Tube-Side Construction

The mechanical construction of the tubes in an air cooler creates some rather nasty problems. Figure 17.4 shows the exterior appearance at either end of an air cooler. The small black circles are threaded steel plugs. They are not connected to the ends of the tubes. Allow me to rotate the air-cooler header box shown in Fig. 17.4 by 90°, and display a cross-sectional view in Fig. 17.5. Note that the plugs are not

• Outlet
• Inlet
• Drain
• Vent

connected to individual tubes. Unscrewing a plug just gives one access to the end of a tube for cleaning purposes. Proper cleaning of an air-cooler tube requires removing two plugs. A large industrial air cooler may have 2000 tubes or 4000 plugs. The labor involved to remove and reinstall all these plugs is formidable. Leaking plugs due to cross-threading is a common start-up problem. Hence, many air coolers are simply never cleaned.
The pass partition baffle shown in Fig. 17.5 makes this cooler a two pass exchanger. These baffles are subject to failure due to corrosion.
More often, they break because of excessive tube-side pressure drop. The differential pressure across a two-pass pass partition baffle equals
the tube-side. Once the pass partition baffle fails, the process fluid may bypass the finned tubes, and cooling efficiency is greatly reduced. This is bad.
But worse yet, during a turnaround of the cooler, there is normally no way to inspect the pass partition baffle. There is no easy way to visually verify the mechanical integrity of this baffle. A few air coolers have removable inspection ports for this purpose; most do not.