Page 178 - Fluid mechanics, heat transfer, and mass transfer

156 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

& Thermal Compressors: These are the ejectors that & Capacity can be controlled by speed variations,
use a high-velocity gas or steam jet to entrain the reducing suction pressure or by inlet vane control.
inflowing gas, then convert the velocity of the mix- & Service factor is so high that only one compressor is
ture to pressure in a diffuser. required even in services requiring three or more
. What are the ranges of pressures obtainable for different years in continuous operation.
types of compressors? & Can handle liquids and solids better than other types
& Centrifugal: Over 345 bar (>5000 psig). of compressors, although it is not desirable to have
liquids and solids in the gas for any type of
& Rotary screw: 20 bar (250 psig).
compressor.
& Sliding vane: 10 bar (150 psig).
. What are the disadvantages of reciprocating
& Reciprocating: 6900 bar (100,000 psig).
compressors?
. What are the ranges of power for different types of
& For continuous duties, more than one machine must
compressors?
be provided to permit servicing.
& Centrifugal (multistage): For light molecular weight
& Large and expensive.
gases more than 15 kW (2000 HP) and for general
& High maintenance costs, especially when handling
hydrocarbon gases more than 4 kW (500 HP).
gases containing liquids, solids, or corrosives.
& Centrifugal (single stage): Low power and low head
& Require large foundations.
applications, for example, blowers.
& Rotary screw: From 74.6 10 3 to 45 kW . How does a diaphragm compressor work? What are its
applications?
(10–6000 HP).
& In a diaphragm compressor, a piston acts indirectly
& Sliding vane: Up to 3 kW (400 HP).
by applying pressure to hydraulic oil, which flexes a
& Reciprocating: Up to 110 kW (15000 HP).
thin metal diaphragm to compress the gas.
. What are the factors to be considered while choosing
& It is used for small flow rates, below the range for
between dry and oil-flooded screw compressors?
reciprocating compressors, and is limited by the
Dry Screw Compressors construction of the diaphragm.
& Dry type is the choice if oil is not compatible with & An advantage of the diaphragm compressor is that
oil or it should be free from oil. Also if the gas leakage of either the gas or the oil into the gas is
contains small particulates, dry compressor is pre- prevented. Thus, the diaphragm compressor is ideal
ferred. for compressing flammable, corrosive, or toxic gases
at high pressures.
Oil-Flooded Compressors
& A disadvantage is its high maintenance cost, mainly
& Used if oil is compatible with gas or if gas carries
because the diaphragm has to be replaced after about
droplets of oil as in natural gas processing systems.
2000 h of operation.
Also oil-flooded compressors are good to minimize
. What are the plus and minus points for rotary screw
or avoid shaft seal leakages.
compressors?
. “Centrifugal compressors are classed as dynamic
machines” Comment. Plus Points
& These do their work byusing inertial forces applied to & Balanced machines and require light foundations.
gas by means of rotating bladed impellers. & As there are no rubbing surfaces, do not contaminate
. What are the advantages of centrifugal compressors? compressed gas with lube oil.
3
& Large capacities: 30–4500 atm m /min) and pres- & Low maintenance costs.
sures up to about 350 bar. & Cheaper than centrifugal and reciprocating
& Efficiencies are in the range of 68–76%. compressors.
& No contamination by lubricating oil, as they do not & Can be used to compress gases with contaminants
have rubbing surfaces. such as liquid droplets, tars, and polymers, with good
& Balanced machines and do not require heavy levels of efficiency.
foundations.
Minus Points
& Low maintenance costs.
& Noisy.
& Within their operating range, initial cost is less than
& Range of capacity variation at constant speed is very
that for reciprocating compressors within the same
small.
operating range.
& Designed for a specified gas and compression ratio.