Q1 2020

Air Compressors and Compressed Air Systems, Part Two

Last issue we discussed the basic components of a reciprocating compressor; motor, pump, and receiver (tank) which you’ll find on every machine in this family of compressor. This time I’ll explain the function of other components which when combined, make up a complete air compressor.

In review, the electric motor is connected to the pump using belts. The rotary motion of the motor turns a flywheel on the pump linked to a crankshaft which moves piston(s) inside the pump cylinder up, compressing air and down sucking air into the cylinder to be compressed during the next compression cycle. The compressed air leaves the cylinder through the top portion called the head and is pushed into a large vessel called a receiver or tank.
In order to familiarize yourself with air compressors, descriptions of the individual component parts are as follow:

  1. To prevent the air from just flowing back from the tank into the cylinder when the piston moves down to suck in more air, there is a check valve (see item one in diagram), generally in the tank, which permits air to flow into the receiver but prevents air from flowing back out.
  2. There is a pressure relief valve (item two) guarding against excessive pressure building up inside the cylinder. When this valve’s pressure setting is exceeded, this safety device opens venting to the atmosphere, thereby relieving excessive pressure inside the cylinder.
  3. A valve provides a means of shutting off the airflow out of the compressor (item three). Ball valves serve nicely for this purpose.
  4. Contaminants in the form of small amounts of oil as well as water accumulate in the tank and must be removed. This is accomplished by a drain valve located at the bottom of the tank which when opened, provides a path for pressurized air to blow contaminants out of the tank (item four). I highly recommend installing an auto drain type valve to accomplish this, however a ball valve will do nicely if you remember to use it.
  5. As air pressure increases inside the tank there needs to be a method to shut off the motor when the desired air pressure is reached. An electrical air pressure control switch (item five) is generally used to accomplish this. This switch opens an electrical circuit either controlling a motor starter (item nine) on larger compressors, or opens the power circuit to the motor on smaller compressors.
  6. When the air pressure in the tank has reached the setting on the pressure control switch, shutting off the motor, pressurized air is trapped between the top of the cylinder and the tank check valve. When the air pressure in the tank drops and the motor restarts, this pressurized air would resist the piston from moving up inside the pump cylinder, placing an extremely high load on the motor while it’s starting. To prevent this, a small valve called an unloader (item 6) is used to bleed off this pressurized air when the pressure control switch opens. In some compressors (usually larger types) the motor continues to run when the desired pressure is reached; however, the cylinder head is unloaded to the atmosphere using an unloader valve (item 6a) so there will be no further increase in tank pressure
  7. Located near the air pressure control switch is a gauge that indicates the pressure inside the tank (item 7).
  8. To protect the tank from excessive pressurization, a relief valve is used. These valves are generally calibrated to open at a preset pressure exhausting to the atmosphere and should never be tampered with and always be replaced by valves having an identical pressure rating (item 8).
  9. To safely control the motor switching on and off a motor starter is used (item 9). The term “motor starter” means a magnetic motor contactor (switch) does the actual switching of the motor as well as some form of motor overload protection, usually in the form of a resettable thermal overload device combined in one control. If excessive current flows through the motor the circuit automatically opens shutting the motor off before damage can occur.

That’s it for this issue. I’ll be back in the next issue with more good stuff on air compressors and compressed air systems.


Bruce Grossman is the Chief of R&D for EZtimers Manufacturing. EZtimers is the manufacturer of the new EZ Level return tank water level control. To prevent boiler scaling and other damage the EZ Level return tank water level control replaces that troublesome ball float valve in the condensate return tank. For saving money on handling waste the SAHARA and DROP IN THE BUCKET line of high purity separator water mister/evaporators provide a thrifty, legal method to get rid of the separator water generated by your dry-cleaning machine. See our Ad in this issue and for further information on EZtimers products visit www.eztimers.com Please address any questions or comments for Bruce to bruce@eztimers.com or call 702-376-6693.

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