Crossflow heat exchangers

Another type of crossflow cooling tower is the wet-dry tower, which consists of a normal crossflow tower over which a few air coils are placed. The hot water is first cooled by an air cooled heat exchanger and then drops to the wet cooling tower where more cooling is obtained by the evaporative mechanism. Figures 5 and 6 provide examples. In contrast, deck-filled towers contain tiers of splash bars or decks to aid in the breakup of water drops to increase the total water surface and, subsequently, the evaporation rate. 

In its simplest form, a heat exchanger may consist of two passages, with the cooling fluid in one passage and the warming fluid in the other. The flow direction of each of the fluids relative to one another may be countercurrent, cocurrent, or crossflow. 

Figure 3 Micro heat exchanger produced by means of mechanical micromachining, (a) Platelet with grooves of 30-p.m depth and 70-p.m width, (b) Assembly of crossflow heat exchanger, (c) Final devices. (Source Forschungszentrum Karlsruhe.)... 

It was seen from the discussion of heat exchangers that the fluid streams are not strictly countercurrent. Baffles on the shell side induce crossflow, and in a two-tube-pass heat exchanger both countercurrent and cocurrent flow occur. To account for deviations from countercurrent flow, the logarithmic-mean teri5)erature difference is multiplied by a correction factor, F. Thus,... 

R. A. Stevens, J. Fernandes, and 1. R. Woolf. Mean Temperatjre DilTerencc in One, TVo, and Three Pass Crossflow Heat Exchangers. Transactions oftheASME 79 (1957), pp. 287-297. 

The heat exchanger is divided into 3 computational cells in parallel-flow and counterflow types, and into 9 cells in crossflow type. The mass and heat balances of the cells are calculated by means of heat balance of a single plate [3,4]. The procedure of the calculation is similar to the case of the flue gas heat exchanger. The surface temperatures and air temperatures leaving the different types of heat exchangers are shown in Fig. 10. The pressure drop of the heat exchanger is calculated according to [5]. 

Pure crossflow is found in flat plate heat exchangers, as indicated by Fig. 1.22. The temperatures of both fluids also change perpendicular to the flow direction. This is schematically shown in Fig. 1.23. Each fluid element that flows in a crossflow heat exchanger experiences its own temperature change, from the entry temperature which is the same for all particles to its individual exit temperature. Crossflow is often applied in a shell-and-tube heat exchanger when one of the fluids is gaseous. The gas flows around the rows of tubes crosswise to the tube axis. The other fluid, normally a liquid, flows inside the tubes. The addition of... 

There are also numerous other special designs for heat exchangers which will not be discussed here. It is possible to combine the three basic flow regimes of countercurrent, cocurrent and crossflow in a number of different ways, which leads to complex calculation procedures. 

Fig. 1.59 Counter crossflow heat exchanger with two rows of tubes and two passageways...

External Flow (Shell Side). Two-phase flow patterns for flow normal to tube bundles (crossflow), such as on the shell side of a shell-and-tube heat exchanger, are much more complex than those inside a plain circular tube. Consequently, prediction of flow patterns in such situations is very difficult. It is important to note that two-phase shellside flow patterns are substantially less analyzed than those for internal flows. A review of the shellside flow pattern is presented by Jensen [68]. The dominant flow patterns (see Fig. 17.51 [69]) may be assessed... 

In Table 17.22, two correlations are presented for shellside two-phase flow pressure drop estimation, based on modifications of the internal flow correlations. The first correlation uses the modified Chrisholm correlation [69, 79], and the second one [80] employs the modified Lockhart-Martinelli correlation. The first correlation is for horizontal crossflow (crossflow in a baffled horizontal heat exchanger with horizontal or vertical baffle cuts). The second one is for vertical crossflow (upflow in a horizontal tube bundle). 

FIGURE 17.54 A single-pass crossflow heat exchanger. 

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. 

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