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# Air requirments for pneumatic filling

## Air requirments for pneumatic filling

(OP)
Hi, I am trying to calculate the SCFM for a compressor used for pneumatic filling of chemical tanks. • I know that there is 15 psi of losses in the line from the truck to the tank, the max pressure is 25-30 psi and that the desired flowrate of the chemical is 180 gpm. I initially thought the volume of air would be the same as the volume of chemical transferred and just did a conversion from gpm of air to SCFM, but the answer I got seemed unreasonably small (~40 SCFM). Then I was thinking perhaps the calculation would be based on mass flowrate instead, but I didn’t reach an answer with my calculations. Can anyone help me out with the calculations here? Apologize if this is a relatively basic question, I am a junior wastewater process engineer and don’t have experience with pneumatic filling. If this isn't the correct place on this forum could someone point me to where I should post the question? thank you.

### RE: Air requirments for pneumatic filling

It is volume, but actual not standard.
So what air flow is equivalent to your flow at the max pressure?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Air requirments for pneumatic filling

(OP)
180 gpm equals 24.1 acfm, which was my initial thought assuming the acfm air flow would be the same as the liquid outflow. I converted to scfm because that's what was asked of me to find. seems too low to me though for this to be correct.

### RE: Air requirments for pneumatic filling

No, it is 24 cfm at 30 psig, you need to correct for the pressure to get scfm.
Look up the gas law.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Air requirments for pneumatic filling

Ed,
He said that he did that.

Samuel.Warner,
Did you account for the hydrostatic head on the tank? If you are filling from the bottom then if the chemical has a specific gravity of around 1.0, every foot of fluid in the tank increases your required pressure by 0.433 psi. If you fill from the top then every gallon will be at a pressure to overcome the friction and the hydrostatic head. If the tank is 20 ft tall, then your required pressure to overcome known losses and hydrostatic pressure is around 25 psig, I'd plan on 30 psig in that case, some dP is required for the fluid to move.

180gpm is 24 ACFm. The density of air at standard pressure is 0.076 lbm/ft3 At 30 psig (assume 44.5 psia) and 60°F density of air is 0.231 lbm/ft3--a factor of 3. Multiply your 24 ACFM by the ratio and get 72.7 SCFm or about 2.2 hp. (with air at standard temperature you could have just used the ratio of pressures, but I always use the ratio of densities to remind me that I don't always work with air).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: Air requirments for pneumatic filling

Samuel.

How did you covert ACFM to scfm?

Did you forget that you need to use absolute pressures to convert?

In simple terms 24 ACFM at 30 psi = 45 psia, so the ratio is 45/15 =3.

3 x 24 = 72 scfm

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Air requirments for pneumatic filling

What are you transferring? Typically, air push is not used with flammables.

A sketch, PFD, or P&ID would be helpful.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Air requirments for pneumatic filling

(OP)
Thank you all for the responses. I guess my original approach was right just not the execution as you all have pointed out.

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