Heat exchanger applications

The two basic types of tubes are (a) plain or bare and (b) finned—external or internal, see Figures 10-4A-E, 10-10, and 10-11. The plain tube is used in the usual heat exchange application. However, the advantages of the more common externally finned tube are becoming better identified. These tubes are performing exceptionally well in applications in which their best features can be used. 

The first type is of interest only when considering fluids of low Prandtl number, and this does not usually exist with normal plate heat exchanger applications. The third is relevant only for fluids such as gases which have a Prandtl number of about one. Therefore, let us consider type two. 

Fouling factors vary with the heat-exchanger application but are typically 0.001 to 0.002 Btu/hr/sq ft/AT. High fouling factors require designers to provide a proportionally larger heat-transfer surface area, which increases capital costs. 

Shah RK, Thonon B, Benforado DM. Opportunities for heat exchanger applications in environmental systems. Appl Thermal Eng 2000 20 631-650. 

In the past, the principles described have been implicitly recognized in several attempts to convert monolithic catalysts into catalytic heat exchangers. While the use of millimeter dimensions and nanoporous ceramic supports meets the primary criteria already mentioned, the parallel channel structure of monoliths is not ideally tailored for heat exchanger applications, and complex header structures are required to uniformly distribute and collect reaction medium and coolant to and from the individual channels (Figure 9). The unsatisfactory interface between the milli- and macroscale has been a major weakness of such concepts. 

A simple but common heat exchanger application in a chemical process plant is cooling a hot liquid or gas product from the process (called the process fluid ) to a temperature low enough that it can be safely stored. The coolant is likely to be air or water, which would be heated in the heat exchanger. If none of the fluids involved reach their boiling or condensing temperatures, no phase change occurs, and the process fluid is sensibly cooled and the coolant sensibly heated. A heat balance relates the inlet and outlet temperatures, the specific heats, and the mass... 

FIGURE 1 Typical temperature profiles for several process heat exchanger applications (a) product cooler (b) feed heater with condensing stream (c) multicomponent feed heater with vaporization and superheating (d) pure-component product condenser (e) multicomponent product condenser (f) typical feed-effluent heat exchanger. 

In heat exchanger applications, cascade loops are configured so that the master detects the process temperature and the slave detects a variable, such as steam pressure, that may upset the process temperature. The cascade loop, responds immediately and corrects for the effect of the upset before it can influence the process temperature. The cascade master adjusts the set point of the slave controller to assist in achieving this. Therefore, the slave must be much faster than the master. A rule of thumb is that the time constant of the primary controller should be ten times that of the secondary, or the period of oscillation of the primary should be three times that of the secondary. One of the quickest (and therefore best) cascade slaves is the simple and inexpensive pressure regulator. 

In heat-exchanger applications, it is frequently important to match heat-transfer requirements with pressure-drop limitations. Assuming a fixed total heat-transfer requirement and a fixed temperature difference between wall and bulk conditions as well as a fixed pressure drop through the tube, derive expressions for the length and diameter of the tube, assuming turbulent flow of a gas with the Prandtl number near unity. 

A specific example of this can be found in the evaluation of composites for heat exchanger applications.13 In this case, the heat exchanger design calls for a tubular construction which will be pressurized. Under these conditions, a flexural stress will be present in service, and consequently, a C-ring test specimen configuration provides a reasonable way to examine the properties of the composite. 

Table 7.1 Examples of commercial microchannel heat exchanger applications [16].

It should be emphasized that, for laminar flow of liquids in tubes, the influence of viscosity and density variations (buoyancy or free convection effects) must be considered simultaneously for heat exchanger applications. Some correlations and work in this area have been summarized by Bergles [59]. 

Copper and its alloys are widely used for fresh and sea water heat-exchanger applications, because they are especially resistant to biofouling. 

Sometimes nonmetallic conductors may act as cathodes in galvanic couples. Both carbon brick in vessels made of common structural metals and impervious graphite in heat-exchanger applications are examples. Conductive films, such as mill scale (Fe203) or iron sulfide on steel, or lead sulfate on lead, are cathodic to the base metal or to some metallic components in their contact. 

This is not always an easy question to resolve. The feedback controller may be asked to perform a number of different services. In the heat-exchanger application it can be useful in correcting for heat loss, in which employment it should add an increment of heat to the process at all loads this would amount to a zero adjustment. Or its principal function might be to correct for variable steam enthalpy, in which case it should apply a span adjustment by setting the coefficient K. In another process, linearity could be the largest unknown factor. But a single feedback controller can hardly be called upon to do all these things. 

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