Page 188 - Handbook of Energy Engineering Calculations

Calculation Procedure:

1. Determine the sources of waste heat available in the typical I-C engine

There are three primary sources of waste heat available in the usual I-C
engine. These are: (1) the exhaust gases from the engine cylinders; (2) the
jacket cooling water; (3) the lubricating oil. Of these three sources, the
quantity of heat available is, in descending order: exhaust gases; jacket

cooling water; lube oil.

2. Show how to compute the heat recoverable from each source
For the exhaust gases, use the relation, H = W(Δt)(c ), where W = rate of
g
A
A
gas flow from the engine, lb/h (kg/h);Δ t = temperature drop of the gas
between the heat exchanger inlet and outlet,°F (°C); c = specific heat of the
g
gas, Btu/lb°F (J/kg°C). For example, if an I-C engine exhausts 100,000 lb/h

(45,400 kg/h) at 700°F (371°C) to a HRSG (heat-recovery steam generator),
leaving the HRSG at 330°F (166°C), and the specific heat of the gas is 0.24
Btu/lb°F (1.0 kJ/kg°C), the heat recoverable, neglecting losses in the HRSG

and connecting piping, is H = 100,000(700 − 330) (0.24) = 8,880,000 Btu/h
A
(2602 MW).
With an average heat of vaporization of 1000 Btu/lb (2330 kJ/kg) of steam,

this exhaust gas flow could generate 8,880,000/1000 = 8880 lb/h (4032 kg/h)
of steam. If oil with a heating value of 145,000 Btu/gal (40,455 kJ/L) were
used to generate this steam, the quantity required would be
8,880,000/145,000 = 61.2 gal/h (232 L/h). At a cost of 90 cents per gallon,

the saving would be $0.90(61.2) = $55.08/h. Assuming 5000 hours of
operation per year, or 57 percent load, the saving in fuel cost would be
5000($55.08) = $275,400. This is a significant saving in any plant. And even
if heat losses in the ductwork and heat-recovery boiler cut the savings in half,

the savings would still exceed $100,000 a year. And as the operating time
increases, so too do the savings.

3. Compute the savings potential in jacket-water and lube-oil heat recovery
A similar relation can be used to compute jacket-water and lube-oil heat

recovery. The flow rate can be expressed in either pounds (kg) per hour or