Pressure heat exchangers, example

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Most nuclear reactors use a heat exchanger to transfer heat from a primary coolant loop through the reactor core to a secondary loop that suppHes steam (qv) to a turbine (see HeaT-EXCHANGETECHNOLOGy). The pressurized water reactor is the most common example. The boiling water reactor, however, generates steam in the core. 

Tanks are used in innumerable ways in the chemical process iadustry, not only to store every conceivable Hquid, vapor, or soHd, but also ia a number of processiag appHcations. For example, as weU as reactors, tanks have served as the vessels for various unit operations such as settling, mixing, crystallisation (qv), phase separation, and heat exchange. Hereia the main focus is on the use of tanks as Hquid storage vessels. The principles outlined, however, can generally be appHed to tanks ia other appHcations as weU as to other pressure-containing equipment. 

The shape of the coohng and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell-sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

Example A ventilation system (Fig. 9.64) handling 20 mVs of air needs to heat the supply air from 10 °C to 20 C. Doubling the number of heat exchangers from one to two increases the heat-recovery efficiency from 50% to 75% and introduces an extra pressure drop of 300 Pa. As we can see from Table 9.19, this is probably a cost-efficient measure. 

Equipment Failure pumps, tubes in heat exchangers and furnaces, turbine drivers and governor, compressor cylinder valves are examples of equipment which might fail and cause overpressure in the process. If an exchanger tube splits or develops a leak, high pressure fluid will enter the low side, overpressuring either the shell or the channels and associated system as the case may be. 

The plate heat exchanger, for example, can be used in laminar flow duties, for the evaporation of fluids with relatively high viscosities, for cooling various gases, and for condensing applications where pressure-drop parameters are not excessively restrictive. 

The added capability of calculating unknown values based on measured inputs will greatly enhance the system capabilities. For example, the neither fouling factor nor efficiency of a heat exchanger can be directly measured. A predictive maintenance system that can automatically calculate these values based on the measured flow, pressure and temperature data would enable the program to automatically trend, log and alarm deviations in these unknown, critical parameters. 

Solution There are several theoretical ways of stabilizing the reactor, but temperature control is the normal choice. The reactor in Example 5.7 was adiabatic. Some form of heat exchange must be added. Possibilities are to control the inlet temperature, to control the pressure in the vapor space thereby allowing reflux of styrene monomer at the desired temperature, or to control the jacket or external heat exchanger temperature. The following example regulates the jacket temperature. Refer to Example 5.7. The component balance on styrene is unchanged from Equation (5.29) ... 

Example 2.1 A new heat exchanger is to be installed as part of a large project. Preliminary sizing of the heat exchanger has estimated its heat transfer area to be 500 m2. Its material of construction is low-grade stainless steel, and its pressure rating is 5 bar. Estimate the contribution of the heat exchanger to the total cost of the project (CE Index of Equipment = 441.9). 

At the first level of detail, it is not necessary to know the internal parameters for all the units, since what is desired is just the overall performance. For example, in a heat exchanger design, it suffices to know the heat duty, the total area, and the temperatures of the output streams the details such as the percentage baffle cut, tube layout, or baffle spacing can be specified later when the details of the proposed plant are better defined. It is important to realize the level of detail modeled by a commercial computer program. For example, a chemical reactor could be modeled as an equilibrium reactor, in which the input stream is brought to a new temperature and pressure and the... 

The tube-in-tube or multitube-in-tube heat exchangers are useful in small Linde lique-fiers or in the final Joule-Thomson stage of any liquefier. The performance of Linde-type exchangers is easy to calculate, and their realization is simple. In the examples shown in Fig. 5.12 (a)-(c), the tubes are concentric and the outer wall contributes appreciably to the pressure drop in the outer stream without contributing to the heat transfer. Usually, the smaller inner tube is used for the high-pressure stream and the low-pressure stream flows through the outer annular space. The tubes in Fig. 5.12 (d) and (e) are solder bonded while that in (f) is flattened and twisted before insertion into an outer tube. 

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