Air Coolers

Air coolers are used where the process temperature is too high to use cooling water (risk of scaling) or the load on the cooling water system would be excessive. There are many possible problems with air cooler performance including:

• air leakage through gaps between tube bundles and side walls;

• excessive clearance between fan blade tips and the plenum chamber. (Tip air recirculation);

• poor fan hub design (center hub air recirculation);

• fans running stalled (excessive pressure drop across the finned tube bundle due to fouling coupled with high angled blades);

• lack of air inlet 'bell-mouth' to assist air flow.

Figure 8.14 illustrates 'bad' versus 'good' design.

On the left there is hub recirculation, tip recirculation and uneven velocity profile. The fan on the right has a correctly designed center disk, zero tip clearance

Figure 8.14 Air cooler fan design improvements.

(due to aluminum honeycomb tip seals), air inlet bell mouth and correct aerodynamic blade profile along the blade radius.

On the left, there is recirculation of air at the fan hub and the blade tips. The blades are simple parallel aerofoil sections, generating a higher pressure ratio towards the tip than the root.

On the right, there is a center seal disk, abradable tip seals and variable chord width blades with decreasing attack angle from hub to tip - giving constant pressure ratio along the blade length.

Figure 8.15 shows the typical effects of:

'A' lack of air inlet bell mouth 'B' inadequate center seal disk 'C' excessive tip clearance 'D' total impact A + B + C.

Note that there are wide differences in air cooler design. In particular, some use relatively small diameter fans with high velocity pressure and other use large diameter fans with lower velocity pressure.

For existing fans, it is worth checking the velocity pressure to see whether modifications are justified.

I f we define velocity pressure (VP) as kinetic energy per unit volume of air passing through the fan, then this is:

Figure 8.15 Effect of air cooler fan improvements. A: lack of air inlet bell mouth; B: inadequate center seal disk; C: excessive tip clearance; D: total impact A + B + C.

Figure 8.16 Air cooler fin types.

Figure 8.16 Air cooler fin types.

where m = mass

V = velocity of air entering the fan mass/volume is density (p).

where the units of VP are pascals; p is kgm-3; Vis ms-1.

Air density is ~1.2kgm-3 and 1 pascal = 0.1mm H2O so the formula simplifies to:

The velocity of air entering the fan can be measured using a hand-held anemometer.

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