Heat exchangers sizing

In summary, the final heat-exchanger sizing should be based on the basic theoretical value plus aU other additional aUowances as described. 

It is recommended that heat-exchanger size and type be designated by numbers and letters. 

Tower and heat exchanger sizing procedures — Peters and Timmerhaus Venture Analysis 11) ... 

Included among the factors is the effectiveness index. It is a measure of the cost-effectiveness of an exchanger, and is defined as /= overall heat-transfer coefficient/heat-ex-changer cost. With coefficients expressed in Biu/(h) ft )CF), and purchase costs in /ft, Elis given as Btu/(h) °F)( ). Such indices are shown in the table, and these are averages for a variety of heat-exchanger sizes (250, 500, 750 and 1,000 ft )... 

In this example, it is quite obvious that the heat exchanger size can be reduced by about 20% if the propane is passed through the tubes and not through the shell side. 

The shell-side coefficient would therefore be hg = 123/0.573 = 215 Btu/hr. sq. ft. °F at the permissible pressure drop, and in this example the heat exchanger size could be reduced by about 35% if the weaker medium is passed through the shell side of the exchanger. 

The field of engineering contains many examples of trade-offs. You have seen some of them in previous courses. In distillation there is the classical trade-off between the number of trays (height) and the reflux ratio (energy and diameter). In heat transfer there is the trade-off between heat exchanger size (area) and pressure drop (pump or compressor work) more pressure drop gives higher heat transfer coefficients and smaller areas but increases energy cost. We have mentioned several trade-offs in this book control valve pressure drop versus pump head, robustness versus performance, etc. 

Figure 33.22 Relative heat exchanger size as a function of design point bypass ratios.

Now we go to sizing the equipment. We start with the heat exchanger. Sizing Heat exchangers We select the first one, equipment number 1 in the flowsheet. The... 

Option for small amounts of helium to improve thermal properties (reduce heat exchanger size)... 

The AHTR has two coupled physical characteristics that may potentially enable the development of a low cost medium sized AHTR using factory assembled modular units with ease of transport. The low pressure liquid cooling for a medium sized reactor implies small reactor vessel and heat exchanger sizes relative to those for gas cooled reactors. The high temperature allows the use of Brayton power cycles. Brayton turbines have much higher power densities than steam turbines and are consequently much smaller in size per unit output. Brayton cycle turbines are typically manufactured and shipped as modular units and have lower costs than traditional steam cycles per unit output. These options have not yet been investigated. 

Thermophysical properties of R-22 and CO2 are compared for the saturation temperatures of -25, -10, and 5°C in Table R-3. The evaporation pressure of CO2 is four to eight times higher than that of R-22. The saturated liquid density of CO2 is approximately 70 percent that of R-22, while the saturated vapor density of CO2 is approximately five times that of R-22. The higher density offers the opportunity to reduce heat exchanger size and weight. CO2 has better heat-transfer characteristics (higher latent heat, specific heat, and thermal conductivity) and lower viscosity than R-22. See Fig. R-4. 

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