# Designing a pressure vessel

1. Oct 24, 2009

### narbij

Im finding it really difficult to calculate the volume of a pressure vessel for my project.

I have been only given values of pressure and temperature, where the pressure is 80psi and temperature is at room temp.

Its difficult using the ideal gas law as i don't know the number of moles, nor i do i know mass of the air. So how do i go about doing these calculation?

Any help would be much appreciated

Cheers mate

nar

Last edited: Oct 24, 2009
2. Oct 25, 2009

### Q_Goest

If you're trying to figure out the volume of the vessel and you don't have mass (or moles) then you have one equation and 2 unknowns, so it's impossible to determine.

3. Oct 27, 2009

### narbij

Thanks for replying and i thought that was the case.

Im having trouble with trying to derive an equation that will enable me to calculate the velocity or time for a compressed air from a tank trying to accelerate a projectile where the acceleration is not assumed to be constant.

I already have some equations like for example:

F(t)= m*(dv/dt)

mass flow rate= density*area of hole* velocity as a function of time

Pressure (t)= density*gas constant*constant temperature

Im just struggling to connect the pressure as a function of time with velocity as a function of time.

I should be able to find for example the time it should take for 30 psi to drop to 20 psi, and the velocity of the projectile at that time assuming the length of the barrel is really long.

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4. Oct 28, 2009

### Q_Goest

No offence, but I'm guessing you don't have much, if any knowledge of thermodynamics, heat transfer, fluid flow or basic physics. Rather than try to make a detailed model of it, which would be very difficult for someone that already knew how to model those things, I'd suggest doing a rough order of magnitude calculation.

If the volume of the tube out of which the projectile is going is small compared to the volume of the vessel (say less than 5% of the vessel size) then I'd suggest you just neglect any loss in vessel pressure (and changes in temperature, frictional flow losses, etc...) and assume the pressure acting on the projectile is constant. Up to a point, that should get you a pretty good number for velocity.