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The interesting point to note is that some of the major manufacturers are switching away from corrosion protection biased coatings over towardscoatings， which are not only oxidation resistant， but also oxidation resistant at higher metal temperatures. Thermal barrier coatings are being used on the first few stages in all the advanced technology units. The use of internal coatings is getting popular due to the high temperature of the compressordischarge， which results in oxidation of the internal surfaces.孔ost of these coatings are aluminide type coatings. The choice is restricted due to accessproblems to slurrybased， or gas phasejchemical vapor deposition. Caremust be taken in production， otherwise internal passages may be blocked. The use of pyrometer technology on some of the advanced turbines has located blades with internal passages blocked causing that blade to operate at temperatures of 95-158 0F (35-70 0C).
Gas Turbine .eat .ecovery
The waste heat recovery system is a critically important subsystem of acogeneration system. In thepast， it was viewed as a separate ""add-on"" item.This view is being changed with the realization that good performance， boththermodynamically and in terms of reliability， grows out of designing the heat recovery system as an integral part of the overall system.
The gas turbine exhaust gases enter the Heat Recovery Steam Generating(HRSG)， where the energy is transferred to the water to produce steam.There are many different configurations of the HRSG units.孔ost HRSG units are divided into the same amount of sections as the steam turbine. Inmost cases， each section of the HRSG has a Pre-heater， an Economizer andFeed-water， and then a Superheater. The steam entering the steam turbine is superheated.
The most common type of an HRSG in a large Combined Cycle Power is the drum type HRSG with forced circulation. These types of HRSGs arevertical， the exhaust gas flow is vertical with horizontal tube bundles sus-pended in the steel structure. The steel structure of the HRSG supports thedrums. In a forced circulation HRSG， the steam water mixture is circulated through evaporator tubes using a pump. These pumps increase the parasitic load and thus detract from the cycle efficiency. In this type of HRSG theheat transfer tubes are horizontal， suspended from un-cooled tube supports located in the hot gas path. Some vertical HRSGs are designed with evap-orators， which operate without the use of circulation pumps.
The 0nce Through Steam Generators (0TSG) are finding quick accept-ance due to the fact that they have smaller foot prints， and can be installed in a much shorter time and lower price. The 0nce Through Steam Generatorsunlike other HRSGs do not have defined economizer， evaporator， or super-heater sections. Figure 1-39 is the schematic of an 0TSGsystem， and a drum-type HRSG. The 0TSG is basically one tube; water enters at one endand steam leaves at the other end， eliminating the drum and circulationpumps. The location of the water to steam interface is free tomove， depend-ing on the total heat input from the gas turbine， and flow rates and pressuresof the Feedwater， in the tube bank. Unlike other HRSGs， the once-through units have no steam drums.
Figure 1-.9. Comparison of a drum type .RS. to a once throu.h steam .enerator. (Courtesy Innovative Steam Technolo.ies.)
Some important points and observations relating to gas turbine waste heat recovery are:
Multipressure Steam Generators-These are becoming increasinglypopular. With a single pressure boiler， there is a limit to the heat recovery because the exhaust gas temperature cannot be reduced below the steam saturation temperature. This problem is avoided by the use of multipressure levels.
Pinch Point-This is defined as the difference between the exhaust gas temperature leaving the evaporator section and the saturation temperatureof the steam. Ideally， the lower the pinchpoint， the more heat recovered，but this calls for more surface areaand， consequently， increases the back-pressure and cost.Also， excessively low pinch points can mean inadequate steam production if the exhaust gas is low in energy (low mass flow or low exhaust gas temperature). General guidelines call for a pinch point of 15-40 0F(8-22 0C). The final choice is obviously based on economic considerations.
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