Air-cooled heat exchangers are employed on a large scale as condensers of distillation columns or pre deg Cess coolers. The approach temperature—the difference between pre deg Cess outlet temperature and dry-bulb air temperature—is typically some 8–14 °C above the temperature of the four consecutive warmest months. By air humidification, this difference can be reduced to 5 °C. Air-cooled heat exchangers are manufactured from finned tubes. Typical ratio of extended to bare tube area is 15:1 to 20:1. Finned tubes are efficient when the heat transfer coefficient outside the tubes is much lower than inside the tubes. The only way to increase the heat transferred on the air-side is to extend the exchange area available. In this way, the extended surface offered by fins increases significantly the heat duty. For example, the outside heat transfer coefficient increases from 10–15 W/m2K for smooth tubes to 100–150 or more when finned tubes are used. Typical overall heat transfer coefficients are given in Table 20.10. The correction factor FT for LMTD is about 0.8.
