Page 370 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 355

cating compressor. There are small compressors that use machined so the cavity becomes smaller as you move
diaphragms instead of pistons to compress the air and along the shaft. An added feature in the compressor
others with synthetic rings that can operate without oil world is a slide that bleeds air back to the suction to re-
(oil-free compressors) but most of the ones you’ll find duce capacity. Screw compressors are used extensively
in a boiler plant use oil. If you’ve never checked the oil in the construction industry, that’s what most of those
in a reciprocating compressor before take this one small little trailers towed behind the contractor’s truck are.
piece of advice; always wait until it has just shut down They also need lubrication because the oil is what seals
before checking the oil. If you just walk up to it and re- the cavities and keeps the metal parts from rubbing each
move the cap on the oil reservoir it’s bound to start and other. Since most construction tools need lubrication
blow oil all over the front of you! Of course that’s advice there’s no problem with what’s carried over with the
from the experienced. air. A screw compressor in a plant is usually followed
Oil is required to lubricate the moving parts of a by an oil separator and coalescing filter to provide the
compressor and except for oil free units, serves to seal specified ‘clean and dry air’ for boiler plant controls and
the space between piston and cylinder so the air can be actuators.
compressed. (By the way, you still have to keep oil in Some rotary compressors are very similar to gear
some oil-free compressors, it’s only the air that has no pumps (Figure 10-97). They simply move air along with
oil in it) little concern for the fact that air rushes into the cavity
Since the oil is coating the cylinder walls, is scraped as it opens to compress the air before it starts flowing
by the piston rings, and exposed to those parts heated out. Vane type rotary compressors (Figure 10-98) use
by the inefficiency, some of it is vaporized and some the eccentrically positioned core to produce a cavity
droplets form to leave the compressor with the air. As that changes volume to compress the air as the chamber
compressors age they tend to load the air with oil more rotates around the shaft. These compressors must be
than when they were new. Your system should have an
oil separator to remove that oil so it doesn’t contaminate
instruments, controls, and tools that use the air. At least
that’s true most of the time, some systems are only used
for tools and the oil helps lubricate them. In that case the
oil should be a non-hazardous type that doesn’t form
poisonous aerosols where it leaves the tool. In addition
you could have an oil coalescing filter which absorbs
the oil. For the sake of your controls, please watch that
coalescing filter and change it when it’s not quite satu-
rated. Also make certain the separator is working to re-
duce the oil loading on the filter.

Other Types of Compressors
Figure 10-97. Lobe type rotary compressor
Centrifugal compressors were touted as the latest
thing about forty years ago but they quickly faded away
because the tip speeds had to be so very high to develop
the necessary pressure. The compressor required large
speed increasing gears to get that high tip speed and
the stresses on the metals at those high speeds made
them vulnerable to all sorts of problems. A reciprocating
compressor, which runs at relatively low speeds, could
take a small drop of water coming off the previous stage,
a high speed whirling impeller couldn’t. I still think a
steam turbine driven centrifugal could be developed
that would be efficient and reliable but nobody has built
one that anyone would buy.
Screw compressors function about the same as a
screw pump. The important difference is the screw is Figure 10-98. Vane type rotary compressor