Fans, key to optimum cooling tower design
The questions most frequently asked of a fan engineer about axial-flow fans for today's wet cooling towers generally cover:
• Corrosion resistance
This article reviews such fundamentals and hopefully gives new insight for optimum tower design.
Optimum. Optimum fan performance is several things other than just delivery of design-air quantity:
• Lowest first cost. This means optimizing the fan diameter and number of blades. Why buy extra blades if they aren't needed?
• Lowest horsepower requirements. The fan should work as close to an efficient operating point as possible.
• Lowest noise without extra cost. Since noise emitted by the fan is a function of tip speed to almost the 6th power, even a small reduction of speed has significant effect on the environment. If this can be accomplished and the required air duty attained at no extra cost, why not?
• Lowest maintenance costs. The optimum choice of fan blades and hub materials ensures against corrosion and subsequent replacement.
Before going into some suggestions on how to achieve the proceeding goals, a review of basic fan principles may be helpful. Starting from ground zero, they are:
• A fan is supposed to move air and do work.
• This air is supposed to be evenly distributed over the entire exit area of the fan.
To move air, the fan must overcome two resistances, which are measured as pressure drops across the fan.
The first is a parasitic loss called the velocity-pressure loss. I see this as the energy required to move the required air quantity without doing any work to overcome the system resistances. Work is done however to move the hot air away from the equipment.
The second resistance is the static pressure loss. It is the accumulated losses due to duct, fill, and mist eliminator pressure drops. This would be the "work" to be accomplished and reflects the design of the total system, including inlet conditions.
Whether the air is distributed evenly across the fan is primarily a function of the blade and hub design. A properly designed blade will have adequate chord width and "twist" to ensure an even distribution of velocity pressure over its entire length.
A properly designed hub will include a center air-seal disk which prevents negative air flow at the center of the fan.