What nozzle diameter are CFM ratings based on? I'm looking at a Porter-Cable 150psi, .8hp air compressor, specifically, and want to figure out how fast air comes out with a 25ft, 1/4" hose and coupling.
Air compressors are generally rated to produce a given volume of air at given pressure. For example, the Porter-Cable C2002 produces 3.5 SCFM (standard cubic feet per minute) of air at 40 PSI (pounds per square inch) and 2.6 SCFM at 90 PSI. Air tools are rated to consume a given amount of air at a given pressure. Basically the two are matched to arrive at a compressor that will meet the needs of the tool(s). Sanders generally consume the most air, followed by impact guns, and finally nailers.
As for the actual velocity of the air coming out of the hose, I would guess there is a mathematical formula to compute the velocity but I have no idea what it would be.
The SCFM ratings for air compressors relate to the ability of the pump to maintain a steady pressure as the tank is bled off. That is why the flow rating at 40 psi is always higher than the 90 psi one. As Adam said, this is used to compare against the tool(s) you plan to use to make sure the compressor has enough capacity to run them.
Air hose will have male pipe threads on both ends. A standard threaded female coupler can be used to connect two hoses together, and these would be threaded on both sides. More commonly used though is a quick connect system, where the hoses can be easily hooked up or disconnected as desired.
When selecting a compressor, the "Specifications" sheet is very instructive.
Here are the specs for a Porter Cable compressor similar to your description.
You'll notice that the "Application Use" specs include a number of functions that require short, periodic bursts of air.
This 3.5 gallon tank will generate 3.5 SCFM @ 40PSI but output drops to 2.6 SCFM @ 90PSI.
So, you could expect normal operation of air tools like roofing nail guns, air brush or craft paint sprayers, or a brad nailer.
But you might not maintain enough air in your tank to drive continuous use tools like car buffers. HOSE LENGTH: An air pressure regulator on your tank outlet, at the end of your hose, or built into your air tool will help maintain the required pressure ... helping eliminate the drop-off normally associated with using longer hoses.
An air filter and pressure regulator in essential if you intend to spray fine finishes on cabinets, furniture, or auto bodies.
This addition will trap moisture and other particles that might contaminate your coating.
It will also give you better control of air flow ... making it easier to adjust consistent flow to your air tools.
your air compressor capacity you need to choose the nozzle with orifice
small enough to maintain productive pressure with 375 CFM compressor.
Otherwise consult a good hvac repair service NJ professional to get your