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FIGURE 6-14. System effect factor
"R." To find the system effect correction value in inches water gauge, we use the fan inlet velocity with Figure 6-27. Since the duct area is 3 .142 ft", the velocity is 1592 fpm (5000 cfm
-7-3.142 ft2= 1592 fpm). From Figure 6-27 we get a correction value of 0.19 "wg. This 0.19" value is added to the fan static pressure when selecting the fan from the multi-rating table.
Select the fan for a static pressure of 8.19 "wg. Interpolating in Table 6-1, we find a selection for 5000 cfm and 8.19 "wg at 2002 rpm and 13.25 hp. This selection for a fan with an elbow at the inlet will result in operation at 5000 cfm and 8
"wg drawing 13.25 hp.
FIGURE 6-15. Fan discharge conditions
Note: The .lystem effixt coefficient compensatesfor the affect on the fan of an irregular air stream. This .system effect coefficient is taken in addition to thefriction loss used to calculate the ,system loss (Figure 5-13.)
Figure 6-15 illustrates typical discharge conditions and the losses which may be anticipated. The magnitude ofthe change in system performance caused by elbows and other obstruc- tions placed too close to a fan inlet or outlet can be estimated for the conditions shown on Figures 6-18 through 6-25.
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Addition to system static pressure is given by reference to lettered curves in all but Figure 6-23. The additional static pressure, in "wg, is determined by obtaining the appropriate system effect coefficient from Figure 6-26 or 6-27 and mul- tiplying it by the fan inlet or discharge velocity pressure.
A vortex or spin of the air stream entering the fan inlet may be created by non-uniform flow conditions as illustrated in Figure 6-24. These conditions may be caused by a poor inlet box, multiple elbows or entries near the inlet, or by other spin-producing conditions. Since the variations resulting in inlet spin are many, no System Effect Coefficients are tabu- lated. Where a vortex or inlet spin cannot be avoided or is discovered at an existing fan inlet, the use of turning vanes, splitter sheets, or egg crate straighteners will reduce the effect.
6.4.2 Inspection and Maintenance: Material accumula- tion or abrasive wear on an impeller can cause a fan to "go out of balance." This unbalance will cause fan vibration. This may result in damage to or failure of the fan impeller, housing, bearings, or pedestal. Periodic cleaning and rebalancing of fans operating in air streams handling high material concen- trations is recommended.
Regular observation offan vibration levels can detect prob- lems before they develop to a damaging amplitude (see Figure 6-17). Modem maintenance equipment permits the inspector to record vibration spectra. Review of changes in these spectra taken over time can indicate specific areas of developing problems with bearings, balance, belts or motors. Electronic or computerized vibration monitors are available to mount on fans used in critical operations. These devices can be set up with automatic alarm functions and/or to provide continuous information about a unit's vibration level.
It is not uncommon, during fan installation or motor/starter maintenance, for the fan impeller rotation direction to be
inadvertently reversed. Since fans do move a fraction of their rated capacity when running backward, incorrect rotation often goes unnoticed in spite of less effective performance of the exhaust system.
Scheduled inspection of fans is recommended. Items checked should include:
I. Bearings for proper operating temperature (lubricate them on the manufacturer's recommended schedule).
2. Excessive vibration of bearings or housing.
3. Belt drives for proper tension and minimum wear.
4. Correct coupling or belt alignment.
5. Fan impeller for proper alignment and rotation.
6. Impeller free from excess wear or material accumula- tion.
7. Tight fan hold-down bolts.
8. Tight fan impeller set screws or bushings, 9. Proper installation of safety guards.
Standard lockout/tagout procedures should be observed when servicing fan equipment or its associated duct. The electrical supply must be shut off and locked out at a discon- nect near the fan. When opening access doors or reaching into the fan inlet or outlet, the fan must be mechanically locked out by blocking the impeller from rotating. A warning tag should be used when blocking a fan. Do not open an access door while the fan is operating or coasting down.
BE SURE to remove any inserted obstructions used to block impeller rotation when servicing is complete,
REFERENCES
6.1. Air Movement and Control Association, Inc.: AMCA Publication 201-90, Fans and Systems. AMCA, Ar- lington Heights, IL (1990).
6.2. Gibson, N.; Lloyd, F.C.; Perry, G.R.: Fire Hazards in Chemical Plants from Friction Sparks Involving the Thermite Reaction. Symposium Series No. 25. Insn.
Chem. Engrs., London (1968).
6.3. Air Movement and Control Association, Inc.: AMCA Publication 99-86, Standards Handbook. AMCA, Ar- lington Heights, IL (1986).
6.4. American Society of Heating, Refrigeration, and Air- Conditioning Engineers, Inc.: 1993 ASHRAE Hand- book, Fundamentals Volume. ASHRAE, Atlanta, GA (1993).
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