POWER February 2016 - 38

HEAT EXCHANGERS
5. Efficient condensation. An uncoated copper condenser tube (top left) is shown next
to a similar tube coated with graphene (top right). When exposed to water vapor at 100C, the
uncoated tube produces an inefficient water film (bottom left), while the coated tube exhibits
the more desirable dropwise condensation (bottom right). Courtesy: MIT
dropwise condensation, while offering chemical
stability and low thermal resistance. The
idea is not completely new; polymer coatings
have been used in the past to enhance droplet
formation, but those coatings tended to degrade
rapidly. When made thicker to compensate
for the degradation, the heat transfer
improvement was negated.
Graphene is the thinnest material known
to man-just one atom thick-but it is also
incredibly strong (about 200 times stronger
than steel). It is very flexible and it is also
an excellent conductor of heat and electricity,
but it isn't cheap. At a price of roughly
$60 per square inch, graphene is currently
too expensive to justify the cost of coating
condenser tubes.
However, the MIT team demonstrated
A Changing Landscape: Air-Cooled Condensers
Air-cooled condensers (ACCs) have become
fairly common in the power generation industry.
China alone has more than 130 GW of
air-cooled coal-fired capacity in service, with
some estimates suggesting that the country's
installed capacity will double by 2020.
In the past, two companies-SPX Cooling
Technologies and GEA Heat Exchangers-
have dominated the ACC market. Recently,
both have experienced major shakeups. On
November 30, the former Power Cooling
Solutions division of GEA, which includes
its ACC product line, changed its name
to ENEXIO as part of a rebranding effort
following a change in ownership. Exactly
two weeks later, SPX announced that it
has agreed to sell its Dry Cooling business
to Kolkata, India-based Paharpur Cooling
Towers Ltd.
If that deal is finalized, as expected by
mid-2016, one of the products Paharpur
will acquire is SPX's ModuleAir ACC (Figure
6). The design is considered a big improvement
over previous ACC models. Construction
costs for the modular style are said
to be up to 25% less than for a standard
A-frame style. The reason is that heat exchanger
bundles, ducting, and structural
components are all factory assembled,
so there is less onsite welding required,
38
6. Time is money. ModuleAir's factoryassembled
components reduced onsite-welding
requirements, speeding the construction
process at the Lisahally biomass-fired power
plant, shown here, in Northern Ireland. Courtesy:
SPX Cooling Technologies
that a graphene coating could enhance heat
transfer fourfold compared to filmwise condensation
(Figure 5), and by some estimates
that could lead to a 2% to 3% improvement
in overall power plant efficiency. Improving
efficiency by that amount would result in annual
fuel savings of more than $1 million for
the average coal-fired power plant. It could
also help plants reduce emissions and meet
Clean Power Plan requirements.
Therefore, when the price of graphene decreases-which
could occur in the next couple
of years as production capacity increases
and production costs decline-graphenecoated
condenser tubes could find a market
eager for the upgrade. And the researchers
aren't done yet; they remain hopeful that by
optimizing operating conditions, the heat
transfer recorded through graphene-coated
tubes could be five to seven times better than
that of noncoated tubes.
which permits faster installation.
Although the ModuleAir ACC operates on
the same working principle as a standard
ACC, integrated steam headers and condensate
manifolds eliminate tube sheet
welding in the field. Its A-frame bundles
are much shorter than classic bundles
too-only two meters rather than 11 meters
long. The result is not only a reduced
height and smaller footprint, but also improved
heat transfer. Steam velocity and
steam-side pressure are reduced in the
heat exchanger, enhancing annual average
plant output.
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Alternative Solutions
With water always a concern for power plants,
some facilities have turned to air-cooled heat
exchanger solutions (see sidebar). HTRI has
its own air-cooled heat exchanger research
program, and the company has conducted
testing on elliptical tubes-used in some aircooled
condensers-using its low-pressure
condensation unit (LPCU).
" There is still a lot to learn about air-cooled
exchanger performance, especially related to
fans off operation, air recirculation, and wind
effects, " Schroeder said.
In addition to the LPCU, HTRI has nine
other operating research units, including
high-temperature fouling units, a liquid-liquid
heat exchanger test unit, a multipurpose
boiling unit, a multipurpose condensation
unit, and a multipurpose visualization unit
(MVU). The MVU allows qualitative and
quantitative flow visualization of two-phase
air/water flows in process heat exchanger
equipment. The test section of the unit is a
POWER | February 2016

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POWER February 2016

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