22/10/2014

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Hi Green Tip #4: Hi Size and Select Fans Near Their Peak Total Efficiency.

Even the most efficient fan models can operate inefficiently if improperly sized.Fans selected close to their peak total efficiency (pTE) will use less energy. The 2012 International Green Construction Code requires selections within 10% of peak efficiency, and ASHRAE Standard 90.1,

Energy Standard for Buildings Except Low-Rise Residential Buildings, is considering language that would require a 15% allowable range. If a fan is selected to operate more than 15 point below its peak efficiency, it is probably undersized to result in the lowest purchase price (first cost). The smaller, less-expense fan will have to run much faster with higher levels of internal turbulence than its larger cousin to meet the required air flow, thus consuming a lot more energy.The cost difference to select a larger fan closer to peak operating efficiency is very small when compared to the energy saved.

Simple payback for 10% selections is usually less than one year. Smaller fans operating faster will also require more maintenance and earlier replacement. Smaller fans generate more noise as well.Below is a table showing the output from a fan manufacturer's sizing and selection program. All of the fans in the table would "do the job" of providing the required airflow at the required pressure.

The fan sizes range from 18-inches in diameter to 36-in. Notice that as the fan diameter increases, the fan speed decreases, as does the fan power (expressed as "brake horsepower"). The red region of the table indicates poor fan selection practice - none of these fans have an actual total efficiency (at the airflow and pressure required) within 15 points of peak total efficiency. The green region indicates proper fan selection process - all have an actual total efficiency within 15 points of peak total efficiency.

Note that the 30-in. diameter fan consumes roughly half the power of the 18-in. fan. The lowest cost fan shown is probably the 20-in. fan, with an efficiency of 49%, 29 points off the peak. If this fan runs 6,000 hours per year at a utility rate of 10 cents per kwh, it will cost $4,300 a year to operate. A more efficient selection might be the 24-in. fan because it is "Class I" and complies with both ASHRAE 90.1 and the Green code requirements. It has an actual efficiency of 69%, 10 points less than the peak efficiency of 79%. This fan would cost $3,100 to operate, which is probably more than the fan itself costs. A more efficient 30 inch selection is only 1 point from its peak efficiency of 83% and will consume only $2,600 per year, saving $500 a year relative to a 24-in. fan, and $1,700 a year over the lowest cost fan. Generally, the difference in initial cost of the most efficient fan selection is paid back in less than 5 years over more common less efficient alternatives. Perhaps this observation will bring it home.

Most fans consume more each year in energy cost than they are worth. So, when you buy a fan, think of it as a liability, not an asset. Your objective should be to make the liability placed on those who will pay future energy bills as low as possible. The leverage implicit in choosing a larger, more efficient fan is much greater than most people appreciate. And fans last a long time – 20 years plus – so choose wisely.The bottom line is this. Right-sizing a fan can yield energy savings and generate a lot of operating cost savings for the facility owner or occupants for many, many years.

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