Compressor CFM and Pressure

An explanation of how airflow speed and pressure relate, and how this affects compressor sizing and distribution pipe system design. June 15, 2014

We are running two, 10 HP piston compressors that generate 34 CFM at 175 #/. They cycle on and off together. We are adding equipment and need more air. I am looking at a used 30 screw compressor that generates 118 cfm at 125#. What is the best way to look at this? Would I get 42% more air? When the use point is far from the compressor will I get pressure drop at 125 psi?

Forum Responses
(Dust Collection and Safety Equipment Forum)
From Contributor E:
Nothing beats a rotary screw compressor for continuous air output. Unlike your piston units that have a duty cycle of probably 35-40% (I'm guessing), the rotary has a 100% duty cycle. That is, it can make air continuously. In fact, rotaries are designed to do just that. I don't think pressure drop related to the length of your air runs should be a big concern. The key is having enough air volume to keep the tools happy. We have about 500' of 1.25" pipe in a loop around the shop feeding .75" drops to the tools. Where necessary we run .375" i.d. hose to the machines' air inlet. Look for high-flow fittings if you're using quick disconnects - the typical fittings you might find at the local home improvement store are too restrictive for air-hungry tools. You might want to keep one of those piston units as a back-up, just in case.

From contributor J:
Pressure drop is related to how fast the air has to move through the pipes to satisfy demand. Air moving quickly through pipes creates more friction and turbulence, therefore more pressure drop. At higher pressures, a given amount of air occupies less space and therefore moves through pipes of a given size more slowly to satisfy a given demand. When pressures are lower, a given amount of air has to move more quickly (assuming same-size pipes) to satisfy that given demand, and that extra velocity creates more pressure drop. You can remedy pressure-drop by using larger pipes.

Think of the various ways to set up an HVLP spray gun. If you're using a turbine (very low pressure air) then the air supply hose is huge. If you're using a conversion gun then inlet pressures are higher, but much lower than typical (~100 PSI and up) line pressures, so you need a regulator somewhere. If you use a little regulator mounted right on the gun then the supply hose can be very small. If the regulator is mounted at the wall then the supply hose needs to be larger. See how this works? To satisfy a given demand without excessive pressure drop, pipes run at lower pressure need to be larger in diameter. Perhaps more obviously, pipes that carry lots of air need to be larger than pipes carrying smaller amounts of air. You are nearly doubling your compressor capacity, and adding some unknown demand to the system. You may need to re-plumb some of your air lines. If you don't need 175psi for any of your equipment then you're wasting money (on electricity) to compress the air to the higher pressures. Those compressors are doing a lot of extra work for nothing.

From the original questioner:
I bought the compressor. It has 8100 hours and I got it for 1/3 the price of new. Its outlet is 1-1/4" pipe. I am planning on running a 1-1/4" main through the building connecting it to our current 3/4" pipe at several locations. My goal is to have the compressor somewhere in the middle of the demand. Is this sound logic? We do not need 175#.

From contributor J:
I dug out a spreadsheet I made up a few years ago when I was educating myself on this topic. I'll spare you the detailed math, but the gist of what I learned is that pressure drop will be negligible so long as you keep the air velocity inside the pipes no faster than 20 feet per second. If you were using your new compressor's entire 118cfm capacity the airspeed through a 1.25" pipe would be about 26.4 fps, which is not quite ideal, but not problematic. Given that you're probably not going to use compressor's full capacity, 1.25" pipe will probably be perfect 98% of the time.

For the drops:
A 3/4" pipe can handle 32cfm at 125psi without significant pressure drop.
A 1/2" pipe is good for 14cfm at 125psi.
A 3/8" hose will do 8cfm at 125psi.
A 1/4" hose will handle 3.5cfm at 125psi.

From contributor F:
Two things you may want to look into:

1. The last shop I worked in had two rotary screw compressors. They run continuously and as such generate heat continuously - a lot of heat! They located the compressors in a side building attached to the shop so that the heat could be kept away from the main floor. The storage tank however was much closer to the main loop.

2. For a larger shop that uses a lot of air I've always heard the airlines should be looped. This helps with the pressure drops.

From Contributor M:
I use an IR Nirvana single screw. What made me go with this one is that it will run at 7.5, 10 or 15 HP. It takes about $1200 to switch between the various settings. My in-rush current with a Champion 7.5 HP 3 phase was 150 A for 5-15 sec. That would have forced me to rebuild it with an ABB VFD or something. Going to the IRN dropped in-rush below FLA (in my case 18 A). Sparky was happy, I was happy - building authority was happy. Not to mention insurance people. Anyway, screw driven systems with either AC --> DC --> AC (VFD) or AC --> DC (VSD) are upgradable and don't cause one to spend huge cash on infrastructure from the start.