If a reciprocating compressor is selected, there are some special features that should be considered. For any compressor that is going to automatically cycle on and off, further consideration should be given to the number of motor starts that may occur every hour.
In compressors of this size range, the number of starts per hour should not exceed 6 to 8. Excessive motor starts may cause extra stress and over-heating of the motor, which may result in reduced life expectancy or premature failure. This can be controlled in a few different ways.
The first way is to ensure that a suitable volume of air is stored when the compressor shuts down to allow it to be off for a reasonable period of time. The air storage will be determined by two factors – the differential pressure between the start and stop of the compressor, and the size of the air receiver(s) used to store air.
A very simple explanation would be to consider the actual volumetric content (in cubic feet) of the air receiver. A 120 US gallon receiver for example has a volumetric content of 16 cubic feet, and a 240 US gallon receiver has 32 cubic feet. For every atmosphere (approximately 14.5 PSI) in differential pressure between the start and stop pressure levels of the compressor, that volume of air (the receiver actual volume) will be stored.
Note – Elevation will play a factor in what an atmosphere actually represents, so we are using general terms which would represent something in the range of 400 feet above sea level.
Therefore, if your start/stop differential pressure is 30 PSIG, a 240 gallon receiver would store just over 64 cubic feet (32 x 2 atmospheres). Based on 64 cubic feet of stored air, if your system demand was 30 cubic feet per minute, your compressor would be off for just over 2 minutes before the receiver pressure fell to the start-up pressure. If you can broaden your differential pressure to 45 PSI, you would store one more atmosphere, an additional 32 cubic feet, or 96 cubic feet in total. That would result in the compressor being off for just over 3 minutes before start up.
Under this scenario, when the compressor restarts, it would basically devote 30 CFM of its capacity to supply air to the system. The balance of its capacity would be devoted to air storage to bring the receiver back up to terminal pressure. Therefore, if the air compressor has a capacity of 50 CFM for a round number, and 30 CFM are required for system demand, the balance of 20 CFM would be required to replace the 96 CFM that were used during the off time. That would require the compressor to operate for approximately 96/20 = 4.8 minutes to reach terminal pressure before shutting down. The sum of the off time and on time (4.8 + 3 minutes) will result in the compressor having to start every 7.8 minutes, or between 7 and 8 times per hour. This is borderline, but within the general limitation requirements for motor starts.
There are situations, where it may not be possible to create a 45 PSI differential between starting and stopping, and if only two atmospheres (approximately 30 PSI) were achieved, that would create higher start/stop numbers that may put the number of starts up to unacceptable levels. You can of course, add additional air receivers to your system to increase air storage capacity, and decrease the number of starts per hour.
The other method of controlling the number of starts and stops, is through the type of control used on the compressor.
A recommended type of control used by APPL has a time-delay off feature. Reciprocating air compressors may run loaded, during which time they compress air, or unloaded, during which time they do not. Unloaded running is accomplished with either the use of free air inlet valve unloaders or a discharge vent valve.
With free air inlet valve unloaders, when terminal pressure is reached in the air receiver, a mechanical finger-type device is pneumatically actuated to hold the compressor inlet valves open, during which time, the air which is taken in on the down-stroke of the piston, is allowed to freely exit the intake valves under no pressure.
Discharge vent unloaders operate in a similar manor, but instead of air being discharged through the inlet valves on the up-stroke or the piston, air is discharged through the pump’s outlet valve and then vented to atmosphere through a vent valve which is tee into the pump discharge line. The vent valve is opened when no-load operation is called for. With either unloading system, the power requirement to operate the compressor is reduced to approximately 20-25% of full load power, and there is very little heat generated in the pump cylinders and heads.
The other advantage of this system, is that during no load operation, cooling air continues to flow across the pump cylinders and heads to dissipate heat while the compressor under no load. Better cooling will be achieved under this no-load condition than if the compressor actually shut off.
Getting back to how this feature is used in the control system – when the compressor is started up, a normally open solenoid valve is energized to vent an air pilot signal at the free air unloaders or vent valve, and the compressor pumps air into the air receiver. When terminal pressure is reached, say at 120 PSIG for example, the solenoid valve is de-energized, and a signal is supplied to the unloaders or vent valve to allow for no-load operation. At the same time, a timer is activated to begin to time the no load time. If there is no air demand from the system, and the air receiver pressure does not fall to the compressor start up level, at say 90 PSIG for example, the compressor will shut off when the preset time period elapses.
The time period is adjustable, but is normally set for between 5 to 8 minutes, which would normally result in the compressor starting up less than 8 times per hour. If the receiver pressure falls to the start-up level of 90 PSIG, the unloader solenoid valve signal is vented, and the compressor will automatically start to pump air into the receiver again. When terminal pressure is reached, the compressor will once again enter the no-load operating condition, and a new delay-off time period will begin.
On some larger compressors, a second timer may be used to delay the venting of the unloader signal to the compressor unloaders or vent valve, which will allow the pump to start up freely from the stopped position until the motor is up to full speed. That time period is normally set for approximately 3-5 seconds. Both of these timing features will greatly reduce motor heat loads, and result in optimum motor life, and better cooling for the air compressor pump.
Contact APPL for information or quotations for these optional controls.