Environmental Engineering Reference
In-Depth Information
ascending from these low rates of theoretical interest to
high rates of practical engineering and environmental
concern. Assuming that during the early 2000s about
90% of U.S. energy (@2.7 TW) was dissipated over im-
pervious surfaces, whose area was roughly 113,000 km 2
(see section 11.1), the average power density would
have been roughly 25 W/m 2 . Clearly, densities calcu-
lated specifically for urban areas, highways, and down-
towns must be of the same order of magnitude, and
those of the busiest expressways and central business dis-
tricts (CBDs) densely built up with skyscrapers will go
1 OM higher (see section 9.3).
Residential areas in North America rate mostly at 15-
25 W/m 2 ; less dense CBDs may have densities of about
50 W/m 2 ; cores of high-rise CBDs in cold climates may
easily surpass 500 W/m 2 in winter; the power densities
of the busiest multilane highways may be as high as 300
W/m 2 in traffic jams; and individual skyscrapers will dis-
sipate more than 1 kW/m 2 of their foundations. Crude
oil refineries dissipate 5%-10% of energy in the processed
oil as heat; with throughput densities of 1-7 kW/m 2 ,
this translates to heat losses of 100-700 W/m 2 . Heat re-
jection by steel mills is commonly in the same range, and
high-rise buildings convert up to 3 kW/m 2 of electricity
into heat (fig. 11.4). All these instances are heat dissi-
pations by default, but there are also many carefully
engineered heat rejection devices whose thermal power
densities reach very high levels. Giant cooling towers
and tall stacks are the most prominent objects in this
category.
Cooling towers dissipate typically half of all energy
consumed by thermal power plants, most of it ( > 70%)
as latent heat. This means that large concrete-shell natu-
ral draft units designed to handle 20-40 kW ei /m 2 (see
section 8.5) will be actually rejecting (with electricity
generation efficiency between 33%-40%) as much as
60 kW t /m 2 of their foundations, and mechanical draft
towers will dispose of 100-125 kW t /m 2 . For common
station sizes (500 MW ei -2 GW ei ), this means that one
to four cooling towers serving a power plant will have
an aggregate flux of 700 MW t -3 GW t . The second
largest heat loss in large power plants is in the form of
hot combustion gases that have not transferred their
energy to water-filled tubes inside a boiler. This flux
amounts typically to about 10% of energy content of the
burned fuel. Part of that heat is recovered by a stack
economizer (to preheat feedwater for the boiler) and by
an air heater (to preheat combustion air), but until the
1950s most plants vented the rest through increasingly
tall stacks ( > 200 m-300 m).
Because their top inside diameters are just 3-7 m
(compared to 30-40 m for large natural draft towers),
such stacks reject heat with densities of up to 3-5
MW t /m 2 of their mouths and about 1 MW t /m 2 of
their foundations. But these discharges have changed
with the increasingly frequent use of flue gas desulfuriza-
tion (FGD) (see section 8.5). Flue gases arrive at an FGD
unit at 120 C-150 C, are cooled to saturation tempera-
ture before getting stripped of SO 2 by reaction with alka-
line compounds, and leave the stack at only 45 C-50 C.
As a result, only 1%-2% of the consumed fuel can leave
through a stack, or about 100 kW t /m 2 of chimney foun-
dations. Even the most efficient household natural gas
furnaces now have heat loss power densities higher than
FGD-equipped thermal power plants. These furnaces
need no chimney as they exhaust warm CO 2 -laden air
that contains only 4%-6% of the input energy through a
short plastic pipe protruding from a wall. Assuming neg-
ligible heat loss along the short pipe, the flux is about
150 W/cm 2 (1.5 MW/m 2 ).
Search WWH ::




Custom Search