Page 292 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:41 PM Page 254

254 Applied Process Design for Chemical and Petrochemical Plants

®
Figure 10-177. Air-cooled Stac-Flo steam condensers illustrating process system. Representative types of tubes are illustrated. (Used by per-
mission: Bul. M-390621 10/90. ©Hudson Products Corporation)

maximum temperature of the exit air must either be lim- number (for emergency removal of blades to obtain balance
ited by specification, or the gears must be rated at the for continued partial operation.)
88
expected air temperature surrounding the case. Remote Fan diameters range from 3–60 ft. The blades may be
lubrication should be provided for gears, bearings, etc., to solid or hollow construction, 251 with the hollow design being
prevent shutdown of the unit. the most popular.
For V-belt drive, the type of belt section and maximum num- The blades are usually fixed pitch up to 48-in. diameter
ber of belts may be specified, as well as the minimum number with applications for adjustable pitch above this size. Fixed
—usually 3. B-sections are most common. V-belts are not con- pitch is used up to 60-in. diameter with aluminum fan blades
sidered for drives over about 50–60 hp, and a minimum ser- when direct-connected to a motor shaft. Variable pitch is
vice factor of 1.4 should be specified for continuous duty. Belts used with belts, gears, etc., between the fan shaft and the dri-
should not be used in any conditions where the surrounding ver to allow for the possibilities of slight unbalance between
temperature is greater than 160°F, with or without fans oper- blades due to pitch angle variation. Aluminum blades are
ating. This is of particular importance in induced draft condi- used up to 300°F, and plastic is limited to about 160°–180°F
tions where belts might be in the exit air stream. air stream temperature.
For general service, the fans are axial flow, propeller type Air noise is usually less with multibladed fans (4 or more)
with 2–20 blades per fan which force or induce the air across than with 2 or 3 blades. In general, noise is not a real prob-
the bundle. Four blades are considered minimum, and an lem when associated with other operating machinery and
even number of blades (2–20) are preferable to an odd when the frequency level is low and nonpenetrating. When