Page 175 - Fluid mechanics, heat transfer, and mass transfer

FANS, BLOWERS, AND COMPRESSORS 153
TABLE 5.8 Troubleshooting Guidelines for Steam Jet Ejectors

Problem Effect Corrective Action
Lower than design motive steam pressure Poor ejector performance Raise steam pressure or bore steam nozzles
Higher than design motive steam pressure Reduced ejector capacity and wasted Reduce motive steam pressure
steam
Higher than design temperature Poor ejector performance Raise steam pressure or bore steam nozzles
Higher than design discharge pressure Poor ejector performance Downstream condenser or ejector problems,
restrictions in discharge piping
Low ejector discharge temperature Reduced capacity/poor performance Insulate steam lines/add moisture separator
to steam lines
Higher than design suction pressure Higher than design load or mechanical Inspect internal dimensions and replace if
problems, forexample,worn parts and necessary. Tighten steam nozzle to steam
internal steam leaks. chest.

& Fans are classified according to the direction of air & In an axial flow fan, air/gas enters and leaves the fan
flow. In a centrifugal fan, air/gas flows along the fan parallel to the shaft.

shaft, turns 90 by the impeller, which imparts kinetic & Centrifugalfansareclassifiedaccordingtotheirblade
energy to the air/gas as it flows radially outward. The geometry—radial,forwardcurved,backwardcurved,
kinetic energy is converted to pressure as it leaves the and airfoil. The major characteristic of radial fan is its
fan parallel to the shaft. ability to compress air/gases to a higher pressure but
delivers lower flow rates than the other fan types.
& The blades are self-cleaning, tending to fling off
TABLE 5.9 Pressures and Capacities Obtainable for Vacuum
particles and thus can be used to pneumatically
Equipment
convey solids.
Lower Single Unit
& Figure 5.36 illustrates different types of fans.
Limit for Capacity
& The backward incline fan design consists of the
Ultimate Process range
3
Type Pressure Applications (m /min) single thickness blade and the airfoil blade. The
single thickness blade can be used for pneumatic
Steam jet ejectors
conveying of solids.
Single stage 50 Torr 75 Torr 0.3–30,000
Two stage 4 Torr 10 Torr & The airfoil type has aerodynamically shaped blades
Three stage 0.8 Torr 1.5 Torr to reduce flow resistance, resulting in a high efficien-
Four stage 0.1 Torr 0.25 Torr cy. Entrained particles will damage the blades, and
Five stage 10 mm 50 mm thus this fan is not suitable for pneumatic conveying.
Six stage 1 mm 3 mm
& For both types as the flow rate increases, the required
Liquid ring pumps 1–500
power increases, reaches a maximum, and then de-
Water sealed (15 C)

creases instead of continuously increasing. When
Single stage 50 Torr 50 Torr
operating under conditions where the flow ratevaries,
Two stage 20 Torr 25 Torr
Oil sealed 1 Torr 10 Torr this characteristic is an asset.
Air ejector first stage 1 Torr 10 Torr & The forward curved blade fan is designed for low to
Dry vacuum pumps medium flow rates at low pressures. Because of the
Three-stage rotary lobe 0.5 Torr 1.5 Torr 1.7–6.8 cup-shaped blades, solids tend to be held in the fan,
Three-stage claw 0.1 Torr 0.3 Torr 1.4–7.6 and thus this fan is also not suitable for pneumatic
Screw compressor 50 mm 0.1 Torr 1.4–40 conveying of solids.
Integrated systems
& Axial fans consist of the tube axial fan and the vane
Booster–liquid 1 Torr 5 Torr 2.8–42.5
ring pump axial fan, which are designed for a wide range of
Booster–rotary 25 mm 0.25 Torr 2.8–42.5 flow rates at low pressures. These fans consist of a
lobe dry pump propeller enclosed in a duct. They are limited to
Booster–claw 10 mm 0.1 Torr 2.8–70 applications where the gas/air does not contain
compressor entrained solids.
Booster–screw <0.1 mm 1 mm 2.8–140
& In a tube axial fan, the discharged flow follows
compressor
a helical path creating turbulence. To reduce