Heat exchanger flow-forced

In extraction processes using supercritical fluids it is of interest to predict the heat-transfer processes that take place in heat exchangers in forced flow. 

Remark 1 Since we cannot bring the two composite curves closer, the pinch point represents the bottleneck for further heat recovery. In fact, it partitions the temperature range into two subnetworks, one above the pinch and one below the pinch. Heat flow cannot cross the pinch since there will be violations in the heat exchange driving forces. As a result, we need a hot utility at the subnetwork above the pinch and a cold utility at the subnetwork below the pinch. In other words, having identified the pinch point, we can now apply the first law analysis to each subnetwork separately and determine the hot and cold utility requirements. These can be read from the T - Q diagram since they correspond to the horizontal segments AG and CD, respectively. Hence, for our example we have ... 

Maximum shell-side heat-transfer rates in forced convection are apparently obtained by cross-flow of the flmd at right angles to the tubes. In order to maximize this type of flow some heat exchangers are built with segmental-cut baffles and with no tubes in the window (or the baffle cutout). Maximum baffle spacing may thus equal maximum unsupported-tube span, while conventional baffle spacing is hmited to one-h f of this span. 

Air cooled heat exchangers are used to transfer heat from a process fluid to ambient air. The process fluid is contained within heat eonducting tubes. Atmospherie air, whieh serves as the eoolant, is caused to flow perpendicularly across the tubes in order to remove heat. In a typical air cooled heat exchanger, the ambient air is either forced or induced by a fan or fans to flow vertically across a horizontal section of tubes. For condensing applications, the bundle may be sloped or vertical. Similarly, for relatively small air cooled heat exchangers, the air flow may be horizontal across vertical tube bundles. 

In this section the correlations used to determine the heat and mass transfer rates are presented. The convection process may be either free or forced convection. In free convection fluid motion is created by buoyancy forces within the fluid. In most industrial processes, forced convection is necessary in order to achieve the most economic heat exchange. The heat transfer correlations for forced convection in external and internal flows are given in Tables 4.8 and 4.9, respectively, for different conditions and geometries. 

Above this size, the flow of air over the condenser surface will be by forced convection, i.e. fans. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 1 and 10 1. 

Shah RK, London AL (1978) Laminar flow forced convection in ducts a source book for compact heat exchanger analytical data. Advances in Heat Transfer, suppl 1. Academic, New York Sher 1, Hetsroni G (2002) An analytical model for nucleate pool boiling with surfactant additives. Int J Multiphase Flow 28 699-706... 

ESDU 93018 (2001) Forced convection heat transfer in straight tubes. Part 2 laminar and transitional flow. ESDU 98003-98007 (1998) Design and performance evaluation of heat exchangers the effectiveness-NTU method. 

Herd, K. G., W. P. Goss, and J. W. Connell, 1983, Correlation of Forced Flow Evaporation Heat Transfer Coefficient in Refrigerant Systems, Paper B2, in Heat Exchangers for Two-Phase Applications, Vol. 27, National Heat Transfer Confi, Seattle, WA. (4)... 

The Rayleigh line is defined by the condition which results from heat exchange in a flow system and requires that the flow force remain constant, in essence for a constant area duct the condition can be written as... 

Any type of input-forcing function can be used steps, pulses, or a sequence of positive and negative pulses. Figure 14.9a shows some typical input/output data from a process. The specific example is a heat exchanger in which the manipulated variable is steam flow rate and the output variable is the temperature of the process steam leaving the exchanger. 

Reboilers are heat exchangers that are used primarily to provide boilup for distillation and similar towers. All types perform partial vaporization of a stream flowing under natural or forced circulation... 

---