Shell Heat Loss Calculations

If not properly designed, heat losses through the attachments to the inner shell, will account for more heat loss than the rest of the surfaces. Proper design will limit heat loss through these components to approximately 20% of the total calculated heat loss. 

Figures 7.6 and 7.7 are calculated shell losses based on refractory type and thickness. Although the actual mechanism of heat loss through a rotary kiln wall is more involved, the obvious result is that...

Table 9.8 covers the calculated heat loss from the shell. 

The product of the cold area and fourth power of absolute temperature of the cold wall is insignificantly small. Thus, the warm area of the external shell governs the heat loss of a dewar. To keep the surface of the warm emitter as small as possible, an efficient design of a dewar vessel must strive for minimum spacing between the cold inner vessel and the warm outer shell. As the work of M. M. Fulk and M. M. Reynolds [2] has shown, the emissivity as a surface property has a minimum which cannot be improved by further polishing or surface treatment. The only way to achieve further reduction of the heat loss is to employ a multiplicity of radiation shields between the warm and cold surfaces. Various techniques have been developed for the installation of such shields between the cool and warm surfaces. Each of these shields, to be effective as a heat transfer barrier, must be allowed to assume proper equilibrium temperature. The heat transfer between each pair of successive shields obeys again the Stefan-Boltzmann law and the over-all heat transfer across any number of shields can be calculated by matrix algebra. 

As seen in Table 2.1, Wlc is an acceptable fallback solution for systems for which W1 calculations are not feasible because of the number of inner-shell orbitals for heats of formation and certainly for ionization potentials, Wlch offers a significant further cost reduction over Wlh at a negligible loss in accuracy. 

The design calculations highlighted the shortcomings of the Kern method of exchanger design. The Kern method fails to account for shell-side inefficiencies such as bypassing, leakage, crossflow losses, and window losses. This leads to a marked overestimate of the shell-side heat-transfer coefficient and shell-side pressure drop. The Bell method is recommended to correct these deficiencies. 

As a consequence of the channel head tube-side inlet and outlet being located on the same end of the exchanger, the lower half of the shell is in co-current flow. Depending on the temperature profile, this typically reduces the LMTD by 5 to 25 percent. To calculate this loss in heat-transfer efficiency due to this problem, we use the F-factor correction factor as presented in your TEMA Data Book. 

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