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SP 5623

Locomotives need an air compressor to operate the air brake systems, horn, bell, and in older locomotives, the "control" air system. Control air is used to operate the reverser, cam switch (in dynamic brake equipped locomotives) and power contactors.  The steam boiler uses main reservoir air and has its own pressure regulator system.

The compressor has 3 cylinders, two low pressure and one high pressure. Air is taken in by a low pressure cylinder's piston's downward stroke. When the piston returns upwards, it compresses the air, circulates it through the intercooler and into the high pressure cylinder which compresses it further and finally, exhausts it into the main reservoirs.

The intercooler is nothing more than a radiator connected between the low and high pressure cylinders. In an air cooled compressor, the intercooler would have fins for air circulation, exactly like an automobile radiator. In this water cooled version, the intercooler has a water jacket and does not rely on air cooling. S.P. was a big believer in water cooled compressors as they are much less prone to failure due to their cooler running nature.  They are, however, subject to freezing ans one does not run antifreeze in the cooling system of a EMD engine.

This the the high pressure cylinder and the exhaust line to the main reservoirs. The small line connected to the center of the head is the unloader line. The unloader system connects to all 3 heads and forces the intake valves open at the proper pressure in order to stop the compressor from pumping. The brass thing on top of the intercooler is a 35 pound safety valve used to protect the intercooler from excessive pressure. The air system has a safety valve set at 150 pounds, mounted near the main reservoirs.

This view from the fireman's side shows a low pressure cylinder and its connection to the intercooler. You can also see the cooling water line running between the high pressure head and this low pressure side. The compressor air intake and air filter are to the right, out of view.

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