Case History No. 6

Industry & Location:

An aluminum plant in the east

Problem:

Inadequate ventilation from the pot line work area

Source of Detection:

Excessive hydrogen fluoride fumes in the pot room area.

Cause of Problem:

I.D. fans with 800 horsepower motors are used to induce hydrogen fluoride fumes from the pot line work area and discharge these fumes into a scrubber system. The scrubber system was originally designed to operate at a pressure loss of from 8-10 inches. With the advent of improved scrubbing technology, i.e., more efficient packing material, it was discovered that the scrubber could be operated at a considerably lower pressure drop of 3-4 inches and still comply with environmental regulations.

I.D. Fans

Since the I.D. fans are driven at a fixed synchronous speed, it was necessary to modulate the inlet dampers in a continuously throttled position to prevent an overload condition at the I.D. fan motors. The original inlet box dampers did not represent state of the art technology and caused excessive turbulence and vortex shedding to the fan inlet when they are partially opened. Such turbulence resulted in reduced fan efficiency and excessive horsepower consumption and less than adequate volumetric flow from the pot-line rooms.

Original Damper Design

New Damper Design

Solution:

Each airfoil fan impeller was replaced with an impeller having a slightly reduced diameter and a modified airfoil blade configuration. The modified fan impellers have the capability of developing the new pressure requirement while allowing the fan inlet box dampers to be operated in a more open position. In addition, the inlet box dampers have been replaced with ones having fewer blades and more aerodynamically efficient profiles, i.e., the individual blade drag coefficient has been reduced. This combination of fan modifications has resulted in an improvement in fan efficiency and the lowering of fan horsepower consumption. In fact, the volumetric flow rate from each fan has increased from 150,000 ACFM to 270,000 ACFM and the horsepower reduced from 800 to 550.

Benefit:

Increased flow rate and reduced horsepower