- ½ HP motors = 14 starts per hour
- 1 to 5 HP motors = 10 starts per hour
This is especially critical for single phase motors. The number of starts per hour will of course be determined by the amount of leakage in the sprinkler system. For older systems with substantial leakage, these numbers could easily be exceeded. One of the few controls the designer has to reduce motor starts, is the size of the air receiver being used with the compressors. Each time the compressor shuts off, the receiver stores air which is used to offset air loss through system leakage.
For sprinkler system compressors, there is a tight differential setting of approximately 15 PSI on the pressure switch, which has the compressor starting up at approximately 45 PSIG and shutting off at approximately 60 PSIG. Through this differential pressure range, the air storage will equal slightly more than the volumetric content of the receiver. The standard 14 gallon receiver has a volumetric content of approximately 1.9 cubic feet. That stored volume can quickly be depleted in a larger system with substantial leakage. A 30 gallon receiver has a volumetric content of approximately 4 cubic feet, and a 60 gallon receiver has a storage capacity of 8 cubic feet. These additional storage capacities may be required for some systems in order to reduce the start/stop frequency of the compressor to acceptable levels.
With this in mind, we have designated the 30 gallon receiver to be the standard size for our 3 HP compressor. We have found that the 3 HP compressor installed on a 14 gallon receiver is required to run for only 6-8 seconds to bring receiver pressure from 45 up to 60 PSIG. If the small volume of stored air is depleted in less than 5 minutes, this may result in motor fatigue and possible premature failure. We highly recommend that our sprinkler system customers evaluate the system being serviced, and monitor the number of start/stops to ensure they are in compliance with the acceptable startup frequency. As a 3 HP compressor is used on larger system, there is greater potential for air usage due to leakage. We have therefore designated the 30 gallon receiver to be the standard for our 3 HP compressor, with possible consideration for 60 gallon when warranted. For those systems already existing with compressors that are starting and stopping excessively, we recommend:
- Correcting leaks
- Adding an air receiver for additional storage to reduce compressor cycling