When air is compressed, heat is generated. This heat causes the air to expand, thus requiring an increase in power for further compression. If this heat is successfully removed between stages of compression, the total power required for additional compression may be reduced by as much as 15 percent. In multistage reciprocating compressors, this heat is removed by means of intercoolers that are heat exchangers placed between each compression stage. Rotary air compressors are cooled by oil and do not use intercoolers.
It is obvious that the presence of water or moisture in an air line is not desirable. The water is carried along through the line into the tool where the water washes away the lubricating oil, causing the tool to run sluggishly and increases maintenance. The effect is particularly pronounced in the case of high-speed tools where the wearing surfaces are limited in size and excessive wear reduces efficiency by creating internal air leakage.
Further problems may result from the decrease in temperature caused by the sudden expansion of air at the tool. This low temperature creates condensation that freezes around the valves, ports, and outlets. This condition obviously impairs the operational efficiency of the tool and cannot be allowed.
The most satisfactory means of minimizing these conditions is the removal of the moisture from the air immediately after compression and before the air enters the distribution system. This may be accomplished in reciprocating compressors through the use of an aftercooler that is an air radiator that transfers heat from the compressed air to the atmosphere. The aftercooler reduces the temperature of the compressed air to the condensation point where most of the moisture is removed. Cooling the air not only eliminates the difficulties which moisture causes at points where air is used but also ensures better distribution.
The receiver tank is of welded steel construction and is installed on the discharge side of the compressor. It acts as a surge tank as well as a condensation chamber for the removal of oil and water vapors. It stores enough air during operation to actuate the pressure control system and is fitted with at least one service valve, a drain or blow-by valve, and a safety valve.
All portable air compressors are governed by a pressure-control system. The control system is designed to balance the compressor's air delivery and engine speed with varied demands for compressed air.
In a reciprocating compressor the pressure-control system causes the engine to idle and the suction valves to remain open when the pressure reaches a set maximum, thus making the compressor unit inoperative. When the air pressure drops below a set minimum, the pressure-control unit causes the engine to increase speed and the suction valves to close, thereby resuming the com pression cycle.
The rotary compressor output is governed by varying the engine speed. The engine will operate at the speed required to compress enough air to supply the demand at a fairly constant pressure. When the engine has slowed to idling speed as a result of low demand, a valve controls the amount of free air that may enter the compressor.
A screw compressor output is governed by automatic control that provides smooth, stepless capacity regulation from full load to no load in response to the demand for air. From a full load down to no load is accomplished by a floating-speed engine control in combination with the variable-inlet compressor.
A number of built-in features that make portable compressors easy to maintain include:
an automatic blowdown valve for releasing air pressure when the engine is stopped,
a valve for draining moisture that accumulates in the receiver tank,
a drain cock at the bottom of the piping at the bottom of the oil storage tank,
an air filter service indicator to show when the filter needs servicing, and
a demister, or special filter, that separates lubricating oil from compressed air.
Remember a good maintenance program is the key to a long machine life. So it is up to both the operator and the mechanic to ensure that the maintenance is performed on time every time.Continue Reading