April 1996, Volume 5
April 1996, Volume 5
Interestingly, Process Equipment Company became involved in such a problem at a large aluminum plant in 1995. Aluminum is manufactured from alumina in "pot lines", i.e., numerous individual furnaces which unfortunately generate fugitive smoke which must be evacuated through a ventilation system to a baghouse and scrubber.
Because of fan limitations, smoke was escaping from the hoods at the individual pots and was accumulating inside the building and activating smoke detectors in the upper area of the building. Process Equipment Company decided that the most practical solution to this problem was to modify the eight individual fans in order to gain additional capacity and subsequently prevent the smoke from accumulating inside the building.
These particular fans were approximately 30 years old and were of the airfoil design, a fan design which would not ordinarily be used for induced draft applications except that, in this case, a mechanical collector is utilized to collect the coarse particles which would otherwise wear out the relatively thin gauge construction of the airfoil impeller. Figure 2 illustrates the tipout procedure to increase the wheel diameter and gain additional fan capacity. Originally, the scope of the project was defined to us as one requiring additional flow capacity without the need for additional static pressure capability. This common request is basically correct except that the additional capacity through the system of this type cannot be realized without a fan modification which will yield some degree of pressure increase. Fluid moving through an existing dust collector, ductwork, baghouse, and scrubber cannot be increased without experiencing some increase in pressure drop. Subsequently, a decision was made to enlarge the fan rotors as shown in Figure 2 so that the 800 horsepower motors would be fully loaded and a gain in both pressure and flow could be realized.
Process Equipment Company initially measured the performance of the fans as summarized in Table 1. The exact design of the tip modification was concluded after these field measurements were evaluated. The cost to perform a field modification of this type is quite inexpensive as compared to the cost associated with completely replacing the fans. Table 2 is a summary of the modified fan performance after the field impeller modification was complete. You can see from Table 2 that the total flow was increased by about 20% and the static pressure head increased by this same amount.
| Fan No. | Flow Rate - ACFM | Fan Pressure Rise | Fan Efficiency | Motor HP |
|---|---|---|---|---|
| 1 | 245,402 | 13.6 | 80.5 | 653 |
| 2 | 274,802 | 10.8 | 76.5 | 611 |
| 3 | 184,081 | 10.2 | 36.9 | 800 |
| 4 | 267,187 | 9.2 | 65.1 | 594 |
| 5 | 292,039 | 8.2 | 65.8 | 574 |
| 6 | 277,230 | 9.3 | 62.6 | 648 |
| 7 | 342,072 | 8.2 | 61.7 | 716 |
| 8 | 185,900 | 10.9 | 39.4 | 808 |
| TOTAL | 2,068,713 |
| Fan No. | Flow Rate - ACFM | Fan Pressure Rise | Fan Efficiency | Motor HP |
|---|---|---|---|---|
| 1 | 294,868 | 15.1 | 88.0 | 796 |
| 2 | 329,941 | 14.3 | 90.3 | 800 |
| 3 | 315,816 | 14.7 | 91.8 | 795 |
| 4 | 293,651 | 14.5 | 84.6 | 792 |
| 5 | 286,635 | 13.5 | 82.6 | 737 |
| 6 | 282,859 | 13.6 | 81.2 | 745 |
| 7 | 306,781 | 13.5 | 82.8 | 787 |
| 8 | 309,664 | 14.5 | 92.7 | 762 |
| TOTAL | 2,411,215 |
The increase in fan capacity eliminated the smoke accumulation problem inside the building and was achieved at a very affordable capital cost price level. Replacement fans and motors on a project of this type would have cost millions of dollars while the field tip out capital cost totalled a fraction of this amount.
For additional information on articles appearing in this newsletter, contact Process Equipment Company at (205) 663-5330 or E-mail to: information@process-equip.com
