Heat exchanger performance

Planti presented a technique for comparative heat exchange performance evaluation that is based on his efficiency method and can include almost any style and application of exchanger. 

Tinker, T. Proceedings of the General Discussion onHeat Transfer, September, 1951, p. 89. Analysis of the fluid flow pattern in shell and tube exchangers and the effect of flow distribution on the heat exchanger performance, (Inst, of Mech. Eng. and Am, Soc. Mech. Eng.). 

Table 12.1 Heat-exchange performance of different reactors [10],... 

It is important to note that the compacityfactor is defined by the ratio of the surface area offered to heat transfer over the volume of the reactive medium. The thermal performances are estimated from the product between this compacity factor and the global heat-transfer coefficient. Consequently, owing to the large value of this factor combined with the conductivity performances of the SiC material, the heat-exchange performances are expected to be very high, which can be noticed from the last column of this table. 

The figure shows results for both HTE and low-temperature electrolysis (LTE). In addition, an efficiency curve for the SI thermochemical process is shown (Brown, 2003). The results presented in Figure 3 indicate that, even when detailed process models are considered, with realistic component efficiencies, heat exchanger performance, and operating conditions, overall hydrogen production efficiencies in excess of 50% can be achieved for HTE with reactor outlet temperatures above 850°C. For reactor outlet temperatures in the range of 600-800°C, the supercritical C02/recompression power... 

Fig. 1.3. The effect of heat exchanger performance, NTU, and heat capacity ratio, h, on the normalized heat loss for heat-integrated processes in (a) countercurrent and (b) counter-cocurrent flow configuration.

Bejan, A. "General Criterion for Rating Heat-Exchanger Performance, Int. J. Heat and Mass Transfer, 21, 5, 655-658, 1978... 

Knowing the efficiency, one can use this value to predict heat exchanger performance for other streams and fluids. Efficiency is based on the maximum amount of heat that can be transferred ... 

Examples 10-4 to 10-8 illustrate the use of the LMTD for calculation of heat-exchanger performance. 

Why does a mixed or unmixed fluid arrangement influence heat-exchanger performance ... 

In order to understand the influence of fouling on compact heat exchanger performance, the following equations for h and Ap are derived from the equations presented earlier for fully developed gas flow in a circular or noncircular tube ... 

R. K. Shah and A. Pignotti, The Influence of a Finite Number of Baffles on the Shell-and-Tube Heat Exchanger Performance, Heat Transfer Eng., Vol. 18, No. 1, pp. 82-94,1997. 

G. D. Bahnke and C. P. Howard, The Effect of Longitudinal Heat Conduction on Periodic-Flow Heat Exchanger Performance, ASME J. Eng. Power, Vol. 86A, pp. 105-120,1964. 

K. Chowdhury and S. Sarangi, The Effect of Flow Maldistribution on Multipassage Heat Exchanger Performance, Heat Transfer Eng, Vol. 6, No. 4, pp. 45-54,1985. 

The purpose of any fouling model is to assist the designer or indeed the operator of heat exchangers, to make an assessment of the impact of fouling on heat exchanger performance given certain operating conditions. Ideally a mathematical interpretation of Equation 4.6 would provide the basis for such an assessment but the inclusion of an extensive set of conditions into one mathematical model would be at best, difficult and even impossible. 

In terms of their effect on heat exchanger performance the measurement of heat transfer reduction or increase in pressure drop provide a direct indication. The simple methods of measuring deposit thickness described earlier are useful, but in general they require that the experiment is terminated so as to provide access to the test sections. Ideally non-intrusive techniques would allow deposition to continue while the experimental conditions are maintained without disturbance. Such techniques include the use of radioactive tracers as already referred to earlier... 

There are in general, two ways in which plant data on fouling may be directly obtained. They involve an assessment of heat exchanger performance and the use of a side stream of process fluid. The use of probes may provide an indirect method of fouling evaluation. 

An improvement on the assessment of individual heat exchanger performance is to pass a sidestream through some kind of monitor. In general monitors resemble the test equipment used in the laboratory. The use of monitors allows a more careful control of the fouling conditions to be exercised particularly in terms of temperature, temperature changes and flow rates, using actual plant streams. The data obtained are more reliable than those obtained directly from the operation of full scale plant. 

The heat exchanger performs two-sided energy and material balance calculations. The heat exchanger is capable of solving the temperatures, pressmes, heat flows (including heat loss and heat leak), material stream flows, and UA. 

Increasing heat transfer coefficients in heat exchangers has been researehed quite extensively, recently reviewed by Anxionnaz et al. [10]. Plate heat exchangers performance can be improved by different surface effects (corrugation), extended surfaces (e.g. plate-fin HX) or inserts (e.g. foams). This last class of heat exchangers with inserts (plate reaetors) ean be used as reactors, as the inserts not only enable increased heat transfer but also better mixing behaviour [10],... 

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