Propeller fan

Fig. 9. Types of axial-flow fans where the dashed arrows denote the direction of air flow (a) propeller fans (b) tube-axial fans (c) vane-axial fans (17).

Propeller Fa.ns, Propeller fans may have from 2 to 6 blades mounted on a central shaft and revolving within a narrow mounting ting, either driven by belt drive or directiy coimected. The form of the blade ia commercial units varies from a basic airfoil to simple flat or curved plates of many shapes. The wheel hub is small ia diameter compared to the wheel. The blades may even be mounted to a spider frame or tube without any hub. The housiag surrounding the blades can range from a simple plate or flat ring to a streamlined or curved beU—mouth orifice. 

A tube axial fan (see Fig. 9.32) is essentially a propeller fan located in a short cylinder housing, the gas flowing in an axial direction. A vane axial fan incorporates specially designed vanes, which are positioned either upstream or downstream of the fan. 

Compact, low-space environment and low weight per unit volume handled make it second only to the propeller fan. 

A propeller fan consists of a propeller or disc wheel within a mounting ring or plate. The propeller fan is designed to move air from one enclosed space to another or from indoors to outdoors or vice versa in a wide range of volumes at low pressure. (The automatic type of shutter illustrated in the cutaway opposite is not a part of the propeller fan but is an auxiliary device to protect the fan when not operating by keeping out wind, rain, snow, and cold). 

Figure 12-125. Square panel, v-belt drive propeller fan. (Used by permission The New York Blower Co. For more information, contact the company at www.nyb.com.)...

Propeller fans are of low capital cost for the volume moved but are used in applications where the resistance is very small such as non-ducted openings through partitions. The power required increases with resistance. Their pressure-volume characteristic changes with the relative position of the blades and mounting plate. 

Large volumes of air at low pressures can be moved by the propeller fan (Figure 2 7.6). The imparted energy is mainly in an axial direction and any large external resistance will cause a high proportion of slip over the blades. 

Figure 27.6 Propeller fan. (a) Construction, (b) Typical performance curves...

The working pressure limits of the propeller fan, depending on its diameter, are of the order of 150 Pa. The characteristic curve has a pronounced trough , which should be avoided in application if at all possible, since wide variations in air flow can occur for a small change in pressure. Performance varies with aperture shape, clearance and position. 

In the smaller cooling tower installations employed for refrigeration and air conditioning service, induced-draft propeller-type air movers are used. A favored design utilizes the rotary centrifugal squirrel cage blower, which is normally driven by electric motors connected to the rotors by V-belts. Because of lower fan efficiency, these often use more electric power to achieve the same results as propeller fans. 

An important characteristic of propeller fans is that their operating smoothness is directly proportional to the number of blades loading and unloading as they pass over the mechanical equipment supports and tower... 

Erosion Similar to abrasion cutting in ductile metal fracture (of brittle material) very small chips or particles (e.g., impellers, propellers, fans) Reduce fluid velocity to eliminate turbulence select harder alloy (high chromium) hard coatings such as cement lined pipe, rubber lining... 

The horsepower of a propeller fan, if the speed remains unchanged, increases as the area of the discharge orifice is decreased. In some designs the horsepower with rated orifice is not over one-quarter of the horsepower with complete restriction. 

The propeller fan is used in heating, ventilating, cooling, drying, etc., rarely for pressures exceeding 1 in. of water Their rated capacities are given in the accompanying table. 

Approximate Horsepower of Propeller Fans.—The horsepower required to drive any propeller fan may be represented by an expression of the form,... 

Table 7.—Constants for Propeller Fans (B. F. Sturtevant Company)...

Axial Flow Propeller fan Tubeaxlal (duct) fan Vane axial fan Moves large quantities of air low static pressure used for relatively clean air and no duct resistance common for general ventilation Fabricated in a round duct used for condensable fumes, pigments and other materials that collect on blades larger diameters at slow speeds are better for abrasives and accumulating material Develops higher pressures than other axial flow fans more economical in horsepower and space... 

Fan stack Cylindrical or modified cylindrical structure in which the fan operates. Fan stacks are used on both induced-draft and forced-draft axial-flow propeller fans. Also known as/on cylinder. Fill The section of the cooling tower consisting of materials which are placed within the coohng tower to effect heat and mass transfer between the circulating water and the air flowing through the tower. 

This type of fan consists of a propeller, or paddle wheel, mounted on a rotating shaft within a ring, panel, or cage. The most widely used propeller fans are found in light- or medium-duty functions, such as ventilation units where air can be moved in any direction. These fans are commonly used in wall mountings to inject air into, or exhaust air from, a space. Figure 13-1 illustrates a belt-driven propeller fan appropriate for medium-duty applications. 

Figure 13.1 Belt-driven propeller fan for medium duty applications...

More efficient and much quieter during operation than propeller fans for static pressures above approximately one inch of water (gauge)... 

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