Factors and Direct Exchange Areas

TABLE 5-4 Normal Total Emissivity of Various Surfaces 

Aluminum Sheet steel, strong rough oxide layer 75 0.80 

Highly polished plate, 98.3% pure 440-1070 0.039-0.057 Dense shiny oxide layer 75 0.82 

Aluminum-surfaced roofing 100 0.216 Cast iron, rough, strongly oxidized 100 80 0.95 

Calorized surfaces, heated at 1110°F. Wrought iron, dull oxidized 70-680 0.94 

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Only one direct-view factor F o of direct-exchange area 12 is needed because Fn equals 1 — Fig and Foo equals 1 — Fo equals 1 — FioA /Ao. Then 11 equals A] — 12 and 22 equals A2 — 21, With these substitutions, Eq, (5-131) becomes... 

In the case of transparent gas, the direct exchange area is equal to the product of the view factor and the surface area, namely... 

Example 7 The Crossed-Strings Method Figure 5-16 depicts the transverse cross section of two infinitely long, parallel circular tubes of diameter D and center-to-center distance of separation C. Use the crossed-strings method to formulate the tube-to-tube direct exchange area and view factor s st and Ft,t, respectively. 

Compute direct exchange areas There are Z = 6 unique direct exchange areas. These are obtained from Eqs. (5-114) and view factor algebra. The final array of direct exchange areas is ... 

Clearly when K = 0, the two direct exchange areas involving a gas zone g[ j and g gj vanish. Computationally it is never necessary to make resort to Eq. (5-155) for calculation of gg,. This is so because sj, gSj, and gg may all be calculated arithmetically from appropriate values of StSj by using associated conservation relations and view factor algebra. 

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