Does anyone know how to work out the volume of air at 100 psi that has been allowed to expand back to 1 atmosphere.this is for a air engine I am trying to build. I don't want the air to have any pressure at the bottom of the stroke but also don't want the valves to close to soon and create a vacuum.also I have heard that expanding air causes a large temperature drop is this correct.my air pressure will be created by a small compressor and a heat source. If you are confused I am trying to build a valved sterling
Air volume calc
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Train_Fan
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You are looking for Boyle's Law, or P1V1=P2V2. (P= Pressure, V=Volume, 1 is pre expansion and 2 is post expansion).
AFAIK this equation may need SI units and is for ideal gasses with no other variables so just be aware of that.
AFAIK this equation may need SI units and is for ideal gasses with no other variables so just be aware of that.
100 psi is 6.9 bar so your compressed air will expand to 6.9 times its compressed volume if it is allowed to expand to atmospheric pressure. This assumes the temperature stays the same. If it doesn't, the following relationship applies:
V2 = 6.9 x V1 x (T2 +273) / (T1 + 273)
Where V1 is the volume of the air at 100 psi
T1 is the temperature of the compressed air
T2 is the temperature of the air after it has expanded.
V2 is the volume of the expanded air.
V2 = 6.9 x V1 x (T2 +273) / (T1 + 273)
Where V1 is the volume of the air at 100 psi
T1 is the temperature of the compressed air
T2 is the temperature of the air after it has expanded.
V2 is the volume of the expanded air.
Entropy455
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Ahh, the good old ideal gas law.
Use caution when applying the ideal gas law within an air engine (PV = NRT).
It is an incorrect assumption that the final air temperature will be the same as the initial air temperature. Reason - unless you are adding heat to the air during the expansion process, the air within the cylinder will be significantly colder after the expansion stroke. This means less final volume, and less final pressure.
The correct equation to use in this instance is the polytrophic process (aka adiabatic and isentropic). The equation is PV^q = c
Where q is the ratio of specific heats, (CP/CV) which for air is 1.4
Using this equation, 100 psig air will expand 4.338 times its initial volume, if expanded down to 14.7 psia with zero external heat input. Assuming an initial air temperature of 86 degrees F, the final air temperature will be -156 degress F.
Note that this cold is short lived, as the air will quickly adsorb heat from the surrounding air. Additionally, there is no such thing as a pure adiabatic process, thus some heat will enter into the expanding air during the expansion stroke. Nonetheless, the polytrophic equation will get you close. Additional note: the reason air valves ice over when compressed air is vented, is because air gets VERY cold as it expands. . . . It's the reason an air engine under sustained use will ice over without an external heat input . . .
Use caution when applying the ideal gas law within an air engine (PV = NRT).
It is an incorrect assumption that the final air temperature will be the same as the initial air temperature. Reason - unless you are adding heat to the air during the expansion process, the air within the cylinder will be significantly colder after the expansion stroke. This means less final volume, and less final pressure.
The correct equation to use in this instance is the polytrophic process (aka adiabatic and isentropic). The equation is PV^q = c
Where q is the ratio of specific heats, (CP/CV) which for air is 1.4
Using this equation, 100 psig air will expand 4.338 times its initial volume, if expanded down to 14.7 psia with zero external heat input. Assuming an initial air temperature of 86 degrees F, the final air temperature will be -156 degress F.
Note that this cold is short lived, as the air will quickly adsorb heat from the surrounding air. Additionally, there is no such thing as a pure adiabatic process, thus some heat will enter into the expanding air during the expansion stroke. Nonetheless, the polytrophic equation will get you close. Additional note: the reason air valves ice over when compressed air is vented, is because air gets VERY cold as it expands. . . . It's the reason an air engine under sustained use will ice over without an external heat input . . .
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