Sizing tank for Lifting air springs.

[kgwhipp]

Hello All!

I'm trying to size a system for lifting a steel platform using air springs. (They're called air springs, but they're essentially used actuators and I'll refer to them as such) These are big rubber bags that are typically mounted for automotive purposes and have a variable volume.

I need the actuators to exert the force corresponding to their rated pressure (100 psig) during most of the cycle. They each have a volume of 250in^2 when fully inflated and there are two actuators. (This is .2864 cubic feet)

What I'm trying to figure out is what size air tank filled to what pressure will inflate the actuators within 2-5 seconds while they are under full load. I'm hoping that a commercially available tank and compressor can be purchased inexpensively that will suit our purposes.

A few sample numbers for CFM:

1 sec: 17.1cfm
3 sec: 5.7cfm
5 sec: 3.43cfm

I see numbers similar to this for the ratings on air compressors but I'd like the operation to be having the compressor run only to refill the tank.

Environment is typical indoor with no extreme requirements. The actuators will do their cycle during their use then the compressor will have at least 2 minutes to bring the tank back to its original pressure. All equipment is located close enough I'm not thinking that line losses will be a concern, but the fittings are likely to be restricted to 1/4" NPTF which may play a small factor.

Thanks for any insight you can give me. I've been toying around with some P1V1=P2V2 and ideal gas equations but it's been a while since I've done any of these calculations...





-Kevin

 

[zdas04]

If I was doing this, I would determine the maximum bag pressure that I was looking for, then determine what pressure would give me critical velocity using the choked-flow equations backwards. That is the end point. The start point is the end point plus the mass that I was putting into the bags. So if I have choked flow (constant velocity) for the whole fill, the mass flow rate is a function of upstream density so it will drop during the fill, but the drop will be manageable if you end up with choked flow. Then the fill time is a strong function of the piping and a weak function of starting pressure.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[kgwhipp]

Thanks for the advice, I tried to interpret best I could but got stuck on a few particulars.

Max bag pressure would be that 100psi. From my understanding this setup is not going to be hitting choked flow as the source will probably be 150psig max, far a multiplicative factor of only 1.5. (From what I can tell, choked flow in this setup needs a ratio of 1.9+)

The tank is more likely to be around 120psig. Thus there's a possible pressure differential only of 20-50psi between the actuators and the tank. I don't know whether a chart like this applies but looking at

http://www.engineeringtoolbox.com/air-discharge-hose-d_1523.html

Seems to indicate I'll get at least 3.5cfm through my 1/4 piping at a pressure differential of 30psi.

The drop in the tank pressure is the bigger concern for me and knowing what size tank I should procure is the first thing I'm toying with sorting out.


-Kevin

 

[zdas04]

I didn't realize that you had an artificial constraint on your max supply pressure. With that constraint none of what I said applies. In your situation I would design the system with a minimum (end of cycle) pressure of 210 psig. I would really question a decision to use 1/4 inch tubing if I wanted the bags to fill quickly.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.