I used to service compressors when I worked for Walker's Hydraulics. One of the mistakes I would see is a shop that used a lot of air with an extra tank to hold "more air". All that does is make your compressor work twice as hard. Look at CFM ratings @ 90 psi. The bigger the better. I have a 5hp 60 gallon upright at home ,and when I run my big sander it will drop to 65 psi and hold that as long as the sander is running. My big compressor is 7.5 hp ,and puts out 33cfm at 90psi. It is a Kaeser air box,so quiet you can talk normally standing right next to it. Ideally you want the compressor to pass you up,and shut off. Don't have the shut off set at over 125#,all you are going to do is work the compressor harder. Most air tools are rated at 90psi, but they usually run at tank pressure at all of the shops I have been into. Good quality tools will use less air ,and give more power due to tighter tolerances reducing internal leakage.
Now when you are talking about blasting you never have enough air. I worked at a place where the blaster used 100cfm @ 100psi. It took 2 screw compressors to keep up. You are basically opening a drain on the compressor. Running too small of a compressor will generate A LOT of heat. This can cause head gaskets to literally burn out on some compressors. Hot air carries more moisture ,so a good place to put the water trap is down range at the blaster where the air has had a chance to cool off some. Don't skimp on a water trap, get a good one(s) that is(are) sized big enough. To get all of the moisture out requires a refrigerated air drier $$$$ The way the piping is done will also affect how much water you get down range. Look around for local auctions, or on
www.craigslist.org under tools for sale. You can find good heavy duty compressors for a decent price. Compressor heads generally only need new rings,and bearings-if they have bearings. Many inexpensive compressors now have just a bored aluminum con rod on the crank without bearings. Bad news if the oil level gets too low,tends to eat everything up. The cylinders usually don't need to be rebored. If you stick with a good name ,parts are still available. Put the compressor in a location where it will get good air flow over the head to help keep it cool. Don't be afraid of 3phase motors. There are ways around that. Also 220V motors will use 1/2 the amperage that a 110V motor will-so you can go with a bigger motor. I would stay away from the direct drive compressors due to the way they scream-at least all of the ones I have heard so far. Look for a good cooling fan-usually built into the pulley, finned compressor head piping, compression release for easier restarts,check the oil, pull it over by hand (electrical turned off) to check for compression,or lack of. Kind of like a car. If you have more air coming out the oil fill you know it needs rings. Low compression can also be valves. If it is not going past the rings it is just getting pushed back out the intake. Some compressors use reed valves,bigger compressors have plate valves. Some compressors are also restricted at the intake/filter- even from the factory. It is just a pump,so anything to help it breath better is good. Piping kinked,flattened,too small,plugged air filters,filters too small, and wrong sized hoses all lead to compressor problems. I saw some pretty jacked up compressors in my time as service tech.
