System heat exchange design

The combined influence of the thermodynamic, chemical/kinetic, physical properties, and application environment define the heat exchange requirements for the entire system. With this information the system designer is able to evaluate various methods for heat transfer within the system to obtain an overall optimized design based on the end-use requirements. 

This is not a comprehensive list of examples, and the designer may determine that a hybrid, or combinatorial, solution is appropriate, depending on the application. 

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Considerable interest has been generated in turbulence promoters for both RO and UF. Equations 4 and 5 show considerable improvements in the mass-transfer coefficient when operating UF in turbulent flow. Of course the penalty in pressure drop incurred in a turbulent flow system is much higher than in laminar flow. Another way to increase the mass-transfer is by introducing turbulence promoters in laminar flow. This procedure is practiced extensively in enhanced heat-exchanger design and is now exploited in membrane hardware design. 

Fig. 11. Liquid recirculation in a flooded system where A = refrigerant circulated through coils as brine, B = direct expansion coils, C = flooded evaporators, andD = special heat-exchanger designs.

The engineering challenges include heat exchanger design, performance and accommodation of high pressures, temperatures and thermal stresses. If successfully developed the technology could be applied in the liquefaction of natural gas to provide a low-cost alternative to diesel fuel. So far one unit is reported built having a liquefaction capacity of about 35 kg/h. In this unit, 30% of the input natural gas stream was consumed as heat input, with a 70% yield of LNG. A future system with a capacity of about 700 kg/h LNG and with a projected liquefaction rate of 85 % of the input gas stream is under development. 

Additionally, highly efficient heat exchangers are required in fuel processors and the micro si rue lured plate heat exchanger design seems to be the best solution so far to maintain the crucial system efficiency competitive. 

Polymers are a viable material option for the fabrication of highly efficient low-temperature heat exchangers, designed to withdraw the last portion of energy out of the fuel processor off-gases before releasing them to the environment For small-scale MEMS-like systems, the materials commonly applied in this field are silicon [20, 71] (as a material with high heat conductivity) and silicon nitride [71] (as an insulation material). 

The results presented In Table III show that as the capital Investment in the tower (Ztower) increases at larger tower heights, the available-energy destruction decreases. Thus the optimal design reflects the classical trade-off between capital investment and fuel cost. It is important to note that heat exchanger design plays a major role in separation systems (16). 

Two ways to control the outlet temperature of a heat exchanger cooler are sketched on the following page. Comment on the relative merits of these two systems from the standpoints of both control and heat exchanger design. 

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