In ATAER Power Plant, fresh air from the outside environment enters into the compressor with the outside temperature, but especially in hot days, the efficiency of the plant decreases thus air is conditioned by evaporative cooling and filtered by air filters before entering into the air compressor. After compression stage, compressed air goes into burning reaction with the natural gas in combustion chamber. As a consequence of the burning reaction, the temperature of combustion gases varies between 850 and 870 1C. The combustion gases expand while leaving gas turbine unit, whose technical specifications is shown in Table 1 [27], and the temperature of combustion gases decrease to between 450 and 460 1C. In order to benefit from high energy and enthalpy rates of those exhaust gases leaving gas turbine unit, heat recovery steam generators are used in the plant. In investigated powerplant there are two different types of heat recovery units. These two units have different construction types, while one of them is heat recovery steam generator and the other one is once through steam generator. Although both of the units are used for producing superheated vapour these units differ from each other with their construction types. While heat recoverysteam generator (HRSG) is constructed with a part called drum which is used to stock certain amount of the produced steam, once through steam generator (OTSG) does not have drum part. The HRSG unit of ATAER Power Plant has multiple pressure levels, which are HP (5 MPa) and IP/LP (0.5 MPa) respectively, for steam production and does not have preheater section. In Fig. 2 [30], HRSG unit of the power plant is shown. In OTSG unit of the power plant, exhaust gases, which come from gas turbine, pass over the tubes carrying the feedwater,