I'd like to keep my air compressor filled for quick tire top-offs, like at gas stations. The manual says to empty the tank after 4 hours. How do gas stations avoid draining this often? Is it possible with a consumer compressor?
This taps the bottom of the tank where water would accumulate. At intervals, it blows some air out of the tank. PSHHHHHT! This ejects most of the water and some air.
This operates at time intervals, when the compressor cycles on or off, or when water is detected.
Your electric bill will not like this
In real world practice, compressors are not leakproof. It will cycle randomly at all hours.
Pressure tanks are usually steel, steel is somewhat flexible, it stretches a bit when filled, if the tank is left full it can shorten the life of the tank, I used to have a small scuba shop and did tank inspections as required annually, with hydrostatic testing every 5 years , the recommended pressure for long term storage for all tanks was 500 psi this is for tanks that range from 2200-3500 psi working pressure. I once purchased 4 steel tanks that had been sitting for close to 15 years at 3300 psi they were 3000 psi tanks, I dumped the air and had them hydrostaticly tested in fact all 4 tanks passed by enough to get the + rating that allowed them to get the 10% over fill. After that I did not pay much attention to that guide line but did tell my students the guideline and my experience so Can it be left full sure it may shorten the life of the tank , just make sure to blow the water out of the tank prior to letting it sit full of air.
That statement means after four hours of run time. Storing air doesn't cause water accumulation.
I'd drain it every few months if you only use it occasionally. The fact of the matter is that there will always be water in it, and when it rusts out you can't do much about it.
Industrial compressors have air drying mechanisms upstream of the intake air, so they mostly eliminate the problem. Either that or they drain them nightly.