
kJ/kg).

Compare the two oils on the basis of cost per 10,000 Btu (10,550 kJ),
because this is the usual way of stating the cost of a fuel. The weight of each
oil was computed in step 1. Thus the 24° API oil weighs 7.578 lb/gal (0.90
kg/L), while the 28° API oil weighs 7.387 lb/gal (0.878 kg/L).

Then the cost per 10,000 Btu (10,550 kJ) = (cost, $/gal)/[(HHV,
Btu/lb)/10,000](oil weight, lb/gal). For the 24° API oil, cost per 10,000 Btu
(10,550 kJ) = (cost, $/gal)/[(HHV, Btu/lb)/10,000] (oil weight, lb/gal). For
the 24° API oil, cost per 10,000 Btu (10,550 kJ) = $0.0825/[(18.941/10,000)

(7.578)] = $0.00574, or 0.574 cent per 10,000 Btu (10,550 kJ). For the 28°
API oil, cost per 10,000 Btu = $0.0910/[(19,157/10,000)(7387)] = $0.00634,
or 0.634 cent per 10,000 Btu (10,550 kJ). Thus, the 24° API is the better buy
because it costs less per 10,000 Btu (10,550 kJ).

Related Calculations. Use this method for engines burning any liquid fuel.

Be certain to check local laws and the latest NBFU recommendations before
ordering fuel storage or day tanks.
Low-sulfur diesel amendments were added to the federal Clean Air Act in

1991. These amendments required diesel engines to use low-sulfur fuel to