Many building in Hawaii still have pneumatic controls systems or a hybrid system utilizing digital and air for controls. Many people do not know about compressor sizing and starts per hour. The compressor is normally sized to operate up to one half of the time to provide normal system air consumption. The air storage tank capacity is matched to the pump and is designed to give a reasonable number of cycles per hour. The storage tank provides sufficient storage capacity to carry peak air consumption loads. To determine the SCFM rating of compressors the load must be determined. The compressor should have a rated SCFM output of at least 33% or 50% of operation.
It may be necessary to determine the effects of sizing, pressure switch settings, and tank size on compressor starts per hour. Air compressors will specify maximum starts per hour. Some smaller sizes will specify no more than 12 while some larger models will enable 30 starts per hour. The minimum bleed off time should also be calculated. Then you can decide on other things like oil less, lubricated, single, or duplex compressors.
Run time testing should be accomplished because in many cases in some buildings the poor air compressors never shut off, due to leaks and improper air settings. A run test is an air compressor performance test that measures the percentage of time that a compressor runs to enable it to maintain a supply of compressed air to the control system. A run time test determines whether an air compressor is sized properly for the job. The run time test is performed under a normal compressed air load on the system. The length of time (in minutes and seconds) that the air compressor is on is measured with the length of time that the air compressor is off. The run time percentage is found by dividing the compressor on time by the compressor on time plus the compressor off time. This number is multiplied by 100 to convert to a percentage. Use the following formula.
For example what is the run time if an air compressor run time if it is on for 6 minutes and off for 12 minutes?
6
---- x 100
6 + 12 =
6
----- x 100 = .33 x 100 = 33%
18
Run time is only useful as a percentage of the total cycle time. For example if an air compressor is on for 6 minutes and off for 18 minutes, the compressor run time is 25%.
{[6/(6 + 18)] x 100 = 25%}. If an air compressor is on for 6 minutes and off for 6 minutes
the rate would be 50%. In both cases the air compressor operates for 6 minutes but the mechanical wear is greater on the compressor having the 50% run time.
Most manufactures recommend that a properly sized and installed air compressor have a high limit of 50% run time. Anything greater cause’s abnormal wear and premature air compressor failure, In addition to abnormal wear and premature compressor failures the oil carryover rate is increased when a compressor’s run time is over 50%. Run time test should be run when the compressor goes in to establish a benchmark and can be referred to make sure the standard is being followed.
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