Heat exchangers thermal performance

Fypass Flow Effects. There are several bypass flows, particularly on the sheUside of a heat exchanger, and these include a bypass flow between the tube bundle and the shell, bypass flow between the baffle plate and the shell, and bypass flow between the shell and the bundle outer shroud. Some high temperature nuclear heat exchangers have shrouds inside the shell to protect the shell from thermal transient effects. The effect of bypass flow is the degradation of the exchanger thermal performance. Therefore additional heat-transfer surface area must be provided to compensate for this performance degradation. 

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. 

The heat exchanger must perform the required thermal changes on the process streams within the allowed pressure drops. 

For flow maldistribution on the Cmax fluid side, the exchanger thermal performance deterioration factor Ae approaches a single value of 0.06 for all C < 1 if NTU approaches zero. The performance deterioration factor decreases as NTU increases. For a balanced heat exchanger (C = 1), the exchanger thermal performance deterioration factor continually increases with NTU. 

Finally, the phenomena described above are accompanied by heat exchanges the performances and lifetimes of the components are very sensitive to temperature. Hence, thermal aspects are of cracial importance for their implementation in electrical systems. 

Effect of Uncertainties in Thermal Design Parameters. The parameters that are used ia the basic siting calculations of a heat exchanger iaclude heat-transfer coefficients tube dimensions, eg, tube diameter and wall thickness and physical properties, eg, thermal conductivity, density, viscosity, and specific heat. Nominal or mean values of these parameters are used ia the basic siting calculations. In reaUty, there are uncertainties ia these nominal values. For example, heat-transfer correlations from which one computes convective heat-transfer coefficients have data spreads around the mean values. Because heat-transfer tubes caimot be produced ia precise dimensions, tube wall thickness varies over a range of the mean value. In addition, the thermal conductivity of tube wall material cannot be measured exactiy, a dding to the uncertainty ia the design and performance calculations. 

If a heat exchanger is sized usiag the mean values of the design parameters, then the probabiUty, or the confidence level, of the exchanger to meet its design thermal duty is only 50%. Therefore, in order to increase the confidence level of the design, a proper uncertainty analysis must be performed for all principal design parameters. 

Eurther research on convective transport under low Reynolds number, quasicontinuum conditions is needed before the optimal design of such a micro heat exchanger is possible. The cooling heat exchanger is usually thermally linked to a relatively massive substrate. The effects of this linkage need to be explored and accurate methods of predicting the heat-transfer and pressure-drop performance need to be developed. 

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

Common to all air cooled heat exchangers is the tube, through which the process fluid flows. To compensate for the poor heat transfer properties of air, which flows across the outside of the tube, and to reduce the overall dimensions of the heat exchanger, external fins are added to the outside of the tube. A wide variety of finned tube types are available for use in air cooled exchangers. These vary in geometry, materials, and methods of construction, which affect both air side thermal performance and air side pressure drop. In addition, particular... 

Data on thermal performance are not readily available on all heat exchangers because of the proprietary nature of the machines. To exemplify typical thermal data, heat transfer can best be described by a Dittus-Boelter type equation ... 

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. 

Emerson, W. H. (1967) Thermal and Hydrodynamic Performance of Plate Heat Exchangers, NEL. Reports Nos. 283, 284, 285, 286 (National Engineering Laboratories, East Kilbride, Glasgow, UK). 

Kroger, D. G. (2004) Air-cooled Heat Exchangers and Cooling Towers Thermal-flow Performance Evaluation and Design, Vol. 1 (PennWell). 

Since heat exchange between the calorimeter vessel and the heat sink is not hindered in a heat-flow calorimeter, the temperature changes produced by the thermal phenomenon under investigation are usually very small (less than 10 4 degree in a Calvet microcalorimeter, for instance) (23). For most practical purposes, measurements in a heat-flow calorimeter may be considered as performed under isothermal conditions. 

---