Compressor Query

It is not unusual to see the same compressor pump used with different Hp motors, so there is a range which possible by changing the motor pulley to run the compressor slower. There can be fairly high starting current with out a unloader, so there are limitations and you need a substantial motor rated for compressor use. At least in the US, it is not uncommon to run up to a 5Hp compressor off of 240VAC single phase. I had a two stage Curtis belt drive with an E-57 pump, this pump would accommodate up to 7.5 Hp. Two stage compressors usually go to 175 PSI tank pressures, so affords you a longer running time before the motor kicks in. The CFM output is usually rated at different PSI, but ultimately is limited to the pump size and the motor Hp with continuous use like with a media blaster cabinet. This usually requires something in the 12-18 CFM range for a smaller cabinet. Single stage piston pumps is usually used in lower CFM units. A good cast iron pump or metal sleeved pump goes a long way toward longevity and helps quiet things down a bit. My biggest issue with most compressors (in the shop) is the noise, better compressors run slower (4 pole) motors and pumps which tends to reduce the noise. Pump design wise, I do not think it matters as to the configuration, bigger pumps tend to be in a V configuration. This also may afford slightly better cooling, not sure on vibration. Most of the better pumps of some form of air to air cooler between and/or after the 2nd stage. Older pumps in most cases can be refurbished and perform well, older compressor tanks, I would be worried.

MozamPete said:

Can I just run a piston compressor at a lower speed without problem?

Click to expand...

Yes, I'm doing it. I made a mistake in ordering my new compressor motor and bought a 1725 motor rather than a 3450 so my compressor runs at half speed. Originally rated at 17 CFM @ 140 PSI. That means that my 5 HP compressor is now a 2.5 HP compressor. Works fine for most of my air needs, except sand blasting.

I am UK-Based, and researching everything compressor with the intention of getting my own (first) compressor.
To get there, I have been sorting through every kind of units conversions (Watts, HP, Litres-> cu ft/min), and trying to get an impression of what really counts. I guess my needs are relatively modest, but I do get the impression that what we here consider quite large for a home garage kit is considered tiny/inadequate in USA.

Getting to a suitable air reservoir size, and air flow rating seems straightforward enough. Even when we get past the difference between cylinder air displacement rating and the real flow at a sustained pressure, things get a little more murky when deciding between oil-lubricated and oil-free types.

High RPM / low RPM - and was lower RMP arrived at with a belt pulley. It that about wear, or noise, or both?

The current one can drag out of a wall socket? What is the surge current? Does brushless motor technology change the game?

And then - for me, there is the noise! There is LOTS you can do for it. I am surprised that the simple and effective solutions to making the kit quiet enough to not be intrusive at all are not available in the purchased products.

One can find the "quiet" types, 68-70dBA, seemingly also oil-free as a pre-condition, like the California Air Tools product, which looks very similar to countless others.
http://www.ultraquietaircompressor.com/
Link -> eBay Silent 65dB 50L (13.2gal)
My conclusions so far are:
1) The oil-free types are shorter-lived than oil lubricated types. They wear out faster.
Balanced against the advantages of oil-free, the extra kit needed to take out the oil can be as effective as one needs, and does not cost so much as to give "oil free" a big advantage. So we go for oil lubricated.

2) I have seen 97dBA described as "among the quietest". This is rubbish! They need to be quieter than 68-70dBA
Understand dBs. They are logarithmic. Every 3dB is a doubling or halving of noise power.
A 97dBA compressor is 27dB louder than a 70dBA kit.
That much is more than 500x louder! (27dB ratio = 501.187).
The ear perception is also logarithmic, so it will seem 27/3 = 9 times louder by ear perception
All the "quiet" and "oil free" offerings come severely limited in flow capacity. Around 4cu ft/min instead of the 9 or 14 cu ft/min one needs for air hose, or bead blast. The price/size seems inflated also.

3) Yes - you can very effectively fashion a muffler, even out of plastic pipe parts, to go on the intakes.
For me, I just have to be impressed by the tons of YouTube videos on how to silence a compressor down to 45dBA or so.
The questions of adequate cooling take care of as well! The stuff is just very very good!
Examples are:
https://www.youtube.com/watch?v=Txs5Q2V85P8
https://www.youtube.com/watch?v=Txs5Q2V85P8

I have more or less settled on something like 50L (13.2gal) 2.5HP or 3HP, oil lubricated, and don't care if it makes a racket, because I am going to make up one of the sound insulated solutions I see on the videos. Apparently they use MDF, foam rubber insulation, glue and screws, and some ex computer PSU 12V fans. Even I can manage something like that!