Pressure drop correlations, heat exchangers

Manglik RM, Bergles AE. Heat transfer and pressure drop correlations for the rectangular offset strip fin compact heat exchanger. Experiments in Thermal Fluid Sci 1995 (10) 171-180. 

R. M. Manglik and A. E. Bergles, Heat Transfer and Pressure Drop Correlations for the Rectangular Offset Strip Fin Compact Heat Exchanger, Experimental Thermal and Fluid Science (10) 171-180,1995. 

H. C. Chai, A Simple Pressure Drop Correlation Equation for Low Finned Tube Crossflow Heat Exchangers, Int. Commun. Heat Mass Transfer, Vol. 15, pp. 95-101,1988. 

Friction factor values are normally plotted as a function of the Reynolds number and can be obtained from manufacturers for their various fin geometries. Proprietary methods for determining the pressure drop in inlet and exit nozzles, distributors, and headers can also be obtained from these vendors. Table 5.4 provides examples of heat transfer and corresponding pressure drop correlations for three typical fin configurations used in plate-fin exchangers. 

Table 5.4. Empirical Heat Transfer and Corresponding Pressure Drop Correlations for Brazed, Aluminum Plate-fin Heat Exchangers (To Match Present Vendor Specifications, Dimensions Are Also Given in Customary English Units)" ... 

The complex flow pattern on the shell-side, and the great number of variables involved, make it difficult to predict the shell-side coefficient and pressure drop with complete assurance. In methods used for the design of exchangers prior to about 1960 no attempt was made to account for the leakage and bypass streams. Correlations were based on the total stream flow, and empirical methods were used to account for the performance of real exchangers compared with that for cross flow over ideal tube banks. Typical of these bulk-flow methods are those of Kern (1950) and Donohue (1955). Reliable predictions can only be achieved by comprehensive analysis of the contribution to heat transfer and pressure drop made by the individual streams shown in Figure 12.26. Tinker (1951, 1958) published the first detailed stream-analysis method for predicting shell-side heat-transfer coefficients and pressure drop, and the methods subsequently developed... 

At the conceptual stage for heat exchanger network synthesis, the calculation of heat transfer coefficient and pressure drop should depend as little as possible on the detailed geometry. Simple models will be developed in which heat transfer coefficient and pressure drop are both related to velocity1. It is thus possible to derive a correlation between the heat transfer coefficient, pressure drop and the surface area by using velocity as a bridge between the two1. 

Steady two-phase flow. In rod (or tube) bundles, such as one usually encounters in reactor cores or heat exchangers, the pressure drop calculations use the correlations for flow in tubes by applying the equivalent diameter concept. Thus, in a square-pitched four-rod cell (Fig. 3.51), the equivalent diameter is given by... 

For plate-fin heat exchangers in single-phase flow, the heat transfer coefficients are related to the developed heat transfer surface, and the area ratio must be taken into account. As related to the projected surface, the overall heat transfer coefficient is very high. Heat transfer and pressure drop can be estimated from correlations (43 44), but these correlations give only an estimate of the performance, because local modification of the fin geometry will affect heat transfer and pressure drop. 

Carey (51) has studied pressure drop and void fraction in different types of compact heat exchangers and has outlined the differences with plain tube geometries. Kreissig and Muller-Steinhagen (52) and Margat et al. (53) have shown that the principles of the methods developed for plain tubes can be used but need to be adapted. The main results of these studies are that the liquid holdup is significantly affected by the mass flow rate (Figure 28) the liquid holdup is underestimated by conventional correlation the two-phase flow multiplier can be estimated from a Chisholm-type correlation. 

Winkelmann et al. (54) have studied air-water flows in a corrugated heat exchanger. Flow visualization and two-phase pressure drop measurements have been performed. The flow visualizations have shown that the flow pattern is complex and that a wavy or a film flow occurs in most cases (Figure 29). The two-phase pressure drop depends on the total flow rate and vapor quality, and Chisholm-type correlation is proposed. More work is required to characterize the flow structure in compact heat exchangers and to develop predictive methods for the frictional pressure drop and the mean void fraction. 

Today contractors and licensors use sophisticated computerized mathematical models which take into account the many variables involved in the physical, chemical, geometrical and mechanical properties of the system. ICI, for example, was one of the first to develop a very versatile and effective model of the primary reformer. The program REFORM [361], [430], [439] can simulate all major types of reformers (see below) top-fired, side-fired, terraced-wall, concentric round configurations, the exchanger reformers (GHR, for example), and so on. The program is based on reaction kinetics, correlations with experimental heat transfer data, pressure drop functions, advanced furnace calculation methods, and a kinetic model of carbon formation [419],... 

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