Diffuser pore size and air flow rate

Diffusers with small pore size generate a large number of small bubbles. The total surface area of bubble is inversely proportional to the bubble size at a given air flow rate. In general, fine bubble diffusers have smaller than 1 mm pores and produces less than 5 mm bubbles. The fine pore diffusers generally incur higher head pressure loss than coarse bubble diffusers, but the head loss can be controlled low if large enough total diffuser surface areas are used. (Note: Surface area of each bubble is proportional to the second power of bubble size, but the number of bubbles is inversely proportional to the third power of bubble size at a given air flow rate. Overall, total available surface area is inversely proportional to the bubble size.)

Oxygen transfer efficiency of a same diffuser can vary widely depending on air flow rate. As air flow rate increases, bubble size increases and total bubble surface area decreases. In addition, at the high air flow rates, the rising bubbles have higher chances to collide and coalesce due to the larger population density.

The effect of bubble coalescence is less significant in coarse pore diffusers because coarse bubbles generate stronger turbulence that can keep the bubbles from coalesce in the event of collision. As a result, alpha factor of coarse pore diffusers does not decrease as fast as that of fine pore diffusers at high air flow rates. Fig. 1 shows the diminishing alpha factor as air flow rate increases. As the performance of fine pore diffusers is significantly affected by air flow rate, manufactures provide SOTE curves as a function of air flow rate.

In one experiment (Ndinisa, 2006) summarized in Table 1, the bubble size distribution measured by image analysis is more sensitive to air flow rate rather than nozzle size. This is apparently caused by the overloading of the nozzles with high air flow under the experimental condition, where the steady state bubble size is determined by the strength of the turbulence rather than diffuser pore sizes. This experimental observation cannot be generalized for field situations, but it suggests that small diffuser pores do not guarantee small bubbles.

Diffuser placement is another factor affecting fine bubble diffuser performance by affecting bubble channelling. In general diffusers are spaced evenly to cover entire floor spaces to minimize short-circuiting of air bubbles as shown in Fig. 2. Inadequate diffuser placement and improper air flow distribution cause short-circuiting of the air, which promote bubble coalescence in the high air flow zones and expedite the escape of the bubbles in the zone.

image002 (1)Fig. 1. Effect of air flow rate on a-factor in field. Each dot shape indicates data from one full scale plant. (Rosso, 2005)

Table 1. Effect of nozzle size and air flow rate on bubble size (Ndinisa, 2006)
image006

portageemptyaeration Fig. 2. Evenly distributed air diffusers covering entire floor (Environmental Engineering Textbook, MTU)

Coarse bubble diffusers normally have bigger than 6 mm pores. Coarse bubbles can break down due to the turbulence and reach steady state sizes while they are rising. The steady state bubble size might be 6-10 mm under a typical condition. The existence of the steady state bubble size in the turbulence field is responsible for relatively stable OTE when air flow rates increases when they are compared with fine pore diffusers. In some cases OTE even slightly increases with air flow rates due to the turbulence (Shammas, 2007). In addition, the turbulence around coarse bubble also increases mass transfer coefficient on bubble surface. Therefore, the efficiency gap between fine bubble diffusers and coarse bubble diffusers in the field is not as big as bubble size suggests. The advantages and disadvantages of fine bubble diffusers are summarized in Table 2 (Shammas, 2007).

Table 2. Comparison of fine bubble diffusers and coarse bubble diffusers. Advantage of one system is disadvantage of other system.

Fine Bubble Diffuser Coarse Bubble Diffuser
Pore Size <2 mm >6 mm
Bubble size 2-5 mm 6-10mm
Pressure loss 0.1-0.2 mH2O (new)

0.2-0.5 m H2O (used)

0.1-0.5 mH2O (New and used)
Advantage ·   Higher OTEs and lower power costs

·   Less volatile organic compound emissions

·   SOTE : 4-6 %/m

·   Less susceptible to fouling

·   Less susceptible to chemical attack

·   Less efficiency loss at high air flow

·   Less susceptible to uneven air distribution

·   Less maintenance costs

·   SOTE : 2-3 %/m

 

1) Rosso, 2007a

© Seong Hoon Yoon