Critical Air Receiver Sizing

[UtilityLouie]

This is the most basic of all questions, but some of the numbers I am coming up with do not make sense.

I am designing some air receivers for new equipment. If the air header loses pressure, I need to maintain enough pressure to continue controlling actuators until the machine shuts down.

For example: (1) set of control items use 456 SCFM peak airflow. I was told that a guess coast down time of this machine is 20 minutes. I don't know if that is true or not. The system running pressure is 100 psig and the minimum pressure that I was told the actuators could operate at was 65. I used a 30 psi delta in my calculations for a little safety factor.

My calculations show that a 4,500 gallon tank is required. This seems way too big. Even if I use 10 minute coast down, this would only cut my volume in half. That still seems too big for emergency services and it is bigger than air receivers on other equipment that I have seen.

Just a quick refresher or some suggestions would help me out a lot! Should I consider the volume of the distribution piping too? I thought that maybe on other machines that I have seen this may have been done to help reduce receiver size.

Thanks for the help experts!

 

[butelja]

I've checked your numbers, and its even WORSE than you thought. I come up with ~4500 ft^3, not 4500 gallon, which is 7.48 times bigger than you came up with. This is based on isothermal tank behavior, and using PV=mRT (actually V = dm*R*T/dP) ideal gas law.

You state that the items use 456 SCFM of PEAK airflow. Is the AVERAGE airflow signicantly smaller? If so, then you can save size there.

If your volume requirements are right, then the tank will be huge. Keep in mind that 456 SCFM is ~100 BHP of air compressor power. Storing 100 HP times 20 minutes is indeed a lot of energy storage.