Page 179 - Fluid mechanics, heat transfer, and mass transfer

FANS, BLOWERS, AND COMPRESSORS 157

. What are the advantages and disadvantages of vane & Pulsations in discharge pressure are low due to
type rotary compressors compared to other rotary com- circulation of liquid inside.
pressors?
Disadvantages
Advantages & Low overall efficiency is 35–50%.
& Low cost. . What is the advantage of a dry screw compressor over an
& Highest compression efficiency and overall efficien- oil-flooded screw compressor?
cy over all types of rotary compressors. & Oil-flooded screw compressor contaminates with
& Very high volumetric efficiency and hence high flow oil the product gas and instrument air handled by
rate with respect to machine size. it.
& Low starting torque requirement. & Oil contamination in air compressors resulted in
several explosions involving oil and air.
Disadvantages
& Such risks are eliminated in dry screw compressors.
& Continuous lubrication of internal rubbing parts is
. Summarize characteristics of different types of
required (10 times more than reciprocating
compressors.
machines).
& Table 5.10 presents a summary of the characteristics
& High maintenance due to wear.
of different types of compressors.
& Sensitive to solids in gas stream and so require filters
. Give equations for theoretical adiabatic and polytropic
in air service.
power for compression.
. What are the advantages and the disadvantages of liquid
& Adiabatic power:
ring rotary compressor?
a
Advantages mz 1 RT 1 ½ðP 2 =P 1 Þ 1ފ=a; ð5:27Þ
& Low discharge temperature capability. Useful for
polymerizing gases such as acetylene and gas mix- where T 1 is the inlet temperature (K), R is the gas
tures with exothermic reactions. constant (8.314 J/mol K), z 1 is the compressibility
& Selection and processing of liquid allows unit opera- factor, m is the molar flow rate, a ¼ (k 1)/k, and
tions such as absorption, scrubbing, and cooling, k ¼ C p /C v .
besides compression. & The outlet for the adiabatic reversible flow is
& Insensitive to liquid and solids fines carry over.
& Oil-free operation.
T 2 ¼ T 1 ðP 2 =P 1 aŠ: ð5:28Þ
& Simple with only one moving part.

& Maintenance in corrosive environments is lower than ➢ Exit temperatures should not exceed 204 C

for other types. (400 F).
& Nearly five times as efficient as ejectors in vacuum ➢ For diatomic gases (C p /C v ¼ 1.4), this corresponds
service. to a compression ratio of about 4.

TABLE 5.10 Summary of Characteristics of Different Types of Compressors
Inlet Flow Compression Maximum Overall
3
Type Rate (1000 m /h) Ratio Temperature (K) Efficiency
Positive Displacement
Reciprocating — 3–4 450–510 0.75–0.85
Diaphragm 0.0051–0.051 20 — —
Rotary
Helical screw 34 2–4 450–510 0.75
Spiral–axial 22 3 450–510 0.70
Straight lobe 52 1.7 450–510 0.68
Sliding vane 10 2–4 450–510 0.72
Liquid ring 22 5 450–510 0.50
Dynamic
Centrifugal 85–340 6–8 111–505 —
Axial 1.3–1000 12–24 590 —