air compressors: The Good, The Bad, and The Noisy

 They're all good compressors, Brent.

 In terms of the pumping technology, there are really just three, well, four in some special cases, er, five, depending on how you define... you know what?  Here is a list of some compressor technologies that I know of:

vane
(orbiting) scroll
reciprocating piston (includes linear-motor designs)
(rotating) screw
Roots™blower (AKA rotating-lobe)
axial turbine

 Note that this is also - roughly, with some overlap - in order of maximum flow rating.  Once you get into turbines, the sky is the limit.  Note also that some of these pumps (vane, screw, scroll, Roots) are - or can be - positive-displacement, which just means they don't leak backward through the pump if you shut the pump off with pressure remaining on the high side.  Some are - or can be - non-positive-disaplacement.  Several can be both, depending on application and pressure.

 Total lifetimes vary wildly.  Vane pumps require a tear-down and major rebuild (or replacement, dammit) after three thousand hours.

 As for reciprocating piston pressure pumps with splash lubrication, well... somewhere in Great Britain or one of its former colonies (coughindiacough) there is probably one of these that has been in continual use since the industrial revolution.  Certainly their lifetimes - with good maintenance - can extend into the tens of thousands of hours, even longer if they start out of good quality and they get rebuilt from time to time.

 Mean Time Between Wear/Maintenance for the others varies wildly, and is also heavily dependent on quality of design and construction.

 I have been shopping as hard as I can (read: seeking the minima in a complex three-dimensional curved map which has various axis including 'lifetime', 'ruggedness', 'purchase price', 'sound level', 'cost of ownership', and 'tech specs'.

 The last one - tech specs - may be removed from the map and taken as a given since we may assume that any compressor I purchase will, in fact, meet my technical needs.*

 Way, WAY more important - to both me and to my wife†, is sound level.

 Reciprocating pumps are the worst offenders because they generate very low frequency sounds which are difficult to block.  One bit of good news however, is that 90% of that low frequency noise is "port noise" from the intakes, resulting in...

 (cough)

PORT NOISE COMPLAINTS

 Ahem.  Sorry.

 The difficulty of sound treatment is why most professional organizations - be they boiler factory or dental office - put their air compressors in a separate, dedicated room (concrete or cinder-block enclosed including roof, and tightly sealed from top to bottom) or more often, a small separate shed or building, ensuring that any sound leaking from the enclosure radiates into the great outdoors and not the building where the people are.  I would do this myself if I had the back yard real estate, but I don't.

 Rotary Screw and (Orbiting) Scroll type pumps are much quieter, but they also require greater precision of machining, and their own special oil handling and moisture-removing (to keep water out of the pump oil) hardware, increasing the cost still further.  "Low price" (meaning, less than about two thousand dollars) versions of these techs have begun appearing on the market, appealing to the "prosumer" market, but my investigation revealed a lot of complaints from early adopters and reviewers which suggest that consumers saving money on the purchase price will pay it out later in cost of ownership, so I'm avoiding those.

 Diaphragm and some vane pumps can be very quiet if their intake ports are muffled, but they have short lifetimes and most of them have cheap motors and high rotor speeds leading to poor duty cycles.  When I am sand-blasting a large part, I don't want to have to set a timer to remind me to take a break after fifteen or twenty minutes of operation to avoid burning up my shop's air compressor.  No, on the contrary, I do not wish to think about my air compressor AT ALL.  I mean, that's why I'm installing an automatic drain valve, ferchrissakes.

 So that leaves Roots blowers.  Just kidding.

 I think I may be back to square one: buy a splash-lubricated, reciprocating-piston compressor, probably a low-rpm, two-stage design with as small a tank as I can find under a 4HP, 10CFM pump.  So far, all I've found in that compressor range has been 60 gallon tanks, which I neither require nor desire, as they take up too much space.  What's even more annoying, the few twenty and thirty gallon tanks setups with large compressors which I have found so far were horizontal, and I absolutely require a vertical design because of my very limited floor space.

 I am seriously considering buying all the parts - motor, pump, tank, and pressure switch - and assembling my own to order.

 But whether I do or don't, at this point it seems clear that my only remaining choice, one which will meet my technical requirements and make me happy in the long term and have a low enough sound level (after mods and an enclosure are added) is going to be the good old-fashioned oil-lubricated piston pump.

 Sometimes, the classics are still the best.

________________________________________________________
*broadly expressed as 10CFM @ 90PSI

†which will make the inevitably-higher price tag easier to swallow.
This is also called Spousal Acceptance Factor.