Air compressor test

I conducted a test with my new air compressor in which I timed the duration between the compressor cycling.

The compressor came set at a 150 psi shut off. I was running my CNC on a fairly long program and the air usage was pretty consistent during my test. The spindle bearings are air over oil so it is constantly using air.

At 150 psi the compressor kicked on every 8 min. 45 sec.

I turned the pressure switch down so the compressor would shut off at 120 psi. Then cycle time was 7 min. 45 sec.

The compressor recovers in 1min 15 sec. (120-150)

My test didn’t reflect what Doyles law suggested as I thought I understood it. Would it be better if the compressor ran longer with a longer duration between cycles? Is it possible to adjust the switch so the kick on and off are further apart like 40 lbs. instead of 30?

My compressor is a 5hp, 80 gal. 635 rpm Devair.

I have a question for ya. What was the recover time after turning down the pressure? The reason I asked is because I had a Air compressor repairman tell there is one big mistake that most people don't understand. Your shop runs on cfm, not pressure. He indicated that the recover time will be shorter on the lower pressure, rather than the higher pressure. The less running time, the less heat buildup. I think heat is your biggest enemy. And keep it filled with oil. Change oil often as it gets dirty fast.

The recover time was the same as before I lowered the pressure. 1 min 15 sec. Pretty fast recovery I thought and it was 103 degrees here, in Oregon!!! This compressor turns pretty slow and the pump barely gets warm on a normal day.

Most air compressors have journal bearings similar to automotive bearings that are so called hydrodynamic bearings. That type bearing has no load capacity at zero rpm and only gets load capacity after the crankshaft has started running. The most wear occurs at start up where there is no lubrication film. For this reason it is better to have longer run times which are relatively easy on the compressor. Frequent cycling is not a good idea 1. because of wear (above). 2. Because of non-optimum running conditions due to rapid temperature change at start which makes for more oil useage and water in the oil. 3. Stopping and starting against a load put heavy stresses on the components. 4. Surge current to the starting motor causes heating of the motor when it draws the much higher amperage. Lowering the cut off pressure makes for less work on the compressor. Lengthen the cycle by increasing the volume of the air tank or adding another tank. A tall vertical receiver made of large diameter pipe with a water release valve at the bottom and output at the top can be put in front of the main tank for a very effective water and oil reducing device and it will also increase air storage volume.

BTW, be sure to make the receiver out of steel pipe and not plastic pipe which will more than likely fail with a big bang that might kill you.