Cycling through fan options
Engineers must match the best fan to its use and performance.
By J. Patrick Banse, PE, LEED AP Senior Mechanical Engineer, Smith Seckman Reid Inc., Houston -- Consulting-Specifying Engineer, 3/1/2008 1:00:00 AM
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Owning and operating costs, noise, environment, availability, service duty, and power are but a few of the factors engineers need to consider when selecting a fan for a long and efficient life.
Matching the best fan selection to use and performance is the primary goal. With new construction, the designer has time to design the air-moving system and select the best fan type for the use. When a fan needs replacing, the schedule may not always allow a new “design” to take place without extensive system downtime.
Table 1 in Chapter 18 of the “2004 ASHRAE Systems and Equipment Handbook” shows different types of fans and fan design characteristics along with performance and usual applications. Fan manufacturers also have selection guidelines to aid in proper fan selection.
Fan service life ranges from 15 years for propeller fans to 25 years for centrifugal fans, according to studies published in Table 4 of Chapter 36 in the “2007 ASHRAE Applications Handbook.” Obviously, factors such as installation, environment, maintenance, and operating conditions affect the service life. In addition to these factors, what other considerations are there in the fan lifecycle selection? Figure 1 shows some items to consider.
Initial costs (owning) and power and maintenance cost (operating) are generally the primary selectors when choosing a fan. For a fan to last 20 years, proper maintenance must be provided, which includes designing adequate installation and maintenance space at new construction. The installation should allow fans to be inspected, disassembled, and greased.
Fans installed with proper duct inlet and outlet connections will lead to higher operating efficiency, less power use, and quieter operation. Fan bearing noise, vibration, and belt condition/slippage are items that should be inspected for regularly in an effective maintenance program.
Where floor space is not available, an inline type fan might be needed. Inline centrifugal fans generally are noisier and require sound attenuators that add to the initial cost life. Maintenance on a suspended fan also may be more difficult due to less-than-desirable access.
The need for flexibility in design, future air volume, and the fan's ability to operate at less than peak air volume (i.e., part-load efficiency) are additional parameters of fan lifecycle selection. The fan laws are mathematical formulas that predict fan performance and power requirements for a series of aerodynamically similar fans based on varying the fan size, fan speed, or air density given the volume or pressure are fixed. The fan laws and explanations for use are available in ASHRAE handbooks and in most manufacturers' fan catalogs. The fan laws also will help identify new performance characteristics of an existing fan to help determine if a more efficient fan of differing size and speed with a better lifecycle cost can be used.
Bigger may be better
Consider a fan operating continuously at 8,760 hours/yr at an energy cost of $0.15/kWh. Airflow is 5,000 cfm at a pressure of 1 in. of water. A manufacturer's catalog for centrifugal utility fan may show a reasonable selection for a 24-in. diameter fan requiring 0.9 bhp at 690 rpm, while a 20-in. diameter fan costing $300 less requires 1.25 bhp at 1,005 rpm.
It would appear that both fans require a 1.5 hp motor, but the larger diameter fan operating at a slower speed will require less power and potentially last longer with fewer bearing problems and operate more quietly, thus making the lifecycle cost more beneficial for the larger diameter fan. Installation and maintenance costs are assumed the same for both fans. The power required includes fan drive losses of 3% to 5%.
The energy cost is calculated by the following method:
number of hours/yr x power (bhp) x 0.746 kW/bhp x dollars/kWh
Using the formula, the energy costs are:
Fan size Energy cost
24 in. $882
20 in. $1,225
Annual savings: $343 (less than 1 year payback).
It is important to consider both the fan and system design when defining the fan lifecycle cost. The operational characteristics may be the driving factor in a long-term scenario, but if the fan cannot be (or is not) properly maintained, the energy performance may not be a factor as the fan may require early replacement. In this case, the fan lifecycle cost is limited to initial cost (owning) and energy cost (operating) for a shorter time period.
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