Page 369 - Boiler_Operators_Handbook,_Second_Edition

354 Boiler Operator’s Handbook

uses when he asks you to say “ah.” valves to remove moisture collected in the compressor
It’s far more complicated than the typical liquid and storage tank. Then, to give the operator time for
(incompressible fluid) pump which fills and discharges. other duties, we tried installing drain traps on them that
You can’t calculate the volume of the stroke and deter- would automatically drain the water off. We quickly
mine the capacity of the compressor because a good por- learned that we couldn’t count on those drain traps en-
tion of the stroke is devoted to recompressing the fluid tirely so the operator still had to check them regularly.
that expanded after the discharge valve closed. It should Most systems today are equipped with timed drain
be obvious to you that the less fluid in the compressor at traps, solenoid valves connected to a timer that opens
the end of its discharge stroke the less that will be there them at preset intervals to drain the liquid. From what
to expand and get in the way of more fluid coming in. I’ve seen of them they drain some liquid and a lot of air,
That’s why compressors are built differently. another waste. The problem is we don’t know what the
The piston and chamber are designed for mini- demand on the compressor is so we set the timers for the
mum clearances at the end of the stroke. There’s very worst (full load) condition. Occasionally the compressor
little room devoted to passages between the chamber will be shut down or run unloaded between drain valve
and the discharge valves. It’s all those close clearances cycles so the only thing it’s going to drain is air. They’re
that create the problem when a little liquid gets into reliable but waste a lot more air than a wise operator.
a compressor, it will pass out through the discharge As time goes by there will probably be a better device
valves but it makes a lot of noise doing it and the ham- invented, but until then…
mering usually results in compressor damage. That’s Reciprocating compressors are designed to start
why it’s so important to remove any liquid that forms unloaded. The typical scheme is use of lube oil pressure
between compressor stages. where a small oil pump eventually builds up pressure
Staging in compressors is similar to staging in as the compressor is started and that pressure is used to
pumps, you let one part of the compressor do part of overcome the force of springs that hold the compressor’s
the job and another finish it (two stage) although more inlet valves open. During normal operation that same oil
stages are common. That little compressor you bought at pressure can be bled off to the crankcase to allow the
the hardware store and keep in your garage is probably springs to hold the inlet valves open for unloading. In
a two stage compressor, not two cylinders each doing compressors with multiple cylinders it’s possible to
the same amount of work. Two, three and four stage unload one set of valves while leaving others in opera-
compressors are all common, some with multiple inter- tion to adjust the capacity of the compressor. That form
coolers. of unloading is normally accomplished with a pressure
Compressors are fitted with ‘intercoolers’ which switch that switches valves in the oil circuits although it
are heat exchangers used to cool the compressed air can be done with an electric switch and solenoid valves.
between stages so the next stage doesn’t get too hot. Staged unloading is common in refrigeration compres-
You’ll probably notice an intercooler buried under the sors. (See Chapter 5.)
belt guard of your control air compressor and the fact You have to be aware of that unloading scheme if
that the sheave for the compressor has spokes formed you proceed to adjust anything in the system. I knew one
like fan blades to force room air over the intercooler to operator that thought he would save money by lowering
remove the heat. Now there’s a cue, if you keep that the compressed air pressure. He lowered the setting of a
screen and all the fins on that surface clean then the pressure control switch but was dumfounded to see that
compressor will run more efficiently. Please be sensible the compressor would run longer. He had simply reset
about it though; I told that to one operator who used the unloading setting so the compressor always ran with
compressed air to blow it all clean every day. Yes, he half the cylinders unloaded. Someone else lowered the
used more energy to clean it than he saved by keeping setting of the on-off pressure control below the unload-
it that clean. ing value of a compressor with hydraulic unloading and
Usually the compression is such that a control air couldn’t understand why the motor burnt up because
compressor will not condense any water out of the air the compressor was constantly starting and stopping.
in the intercooler so there are no drains on it. Larger The partial unloader or unloaders must operate within
compressors will be cooled to the degree that water has the span of the on-off control switch. If the unloading
to be separated, collected, and drained from the outlet settings are not in the operating range of the compressor
of each intercooler. they won’t work.
We used to count on the operator to open drain Oil almost always requires attention in a recipro-