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Handbook of air conditioning and refrigeration / Shan K

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15.12 CHAPTER FIFTEEN<br />

Tubular or In-line Fans<br />

A tubular or in-line centrifugal fan generally consists <strong>of</strong> an impeller with <strong>air</strong>foil blades, an inner<br />

cone, a set <strong>of</strong> fixed vanes, <strong>and</strong> a cylindrical casing, as shown in Fig. 15.9. Air enters the tubular fan<br />

at the inlet <strong>and</strong> then flows through the blade passages <strong>of</strong> the impeller. It is discharged radially<br />

against the inner surface <strong>of</strong> the cylindrical casing <strong>and</strong> is then deflected in a direction parallel to the<br />

axle <strong>of</strong> the fan, in order to produce a straight-through flow. The impeller <strong>of</strong> a tubular centrifugal fan<br />

may have 6 to 12 blades. Fixed vanes are used to convert velocity pressure to static pressure to prevent<br />

swirls <strong>and</strong> straighten the <strong>air</strong>flow.<br />

In Fig. 15.8, it can be seen that the �pt-V˙ curve for a tubular centrifugal fan is similar to that <strong>of</strong> a<br />

backward-curved fan. Because <strong>of</strong> the 90° turn <strong>of</strong> the <strong>air</strong>flow after the impeller, both total pressure<br />

<strong>and</strong> total efficiency are lower than those <strong>of</strong> a backward-curved fan, <strong>and</strong> the sound power level is<br />

greater. The fan power P-V˙ curve <strong>of</strong> this type <strong>of</strong> fan is also a nonoverloading curve. The main advantage<br />

<strong>of</strong> the tubular fan is its straight-through <strong>air</strong>flow.<br />

Unhoused Plug/Plenum Fans<br />

FIGURE 15.7 Velocity triangles <strong>of</strong> radial-bladed <strong>and</strong> forwardcurved<br />

centrifugal fan: (a) radial-bladed; (b) forward-curved.<br />

Unhoused plug/plenum fans are centrifugal fans with no outer casing or scroll. The impeller is<br />

mounted in a plenum with a flexibility <strong>of</strong> the location <strong>of</strong> the discharged outlet. Unhoused centrifugal<br />

fans are simpler in construction <strong>and</strong> can plug into an <strong>air</strong> system where a plenum can be used as the<br />

outer casing <strong>of</strong> the impeller, <strong>and</strong> the two ends <strong>of</strong> the plenum used as the inlet <strong>and</strong> outlet <strong>of</strong> the fan.

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