Performance curve, fans

FAN PERFORMANCE CURVE - Fan performance curve refers to the constant speed performance curve. This is a graphical presentation of static or total pressure and power input over a range of air volume flow rate at a stated inlet density and fan speed. It may include static and mechanical efficiency curves. The range of air volume flow rate which is covered generally ex tends from shutoff (zero air volume flow rate) to free delivery (zero fan static pressure). The pressure curves are generally referred to as the pressure-volume curves. 

Fig. 2. Fan performance (a) a typical test arrangement (b) performance curve where the soHd line showing data points is the pressure the dashed line is the power. At point A the duct is blanked off, and at point B the flow is wide open the data points in between represent progressively less restricted flow.

Types and Characteristics. The four basic fan wheel and blade designs and the corresponding performance curves are illustrated in Figure. 

Fig. 3. Shape of fan blades and typical performance curves (a) forward-curved blades (b) backward-inclined blades, at left, straight backward blades at top,...

Fan Rating. Axial fans have the capabiUty to do work, ie, static pressure capabiUty, based on their diameter, tip speed, number of blades, and width of blades. A typical fan used in the petrochemical industry has four blades, operates neat 61 m/s tip speed, and can operate against 248.8 Pa (1 in. H2O). A typical performance curve is shown in Figure 11 where both total pressure and velocity pressure are shown, but not static pressure. However, total pressure minus velocity pressure equals static pressure. Velocity pressure is the work done just to collect the air in front of the fan inlet and propel it into the fan throat. No useflil work is done but work is expended. This is called a parasitic loss and must be accounted for when determining power requirements. Some manufacturers fan curves only show pressure capabiUty in terms of static pressure vs flow rate, ignoring the velocity pressure requirement. This can lead to grossly underestimating power requirements. 

Performance Curves. Pan manufacturers furnish fan performance curves for each type fan available. These are typically based on 61 m/s (12,000 ft/min) tip speed and 1.20 kg/m (0.075 lb /ft ) density. To select a fan for a specific duty requires knowledge of the flow, static pressure resistance, and density of the actual operating conditions. Usually the fan diameter is known as well as some idea of operating speed a 61 m/s tip speed can often be assumed. 

FIGURE 9.42 Typical performance curve of centrifugal fan, - q, chart,... 

These performance curves reflect the general characteristics of various fans as commonly employed. They are not intended to provide (he complete selection criteria for application purposes, since other parameters >uch as diameter, speed, etc. are not defined. 

Figure 12-133A. Performance curve for axial-type fans. (Used by permission The Howden Fan Company.)...

Bard presents an unusual analysis with typical performance curves of the problems associated with various condidons that can develop with parallel fan operadon. 

Figure 27.5 Centrifugal fan. (a) Construction, (b) Forward-curved blades and typical performance curves, (c) Backward-curved blades and typical performance curves...

Figure 7.24. Performances of fans with various-shaped blades (Green Fuel Economizer Co.) (a) Backward curved blades, (b) Straight radial blades, (c) Forward curved blades, (d) Comparison of characteristics of the several blade types (Sturtevant).

The manufacturer s proposal shall include performance curves of the tower, based on design fan horsepower, showing cold water temperature versus air wet-bulb temperature for 90%, 100% and 110% of design water flow. The curves shall cover cooling ranges of 15°F, 20°F, 25°F, 30°F and design range. 

FIGURE 6 Fan performance curve. Source Courtesy of Fluid Air Incorporated. 

The existing dust filter has a filtration area of 15 m (Figure 16.12) and is fitted with a fan and polyester needle-felt bags. From performance curves, the fan can extract 33.3m%iin of air at 150 mm H20 = 1.5kPa. At first glance, this appears to cover quite adequately the blower capacity of 21.7 m /min. However, transient effects and filtration efficiency also have to be evaluated. 

The fan-performance curves shown in Figures 13.3 and 13.4 show the performance of the same fan type designed for different volumetric-flow rates, operating in the same duct system handling air at the same density. 

Fan operation is indicated on a performance curve, as shown in Fig. 19.3. The head developed by the fan is equivalent to 5 or 10 inch of water. As the fan air flow is pretty constant, the fan s head is also constant. Another way of stating this is to say that as a tube bundle fouls, the resistance to air flow increases. This reduces the air flow through the bundle, but the pressure loss of the air flow through the tube bundle does not change. 

The wisest fan choice is frequently not the cheapest fan. A small fan operates well on its curve but may not have adequate capacity for maximum flow control, future needs, or process upset conditions. It may be so lightly constmcted that it is operating near its peak speed with no provision for speed increases in the future, if needed. As fan size is increased, efficiency generally improves and wheel speed is lower. These factors decrease operating cost and provide reserve capacity for the future. However, it is also possible to oversize a fan and impair its performance. 

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