Evaporative gas turbines

The work required to drive the turbine eompressor is reduced by lowering the compressor inlet temperature thus increasing the output work of the turbine. Figure 2-35 is a schematic of the evaporative gas turbine and its effect on the Brayton cycle. The volumetric flow of most turbines is constant and therefore by increasing the mass flow, power increases in an inverse proportion to the temperature of the inlet air. The psychometric chart shown shows that the cooling is limited especially in high humid conditions. It is a very low cost option and can be installed very easily. This technique does not however increase the efficiency of the turbine. The turbine inlet temperature is lowered by about 18 °F (10 °C), if the outside temperature is around 90 °F (32 °C). The cost of an evaporative cooling system runs around 50/kw. 

Fig. 6. lb shows diagrammatically the evaporative gas turbine (EGT) in which water is injected into the compressor outlet and is evaporated there the mixture may then be further heated in the cold side of a heat exchanger. It enters the combustion chamber and then pas.ses through the turbine and the hot side of the heat exchanger. 

Horlock. J.H. (1998), Heal exchanger performance with water injection (with relevance to evaporative gas turbine (EGT) cycles). Energy Conver Mginl 19( 16-18). 1621 - 1630. 

De Ruyck, J., Allard, G. Maniatis K, (1996), An externally fired evaporative gas turbine cycle for small scale biomass CHP production.. In Proceedings 9th European Bioenergy conference, Copenhagen (Ed. P. Chattier et al.) Pergamon, Oxford. 

Overlay coatings onto gas-turbine blades and vanes of M—Cr—Al—Y type alloys by electron beam evaporation is estimated at 10 x 10 to coat 200,000 parts at an average cost of 50 per part. 

Coal is fed as a paste containing 25 wt % water, and sorbent is fed diy by a lock-hopper system with pneumatic conveying. The top size of each feedstock is 3 mm in). The latent heat lost evaporating the water fed with the paste is compensated by increased gas turbine power output resulting from the increased flue-gas mass flow rate. For the 80-MWe unit, there are six coal feed points (one per 4.5 m" [48 ft"]) and four sorbent feed points (one per 6.7 m" [72 ft"]), all entering beneath the tube bank along one wall. The bed depth is... 

The gas turbine is a high-volume air machine. The compressor air power required is usually between 50-70 percent of the total power produced by the turbine. Thus, the ambient temperature affects the output of the gas turbine. On hot days, the gas turbine produces less output than on cold days. In dry climates, the use of evaporative cooling in the gas turbine decreases the effective inlet temperature and increases the power output of the unit. 

Evaporative Cooling of the Turbine. Traditional evaporative coolers that use media for evaporation of the water have been widely used in the gas turbine industry over the years, especially in hot climates with low humidity areas. The low capital cost, installation, and operating costs make it attractive for many turbine-operating scenarios. Evaporation coolers consist of water being sprayed over the media blocks, which are made of fibrous corrugated material. The airflow through these media blocks, evaporates the water, as water evaporates, it consumes about 1059 BTU (1117kJ) (latent heat of vaporization) at 60 °F (15 °C). This results in the reduction... 

Refrigerated Inlets for the Gas Turbines. The refrigerated inlets are more effeetive than the previous evaporative eooling systems as they ean lower the temperatures by about 45-55 °F (25-30 °C). Two teehniques for refrigerating the inlet of a gas turbine are vapor eompression (meehanieal refrigeration) and absorption refrigeration. 

Figure 2-44. Evaporative eooiing and steam injeetion in a gas turbine.

Evaporators require a source of heat to operate. This heat may be supplied from a boiler, gas turbine, vapor compressor, other evaporator, or a combination of sources. Multiple effect evaporators are very popular when cheap, high pressure steam is available to heat the system. A Mechanical Vapor Recompression evaporator would use electricity or a gas turbine to drive a compressor that recycles the heat in the evaporator. 

In the search for higher plant thermal efficiency, the simplicity of the two basic STIG and EGT cycles, as described by Frutschi and Plancherel, has to some extent been lost in the substantial modifications described above. But there have been other less complex proposals for water injection into the simple unrecuperated open cycle gas turbine one simply involves water injection at entry to the compressor, and is usually known as inlet fog boosting (IFB) the other involves the front part of an RWI cycle, i.e. water injection in an evaporative intercooler, usually in a high pressure ratio aero-derivative gas turbine plant. 

De Ruyck, J., Bram, S. and Allard, G. (1997), REVAP cycle A new evaporative cycle without saturation tower, ASME J. Engng Gas Turbines Power 119, 893-897. 

Utamura, M., Takaaki, K., Murata, H. and Nobuyuki, H. (1999), Effects of intensive evaporative cooling on performance characteristics of land-based gas turbine, PWR-Vol. 34, Joint Power Generation Conference, pp. 321-328. 

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ). ... 

Dual pressure For comparison, a combined cycle scheme with dual pressure is shown in Figure 15.13. In this case, the waste heat recovery boiler also incorporates a low-pressure steam generator, with evaporator and superheater. The LP steam is fed to the turbine at an intermediate stage. As the LP steam boils at a lower temperature than the HP steam, there exists two pinch points between the exhaust gas and the saturated steam temperatures. The addition of the LP circuit gives much higher combined cycle efficiencies with typically 15 per cent more steam turbine output than the single pressure for the same gas turbine. 

---