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To enhance the internal heat transfer coefficient, the velocity of the fluid inside the tube needs to be high, putting a limit on the number of parallel tube passes, at the same time necessitating a certain pressure loss to be tolerated even when the tube length is kept as short as the aspect ratio and manufacturing costs allow. Internal heat transfer coefficients are highest in the evaporator section, where a nucleate and convective boiling regime prevails. The lowest heat transfer coefficients are found in the economizer section. The working fluid bulk temperature design point at the heat exchanger outlet was set to 250°C to allow ample margin for the wall temperature to stay below 300°C. The highest fluid temperature, 230°C–250°C, is encountered in the superheater section of the boiler. Within the GE proprietary concept of “protective staging,” the superheater section is placed in the exhaust gas stream behind the evaporator section (see Figure 14), where the exhaust gas is already cooled although still hot enough to ensure reasonable heat transfer performance in a counterflow layout. The evaporator section is placed first in the exhaust stream in a parallel flow arrangement, such that the highest exhaust gas temperature is met by a fluid bulk temperature at the boiling point, but at low vapor quality. In this regime the heat transfer coefficients are highest and can only increase with the heat flux. The danger of local wall dryout (that leads to sharp wall temperature spikes as the heat transfer coefficient collapses) is minimal because of the low vapor quality. As the vapor quality increases along the evaporator, the exhaust gas temperature and the heat flux decrease, by the time dryout is expected, the high-volume flow rate has increased the gas phase heat transfer coefficient while the exhaust gas temperature is low enough so that the wall temperature limit is not exceeded. The implementation of such a concept is schematically shown in Figure 15. A further measure to limit the heat flux and decrease the inner wall temperature where it can become critical is to reduce the number and area of the fins on the tube outside. This concept of “variable finning” of the tubes (see Figure 16) throughout the length of the heat exchanger includes smaller and less densely spaced fins in the superheater and evaporator section where the temperatures are high and the internal heat flux needs to be limited. Figures 16 (a) through (d) show four different types of fins (internal and external) that can be used in the different sections of the Direct Evaporator. However, in the economizer section where the temperatures are lower and the wall temperature is not critical, larger diameter and serrated fins with tighter fin spacing are used to improve heat transfer performance and reduce the number of rows necessary to achieve the required duty. Working Fluid ~110°C Preheater (Counter Flow) Heat Source / Hot Exhaust Evaporator (Parallel Flow) Superheater (Counter Flow) ~230°C ~230°C Figure 14. Concept of protective staging applied to a gas turbine exhaust stream. 15PDF Image | Final Report Modifications and Optimization of the Organic Rankine Cycle to Improve the Recovery of Waste Heat
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