|
TimV
05-20-2006 05:44:57
|
|
Re: Another Air Compressor Question in reply to ldj, 05-19-2006 21:19:30
|
|
| |
ldj: The thing I love about this board is the ability to have a difference of opinion with someone without name-calling, hair-pulling and other various dramatics! In that light, I want to mention one thing about CFM, which is to say that in today's world, relying on a bare CFM number to make a decision on which machine to buy is dangerous. CFM is a measure of PUMP capacity, not UNIT capacity. It is also tweaked, poked, prodded, massaged, and in general shamelessly misused by many compressor companies (as are their motor horsepower numbers) in order to sell machines. The reason that CFM ratings go down as pressure goes up is due to the fact that it takes more mechanical power to compress air the more you compress it. Therefore, as the outlet pressure increases, the outlet volume decreases. Granted that that sounds obvious, but it's the foundation for what I'm driving at. The CFM number of the pump bears very little relevance to the CFM rating you'll actually see at the end use. If these two units were hooked directly to the sandblaster (or other end use) without any air reservoir, and if that end use always drew exactly the same amount of air (pressure and volume) all the time, then a comparison of the two units on the basis of pure CFM ratings might make sense. However, I'd be willing to bet next week's beer money (though considering I've still got four beers in my fridge from a 6-pack I bought last Thanksgiving, you wouldn't get much if you won!) that the 2-stage unit will actually deliver more air TO THE END USE (downstream of the tank and any associated piping) at a given PSI than the single-stage one because of its higher amount of storage capacity, even though its bare CFM rating is lower. In addition, it will last longer, run quieter, and take less electrical power to do it. Compressor companies rely on the fact that many uneducated consumers will look at the bare CFM and horsepower numbers to make their decision, when indeed a wise consumer will look much further (checking amp draws and efficiency ratings of the motors to determine actual running horsepower vs. advertised horsepower for instance) before buying. While I can't speak intelligently about which machine Tim should buy without knowing his intended use, in general, there's a reason why the two-stage unit costs $300 more, and if he's planning on running the unit for extended lengths of time I would lean toward the two-stage unit, as he'll save the initial difference in purchase price back several times over in maintenance and electrical costs.
|
|
|
