Page 141 - Handbook of Energy Engineering Calculations

of both the GT and—because of energy added to the GT exhaust—the steam
turbine/generator. Overall, plant output for case 1 is increased by 5.8 MW GT
output + 0.9 MW ST output—plant auxiliary load of 0.9 MW = 6.65 MW, or
3.3 percent. The CC heat rate is improved 0.2 percent, or 15 Btu/kWh (14.2
kJ/kWh). The total installed cost for the evaporative cooling system, based on

estimates provided by contractors and staff, is $1.2 million. The incremental
cost is $1,200,000/6650 kW = $180.45/kW for this ambient condition.
The effectiveness of the same system operating in less-humid conditions—

say 95°F DB (35°C) and 40 percent RH—is much greater. In this case, the
same evaporative cooler can reduce inlet-air temperature to 75°F DB
(23.9°C) by increasing RH to 88 percent. Here, CC output is increased by 7
percent, heat rate is improved (reduced) by 1.9 percent, and the incremental
installed cost is $85/kW, computed as above. As you can clearly see, the

effectiveness of evaporative cooling is directly related to reduced RH.
Water-treatment requirements must also be recognized for this Case, No. 1.
Because demineralized water degrades the integrity of evaporative-cooler

film media, manufacturers may suggest that only raw or filtered water be
used for cooling purposes. However, both GT and evaporative-cooler
suppliers specify limits for turbidity, pH, hardness, and sodium (Na) and
potassium (K) concentrations in the injected water. Thus, a nominal increase
in water-treatment costs can be expected. In particular, the cooling water

requires periodic blowdown to limit solids buildup and system scaling.
Overall, the evaporation process can significantly increase a plant’s makeup-
water feed rate, treatment, and blowdown requirements. Compared to the

base case, water supply costs increase by $15/h of operation for the first
approach, and $20/h for the second, lower RH mode. Disposal of
evaporative-cooler blowdown costs $l/h in the first mode, $2/h in the second.
Evaporative cooling has little or no effect on the design of the steam turbine.

3. Evaluate the economics of inlet-air chilling

The effectiveness of evaporative cooling is limited by the RH of the ambient
air. Further, the inlet air cannot be cooled below the wet-bulb (WB)
temperature of the inlet air. Thus, chillers may be used for further cooling of
the inlet air below the wet-bulb temperature. To achieve this goal, industrial-

grade mechanical or absorption air-conditioning systems are used, Fig. 9.
Both consist of a cooling medium (water or a refrigerant), an energy source