Axial fans

When the aim is to build a cheap fan system and have a small pressure rise, then the nozzle and diffuser are not usually installed in the system. 

The performance of an axial fan is based on the external force to drive the propeller, whose blades change the direction of airflow when flowing from the inlet edge to the outlet edge. 

Since P is positive, it must be that C2 when the circumference speed IS chosen as the positive direction. Equation (9.101) is valid for every circumference speed between the hub circumference and the outer impeller circumference. 

In the following discussion, u is the average circumference speed. 1 he absolute velocity in the shaft direction is denoted by c.  

Equation (9.102) gives = C2a- The axial component does not change, but when C2 then C2 Cj. Thus, the axial fan increases the absolute velocity of airflow. 

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Axial fans are classified as propeller, tube-axial, and vane-axial (Fig. 9). The choice of fan requited is determined by the resistance (static pressure) the fan must work against as well as the volume flow requited. PtopeUet fans usually discharge iato a plenum or directly iato the atmosphere. Tube-axial fans are usually mounted ia ducts as ia an air conditioning system. Vane-axial fans are also mounted ia ducts but feature a stationary guide vane on the discharge side that straightens the air flow to improve efficiency. Tube-axial fans can work at static pressures up to 623 Pa (2.5 ia. H2O) vane-axial fans can work up to 2000 Pa (8 ia. H2O). 

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).

Fig. 10. Performance characteristics for axial flow fans ( ndashrule ), ptopeUet fan (—), tube-axial fan and ( ), siagle-stage and (.), two-stage...

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. 

Efficiency. Fan efficiency describes a fan s abiUty to do work and is calculated as total efficiency (Eff) using total pressure (TP) or static efficiency (Eff) using static pressure (SP). Total efficiencies for axial fans range from 55 to 80% static efficiencies range from about 40 to 65%. When pressure is in pascals and flow rate in irf /s,... 

An axial fan is a constant volume device. That is, a fan at a certain pitch moves a constant volume of air or gas at a constant speed and resistance (static pressure). If the density changes, the static pressure and wattage change, but the volume remains constant, ie, if the density (temperature) decreases, the static pressure and kW go up, but air flow remains the same. 

Application Criteria. The desiga and constmction of axial fans is dictated by size and function. Small fans ate usually molded plastic having... 

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. 

Vane axial fans develop a greater static pressure than tube axial fans. They are constructed from a variety of materials, depending on the application. They may use either a direct drive or a belt drive. More expensive models... 

For equal pressure and volume, axial fans weight and size are greater for the backward-curved fan... 

Figure 9.43 shows the schematic diagram of an axial fan system. The air flows through a nozzle toward the impeller, where static pressure rises. The impeller is attached to a hub. The impeller is also called the propeller. The propeller is followed by a diffuser. 

We restrict ourselves to investigate an axial fan, where there is only an impeller. The incoming air has the direction of the axis of the propeller. 

At the exit the absolute velocity has velocity component C2 on the large circumference parallel to the shaft of Example 3. Component is of no advantage if a duct is connected to the axial fan, since it disappears due to the friction between the walls of the duct and gas flow. 

In the previous sections, the centrifugal and axial fans were investigated for a constant rotational speed. 

In this section, the rotational velocity is directly proportional to the rotational velocity n according to the equation u — irDn. The impeller blade angles remain the same regardless of the rotational velocity of the impeller. Hence, the inlet and exit velocity triangles have the same form. The axial velocity of an axial fan changes directly proportionally to the circumference velocity u. This is also valid for the radial velocity at the outer circumference of a radial impeller fan. These velocities are directly proportional to the fan flow volume hence. 

In some instances the fan has a free discharge. Typical is the axial fan installed in a wall opening (wall fan). In most cases the fan is connected to a duct run in this instance the total pressure difference and volume flow are determined from both the fan and duct network characteristics. 

The regulation of axial fan blade angle also influences the inlet and exit velocity triangles in such a way that the axial velocity and thus the volume flow change. When the relative velocity remains parallel to the blade, the efficiency remains high (Fig. 9.52). 

Panel fans A simple form of axial fan with its impeller mounted in a ring or diaphragm it discharges air both axially and radially. 

Tube-axial fans An axial flow impeller mounted in a tubular housing, which contains the rotational velocity. 

Figure 12-118C. Common names associated with axial fan components. (Used by written permission ASHRAE Handbook and Product Directory, 1979, pp. 3.4. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. All rights reserved.) Also see 1996 Edition, HVAC Systems and Equipment.

The axial fan is being built in larger capacities and for higher pressure characteristics and is finding increasing application. Figure 12-124 shows a vaneaxial unit. A tubeax-... 

The vane axial and tube axial can be selected for higher outlet velocities than the centrifugal, approximately 2,000-4,000 ft/min through the fan casing as compared to 2,000-3,500 (a few to 6,600) ft/min outlet velocity for the centrifugal. The axial fans should be connected to ducts by tapered cone connections. 

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