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trials. In practice, this “minimum T” will not measure less than a certain value, so if in any case the chosen LMTD would have forced a minimum T of less than 10°C, the LMTD was increased until a 10°C minimum T was reached. A T of 10°C at the pinch point is fairly typical for large industrial heat exchangers. 2.3 Direct Evaporator Design A successful design of the direct evaporator must satisfy the required duty (i.e., the amount of heat to be transferred per unit time given the inlet temperatures and mass flows), and meet certain constrains specific to the working fluid and application. For extraction of heat from a low-pressure gas by a high-pressure fluid, finned-tube heat exchangers are employed because of their suitable characteristics of low-pressure loss on the gas side along with high-surface-area ratio between the fins, where the heat transfer coefficient is low, and the tube inside, where the heat transfer coefficients of the fluid are typically about two orders of magnitude larger, leading to a high overall heat transfer coefficient with a relatively compact volume. Optimizing the heat exchanger design demands a compromise between size (i.e., capital-intensive heat exchange area), and tolerable pressure losses in each of the fluids streams. However, in this case specific constraints require a distinctive approach in regard to dimensions, geometry, and layout. The primary constraints imposed to the heat exchangers of the direct evaporator by the working fluid are: Limiting working fluid maximum temperature to avoid excessive working fluid degradation Ensuring safety in the event of working fluid leak Observing fin surface temperature lower limit Maintaining TEG temperature above dew point temperature for nitric acid formation (otherwise, cannot use carbon steel tubes) Limiting backpressure from the ORC to within allowable limits to avoid choking the GT. The most severe design constraint is the upper limit imposed upon fluid temperature above which decomposition is accelerated. As the highest fluid temperature is found in the boundary layer of the fluid close to the wall of an externally heated duct, the inside wall temperature of all heat exchanger pipes must remain below this temperature limit at all times. The thermal stability of the fluid determines the lifetime of the working fluid, affecting life-cycle cost, and has safety implications if undesirable chemical decomposition products are generated. The dehydrogenation reaction results in hydrogen evolution that, since hydrogen in non-condensable, dramatically reduces expansion pressure ratio, maximum output power and efficiency. Longer-chain hydrocarbons may form, which can leave a gummy or coke type of residue that is deleterious to system components (especially the pump and heat exchanger). Undesired reaction products, including non-condensables, should be periodically or continuously removed from the heat transfer loop. Avoiding oxygen ingress into some working fluid is critical, since experiments conducted at INL show that decomposition products (measured in solution) increase five-fold upon a bulk temperature increase from 300°C to 350°C, whereas solid product deposition is three times higher. Also, avoiding materials of construction or contaminants that contain catalysts that can promote working fluid degradation is recommended. 2.3.1 Strategies for Tube Wall Temperature Control The inside tube wall temperature depends on the fluid bulk temperature, the heat transfer coefficient, and the heat flux. All three of these parameters are designed such that the inside wall temperature remains below 300°C. 14PDF Image | Final Report Modifications and Optimization of the Organic Rankine Cycle to Improve the Recovery of Waste Heat
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