Accumulator Precharge Pressure

[jscotty29]

i'm designing a very basic form of a pneumatic system which involves a telescoping cylinder with a pressurized air chamber acting as an accumulator. what i'm having trouble figuring out is how much pressure to charge the accumulator with to extend the cylinder and hold it. i want to keep the pressure to a minimum, just what's required (of course with a safety factor included).

Volume of Extended Cylinder = 12.5 gal
Volume of Accumulator = 12.98 gal
Accumulator fill temp. = 70F
Accumulator operating temp. = -45F
Pressure required to raise cylinder = 12 psig

i've been using the Ideal Gas Law P1V1 = P2V2 and P1/T1 = P2/T2 but am not sure if this is giving me the results i'm looking for. any help would be much appreciated. Thanks

 

[HEC]

Jscotty29
You are close applying the gas laws, one more step to get there would be the mass of gas involved. Presuming the the system is sealed, this will not change. Determin the mass of gas present in the volume at the extended state;

m=PV/RT

Use this mass as the basis for determining the pressure at teh smaller volume of the accumulator. The mass must remain constant.

Cheers

Mark Hutton

 

[hydtools]

If mass is constant, then P1V1/T1 = P2V2/T2. Back to the gas laws. Also, use absolute pressures and temperatures.

Ted

 

[hydtools]

You should also consider the process of the expansion. In the short time it is probably isentropic. PV^k = const.

Ted

 

[hydtools]

I couldn't leave the problem alone.

Set up three conditions.

1. Fill the accumulator: P1, V1, T1
V1 = 12.98gal , T1 = 534.69R

2. Cool the system, a constant volume process: P2, V2, T2
V2 = V1; P1 = P2 * T1/T2
T2 = 414.69R

3. Expand the accumulator into the cyclinder, an isentropic process: P3, V3, T3
P2 = P3*(V3/V2)^k , k=1.3
V3 = 25.48gal , P3 = 26.7psia, min.

Solve for P1: P1 = P3*(T1/T2)*(V3/V2)^k = 82.74psia or 68psig.

Ted

 

[jscotty29]

Thanks everyone, i've got it now. All I have to do is figure out how to defy the Laws of Physics. you see, what I was trying to do is have a high pressure side of the accumulator and a low pressure side of the accumulator. my thought was if i extended the cylinder vertically, by opening a valve on the HPA, then I could use the weight of the rod and open a valve on the LPA side. With some help from the LPA i could retract it. i wasn't completely trying to create perpetual motion i did figure i'd have a limit to the number of cycles, i was hoping 5 or 6. Boy i feel like an idiot. It never sunk in my head why everytime the cylinder would cycle, the next cycle would require more pressure and so on. of course all your help hasn't gone to waste. I still have REALISTIC applications for these equations. thanks again.