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!

 

[whylie]

Hi UtilityLouie,

I think you should use 10 mins of ACFM (@100psi) rather than (SCFM ) all your calculations will be 1/6th..

Let me know and maybe I can elaborate more details.

Regards,

whylie

 

[TD2K]

You need to go back to the various design groups, get them to sit down and go over this. I think the problem is that each group is giving you the worse case and you get stuck with a huge vessel (for your numbers, though I haven't checked them, they don't seem unreasonable for a peak air flow of 456 scfm).

Basically, you need to get an average (not peak) air useage for the 20 minute coast down period. Then, use this to size the air receiver which will be x scf. In this case, confirm what is to be used for the starting pressure (average instrument air pressure, minimum, etc) and the minimum the actuators need.

Your receiver is then sized to hold this many scf when it drops from 100 psig to 65 psig. I can tell you that 100 ft3 of air doesn't hold a lot of scf as it drops in pressure by 35 psi which is why you are getting a good sized vessel.