(70) Therefore, the risk of sulphuric acid dew point attack ... - DTI Home

application for which OTSGs may be particularly suitable. The OTSG can be
designed to run dry at full exhaust temperature, removing the need for a
bypass stack and providing a more simple and robust system.
4.3.4 Gas Turbine Inlet Air Chilling
Air intake cooling technology is used to enhance combined cycle power
output and is particularly relevant for utility plant situated in warm climates
where the air is dry and hot. The cooling of the air at the inlet results in a
noticeable increase in mass flow through the gas turbine. This in turn results in
a higher gas turbine power output as well as a slight increase in the steam
production in the downstream HRSG.
The cooling of the inlet air is achieved by means of a refrigeration system
similar to the type employed in large building air conditioning units. The heat
exchanger used to cool the incoming gas turbine air is formed from a coil of
finned tubes located in the inlet housing of the gas turbine. Cooled water is
circulated through the tubes. The prior cooling of the water is achieved by
chillers which are mass produced pieces of equipment and essentially fall into
two categories:-
• Centrifugal chillers
• Absorption chillers
In general, the absorption chiller delivers a lower gross plant power output
than the centrifugal chiller due to the use of steam bleed from the HRSG to
drive it. However in terms of plant net power outputs the two systems are
approximately the same. This is because the centrifugal chiller requires an
electric pump for circulation and therefore consumes a far greater amount of
auxiliary power as opposed to the steam driven circulation for the absorption
method. Further differences between these two systems are described in
greater detail by Elmasri [62] .
Chilling the inlet of a large combined cycle allows extra power to be obtained
from a plant (~ 5% increase in the net kWe output). The cost of that additional
power output is the additional expense of the capital and operational costs of
the upstream chilling unit and other necessary modifications to the gas turbine
itself. These have been estimated by Elmasri [62] at ~$250 per kWe of capacity
gained above the initial hot design condition.
The small-scale OTSG design may also find application in power
augmentation for existing open cycle GT plant by GT air inlet chilling, as
described above. A small OTSG unit can be added to an open cycle GT to
provide steam for an absorption chiller. Again the advantage of the OTSG
design is that protection in the event of a boiler trip is not required as the
OTSG may be designed to run dry at full exhaust temperature.
4.3.5 Increases in Gas Turbine Exhaust Temperature
The pursuit of higher efficiency CCGT plant has driven the rapid increases in
GT exhaust temperature and mass flow rate imposed on HRSGs. The exhaust
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