Air Pressure and flow to power calculations?

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Discussion Overview

The discussion revolves around calculating the power stored in air based on its pressure and flow rate. Participants explore various equations and concepts related to fluid dynamics and thermodynamics, specifically in the context of air as a compressible fluid.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about a straightforward method to calculate power stored in air at a specified pressure (30 psi) and flow rate (100 cfm).
  • Another participant suggests using Bernoulli's Equation, providing the formula for potential energy and identifying variables such as atmospheric pressure, density, gravity, height, and velocity.
  • A participant expresses uncertainty about the variables in Bernoulli's Equation, asking for clarification on atmospheric pressure, density, gravity, height, and velocity.
  • Clarifications are provided regarding the meanings of atmospheric pressure, gravity, and height.
  • One participant questions the purpose of the calculations, hinting at a potential application in hydraulic systems, and notes that the compressibility of air may introduce significant errors compared to hydraulic fluids.
  • Another participant reiterates the initial question about calculating power stored in air and introduces the concept of total enthalpy, mentioning the need for velocity and thermodynamic variables (pressure and temperature) for an ideal gas.
  • A participant seeks further clarification on the margin of error in calculations and expresses uncertainty about the units for pressure and flow rate.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a specific method for calculating power stored in air, and multiple approaches and equations are discussed without resolution. Uncertainty regarding variable definitions and the implications of compressibility in calculations is evident.

Contextual Notes

Participants express varying levels of understanding regarding the variables involved in the equations discussed, and there are unresolved questions about the accuracy and applicability of the proposed methods.

infamous_Q
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is there any easy way, or relatively easy way, to calculate how much power is stored in a certain amount of air with a certain amount of pressure and flow? i know that's VERY vague, so let's say (random number's being chosen...) 30 psi at 100 cfm.
 
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use Bernoulis Equation.
PE = P_{atm} + \rho gh + \frac{1}{2} \rho v^2

Regards,

Nenad
 
hmm...call me an idiot if you wish. but i guess I am going to have to guess at these variables:

Patm...no idea

p = density
g = no idea
h = no idea
v = velocity

thats bad i know..but could you maybe help me fill in the blanks?
 
Patm is Atomspheric Pressure
g is gravity of course
h is height
 
What exactly are you trying to get to using this? I have a sneaky suspicion I know, but I'd rather know for sure.

In hydraulic systems, one simply uses

Power = p * Q
Where:

p = pressure
Q = Volumetric flow rate

You can do that here, but you'll have some pretty decent errors due to the high compressibility of air vs. hydraulic fluid and availability to do work.
 
infamous_Q said:
is there any easy way, or relatively easy way, to calculate how much power is stored in a certain amount of air with a certain amount of pressure and flow? i know that's VERY vague, so let's say (random number's being chosen...) 30 psi at 100 cfm.

The amount of energy stored by a fluid is its total entalphy:

h_t=e+v^2/2+P/\rho=c_pT+v^2/2(J/Kg) in the case of an ideal gas.

In order to determine the total content of energy of a gas you need a mechanic variable such us velocity and two thermodynamic variables (P,T). If the flow is at low Mach numbers, it is only needed one thermodynamic variable and one mechanic variable because thermal and mechanical states become discoupled.
 
thanks guys. hey Fred...what's this sneaky suspicion you have? lol. also..how big would that margin of error be? and I'm assuming pressure is kpa and flow rate is m^3/s...(although i really think I'm wrong with the Q unit)
 

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