Effectiveness factors, heat exchangers

This demonstration plant will normally operate at a first-effect boiling point of 250° F., a last-effect boiling point of 120° F., and a discharge sea water concentration factor of 4. The primary control of the process is accomplished by automatic control of steam flow rate, sea water flow rate, and last-effect vacuum. No control is needed for temperature or pressure in the individual effects and heat exchangers, since these achieve their own levels, influenced only by the proportioning of the equipment. The demonstration plant is rather heavily instrumented to permit close surveillance of operating conditions and carrying out of special tests. 

It is generally accepted that the presence of flux residues on a heat exchanger enhances its corrosion resistance [80,81,82]. However, it has always been difficult to quantify the level of corrosion resistance enhancement. In corrosion testing of flat panels or coupons coated with flux residue, there is no doubt that there is a beneficial effect. With heat exchangers on the other hand, the general trend shows a longer corrosion life but factors such as uniformity of flux residue coverage and variations in flux load sometimes confuse the corrosion test data. 

Frequently, the difference ia exchanger type does not influence the desired topology to any significant extent. For iadustrial problems, however, it is necessary to consider iadividual heat-exchanger shells rather than just the match that is called the heat exchanger. If a high level of heat recovery is desired, the effect of the F factor can be important. This problem has been solved but is beyond the scope of this article. 

Form view factor A factor which describes the effects of the relative area of two surfaces, the geometry of the surfaces in relation to each other, and the two emissivities on radiation heat exchange between the surfaces. 

Typical velocities in plate heat exchangers for waterlike fluids in turbulent flow are 0.3-0.9 meters per second (m/s) but true velocities in certain regions will be higher by a factor of up to 4 due to the effect of the corrugations. All heat transfer and pressure drop relationships are, however, based on either a velocity calculated from the average plate gap or on the flow rate per passage. 

A further advantage of the plate heat exchanger is that the effective mean temperature difference is usually higher than with the tubular unit. Since the tubular is always a mixture of cross and contra-flow in multi-pass arrangements, substantial correction factors have to be applied to the log mean temperature difference (LMTD). In the plate... 

In Chapter 15, the Fr correction factor was correlated in terms of two dimensionless ratios, the ratio of the two heat capacity flowrates (R) and the thermal effectiveness of the exchanger (P). Practical designs were limited to some fraction of Pmax, that is7 ... 

To offset the effects of fouling on both sides of the heat exchanger to some degree, designers build various fouling factor tolerances into the heat exchangers. 

Major equipment factor estimates. Major equipment factor estimates are made by applying multipliers to the costs of all major equipment required for the plant or process facility. Different factors are applicable to different types of equipment, such as pumps, heat exchangers, and pressure vessels. Equipment size also has an effect on the factors. 

Duration of a cycle of HHP operation is defined as time required for reaction hydrogenation/dehydrogenation in pair hydride system. This time determines heat capacity of HHP. Duration of a cycle depends on kinetics of hydrogenation reactions, a heat transfer between the heated up and cooling environment, heat conductivities of hydride beds. Rates of reactions are proportional to a difference of dynamic pressure of hydrogen in sorbers of HHP and to constants of chemical reaction of hydrogenation. The relation of dynamic pressure is adjusted by characteristics of a heat emission in beds of metal hydride particles (the heat emission of a hydride bed depends on its effective specific heat conductivity) and connected to total factor of a heat transfer of system a sorber-heat exchanger. The modified constant of speed, as function of temperature in isobaric process [1], can characterize kinetics of sorption reactions. In HHP it is not sense to use hydrides with a low kinetics of reactions. The basic condition of an acceptability of hydride for HHP is a condition of forward rate of chemical reactions in relation to rate of a heat transmission. 

Specific heats of metals and hydrides are easily determined and typically fall in the range of 0.1-0.2 cal/g°C. Thermal conductivity is a little more difficult to determine. The conductivity of the metal or hydride phase is not sufficient the effective conductivity of the bed must be determined. This depends on alloy, particle size, packing, void space, etc. Relatively little data of an engineering nature is now available and must be generated for container optimization. Techniques to improve thermal conductivity of hydride beds are needed. As pointed out earlier, good heat exchange is the most important factor in rapid cycling. 

It is convenient to express the pressure drop for heat exchangers in a form similar to the Fanning equation as presented in Chap. 14. Because the transfer of heat is involved, a factor must be included for the effect of temperature... 

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