Page 189 - Handbook of Energy Engineering Calculations

gallons (L) per minute, depending on the designer’s choice.
Since water has a specific heat of unity, the heat-recovery potential of the
jacket water is H = w(Δt ), where w = weight of water flow, lb/h (kg/h);Δt w
w
w
= change in temperature of the jacket water when flowing through the heat
exchanger,°F (°C). Thus, if the jacket-water flow is 25,000 lb/h (11,350 kg/h)
and the temperature change during flow of the jacket water through and
external heat exchanger is 190 to 70°F (88 to 21°C), the heat given up by the

jacket water, neglecting losses, is H = 25,000(190 − 70) = 3,000,000 Btu/h
w
(879 MW). During 25 hours the heat recovery will be 24(3,000,000) =
72,000,000 Btu (75,960 MJ). This is a significant amount of heat which can

be used in process or space heating, or to drive an air-conditioning unit.
If the jacket-water flow rate is expressed in gallons per minute instead of
pounds per hour (L/min instead of kg/h), the heat-recovery potential, H =
wg
gpm(Δ t)(8.33) where 8.33 = lb/gal of water. With a water flow rate of 50
gpm and the same temperature range as above, H = 50(120)(8.33) = 49,980
wg
Btu/min (52,279 kJ/min).

4. Find the amount of heat recoverable from the lube oil
During I-C engine operation, lube-oil temperature can reach high levels—in
the 300 to 400°F (149 to 201°C) range. And with oil having a typical specific

heat of 0.5 Btu/lb°F (2.1 kJ/kg°C), the heat-recovery potential for the lube oil
is H = w (Δt)(c ), where w = oil flow in lb/h (kg/h);Δt = temperature
o
wo
o
o
change of the oil during flow through the heat-recovery heat exchanger = oil
inlet temperature − oil outlet temperature,°F or°C; c = specific heat of oil =
o
0.5 Btu/lb°F (kJ/kg°C). With an oil flow of 2000 lb/h (908 kg/h), a
temperature change of 140°F (77.7°C), H = 2000(140) (0.50) = 140,000
o
Btu/h (41 kW). Thus, as mentioned earlier, the heat recoverable from the lube
oil is usually the lowest of the three sources.

With the heat flow rates computed here, an I-C engine cogeneration facility
can be easily justified, especially where frequent startups and shutdowns are
anticipated. Reciprocating diesel engines are preferred over gas and steam

turbines where frequent startups and shutdowns are required. Just the fuel
savings anticipated for recovery of heat in the exhaust gases of this engine
could pay for it in a relatively short time.