Last month I discussed the essential components of a reciprocating compressor; motor, pump, and receiver (tank), which you’ll find on every machine in this family of compressor. This month I’ll be explaining the function of other components that, when combined, make up a complete air compressor.
In review, the electric motor is connected to the pump using belts. The motor’s rotary motion 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 parts are as follows:
- There is a check valve (item #1),generally located in the tank, which permits the air to flow into the receiver but prevents the air from flowing back out. This check valve stops 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 pressure relief valve (item #2) guarding against excessive pressure building up inside the receiver. When the pressure setting of this valve exceeds, this safety device opens venting to the atmosphere, relieving excessive pressure inside the receiver.
- A valve provides a means of shutting off the airflow out of the compressor (item #3). Ball valves serve nicely for this purpose.
- Contaminants in the form of small amounts of oil and water accumulate in the tank and must be removed. Removal of contaminants is accomplished by using a drain valve located at the bottom of the tank, which provides a path for pressurized air to blow contaminants out of the tank (item #4). I highly recommend installing an auto drain type valve to accomplish this. Failing that, a ball valve will do nicely if you remember to use it.
- 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 #5) is generally used to accomplish this. This switch opens an electrical circuit that either controls a motor starter (item #9) on larger compressors or directly opens the power circuit to the motor on smaller compressors.
- 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. A small valve called an unloader (item #6) is used to prevent this by bleeding off this high-pressure air when the pressure control switch opens. There are some compressors (usually larger types); when the desired pressure is reached, the motor continues to run; 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
- Located near the air pressure control switch is a gauge indicating the pressure inside the tank (item #7).
- A relief valve is used to protect the tank from excessive pressurization. These valves are generally calibrated to open at a preset pressure exhausting compressed air in the tank to the atmosphere, and should never be tampered with and always be replaced by valves having an identical pressure rating (item #8).
- To safely control the motor switching on and off, a motor starter is used (item #9). The term “motor starter” means that a magnetic motor contactor (switch) does the actual switching of the motor, and some form of motor overload protection, usually in the form of a resettable thermal overload device, are 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. Look for my AD on innovative products under EZtimers Manufacturing.