Calculating the effect of humidity volume and temp of compressed air

In summary, the conversation discusses the role of humidity in compressed air for an automotive application and how to calculate the temperature and volume of compressed air with different humidity values. The solution for dry air is also provided and a link to psychrometric charts is given for further assistance. Ultimately, the calculation of humidity involves using a slightly different value of k and taking into account the mole fraction of water vapor in the gas phase.
  • #1
InquisitiveOne
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1
This isn't homework, I'm a hobbyist, not a student. Posting here because questions I've previously posted were moved to this section. My intent is to better understand humidity and it's role in compressed air for an automotive application. All help and direction is greatly appreciated. Thank you in advance.


Homework Statement



A turbocharger compresses air which is then cooled, what is the new temperature?

How do I calculate the temperature of compressed air with different humidity values?
How do I calculate the volume or density of compressed air with different humidity values?

Atmospheric pressure - 14.7
Atmospheric air temperature - 77°F
Relative Humidity - 77%
Compressor efficiency - 60%
Compressed air cooler efficiency - 80%
Compressed air cooler media (water) temperature- 70°F
Air compressed to - 14.7psig/29.4psia/2bar

Homework Equations



I haven't any idea where to start.

The Attempt at a Solution


Pretty sure I've figured out how to accurately calculate for dry air, where using the following method and values above result in a temperature of 110.62°F and a volume of 0.53 it's original with 0% humidity:

Solving for temperature

06ff0f83f9253aa25f1dba86e7c714f0.png


(k) 1.4 -1 = 0.4 / 1.4 = 0.286
(T1) 77°F + 459.69 = 536.69°Ra
(P1) 14.7psia
(P2) 29.4psia

T2= 654.36°Ra - 459.69 = 194.67°F

-----------

CE - Compressor Efficiency = 60%

T3 = (T2 - T1) / CE + T1
(654.36-536.69) / 0.60 + 536.69 = 732.81°Ra or T3 = 273.1°F

-----------

HE - Cooler efficiency - 80%
WT - Cooler media (water) temperature - 70°F (529.69°Ra)

T4 = T3 - ((T3 - WT) * HE)
732.81 - ((732.81-529.69) * 0.80)
732.81 - 162.50 = 570.31°Ra T4 = 110.62°F

----------

Solving for volume or compression ratio

CR = T1 / T2 x PR

Using T1, T4 and pressure ratio of 2 from above...

536.69 / 570.31 x 2 = 1.88:1 Compression Ratio

1 / 1.88 = 0.53

So, what was 1 cubic foot of 77°F air should now be 0.53 cubic feet and 110.62°F
Any links or insight on how to account for humidity and the effects of latent heat is very much appreciated.

Thank you again.
 
Last edited:
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  • #3
You pretty much do it the same thing as for dry air, except you use a slightly different value of k. If you know the relative humidity and the temperature, then you know the mole fraction of water vapor in the gas phase, correct. If you know the mole fraction of water vapor x, then you can calculate the weighted average molar heat capacity at constant volume Cv (weighted by the mole fractions of air and water vapor). The weighted average heat capacity at constant pressure is obtained by adding R. Then you can calculate k for the moist air. If at any point you need to use the ideal gas law, you need to use a molar weighted average of the molecular weight.
 
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What is the formula for calculating the effect of humidity volume and temperature on compressed air?

The formula for calculating the effect of humidity volume and temperature on compressed air is: P1V1/T1 = P2V2/T2, where P1 and T1 are the initial pressure and temperature, and P2 and T2 are the final pressure and temperature.

How does humidity affect the volume of compressed air?

As humidity increases, the volume of compressed air also increases. This is because water vapor occupies a larger space compared to dry air at the same pressure and temperature.

What is the ideal temperature for compressed air?

The ideal temperature for compressed air is around 68-72°F (20-22°C). This is because at this temperature, the moisture in the air will not condense, which can cause damage to equipment and affect the quality of the compressed air.

What are the units used for measuring humidity, volume, and temperature in compressed air calculations?

Humidity is measured in percentage, volume in cubic feet or cubic meters, and temperature in degrees Fahrenheit or Celsius.

How can I calculate the effect of humidity, volume, and temperature on compressed air in a real-life scenario?

To calculate the effect of humidity, volume, and temperature on compressed air in a real-life scenario, you will need to measure the initial and final pressure, volume, and temperature of the compressed air using appropriate tools. Then, you can plug these values into the formula P1V1/T1 = P2V2/T2 to determine the change in volume or pressure caused by changes in humidity and temperature.

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