Page 173 - Boiler_Operators_Handbook,_Second_Edition

158 Boiler Operator’s Handbook

Figure 5-13. Screw compressor

cated as well. Some screw compressors will have a shaft fixed speed motor and inlet vanes (Figure 5-14). The in-
mounted oil pump while others will rely on a separate let vanes are typically operated by a small motor mount-
electric pump at all times. ed outside the compressor housing. They reduce flow
Although I’ve mentioned that a plant noise level through the compressor by closing off on the inlet while
should be restricted so you don’t need to wear earplugs simultaneously creating a whirl in the flow of the gas in
or earmuffs I have to admit I have never been near a the direction of the impeller rotation to further reduce
screw compressor without one or the other. Screw com- the motor horsepower required at the reduced flows.
pressors are inherently noisy; so noisy that you should Centrifugal compressors can be open or hermetic
never go near them without ear protection. but are normally hermetic when powered by an elec-
tric motor. You may encounter one that’s powered by
Centrifugal Compressors a steam turbine or diesel engine. The principal use of
If you’ve ever wondered why a facility would uti-
lize chilled water for cooling and air-conditioning pur-
poses instead of individual local refrigeration equip-
ment it’s because chillers, which are discussed later, are
so efficient. Refrigeration using a centrifugal compressor
has energy requirements that are fractional compared to
other types of compressors. With the advances in design
that include variable speed drives and magnetic bear-
ings a centrifugal compressor today has half the oper-
ating cost of one from 20 years ago and those still pro-
duce cooling at half the power cost of local refrigeration
equipment. A centrifugal compressor is a high tech piece
of equipment and must be built in large sizes to be eco-
nomical. A centrifugal compressor is very much like a
blower (their forms are discussed in Chapter 10 under
fans and blowers).
Considerable turndown is achieved with a centrif-
ugal compressor using variable speed drives. However
you may find yourself operating an older one that has a Figure 5-14. Centrifugal inlet vanes