Page 143 - Handbook of Energy Engineering Calculations

A centrifugal-compressor-based chiller can easily reduce the temperature
of the GT inlet air from 95°F (35°C) to 60°F (15.6°C) DB—a level that is
generally accepted as a safe lower limit for preventing icing on compressor
inlet blades—and achieve 100 percent RH. This increases plant output by
20.2 MW for GT + 2.4 MW for ST − 4.5 MW plant auxiliary load = 18.1

MW, or 8.9 percent. But it degrades the net CC heat rate by 0.8 percent and
results in a 1.5-in-(3.8-cm)-H O inlet-air pressure drop because of heat-
2
exchanger equipment located in the inlet-air stream.
Cooling requirements of the chilling system increase the plant’s required
3
circulating water flow by 12,500 gal/min (47.3 m /min). Combined with the
need for increased steam condensing capacity, use of a chiller may

necessitate a heat sink 25 percent larger than the base case. The total installed
cost for the mechanical chilling system for Case 2 is $3-million, or about
$3,000,000/18,100 kW = $165.75/kW of added output. Again, costs come

from contractor and staff studies.
Raw-water consumption increase the plant’s overall O&M costs by $35/h
when the chiller is operating. Disposal of additional cooling-tower blow-
down costs $17/h. The compressor used in Case 2 consumes about 4 MW of
auxiliary power to handle the plant’s 68-million Btu/h (19.9 MW) cooling

load.

4. Analyze an absorption chilling system
Absorption chilling systems are somewhat more complex than mechanical
chillers. They use steam or hot water as the cooling motive force. To achieve

the same inlet-air conditions as the mechanical chiller (60°F DB, 100 percent
RH) (15.6°C, 100 percent RH), an absorption chiller requires about 111,400
2
lb/h (50,576 kg/h) of 10.3-lb/in (gage) (70.9-kPa) saturated steam, or 6830
3
gal/min (25.9 m /min) of 370°F (188°C) hot water.
Cost-effective supply of this steam or hot water requires a redesign of the
reference plant. Steam is extracted from the low-pressure (l-p) steam turbine
2
at 20.3 lb/in (gage) (139.9 kPa) and attemperated until it is saturated. In this
case, the absorption chiller increases plant output by 8.7 percent or 17.4 MW
but degrades the plant’s heat rate by 1 percent.
Although the capacity of the absorption cooling system’s cooling-water

loop must be twice that of the mechanical chiller’s, the size of the plant’s