Page 176 - Fluid mechanics, heat transfer, and mass transfer

154 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

FIGURE 5.36 Types of fans.

turbulence and increase the fan efficiency, the vane & The performance of a centrifugal fan varies with
axial fan contains flow-straightening vanes. changes in conditions such as temperature, speed,
. What are the characteristics and applications of fans? and density of the gas being handled.
& Fans are used for high flow and low-pressure appli- . “Axial flow fans and blowers have high specific speeds
cations to move large volumes of air or gas through compared to centrifugal compressors.” True/False?
ducts at near atmospheric pressures, supplying & True.
air for drying, cooling towers, removing fumes, and . “Mixed flow fans and blowers have higher specific
so on. speeds than axial flow types.” True/False?
➢ Because the clearances between the impeller and & False.
the casing are large, the pressure developed is low,
. What are the advantages and disadvantages of straight
between 1.01 and 1.15 bar.
lobe blowers?
& In large buildings, blowers are often used due to
& Low cost.
the high delivery pressures needed to overcome the
& Low maintenance requirements.
pressure drop in the ventilation system. Most of these
blowers are of the centrifugal type. Blowers are also & Applicable for low-pressure operations.
used to supply draft air to boilers and furnaces. & Poor efficiency compared to screw compressors
because of slip and high-frequency flow direction
. What are the pressure increases involved in fans,
reversal, which takes place at the discharge port.
blowers, and compressors?
& Solid contaminants deteriorate critical internal
& Fans: Low delivery pressures: 3% (<40 kPa) (30 cm
clearances.
H 2 O).
. Classify different types of compressors.
& Blowers: <300 kPa (<2.75 barg).
& Figure 5.37 gives classification of different types of
& Compressors: Higher pressures: >300 kPa
compressors.
(>2.75 barg).
& Positive Displacement Compressors:
. What is a jet compressor?
➢ Reciprocating compressors are used mainly
& Jet compressors utilize a high-pressure gas to raise
when high-pressure head is required at a low
other gases at low pressure to some intermediate
flow.
value by mixing with them.
➢ Reciprocating compressor ratings vary from frac-
. What are the factors that affect performance of
tional to more than 30,000 kW (40,000 hp) per
centrifugal fans?