Counter-current-flow heat exchange

Taking into account typical numbers for a and D, this underlines that the channel width should be considerably smaller than 1 mm (1000 pm) in order to achieve short residence times. Actually, heat exchangers of such small dimensions are not completely new, because liquid cooled microchannel heat sinks for electronic applications allowing heat fluxes of 790 watts/cm2 were already known in 1981 [46]. About 9 years later a 1 cm3 cross flow heat exchanger with a high aspect ratio and channel widths between 80 and 100 pm was fabricated by KFK [10, 47]. The overall heat transport for this system was reported to be 20 kW. This concept of multiple, parallel channels of short length to obtain small pressure drops has also been realized by other workers, e.g. by PNNL and IMM. IMM has reported a counter-current flow heat exchanger with heat transfer coefficients of up to 2.4 kW/m2 K [45] (see Fig. 3). 

In a counter-current-flow heat exchanger, the fluids flow in opposite directions, as shown in Figure 10. In this case, the hot and cold fluids enter the heat exchanger from opposite ends. In both the parallel and counter-flow configurations, the two fluids are forced to flow along the heat exchanger by either pumps or fans. 

Figure 10 Schematic representation of a counter-current—flow heat exchanger...

Figure 12 Temperature profile - counter-current-flow heat exchanger...

The procedure to obtain Arlm is the same for a counter-current-flow heat exchanger. It is again given by Equation (43) however in this case, ATi is the temperature difference between the inlet and outlet temperature of the hot fluid and the cold fluid respectively at one end of the exchanger, and A T2 is the temperature difference between the hot fluid and the cold fluid at the other end, as shown in Figure 15. 

Comparison between Co-Current-Flow and Counter-Current-Flow Heat Exchangers. The worked examples reported below will demonstrate the different efficiencies of the two heat exchanger configurations in terms of the heat transfer area required in both cases for the same U. Let us consider two fluids between which heat is being... 

Heat exchanger, counter flow or counter current A heat exchanger in which the flow inlet of one fluid is adjacent to the outlet of the second fluid and vice versa the fluids flow in opposite directions. 

Figure 2.31 Characteristic temperature profiles in a counter-current micro heat exchanger for a very low (left), intermediate (middle) and very high (right) thermal conductivity of the wall material and equal volume flows inside the two channels, reproduced from [125],...

The best known use of the hairpin is its operation in true counter-current flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted me advantages gained from countercurrent flow. 

BATCH COOLING EXTERNAL HEAT EXCHANGER (COUNTER-CURRENT FLOW), NON-ISOTHERMAL COOLING MEDIUM... 

Batch heating/cooling of fluids external heat exchanger (counter-current flow) non-isothermal cooling medium... 

Before equation 12.1 can be used to determine the heat transfer area required for a given duty, an estimate of the mean temperature difference A Tm must be made. This will normally be calculated from the terminal temperature differences the difference in the fluid temperatures at the inlet and outlet of the exchanger. The well-known logarithmic mean temperature difference (see Volume 1, Chapter 9) is only applicable to sensible heat transfer in true co-current or counter-current flow (linear temperature-enthalpy curves). For counter-current flow, Figure 12.18a, the logarithmic mean temperature is given by ... 

The temperature correction factor, Ft, will normally be higher with plate heat exchangers, as the flow is closer to true counter-current flow. 

The flow arrangement in shell-and-tube heat exchangers can involve both co-current-flow and counter-current-flow, as shown in the schematic in Figure 17. 

Most of the simulators allow heat input or removal from a plug-flow reactor. Heat transfer can be with a constant wall temperature (as encountered in a fired tube, steam-jacketed pipe, or immersed coil) or with counter-current flow of a utility stream (as in a heat exchanger tube or jacketed pipe with cooling water). 

Integrating (2) as a function of x is difficult since we d have to predict AZ (x). There is a easier way (but a trick is required). Consider steady-state counter-current flow through a double-pipe heat exchanger (shown at right). A steady-state energy balance on the hot fluid gives... 

The IHX is a vertical, counter current flow, shell and tube heat exchanger (fig. 6). Each IHX has 3000 straight tubes (19 mm OD x 0.8 mm WT) with primary sodium on shell side and secondary sodium on the tube side. The tubes are arranged in circumferential pitch. A variable flow distribution is provided inside the IHX tubes with a higher flow on the outer rows to improve the thermohydraulic behaviour of the tube bundle. A mixing device is also provided at the secondary outlet to reduce the temperature differences between inner and outer shell of the secondary outlet header. Absence of flow induced vibration of tube bundle and the drain pipe in the downcomer have been verified by theoretical analysis. 

The following equations are used to calculate outlet temperatures of each heat exchanger present in the HEN. Counter-current flow is assumed in each exchanger and h gi and represent the film heat transfer coefficient on the hot and cold side, respectively. 

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