# Design Point Performance and Optimization of Humid Air Turbine Power Plants
## 3 Results and Discussion
### Impact of Heat Exchanger Effectiveness on Cycle Performance
Hence, the fraction of gas that bypasses the saturator (\(\beta\)) decreases by 19% (Figure 5a) and the humidity at the inlet of the combustion chamber also decreases from 20% to 18% (Figure 5b). The reduction in heat recuperated at low temperature in the cycle, due to the aftercooler, affects the thermal efficiency of the cycle, with a 0.48 pp reduction between cycles \(A_{5}^{40}\) and \(D_{5}^{40}\) (Figure 5c). When the economizer effectiveness reduces from 0.95 to 0.75 (\(A_{5}^{40}\)\(\rightarrow\)\(E_{5}^{40}\)--see Figure 5), the mass flow of the water heated in the heat exchanger falls by 22%, whereas the water outlet temperature falls only by 0.3%--i.e., \(C_{EC}^{*}\) increases. As for the aftercooler, the saturator bypass \(\beta\) decreases by 26% as shown in (Figure 5a) and the humidity at the inlet of the combustion chamber decreases from 20% to 15.7% (Figure 5b). This reduction in heat recuperated in the cycle at low temperature, as for the aftercooler, entails a decrease in thermal efficiency of 1 pp between cycles \(A_{5}^{40}\) and \(E_{5}^{40}\) (Figure 5c). The effectiveness of the intercooler affects the amount of waste heat that is recuperated both at high and low temperatures within the cycle. When \(\varepsilon_{IC}\) becomes 0.75 from 0.95 (\(A_{5}^{40}\)\(\rightarrow\)\(E_{5}^{40}\)--see  
Figure 5: (**a**) normalized heat exchanger effectiveness vs. mass flow fraction through the saturator and aftercooler; (**b**) normalized heat exchanger effectiveness vs. absolute humidity at the inlet of the combustion chamber; and (**c**) normalized heat exchanger effectiveness vs. cycle thermal efficiency—\(OPR\) = 40 & \(\Delta T_{ps}\) = 5 K.  
Figure 5), the total amount of water injected in the saturator increases by 10% due to the increase of heat available in the gas at the inlet of the economizer--i.e., the temperature at the outlet of the recuperator on the hot side increases due to the increase in temperature of the gas at the inlet of the cold side. Therefore, the mass flow ratio of gas that bypasses the saturator decreases by 2% (Figure 5a) and the humidity at the inlet of the combustion chamber increases by 5% (Figure 5b). This highlights the importance of the intercooling process, since it not only reduces the compression work required but also the heat extracted from the gas between the two compressors is partly recovered in the saturator and not rejected entirely as in most common intercooled gas turbines. Thus, the reduction of the effectiveness of the intercooler from 0.95 to 0.75 entails a reduction of 2 pp in the cycle thermal efficiency (\(A_{5}^{40}\)\(\rightarrow\)\(F_{5}^{40}\)--see Figure 5c). The recuperator is the heat exchanger whose effectiveness change has the most severe effects on the thermal efficiency of the cycle, with a reduction of 2.28 pp in thermal efficiency when it is reduced to 0.8 from 0.9 (\(A_{5}^{40}\)\(\rightarrow\)\(G_{5}^{40}\)--see Figure 5), due to the reduction of high quality heat recovery--i.e., heat recovery at high temperature--within the cycle. Nevertheless, this reduction in recuperator effectiveness results in higher enthalpy gas at the inlet of the economizer and entails 22% higher amount of heat transfer in the latter heat exchanger. Therefore, the water mass flow injected into the saturator increases by 10%, the mass flow ratio of gas that bypasses the saturator decreases--i.e., \(\beta\) increases by 4.7% as shown in Figure 5a--and the humidity at the inlet of the combustion chamber increases from 20% to 23% (Figure 5b). Figure 6 shows the impact of the heat exchanger effectiveness on the heat transfer coefficient times the heat transfer area (\(k\times A\)) of each heat exchanger for constant _OPR_ (40) and saturator pinch temperature difference (5 K). For similar inlet conditions, the variation in \(k\times A\) of a heat exchanger is a useful parameter to evaluate the impact of the of the cycle \"design vector\" on the variation in size independently from the design choices in sizing the heat exchanger.