Handbook of air conditioning and refrigeration / Shan K

15.26 CHAPTER FIFTEEN
Fan Width and Inlets
FIGURE 15.21 Class standards of fans.
Class standard specifies the construction of the fans as heavy, medium, or light structure. Construction
is closely related to the static pressure developed �p s (in in. WC) by the fan and the fan
outlet velocity v o (in fpm or m/s), as shown in Fig. 15.21. Two classes of fans are widely used in
HVAC&R applications: class I and class II. As shown in Fig. 15.21, a class I fan must provide 5 in.
WC (1250 Pa) static pressure at an outlet velocity of 2300 fpm (11.5 m/s) and a static pressure of
2.5 in. WC (625 Pa) at an outlet velocity of 3200 fpm (16 m/s). A class II fan must provide 8.5 in.
WC (925 Pa) static pressure at an outlet velocity of 3000 fpm (15 m/s) and 4.25 in. WC (1063 Pa)
static pressure at 4175 fpm (20.9 m/s).
For centrifugal fans, air velocity at the fan inlet v i should be optimal. A lower v i means a large inlet.
A higher v i results in a greater energy loss and therefore a lower fan total efficiency. A suitable relationship
is v i/u 2 � 0.35 to 0.4.
A single-width single-inlet (SWSI) centrifugal fan has a single inlet cone connected to the
eye of the impeller, as shown in Fig. 15.22a. A double-width double-inlet (DWDI) centrifugal
fan has a double-width impeller and provides double inlets from both sides, as shown in
Fig. 15.22b. In such an arrangement, v i can still be maintained at an optimal value, and the
volume flow rate is approximately doubled. Because of the interaction of the two airstreams in
the impeller and the scroll, the fan efficiency of a DWDI fan may be 1 to 2 percent lower than
that of an SWSI fan.
Drive Arrangements and Direction of Discharge
Fans can be driven by motor directly or through V-belt and sheaves. In a belt-drive arrangement, fan
speed can be changed by varying the diameter of the sheaves. On the other hand, belt drive requires
3 to 5 percent more energy input than direct drive.