تجزیه و تحلیل هزینه و اندازه یابی بهینه ساده شده برای مبدل های حرارتی فشرده در VHTR

In this study, the optimum size of the compact heat exchanger has been developed based on its weight and pumping power for the reference design of 600 MWt very high temperature gas-cooled reactor (VHTR) system. Alloy 617 was selected as a construction material. The optimum size and a number of configurations for the reference design of the VHTR with an intermediate heat exchanger (IHX) were investigated and our initial calculations indicated that it has an unrealistically too large aspect ratio of the length and height due to its small-sized channels, which might cause manifolding problems and a large number of parallel modules with high thermal stress. The flow area and channel diameter were then adjusted to achieve a smaller aspect ratio in this paper. Achievement of this aspect ratio resulted in higher cost, but the cost increase was less than would have occurred by simply reducing the flow area by itself. The appropriate channel diameter is estimated to be less than 5.00 mm for the reference system. The effect of channel waviness enhanced the compactness and heat transfer performance, but unfavorably increased the aspect ratio. Therefore, the waviness should be carefully determined based on performance and economics. In this study, the waviness of the IHX is recommended to be selected between 1.0 and 2.5. Calculations show that reducing the duty dramatically decreases the aspect ratio, indicating that the compact heat exchanger is easy to be optimally designed for low duty, but many modules are required for high duty operation proportional to the operating power. Finally, the effect of working fluids was investigated, and it reveals that using carbon dioxide instead of helium in the secondary side reduces the size and cost by about 20% due to the lower pumping power in spite of its lower heat transfer capability by a factor of 4.

The Idaho National Laboratory and the U.S. Department of Energy are developing a Next Generation Nuclear Plant (NGNP) as part of the Generation IV program. A very high temperature reactor (VHTR) with a closed gas turbine cycle is envisioned as one of the most promising nuclear reactor technologies due to its passive safety features and capability to supply high temperature heat to hydrogen production plants and other potential industrial users. The efficiency of the power conversion system (PCS) for the NGNP will be enhanced over those used in the current generation of light water reactors due to the significantly higher outlet temperatures of the VHTR. Besides demonstrating a system design that can be used directly for subsequent commercial deployment, the NGNP will demonstrate key technology elements that can be used in advanced power conversion systems for other Generation IV reactors. In this type of reactor, an intermediate heat exchanger (IHX) transfers heat from the reactor core to an electricity or hydrogen production system. The IHX is a key component because its effectiveness is directly related to the system overall efficiency. In VHTRs, the gaseous coolant (i.e., helium) generally has poor heat transfer capability that requires very large surface area for the given conditions. To meet this large surface area requirement, the compact heat exchanger (CHE), which is widely used in the chemical and petroleum refining industries for gas-to-gas and gas-to-liquid heat exchange, is considered as a potential candidate for an IHX as a replacement for the classical shell and tube type heat exchanger. A compact heat exchanger is arbitrary assumed to be a heat exchanger having a surface area density greater than 700 m2/m3. This high compactness is usually achieved by fins and microchannels, and leads to an enormous heat transfer enhancement with an overall size reduction. For decades, much research has been carried out on the design and performance of compact heat exchangers. Kays and London (1984) have studied and published a book about the various types of the compact heat exchangers. They categorized the configurations of compact heat exchangers and provided experimental data on heat transfer and friction factors. Hesselgreaves (2001) improved the heat transfer and friction factor correlations and provided more generalized forms that considered the geometrical configurations. Dostal et al. (2004) performed simple cost estimation and design calculation for an IHX used with a supercritical CO2 reactor, on the basis of the weight. In their study, a printed circuit heat exchanger (PCHE) manufactured by HEATRIC was employed and estimated extensively. A PCHE is a special type of compact heat exchanger that is manufactured in two steps. First, individual plates are chemically etched to form the flow channels, and then the plates are diffusion bonded together to form a monolithic block. The shapes of channels are generally wavy. Nikitin et al. (2006) experimentally investigated the performance of a PCHE using supercritical CO2. They measured heat transfer and flow data and developed heat transfer and friction factor correlations. In addition, they compared the experimental data with CFD simulations for thermal hydraulic analysis. Song (2005) also performed experiments for a PCHE using air. He obtained data at low Reynolds numbers and developed heat transfer and friction factor correlations. He estimated the adaptability of Hesselgreaves (2001)'s universal correlation to PCHE type heat exchanger. Generally, in the VHTR, the IHX influences plant economics due to its large size and costly materials. In the current study, we focused on the optimum sizing and cost for CHEs. The cost of a heat exchanger can be described as the summation of capital cost and operating cost. The capital cost is associated with the heat exchanger size while the operating cost is associated with pumping power. Generally, the capital and operating costs are negatively correlated. For example, if the size of a heat exchanger is reduced to lower capital cost, operating cost will tend to increase due to increased pressure drop. Therefore, the size of the heat exchanger should be carefully determined from the economic viewpoint. Until recently, much research has been carried out to estimate CHE heat transfer performance and friction loss, but little attention has been given to the optimum size and cost based on performance and economic considerations. In this study, we developed an analytic model for the optimum size of compact heat exchangers, and evaluated them in the context of VHTR systems.

In this study, the optimum size of the compact heat exchanger for VHTR has been investigated from the economic point of view. To avoid complications, we developed an optimum sizing model by analytical method considering only size and friction loss effects, which are considered to be two dominant factors determining heat exchanger cost. For this reason, our model might not give the most accurate optimum design point, but would give quite reasonable prediction even by simple hand calculations, saving computational cost. In this work, we focused on investigating the effects of various design parameters on the optimum design rather than providing the accurate optimum point because the real design and selection process is much more complicated and plant-specific than we could achieve here with generalized calculations. In the larger view, the IHX cost and design are not independent of the full plant economics. Therefore, the optimum size estimated here might be a little bit different from the real optimum point, which considers the full plant economics. Despite that, our model is still considered to be useful for developing preliminary estimations of optimum heat exchanger sizes before detailed plant design can begin. In addition, this model gives good insight on the optimum sizing of compact heat exchangers. Our model was applied to the reference 600 MWt VHTR system. As a result, we could obtain an optimum overall geometry and size of the IHX, but we found that the optimal IHX has a too large aspect ratio with short flow length. This large aspect ratio is due to the size of the microchannels. In compact heat exchangers, the use of microchannels dramatically enhances the surface area density (surface area per unit heat exchanger volume) and heat transfer capability, but consumes more power due to higher pressure drop and fluid friction factors. To increase the aspect ratio, we took two approaches. One was to reduce cross-sectional flow area, and the other was to increase channel diameter. When the flow area was reduced, the compactness could be maintained, but the pressure drop was sharply increased. As a result, at 1.00 of the aspect ratio, the total IHX cost increased up to 100 times, which is not economical. When we increased the channel diameter, the compactness was highly reduced, but the aspect ratio was exponentially decreased. As the aspect ratio becomes 1.00, the total cost is increased only 4 times, which is better than could be achieved by reducing the flow area. Therefore, we conclude that changing channel diameters is the more cost-effective method to achieving a heat exchanger with acceptable size and aspect ratio. However, since the effect of increasing the channel diameter on the heat exchanger aspect ratio is diminished as the channel diameter gets larger, changes in the diameter must be balanced by changes in heat exchanger volumes. For the reference IHX (600 MWt), the recommended channel diameter is below 5.00 mm. The effect of waviness enhanced the compactness and heat transfer, but unfavorable aspect ratio increase was observed. For the reference IHX, the recommended waviness between 1.0 and 2.5. The duty of heat exchanger was also investigated. As the duty decreases, the aspect ratio was dramatically decreased. This means that a optimum compact heat exchanger can be easier to be designed at lower duties. At high duty, as mentioned above, the optimum design requires too large number of modules. Finally we investigated the effect of choosing different working fluids. Using CO2 results in a size and cost reduction of about 20% in comparison to the use of helium coolant. This drop in cost is due to a lower pumping power requirement for CO2.