Page 368 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 353

compressor people don’t understand anything but scfm. with microprocessor based controls in some plants has
Of course I only made that stupid error once! eliminated a lot of the waste but there’s still more to do.
Normally all we deal with in a boiler plant is com- A wise operator can realize the opportunities for
pressing air. It has its problems but it isn’t as critical a cost savings by locating and repairing leaks in air sys-
process as compressing oxygen where the hydrocarbons tems and eliminating wasteful use of compressed air.
from your fingerprint on one part can catch on fire in Waste can account for about 60% to 80% of the consump-
the compressor and do damage. Be aware of the hazards tion of compressed air.
associated with any fluid you’re compressing. Best way,
of course, is to read the instruction manual. Just because Reciprocating Compressors
the fluid is flammable or hazardous it’s not something Just like reciprocating pumps reciprocating com-
you should shy away from, with proper training and pressors use a piston that changes the volume of a
sensible operation you should be able to handle any chamber to move the fluid. Intake valves are required to
compressed fluid. open as the piston moves down the chamber, increasing
We regularly use gas compressors, used to boost its volume, so the air can enter the chamber. They close
the pressure of natural gas high enough to fire in our as soon as the flow stops. Unlike a reciprocating pump
boilers. The key to their use is that the gas is all gas; it’s the fluid doesn’t start to leave the chamber as the piston
so fuel rich that it can’t burn inside the unit. All you moves up to reduce the volume, the fluid is compressed
have to be concerned with is any leak that might form a in the chamber instead. Not until the pressure is higher
flammable mixture and accumulate somewhere. in the chamber than in the discharge piping connecting
A unique feature of compressors that is not associ- the compressor to its storage tank will the fluid begin
ated with other fluid handling equipment is the func- to leave the chamber. When the piston reaches the end
tion of ‘unloading.’ Unloading a compressor consists of of its stroke there’s no difference in pressure so the
bypasses, valves held open, or other methods built into discharge valves close. As the piston moves down the
the machinery that prevents compression occurring but chamber to increase its volume the fluid expands until
does not require stopping the compressor. It’s not effi- the pressure in the chamber is lower than the pressure at
cient operation because the compressor isn’t doing any- the inlet. Then the fluid will flow into the chamber until
thing but moving its parts around but the wear and tear the piston reaches the end of its stoke. The progression
of full blown starts and stops is eliminated to make life is depicted in Figure 10-96.
easier on the compressor and driver. Some equipment The typical air compressor valve looks something
even has staged unloading where part of the compres- like a metal popsicle stick. For those of you who have
sor is actually working while the other part or parts are never enjoyed a popsicle on a hot summer day, the valve
unloaded. The original purpose of unloading had noth- looks something like the tongue depressor the doctor
ing to do with continuing compressor operation, it still
serves that purpose today; it allows the driver to bring
the compressor up to speed before it starts compressing
fluid. Even the smallest compressors have that feature.
Almost every boiler plant has a reciprocating
compressor to produce compressed air for controls and
actuators. That will probably be the case for a few more
years until microprocessor based controls and electri-
cally powered actuators are fully developed to eliminate
both the compressor and all the compressed air distribu-
tion piping. You can pick any other system in the plant
and you won’t find one that is more inefficient than the
compressed air. We compress air to 80 to 120 psig then
use most of it at 18 to 30 psig.
I don’t understand why I can’t convince plants to
install little compressors to produce air at about 25 psig
and distribute that to all the controls then leave the other
one to serve actuators that need it and provide atomizing Figure 10-96. Reciprocating compressor operating
medium for emergencies. Replacing pneumatic controls stages