# Design Point Performance and Optimization of Humid Air Turbine Power Plants
## Appendix B Heat Exchanger Design  
A counter-current plate-fin architecture was chosen for the intercooler and aftercooler design. For the airside, a serrated fin arrangement was selected based on its heat transfer and mechanical properties as discussed in [31]. On the waterside, stainless steel plain fins were employed to minimize pumping work, decrease the risk of fouling or passage blocking and avoid potential corrosion problems. The effectiveness-Number of Transfer Units (\(\varepsilon\)-NTU) method was followed to estimate the heat transfer area as described by [PERSON] and [PERSON] [32]. The number of transfer units was obtained by means of the correlations provided by the Engineering Society Data Unit (ESDU) in [33]. To obtain the main heat exchanger dimensions, the heat transfer coefficient of the airside was estimated using the expressions provided by [PERSON] and [PERSON] [34]. For the waterside, the equations suggested in [35] were employed to evaluate the heat transfer coefficient for turbulent flow, and the equations in [36] were utilized for the laminar flow. The recuperator is an air-air heat exchanger and, as such, a serrated-fin configuration was considered for both sides. In addition, the recuperator was arranged in a counter-current multi-pass cross-flow configuration. Due to the presence of the reheater, the maximum temperatures that the recuperator has to be able to stand can go up to 900 \({}^{\circ}\)C. Previous research by [PERSON] [37], [PERSON] [38], and [PERSON] [39] has stated that Inconel 625 presents excellent creep resistance properties. Therefore, the passes with a temperature higher than 800 \({}^{\circ}\)C were assumed to be made of Inconel 625, whereas the rest of the heat exchanger material was assumed to be stainless steel. For the calculation of the heat transfer area, the \(\varepsilon\)-NTU method was followed [32]. The number of transfer units was obtained by means of the ESDU correlations [33] and the heat transfer coefficient was estimated with the equations purposed by [PERSON] and Li [34]. In the economizer, up to 25% percent of the moisture in the air condenses during design point operation. Hence, to avoid issues associated with passage blocking due to the presence of contaminated air and water inside its passages, a mixture of finned-tube and plain-tube configuration was selected. The first part of the heat exchanger (dry part), where no condensation takes place, was designed using a finned-tube configuration. The later part (wet part), where the condensation begins to occur, was designed in plain-tube to ease the flow of the condensing moisture. The tubes are arranged in a counter-current multi-pass cross-flow configuration, with the air flowing over the tubes and the water through them in an effort to minimize passage blocking and fouling and ease maintenance procedures. The \(\varepsilon\)-NTU method [32] was followed to calculate the number of transfer units. Then, to calculate the heat transfer coefficient of the dry airside flow, the correlations given in the ESDU items [40] were utilized. However, in the wet part, the heat and mass transfer process was simulated following the model developed in [41]. The heat transfer coefficient of the water was evaluated as dictated in the ESDU items [35]. An index of the acquisition cost of the four heat exchangers specified above was estimated using heat transfer area based correlations. For the plate-fin geometries, the expressions proposed by [PERSON] and [PERSON] [42] were used. These correlations were originally derived for heat exchangers made of aluminum alloys; consequently, the material correction factors suggested by [PERSON] and [PERSON] [43] were applied to consider the different alloys employed. For the economizer, the correlation presented by [PERSON] [44] was utilized for estimating its acquisition price. All cost estimates were updated to 2015 by means of the Chemical Engineering Plant Cost Index (CEPCI) [45].