Table 5. Characteristics of micro-fin tube designs

Table 5. Characteristics of micro-fin tube designs.

α

β

e*

( )

Design

(mm)

*e = fin height in mm, p = fin pitch, α = helix angle, β = included angle of fin cross-section,

n = number of fins in the tube, A = surface area of micro-fin tube, Wt = weight of the tube,

h = condensation heat transfer coefficient, and the subscript p denotes the corresponding

values for the plain tube.

Yasuda et al. (1990), shows the cross-sections of different micro-fin tubes. Table 5 gives

information on geometry, weight, enhancement ratio (for condensation of R-22) for the

four micro-fin tube designs (tube outside diameter = 9.52 mm).

The last column of Table 5 shows that the original micro-fin tube improves the conden-

sation coefficient for R-22 by 80% over a plain tube. The subsequent designs, EX and HEX,

raise the improvement figure to 140 and 150%, respectively. The HEX-C gives the highest

performance with an improvement of 210% over a plain tube. The improved designs have

greater fin heights (e) and smaller fin included angles (β). Condensation experiments with

micro-fin tubes conducted by Shinohara and Tobe (1985), Shinohara et al. (1987), and

Schlager et al. (1990c) have shown that the heat transfer coefficient for R-22 gradually

increases with helix angle (α), from 7, up to 30. Although these results do not clearly

identify the optimum helix angle (α), it does appear that α = 30 results in the best conden-

sation performance.

Figure 28 shows the R-22 condensation heat transfer coefficient as a function of mass

flux (velocity) for the tube geometries in Table 5, and demonstrates graphically the

superiority of micro-fin tubes over plain tubes. The HEX-C design stands out as distinctly

superior to HEX and EX designs for R-22 condensation. The tube outside diameter ap-

pears to have little effect on the heat transfer performance.

9.52 mm o.d.

7.54

10

HEX-C

HEX

EX

Plain

1

Figure 28. Heat transfer coefficient for R-22

100

200

300

400

500 600

condensing in micro-fin tubes.

2 s))

2

G,,Mass Velocitty((kgm / s

G Mass Velociy kg//m

44