When the fins are root-grounded on the base bare tube, in the case of heat from inside to outside, the heat will be transferred from fin root along fin height. It is also continuously transmitted to the surrounding fluid by convective heat transfer. As a result, the fin temperature gradually decreases along the altitude. This also illustrates that difference between fin temperature and ambient fluid temperature is reducing gradually and the heat change per unit is shrinking. Therefore, the effect of fin surface area on enhanced heat transfer is decreasing. The higher the fin, the contribution of the increased area to heat exchange is smaller.
Generally speaking, as for the high frequency welded fin tube applied in engineering project, when fin height is 15mm, the fin efficiency is about 0.8; when fin height is 20mm, the fin efficiency is decreased to 0.7. Based on this, 15mm is the best height. If fins height above 20mm, the fin efficiency will be very bad, so generally not adopted. However, for the aluminum fin on air cooler, height at 22-25mm are always adopted due to much better heat conductivity coefficient of aluminum than carbon steel.
How will fin pitch affect fin ratio?
Usually smaller pitch can effectively increase fin ratio. While considering the flow gas property and ash deposit , we should pay attention to following factors.
A. Serious heavy ash deposit
Such as electric furnace and converter in steel works and exhaust of industrial cellar furnace, the ash content is heavy. If fin tubes are used for heat exchange, larger fin pitch will be suggested. For example, if pitch above 10mm, it is necessary to add a air discharge and choose an air blower.
B. Occasion with small ash deposit but should also be cared.
Take exhaust on plant boiler and industrial boiler as example, 8mm fin pitch is suitable, but should be designed with self-blowing ability.
C. Occasion with no dust or light dust.
Such as exhaust on burning natural gas equipment or air cooler, fin pitch at 4-6mm is OK. For aluminum air cooler, 3mm as fin pitch is also chosen.
The choice on fin thickness depends on corrosion and abrasion of fluid gas. Usually thicker fin is used on site with heavy corrosion.
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